29 replies to this topic

[Canuck]

In the ever-brewing kettle that is my hamster wheel, I was thinking about conrod stresses and the steady upward trajectory of output in motorcycles (basically the relationship between piston speed and output). The 2018 KTM Super Duke 1290 twin has a displacement of 1.3 liters (108 x 71 / 4.25 x 2.795) and a remarkable (claimed) 177 hp. That's 132 hp/liter. For reference, the GM LS9 with it's rated 638 hp is "just" 102 hp/liter on a smaller 4.065 bore. Clearly we're not talking about area under the curve and how much fun one feels over another - regardless.

 

The KTM piston however has almost no skirt. Micro-mini skirt. How does a piston so short survive the warranty period, much less long-term use? What's happening - or not happening, that allows for such a slim design?

[Joe Bosworth]

Canuck

 

If I have done my search for good data well enough I find that the answer derives from a calculation of BMEP.

 

 

BMEP is a calculated average (mean) pressure on the head of the piston over its power stroke.

 

The LS9 bmep is 22% higher than the Duke's BMEP.

 

I also suspect that the Duke's rod length to stroke ratio is a lot better than the LS9's but I haven't yet found the rod length for either.

 

I might continue my information search after a beer and watching the afternoon news.

 

Regards

[Bloggsworth]

I suspect the far tighter manufacturing tolerances may have something to do with it.

[gruntguru]

I doubt the tolerance on piston/bore clearance would be much different.

 

Joe, as you say, I think rod/stroke might be key. Larger l/r reduces side force and side force plus friction is the main source of rocking moment applied to the piston. Still find it hard to get my head around such a short skirt.

[SarbaV]

Canuck

If I have done my search for good data well enough I find that the answer derives from a calculation of BMEP.


BMEP is a calculated average (mean) pressure on the head of the piston over its power stroke.

The LS9 bmep is 22% higher than the Duke's BMEP.

I also suspect that the Duke's rod length to stroke ratio is a lot better than the LS9's but I haven't yet found the rod length for either.

I might continue my information search after a beer and watching the afternoon news.

Regards

How will you calculate the stress on the skirts.
You will need to model that piston and do analysis on software?
Or can you do it analytically?

[Lee Nicolle]

Motorbike engines do not last as long as car engines,, that is why they are getting away with a  short piston skirt. Though stroke is ofcourse very short as well.

But having the pin go through the oil ring area is fairly common. Usually and extra 'spacer' under the ring too support the ring. Stroker engines or engines using very long rods do it like that. A 347 Ford [302W stroked .4 of an inch] has the oil ring very similar.As is the skirt length also,, the skirts hang well out the bottom on a 302 at best!

The oil ring on that piston is very small though the top ring is fairly well down.

Edited by Lee Nicolle, 13 July 2018 - 05:40.

[Joe Bosworth]

I will take another try at analysing the short skirt length; in the case that Grunt started, comparing the GM LS9 to the KTM Super Duke.

 

The BMEP info that I supplied above provides one comparison that I will carry to the next step as BMEP is a reather abstract measure that most on this forum has no background on or for that matter any comparative data.

 

Sarba asks a couple of good questions.  Unfortunately I have never seen any real force measurement data on a running engine.  Hence people using things like BMEP in order to get a feel for relativer differences between engines.  I have done  a fair bit of dyno work  and cn assure you that to instrument an engine to get real live data is very heavy on technology and money.  The big companies can afford it but don't appear to share anything.  I also seriously doubt that their are any publicly available computer programmes but in reality if you and I used them  the results would be meaningless to us so I have never looked.

 

I have written graphic programmes that show the effect of rod/stroke ratios and this is reasonably profound.  The real problem is that without a force-time measure we don't know how far down the bore that the max force exists.  I have good data on flame front propogation at i atmosphere pressure but have never tried to use it as the propogation at many atmos pressure and high temperature will be quite different and the results useless to anybody.

 

Modern lubricants is part of how skirt lengths have shortened, it is many years since I have played with castor oil additives though I do know first hand how some metallic bases additives were once useful.

