13 replies to this topic

[Canuck]

Trying to calculate the theoretical specs for a NA engine with an output in the 80-85hp/l range. Base is the BMW M30 6-cylinder with a 94mm bore and a 90mm stroke. ~3970ft/min piston speed means ~6700rpm which is in the realm of valve train speed.

Taylor's volume, in discussing inlet mach index and Ve gave the formula z=(Ap/Ai)X(s/Ci x a), where Ap is piston area, Ai = total intake valve area per cyl, s = piston speed in m/s, Ci = mean valve flow co-ef and of course a = velocity of sound in the intake gases. The theory being that an engine should be designed such that at peak rpm, Z >= .6 as Ve drops off rapidly past that point.

Firing that up, I get a valve size of 52mm (which exceeds the .53*bore rule-of-thumb but that's not my issue). The port velocity formula of RPM=(23.275*Gas Speed (m/s)*(.9*Valve dia)^2)/cyl vol(cc) gives me a valve size of 51.5mm at 80m/s, but during the port velocity discussion, it was made reasonably clear that 100-120m/s was a good mid-range velocity. At 110m/s, that formula gives me a 44mm valve size which seems awfully small considering the stock head is 47mm.

What gives? Am I asking for something unreasonable there?

[McGuire]

I don't know about the .9*valve OD deal.

[Canuck]

That should have been Z <= .6 - These 5am start (@ work) 12 hour days are killing me.

If I go the valve size -1mm, things get even more skewed and I end up with a 42mm intake valve.

[McGuire]

In a high-performance two-valve engine, for practical purposes the ideal intake valve diameter is the largest that will fit in the chamber and bore and still leave room for the exhaust valve. So how large should the exhaust valve be? Somewhere between 66 and 80 percent as large as the intake valve... another interesting question, and another one based on compromise as much as first-order airflow considerations. We can tackle it while we are also asking: how much should valve diameter compromise combustion chamber design, and vice versa?

As I see it, an intake valve can't be too big. It can be too heavy, or it can be open too far, but it can't be too large. As proof of that statement please refer to the four-valve engine: More valve area than any two-valve and it still works better.

I lean toward dubious regarding the value of formulas based on zero-dimensional flow calculations. I believe you could select the 50 most meritorious two-valve engines ever built (or the 50 most applicable in comparison for a particular project) and calculate the average bore-to-intake-valve ratio for the group, and the resultant figure would be just as useful as anything derived from flow-based math. The approach may not seem terribly scientific but it would be powerfully empirical. And really, both are based on other than first-order premises.

[Canuck]

Thanks McGuire. It's my understanding that Joe's formula was developed this way, or verified - via collecting and analyzing information from other engines and strangely enough when using his 80m/s target figure, it jibes closely with the other. I'll gather up some specs and see what sort of data I can put together.

[Joe Bosworth]

Canuck

You have me somewhat puzzled with the givens in your first post,
"Trying to calculate the theoretical specs for a NA engine with an output in the 80-85hp/l range. Base is the BMW M30 6-cylinder with a 94mm bore and a 90mm stroke. ~3970ft/min piston speed means ~6700rpm which is in the realm of valve train speed."

Let me explain my puzzle. I am far from a BMW expert, they cost too much to play with, at least where I am. Having said that, I have googled a bit and find that your engine is likely to be the 94.6 mm bore 4V engine going back to about 1992 and referred to as the M88 engine. Except that you refer to a 52 mm valve that indicates a 2V configuration.

I offer my analysis for a 2V engine first. If I was building such an engine wanting to make best HP at 6700 rpm I would apply a 53 mm inlet valve and expect to get something like 115 bhp/L. (Way more than your expectation). If I took such an engine and decided to optimise for 85 bhp/L I would apply a 46 mm valve and expect to get my HP at about 5000 rpm.

