Old SStroker

No, not all of us.

For a given displacement and rpm, torque isn't necessarily proportional to stroke. It depends more on how much air the engine is pumping and effectively burning.

More BMEP at a given rpm and given displacement = more torque and therefore more power at that rpm.

Race engines with displacement and max rpm limited by rules might be good examples. Most go with the largest bore allowed and therefore the shortest stroke to make most power (and therefore torque) at a given rpm. Ask yourself why they do this.

racer7088

With big bore motors of the same displacement the torque does not usually go up but the potential power and rpm ceiling do because of the larger valves and the larger ports behind those valves.

At the same CID the longer stroke smaller bore engines will usually return slightly more tq and power at the lower rpms (for many reasons) but will become breathing limited at higher rpms due to the smaller valves they must run due to the smaller bore and thus ultimately they will make less power than something with a bigger bore of the same displacement.

At the same CID the shorter stroke bigger bore engines will have more airflow capability and will keep making similar tq but can maintain it much higher in rpm so they will always have the potential to make more power higher up in rpm. Certainly they will make more tq past peak and higher than the smaller headed engine will.

People run big bores and smaller strokes when they are displacement limited to allow more rpm and therefore more power and more average power and then they gear the car accordingly. At lower rpms it's not so clear cut and most run smaller bores and more stroke as we see everyday in OEM and especially commercial engines that must make continuous power for extended periods and must be totally reliable (AKA Low RPM).

LSs1Power

iTrader: (5)

The way i see it after all the discussion here

Picking a setup based on the application would be something like this

Longer Stroke Ex. a 382-392 cid with 3.900 Bore x 4.100 Stroke = off road application or a peaky SC setup (TQ wanted)

Larger Bore Ex. a 387 cid 4.125 Bore x 3.622 Stroke = High RPM application, Nitrous setup or Turbo Setup (More RPM wanted + better exhaust valve flow due to the ability to run larger valves)
Nitrous and Turbo will make up for the TQ loss by the smaller Stroke.

Equal Bore and Stroke Ex. a 402-408 cid with 4" bore and Stroke = N/A setup, Roots blower or a high step up ratio Cenrti SC. (Compromise between the Large Stroke, Big Bore setup)


One more question: Which would spool a Turbo faster... A larger Bore or More Stroke?

Im thinking a larger bore would spool a turbo faster due to larger valves... Also the Larger Bore setup does have a longer power band so it will use more of the turbo range.

racer7088

LS1Power,

I don't think it matters that much but the power adder people seem to like smaller bores usually for the detonation resistance and the thicker cylinder walls for the most part. I would think the spooling would mostly be independent of the bore stroke thing but I have not really put much thought into that. I know a lot of the outlaw people think stroke spools the turbos more but I seriously do not fully understand why?

Old SStroker

No, the 3.30/9000rpm engine will have about 7% more piston gs than the 3.57/8300 rpm engine if both have the same rod length.

Loads are proportional to mass x acceleration so if all the parts weigh the same loads on the rod and crank will also be 7% higher on the 3.3/9000. If the part weights vary, the loads will vary directly with the part weight.

FWIW: "load" is force (pounds); "stress" is load/area (pounds/square inch or psi); "strain" is the amount of deformation (inches). No offense intended. People often misuse /confuse/interchange these terms.

LSs1Power

iTrader: (5)

None taken... This is the point of the thread.

Old SStroker

I disagree with your reasons for choosing bore and stroke combinations. Stroke length doesn't make all that much difference in torque which is probably counter-intuitive to many folks. It is the combination of displacement and breathing that makes the torque and therefore the power.

If you want more torque, increase the displacement. If the block limits the bore size, stroke will get you the cubes. It's not the longer stroke that makes the torque, it's the extra cubes resulting from the extra stroke. IOW, the extra stroke is the cause of the extra displacement which results in extra torque and power.

Draco

So a longer stroke doesn't apply any extra leverage, thereby increasing TQ? Obviously stroke to rod ratio comes into play as well, but I thought a longer stroke would in general increase the mechanical advantage at the crank. Is this gain in leverage somewhat offset by a drop in VE due to the breathing issues mentioned above?

KillerBrink182

I'm not sure if this was mentioned or if it's even relevant now, but i thought someone might get something from this. Rod length doesn't necessarilly affect piston speed, BUT it does affect PS characteristics. Mainly how long the piston stays at TDC and BDC and how quickly the piston achieves maximum velocity. When comparing two different rod lengths with the same stroke length, the longer rod will cause the piston to remain at TDC and BDC for a longer time and will also increase the time it takes to reach maximum velocity. Besides PS, rod length will inturn affect ignition spark requirements, cylinder pressure conditions, optimum valve timing, and net torque.

99% of my information comes from an article in the '06 edition of Engine Masters Racing. I won't be able to scan it anytime soon to share it here, but if anyone here is has the magazine maybe they can help out. Otherwise it'll be awhile before i can get it up.

SStrokerAce

iTrader: (2)

Yep, if you are talking cubes being the same the motor doesn't care how they get there in terms of TQ production in the RPM range of the motor.

