combustion chamber

[lx5091]

Why does it seem that some set-ups work better with a smaller chamber. Could you give an example of a setup that would benefit from a 58cc combustion chamber, and a setup that would require a 64cc chamber. What is the deciding factor?

[88workcar]

A small # of cc chamber makes more compression. Naturally asporated this is what you want. Nos or forced induction use the larger cc heads. If you are planning a purchase and will bolt them on a stock motor, get the 58cc, as long as you do not plan on forcing induction. For up to 150hp shot of NOS the 58cc will be OK

[lx5091]

thanks, makes sense

What is the relationship of intake and exhaust port sizes to the size of the combustion chamber.

What I mean is GT-40X's with a 58cc combustion chamber flow 240cfm @ .550 lift utilizing 1.94" intake and 1.54" exhaust valves.

While the afr 185's with a 58cc combustion chamber flow 273cfm @ .550 lift utilizing 2.02" intake and 1.60" exhaust valves.

Are the AFR 185's with a 58cc chamber overkill? Do you need valves that big with this size chamber?

Better yet, what is the maximum size valves neccessary to fill a 58cc chamber on a naturally aspirated motor? Could an intake keep up (any brand)? Or is that even the point?

[Stang Runner]

the champer CC dont afect the Flow of a head the CC of the Runners do tho.

[HotRoddin]

Originally posted by lx5091
Better yet, what is the maximum size valves neccessary to fill a 58cc chamber on a naturally aspirated motor? Could an intake keep up (any brand)? Or is that even the point?

You aren't just filling the combustion chamber lx you're filling that entire space from the top of the piston when its all the way down in the bore (bottom dead center) all the way up to and including the cavity in the head (combustion chamber), and at 6,000 RPM you're filling that space somewhere in the neighborhood of 50 times a second per cylinder. So that valve has to be big enough to flow quite a bit of air and fuel ! You want the intake valve and intake port to be big enough to not severely restrict flow, but at the same time it can't be so big that you lose the speed of that flow (everything in a racing motor is a trade off ). What you really want is a head that has valves and ports big enough to maintain good flow numbers but still maintains good air fuel velocity through the intake ports and on into the combustion chamber. The exhaust valves and ports just need to flow well throughout the cams lift range ... velocity isn't very important on the exhaust side, just good flow. You also want a combustion chamber shape that creates the proper turbulence to promote a fast clean complete burn of the air fuel mixture when it ignites.
The size of the chamber primarily effects the compression ratio. The smaller the chamber, the higher the compression ratio (that is if everything else remains the same).
Sorry that was so long winded but all those things are so inner related its difficult to discuss it without writing a book!