Stroke or Bore?

[rvanderpyl]

Given everything else is the same, compression ratio, fuel system, exhaust etc. What would make more horespower, or perhaps more responsive horsepower?

1. A shortstroke (66mm) 93mm bore 2.7 liter
2. A longstroke (74mm) 90mm bore 2.7 liter

Mahle has some new Big Bore 93mm P/C sets with a 9.5:1 compression ratio, with a 66mm crank this should end up giving around a 8:1 ratio, the same ratio that regular 2.7 motors came with.

Which would be more responsive? Any experience from anyone on the bore vs stroke method of creating displacement?

[Chuck Moreland]

In general a short stroke can rev to a higher redline, provided the valve train is up to snuff, and therefore produce more HP. This is due to lower piston speeds.

A longer stroke will give more torque, but a lower redline.

[masraum]

And generally a longer stroke will give more torque which is what kicks you in the pants, so you would most likely feel that more.

[Wayne 962]

Right-o. Short-stroke = good top speed. Long stroke = more off-the-line performance...

-Wayne

[jluetjen]

Gentlemen -- I beg to defer.

While the "Long stroke = more torque" myth is always pulled out when this question is asked -- there is no data that supports it. I know of a classic case (911 no less) where porsche made the 2.5 ST first with a long stroke (70.4 mm) and then with a short (66mm) while keeping everything else the same. The HP and torque figures were the same, and they occurred at the same engine speed no matter what the configuration.

Please show me an example where there are two engines with the same capacity and the same state of tune where the long stroke version makes more torque then the short stroke version. Since BMEP is directly proportional to torque, I'd be happy to see two different sized motors in similar states of tune where the BMEP of the long stroke version is greater then the BMEP of the short stroke version.

Keep in mind that engine designers often increase the stroke to increase capacity without changing the block. If they keep everything else the same (CR, valves, cams, ports, etc) the increase in capacity will cause more torque while the comparatively restrictive valves/ports will move the peak HP engine speed lower in the rev range. Short stroke engines are generally used in capacity limited classes because they allow faster engine speeds which means more HP.

Anyhow -- I've showed my example - let's see yours!

R; My response is that it won't make a difference. The long stroke engine could possibly show a little more cylinder wear over the life of the engine due to the higher piston speeds, but I'm guessing the affects will be less then the difference in wear caused by your chosen fuel, lubricants and driving style.

[BK911]

Hey John!

One of the questions on my midterm was... For the same dispacement, why do short stroke large bore engines make more power than long stroke small bore engines. The answer was (I hope!) that for a constant mean piston speed, a shorter stroke allows higher RPMS. BMEP is directly proportional to RPM's as well, so higher RPM's give higher power output. Plus there's something about the greater area of the piston yields more force for the same pressure in the cylinder. (PA=F)

I'm not saying you're wrong, but I hope I am right for my grades sake!

BK

[jluetjen]

Hey BK - let me know what the right answer was!

I was thinking of this a couple of nights ago while trying to fall asleep and had a couple of thoughts (I wouldn't justify them with the term insights!)

1) The long stroke cranks on the early motors were 70.4 mm's versus the short stroke 66mm's. This represents roughly a 7% increase in length of the crank throw, so I guess the long stroke motor should make 7% more torque for a given force on the piston.

2) Ahhh force! The force will equal the pressure * area. The area of long stroke's piston would be pi * square(86.7mm) = 23.6K square mm. In the case of the short stroke engine with an 89mm bore, the same calculation would be 24.8K square mm. So given a certain pressure on the piston of X psi, the short stroke engine would generate 24.8K/23.6K = 1.053 or 5.3% more force.

3) So the short stroke engine will generate 5.3% more force on the crank, but 93.7% of the torque as a result of the shorter lever on the crank, resulting in 98.6% of the usable torque of the long stroke engine.

4) But, how about the difference in ring drag between the two versions?

Keep in mind, I got both of my degees in B-School. I just do this as a hobby. If there are any engineers out there who want to refute my attempts at engineering -- please do!

BTW: I hope that you didn't blow it on this part: "...BMEP is directly proportional to RPM's as well, so higher RPM's give higher power output.". I didn't think that BMEP was proportional to RPM's (excluding the affects of tuning the induction and exhaust systems). I thought that given a certain BMEP (which is proportional to a torque), that HP was directly proportional to RPM's.

[DavidPorter]

Interesting debate, good to see old chestnuts hauled out once in a while to see if they're rotten.

It seems "obvious" that higher revs achieved by shortening the stroke and thus keeping piston speeds manageable would result in more power, at least at high rpm, assuming piston speed is a limiting factor. If displacement is restricted you also get to use bigger valves as part of the trade-off since bore can be increased. That's how they get over 100 hp out of a 600cc Japanese sportbike.

What about the fact that long-stroke motors generally have more crankpin offset? To my non-mathematical brain that gives the piston a longer lever arm to exert torque on the crankshaft.

