26 replies to this topic

[Paul Vanderheijden]

I have to admit that this is an area that I need to have a much greater understanding. In reading the comparison between Atkinson cycle (early closing) and Miller cycle (late closing) it would appear that using an early closing scheme would raise the "Dynamic Compression Ratio" (DCR), without changing the computed compression ratio. This has obvious implications for knock and the octane level requirement to keep the engine from detonating.

The particular engine that I am working with has a 68mm bore, 74mm stroke and could use a rod anywhere between 110 and 118mm in length. The camshaft that I would propose to use has the following specifications:

Intake Duration @ 0.050 246 deg
Exhaust Duration @ 0.050 254 deg
Lobe center 107 deg
Intake installed on 104 deg
Intake closes at 69 deg ABDC
Static compression ratio 13.5:1
Dynamic compression ratio 10.43

Using a 110mm long connecting rod this would produce a crankpin angle of 268.5 deg ( approx. 1.5 deg before being at right angle to the centerline of the engine).

Earlier intake valve closing of course changes the lever arm effectiveness of the crankshaft throw. I would pose the following questions?

1. At what crankpin angle does the energy required for the additional compression volume (pumping losses) outweigh any resultant increase in power due to higher DCR?
2. At a DCR of 10.43:1, what is the likely octane requirement to forestall detonation?

Additional comments would be greatly appreciated.

Paul Vanderheijden

[J. Edlund]

What an early intake valve closing will do is

1. Reduce the airmass/combustion reducing torque and power output
2. As airmass/combustion have been reduced so will cylinder pressures and with it the risk for knock
3. Improve part load efficiency as means of reduced pumping losses

BMW is using this on their valvetronic engines as a means of thotteling instead of an ordinary throttle plate. Early opening will not affect compression ratio. However, early intake valve closing together with an ordinary intake duration means that the intake valve will open very early, while the engine is doing it's exhaust stroke, and thereby increase internal EGR.

Late intake valve closing increase efficiency by decreasing the "effective" compression ratio in relation to the expansion ratio as some air is allowed to leak back into the intake early in the compression stroke. But note that a late closing isn't automatically the same as allowing air to leak back into the intake as due to dynamic effects the pressure in the intake may be higher than in the cylinder.

Atkinson cycle engines as found in for instance Toyota Prius uses a late intake valve closing, increasing the compression ratio compared to the expansion ratio. This does however come at the cost of some power and increased friction losses.

[Paul Vanderheijden]

Perhaps I did not state clearly what my concern is. I have done a comparison of a number of camshafts. The engine is a 1050cc, in-line 4, pushrod OHV, 2V per cyl. I used a standardized Computed Compression of 12.1:1 for this comparison. See below

Camshaft Duration/Lift L/C Overlap In. Int. Closing Deg Dynamic Displ (DD)/DCR
SLR300S 290/300/12.4mm 107 deg 81 deg. 69 deg. ABDC 804cc/9.49:1
SLR300 300/12.4mm 108 deg 84 deg 74 deg. ABDC 765cc/9.07:1
Kent FT 6 304/10.8mm 106 deg 92 deg 78 deg. ABDC 732cc/8.73:1
PBS A8 305/10.6mm 108 deg 89 deg 76.5 deg. ABDC 745cc/8.86:1
CatCams 305/11.45mm 108 deg 89 deg 80.5 deg. ABDC 711cc/8.50:1
CatCams 310/11.45mm 108 deg 94 deg 83 deg. ABDC 689cc/8.27:1
Abarth 316 316/10.4mm 105 deg 96 deg 88 deg. ABDC 644cc/7.80:1
Laur 319 319/10.5mm 108 deg 103 deg 87.5 deg. ABDC 649cc/7.84:1
Abarth 336 336/11.7mm 105 deg 126 deg 93 deg. ABDC 634cc/7.70:1

All of these cams are in use within the Abarth community. Durations range from 290 -336 deg. As one might imagine, the long duration camshafts have the largest overlap and generally develop their horsepower in the higher RPM portion of the usable RPM band (7500-9000 RPM). By contrast the shortest duration camshaft produces it power best at a lower pointin the RPM band (5700-7500).

