aluminum heads = less comp. ?

[68chargerboy]

hi i always thought that aluminum heads were the best thing in the world until yesterday.  the guy at my engine shop was telling me that aluminumn heads dont hold the heat as well as cast iorn therefore i will loose compresion.  is this true.  my comp. will be around 11.1 will it drop significantly with aluminum heads.  thanks in advance



                                    Zach

[Mefirst]

With aluminium heads you will loose about 1.0:1 in comp. Reason is aluminium doesn't contain the heat like an iron head does... But don't worry, even if you do loose 1 in comp. The aluminium heads will give equally or more Hp... You can of course do the math and check what cc aluminium heads you need to maintain that 11:1 in comp if you want...

/Tom

[Plumcrazy]

Your compression ratio is a determined by the piston design/deck height, the thickness of the headgasket and the size of the combustion chamber all else being equal.
If you remove a cast iron head with a 70cc combustion chamber and replace it with an aluminum head that also has a 70cc chamber, the compression ratio will remain the same.

However, most aftermarket aluminum heads are designed with a smaller chamber than the factory head they are designed to replace.  This will increase the compression ratio about 1 point which compensates for aluminums heat difference while still being able to run the same octane fuel as before.

[firefighter3931]

  The increased flow and better chamber design will ultimately make more power....lots more. 


Ron

[68chargerboy]

the pistons that i was originally going to get are about 10.9 comp. so if i want to stay around 11.? comp should i go with the domed pistons that say 12.53 comp.  also the heads i will be use for the higher comp will be the e-heads 88cc combustion chambers.  thanks again



                             Zach

[firefighter3931]

Zach, using a domed piston is a step in the wrong direction. The 88cc open chamber heads are also wrong as well if you want to run it on pump gas. For a true pump gas build the best combination will be a flattop piston with a closed chamber head.

Ron

[Chryco Psycho]

the mathmatical compression will remain the same with a 80 cc chanber either alum or iron But alum absorbs more of the combustion heat so the effective compression after heat loss is equal to 1 point less compression so if you have 11:1 mathmatical compression & use Alum heads your effective compression will be 10:1 so you should still be able to just get by on premium  pump gas without detonation
a flat top piston & a smaller chamber will always make more power then a dome piston in a larger combustion chamber due to better flame propagation

[kamkuda]

You do not effectively loose compression as Cryco stated.  The cranking compression will be the same regardless of the whether iron or aluminum heads are employed. Most of the builders I have discussed this issue with state that you should not see any appreciable difference between aluminum and iron if all things were the same from a HP standpoint, but you will be able to run more compression due to better thermal effeciancy of aluminum,  Also as Firefighter stated, the combustion chamber design of aluminum heads are usually better than any iron heads you may be using thus allowing you better performance.

Plus you are saving some good weight from the front end of your car.

[deputycrawford]

There is no doubt that If you have the money then go with the aluminum heads. You will come out the winner in the end.

[Mike DC]

 
I remember reading something in a car magazine recently where they tested aluminum versus iron heads.  I think they found the octane/compression differences were actually very small.

   

[Chryco Psycho]

I built a real 11:1 alum head 440 & it just slightly pings on 93 octane fuel , very similar to a real 10:1 iron head 440 would

[71charger_fan]

There was a tech article in Mopar Performance News many years ago. The net of it was to bump the compression a bit if going to aluminum heads because of aluminum's propensity to move heat away from the chamber. Essentially, the aluminum made the engine perform as if the compression ratio had dropped. I'm going to assume whichever Mopar engineer wrote that knows more about it than I do. Unless your aluminum head is an exact copy of your iron one I don't believe the one point of compression is a figure cast in stone. If your new head is more efficient, than that should balance out the other side of the equation at least somewhat.

[Silver R/T]

thats why they're perfect for boost