There have been some great articles in mags about bore vs. stroke and the advantages and disadvantages of each.

Think about this a 500 cube Pro Stock motor which has a stroke smaller than a 396/427 big block Chevy makes about the most TQ per cube of any motor out there period. 830ft/lbs out of a 500 cube motor is really impressive. Now throw in that it occurs at 8,000rpm and there is a lot of friction in there bringing that number down. If that same TQ occured at 3,000rpm lower then those numbers would be even higher.

Cubes and VE make TQ not stroke.

Bret

blackz93

iTrader: (2)

I was under the same impression until about a year ago when the mags came out. They did some back to back testing if I remember correctly.

joecar

So basically, if you keep bore size the same (say it's maximum you can go) and increase stroke, the additional cubes allow more torque to be made.

Old SStroker

Yes. Size matters.

Of course you still have to do the combination of engine parts correctly.

racer7088

The longer rod goes a little slower at TDC but a little faster at BDC than the shorter rod.

All pistons and rods dwell longer at BDC. The longer the rod gets the more the motion at both ends becomes the same. The shorter the rod gets the more the rod swings through TDC faster and dwells longer at BDC.

LSs1Power

iTrader: (5)

I agree that the more Cubes you have the better TQ and HP it will make, but the point of my post above #24 is to show setups with similar cost (around 5K or so) which is why i didn't list anything above 408cid. If you notice with the 4.125 Bore (aprox. cost $2000) i used a stock crank and with the 4.100 Stroke (aprox. cost $2000) i used stock Bore.

So in that aspect do you agree with the reasoning listed?

racer7088

I go by the same overall philosophy but in actuallity the stroke does add torque through leverage since the piston area and force from that is the same as on a shorter stroke engine.

To me it's like people that think a turbo works off heat energy. It's a way to look at a turbo or analyze it but in reality the turbo is pressure drop driven and the pressure drop is from the heat changes and vice versa. In the end physics says for the turbine to spin a force must be acting on it.

The big stroke simply gives the rod a larger lever arm to act on so the torque goes up and so does the power at that rpm but still for that to happen and the piston top area to stay the same the engine also HAD TO BECOME LARGER as OldSStroker and StrokerAce are saying so I agree with both.

Still TQ is almost totally proportional to engine size so thats the important thing to consider. A big engine will make big tq. A small engine will make small tq. Either may make more power depending on the heads and valvetrain and intakes etc. that they are running.

XxGarbSxX

I agree with this, and I'd like to throw in a little something about the rod length with the stroke, as I find it hard to talk about stroke without taking into account rod length, to some extent, seeing as how they're interrelated.

An important concern with increasing stroke is the increasing rod angle. Longer rods will decrease this rod angle, which should contribute to reliability and longevity as well as decreasing piston acceleration. Also since the rod angle is smaller, it will exert a force more parallel to the piston on the crank than a shorter rod at a larger angle will, which will also contribute to the mechanical advantage of larger stroke. Take a ratchet for example. Same basic principle. If you have the torque arm at around horizontal, and the force is coming from the same vertical line the bolt is on, you would want the location of that force to be as high as possible to decrease the angle on the end of the ratchet, getting it as vertical as possible.

Well, that's about as far as my knowledge base takes me so that's probably a good place to shut up.

Old SStroker

I missed that you were throwing cost factors at this discussion. Did you delete that part in a previous post? Maybe I just can't find it.

Explain to me again what you are trying to do. If it's make a lot of torque and power from a 408 LS engine, the large bore option will allow the heads to work better, especially if you choose your heads and porting carefully. You'll also be able to spin it higher for the same number of piston gs than the small bore/longer stroke version, so if each option has about the same piston/ring/pin weight, there will be less load on the crank and rods at every rpm for the shorter stroke version. What part of that isn't good?

Either engine will produce a similar torque curve, but the big bore engine, due to better breathing (larger bore), has the potential to make more torque than the small bore/long stroke 408. Believe it or don't.

Whenever an engine is designed or modified, you need to look at ALL aspects of the engine and it's intended use. The title of this thread is: Piston speed, Displacment vs HP?

So:

Piston speed is directly proportional to stroke and rpm.

It's easier to get more torque and more horsepower from a bigger displacement engine than from a smaller displacement engine.

Higher piston speeds mean more friction.

If you choose a limiting piston speed, your stroke determines your max rpm, so a shorter stroke allows you to twist it higher (for your chosen max piston speed).

A higher rpm limit gives you the opportunity to produce more peak hp.

OK?

LSs1Power

iTrader: (5)

You didnt miss anything... I just never clearly said cost is a factor... I just avoided including anything larger than a 408cid. Also you really dont have to be defensive... We are just discussing the relationship between PS, D and HP or TQ. Thats all

Old SStroker

I was trying to be nice. It was a just a more PC way of saying "Why the hell are you bringing cost into it now? Or was it always there?"

If you were thinking about cost and a 408 max LS, why not say that in the beginning?

So do you prefer the big bore or long stroke 408 now that you heard a few varying opinions and some basic physical facts? Inquiring minds want to know.