Due to the later closing of the intake valve in the Abarth 336 cam (as the most extreme example), the effective dynamic displacement of a 260cc cylinder is 158.5cc. Whereas at the other end of the scale the SLR 300S camshaft, with the intake valve closing 24 degrees earlier, has an effective dynamic displacement of 201cc for the same 260cc cylinder. This computes to a dynamic compression ratio of 7.70:1 and 9.49:1 respectively for these two camshafts, for what is dimensionally the same engine.

Obviously, if an engine running with a DCR of 7.7:1 will "just" run on say 98 octane fuel (with a total spark advance of 30 degrees) without detonating, then it is perhaps equally evident that a duplicate engine using the SLR300S camshaft (with equivalent ignition advance) may suffer from detonation at 9.49:1.

Given the bore/stroke/rod length combination of this motor, a point 71 deg. ABDC will put the crankpin at 90 degrees to vertical. Can anyone point me toward any literature that discusses the lever arm effectiveness of the crankpin in relation to intake valve closing in terms of producing power most efficiently?

Secondarily, any ideas on the relationship between octane rating and dynamic compression would appreciated.

I apologize if the chart did not come out as I expected.

The SLR cams have the shortest duration/highest lift, and earliest intake closing

[cheapracer]

From memory Grumpy Jenkins books would give you a lot of help for what you seek about cam closing events and results, conrod length effects etc. Very simple explanations. You should get more midrange with earlier closing but on a 1050cc 4 why would you bother unless you have gearbox ratio restrictions? Otherwise REV it!!

Cam manufacturers/grinders are very helpful people, write to them with all your specs.

[Joe Bosworth]

Paul

I really can't relate to "lever arm effectiveness of the crankpin in relation to intake valve closing in terms of producing power most efficiently?" nor for that matter "At what crankpin angle does the energy required for the additional compression volume (pumping losses) outweigh any resultant increase in power due to higher DCR?"


Nor can I relate to "relationship between octane rating and dynamic compression would appreciated."

I have never seen any correlation of these things to the real world., May be that my experiences aren't broad enough!!

However, having said that I have a whole bunch of rules of thumb that I have developed over the years that seem to relate to real world HP. Let me offer the following.

1. Run the longest rod that you can. You say that is 118 mm which is only 1.59 times the stroke length. Even this is way short of what real engines run so I have to think that anything any shorter is getting into real bad territory for side thrust and friction HP.

2. Obviously your bore/stroke ratio is terrible for HP. But if that is what you have then you have to do the best with what you are dealt. Intake valve diameter is quite compromised and in particular getting breathing room around the valve circumference. Not knowing your valve guide centers, (which can be changed if you really go to the trouble and expense) I have to make a guess at the max intake valve diameter you can run. It has to be in the 34 1/2 to 36 mm range. Sculpting the high side of the head and putting ears in the top of the bore to improve flow is a very likely benefit at these valve diameters but I say this not having seen your configuation. Sculpting and ears obviously reduces compression ration but it is no good trying to squeeze that which hasn't gotten into your cylinder in the first place. Less CR and better flow usually reates to more HP.

3. For valves in the size range of 34 1/2 to 36 you want to run a cam grind of 284 degrees for the smaller to 298 degrees for the larger. (These are real durations from valve opening to closing). Starting with symetrical timings these durations equate to running 23 80 80 23 to 30 88 88 30 timings. You might gain performance by rapping the exhaust durations back about 5 or 10 degrees or even taking a few exhaust degrees out but I believe that you can only find this out on the dyno.

4. Static Compression ratios to detonation don't mean much because detonation is far more related to cylinder mixing and ignition timing. I don't know of any generic computer ptrogramme that gives realiable information for the range of mixing and timing relationships that exist. Again may be I have a lot to learn on this front but I still go back to the dyno and real life experience of varying one thing at a time.

Regards and hope this helps you get a direction

[Joe Bosworth]

PS: Forgot to add that for those valve sizes you probably want lifts at the valve of 8.6 to 9.4 mm for that range of valve sizes assuming that you can get that with the grinds and piston-valve clearances that you have.

If you can't get those lifts then another set of compromises set in.

Regards again

[Paul Vanderheijden]

Joe,

Thanks for your reply. I have provided some more information about the current build of the motor, as it may provide you with some additional insight.

For Cheapracer - At the moment we buzz these little motors to just under 9000 RPM. The block has only three main bearings, and once consquence of using more RPM than this is that the center web usually fails. Needles to say the end result is catastrophic.

