Aluminium v cast iron cylinder head

[shane]

Hi

What are the advantages over the aluminum head over a cast iron on a high-performance engine?

Is it just weight or are there thermal advantages with aluminum?

Are the new aluminum heads better for gas flow? 

Can a standard iron head be modified to give equal performance to the aluminum head?

Sorry for the long list but I am trying to use modified original parts where possible but not at a significant performance disadvantage.

Best Wishes

Shane

[Bob Culver]

In the days of low octane fuel an alloy head was reckoned to allow about a compression ratio higher, as with the pre war sv Ford V8. With current fuels breathing rather than pinging tends to set the cr limit and it is usually within the workable range for c.i so the advantages of alloy are now reduced.

[Ian Williams]

I am not sure that there is a conclusive answer, Aluminium heads are supposed to reduce maximum horse power due the the improved thermal properties conducting too much heat from the combustion chambers, however I have read reports that on dyno tests they produce more torque than cast iron heads. I stress however that the tests I have read are all on bigger OHV engines, as I see it the big advantage of alloy on our engines is that it can be cast more cost effectively so you can get a better designed combustion chamber at an affordable price. If two identical heads were cast one in alloy and another in steel then dyno tested back to back on the same A7 engine I am not sure I would wager which would be better, I have my hunch but perhaps some of our more enlightened contributors have a definitive answer.

[Austin in the Shed]

I think if you asked 10 owners you might get 3 different answers of which cylinder head they prefer.
CI, there's 3 choices Low comp.Late Ruby or sports 9E 1. Numerous Alloy heads to choose from.
I am not too sure on wedging early heads but it does raise the compression ratio.
There is always a trade off on compression ratio and gas flow transfer to the cylinders on a side valve.
One Seven owner I know dyno tested a modified late ruby against a Ricardo and saw a 3 BHP improvement with the alloy one.

[Ian Williams]

Quote "One Seven owner I know dyno tested a modified late ruby against a Ricardo and saw a 3 BHP improvement with the alloy one."
Yes but this is not comparing apples for apples is it, the Ricardo has a very different combustion chamber so one would expect different results from a Ruby head. This is the case with almost every Alloy head there has been, although some are good some are bad, as I said the really interesting comparison would be between and Iron and Alloy of identical design. The only ones I have come across which are exactly the same are the pattern 9E sports items, I actually have Iron and Alloy versions of these and there were a few cast in the 80's from bronze. I have never bothered to dyno the two on the same engine, but I have also never noticed a really discernible difference in the performance of either, and even if i did it would probably be subjective.

[Alan]

I know that theoretically the greater thermal conductivity of aluminium should reduce the thermal efficiency of an engine using an alloy head but in practice I don’t think it would matter much to a 7 engine.
Over the years I’ve played with many cylinder heads and to be honest never really seen a marked improvement over the ‘37 head. Provided your bores and rings are in good condition I’d use one of those with 60 thou or so shaved off.

[Chris KC]

A spectrum of views to be expected here. I wouldn't advocate modifying original parts when there is a range of good aftermarket ones available (some of which look a lot like original or at least contemporary heads; some are classics in their own right). Most are aluminium mainly because it's lighter and better for cooling. The minor(?) downside of aluminium heads is they are easier to warp. Personally I follow Bill Williams' advice and wouldn't run a compression ratio above 6.5 or so on a car I aspired to get home in, not unless it had some bottom-end enhancement anyway. Fastest head I ever ran personally was a Speedex, but at 10:1 or so CR I figured the fun wouldn't last, have mostly used a standard cast iron '37 head since.

[dickie65]

Hi,
The side valve Seven engine head choice is complex.
There are two directions to go in.

High Compression which reduces volumetric efficiency but good for torque at low revs.
Good breathing head, higher volumetric efficiency and the gains the ability to rev higher.

The ideal is to gain CR and have good breathing.


The use you put your Seven too will affect your choice.

My car runs better with a Ricardo Replica rather than with its original 9E head. ( My engine will run up to 6500 Rpm )

The Late Ruby head is also a good choice as mentioned by Chris KC