[BK911]

On paper it looks pretty simple:

Since Mean Piston Speed is a major limiting factor is designing a race engine, and since;

MPS = 2 * Stroke * RPM's,

a constant MPS will allow a higher RPM with a shorter stroke. And since;

BMEP = P / (disp * RPM's),

with disp and BMEP constant, higher RPM's yield more power.

But more power requires more air, so the cams, valves and heads will probably need to be modified.

A larger stroke does exert more torque on the crank, but less torque is required to compress the other cylinders with a smaller stroke. This could almost be a wash. And the increased ring drag area for a shorter distance may balance out with less ring drag area for a greater distance.

I wish we did this before my test! Some interesting points I could have used.

Only one way to be sure. Sounds like a good thesis project!

[Tyson Schmidt]

The larger the bore for a given displacement, also allows larger valves and therefore potentially better breathing. So you have higher potential engine speeds, and better potential engine breathing.

The short stroke is the better race motor.

[dickster]

just read about this in "maximum boost".

longer stroke = more torque, something to do with inertia if memory serves.......no, this is crap, just looked it up see below!

[Bobboloo]

It would seem that there would be some scientific correlation between short vs. long stroke and guns. The pistol being the short stroke and the rifle being the long stroke.
Therefore, since the pistol is good for killin' at close range and the rifle is better for killin' at long range then the rifle is better. Cause it can kill ya up close too.

Therfore Rifle=Longstroke is better.

There. It's settled.

Goodnight.

[jluetjen]

Bobboloo;
I get it now. So the >>>BEST<<< 911 motor is the one which has blown a rod out of the motor. You know -- just like the bullet in your rifle analogy!!!


I'm the first to admit that I don't understand the gun thing, but that was another thread...

Bye now!

[724doorE]

John, I'm not much of a marksman either, but what I do question is this: Porsche claims of 275hp @7800 or 8000 ( I've seen both quoted) for the long and short stroke engines. This is with the same heads, valves, ports, and cams.

I kind of figure that: the long stroke came first (I think this is common Knowledge) but of course they had problems with the crank throwing the flywheel(early 6 bolt crank) which was finally solved by going to 70.4mm crank with a larger flywheel hub and generous fillets (the larger radius fillets create a lower stress concentration). The 66mm crank was used in the interum period with 89mm pistons to get the 2.5 capacity (2466cc). I think they just labeled the short stroke the same as the long stroke, power wise and RPM wise for continuity. I think we all agree the shorty should make more peak hp and turn a few more rpms to do it while maybe losing a few ft-lbs of torque. The question is how much.

An apples to half apple-half orange comparison might be 2.2S to 2.4S peak hp RPM. The hp is different due to the overall capacity increase, but we might loosely assume that the difference in peak rpms should approximate a similar difference in the peak rpms of the short and long stroke 2.5.

Just throwing wood on the fire

[jluetjen]

Dennis;
I was with you until you said...

I think we all agree the shorty should make more peak hp and turn a few more rpms to do it while maybe losing a few ft-lbs of torque. The question is how much.

I'm arguing that the "shorty" will not make any more HP then the long stroke, nor will the long stroke have any different torque charactoristics then the short stroke (all other things being equal). Here is a hypothetical example:

Let's assume that using a 906 cam and with the proper porting, the long stroke ST motor produced a BMEP of 173 lb/square inch. (Incidentally, this is actually the BMEP of the 2.8 RSR which gives you a clue about how similar in concept the engines are.)

911 2.5 ST with 70.4mm crank and 86.7 mm pistons for a 2.45 liter engine: a BMEP of 173 lb/sq-in at 8000 RPM equals 266 HP at 8000 RPM and a piston speed of 3696 fpm.

911 2.5 ST with a 66 mm crank and 89mm pistons for a 2.42 liter engine: a BMEP of 173 lb/sq-in at 8000 RPM equals 263 HP at 800 RPM and a piston speed of 3465 fpm.

Let's make believe that our conn rods will immediately separate when the piston speeds equal 4000 fpm. How many HP can we get from either engine?

2.5 long stroke: You'll be able to rev this engine to about 8650 (3998 fpm) safely and it will produce about 288 HP. Of course the heads, valves and ports will need to be modified to breath and survive at those engine speeds.

2.5 short stroke at 8650 (3746 fpm) is also producing 284 HP (keep in mind that it is marginally smaller then the long stroke). As I mentioned, it will have the similar head mods to the above 2.5 long stroke at 8650 RPM in order to produce this HP.

max'd out 2.5 short stroke: You'll be able to rev this engine to about 9230 (3997 fpm) safely and it will produce about 303 HP. Of course the heads, valves and ports will need to be modified above and beyond those required for 8650 RPM in order for the engine to breath and survive at those high engine speeds.

So the short stroke engine can make more HP if it is spun faster (which it can be). My theory(?) still remains the same, if the rev's are the same, then both engines should produce essentially the same torque and HP charactoristics.

