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Why use aluminum heads?

19K views 17 replies 12 participants last post by  pmeisel 
#1 ·
Ok, I am NOT trying to start a war here but I would like to know why people use aluminum heads.

At first I thought it was a great idea because you can run higher compression. Then several people on this forum, whose opinions I trust, tell me that you actually don't gain that much horsepower from the compression increase because most of it is lost through the better heat dissapation of the aluminum.

I do realize that the aluminum is easier to port and repair. I also understand that there is a 40-50 lb weight savings.

These seem like nominal gains for the extra $500 or so cost in the aluminum heads.

So please explain to me the advantages of aluminum heads because I seem to be in the dark on this subject.
 
#2 ·
"Cause we're thickheaded numbskulls that like the shiny stuff, one of he machinists here said once ..........

"Never underestimate the power of shiny stuff".

I think that was it anyway.

All that aside, aluminum heads usually are thicker castings, that leaves a few options open when doing serious portwork. If you take advantage of the better heat dissipation that aluminum offers you can make a bit more power. More static compression allows the use of a larger camshaft, so on and so forth. Get the combination right while taking advantage of the heat absorbing properties aluminum and you can make more power.

Swap a set aluminum heads in place of iron heads and, all else being equal, it probably won't make a bit of difference.

Think about the LT1/LT4 engines that chevy built a few years ago. 11-1 compression on pump gas. Sure the extremely wide LSA the camshaft was built with helped a great deal but so did the reverse flow cooling and aluminum heads. Stab a larger cam in it with a 110-112 LSA, make the necessary changes to take advantage of the camshaft and that engine would wake up big time.

It all boils down to the combination of parts.

Larry
 
#4 ·
I would like to make a note, a wide LSA increases cylinder pressure and a tight LSA lowers cylinder pressure, generally speaking. So GM was able to pull off using a wide LSA for proper street manners and top end power while at the same time using high compression for street manner and top end power.

You've heard "everything is a compromise" but some how GM pulled it off with "no compromise"!! 11:1 compression and a 114LSA !!! magic!
 
#5 ·
That's odd. I was under the impression that when the intake valve opens and closes later, cylinder pressure goes down.

A single pattern cam with 280 degrees duration and a 106 LSA closes at 66*BTDC. The same cam with a 114LSA closes at 74*BTDC. Early closing = more pressure, later closing = less pressure.

You have less overlap with a wide LSA which, to a small degree, helps reduce cylinder filling at low revs. This also helps if you have really good heads and don't need any more low rev filling than is necessary. Higher revs aren't as much of a concern because there's less time for bad things (detonation) to happen.

Tight LSA's have more overlap and help with cylinder filling at low revs, especially if you have crappy heads. A tight LSA and good heads = good cylinder pressure, in a tight rpm range. But you have to be careful not to end up with too much pressure. You'll have a wimpy race engine/hot street engine that won't run on pump gas.

I believe these are some of the reasons that most good street cams have a 110LSA. It's a good compromise.

I may be way off base here and none of that is set in stone but I have had plenty of success using these as guidelines when making decisions on a particular engine and it's application.

The combination makes the difference.

Larry

PS, The '96 LT4 cam had a 115.5* LSA.
 
#6 · (Edited)
dammit coldknock! i have a pontiac 326 with famously crappy 2bbl heads, and you go and tell me my summit 2800 i'm putting in with a 112 lsa is all wrong. why do they say it's operating range is only 2000-4500 rpm? i am not going to use headers cause i was told the exhaust manifolds are the same from 326 to 455 cid so headers dont seem like they are necessary. you are telling me ideally i would want a cam with a smaller lsa. i would probably have to have one custom ground as the tightest i've seen is only 110

anyway back to the subject dont forget the obvious-weigt. although that's not really much of a factor for a street car. i can get some great poncho iron heads race prepped from SDperformance for like $1450 assembled. and for the same price i can get the same in aluminum from wenzler pontiac. i was thinking of aluminum just for the higher c.r. like you said, higher cr then more radical cam= more HP and pump gas. so with my imaginary 455 with aluminum heads. what lsa? RAIV cam is at 114 ( and edel rpm cam is 113) and a Comp 280 is 110. lean towards the 114?
 