If we are talking of a 4V engine wanting to make best HP at 6700 rpm I would apply a 37.5 mm inlet valve and expect to get something like 125 bhp/L. If I took such an engine and decided to optimise for 85 bhp/L I would apply a 30.5 mm valve and expect to get my HP at about 4400 rpm.

You also mention, "it was made reasonably clear that 100-120m/s was a good mid-range velocity".

In my data library I have good HP and valve size data on some 200 engines over time. I have developed my measure based on this history base and can find no calculation that verifies justification for using such average velocities over the inlet phase. All data supports my contention that 80 m/s is a good starting point though there are some good engines that exceed that by various values up to plus 10%

Having said that, there are a few exceptions such as NASCAR engines that have been optimised within regulations that force strange such gas velocity results. The NASCAR engines are built to standards far from the free thought that most of us utilise. They work because there have been tens of thousands of iterations tested and developed over decades to a rather restrictive formula.

We rather well discussed this back in 2006 where these ideas were first foisted on this forum.

Regards

[Canuck]

Hi Joe Was hoping you'd pop in. I have a tendency to be vague for no particular good reason, hence your confusion on things. I'd like to build an M88 but I don't have one at hand . I have however managed to amass a little collection M30 engines - 2 of each 2.8L and 3.5L (3.4 if we're honest), M30 series pieces. While digging around in old BMW world, the question arose Can you build a 300hp NA daily driven M30? , so I started looking at valve sizes given the somewhat restrictive nature of the M30 cylinder heads. The 26' valve angle makes it interesting to get a 50mm valve in (valve to valve clearance) and I suspect that anything larger than that would require fairly significant surgery. The concept had evolved to a 94mm x 90mm b/s, the idea being that would allow the engine speed to remain under 7000 and the piston speed under 4000fpm which should provide the necessary airflow (assuming a valve flow coefficient of .3)

I must have confused my formula translation in Excel. With a per cylinder displacement of 625cc, 80m/s, 6700rpm and using valve dia - 1mm, I'm getting 48.5mm. I'm clearly missing something - can you walk me through the the scenarios you put together?

This is a purely theoretical exercise however. The output of a turbo-charged M30 is quite remarkable. The reading I've done suggests ARP cylinder stud kit, stock (or Cometic MLS) headgasket, stock internals and folks claim anywhere from 300-400hp at the wheels on 15-22psi. The few video'd dyno runs and posted charts appear to back that up with a recent posted run showing 370 at the wheels on 18psi. The Scandinavian guys are taking things differently and running huge boost up top with much larger turbos and cranking out staggering numbers (or, claims at least) on the stock engine. I'm thinking a solid 350hp at the wheels is a respectable daily driver and $/hp vs. the NA route doesn't even come close.

[gary76]

I have been trying to post a reply with some information attached. The document is made up from MS Word and some MS Excel data but I am bu...ed if I can fathom a way to attach it into the thread. If somebody can give me a clue (in simple language!) I would be grateful and you can all read it.
Regards
Livingfossil

[Canuck]

Simplest way for the word document is cut-and-paste the text - same for Excel. If you have some means to upload them to a webhost, you can link to them in the post but you can't directly attach them as far as I know.

[johnny yuma]

Another "rule of thumb" I heard somewhere was intake valve to match the throat size of carburettors/s.Don't know how this relates to injected engines,probably port size at the nozzle.

In Australia a very common,cheap 3.3 litre 6 cylinder "old school" engine has been raced and modified by half the population.A reputable company has been making after market heads for decades and they have 45mm intake and 39mm exhaust valves(not that much bigger than standard,but circle geometry provides big area increase for slight diameter increase) Interestingly the carburettor setup of choice is 3 X 45mm webers which neatly fits the inlet valve=carby throat rule of thumb.Ditto poverty pack 3 X 1.75inch SUs.

These engines operate in the 5000-7000rpm area and the very best of them are rumoured to produce 290 bhp.
The bore/stroke are 92mm/82.5mm. Pushrod valve actuation.

