R65 heads on R75 motor

[Rokoneer]

The Monster GS I recently aquired has an '72 R75 block and cylinders and the builder decided to use '79 R65 heads as they have smaller valves than the R75. The reason given for doing this was that the smaller valves in the R65 head would produce more low end power and torque than the R75 head would have.

I'm having trouble understanding why this would work (change the power charcteristics of the motor), if indeed you agree that it will do what is claimed by using the smaller valves.

Secondly, in my feeble mind it would seem that the motor would not operate as efficently with it 'breathing' less due to the lesser amount of air going through it due to the smaller valves. Would this not create a richer fuel mixture from the carbs? I guess I'm thinking that the carbs will need to be jetted leaner due to less demand for fuel with less airflow going through the cylinder?

Feel free to insult my bizarre way of thinking of this as needed, but I'd appreciate your thoughts on whether something else should be done when using smaller valves than the motor was designed for.............


Thanks

[nrpetersen]

I'd bet it would take a careful dyno run to tell the difference.

The smaller intake ports would cut the max hp output a few percent. On the other hand there would be a lilttle better transient throttle response since there would be less liquid fuel surface area in the intake manifold. Again it would take a well calibrated dyno to detect the difference though.

At least that's my mechanical engineering take on it. Others?

[kbasa]

Larger ports work better when there is large amounts of air flowing through them. Smaller ports can help keep port velocity up, even with less air flowing through them, just like when you put your finger over the end of a hose.

Intake tract length can have an effect as well, which is why some modern cars have variable length intakes. A longer tract will work better at high volume/high rpm operation and a shorter tract will work better at lower volume/rpm operation.

So, I can see why the PO put the 65 heads on.

[lkchris]

Originally posted by Rokoneer
... due to the lesser amount of air going through it due to the smaller valves. Would this not create a richer fuel mixture from the carbs?


Your carbs mix fuel in the proper 15-1 ratio or thereabouts.

If the engine demands less, the carbs provide less.

Recall also that your piston size isn't changed, but your combustion chamber size is probably changed just a little. The sum of these two volumes is the amount demanded. Whether or not it can get it through slightly smaller valves may or may not change. Clearly, the velocity may be higher with the smaller valves and this could then provide the same amount.

Whether an increase in velocity will raise the slide higher is an interesting question, but if it does the carb will provide more fuel.

Given that your comb chamber size may be a bit smaller, you may then have a higher compression ratio. It wouldn't be so good to lean the mixture, then, when compression is higher, as this risks detonation.

Bottom line--look at your plugs to evaluate mixture.