12 replies to this topic

[Group B]

Good evening all. I have a question which has been niggling me for a while, I have a decent grasp of engineering, mechanics, etc, but once we get into complex stuff, fluid dynamics, etc I'm lost. The basic upshot is that 30 years of observations tells me that small engines generally put out more hp/cc than large ones - in moto GP for instance we're led to believe figures of 55/60hp for 125s, 100/105 for 250s and around 190 for 500s. My friend with an A level in physics a ½ a degree in mechanical engineering (had a bit of beer trouble) tells me there is no reason for this, and in fact it should be the other way around.

The recent appearence of Honda's 1000 GP 4 stroke emphasises the question, since being the 2 wheeled pinnacle we must assume they are giving it all they can just as they do their F1 engine. The 1000cc bike engine apparently puts out 230 bhp (x3 = 690) and the 3000cc car engine around 800bhp, a considerable difference. So are Honda's bike tuners a bit crap or is there a real advantage for bigger engines after all?

[Chui]

A two stroke engine will make close to twice the power of a four stroke. This is simple physics: It takes 720 crank angle degrees for a four stroke engine to fire all cylinders while a two stroke needs half that number.

As far as the Honda bike engine is concerned the limit is TRACTION. Little traction = little USEABLE bhp. More could be obtained, obviously, but power delivery is much more important when you only have one contact patch versus two.

[H. Eckener]

Geo. Similar Engine Comparison

[schuy]

Good discussion.
Thanks to all those who gave useful information.

[Group B]

Yeh, thanks for the info guys. So you think (Chui) that Honda could get 260-270bhp (1/3 of 800) from the V5 if they wanted to? but it simply isn't usable?

[Aubwi]

So they're getting wheelspin in top gear? Yikes.

[AndreasNystrom]

They talked about Ducatis engine in MotoGP btw, that there was a mysterious box they needed to start the engine. Probably are they using pneumatic valves they said.

And about getting away wheelspin, they had the usable torque moved down, and not as many hp cause when the tire loose traction, the wheel and engine spins up, but the torque lowers dramaticly, making the wheel come back to traction, instead of before, when all the hp and torque was really high up in the register.

Its a sort of "natural tractioncontroll" they said.

[Yelnats]

Yes a smaller displacement engine (per cylinder) has several reasons why it can make more power.

1. Better Breathing.... If you take a 500 cc engine and shrink it proporionally in alll dimensions so that the stroke, bore ratio and valve/port sizes remain proportionate, the valve/port area per unit of displacment will increase allowing the cylinder to be filled in a shorter time for a given port flow velocity allowing better output at higher rpms.

2. Higer RPMs.... Following the same rules as above, the shortened stroke will allow higher rpms for a given piston acceleration and speed thus allowing the engine to operate at higher rpms to take advantage of the better breathing provided by the greater port area per unit displacment. With more power strokes per unit of time and a greater fuel air/throughput a higher maximum power can be achieved compared to a similar unit of larger displacement.

[Group B]

Thanx Yelnats, that's exactly what I was trying to ask in my roundabout way. Am I to assume though that there are practical limitations or compromises to the multi-cylinder approach that stop us from seeing 3 litre V24s ? or is it only the rules that forbid such insanity? (would be kinda fun - sound nice too..)

[Yelnats]

The rules of dimminishing returns apply as engine complexity increases. A 24 cylinder (if F1 rules permitted) engine operating at the same rpms as a 12 cylinder with half the displacement would be twice as vulnerable to failure due to manufacturing tolerences/defects. Added to this would be additional weight and packaging limitations that restrict exhaust and intake systems design, a no-go from the start.

[Pioneer]

Not to mention increased friction losses.

[Christiaan]

and the fact that as the rpms get higher air has less time to fill up the cylinders and therefore the volumetric efficiency is lowered

[Chui]

Originally posted by Yelnats
The rules of dimminishing returns apply as engine complexity increases. A 24 cylinder (if F1 rules permitted) engine operating at the same rpms as a 12 cylinder with half the displacement would be twice as vulnerable to failure due to manufacturing tolerences/defects. Added to this would be additional weight and packaging limitations that restrict exhaust and intake systems design, a no-go from the start.

It would also make less bhp. It would have to rev higher than the current engines to make more power. And it would. But it, too, would be less driveable. I don't think it would be any less reliable with the correct material/manufacturing choices at hand.