david97gsxr

i was reading on another board (www.hotrodders.com) and a guy posted a question about the difference between the two.

and someone said that aluminum heads lower the compression ratio 1 full point "due to lack of thermal efficiency". given a set of iron heads and aluminum heads both have the same combustion chamber CC.

and no one said different... i've never heard of this? i know you can run more compression with aluminum heads, due to heat dissipation. but i was unaware that they 'lower' the compression ratio.

Kevin97ss

Al. heads don't lower CR but because they are more inefficent than an iron head they need about 1 point more CR to be equal as far as combustion efficency

10 seconds

I will take a 10 to 1 motor over a 9 to 1 motor, all things but head material being equal anyday.

oil pan 4

Well I knew there was a difference in Iron and aluminium but could never put my finger on what it was.
So there is a combustion chamber comperssion tempture difference theory it looks like.
To me it makes a lot of sence.
Al counducts heat 6X better than Iron and if the compression charge is the same volume but the temp is different (lets say cooler) your pressure will be lower.
Well that gave me some thing to think about to day.
The comperssoin ratio is cylinder volume @ BDC / cylinder volume @ TDC
If you have 64cc Iron heads with say 2.02 and 1.60 tuliped valves then replaced them with 64cc aluminium heads with the same head gasket and valves and keep the same type of spark plug your compression raito will stay the same but the engine will now run a little different now.
I knew that an engine would run a little different (usaly better) after a Al head swap but never realy knew why.

Stephen 87 IROC

If you built an iron head 10:1 engine that experiences detonation with 92 octane, switching to aluminum heads with the same size combustion chamber will probably eliminate the detonation. The fuel detonation would react like the engine was only 9:1 so you could run a higher compression ratio with less octane using an aluminum head.

dkeers

So if you had two heads that were identical except for material (ie same chambers, ports valves etc.) If both were ran on the same engine and no other changes were made, what would perform better, iron or aluminum? Does the aluminum head lose more performance because of the heat escaping throught the chamber, it seems like it would.

In another case:
What if you have two identical engines (like above) one having aluminum and the other having iron heads(identical except for material), and you can change the compression ratio/timing curves to get both engines to run at max performance, right at the edge of detonating? What engine could you get more performance out of? Ignoring flame propagation problems etc caused by altering the compresion ratio with a dome.

My one semester of thermo tells me that you would want the iron so you could keep as much heat as possible in the chamber. What am I missing?

Dustin

rskrause

At the same CR, assuming no detonation, iron heads do make a little more power. You can coat the chambers of Al heads with a thermal barrier though and get the best of both worlds.

Rich

LameRandomName

I love modern chemistry!

viperkiller210

I heard that the iron head flows a little better...
Anybody know if there is any truth to this?

rskrause

Not true in general, if the ports are the same size and shape they will flow the same. The misconception is based on the LT1 iron v. aluminum heads. It so happens that the iron heads have a slightly more refined port shape and do flow a little better.

Rich

Steve in Seattle

yep. In practise the big advantage of aluminum is:

a) the weight savings over the front of the car
b) the ease of machining/porting aluminum over iron
c) the huge aftermarket possibilities with aluminum heads (almost none in iron).

AdioSS

d) it is a LOT easier to "fix" aluminum heads than iron heads.

Personally, I'll give up that very small power difference for the weight!

10 seconds

DITTO

oil pan 4

Higher compression is all ways good if you can run it.

arnie

The actual compression ratio is relevant at best. It is the actual amount of 'push' exerted on the piston crown that is what is important. That is what the discussion, in this thread is about. It doesn't matter that the engine CR is 20 to 1. If 90% of that (potential) energy is absorbed into surrounding surfaces (and subsequently into oil and coolant), the engine will not be very efficient.