Bore vs. Stroke, a little math

[vkh]

I did a wack of Googling to compare a 377 (400/350crank) and a 383 (350/400crank). The consensus - especially among the circle track crowd - is that the 383 just has a whole lot more usable torque, despite more valve shrouding. And if you had a 400 block, just build a 408.

The 377's beat all the 350's, then the 383's beat all the 377's, then the 408's beat all the 383's.

I went looking because I have a line on a good 400 block, and I have a good 350 crank, and got curious.

Huh, well maybe I was wrong. I'll have to do some more reading myself. Doesn't a 400 have larger mains than a 350?

 

[shiftpro]

Were not the 400 blocks inferior with the siamese cylinder walls? Like get hot and can't keep bore round = compromised ring seal. ??

 

[shiftpro]

I sure like how a 383 feels under my foot...

 

[shiftpro]

So this morning I got to thinking about bore vs. stroke. I'd always heard that the longer stroke gives more torque due to the leverage, made sense to me so I never thought past that. But this morning I was thinking about how at the same displacement and compression engines with different bore/stroke combos should have the same cylinder pressures as they run. The larger piston would receive more force since Force = Pressure * Area. Now this is just some quick and dirty math to check an idea. In the real world there is friction to worry about, rod ratios, and all kinds of fun stuff that factor in.

For my example I'll use 2 500 CI engines (not based on any real ones)
Engine 1: 4.5 in stroke 4.205 in bore piston area: 13.89 in^2
Engine 2: 3 in stroke 5.15 in bore piston area: 20.83 in^2
Peak cylinder pressure : 1000 psi

Forces on the pistons:
Engine 1 = 1000*13.89 = 13890 pounds
Engine 2 = 1000*20.83 = 20830 pounds

Now to factor in the leverage to get torque.
Engine 1 = 13890*4.5/12 = 5208.75 lbft
Engine 2 = 20830*3/12 = 5207.5 lbft

Basically identical. Now of course if you wanted to wing these engines out to 7000 rpm the shorter the stroke the easier it would be on the rotating assembly. And the torques are crazy high, this was just to see if there would be much of a difference purely from bore and stroke. I could make this more accurate with calculus and all that fun but I believe the relationship would be just the same. So let me know what you guys think.

Should there not be other factors such a friction? And the force of inertia on the crank blah blah..?

 

[vkh]

Should there not be other factors such a friction? And the force of inertia on the crank blah blah..?

There are a ton of factors, my example was purely hypothetical pretending everything was equivalent except bore and stroke. Honestly what got me on this was wondering if large bore small stroke was actually as bad for torque as I've heard. I mean the 454, 460, and 440 were all over square and mostly used in heavy applications. I think if you were arbitrarily limited on cubes you could make a short stroke engine perform admirably in a towing or low end torque application. But in real life I see no reason to limit stroke as long as rod ratios stay reasonable.

 

[HotRodPC]

Should there not be other factors such a friction? And the force of inertia on the crank blah blah..?

Yes about the siamese cylinders and such you mentioned not to mention SBC 400's are hard to find. The 4 bolt blocks are even more prone to crack than the 2 bolt blocks, so the 2 bolt blocks are just as valuable, moreso IMO since you can add splayed mains to the 2 bolt block and really have some added strenght. But for those like me, that don't really care for SBC400's, that's why the 383 stroker exists. Stroke of the 400 to add that torque in an easily found 350 4 bolt in a high nickel block.

These days you can also buy that stroker 383 kit in a 6 inch rod instead of the standard short 400 rod. That helps keep the rod from being so busy and takes strain off them making for a stronger bottom end and a bit less parasitic hp/torque loss.

 

[Honky Kong jr]

You know what you never see....people factor in piston / cylinder drag. Only way to really combat that is to run ported pistons and loose rings but for longevity that isn’t a viable option. I think people would be shocked to see what kinda drag is on a piston when pulled up out of a bore with a scale attached.

 

[MikeB]

I was always under the impression the additional torque was created by the mass of the heavier stroker crank once it gets spinning. E=mc^2 (mass x velocity squared equals energy, or torque.

Nope. More rotating mass will make for a smoother operation a low RPMs, but it is leverage that makes torque. Same is true of a heavy flywheel. It won't affect torque, just inertia.