#8 ·
coldknock said:
"Cause we're thickheaded numbskulls that like the shiny stuff, one of he machinists here said once ..........

"Never underestimate the power of shiny stuff".

i agree

when i was 13 or so me and my dad use to tell each other that the more chrome and shiny stuff that we put on the car it would make it go that much faster. We were just kidding around of course

J
 
#9 ·
I was under the impression that when the intake valve opens and closes later, cylinder pressure goes down.
That is also correct, notice in my post I said "generally"...not always.
Tight LSA's have more overlap and help with cylinder filling at low revs
that is backwards. a wide LSA results in a more efficient cam and more complete cylinder filling at lower rpms AND higher rpms. a tight lsa results in more overlap, a less efficient cam that losses cylinder pressure at all but the tightest of midrange rpm.{due to the fact that the intake and exhaust valve are open at the same time it sucks spent gasses back nto the cylinder...only once intake velocity has increased to the point that the air/fuel mix continues pushing against the intake valve so as to sweep the cylinder clean when the intake valve reopens} this occus when the intake velocity continues to fill the cylinder even though the piston is on its way up.

it losses high rpm power b/c of sonic reversion in the intake port{I think}

off on cams from aluminum heads! lol, but it was to show that a wide LSA cam that is good in lower rpms & higher rpms {b/c of cylinder pressure!} is able to be used with aluminum heads.
 
#13 ·
When building a high performance engine you choose a head that:
A) Has the runner size you are looking for,
B) Has a modern highly efficient combustion chamber,
C) Has the best flow numbers you can afford in the cam lift range you can reasonably be expected to run for your application,
D) Has the valve angle you are looking for.
E) All the above are targeted to the amount of HP you want to run in a particular powerband with a given amount of cubic inches.

So, lets say you want a 220cc runner for a 434 small block, you want to make 550 HP and want to run it on the street with a max RPM of 6200. You suspect you'll need a camshaft in the 240 duration range and maybe lift it .650. You calculate the amount of airflow you need and start perusing the catalogs. With iron heads your limited to basically an Iron Eagle. a 215cc runner. With Aluminum your options are vast, Brodix, AFR, All Pro, Trick Flow all make a variety of heads that will work, BETTER. Better because the all flow better. All these heads in aluminum flow near or over 300 (some well over) CFM. That extra CFM, 30 to 50, will allow you to make more power with less camshaft.

So the point being is many of us choose aluminium not because it's made out of aluminium but because the head is better at meeting our goals.

Manufacturers make the heads out of aluminium because it's easier or manufacture. The weight for sure is nice as well as the abilty to repair if damaged.
 
#14 ·
inspiron said:
Don't aluminum heads expand and contract diffrently than iron causeing all kinds of head gasket probloms.

If I'm not mistaken thats why the GM 3.1 V6 engines have headgasket probloms.
This is not a problem. head gasket technology is well sorted out on this type of application.
 
#15 ·
jesse01 said:
i agree

when i was 13 or so me and my dad use to tell each other that the more chrome and shiny stuff that we put on the car it would make it go that much faster. We were just kidding around of course

J
Cometh forth Nairb, and deliver the commandment:

Never underestimate the desirability of shiny things.
Therefore it was spoken and now is proclaimed to be hotrodding truth.


Nairb has spoken
 
#18 ·
Rick WI said:
When building a high performance engine you choose a head that:
A) Has the runner size you are looking for,
B) Has a modern highly efficient combustion chamber,
C) Has the best flow numbers you can afford in the cam lift range you can reasonably be expected to run for your application....

Manufacturers make the heads out of aluminium because it's easier or manufacture. The weight for sure is nice as well as the abilty to repair if damaged.
Absolutely. It started because of weight but aluminum's machinability allows the manufacturer to do things in aluminum that were difficult to do in cast iron at production volume on automated machining lines.
 
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