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Do aluminum heads really reduce the octane requirement...

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Do aluminum heads really reduce the octane requirement...

Postby plovett » Wed Mar 16, 2011 7:57 pm

compared to otherwise identical iron heads on an otherwise identical engine? Or do they allow more compression ratio to be used with the same octane fuel?

This has been said for decades. Anybody know by testing that it's true?

thanks,

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Re: Do aluminum heads really reduce the octane requirement...

Postby ProPower engines » Wed Mar 16, 2011 8:03 pm

The heat reducing ability of aluminum head will allow more comp. for a given fuel. That said If you had an engine with iron heads and switched to aluminum heads the were in every way identical you would loose HP because of the heat loss in the chamber. Seen that lots of times but the same heads would allow you more static comp increase over iron heads before detonation. Thats as long as they are identical in every way IE chamber shape valve location flow etc. JMO
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Re: Do aluminum heads really reduce the octane requirement...

Postby F-BIRD'88 » Wed Mar 16, 2011 8:16 pm

Aluminum conducts heat (a change in heat) faster than cast iron.
But hot aluminum is just as hot as hot cast iron. So when a motor goes WOT it starts making heat. The alumnium sucks the heat away at first (to the cooling system) but once heat soaked the cast iron catches up. Once you let off the gas the cast iron head retains the heat longer. So the aluminum head has a short period where it will have a small knock threshold advantage and small power diference. But the two materials even out eventually. Hot is hot.

So for drag racing it has a small brief advantage (or disadvantage depending on how you look at it) and can tend to tolerate a bit more cr.

Once running WOT steady state there is no difference.

Many people make way too much of any practical difference.
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Re: Do aluminum heads really reduce the octane requirement...

Postby bigjoe1 » Wed Mar 16, 2011 8:48 pm

In testing I have seen, an iron head that is identical to the same head in aluminum, will make 20 MORE HORSEPOWER unless you raise the compression with the alu head, Then they will make the same power. It takes almost one full ratio more compression with the alu head to equal the out put of the iron head.


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Re: Do aluminum heads really reduce the octane requirement...

Postby Troy Patterson » Wed Mar 16, 2011 8:55 pm

F-BIRD'88 wrote:Aluminum conducts heat (a change in heat) faster than cast iron.
But hot aluminum is just as hot as hot cast iron. So when a motor goes WOT it starts making heat. The alumnium sucks the heat away at first (to the cooling system) but once heat soaked the cast iron catches up. Once you let off the gas the cast iron head retains the heat longer. So the aluminum head has a short period where it will have a small knock threshold advantage and small power diference. But the two materials even out eventually. Hot is hot.

So for drag racing it has a small brief advantage (or disadvantage depending on how you look at it) and can tend to tolerate a bit more cr.

Once running WOT steady state there is no difference.

Many people make way too much of any practical difference.


I'd say it depends on the cooling system, the head in question and the coolant temperature. Some heads have inherently better cooling passages. Cold head equal more heat loss to the cooling system, but a colder, denser induction charge. Does it balance? Depends.

Troy Patterson blog tmpcarbs.blogspot website TMPCarbs.net Engine Professionals Magazine's TMP Tuning article, pg. 44
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Re: Do aluminum heads really reduce the octane requirement...

Postby F-BIRD'88 » Wed Mar 16, 2011 11:40 pm

In a 1/4 mile blast or a 10 sec dyno pull when you stage with a cold engine ya there is a difference.

The trouble starts when transferring that thinking to a street car or marine application. 10 seconds is a very short ride.

Yes fuel vapourization in the intake runners cools the runners but it takes a good 30+++seconds to cool off the (hot) runners at WOT.

I bet you Joe Sherman can tell you exactly how long it takes (for the intake manifold runners to cool off) on a WOT dyno pull.

Alcohol motors cool the intake a lot more and a lot faster. (almost twice the fuel volume is changing from liquid to a gas)
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Re: Do aluminum heads really reduce the octane requirement...

Postby ProPower engines » Thu Mar 17, 2011 12:50 am

bigjoe1 wrote:In testing I have seen, an iron head that is identical to the same head in aluminum, will make 20 MORE HORSEPOWER unless you raise the compression with the alu head, Then they will make the same power. It takes almost one full ratio more compression with the alu head to equal the out put of the iron head.


