Big bore, short stroke or narrow bore, long stroke?
Discussion
I've just been reading about a two identical engines, both 2.5 inline fours.
Identical specification except for one thing, they changed the bore/stroke ratio.
The LS was 100 stroke, 89 bore
The SS was 94 stroke, 92 bore.
If the RPM limit was the same on both (7k) would the power outputs be the same?
Yes I know the SS will have a more 'revvy' nature, the potential to rev further safely and make more power, but if it hasn't been exploited (with a change in spec) would it give the same power output?
Identical specification except for one thing, they changed the bore/stroke ratio.
The LS was 100 stroke, 89 bore
The SS was 94 stroke, 92 bore.
If the RPM limit was the same on both (7k) would the power outputs be the same?
Yes I know the SS will have a more 'revvy' nature, the potential to rev further safely and make more power, but if it hasn't been exploited (with a change in spec) would it give the same power output?
not a clue but interested to see what the responses are assuming everything else stayed the same
I'd expect there is a point where stroke becomes too long for the rpm and piston speeds causing issues for reliability but where that would affect power wouldnt care to guess.
changing bore/stroke would affect compression too i'd assume
nope cant make a guess
I'd expect there is a point where stroke becomes too long for the rpm and piston speeds causing issues for reliability but where that would affect power wouldnt care to guess.
changing bore/stroke would affect compression too i'd assume
nope cant make a guess
Power as BHP or Torque as foot-lbs?
A long stroke engine delivers more torque in the midrange, but cannot be revved, as the mean piston velocity will soon exceed the safe limits for the materials. Unless you go to exotics.
A short stroke engine can be revved and within the breathing limits of the motor, power lies there, just from more explosions/minute, but will have less torque lower down. If the rev limit is the same for both then the same power.
But so many other things can be varied, timing, ignition and valve in particular, and the differences you quote are not great.
John
A long stroke engine delivers more torque in the midrange, but cannot be revved, as the mean piston velocity will soon exceed the safe limits for the materials. Unless you go to exotics.
A short stroke engine can be revved and within the breathing limits of the motor, power lies there, just from more explosions/minute, but will have less torque lower down. If the rev limit is the same for both then the same power.
But so many other things can be varied, timing, ignition and valve in particular, and the differences you quote are not great.
John
GreenV8S said:
Same rod/stroke ratio, cam profile, intake and exhaust characteristics?
Cam profile, intake and exhaust are the same, even timed the same.Rod/stroke ratio is a bit better on the SS, but is that much of an issue? A lot of people say the dwell at TDC makes more power, but then the more modern thinkers say not, or at least if it does it's only worth about 1 - 3 bhp on a 2 -- 2.5 litre engine. If you'd like to debate that (and it's an important part of this thread) i'd like to see some hard evidence that it works.
For any given cylinder capacity, all the "short stroke" architecture is able to make more power because:
1) A bigger bore gives more room for bigger valves. Bigger valves support a greater cylinder filling at any given speed, and hence increase BMEP
2) The shorter stroke reduces piston speed, which reduces friction, and ultimately, peak piston speed becomes limiting
(There is no geometric advantage, because for a given CC, stroke and piston area cancel)
1) A bigger bore gives more room for bigger valves. Bigger valves support a greater cylinder filling at any given speed, and hence increase BMEP
2) The shorter stroke reduces piston speed, which reduces friction, and ultimately, peak piston speed becomes limiting
(There is no geometric advantage, because for a given CC, stroke and piston area cancel)
Max_Torque said:
For any given cylinder capacity, all the "short stroke" architecture is able to make more power because:
Yes it has more "power potential" but if cam duration, valve size, port geometry, CR etc. are the same, then that potential won't be utilised.The short stroke engine suffers from increased heat loss due to greater cylinder/combustion chamber area. On the plus side it has a slightly more displacement (0.44% from the OP's numbers) and a more favourable rod ratio. It may also provide less valve shrouding depending on head design.
In short it's not possible to give a definite answer, still too many variables even though the engines are nominally the same apart from bore/stroke ratio.
Mignon said:
Yes.
Yes I think so too.Max_Torque said:
For any given cylinder capacity, all the "short stroke" architecture is able to make more power because:
1) A bigger bore gives more room for bigger valves. Bigger valves support a greater cylinder filling at any given speed, and hence increase BMEP
2) The shorter stroke reduces piston speed, which reduces friction, and ultimately, peak piston speed becomes limiting
(There is no geometric advantage, because for a given CC, stroke and piston area cancel)
2. There will be greater friction from the bigger bore, whether it will reduce or cancel out any of the gains I don't know.1) A bigger bore gives more room for bigger valves. Bigger valves support a greater cylinder filling at any given speed, and hence increase BMEP
2) The shorter stroke reduces piston speed, which reduces friction, and ultimately, peak piston speed becomes limiting
(There is no geometric advantage, because for a given CC, stroke and piston area cancel)
Mignon said:
tapkaJohnD said:
A long stroke engine delivers more torque in the midrange
No it doesn't.Mr2Mike said:
227bhp said:
Bigger circumference = more ring contact = more friction.
For the same ring tension force per unit area will be lower, cancelling the greater area 
I remember doing this experiment at school with a steel block and a surface table.
Your assumptions are in error. Friction is only independent of surface area for an object of a given mass, or for a piston ring of a given total radial tension. For effective bore sealing the ring tension will be proportional to bore diameter (or more properly ring contact area) so yes friction will increase with bore size but for a given cylinder capacity will reduce again proportionally with shorter stroke as I said earlier.
I wasn't responding to your post Mignon but to the simple suggestion I quoted that an increase in surface area causes an increase in friction, (all other parameters being equal).
However, I dispute your suggestion that a longer stroke equates to an increase in friction. In the block diagram above, the force to keep the block moving will stay constant whether the block is being moved an inch or a mile, assuming the nature of the surfaces remains constant. If the force remains constant, then the friction being overcome must also remain constant.
What does increase with distance is work.
Work against friction = Force of friction x Distance
However, I dispute your suggestion that a longer stroke equates to an increase in friction. In the block diagram above, the force to keep the block moving will stay constant whether the block is being moved an inch or a mile, assuming the nature of the surfaces remains constant. If the force remains constant, then the friction being overcome must also remain constant.
What does increase with distance is work.
Work against friction = Force of friction x Distance
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