Discussion
This is essentially a pub argument that led on to to various musings, however...
Does a larger bore mean better torque (let's restrict the argument to normally aspirated engines for now)
I realise simple factors re bore/stroke may not play the whole part, but given equal fuelling for same capacities what are the rough benefits of both?
Dave
Does a larger bore mean better torque (let's restrict the argument to normally aspirated engines for now)
I realise simple factors re bore/stroke may not play the whole part, but given equal fuelling for same capacities what are the rough benefits of both?
Dave
As a general rule for the same given capacity a longer stroke will make more torque, and a larger bore will make more bhp, but there are so many factors you can easily lose torque by a longer stroke too!! for instance if you run a longer stroke but then run a shorter conrod you have just lost what you have gained! but generaly yes bore= bhp stroke= torque
For a given capacity and BMEP the bore/stroke tradeoff makes no difference to torque. The change in leverage on the crank is exactly counterbalanced by the change in piston area.
However, a larger bore allows a larger valve area in relation to the cylinder volume, which means that volumetric efficiency does not fall off so much at high revs, so an oversquare engine generally makes more power and has a torque curve that peaks higher in the rev range than an undersquare one.
However, a larger bore allows a larger valve area in relation to the cylinder volume, which means that volumetric efficiency does not fall off so much at high revs, so an oversquare engine generally makes more power and has a torque curve that peaks higher in the rev range than an undersquare one.
Pigeon said:
For a given capacity and BMEP the bore/stroke tradeoff makes no difference to torque. The change in leverage on the crank is exactly counterbalanced by the change in piston area.
Um, isn't that just saying that two engines of the same capacity producing the same power produce the same power?
Changing the bore/stroke ratio may itself change the BMEP, perhaps due to the effects of different con rod length/stroke ratios, and/or squish characteristics and/or valve shrouding effects etc.
Edited by GreenV8S on Tuesday 18th July 00:08
long stroke engines are usually torqueier and lower reving
the bigger the bore and the shorter the stroke to faster it can rev
but revs kill engines so reducing the stroke puts less pressure on the big end due to the fact it dosnt have to accellerate the piston as fast to travel the greater distances in a long stroke engine
basically the bore and stroke make up the charecteristics of the engine for the purpose it was designed for
BMW play with stroking engines to give more effiency
ie the bmw 525 can be stroked to 2.7 to give a greater effiency due to the long stroke
these are all options available to the engine designer
but the limiting factor on revs on any engine will allways be the conrod and piston trying to leave due to revs
once you know the safe max revs then the only thing you can do to increase power is to increase volumetric effiency
so if you start with a honda vtec engine that revs to 9000 rpm you have a lot of scope to play with volumetric effiency
the bigger the bore and the shorter the stroke to faster it can rev
but revs kill engines so reducing the stroke puts less pressure on the big end due to the fact it dosnt have to accellerate the piston as fast to travel the greater distances in a long stroke engine
basically the bore and stroke make up the charecteristics of the engine for the purpose it was designed for
BMW play with stroking engines to give more effiency
ie the bmw 525 can be stroked to 2.7 to give a greater effiency due to the long stroke
these are all options available to the engine designer
but the limiting factor on revs on any engine will allways be the conrod and piston trying to leave due to revs
once you know the safe max revs then the only thing you can do to increase power is to increase volumetric effiency
so if you start with a honda vtec engine that revs to 9000 rpm you have a lot of scope to play with volumetric effiency
GreenV8S said:
Pigeon said:
For a given capacity and BMEP the bore/stroke tradeoff makes no difference to torque. The change in leverage on the crank is exactly counterbalanced by the change in piston area.
Um, isn't that just saying that two engines of the same capacity producing the same power produce the same power?
Changing the bore/stroke ratio may itself change the BMEP, perhaps due to the effects of different con rod length/stroke ratios, and/or squish characteristics and/or valve shrouding effects etc.
Yes, probably not one of my clearest posts
I'm just trying to say that if ALL you change is the bore/stroke ratio it doesn't make any difference. I agree that in the real world, changing the bore/stroke ratio without changing anything else is not what usually happens 
Contrary to popular belief, ALL THINGS BEING EQUAL, bore & stroke make no difference. Two engines, of differing bores & strokes, but the same displacement, using the same heads etc, will make the same power.
