Topic: jetting and compression question

[01Thomas]

Hi
Just a generic question: we know that increasing the compression in an engine will increase the engines' performance, up to a point (practicality and diminishing returns).

Assuming everything else is left untouched and aside from pinking and detonation and other (mechanical) issues, what effect will raising the compression have on the carb jetting to ensure an optimal mixture?

Does a higher compression result in better, more efficient combustion, thereby requiring the mixture to be made leaner?

regards
Thomas

[PeWe]

I could not resist to search when I thought I almost knew...  "compression ratio vs air/fuel ratio"
http://auto.howstuffworks.com/compression-ratio-fuel-economy.htm

[calj737]

It has always been positioned to me that one has little to do with the other. The carb settings control the mixture of the fuel, and compression doesn't care about that. What does matter, is the volume of air you shove into the motor as a result of the compression increase to achieve performance benefits. That has an effect in the carb settings by virtue of needing to tree more air and fuel, in effect jetting changes.

If you kept the same volume of air, but compressed it at a higher ratio only, you need only insure the quality of the fuel is appropriate for the compression ratio.

Or so I've been told...

[PeWe]

I got pinging on very low throttle lift due to lean pilot circuit. Richer jetting - no pinging.
(Have got it due to lean main jets too)
Higher CR will probably give more power out of the fuel. More bang for the buck. My experience is higher CR is more sensitive for lean carbs. I have RC 836 pistons (CR 10.5:1) with holes as a reminder.

[turboguzzi]

if A/F ratio was spot on already, then higher CR shouldnt require jetting changes, maybe a little richer if you were lean to begin with as PW says.
what is totally missing from your evaluation is the squish area treatment. if you just raise CR the old skool way of just bumping the crown in the center, very likely the engine will make more power but it will also be very prone to detonation due to low octane fuel. if instead you raise the CR smartly by building large squish areas, not only efficiency will be better, you will get quicker burning and lower operating temps due to the turbulence in the CC. 

[MRieck]

TG was talking before about the use of flattop pistons or those with squish bands built into the OD of the piston. I don't have any problems with "pinking" with 71.25mm pistons and approx. 10.2 CR. The chamber is small with a nice squish band....the charge is really concentrated towards the center of the chamber on the compression stroke. Another benefit to that bore size and flat top piston is great deshrouding on the intake side.
 Large chambers and domed pistons play havoc with charge. More fuel will more than likely be necessary to prevent the lean situation PeWe talked about. It also helps with cooling in an air cooled engine.
 Look at modern engines.....small, compact chambers, small valve angles and flat pistons, liquid cooling and CR's around 13.5 for some bikes.
 Compression is good and works in conjunction with a cam so you can have good torque and good horsepower......you just have to know it works together.

[01Thomas]

Thanks, this makes perfect sense.

It has always been positioned to me that one has little to do with the other. The carb settings control the mixture of the fuel, and compression doesn't care about that. What does matter, is the volume of air you shove into the motor as a result of the compression increase to achieve performance benefits. That has an effect in the carb settings by virtue of needing to tree more air and fuel, in effect jetting changes.

If you kept the same volume of air, but compressed it at a higher ratio only, you need only insure the quality of the fuel is appropriate for the compression ratio.

Or so I've been told...

[01Thomas]

Thanks for the link, PeWe

I could not resist to search when I thought I almost knew...  "compression ratio vs air/fuel ratio"
http://auto.howstuffworks.com/compression-ratio-fuel-economy.htm