As you may have guessed, the configuration of the motor is not what we would expect in "modern" terms. Yes, I would like to get a 1.75 rod ratio too, but with a 74mm stroke it just is not possible. When Carlo Abarth rubbed his magic wand on these motors in the 60s, the easiest way to get more displacement was to lengthen the stroke. The ABSOLUTE largest intake valve that can be managed without moving the valve guide, is 32mm, with the attendant exhaust being 28mm.

As far as notching the block is concerned, at stock configuration the distance between the adjacent cylinders is approximately 5.5mm, so there is precious little to work with.

I seal the cylinders with a combination of a solid 0.027 solid copper head gasket and four, 66mm "Coopers" type sealing rings. These 0.031 hollow rings are "gas-filled" and as they heat up the pressure inside them increases to increase the sealing pressure. They compress 0.004 to the same thickness as the copper head gasket. This also means that with the pistons installed with the squish area at deck height, that the squish distance is 0.027 inch. As such the copper head gasket only has to deal with sealing water and oil passages, and the rings seal the cylinders.

I have even given some serious thought to drydecking the block and running a modified cooling system with two discreet flow paths, one for the head and one for the block. The only passage remaining in the head gasket would then be the oil passage that feeds the rocker arm shaft.


Here is a photo of the cylinder head and combustion chamber.




The combustion chamber photo is one with 31 and 27mm valves respectively. The chamber gets a little more work before completed. As you can see if you take each valve out an additional mm, then the seats are nested in one another. The layout is a wedge configuration, with the valves canted at 11 degrees. There is a "squish area" opposite the spark plug.

The induction system consists of four 39mm Keihin slide throttle carburetors. These are then mounted on a special CNC machined intake manifold (see drawing below). The carburetor bodies are mated to the intake manifold using four Adel Wiggins clamps, for ease installation. One o-ringed end is welded to the intake manifold, and the other o-ringed end is the screwed into the Keihin carburetor.






The piston used is illustrated below.



The dome on this piston can me modified to provide whatever compression ratio desired. In addition part of the final piston treatment is to machine a fire-slot for the spark plug. Once all of the modifications are complete, then the piston is sent off to have the skirt polished and DLC coated, and the crown ceramic coated.

The camshaft that I am running at the moment is the SLR 300S. This is 290 degrees duration (seat to seat) on the intake and 300 degrees duration on the exhaust. Valve lift at the CAMSHAFT is 8.4mm with a 1.45mm rocker arm, so this means 12.18mm at the valve. According to my calculations, this is right at the limit for a 20mm diameter flat tappet lifter, which is what we have to work with.

I would be pleased to hear further thoughts.

[cheapracer]

Gee that looks like a Datsun 1000/1200 setup!

Either twin plug it or at least use a smaller plug which you could get closer to the roof and grind a flame path across the piston crown, I reckon you have to be suffering some plug masking.

Is your tappet diameter and/or radius increasable to get greater valve acceleration? If so you may be able to go with less duration.

You have a lot of weight in those tappet adjusters still, with my Datsuns (thats a Nissan Kids) I ground the squares off, hacksawed a screwdriver groove in and used thinner locknuts.

[Joe Bosworth]

Paul

This is a wonderful world we live in. I am writing this watching the end of the French F1 race and awaiting the beginning of the British Moto GP. I am sitting in Australia watching the races on a South African station via satellite because that is the easiest way to watch these live. Here we are talking about an Italian engine with someone that sounds Dutch living in the US. All on an English domiciled forum.

Thanks for posting more particulars on your engine, that helps to make sense of what’s happening.

I am a great believer that the best engines are the result of matching inlet systems, porting, valve sizes, cam configuration, and exhaust systems so that all match. Going too big or too small on anything in the system means that either HP or torque is being sacrificed.

Let me digress for a moment and say that I have had good results in sealing head to block with the very simple system of facing the head and block and making a copper sheet gasket of the desired thickness. Just anneal the copper back to dead soft after cutting the desired shape and openings. Using this you can get by with some pretty narrow bore spacings and save some of the efforts that you are going to now. However what you are doing works and it may be that you don’t want to go in another direction.

Back to where we were. I believe that you may be running quite a mis-matched system. I would love to see a dyno sheet or two just to convince myself that you haven’t found some formula that falls well outside my parameters and still works. Whether you want to share such info is up to you, OK to send these to me by private email if you are sharing.