 

As Lee says the ring stack height has shrunk tremendously with time for a variety of quite good reasons including ring design.

 

Let me give another comparison of the Duke and LS9.  Lets look at one cylinder in isolation. Max force has to occur pretty close to the RPM for max torque so I will look at that point.

 

The Duke makes about 30% less torque per cylinder than the LS9, 70.5 NM to 102.4

 

The duke makes it max torque at 1.97 times the RPM of the LS9, 7500 vs 3800.  This effectively means that each Duke power pulse needs to only make 15% of the torque that a LS9 does.

 

We start to see that each torque (force) making pulse of the LS( is about six times stronger than the Duke.

 

I suggest that this ays about all that needs to be said as to how they get away with such small skirts, ( that with Lee's commen t on relative engine life).  Let,s just say that both are fit for purpose.

 

Regards

 

[Lee Nicolle]

Piston material has improved out of sight in the last 40 years, as has rings. Those little narrow rings last forever in a serviceable state and do not cut up the bores. When I say serviceable I mean road car serviceable where a few percent leakdown seldom effects performance. A race engine turning a lot harder with more compression will leakdown about the same but does make it lazy. Though even then bores do not cut up much at all. Though hone finish is far more important.

And oils are so much better as well ofcourse.

All of this on what is considered 'old school' engines. 

[Canuck]

I came across a video discuss with Reher-Morrison folks and the question of r/s ratio was brought up. From a performance perspective, as far as they were concerned it was the last thing on the list to consider. They'd tried more combinationa and ratios than they could count, and that just didn't do anything at the end of the day. Of course, lots of things that apply to race engines don't apply to road engines. I'll be damned if I can find that talk again however.

 

Perhaps the 4.25" diameter requires less skirt to maintain stability as rocking motion at the pin axis is much larger at the extreme ends, pushing the skirt against the wall sooner.  Or does the skirt only work under compression loads? Is it just along for the ride during overlap and intake?

[Lee Nicolle]

Long rods will wear  pistons out faster. The reason most road engines are fairly conservative. Long rods park the piston at and near TDC longer which makes torque, though you have to be a little more conservative with timing.

Longest production engine rods I know per bore and stroke is both A & LA Chryslers with 6.123 long rods for a 3.91, 4" ,4,040" bore and either 3.31 or 3.58 stroke. 

And that goes back to the 50s.

I am sure there is other petrol engines with similar length but those Mopars were all torquey things and that is the reason why. And they have never had an appetite for pistons either. 

My 400 Ford has 6.55" rods but a 4" stroke [and bore]

My Cleveland race engine has 6.030 rods [from a 302C] and the approriate pistons and the torque is hard to get moving with fairly skinny tyres.

I believe Nascar 'proper' Chevs were using 6 1/8 long rods. And a Chev is a shorter deck than a Mopar though not by much.

The baby little 289/302 Fords with the 5.4" rods require grinding the pan rails to fit in the engine with the 3.4" [347ci]  long stroke.

My brother just called, he runs a bike shop workshop and evidently pistons behind the rings is not uncommon though 4 1/4" bore is. But it is only a twin!

[Greg Locock]

The reason production cars have long skirts (these days) is for heat transfer to the cylinder walls to stop the top of the piston burning, and to control piston slap. Neither is of much concern in a motorbike engine, which is rather likely to be rebuilt before 100000 miles, and whose owner is oblivious to piston slap, which is purely an audible issue, it does not affect functionality.

Edited by Greg Locock, 15 July 2018 - 09:48.

[Canuck]

Thanks Greg - that's interesting. I had neglected heat transfer entirely in my pondering. I'd be curious to see if they have piston oil squirters in there - they do seem to be more common these days, even HD started with them when they introduced the Twin Cam series in the late 90's.

