Are HP gains linear with displacment gains?

[OB_easyrider]

This is a repost of a conversation that started on the ZX-12 site. However there seem to be more people on this site with technical information regarding engines. I hope I can get some information regarding engine performance with minimal flaming due to the bikes used for the example. Assume both bikes have the same stroke (which is true).
"Many people think the ZX-12 is maxed out performance wise due to the small displacement. If the ZX-9 is putting out 140HP to the wheel and the ZX-12 is rumoured to only put out 160HP to the wheel then assuming linear HP/CU the ZX-12 should be capable of 185+HP at the rear wheel with similar techniques (math: 140hp*1200cc/900cc = 187hp). Therefore perhaps the ZX-12 has some relatively serious after market potential after all, without over boring."
My question is: does HP go up linearly with displacement? If so, what has the ZX-9 done to get better HP efficiency vs. Displacement than the ZX-12. Politically we can say they capped it due to anti-HP lobbying. Or can we expect some simple ECU re-mapping to get us the big gains? One of the pre-release ZX-12 was dyno'ed at 161HP apparently so I think the numbers above are reasonably valid for arguements sake.
Does anyone have any comments on this?


[This message has been edited by easyrider (edited 12 October 1999).]

 

[Gixx1300R]

i have personally Dynoed 3 ZX9s.2 of them had 130 horsepower and one had 124 horsepower.dont believe everything you read in the Magazines about rear wheel horsepower.

 

[OB_Dr Busa]

A larger engine has more moment of inertia and more friction than a smaller engine thus can it not produce linear HP with regards to displacement even if the energy in the combustion chamber is linearly greater. But if the big displacement engine had the same moment of inertia and friction as the smaller you could probably use the sort of simple math mentioned in the top post. However there are things such as gasflow and other complicated stuff to be considered as well but i think they are smaller problems in comparison to the other stuff i mentioned.

An engine with greater moment of inertia will generate a higher topspeed even if it shows the same numbers on the dyno as a machine with less M of I. I can explain more but i guess no one is interested.......

 

[OB_maui]

Generally, no. Hp does not follow displacement in a linear way. As engine size increases, so does the mass of internal parts. Rods, pistons cranks, valves, etc. The forces that act here on the internal parts increase exponentially as speed increases. So parts need to be bigger ( disproportional) to handle these forces.

However, these power differences should be fairly small. As a rule, bigger is better. And as rules go, there is always an exception. Bigger also starts to throw parts out of the case faster! Also exterior size is a big factor. If you could build to scale a bigger motor, then the differences are hard to measure. But, will it still fit in the frame? No. So what engineers are faced with is the task of bigger engines with out increasing exterior dimensions. Stroked cranks and big bore stuff. As you increase bore, you lose some strength. Your engine can't cool well, you get hot spots in the bore, bores can become out of round. As cranks get bigger, we start cheating journal size to get the stroke we need. Connecting rods get shorter in stroked motors. Piston pins can be moved up to try a counter this, but then you compromise piston positions. Blah blah blah. There is always a trade off. The Hayabusa has a rod ratio of 1.896 to 1. This is very impressive. Start to shorten the rod, lose the rpms.

Simple huh?

 

[OB_easyrider]

Dr. Busa I would be interested in any technical details you have to share.
I understand your point though, strength increases proportionally to surface area (x^2) and weight increases proportionally to volume (x^3). Therefore to compensate for extra HP the components gain weight exponentially with respect to strength, you lose efficiency as you get bigger, however the question is how much do you lose with respect to the gains. If the new (not 1999) ZX-9 has 140HP at the wheel what could we expect with similar technology for a ZX-12 assuming re-mapping and minor pipe mod's?

 

[Guest]

Inorder to make the comparison that you are trying to make, the two engines would have to be exact replicas of each other only scaled up. this is not possible due to meterial strenghts and design safety factors.
also when enlarging the head you would get different flow characteristics due to the increased mass of the air.

Flow characteristics = HP

Also two apparently identical heads will flow differently on the same flow bench due to unknown / un descernable small variations in castings / mods

 

[OB_JohnnyB]

on the other hand, do two ZX9 engines make twice as much HP as one ZX9 engine?
Yes.
Are they double the displacement?
Yes.
Is that linear?
Yes, with a delta of 1

go figure?

[This message has been edited by JohnnyB (edited 13 October 1999).]

 

[Guest]

Larger pistons/cylinders are less efficient,
so a 1000cc twin needs a lot more high-tech goodies to make the horsepower of a stock 1000cc four. I ride a twin, this I know.

JohhnyB with the funny example of two ZX-9 motors, yep, you are right about the power, but you have suddenly gone to a 8 cylinder motor with the same size and efficiency cylinders.

Smaller motors are slightly more efficient. My old Suzuki RGV250 two-stroke had 250cc motor, stock with 65 horsepower. That equates to 338 horsepower on a 1300cc 'Busa (two stroke). Imagine the blue smoke! And the sound! Braaaaap... Ha ha! Even last year's Honda CBR250 was about 43hp, (equiv to a 224hp 'busa)...

The main thing wrong here is that (yes I collect dyno charts) a stock ZX-9 is always 120-127rwhp, NOT 140. By your simple linear calculation we could expect the ZX-12 to be about 165rwhp, but being slightly less efficient than the 900cc bike about 157-162rwhp would be my guess.

 

[OB_easyrider]

Thanks for all the information, this was good feedback with minimal flaming due to the ZX-12/ZX-9 example.

Thanks,

I guess FC must be out of town as well as out of his mind.

 

[Guest]

He's always gone - one way or another!