How does compression effect jetting?

[Steven]

Does increasing the compression ratio increase or decrease an engines demand for fuel?

It would appear that my engine wants me to jet it leaner, going from a 9.6:1 to 11.5:1 compression ratio.

Both the needle and main have been effected by the increase in compression ratio.

Can this be true?

Steve

[lex luger]

no

[Steven]

anyone else have an opinion?

[jchevy]

Raising compression does make the combustion chamber smaller.

So i guess you might run lean. Dont really know for sure.

It would appear that my engine wants me to jet it leaner, going from a 9.6:1 to 11.5:1 compression ratio.

the comp wont do much but the higher octane fuel will fatten it up.


anyone else have an opinion? i know this is a trick question :)


Raising compression does make the combustion chamber smaller.

not 100% true as it relates to the amount of cc's etc. because the chamber remains the same but the piston dome or overall height at TDC is where the changes were made. If you were to in fact had the chamber machined etc then that would add to the overall volume but since that wasnt done the combustion chamber actually remained the same.

Hope I said that right :) it is a little confusing.

But other than the potential change in fuel the only thing I can think of would be the cam or weather change.

[Steven]

I called my local professional engine builder and asked him. He is the guy who flowed my head and did the machine work.

He said that compression DOES effect jetting because the size of the combustion chamber changes.

He says that the smaller the chamber and higher the compression, the more efficient the engine becomes resulting in less demand for fuel.

What do you guys think about that statement?

[jchevy]

thats what i was thinking

dude thats my story and I am sticking to it, unless some can explain different that makes sense. :)

If I am not getting too tired as far as I remember an internal combustion engine is basically nothing but an air pump that needs a fuel/air mix just to keep it pumping and the force or amount of the compression isnt putting any extra air into the motor so where is it calling for more fuel. I know that jetting needs will change with efficiency etc but I am thinking your porting may have more of an affect than that.

Not saying I am 100% on this but best I can remember right now.

[Steven]

Maybe its the the way I have the air box set up that is choking the engine. I have three 1" holes drilled in the air box lid and pre-filter fabric over the hole where the snorkle entred the air box.

Is this sufficient?

Anyway, anyone else have a thought on this?

I'm going to do some more testing this weekend.

Sounds like your air box set up is restricting the flow a whole lot.

Try running it with the lid removed and see what happens :)

[KandK952]

i think if it seems like it needs to use a more lean jetting, than take the lid off and see how it runs.

[Dune Surfer]

We have basicly the same set up. I run with the lid off and a 172. I could be a little rich. With your lid on you are for sure. Take it off and ride it, you will like it better

oh pull the cover and tell us what happens

[Steven]

I will Lenny.