 

Lee - the stock rods in an H-D Evolution series engine are 7.44" on a 4.25" stroke. The Twin Cam is a 7.6 on a 4.00 stroke. There's some new longest production engine ratios for you. Given that there's a lot of pressure on H-D to reduce the overall noise of their air-cooled (less and less) engines, it would be odd if they made changes that increased the mechanical noise and promoted accelerated wear problems.

Edited by Canuck, 16 July 2018 - 00:37.

[404KF2]

I love the magical alignment of the ring gaps!  Uh oh!

[Joe Bosworth]

Lee

 

Up in post #10 you quote, "Long rods will wear  pistons out faster".

 

I have never seen empirical evidence that this statement is necessarily true.  By the same token I have never seen evidence that it is false nor evidence either way for that matter! In fact the answer might be quite different between a race designed application and a road application.

 

It can generally be said that nearly all IC petrol engine applications have rod length to stroke ratios between about 1.6 and 1.9. 

 

You give good examples of road applications between 1.59 and 1.85 and it is safe to say that owners of those road used engines have never had a piston failure in the 100k to 200k miles that they use the engines before rebuild.

 

Likewise, I have built or had built for me dozens of race based engines over the years and have always set a rebuild target of 15 hours.  I have never seen this affected by r/s ratio.  Even today's GP engines meet this standard. It would be fascinating to get a peek inside one of these.  I still have as a momento a piston from a Coventry Climax engine circa 1960s that used to hang off a rod with a ratio of 1.79 that is in perfect condition.  I still have dyno sheets for bunches of Ford stuff with r/s ratios as low as 1.45 and as high as 1.8.  Never saw any life differences and it is truely hard to correlate to HP readings either.  Number 1 son is presently racing historic Corvettes and this month was placed 3rd at the Indi Pro-Am historics.  His engine uses 1.75 ratio rods and is race reliable with adequate HP for modern times.

 

You are quite correct that longer rods provide longer dwell times at TDC and BDC.  I am not sure how fast the ignition flame front is but I doubt that full charge ignition occurs during that dwell period. Of course, once the piston starts down pressures become smaller due to PV/MRT physics laws. Pressures resolve themselves into forces due to piston area. I do know that rod length changes need both cam and ignition timing changes.  Simple force diagram analysis provides insight that longer rods result in smaller side forces at any point during the power stroke.

 

Of course cylinder flame front progress is also strongly affected by cylinder turbulence/combustion chamber shape.  While race tuning production based engines for racing I almost always used to alter bowl type chambers to flat plus slanted to get better turbulence hence HP.  In the modern day we seldom see bowl style chambers for that reason

 

The whole answer to the original question gets too complex unless you keep the analysis simple and in the realm of that which we mortals can relate to real data and information.

 

Regards

 

[gruntguru]

I haven't seen any evidence either way, but my head tells me that low l/r ratios increase side force on the piston and would be expected to produce more piston skirt wear than a longer rod.

[Greg Locock]

The Falcon 3.9 litre I6 often had a long rod, short stroke cousin, the 3.2 litre. I don't think this ever made it into production as the take rate would have been ~0. However it was a nicer sounding engine than the 3.9, which we attributed to the L/r ratio. In retrospect a 3.2 litre turbo would have been a great compromise.

Edited by Greg Locock, 17 July 2018 - 03:09.

[GreenMachine]

Long stroke Greg, or long rod?

[Greg Locock]

Indeed, rod. (I'll edit the post as well)

[Lee Nicolle]

The Falcon 3.9 litre I6 often had a long rod, short stroke cousin, the 3.2 litre. I don't think this ever made it into production as the take rate would have been ~0. However it was a nicer sounding engine than the 3.9, which we attributed to the L/r ratio. In retrospect a 3.2 litre turbo would have been a great compromise.

3.2 was EA only. I have seen one. 3.9 and 250 [4.1] share a crankshaft and rods, as do 3.2 and 200. Smaller bore.

Using 200 rods in a 4.1 is again very torquey though the ACL race pistons to do so were single compression ring which had a very short life.

There was other pistons as well, with a narrow and high ring stack.

