Canuck YYC is a lying heretic! (Stroke doesn't make torque!)

[Canuck YYC]

Go here for the run-up if you need.

Here’s the deal – there’s lots of internet experts out there, so I generally attempt to keep my mouth shut if I don’t have a clue what I’m talking about, but have a tendency to speak my mind and present facts when there are obvious mistakes. This leaves me with alcoholism / addiction, engines, the specific tasks of my day job and with three kids on the go, dirty diapers as my areas of experience and knowledge. I make no presumptions about anyone else’s education or experience in any given topic beyond what they’ve shared publicly. In this forum section in particular, which could rightly be termed the Performance Forum as much as it is the Forced induction forum, I’ve been under the assumption that folks have a desire not just to go fast but to understand, learn and know. If this doesn’t apply to you, forgive my obnoxious willingness to share knowledge and put me on ignore.

Statement (mine) – More stroke produces more torque is a widely-held falsehood.
Reaction: Prove it.

Okay then – here’s hoping the old synapses are up to the task.

There are two formulas I’m familiar with for predicting power:
(Ap * BMEP * Sp)/132,000 where
Ap = Total Piston Area
BEMP = Break Mean Effective Pressure in PSI
Sp = Mean piston speed in feet per minute

or the infinitely more convoluted;
P=((Cp * BMEP)/323.3) * (cube root (ncyl * (Kbs * Vtsv)^2))

where
Cp = Mean Piston speed in Meters/second
BMEP = Same as above but in Bar
ncyl = Number of cylinders
Kbs = Bore/Stroke ratio
Vtsv = Total displacement in CCs

The counter-argument to the stroke != torque (thus far) has been:
Stroke is connected to piston speed, more piston speed according to the formula = more power therefore more stroke = more torque.

1st - if you increase stroke, who says you get to increase piston speed? Yes, stroke is connected to piston speed, but so is RPM. Who says that your new long-stroke crank/rod/piston assembly is sufficiently strong enough to withstand the same rpm? Hell, if we’re just after more piston speed, and piston speed is a function of stroke and rpm, why don’t we just crank up the rev limiter? What? Oh – it’s a high-performance assembly – can withstand serious kinetic forces – lots and lots of piston speed! HUGE RPM!!

Okay then – let’s have a peek at some real-world piston speeds from the realm of ultimate street engines – the sportbike. A 2008 Honda CBR 1000 is (obviously) a 1 litre 4 cylinder that produces 178hp at peak power with a Mean Piston speed of 4340fpm. So let’s say your new crank can duplicate that.

Let’s try something using some known figures, some real-world assumptions and our simple formula.

Stock M30 B34: 182hp@5400 rpm
Bore: 92mm (~3.622”)
Stroke: 86mm (~3.386”)
Mean Piston Speed at peak hp: 3047.2 f/m
Ap: 61.823

If we work it backwards we find a stock BMEP in the realm of 128psi which is within the realm of normal if not a little low by today’s standards. Typical "factory hot" passenger car BMEP is now in the 150psi range while more serious endeavours are upwards of 184. I’ve not yet seen a naturally aspirated street engine with a BMEP exceeding 200psi (even when the N/A hp\l exceeds 200). I digress…for reference sake, the ’08 M5 has a BMEP of ~ 168psi. We’ll pretend we’re all fantastic tuners and we’ve managed to bring our 2-valve M30 into the realm of modern and it too now produces 168psi BMEP – and we have a whizzy new 90mm crank that can spin up to 4340fpm.

What do have now? 61.823 Ap * 168 BMEP * 4340 / 132,000? 342hp – almost double the stock output. Nicely done! You have a redline of 7350rpm which is almost 2000rpm over stock and a displacement of 3590cc – almost 3.6 litres. Impressive.

Now what if we leave the stock stroke and increase the bore to get 3.6l?
94.1mm bore for 64.677” of piston area, stock stroke, 168 BEMP. We get…250 hp. Wait…what’s this? What’s going on? How can that be? Oh yeah… we’re only turning 5400 rpm – the stock peak. We said we wouldn’t increase the stroke but we didn’t say we couldn’t increase piston speed did we – ‘cause we can increase piston speed without increasing stroke right?

What’s a fair increase? Same engine RPM as the 90mm crank or same piston speed? Let’s try both!
64.677 * 168 * 4147 / 132000 = Well look at that…341hp at the same RPM (4147frpm @ 86mm stroke = 7350 rpm).

64.677 * 168 * 4340 / 132000 = 357hp. Huh. Same piston speed, same displacement, same BMEP, shorter stroke, more power. Go figure. Since we know that hp = torque x rpm / 5252, we know that as hp goes up, torque goes up (for a given RPM point). More HP = more torque. Yay.

Damn eh?