Let me specify things that I find non-matching in what you say you are doing:

1. You are running an exhaust valve way bigger than needed in relation to the inlet valve. Exhaust of 89% of inlet is enormous.

2. Following from the above use your space by increasing the inlet valve size. Revving to near 9000 with 31mm valves means that you are using monstrous valve pocket velocities; way past effective speeds. For someone who is capable of fine machining and interesting searches for performance use some of your efforts to getting larger inlet valves. On seeing your photo my immediate thought was that you can go up a mm without doing much of anything. By nesting valve seats you can get even more and if you decrease exhaust valve you can start to get to sizes close to those I recommended even before you move valve guides, which is what I would do in your situation.

3. A valve lift of 39% of valve diameter is way more lift than that does any good. Using 45 degree seats you are using all of your valve pocket area with a lift of 25%. One needs to use a bit more lift because all of the valve circumference isn’t being effectively used but I have convinced myself years ago that you want to maintain port velocity through the valve. Among other things, making big valve lifts cost internal HP that is better let loose to the flywheel.

4. While talking effective valve circumference use, I go back to my earlier comment of scalloping and earing. It doesn’t look as if you are taking advantage of either and I am sure that you have room for some of both. I think that 5.5mm is plenty to work with and you have a fair bit of valve masking.

5. Back to camshafts which is where we started this whole thing. The SLR 300S cam is way too big especially for the 31mm inlet valve you are using. I strongly suspect that you are sacrificing heaps of torque in running that wild and while the valve size and gas velocity is choking your HP at the other side of things as well. I haven’t been around the Abarth community for years but when I was they were well known for having outrageous HP claims and gutless torque. If still true it follows from what you describe but I apologise in advance if I am wrong. If it were me, I would both increase inlet valve size and cut down cam specs by a bunch. It would be very interesting to see what a 30 60 or 30 65 grind cam with valve lifts that I specified. Stuff that into the 31mm engine and see what happens. I suspect no lose of HP and a big torque gain. You might have to cut some revs off the top end but this will not lose you on-track performance unless you are really on the edge of now gearing absolutely perfectly for what you have. Damn hard to do that unless you are working through a Hewland or the like.

6. You mention again playing with water flow things. I would save the effort unless checking exhaust gas temperatures near exhaust port faces show significant differences between ports. Having said that, once upon a time I had an engine for which I took a rear auxiliary water pipe off of and that evened things out with no more effort.

7. I like to play with early exhaust closings to build torque but I won’t go there for now. Early closing works on big exhaust durations seemingly because of maintaining CR.

Please feel free to tell me that your experiences indicate that I am UTS. I/m always able to learn more from the experiences of others; There are sometimes more than one way to skin a cat.

Regards

[Paul Vanderheijden]

Joe,

I took what you had suggested to heart and did some computer modeling of your suggestions. I used head flow specifications that I had measured on our air-flow bench for the 8 port head some time ago and used those as a base line. In addition, I used the exhaust system flow characteristics that we had established for the following header.




Per your suggestion I used a duration of 275 (Adv) with a total lift at the valve of 10.7mm. Intake valve size was 33mm and exhaust 26mm. Total ignition advance was set at 32 deg. Hear are the results:

Item -275- -300-

Overlap in deg/sq inches -2.2- -5.5-
Total Intake/Exhaust % -67- -65.1-
Overlap 47 deg. 66 deg.
Dynamic Comp Ratio 10.32:1 9.78:1
Intake Valve Area in deg/sq inches -124.6- -137.5-
Exhaust Valve Area in deg/sq inches -81.1- -92.1-
Theoretical Crank Compression 291 PSI 270 PSI

Avg Torque 4000-8500 RPM 77.5 ft/lb 79.1 ft/lb
Avg HP 4000-8500 RPM -91.5- -94.6-
Peak HP @ 7500 RPM -108HP- -118HP-
Max volumetric efficiency 101% @ 6000 RPM 104.3% @6000 RPM
Peak BMEP 209@5500 RPM 216@6000 RPM

[J. Edlund]

[quote]Originally posted by Paul Vanderheijden
All of these cams are in use within the Abarth community. Durations range from 290 -336 deg. As one might imagine, the long duration camshafts have the largest overlap and generally develop their horsepower in the higher RPM portion of the usable RPM band (7500-9000 RPM). By contrast the shortest duration camshaft produces it power best at a lower pointin the RPM band (5700-7500).[/quote]

1. With a long duration cam the intake port is closed very late. At low speed, the air velocity in the intake port is low, thus there isn't enough "ram air effect" to overcome the pressure build up in the cylinder before the intake valve has closed. This force air out of the cylinder back into the intake manifold and with that VE and torque output decrease. In addition to that, exhaust pulses can arrive during the overlap period (when the intake valve has just opened) since the length of the exhaust pipes are calculated for high speed use. These exhaust pulses can be powerful enough that exhaust gas is forced all the way back into the intake manifold. In any case, it will reduce VE and torque.

