bore and stroke

[Daffyflyer]

killrob, you are saying that if you increase stroke on an engine and bore stays the saame, it produces more torque , explained by you to be a result of larger capacity.
what if you inrease stroke and decrease bore to mantain the same cap.? In the article posted by bishop it tells the exact thing i said; peek burn and peek pressure happen long before 70-80 degrees ATDC, therefore the longer stroke has very little influence on torque

Yep, You, Bishop, Killrob and I are all correct that it stroke has little influence on torque. The only real torque increase is from any capacity you gain. But in terms of just the bore to stroke ratio, you're more likely to make a little more torque with a larger bore if anything, as you've got room for larger valves that flow a bit nicer (or better placement of small valves)

[Killrob]

Yes, true true. For very undersquare engines with little valve area you would run into breathing problems, thus losing torque while increasing stroke (keeping capacity constant). Assuming that there are no flow problems at all, more stroke does (to first order) exactly nothing while keeping capacity constant.

[Daffyflyer]

And due to increasing piston speed, also makes it more likely you're going to experiment with orbital conrod deployment.

[darkjedi]

honda proved otherwise

[FordManFromHell]

So (if the engine size stays the same), bigger bore = more hp and Nm. Bigger stroke = basically just better smoothness.

Am I right?

I suspect diesel engines gain more from enlarged stroke than petrol engines do, simply because diesel burns much slower than petrol, and therefore the peak in maximum burn takes place much later on than in petrol engines.

honda proved otherwise

Honda proved what otherwise? Did Honda found a way to ignore, or just simply bend the rules of physics?

[Daffyflyer]

Yeah, the only real reason for bigger stroke is you can make narrower/thinner engines. Also cars in the UK are taxed on bore.

[darkjedi]

@Ford dude
Honda has a lot of high revving narrow/ long stroke engines. ask them if they bent the rules of physics not me.

[FordManFromHell]

Im not too sure what you are trying to say there darkjedi, but I can assure you that those Honda engines you brought up would rev alot more if they would have made them with shorter stroke and bigger bore. Simply because in that case the distance the piston has to travel is shorter, there's some physics for you.

So care to elaborate, what did Honda prove otherwise?

[Killrob]

So (if the engine size stays the same), bigger bore = more hp and Nm. Bigger stroke = basically just better smoothness.

For constant size (breathing limited at valves)
bigger bore = significantly more power, more smooth, heavier, larger, more expensive, higher avg. octane requirement, higher max rev, but earlier valve float.
bigger stroke = even less power, less smooth, lighter, smaller, less expensive, lower avg. octane requirement, lower max rev, but later valve float.

At constant size, while not breathing limited at the valves, you take away the effect on power, otherwise things stay the same.
Daffy may correct me when I got something wrong, but that should be most dependencies.

[darkjedi]

Im not too sure what you are trying to say there darkjedi, but I can assure you that those Honda engines you brought up would rev alot more if they would have made them with shorter stroke and bigger bore. Simply because in that case the distance the piston has to travel is shorter, there's some physics for you.

So care to elaborate, what did Honda prove otherwise?

they made some engines, not all of them with very long strokes and 7500-8000 rpm limit. i've seen some modded versions hit 9500. i meant they were able to make long stroke engines that rev high and yet still be reliable. jd power said 3 engines in 1000 have a problem. i don't know their exact reasons for making them like that , but this is what they did.

[FordManFromHell]

For constant size (breathing limited at valves)
bigger bore = significantly more power, more smooth, heavier, larger, more expensive, higher avg. octane requirement, higher max rev, but earlier valve float.
bigger stroke = even less power, less smooth, lighter, smaller, less expensive, lower avg. octane requirement, lower max rev, but later valve float.

At constant size, while not breathing limited at the valves, you take away the effect on power, otherwise things stay the same.
Daffy may correct me when I got something wrong, but that should be most dependencies.

Yes of course. I somehow earlier managed to actually understand that when building an engine, increasing bore would be a “better” way to achieve more power from the engine vs increasing the stroke. But as for the power alone, it really does not matter (given that the valve size also scales with the changing bore&stroke ratio) which one do you increase, bore or stroke. The differences between those two are in other areas, engine size, octaine requirement, revs and so on. This has been very educating thread!

they made some engines, not all of them with very long strokes and 7500-8000 rpm limit. i've seen some modded versions hit 9500. i meant they were able to make long stroke engines that rev high and yet still be reliable. jd power said 3 engines in 1000 have a problem. i don't know their exact reasons for making them like that , but this is what they did.

Maybe what Daffy said earlier about the cars being taxed by bore in the UK has something to do with it? Anyhow, dont really see what is it that Honda would have proven otherwise with those engines you mentioned..