Yes BUT!! If stroke is connected to piston speed, then more stroke = more piston speed = more power - you still haven't proven that wrong
Well...that's because the argument itself is incorrect. Stroke doesn't = piston speed, stroke is a component of. Just like engine speed, stroke is only a portion of what comprises piston speed. Take BMEP for example - valve curtain area, cam timing, port shape and size, manifold characteristics, air cleaners, ring friction, valve springs, heat...BMEP is comprised of all of these things but we wouldn't say "bigger valves raise BMEP so they make more power!" because as anyone who's installed bigger valves in an engine that isn't curtain-area limited knows, that might not accurate for them.

It's a pretty simple formula for a pretty complex interaction but it works rather well. It takes all of the assumptions (good Ve, cam timing, burn, ignition timing etc etc etc) and rolls them into BMEP. You want to make 500 hp? Then this is what you need to achieve...

So - where would I ever use this anyway? How is it relevant?Why do I care

I don't know - maybe you don't. That's cool. On the other hand, if you don't know how to calculate power, how do you know how to achieve your power goals? How do you know what qualifies as a realistic expectation? How sedate or undriveable will it be? If I'm limited to X piston area and Y piston speed, how much BMEP do I need to make my power goals? (Where and how to find the BMEP is an all together different topic.)

What about that lever deal - the crank's throw is a lever!. Yes. Now here's my one back-track on this topic - I said it wasn't a lever but it is, it's just...well...it's not so much a lever like you're thinking. In order for a lever to effect a mechanical advantage, it must have a motion ratio. In a crankshaft, one rotation of the crank will always produce the stroke - no multiplier. It's a 1 to 1 lever.

**Not so fast Canuck! What about not changing the rpm in those first expamples, just the stroke. Trying to be sneaky?

Going back to our 90mm crank, we find
61.823 * 168 * 3189 / 132,000 = 251 hp
Big bore at stock stroke and rpm = 64.677 * 168 * 3047 / 132,000 = 251 hp
Big bore at 3189 fpm = 262hp

Anti-Internet-Expert Task Force Reference Guide:
The Internal Combustion Engine in Theory and Practice, specifically Volume 2, Engine Design 1 and 2. Charles Fayette Taylor, MIT Press

Vehicular Engine Design Kevin L. Hoag. SpringerWien New York (via SAE)

Design of Racing and High Performance Engines. It's really a collection of SAE papers from the days when they weren't advertisements but genuine research documents.

The Influence of Stroke-to-Bore Ratio and Combustion Chamber Design on Formula One Engines Performance
G.M. Bianchi,University of Bologna; G. Cantore and E. Mattarelli, University of Modena; G. Guerrini, Ferrari Racing Division; F. Papetti, IBM SEMEA
SAE

[turbodan]

Theres the mechanical leverage aspect and theres the dynamic aspect. I dont understand the math behind the mechanical leverage enough to argue with you about that, but if we're going to get this far into detail on the subject we should probably touch on the effect this has on flow through the motor.

Even if not for additional mechanical leverage, a longer stroke and higher piston speed produces more torque because it produces a stronger intake signal. By increasing VE you're going to get more torque. Thats a lot harder to account for by calculation, but its going to improve BMEP.

In the context of an M20B25 vs an M20B27, I believe the longer stroke of the B27 is going to make more power. These engines leave quite a bit on the table in the interest of reliability, and an increase in piston speed would most likely be an improvement. I would bet that an M20B20 block in an 80x81mm configuration would make more torque than an M20B25 at 84x75mm. If I had infinite resources I'd totally build that in the interest of science.

[Coldswede]

Well, there is the unpopular opinion that torque is the real output value of an engine and there is no real such thing as horsepower. That horsepower is a mathematical derivative used for convention and comparison.

To simplify the whole of the math would be to say Displacement is the key to power. A big displacement engine will always produce more power per than a small engine, given the same technology. Displacement can only come from two places Bore and/or Stroke. Longer stroke engines are used a lot now days for emissions reasons, the longer the stroke the more time for a complete burn within the tightly controlled confines of the engine.

Some old Hot Rodder tenets simply work, ie:
Bore will increase your flow rates therefore rpm which will increase your Horsepower.
Stroke will increase your Torque,

And my favorite:
Big bore, short stroke engines are good for the soul, but bad for emissions.

[turbodan]

HP is useful to describe work over time. Thats quite relevant when it comes to performance.

[Gunni]

Airflow wise .

long stroke engines vs big bore same capacity same cylinder count.

long stroke engine will make more torque at low rpm
short stroke engine will actually make same torque at high rpm.

3liter I6´s make just about the same torque as V10 F1 engines when they where 3liters.

Its not really more complicated then this.