At high speed, the exhaust pulse arrive correctly creating a pressure at the exhaust port that is below atmospheric. This leads to good exhaust scavenging; less exhaust will be left in the cylinder and more air will be drawn in. The high air velocity in the inlet ports together with the correctly tuned intake runner lengths will cause a pressure above atmospheric in the intake ports towards intake valve closing at high speeds. This high pressure means that air will be forced into the cylinder even after bottom dead center while the charge is being compressed.

[quote]Originally posted by Paul Vanderheijden
Due to the later closing of the intake valve in the Abarth 336 cam (as the most extreme example), the effective dynamic displacement of a 260cc cylinder is 158.5cc. Whereas at the other end of the scale the SLR 300S camshaft, with the intake valve closing 24 degrees earlier, has an effective dynamic displacement of 201cc for the same 260cc cylinder. This computes to a dynamic compression ratio of 7.70:1 and 9.49:1 respectively for these two camshafts, for what is dimensionally the same engine.[/quote]

2. "Dynamic displacement" and "dynamic compression ratios" has no real value, the best you can do is o forget about them. They only confuse things. The answer why they has no real value is in between the lines in answer 1. A hint: think dynamic, not static.

[quote]Originally posted by Paul Vanderheijden
Obviously, if an engine running with a DCR of 7.7:1 will "just" run on say 98 octane fuel (with a total spark advance of 30 degrees) without detonating, then it is perhaps equally evident that a duplicate engine using the SLR300S camshaft (with equivalent ignition advance) may suffer from detonation at 9.49:1.[/quote]

3. No, you can't use these figures like this, not to mention that ideal ignition timing can differ between the cams. Like I said earlier, forget about those "dynamic" figures.

Knock will simply occur if flame arrival time exceeds end gas induction time. A complex subject!

[quote]Originally posted by Paul Vanderheijden
Given the bore/stroke/rod length combination of this motor, a point 71 deg. ABDC will put the crankpin at 90 degrees to vertical. Can anyone point me toward any literature that discusses the lever arm effectiveness of the crankpin in relation to intake valve closing in terms of producing power most efficiently?[/quote]

4. There is no such thing. What you want is a long conrod giving you a low rod angle. This reduce piston side load and friction while also reducing peak acceleration around TDC (which has some additional benefits).

A long rod also gives you a piston with a low compression height (for a given deck height). Low compression height pistons can be made lighter, reducing stress, and with that con rod weight and once again piston side load and frictional losses.

Obviously, at some point the increased length of the rod will do more bad in terms of increased weight than good, but since you use a stock block with a given deck height there is no need to worry about that.

[QUOTE]Originally posted by Paul Vanderheijden
[B]Secondarily, any ideas on the relationship between octane rating and dynamic compression would appreciated.

5. See 3

[cheapracer]

Duration of a cam has nothing to do with the closing point, that is something you determine when dialing the cam in (of course longer duration may then give you opening time problems) and when you order the can what center you have it ground on (single cam motor speak) that also dertermines the overlap.

Indeed a valve has a maximum flow lift point that it will flow no more but it is not a waste of time opening the valve further because that lengthens the duration of time that the valve is at its best flow.

Paul why does ur inlet port porting look crappy, refering to the 'bump' at the guide - do you have meat restrictions there or is it the angle of the photo etc?

[J. Edlund]

Quote

Originally posted by cheapracer
Duration of a cam has nothing to do with the closing point, that is something you determine when dialing the cam in (of course longer duration may then give you opening time problems) and when you order the can what center you have it ground on (single cam motor speak) that also dertermines the overlap.

Indeed a valve has a maximum flow lift point that it will flow no more but it is not a waste of time opening the valve further because that lengthens the duration of time that the valve is at its best flow.