Torque is a function of displacement & volumetric efficiency. It has nothing to do with leverage etc, as far as an engine is concerned. Whether the bore is big with a short stroke, or vice-versa, it's pressure over volume, however you cut the cake.
There may be advantages to going to bigger bores & shorter strokes,rod ratios etc, but ALL THINGS BEING EQUAL apart from that, it makes no difference.
Torque is a function of displacement & volumetric efficiency. It has nothing to do with leverage etc, as far as an engine is concerned. Whether the bore is big with a short stroke, or vice-versa, it's pressure over volume, however you cut the cake.
There may be advantages to going to bigger bores & shorter strokes,rod ratios etc, but ALL THINGS BEING EQUAL apart from that, it makes no difference.
p15ton said:
Contrary to popular belief, ALL THINGS BEING EQUAL, bore & stroke make no difference. Two engines, of differing bores & strokes, but the same displacement, using the same heads etc, will make the same power.
Torque is a function of displacement & volumetric efficiency. It has nothing to do with leverage etc, as far as an engine is concerned. Whether the bore is big with a short stroke, or vice-versa, it's pressure over volume, however you cut the cake.
There may be advantages to going to bigger bores & shorter strokes,rod ratios etc, but ALL THINGS BEING EQUAL apart from that, it makes no difference.
Torque is a function of displacement & volumetric efficiency. It has nothing to do with leverage etc, as far as an engine is concerned. Whether the bore is big with a short stroke, or vice-versa, it's pressure over volume, however you cut the cake.
There may be advantages to going to bigger bores & shorter strokes,rod ratios etc, but ALL THINGS BEING EQUAL apart from that, it makes no difference.
If you are thinking in terms of an idealised engine, that's true. In practical terms though there are various ways that bore/stroke changes can change the efficiency of the engine and hence affect the power output, in other words other things are not actually equal.
Thanks guys, some good thoughts to ponder further there.
One thing I realised last few days (irrespective of bore or stroke) is that despite the weather beating at you to the contrary you lose a bit of power due to low air pressure. Plus everything is much hotter; not the best way for a TVR.
Shoot me down if this is another daft question but would a shorter stroke engine suffer less from diminished external air pressure?
Dave
One thing I realised last few days (irrespective of bore or stroke) is that despite the weather beating at you to the contrary you lose a bit of power due to low air pressure. Plus everything is much hotter; not the best way for a TVR.
Shoot me down if this is another daft question but would a shorter stroke engine suffer less from diminished external air pressure?
Dave
Pigeon said:
Nope, make no difference at all.
Read the article in the news section about the JCB plants developed for their land speed record [url]www.pistonheads.com/news/default.asp?storyId=14540[/url]
There was something perhaps in popularhotrodding.com or somewhere comparing may engines of the same displacement with varying rod, bore and stroke changes. Basically almost no difference.
You do get extra side loading with longer strokes and short rods. A larger bore allows larger valves so should breathe better. But a longer stroke creates a deeper intake stroke which may allow better induction. Though really induction is optimised by valve events (eg overlap)
So yeah, much of a muchness. Larger bore should be more duarable due to less side loading and lower piston velocity.
You do get extra side loading with longer strokes and short rods. A larger bore allows larger valves so should breathe better. But a longer stroke creates a deeper intake stroke which may allow better induction. Though really induction is optimised by valve events (eg overlap)
So yeah, much of a muchness. Larger bore should be more duarable due to less side loading and lower piston velocity.
Pigeon said:
It doesn't say anything about bore/stroke ratio having any effect on the engine's performance under conditions of reduced air pressure - or indeed reduced air density due to increased temperature, which is what really causes the effect donatien's on about.
Yeah Pigeon, I think that's what I meant. I was referring to hot weather reducing air density so less fuel/air mixture going in to the cylinders for combustion. How marginal is this? can't be much more than a few %
Lets take an example - a Ferrari flat plane V8 at sea level in the cold against a drive on a hot day at altitude. Would there be a discernible difference?
That said, I can feel (or perceive) a slight difference in the Griff on hot and cold days. The roof down over-compensates for the slight loss of bhp though
Dave
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