[Jeremy]

The formulae you provided are not incorrect. However, they don't tell 100% of the story. Similar motors with identical stroke, bore, and displacement from different manufacturers or with different valve configurations (2,3,4, or 5 per cylinder) all make different amounts of torque and peak at different points in the allowable rev range.

The secret ingredient is VE, which influences BMEP. As Dan said, increased stroke gives a stronger signal and leads to increased VE, which increases BMEP.

Stroke DOES matter when considering your power goals and desires for a motor. The formulae presented are a good starting point, but they don't tell the entire story.

Jeremy

[joegreen]

I'm sure some one has taken a stock motor dinoed it got a value, and stroked the same motor to come up with more hp, and torque...hence stroker makes torque, also as was stated earlier in dif words. If you can turn it get a bigger wrench, so you can get more stroke there fore more torque. I think...

[Canuck YYC]

The formulae you provided are not incorrect. However, they don't tell 100% of the story. Similar motors with identical stroke, bore, and displacement from different manufacturers or with different valve configurations (2,3,4, or 5 per cylinder) all make different amounts of torque and peak at different points in the allowable rev range.

The secret ingredient is VE, which influences BMEP. <snip>.

I would argue the secret ingredient is BMEP, not Ve. Once again, like stroke to piston speed, Ve is acomponent of BMEP, it's not BMEP. Can you increase Ve and and end up with lower BMEP? Yes - by screwing with the other factors that comprise BMEP. Can you decrease Ve and have a higher BMEP? Obviously - the inverse of the former scenario. In the same way that I can increase piston speed without increasing stroke, I can increase BMEP without increasing Ve.

Stroke DOES matter when considering your power goals and desires for a motor. The formulae presented are a good starting point, but they don't tell the entire story.

Jeremy

If you can point me to where I said stroke doesn't matter, I'll give you a nickle. The statement was, and is, stroke does not equal torque.

As Dan said...

Ahhh....yes, then clearly my post, the formulae and the PhDs that came up with all that are wrong and you are right, because Dan said so. My apologies.

[Canuck YYC]

...a longer stroke and higher piston speed produces more torque because it produces a stronger intake signal. By increasing VE you're going to get more torque.

I don't think that's wrong necessarily, but I think we're now talking entirely different animals. Previously, stroke was a lever in your argument which was why it made more torque. Now you're saying even if that's not the case, it still makes more torque because it changes Ve. Well certainly - changes in Ve can result in more BMEP and thus more torque. However...A longer stroke that results in a higher piston speed obviously travels more distance in a given span and as such generates more friction - trade-off. If you said stroke influences Ve and BMEP and hence torque I wouldn't argue with you, but that's not what you said (or at least that's not what I heard).

Clearly stroke plays a role in an engine - no stroke, no anything, but increasing stroke doesn't "make more torque" than increasing the bore to equivalent final displacements.

The B25/27 post next...

[Jeremy]

Well, sure, if you mess with multiple variables simultaneously, I'm very certain you can get whatever result you'd like in order to prove whatever point you're trying to make.

However, if you keep everything else the same and increase the VE only, BMEP goes up. Since you've already shown that stroke and bore don't matter otherwise, increasing BMEP by increasing stroke (even when keeping displacement constant) increases torque via the improvement in VE, particularly at lower engine speeds. The amount of improvement might be small, but it's certainly present, and it's not entirely canceled by increased friction. If stroke were as inconsequential as you claim it to be, somebody really needs to talk to all the tractor and heavy equipment manufacturers. They seem pretty convinced that really long strokes are more appropriate for their application.

Ahhh....yes, then clearly my post, the formulae and the PhDs that came up with all that are wrong and you are right, because Dan said so. My apologies.

Do you always get this pissy when you're wrong? Don't accuse me of things I didn't say and I won't accuse you of the same. I specifically stated that the formulae were correct but didn't tell the full story. You said:

That’s wrong. Full stop.

and proceeded to go on to show how your magic formulae tell you that stroke doesn't matter one bit in torque production.

Nowhere in that, or any other mathematical formula for the production of power, is there a link to crankshaft stroke. None. It’s not a “determiner” in torque production.

This post.

If you can point me to where I said stroke doesn't matter, I'll give you a nickle.

Where's my nickle?

[mooseheadm5]

Forget it, Jeremy. It's like arguing with a wall.

[Canuck YYC]

Okay - let's say I'm talking out of my ass - I'm not overly attached to my argument and quite okay with being wrong (I am married after all). I generally come here to learn from other people and I've found being open-minded to new information serves me well more often than not. You've said increased stroke increases Ve. Please show me how that works because that's not my experience. Or direct me to a source of information that isn't some random internet post. A paper, a study, ...anything with some legitimacy.