Paul why does ur inlet port porting look crappy, refering to the 'bump' at the guide - do you have meat restrictions there or is it the angle of the photo etc?

A longer duration generally means the valve will be closing later and open earlier; that if we talk about functional setups.

Max flow is generally reached at a quite high lift, about 40% of inner seat diameter. More lift than that can can also be of benefit.

[cheapracer]

Quote

Originally posted by Paul Vanderheijden
Joe,

I just noticed your megaphone. Everybody I have known who has tried an open megaphone has had nothing but trouble, mostly impossible to tune carburation, peaky and or flat spots/missing. All went back to straight thru although a couple had some success with reverse cone megaphones.

[Joe Bosworth]

Paul

Cheap has made a couple of points that for the purposes of general education of readers I feel I should be commented on:

1. Cheap, “I just noticed your megaphone. Everybody I have known who has tried an open megaphone has had nothing but trouble, mostly impossible to tune carburation, peaky and or flat spots/missing. All went back to straight thru although a couple had some success with reverse cone megaphones.” Comment, Way back in the dark ages of running an FWA Climax I ran a mega exhaust that I put a very small reverse on with very good affect. But Cheap is correct that great care needs to be taken so that some really bad RPM points aren’t developed. On first view I judged your primary header length to be about 40 inches which is in the correct ball park so I didn’t comment and didn’t want to go to the bother of calculating mega length and diversion. But I previously suggested that you post dyno data because you are getting into areas of cam, valve size and exhaust tune that can only be proven on the dyno. Funny bumps and lumps show up clearly on the dyno. No amount of desktop calculation shows what you need to know. The desktop programmes only point the directions to take or not take and need to be correlated with real data.

2. Cheap, “Paul why does ur inlet port porting look crappy, refering to the 'bump' at the guide - do you have meat restrictions there or is it the angle of the photo etc?” Comment, I wondered as well but chose to remain silent. Worth having a look at.

3. Cheap, “Indeed a valve has a maximum flow lift point that it will flow no more but it is not a waste of time opening the valve further because that lengthens the duration of time that the valve is at its best flow.” Comment, Cheap is correct about the max flow point of a valve lift but I feel is quite incorrect on the rest of his point! For a correctly selected cam, the duration of time that the valve is at best flow has nothing to do with lifting beyond the optimum point. But more importantly, a great part of internal HP losses come from valve actuation. You can make some calculations of your own and you will find that even 2 or 3 mm of over-lift of valves takes much HP that is better utilised at the flywheel. If added valve lift increases HP fine, but over-lifting does greatly increase internal HP losses.

Regards

[Paul Vanderheijden]

"Joe" and "CHeap"

Thanks for the comments and insight. I should explain that the photo of the head was used to indicate the "general" shape of the combustion chamber. My finished heads look somewhat different. The intake valves have some additional relief, the intake bowls and guide area is much more streamlined. I will post some photos the next time the head is off.

I have just spent some time on the dyno testing various cam/intake/exhaust combinations. Some interesting results were obtained. I will post some data later.

Thanks for the suggestions.

[Paul Vanderheijden]

Hello Joe,

Got your email. Unfortunately due to other projects being on the dyno i have not had time to get back to this.

I have however had a look at the issue of parasitic losses associated with valve lift (or excessive valve lift). In conjuction with my cam grinder I looked at the spring pressures currently associated with the SLR300 cam lift profile, and whether these were appropriate. Keeping in mind that cam grinder many times recommend a spring combination that is a "safe" combination, but may depending on valve train weights be more than what is required.

In this case the spring being recommended was a dual coil design with an installed spring height of 1.350 inches. At this height it has a seat pressure of around 90 lbs. At full lift the pressure across the nose was measured at 245 lbs., for a spring rate of 310 lb/inch.

One thing that I could attest to is that this combination does not float or loft valves !! It also does not do any damage to the camshaft lobe, as these always look good when they are inspected after some time of running. So while perhaps being heavy handed, it was not enough to affect longevity of the camshaft. Of course, it might have deleterious effects on other parts of the valve train such as the rocker arm/shaft interface, but I will make some conclusion on that later.

