Compression ratio difference w/ aluminum Heads???
 

I have a pair of 575sci's that I put aluminum Merlin heads on. If the combustion chamber volume is the same, will my compression ratio remain the same? I was told that when using aluminum heads, you'll lose a full point of compression because of how fast they dissipate heat.

Stock my motors are at 7.5:1. Is it possible they have dropped to 6.5:1 by just using aluminum heads?

Probably not if the combustion chamber volume is the same. An advantage of the aluminum heads is that because they dissipate heat they are less likely to allow detonation WITH a higher compression ratio. I have never changed an engine from cast-iron to aluminum heads and not felt the engine come alive.

Actual comp ratio stays the same. Like Ed said, It is the heat dissipation that changes and allows you to raise the comp ratio.

It is likely that you can add a little more boost to that engine now and really wake it up. I don't mess with the EFI's much, but there is probably someone on the board that can tell you how to get that done without killing the engine.

heat has nothing to do compression-ratio.lowering the compression ratio will increase the area in the cylinder and allow you to run more boost.

I thought less heat would reduce cylinder head pressure. I am still pretty new to the theory of supercharging. I am getting there though.

It is my understanding that aluminum heads produce less HP at the same compression ratio than equivalent design iron heads because the better thermal conductivity of the aluminum heads reduces the temperature of the combustion charge and therefore the power output as well. The increase in compression ratio with aluminum heads is not only allowed but is necessary to regain the lost power. What I have not heard is if there is a net gain in power with aluminum heads ie does the increase in compression ratio allowed produce more hp than is lost by the combustion charge being cooler?

I doubt that's true as a rule because most aftermarket aluminum heads flow better than the stock cast-iron heads, are therefore more efficient and make more power. I learned this straight from Randy Garciga from Cobra Power, so it is not something I'm making up.

Compression ratio is physical measured number. It can not be changed unless there is a volume change in the cylinder. This can happen with a combustion chamber size change, a piston head configuration change, or a gasket thickness change.

Aluminum heads do dissipate heat more quickly. the effect that has on an engine is that you can run a little higher compression on the same octane fuel with less likelihood of detonating, or you can run lower octane fuel on the same compression.

I think when someone says you "gain or loose a point of compression", what they are referring to is the type of fuel required to keep the engine from detonating. I have never heard that the aluminum heads will actually dissipate enough heat to make the combustion less efficient than an iron head. I doubt if that is the case.

So...if the heads flow the same, the CR is the same and the combustion chambers are the same CC, is the power the same with alum vs. cast iron?
In the above example, which is the better product for a marine application? Why?

There is no majic head that will work for all applications. You have to look at each specific aplication and combination individually.

In your case, you might see some extra power inthe mid range due to better flow numbers, but at WOT it shouldn't improve much because the blower overrides a bad head design. The boost will turn a head that won't flow into one that will.

Now if you had a bigger engine, differnet cam, etc, then heads will change everything.

I think your decision to go with a better than stock aftermarket head was a good move.

Good luck with the project