Combustion chamber size
 

What advantages are there to using heads with smaller combstion chambers.I know that smaller combustion chambers give you a higher compression ratio but other than that what are the pro's and cons. The reason I am asking is I have 76cc heads with 1.94/1.50 valves on my engine right now but I am planning on buying Edelbrock aluminum heads with 70cc combustion chambers. My compression ratio now is 10:1. How much will the smaller combustion chambers change this number, and what other changes could I expect to see from this swap?

Smaller chambers = more rapid flame front propagation = less ignition advance needed.

Smaller chambers = less surface area to absorb heat = more thermal efficiency and more power.

Rich Krause

I asked this question on this board a while back. I asked if going with a larger combustion chamber or smaller combustion chamber was better, considering different pistons to achieve the same C/R with both heads. I was told the larger combustion chamber will have better flame front travel...but Rich states the opposite...:confused:

A flat top piston with a smaller chamber will make more power. The only way an open chamber could make more power is if there is a DRAMATIC flow diff. in the heads due to the closed chamber shrouding the valves. If the flow is even close the smaller chamber and flat top piston will out perform the larger chamber-dome piston. This is assuming the deck heighth (quench) is the same on both heads. This is the way I understand it.

NightTrain66

So flat tops and relatively smaller combustion chambers make more power. How in the heck did the Mopar Hemi (domed pistons and hemispherical heads) make so much power prior to computer technology and fuel injection?...:think:

Cajun: you are comparing the proverbial "apples and oranges". There are just too many variables between say an LT1 and a Hemi. All other things being equal, if the CR is the same and the head flow is the same, the smaller CC design will make a little for more power for the reasons stated especially if the comparison is between a small CC/dished piston v. large CC and domed piston. The flat-top piston/medium CC size would be in the middle.

There are a lot of "whys" to this and I am sure I don't know them all. Suffice to say the the small CC/dish give better flame propagation and more swirl.

Rich Krause

my understanding is that a Hemi is king when flow is king, meaning a blower is used to cram pack the cylinders and the hemi head flows greater than a wedge. normally aspirated heads benefit from a "quench" area and on a closed chamber there is more quench area than an open chamber. The hemi has no quench area but makes "much" power when blown because the flow is all that is required and a blower will provide the turbulence and combustion efficiency that is missing from no quench area. The Dodge pro stocks run a so-called hemi motor. If you have seen the heads it is a wedge design. If a Hemi were better naturally aspirated they would have hemispherical chambers since they are out to run as quick as possible.
I thought the ? Was comparing 2 SBChevy 350s with a certain comp ratio (12 to 1 ???) with one having an open chamber (76cc) head and a dome piston and the other having a closed chamber (58cc) head and a flat top piston. As long as the deck heighth of the motor puts the piston .050 (or closer) from the quench pad of the head. If the piston is .100 (or more) down in the bore there would be less advantage for the closed chamber head since the quench is not working at all. The advantages for the flat top motor would 1-no dome to hurt flame travel 2-no dome to restrict flow 3-more quench area to create turbulence 4-(as stated earlier) a smaller area to ignite and less prone to detonation.
As long as the smaller chamber is not shrouding the valves too much and restricting flow such as on a BBChevy. The BBChevy closed chamber heads make TONS of torque when used with a flat top piston and zero deck heighth but will lay over at 6000rpm due to the restriction of the chamber. The open chamber BBChevy head will make more power above that and keep going providing the intake, cam, etc will go higher. the flow or restriction of the chamber will not hurt HP like on the closed chamber BBChevy.
This is the way that I understand it.

NightTrain66

Duke,
From a 76 to a 70 cc head you will gain ABOUT 6 tenths of a point. So if it was 10.0 to 1....it will now be ABOUT 10.6 to 1.
The other heads were cast iron (I assume) and the Edelbrocks are aluminum so SHOULD BE safe for pump gas. Aluminum cylinder heads transfer heat alot more than cast iron so any compression gain would be a trade off for the loss in thermal efficiency. The edelbrock chamber is a better design than the open style GM cast iron head (like an 882). The edelbrocks flow better than unported iron open chambers also.
I hope this helps.
NightTrain66

Thank you guys for all the reply's.