[Joe Bosworth]

Johnny

I believe that it is better to get fix your valve size first and let your venturi size follow.

The first reason is that the valve size is absolutey vital to gatting an induction/cam package working in sync with one another.

The second reason is that changing valve sizes is one, almost ir-reversible and second, very expensive and work intensive to change. Venturis change in a few minutes for very little, relative, cost.

Regards

[Joe Bosworth]

Canuck

Let’s try again.

I confirm that the formula that you are using, (23.275*gas vel ... etc), is correct as I gave you in 2006.

Let me also say that I keep the formula on about four computers in Lotus ‘cause I like using their solver capability. That is you can input any combination of inputs and get any output value at the push of one button correct to any number of decimal places.

(I use Lotus because I started this well before Excel was even dreamed of. In fact I used to have it (mid-seventies) on a programmable HP, the first of the genre). In further fact I first hit on this back about 1964 or ‘65 when you needed to be fast with a slide rule!)

Also for those who missed our first Forum go around on this over two years ago, I developed the formula by observing known data from a wide number of high performance engines starting about the time of the Coventry Climax FWA and continuing through dozens and dozens of further engines including 2.5 L CCs, Cosworth DFVs and continuing through a whole series right up to 2008 1000cc high performance road bikes and a couple of Superbikes. Interspersed with these are a bunch of stock block derived engines starting with Formula Junior of the early sixties and continuing with road and drag Chevs and Fords. The 80 mps gas velocity thing keeps coming back. It just seems that great minds all come to a near consensus on engine design.

McGuire says he, “I don't know about the .9*valve OD deal”. The .9 deal came about because what really controls the gas velocity is the diameter of the port pocket just behind the valve. It is relatively easy to get valve size but very difficult to get pocket diameters. You get a fair few opportunities to either measure valve size, get it from published in articles and/or you can go into parts/data sheets. I simply came into using .9 as a general close approximation of the ratio of valve to pocket diameters to plug into my database. In fact it seems to be sufficiently close as it hasn’t put any noticable warp into the results. (Never the less I would never go about building to that ratio. Rather I would work to put the optimum port shape and size under the valve selected.)

Having said all of that, when I plug 6700 rpm, 80 mps, 625 cc per cylinder into my spreadsheet a 52.692 mm valve diameter spits out. I can’t see how you get your number out of the same formula unless you are rounding as you go, in which case some accumulated errors might come into the calculation.

As to how I come up with the recommendations for the 85 bhp/L or the HP at 6700 rpm I am working backwards from some of the information base from my same database. I have developed some generic volumetric efficiency factors for parallel 2V, opposed 2V and 4V configurations. From these I can calculate how many liters per minute must pass through an efficient engine of a given configuration. From there it is an easy computation, (again kept on a spreadsheet with solver), to calculate what rpm you must turn for a given engine configuration. For that rpm I then go back to my valve size formula and voila you get the answers that I previously provided. (I have also developed some camshaft suggestions, but I won’t go there for now.)

All of this goes back to my heart felt belief that the best engines are those that have induction tracts optimally selected for the duty that you wish to acheive. For instance, you can put in valves or ports too big for the job rpm you are safely capable of developing and you mostly lose torque when you want it, (and V-V).

Boat anchors are developed from mis-matched parts.

McGuire is mostly correct on his point, “an intake valve can't be too big.” The bigger the valve the higher the revs you can pull, the more air that you can pass, the more HP you can make. That is until your valve size exceeds your ability to screw together a bottom end/piston deal that holds together at the rpm you are trying to get to. I can confirm that I have raced a couple of boat anchors from time to time myself by people going too big.

Regards

[johnny yuma]

Canuck-the bore/stroke figures in your first post give a capacity of 3749cc.Is this right? Or are you using wrong data ?

[Canuck]

It is - 94 x 90 (x 6)

BSA Ports.doc courtesy of Gary.