ACL and others made pistons for 302 rods in a 351. But ACL used .040 top rings which they no longer sell so I had to get the grooves machined to 1.5mm

[Lee Nicolle]

Lee

 

Up in post #10 you quote, "Long rods will wear  pistons out faster".

 

I have never seen empirical evidence that this statement is necessarily true.  By the same token I have never seen evidence that it is false nor evidence either way for that matter! In fact the answer might be quite different between a race designed application and a road application.

 

It can generally be said that nearly all IC petrol engine applications have rod length to stroke ratios between about 1.6 and 1.9. 

 

You give good examples of road applications between 1.59 and 1.85 and it is safe to say that owners of those road used engines have never had a piston failure in the 100k to 200k miles that they use the engines before rebuild.

 

Likewise, I have built or had built for me dozens of race based engines over the years and have always set a rebuild target of 15 hours.  I have never seen this affected by r/s ratio.  Even today's GP engines meet this standard. It would be fascinating to get a peek inside one of these.  I still have as a momento a piston from a Coventry Climax engine circa 1960s that used to hang off a rod with a ratio of 1.79 that is in perfect condition.  I still have dyno sheets for bunches of Ford stuff with r/s ratios as low as 1.45 and as high as 1.8.  Never saw any life differences and it is truely hard to correlate to HP readings either.  Number 1 son is presently racing historic Corvettes and this month was placed 3rd at the Indi Pro-Am historics.  His engine uses 1.75 ratio rods and is race reliable with adequate HP for modern times.

 

You are quite correct that longer rods provide longer dwell times at TDC and BDC.  I am not sure how fast the ignition flame front is but I doubt that full charge ignition occurs during that dwell period. Of course, once the piston starts down pressures become smaller due to PV/MRT physics laws. Pressures resolve themselves into forces due to piston area. I do know that rod length changes need both cam and ignition timing changes.  Simple force diagram analysis provides insight that longer rods result in smaller side forces at any point during the power stroke.

 

Of course cylinder flame front progress is also strongly affected by cylinder turbulence/combustion chamber shape.  While race tuning production based engines for racing I almost always used to alter bowl type chambers to flat plus slanted to get better turbulence hence HP.  In the modern day we seldom see bowl style chambers for that reason

 

The whole answer to the original question gets too complex unless you keep the analysis simple and in the realm of that which we mortals can relate to real data and information.

 

Regards

 

I have seen dozens of OEM piston failures at fairly low km. Well under a 100. From most mainstream manufacturers. Sometimes poor assembly, or poor machining but most often the skirts are worn and crack. I once pulled a  running and rattly 302W apart with all eight piston skirts in the sump. Numerous others with cracked skirts. GM Ford Mopar, popular Jap etc.Surprisingly never a Chev. The rings gummed in a piston or burnt top ring lands yes. The Mopar engines seldom cause any problem unless they are driven hard, and then yes they do. That is hard, not abuse. 360ss in particular. I have replaced a set of pistons in one of those at around 100000km. Towing horse floats. This in the early 80s

Unleaded engines generally are better, or the pistons at least are,,not simple cast pistons but generally hyperuetectic pieces. And generally a proper valve stem seal as well so less detonation,, plus all the electronic nannys as well.

But for the beancounters an unleaded engine is a LOT more expensive to make. Though warranty claims cost a LOT more in man hours than in the past.

As for road race engines I have worked on km. 2500 was the target for a freshen up. Reused pistons at least once. And always the engines had further upgrades and or mods at that time. 

Speedway engines are number of shows, or when they feel lazy. Which will never do the km of a road race engine. Even with good filtration rings wear out faster as it seems do valve seats as well. Bearings however usually do not, but are ofcourse replaced.

Though 20 years ago a 372 Sprintcar engine would last a season with maybe replacing some valve springs. Then they went 410 and engine life is half!

360s 10 years ago too lasted a season but now too about half. Max7500rpm was a season now 8300 is half!! Though still a lot cheaper too rebuild.