So how much lower spring pressure could one use with the SLR300 camshaft and not induce either valve float, bounce or lofting? Well, we know that a spring with a closed pressure of 50 lbs and a nose pressure of 151 lbs will not work, as I had a customer try this and even with 0.100 valve-to-piston clearance he managed to bend three of the four exhaust valves. I modeled his spring combination and sure enough at over 7500 RPM he was lofting the valve into spring bind (some 0.120 inch beyond maximum lift of the cam), so this would account for around 0.020 of valve/piston interference not taking into account anything for rod stretch. I posted graphs of this modeling on my website under the FAQ section.

As far as dyno testing, I am going to hold off until we have some new hard-chromed rocker shafts to work with (in production now). I have already modified some standard rocker arms with bronze bushings.

I have posed a computational problem as a new post that I would appreciate some help with, perhaps you could take a look at it.

Regards
Paul

So using the same component weights just what spring combination is likely to work well? It turns out that a valve spring with a rate of 220 lb/in would probably suffice, but in the interests of reliability I would probably opt for something perhaps 10 lbs more.

This is a 25% reduction of pressure and would have a significant impact on boundary interfaces at the rocker arm, push rod tip and cam lobe. Of course this also has significant impact on parasitic losses, as the valve train is one of the most critical areas.

I am working with a spring manufacturer at the moment to make some springs that meet the above criteria. I will test some more when we get our hands on them.

[cheapracer]

Quote

Originally posted by Joe Bosworth
Paul

Cheap has made a couple of points that for the purposes of general education of readers I feel I should be commented on:

3. is quite incorrect on the rest of his point! For a correctly selected cam, the duration of time that the valve is at best flow has nothing to do with lifting beyond the optimum point. s.

Great thing about democracy is that anyone is entitled to his opinion no matter how wrong it is!! (thats a joke Joe)

The duration of maximum flow has everything to do with lifting past the optimum lift because you can't have a square lobe (within reason, the first Group A cars had a "same as production lift" rule then and they had some mighty square looking lobes!). You have to continue to lift past the optimum flow point for some time to extend the duration of maximum flow.

I'm sorry if you disagree but its just fact that can't be denied and racers, particualy drag racers, have proven that the valve train hp loss is far outweighed by the hp gained.

[rms]

Quote

Originally posted by cheapracer


You have to continue to lift past the optimum flow point for some time to extend the duration of maximum flow.

So what happens when you lose flow by going past the optimum point ?

[Joe Bosworth]

rms

You don't loose flow, you just don't get any more. This is easily seen in flow bench work. You can graph flow versus lift and see that as you incrementally lift higher past about 2/3 open that flows increases get smaller until you get to a max flow.

The real problem is that opening valves cost internal HP.

It is not often appreciated by most people that only about 1/3 of the energy input to an engine actually gets to the flywheel. The 2/3 are lost to heat, friction and what I will call parasitic losses. The parasitic lasses are in pumping oils and water, flowing air, and the big ones accelerating and de-accelerating pistons and valve systems.

The biggest gains in flywheel HP over recent years is in decreasing parasitic losses. Lower oil p

[Joe Bosworth]

Oops, the post above was sent without me consciously doing anything. Will continue:

Lower oil pressures, lighter oils, less oil windage, lighter pistons and rings, and reducing valve system losses. Learning to run engines at higher temperqatures has also been a big contributor. Contrary to most peoples thinking a hooter engine makes more HP as long as you keep the inlet air charge cool. (Back in the fifties, we helped cool flat head Ford inlet charges by putting pennies in the heat risers at the manifold faces. Ran good once warmed up; bad at first start on a cold day!)

Quite a measurable and consequentual part of parasitic valve HP loss is used up in opening the valves. At some point in the lift cycle the added lift starts to cast more HP than that gained by increased air availability from the lift.

I only raised the case with Paul because the data he provided, valve diameter and valve lift that my intuition and experience says that he is likely to be lifting too far. This can only be proven by dyno work on different cam shafts. Once upon a time I engaged in a cam development programm and tested a cam a week until i found happiness. I have seen it written, don't remember where, that one team tested 50 cams to find the sweet one for their engine.

Once you have done enough testing you start to get a feel how far you can go with any engine. I have never worked on a Fiat as Paul is doing so I can only give educated guesses as to directions to take. Hence my posts earlier in the thread are meant only to help steer him in the right direction. Some day when he gets real resultd we will then know the truth.

The issue of square top cams is interesting.