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We did an all aluminum 360inch engine for an unlimited type engine just for weight. Although the engines are sorta restricted by injector stack diameter to 2 3/16" we lost a wopping 235hp over the same engine combo with an iron block.
That said 85hp lost with the alum. heads over iron. We had to totally rethink the advantage with weight loss V HP loss.
From 14-1 to 16-1 was what we had to do and that was as far as I could along with a host of other things get to get back 167 HP we lost by using the lighter block.
Yes it was an Alky engine but still apples and apples.
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Re: Do aluminum heads really reduce the octane requirement...

Postby F-BIRD'88 » Thu Mar 17, 2011 12:54 am

ProPower engines wrote:
bigjoe1 wrote:In testing I have seen, an iron head that is identical to the same head in aluminum, will make 20 MORE HORSEPOWER unless you raise the compression with the alu head, Then they will make the same power. It takes almost one full ratio more compression with the alu head to equal the out put of the iron head.


JOE SHERMAN RACING ENGINES

We did an all aluminum 360inch engine for an unlimited type engine just for weight. Although the engines are sorta restricted by injector stack diameter to 2 3/16" we lost a wopping 235hp over the same engine combo with an iron block.
That said 85hp lost with the alum. heads over iron. We had to totally rethink the advantage with weight loss V HP loss.
From 14-1 to 16-1 was what we had to do and that was as far as I could along with a host of other things get to get back 167 HP we lost by using the lighter block.
Yes it was an Alky engine but still apples and apples.


Does the all aluminum motor 's block deck clearance grow a lot when running at WOT, reducing the real compression ratio?
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Re: Do aluminum heads really reduce the octane requirement...

Postby ProPower engines » Thu Mar 17, 2011 1:57 am

That was what were determined. The increase in comp. was to compensate for that.
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Re: Do aluminum heads really reduce the octane requirement...

Postby Adger Smith » Thu Mar 17, 2011 2:38 am

I had an all alum tall KB NOS engine that grew a whopping .017...(water block/Brodix wet heads) I have a customer with an A/Fuel AJ solid block and heads that grows about .013. I had a customer call me the other day and couldn't believe his (Wet) Brodix Alum 632 w/Brodix (Wet) dukes on it had to have the valves set cold at .008 and .011. I just told him to try "not" doing it like that and see how much MPH you loose and how short the lifter life will be.
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Re: Do aluminum heads really reduce the octane requirement...

Postby bigjoe1 » Thu Mar 17, 2011 9:46 am

My example was on a very mild ( compared to these ) street engine. I think it was only at 400 horsepower and 9 to one compression=


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Re: Do aluminum heads really reduce the octane requirement...

Postby ap72 » Thu Mar 17, 2011 10:07 am

F-BIRD'88 wrote:Aluminum conducts heat (a change in heat) faster than cast iron.
But hot aluminum is just as hot as hot cast iron. So when a motor goes WOT it starts making heat. The alumnium sucks the heat away at first (to the cooling system) but once heat soaked the cast iron catches up. Once you let off the gas the cast iron head retains the heat longer. So the aluminum head has a short period where it will have a small knock threshold advantage and small power diference. But the two materials even out eventually. Hot is hot.

So for drag racing it has a small brief advantage (or disadvantage depending on how you look at it) and can tend to tolerate a bit more cr.

Once running WOT steady state there is no difference.

Many people make way too much of any practical difference.


You will see a significant difference at WOT steady state, the heat transfer of the aluminum will cause a lot more heat dissapation into the cooling system, lowering the temp inside the chamber.
LOL, according to the post count I'm an "expert." The only thing I'm an expert at is asking questions.
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Re: Do aluminum heads really reduce the octane requirement...

Postby plovett » Thu Mar 17, 2011 10:57 am

Thanks guys! Very useful info.

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Re: Do aluminum heads really reduce the octane requirement...

Postby pdq67 » Thu Mar 17, 2011 6:36 pm

I have read that we can paint the waterjackets of aluminum heads with waterglass and bake out each coating until the built-up thickness of the waterglass insulate's the heads closer to the heat retention of the heavier cast-iron heads.

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Re: Do aluminum heads really reduce the octane requirement...

Postby MadBill » Thu Mar 17, 2011 8:37 pm

Per a Smokey Yunick article, I did that 30-some years back on my aluminum 495 BBC, but it's never been on the dyno to know how well it worked..
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