Ideally an engine does not have those big lumps on pistons, though at least for a Chev it seldom causes a drama.But you are starting to light the fire at around 36 degrees so it can get decent flame travel,, but that piston parked at or near TDC is building up a LOT more pressure and belts it down harder. Obviously a multi spark ignition  helps there, though with methanol a big fat magneto spark seems to do the job.

[Greg Locock]

Lee, this would have been after EA, I didn't do crankshafts for EA (since I didn't work for ford then).

Edited by Greg Locock, 17 July 2018 - 08:11.

[Kelpiecross]

3.2 was EA only. I have seen one. 3.9 and 250 [4.1] share a crankshaft and rods, as do 3.2 and 200. Smaller bore.
Using 200 rods in a 4.1 is again very torquey though the ACL race pistons to do so were single compression ring which had a very short life.
There was other pistons as well, with a narrow and high ring stack.
ACL and others made pistons for 302 rods in a 351. But ACL used .040 top rings which they no longer sell so I had to get the grooves machined to 1.5mm

I think the 3.2 has the same bore - but the stroke is a full 20mm shorter. I have a 3.2 engine in the garage I used for testing various things. It is a bit like the situation with the 2.4 and 3.8 (or even 4.2) Jag engines - there doesn't seem to be any real advantage in using the smaller engine - and the big ones are a hell of a lot more powerful.

[Greg Locock]

I must admit, a completely new cylinder machining setup and head design does seem an arse about way of reducing engine size.

[Lee Nicolle]

I repeat, EA was the only model with 3.2 as an option. Export models may be different.

I would have been interested to try a 3.3 engine with the short stroke long rod for speedway Modified Sedan. Gear it down and turn it to 7500 The head is good enough and I feel it would be a better engine than a 202 Holden. Which won quite a lot. The 4.1 was a torquier slugger that about 6500 was the safe limit. Better on a heavy track however. Actually seemed better than the 4 litres on a heavy hooky track.

[Lee Nicolle]

I think the 3.2 has the same bore - but the stroke is a full 20mm shorter. I have a 3.2 engine in the garage I used for testing various things. It is a bit like the situation with the 2.4 and 3.8 (or even 4.2) Jag engines - there doesn't seem to be any real advantage in using the smaller engine - and the big ones are a hell of a lot more powerful.

Yeah, 200/ 3.2 are the same bore and short stroke to their bigger brothers. I have ever only seen one 3.2 ever. In an early EA. Same bore as a 3.9 with different rods and crank as Ford has always done. And I had all the engine/ trans codes in my wallet for Fords when I was buying them very regularly in the 80s and 90s.

The 4 litre is slightly bigger bore and I feel any short stroke version would have been closer to 3.3.

Those Falcon  sixes were still the same basic architecture from 59 right through to 2016 in the block. So the homely 144 has a turbo Barra as its grandson!

[gruntguru]

The Falcon 3.9 litre I6 often had a long rod, short stroke cousin, the 3.2 litre. I don't think this ever made it into production as the take rate would have been ~0. However it was a nicer sounding engine than the 3.9, which we attributed to the L/r ratio. In retrospect a 3.2 litre turbo would have been a great compromise.

 

I repeat, EA was the only model with 3.2 as an option. Export models may be different.

Lee, I don't think you read Greg's original post properly.

[Greg Locock]

Lee's right about the 3.2 on EA, it actually got into production, I was talking about models after that. We kept it alive as a development project because (a) NVH (b) mpg © emissions. Unfortunately b and c were trivially small improvements and (d) power suffered a lot (though it may have led the way on the increase in red line from 5500 to 6000).  "Smaller bore." is almost certainly wrong for the Australian 3.2 whether EA or later. It's too hard.

[Charlieman]

My bedtime reading has recently included a 1920s book on engine design. It's a good starting point for understanding how we got to where we are now.

 

IIRC, three piston types were discussed: cast iron (for mass manufacture or slow engine speeds), aluminium alloy (for high speed engines) and built-up pistons for specialist use. Depth of the piston varied from about 1.2x diameter to 1.8x diameter.

