I have read that the use of aluminium heads requires increased combustion ratio due to the thermal dynamics of this metal. I read that under these circumstances you should increase your compression ratio by one full point (i.e. from 9.3 to about 10.3). Cast iron retains heat and redirects it into the combustion chamber while aluminium dissipates heat better.

Therefore, if stock 305 heads have a combustion chamber of 58cc the use of aluminium heads with 58cc or 54 cc would not be good for this size engine. I know people have used aluminium heads with this size combustion chamber on 305 engines.

What do you think?

 

It is true that aluminum dissipates more heat, and you can run more compression with them. I have hears that an iron head will make more power with the same compression and all else being equal, but have never seen any dyno test to prove it, the only time I have seen a true head to head test was in car craft, they tested dart pro1 platinum to dart iron eagle platinum. Both heads flowed equally and had the same combustion chamber. In there dyno test they saw absolutely no difference anywhere. Now a Car Craft article is not gospel, but this being the only real test I have ever seen, I will have to believe them.
If you are putting these heads on a 305, you should start by seeing what pistons you have, you need to know the size of the dish. From this you should be able to figure what combustion chamber size you need to get your desired compression ratio.
I am assuming this is a stock bottom end, so your pistons will be approx. .020 below the deck, so if you use a standard .040 thickness head gasket you will have .060 clearance, this will kill the qhuench and hurt your compression as well. If I were you I would use an .025 head gasket to get a .045 clearance and about a half point additional compression.
Now, If you know the cc of your piston, you can calculate a good compression ratio. With aluminum heads I would shoot for 10:1, I would Imagine you will need a relitively small chamber to acchieve this.I do not know of any aluminum heads offered in that small of a chamber, there are iron heads with 49 cc chambers but that might be to much compression.

 

If the concern is power loss, your only realistic option is to change the pistons or cut the deck. Using a 'thinner' head gasket wont help because the factory did not use .040 gaskets but rather .021.

Most people doing a swap to the corvette AL heads on a 305 dont bother with doing anything else, which is probably the best idea.

 

yeah ideally i'd say jump a full point or two I'm at 11 to 1 on my 383 and no problems on pump gas. Aluminum you can run some compression as long as your tune is nice and the cam is large enough to lower the dynamic compression

But for a 305, i wouldnt go thru the trouble of shaving the heads UNLESS you feel it necessary to ensure a flat surface, and to clean the head deck itself. All good ideas on some old aluminum heads. I'd see no problem in going to 54cc. You probly could shave it down that far and still be fine. That should bring a 9.3 to 1 up to about 9.8 to 1. Thats not bad at all

 

edelbrock told me to run 58cc on their vortec replacement heads, which is the only thing that will take with my tpi.
just dont by anything that is cast in china. their quality control sucks - their cranks break, their pistons crack and explode, if they cast it, you can NEVER rely on it.
i may buy other things made in china but darn few and never anything that has to take a beating.

 

Thank you for this. I too saw that Car Craft article. I have seen this issue mentioned in another article without all the testing the Car Craft article did.

 

I don't know how consistent the 305 motors were from the factory. I found it interesting a few years ago when Chevy High Performance was doing a series on various build ups starting with a Mr Goodwrench 350 that was supposed to have 8.5:1 compression from the factory that no 2 cylinders in that engine were equal and none were up to 8.5 if I remember correctly. They had to tear down the engine and deck the block to get a consistent 8.5 cr before they started on the tests.