From a flow standpoint a cam approaching completely square would make the most HP. (That is a bit tongue in cheek but pretty close.) The only problem is that one is constrained by mechanical problems Most of these are in valve spring design that sets certain limits though this is another area in which wonderful advances have been made since I started. The problems of cam ramp to lifter interface is another problem but small compared to spring detail.

There have been several test engines made using either rotary valve or electro-pnuematic/hydraulic valve actuation. These innovations allow one to get closer and closer to a square lobe configuration and almost always show great promise. The big problem is that virtually all rule makers legislate against such developments.

The reality is that a cam that stops lifting at the optimum point will make more HP than one that lifts too far. A relatively flat top cam is easy to do and many are available.

While I am waffling; one of the big problems with getting HP is that for most people the adage is that more or bigger is better. A cam of 290 degree duration is better than one of 280 degrees. One of 13 mm lift is better than 11 mm and so on. A really big port is better than one a bit smaller and so on. All very wrong

The bullshit only really stops at the track. Some of the BS gets sorted at the dyno. Those who choose not to believe do so at their one peril.

[rms]

Joe,
I have seen quite a few heads on the bench that actualy lose flow when lifting past optimum flow point.
Lifting past this point can cost you BHP from both a parasitic and flow point of view.

It seems the airflow seperates from the valve underhead and becomes turbulent, you can hear it on the bench.

[Joe Bosworth]

RMS:



Regards

[cheapracer]

Take your arguement up with Smokey or Grumpy, their bullshit about lifting past maximum flow has been proven over and over.

They are both pioneers of true windage systems and HP loss theorys turned into practice by the way.

I reported about the team testing 50 cams over a month period, they weren't looking for a sweet "one ", they use different cams at different tracks depending on requirements.

[cheapracer]

Quote

Originally posted by rms

So what happens when you lose flow by going past the optimum point ?

The same as what happens when you don't lift the valve to it's optimum point, but it's there for a lot less time, I don't work with engines that have static valve.

By the way, what valve shape, combustion chamber type, port shape, piston crown shape, cam overlap, conrod ratio etc, etc,.........were we talking about?

Need a licence for a gun but anyone can own a flowbench.

[Joe Bosworth]

I was really going to but out of this discussion with my next to last post but I have been sucked in to a technical arguement that now tells me I should argue with Smokey Yunick.

I hadn't picked up Yunick in the last 20 years but I now have gone back to his book that I got in about 1983.

I will slightly paraphrase from his writings on valve lift contained on pages 75 and 76 where he is discussing Chev small block practice using a 2.05 inch - 52.1 mm valve.

Paul is using a 32 mm valve. He has been playing around with valve lifts up 39% of valve diameter.

Smokey states that when the valve lifts past 32% of valve diameter there is absolutely no flow increase.

He does go on to say that adding dwell at max flow is useful and that going to 33% lift has accounted for for the increased HP currently available.

This is fully within the bounds of what I have been saying. My additional point is that since 1983 that you can now get cam grinds that give you the increased max flow dwells without the further lift.

There is no doubt that on the BS scale flow bench work falls behind dyno results that falls behind lap time on the watch. Unforunately computer forecasted HP falls even further behind which was the point of one of my earliest posts on this thread.

All of my points are for naturally aspirated engines. The drag guys and others running various types of puffers are a completely different deal not applicqable to Paul's endeavours.

The last few posts have been triggered by my private mail to Paul asking if he has any dyno data yet. I will be fascinated how my thoughts correlate with his Fiat work for which I have zero experience. It's all about adding to one's knowledge with new and broader information.

Regards and over and out til real data appears

[cheapracer]

Quote

Originally posted by Joe Bosworth

All of my points are for naturally aspirated engines. The drag guys and others running various types of puffers are a completely different deal not applicqable to Paul's endeavours.

I haven't the faintest clue why you immediately think of puffers just because the word drag racing was mentioned. Grumpy Jenkins is one of the all time legends of Pro Stock N/A class.

Anyway its turning into a pissing competition and all i'm interested is Pauls outcome. One reason is it brings back memories of my Datsun 1000 A series motors which I would have to say that the Japanese had a bit of a gander at the FIAT motor first! The similarities are just too much of a coincence and Paul, it may be worth investigating that side of it too as the A series is very well known mod wise.

And there still is no such thing as a square cam, I don't care what the ads say and 32% is specific to an SBC not that one should work in percentages unless history on popular makes gives you an idea of a proven starting point, I don't think Pauls rig fits that bill :-)