 

Long skirts were essential for aluminium pistons owing to differential thermal expansion. The skirt was wider than the top (piston ring region) and most only had rings at the top. A 1920s racing piston was very much like a 1970s road car design (I think modified Renault 4 pistons turned up in a few vintage racers).

 

Some cast iron and built-up pistons used long skirts for oil control. The middle part of the skirt was sometimes reduced in diameter with oil vents. Some had a ring below the skirt waist.

---

As others have commented, designers of a modern motorcycle engine aren't too worried about piston slap from a cold engine. Modern materials and manufacturing mitigate many of the problems which required long skirts.

 

For any IC engine, the hottest part of the combustion chamber which is in direct contact with oil is the top piston ring. From memory, the maximum permissible temperature is about 1100 degrees centigrade? We can assume that the piston shown above warms up very quickly but combustion chamber temperature doesn't burn oil.

[Lee Nicolle]

Lee's right about the 3.2 on EA, it actually got into production, I was talking about models after that. We kept it alive as a development project because (a) NVH (b) mpg © emissions. Unfortunately b and c were trivially small improvements and (d) power suffered a lot (though it may have led the way on the increase in red line from 5500 to 6000).  "Smaller bore." is almost certainly wrong for the Australian 3.2 whether EA or later. It's too hard.

As an aside. Many years ago I traded an XE 3.3 5 speed wagon from the country and had to go pick it up.

I was actually very impressed as it was quite quick  and responsive and pulling fifth kept the RPM down. And unlike its big brother freer revving

Though efi 4.1 XE and early XF went quite hard as well. Unleaded ones were hopeless!! 

Though for someone building a performance Falcon the E2 alloy head from the asthmatic efi unleaded engine is by far the best head as valves and ports are all larger. 

For a speedway engine it equates to about 500 more useable RPM.

[Lee Nicolle]

My bedtime reading has recently included a 1920s book on engine design. It's a good starting point for understanding how we got to where we are now.

 

IIRC, three piston types were discussed: cast iron (for mass manufacture or slow engine speeds), aluminium alloy (for high speed engines) and built-up pistons for specialist use. Depth of the piston varied from about 1.2x diameter to 1.8x diameter.

 

Long skirts were essential for aluminium pistons owing to differential thermal expansion. The skirt was wider than the top (piston ring region) and most only had rings at the top. A 1920s racing piston was very much like a 1970s road car design (I think modified Renault 4 pistons turned up in a few vintage racers).

 

Some cast iron and built-up pistons used long skirts for oil control. The middle part of the skirt was sometimes reduced in diameter with oil vents. Some had a ring below the skirt waist.

---

As others have commented, designers of a modern motorcycle engine aren't too worried about piston slap from a cold engine. Modern materials and manufacturing mitigate many of the problems which required long skirts.

 

For any IC engine, the hottest part of the combustion chamber which is in direct contact with oil is the top piston ring. From memory, the maximum permissible temperature is about 1100 degrees centigrade? We can assume that the piston shown above warms up very quickly but combustion chamber temperature doesn't burn oil.

Modern alloys for pistons seem a lot more stable. In decades past 7 thou was the piston clearance for a 4" bore forged V8 piston., modern ones as little as three hence they can get away with shorter skirts.

My 4.7 V8 petrol Landcruiser rattles quite badly when cold,,, like a old forged piston Chev. But after 500 metres of driving the noise has gone. Alloy block with iron liners and I believe quite short pistons. Evidently they all do this, this from a Toyota dealer mechanic.

 I built a 347 fairly recently from a 1989 302 F100 engine. Pistons were shorter, lighter and tighter hypereutectic with 1.5 1.5 3mm rings factory. And bores were 98%,Supposedly had done 200000 and it was really a pity to modify it as it was in such good order. Those pistons and rods are now in a ski boat.

A huge improvement on the 70s 302s which all had cracked pistons.

Edited by Lee Nicolle, 23 July 2018 - 04:44.