Bore vs. Stroke, a little math

[vkh]

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.

 

[fussfeld]

That **** just makes my head spin.

i'm generally a fan of longer stroke both on the street for the torque---better for towing/hauling/general driving below 3000rpm. AND at the strip; i still think longer stroke prevails despite the possibility of more friction for a longer stroke vs. larger bore (and i'm not totally convinced this is the case either).

 

[MrMarty51]

I think, long as You dont throw a bunch of alphabets into the equation, I wont get all frustrated and confused.

 

[bucket]

Aaacckkk!!! Numbers!!!!

 

[crazy4offroad]

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. Increasing either mass which we can interpret as the weight of the crank, or velocity which we can interpret as rpms will result in more torque). The same as a loaded 200 car freight train takes a mile to stop where the same train unloaded may only take 1/4 mile, once that mass is in motion it takes a lot to stop it. Much like a torque converter makes more torque than a standard's flywheel, more mass in motion creates more force. They say there's no replacement for displacement though, I'm sure there are a lot of other dynamics that have to be factored in when you consider a simple cam swap can result in more torque. Or does it just shift the torque curve to a lower rpm range that you feel seat-of-the-pants sooner? But I digress, I'm beginning to ramble lol.

 

[SkinnyG]

A larger bore will also allow bigger valves, which allow better cylinder filling.

A longer connecting rod will have less angularity on the crankshaft throw, allowing a longer application of torque during the rotation.

 

[HotRodPC]

E = MC squared with that little 2 doo hickey up high on the C. That's all the scientific ******** I care to know.

Sooo, don't ask me. I'm good at math, but for what works best blah blah ********. I've to have always heard longer stroke brings the torque but also takes from the higher rpm. Now days due to this scientific research ******** you're trying to make the rest of us look stupid with, I know we can get longer rods with wrist pins moved up in the pistons to allow the longer length which takes strain off the rotating assembly with an easier break over on the rods to which higher rpms can be achieved. So there you go, I just buy the damn parts and bolt them in and leave the engineering up the parts designer and builder.

 

[HotRodPC]

Ohhh, and I've also heard that comparing all the GM 350's to each other, cuz other than Cad, there was a Chevy, Buick, Olds, and Pontiac 350, and if asked which one of the 4 would put out the best power all things considered being alike. It's said the Olds 350 should perform best due to being so close to a Square bore x stroke ratio as in for example without looking it up, 3.75 bore x 3.75stroke. So if that's true, your 4.5 x 4.2stroke combo would be the one to use Instead of the 3 x 5 combo.

 

[Honky Kong jr]

BIG BLOCK!!!

 

[HotRodPC]

BIG BLOCK!!!

Has nothing to due with the science he's speaking of. Science is science whether in a SB, BB, I6, V6 etc.

Has anyone else heard that Doc on Street Outlaws unveiled his new TRE built 959 cu inch 5 kit Nitrous engine?

 

[HotRodPC]

BTW, that's not a typo. It is a NINE HUNDRED and FIFTY NINE cubes.

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[Honky Kong jr]

Has nothing to due with the science he's speaking of. Science is science whether in a SB, BB, I6, V6 etc.

Has anyone else heard that Doc on Street Outlaws unveiled his new TRC built 959 cu inch 5 kit Nitrous engine?

So Larry Larson can still beat him.....by rights that show should be done then no need for the list if there is no replacement for displacement. That saying is true to a point and it’s disproved by the need to spray it.

 

[shiftpro]

BTW, that's not a typo. It is a NINE HUNDRED and FIFTY NINE cubes.

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Is there an image for "**** my pants"?

 

[HotRodPC]

So Larry Larson can still beat him.....by rights that show should be done then no need for the list if there is no replacement for displacement. That saying is true to a point and it’s disproved by the need to spray it.

Larry Larson didn't beat him on the STREET. There's many that beat Doc. He's got all that power and can't seem to get it to the ground. Doc isn't all that, he's just one that always seems to need a mountain motor to get down the track and it takes him half a season to get adjustments made to get down the street without burning up a set of slicks it seems. Don't forget street racing is way different than track racing. Anyone can bolt in a store bought fast motor with all the adders, but it's not a who has the most horsepower competition. It's who can get down the street faster all things considered. Most of those guys are extremely de-tuned and have alot more if they can figure a way to get it to the ground. Look what Chuck, Shane and Justin (Big Chief) do with small blocks in the 400's cubes get done.

 

[Honky Kong jr]

Larry Larson didn't beat him on the STREET. There's many that beat Doc. He's got all that power and can't seem to get it to the ground. Doc isn't all that, he's just one that always seems to need a mountain motor to get down the track and it takes him half a season to get adjustments made to get down the street without burning up a set of slicks it seems. Don't forget street racing is way different than track racing. Anyone can bolt in a store bought fast motor with all the adders, but it's not a who has the most horsepower competition. It's who can get down the street faster all things considered. Most of those guys are extremely de-tuned and have alot more if they can figure a way to get it to the ground. Look what Chuck, Shane and Justin (Big Chief) do with small blocks in the 400's cubes get done.

I don’t watch the show I tried but couldn’t do it. Light stands and ambulances is not street racing.