making rotors higher compression...
05-09-2008, 09:30 PM
#31
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There's one thing commonly overlooked in the long bore/short stroke vs. small bore/long stroke conundrum.
It's somewhat common knowledge that a bigger bore means more potential valve area and therefore more power potential. Just geometrically there is more room for more valve. And a smaller bore means more efficient combustion because the combustion space is smaller and less flattened out.
But a longer stroke/smaller bore engine also works the ports harder. Theory is that the air has to move faster to get to the middle of the cylinder (middle being the "average") because it's always further away from the valves than in a big bore/short stroke combo.
It also means more port velocity, which is a Good Thing
It's somewhat common knowledge that a bigger bore means more potential valve area and therefore more power potential. Just geometrically there is more room for more valve. And a smaller bore means more efficient combustion because the combustion space is smaller and less flattened out.
But a longer stroke/smaller bore engine also works the ports harder. Theory is that the air has to move faster to get to the middle of the cylinder (middle being the "average") because it's always further away from the valves than in a big bore/short stroke combo.
It also means more port velocity, which is a Good Thing
This is what I was trying to get across
Get any engine and put small port in it and it will feel torquey down low, put large ports on the same engine and it will feel dead down low and have high HP at high revs.
Just look at any early 4WD like the Toyota 3Y-4Y look how small the ports are
Well the premise is off a bit here. To have the same displacement, the longer stroke engine must have a smaller bore size
The premise is right, 2 engines with the same capacity but different strokes must have different bore diameters to make the same capacity
I'm afraid Smokey was a long time ago and doesn't reflect trends today. Current trends in NHRA Pro Stock, they don't care what length of rod the use. They try for the biggest bore diameter so they can use the largest valves, this bore diameter fixes the stroke. Then they have to use a certain ring stack height for the rings to seal and work effectively, most don't run the ring stack in the pin bore, this works out the piston compression height and with a limited number of deck heights, they just use a rod length that connects the piston to the crank. The ring stack has a higher importance than rod length
I don't know if rotaries are less torquey, look at a 4G63(100bhp peak Tq 7000rpm, peak HP 8000rpm)(very large ports) against a S2 12A(97bhp TQ 4500rpm HP 6750rpm). The 12A makes more torque in the mid range and peak power is at a lower rpm
A PP13B makes about 300-350hp and a 2L BTTC engine makes 300hp rev limited, unlimited probably 350hp. Remember the stroke is 70mm more in the 2L but I would say the curves should be close. The port area of the 2L is around 2sq" the PP13B about 3sq". the higher velocity in the 2L should make a little more torque in the mid range
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