Admittedly, this is a simple example since the BMEP for an engine is not flat across the rev range. When I get a second I'll do a comparison between a 2.0 S (160 HP) and a 2.2 S (180 HP) at incremental rev's and let you know how the BMEP changes across the rev range. It would be interesting if someone could print a HP graph for a 2.4S (I don't have one in my shop manuals) and see how that compares.

[dickster]

i had to look it up it was bugging the hell out of me - well i got it wrong - i think.

i'll let someone else can do the calcs.

torque = (bmep x bore area) x stroke

power = torque x rpm

[Scott Clarke]

Compression ratio has been left out of this dicussion. rvanderpyl has presented two scenarios with substantially different compression ratios. He says he will have 9.5:1 with the 70.4 stroke, 90mm combination, and 8:1 or so with the 66/93. Why not 70.4/93? That would end the argument! Also, if the short stroke path is taken, what about Mahle "race" pitons that are rated at 10.5:1 @ 70.4 stroke. They cost a little more, but should produce an acceptable CR for pump gas with a 66mm crank.

Given the same size valves, I would expect a larger bore size to increase the breathing ability through the valves. This is a result of decreasing the severityof the "bottleneck" between the outside edge of the valves and the cylinder walls. If I remember back to my Datsun days, I seem to recall that it was accepted practice, for this reason, to "eyebrow" the cylinder walls above the path of the upermost piston ring in the vicinity of the intake valves.

An addditinal item for discusion in the bore/stroke debate is the time required for the flame front to travel accross the combustion chamber. Larger bores result in a greater amount of combustion time. Porsche, of course, solved this dilema with twin plugs. Given a single plug, is this a factor at, say, 93mm? How about @ 90mm?

[pete98m3]

hp = ( torque * rpm ) / 5252



just a fyi


other thing to think of is if the short stroke motor will be able to do as efficient a job filling the cylinder with that extra rpms... ie will the fuel / air mixture completely fill the cylinder in time or will it be an incomplete fill... that would severely cut down on power gains...

that problem would be addressed though in the reworking of the heads / valves / cams / induction system that would go along with teh redesign of the motor.


typically a short stroke design is capable of more peak power though because it helps push the limits of the rods further. this is how f1 motors spin up near 18k rpms... they use an oversquare design with an extremely short stroke... (and of course lots of voodoo and other valvetrain tricks so they don't just grenade)

[724doorE]

I recently spoke to Peter Sanchez of Andial, and both long and short stroke piston sets are available as OEM Mahle sets, price was about $3200 each.

Ok, John, nice stuff...but my input was, or was meant to be, that the shorty 2.5 would make more power at a higher rpm. Maybe we should also note that it isn't highly suggested that you rev a six bolt 70.4mm crank much above 7600-7800 (depending on your sorce). Doing so would introduce you to the problems the factory had with their long stroke 2.5....

So... start with a 2.2 E or S and get the 89mm piston set from Andial for 3200 and a set of 906 cams from webcam for 800 have your crank checked and nitrided, and do the shuffle pin mod on your case, oh and don't forget the head work, 41mm ports as per factory details. You could use carbs or mfi and probably get that 265-275 hp at a fairly reliable 7800-8000 rpms. Now the 300hp costs you some more money, aftermarket rods and valvetrain to name just two very important areas.

The plot thickens

[jluetjen]

Here's some stuff to think about. I plotted the BMEP's and intake gas speeds for most of the "early" 911's. You'll notice that the BMEP graphs very similar to the respective torque graphs. You are right, you can think of BMEP as Torque normalized so that we can compare engines of different sizes. A couple of conclusions:

1) I don't see any appreciable difference between the shape of the the BMEP curves for engines with 66 mm cranks and those with 70.4 mm cranks. So I reiterate that stroke has no impact on the shape of the torque curve.

2) The 2.7RS BMEP curve has a very different shape then the other S cam'ed motors. But also notice that the gas speeds are quite a bit higher then the S's. It seems like once the gas speed goes above 80+/- meters per second, that the BMEP drops off. There is a similar affect with the pre-MFI 911 Normal. In this case I suspect that the carb are the restriction, but the affect is the same. Anyway, I would expect that there would be some pick-up of BMEP on the bottom end of the rev range as the gas speeds at low rev's is higher thus contributing to improved mix. That's still my theory.

In a related fashion, note that all of the T's have similar curves. But they're all past their prime by the time the gas speeds reach 80 m/s. The feature that stands out is the extra power from the TE's MFI at higher rev's. I didn't include the TK since that engine had a different cam which makes it a significantly different animal.

3) I'm surprised to see that the BMEP of the 2.2 and 2.4 motors was really no worse then the BMEP of the 2.0's, in spite of the CR dropping. I wonder if the improved (read shallower chamber) combustion chamber resulted in better combustion and thus a higher BMEP?


So does anyone see any indications that the long stroke motors pull better down low? Or that the short stroke motors do better at higher rev's??