Corrected compression vs torque

[Sportzilla]

My assumption is that there is a relationship between corrected compression and torque but I have been wondering....will a 1200cc with a corrected compression of 9.15 make the same power as a 1250cc with a corrected compression of 8.8 with all other things being equal like intake close timing?? Thanks for any input.

[maru]

There is definitely a relationship between compression and tq but there are many other variables that need to be considered. Depending on when you close the intake valve a given corrected compression ratio can be the result of widely different static compression ratios and the two engines will not have equal tq.

One also have to consider fuel capability and the ignition timing needed to to make it work.
If you have to pull enough timing for a high compression ratio, that can negatively affect tq and in extreme cases the higher compression ratio could end up with the same or even less tq as a result. There is a sweet spot when it comes to pump fuel and not a linear relationship in terms of tq and compression, corrected or not.

The corrected compression ratios in your example are well withing the range suited for pump fuels, but in isolation that one variable does not tell you much. Having said that a 1250 can has been know to make 3 ft pounds more tq than a 1200 in some combinations. The small amount of extra correction compression in your example would not give you an equal bang for your buck in my experience. Common sense tells you at at a given displacement your higher figure would make more power but I doubt it would be enough to feel. It might even be so close it would come within the range of difficult to validate on a dyno due to run to run variances.

In my experience you can feel a fUll point of added compression. A third of a point, not so much.

[Sportzilla]

Originally Posted by maru

There is definitely a relationship between compression and tq but there are many other variables that need to be considered. Depending on when you close the intake valve a given corrected compression ratio can be the result of widely different static compression ratios and the two engines will not have equal tq.

One also have to consider fuel capability and the ignition timing needed to to make it work.
If you have to pull enough timing for a high compression ratio, that can negatively affect tq and in extreme cases the higher compression ratio could end up with the same or even less tq as a result. There is a sweet spot when it comes to pump fuel and not a linear relationship in terms of tq and compression, corrected or not.

The corrected compression ratios in your example are well withing the range suited for pump fuels, but in isolation that one variable does not tell you much. Having said that a 1250 can has been know to make 3 ft pounds more tq than a 1200 in some combinations. The small amount of extra correction compression in your example would not give you an equal bang for your buck in my experience. Common sense tells you at at a given displacement your higher figure would make more power but I doubt it would be enough to feel. It might even be so close it would come within the range of difficult to validate on a dyno due to run to run variances.

In my experience you can feel a fUll point of added compression. A third of a point, not so much.

So how do you find the "sweet spot"? And are you saying that a 1250 may not be worth the leap from a 1200 or that trying to squeeze out extra compression may not be worth it? Just trying to clarify

[maru]

I think a 1250 would be stronger than a 1200 with a corrected compression ratio with only three tents of a point higher.

The sweat spot for pump fuel on an Xl in terms of compression/corrected compression results in cylinder pressures between 190 and 195 psi. There are formulas available to determine what static compression and corrected will give you that. One of the key variables is when you close the intake valve. The later you close it, the higher static compression you can run and still remain in this range.

A other thing to consider is that most folks in the US live near the coast and at an altitude near seal level. If you live well above sea level, you need more compression to achieve the same cylinder pressure . There are formulas that reflect the changes for a given altitude. A few thousand feet don't matter that much, but if you are at 5000 feet elevation or higher you can get away with more compression and will need it to develop comparable cranking pressure.

I have run one of my engines well above 195 psi but when you do that they become increasingly difficult to tune for pump fuel. As often as not you lose more power than you gain because you end up pulling enough timing in order to keep it from knocking itself to death that it hurts the power. If you run the thing with it knocking and pinging to beat the band, you stand a very good chance of hurting it.

I think for a practical every day rider, one would build a 1250 and shoot for around a hundred and ninety pounds of pumping pressure. That is likely to give you the best balance of power or torque if you will for the money.

For the record the newest bikes leave the factory at or above what I would call the sweat spot due to the early intake close of the w cam.

[Sportzilla]

Thanks Maru, I appreciate you taking the time to share that.