Because this is written/printed word and not a face-to-face communication, so much gets lost. I assume I understand your position and I assume mine is understood. However in looking at our conversation it's clear to me that I've not adequately communicated my position well. I'm trying to say that the idea that stroke builds an increase in torque that an increase in bore will not is false. Further, that any influence on Ve from an increase in stroke is so...slight as to be inconsequential and in no fashion a significant generator of torque.

As for your nickle - my lack of complete communication stymies me again. I stand by my first post - stroke is not a determiner of torque. Stroke matters because the engine is clearly a system and the package as a whole has to work together - deck height, rod angles and yes, it's influence on piston speed. Does stroke make torque? Not in any significant fashion. Does stroke matter in an engine? Yes. If you still feel I owe you a nickel I'll not argue. As for being pissy, it was you that hopped all over my head, accused me changing me story, denigrating your education and experience and not knowing what I was talking about. All I did was challenge the belief that stroke makes torque.

I've laid out a huge explanation while you stand there and say "you're wrong" while quoting Dan (who thus far has the most reasonable responses even if I still disagree with them). If you don't like what I'm saying, what the formulae say, what the university profs, F1 engineers and OEM engineers agree on, then fine - I'd love to be proven wrong. Show me. Teach me. I hate wandering around thinking I know stuff only to find out I'm quite wrong.

You've said my position is wrong, so educate me.

[shocka]

Wow, super pissing contest.

[Slaytanic-666]

I liiike it!

[Gunni]

Airflow wise .

long stroke engines vs big bore same capacity same cylinder count.

long stroke engine will make more torque at low rpm
short stroke engine will actually make same torque at high rpm.

3liter I6´s make just about the same torque as V10 F1 engines when they where 3liters.

Its not really more complicated then this.

I´m going to quote myself again.

Look at dyno charts, LOTS of them, and you´ll see.

Engine size is the #1 factor in creating torque.
Torque being work per cycle.

Engines with long strokes don´t make more torque then short stroke ones, they just make them at different rpm´s.

Good example are

M20B27 - 81mm stroke - 62.85lb/ft / Liter torque
M20B25 - 75mm stroke - 66.70lb/ft / Liter torque

B27 having longer stroke makes less torque per capacity.

The difference in torque between them is 6lbs/ft.

Stroke doesn´t MAKE torque.

Camshafts and stroke lengths dictate at what rpm the peak torque occurs.

[Canuck YYC]

I'm in general agreement with one hair-splitting exception. The only reason a shorter stroke engine makes it's peak torque at a higher rpm is because we've manipulated it with cam timing, intake optimization and the like to take advantage of the higher horsepower potential. A great deal of low speed power is sacrificed keeping the intake valve open longer than it should be so that at higher rpm, the valve events are right on the money. Everything is a compromise.

For the sake of transparency, there are two schools of rather entrenched thought as to whether stroke functions like a lever and I'm clearly biased to the no side though not committed to it. However-both sides are in agreement that irrespective of whether it is or not, the end result is the same - torque is displacement-based, not stroke biased.

To add a little more because I'm home sick with a head full of snot and have the time on my hands...
Fuel consumption is based on engine speed (as opposed to piston speed) and load. If I'm making a tractor where hp is not the goal, but torque, it makes sense to design the engine for low rpm operation. It's easier on the valve train and easier on fuel. Once I have a torque @ rpm target, I can determine the necessary displacement based on a BMEP pressure that I know is reasonable. The new displacement and the rpm target will then dictate the bore and stroke. Everything is based off the target power and speed (or relative cost, or packaging constraints or customer demand for certain architectures like pushrod v-twins or DOHC V-8s).

That's where this formula (or any of the others that dictate power) comes into play. If you way to make 500 hp at 6500 rpm, how do you get there from here? If I'm limited to 2-valve heads, pump fuel and no forced induction, is it possible from my 3.5 litres? With my current block?

It's the most basic of road-maps to obtaining your power goals.

[Gunni]

. If I'm making a tractor where hp is not the goal, but torque,

This is the typical wrong mindset of
hp is one thing, torque is another thing and they have not interelated meaning, one is good for towing the other accelarating.

Which is just a load of crap.

When you need to tow you need hp in that area, sure that means high torque at that rpm, but the final end use of the rpm and torque is HP.

The only thing that matters coming out of the flywheel is HP.
What´s making the hp doesn´t matter when if comes to towing or accelaration.

As soon as people start thinking about HP curves / area under the curve and not the max values they understand said engine more.

[Canuck YYC]

Your right. Poor use of the words on my part. That doesn't change any of the rest of the post however.

[Gunni]

If these two don´t explain it well enough for people
then nothing will.

http://craig.backfire.ca/pages/autos/horsepower

http://craig.backfire.ca/pages/autos/making-power

[Canuck YYC]

Nice links.