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Old 04-20-2005, 04:24 PM
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Aluminum vs. Cast Iron heads

Is it true that you can run more compresison with an aluminum cylinder head than a cast iron head without detonation? What I am trying to say is will the max point of compression before detonation on an aluminum head is higher than that of a cast iron head?

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Old 04-20-2005, 04:42 PM
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Yes, by about one point. Iron is around 9.5:1 aluminum is around 10.5:1.

Adam
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Old 04-20-2005, 05:05 PM
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Yes you can run more compression. You "need" to run more compression for things to be equal. Meaning if you had an iron head and aluminum head of the same specs, you would probably make more power on the iron heads at lower compressions, as the iron will retain more heat. The aluminum bleeds off heat better/faster, so running 10:1 compression on aluminum heads is more like 9+:1 compression. Chevy Hi Performance did an article on this awhile back. This has always been my general understanding. Feel free to correct me if I am wrong.
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Old 04-20-2005, 05:20 PM
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Siggy, you are right about that, but here is something I was thinking about. What causes the iron heads to make more power is the additional heat of combustion if both situations have the same compression. Now think about the aluminum heads with higher compression. It seems to me that the aluminum heads with higher compression would make more hp than the iron heads with the lower compression, not just be equal with them. This is because at this point the aluminum heads now have the same heat of combustion as the lower compression iron heads, but they also have an additional point of compression. Heat and compression add hp, not just heat. I agree that at 9.5:1 compression, the iron heads would make more hp, but I think at 10.5:1 the aluminum heads would make more hp because of the added compression, and equal heat.

Adam
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Old 04-20-2005, 06:53 PM
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thanks for all the help and the other threads guys.

Basically, what I have figured out from most of the threads is that it depends on your application and preferance so I wanted to ask for your opinion on my application.

350 sbc bored .020
Stock type pistons, flat top
Comp Cams Magum 280 (Dur. @ .050 is 230/230, lift @ .050 is .480/.480)
Comp Cams Magnum Roller Rockers
Edelbrock Performer RPM Air-Gap
Edelbrock Perforer Series carb 600cfm
Hedman shorty headers
Stock '76 pickup heads 76cc 1.94/1.60

The motor is in a '69 Nova that is daily driven, but sees the track every once in awhile. I would like to be able to use pump gas 87octane. Any suggestions would be greatly appreciated.
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Old 04-20-2005, 06:56 PM
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Quote:
Originally Posted by Siggy_Freud
Yes you can run more compression. You "need" to run more compression for things to be equal. Meaning if you had an iron head and aluminum head of the same specs, you would probably make more power on the iron heads at lower compressions, as the iron will retain more heat. The aluminum bleeds off heat better/faster, so running 10:1 compression on aluminum heads is more like 9+:1 compression. Chevy Hi Performance did an article on this awhile back. This has always been my general understanding. Feel free to correct me if I am wrong.
I guess this means if my number 1 spark plug is igniting fuel at a rate of 7 times per second, the chamber in my aluminum head will be appreciabley cooler then if I had a cast iron head due to heat disipation?

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Old 04-20-2005, 07:18 PM
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I would thing you would be at around 9:1 right now with the 76cc heads and flat top pistons. That will without a doubt run fine on premium, as far as 87 goes, I think it will work, all you can do is try it out.

Adam
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Old 04-20-2005, 07:34 PM
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Firestone I totally agree with you on your point about higher compression in the aluminum heads. Undoubtably, given the same amount of heat in a combustion chamber, the higher compression setup should make more power. Thats why I think Iron heads are actually a better choice, until the point where you get enough heat (due to compression) that you can no longer run the gas you want. At that point aluminum heads take the cake.
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Old 04-21-2005, 06:24 AM
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Quote:
Originally Posted by firestone
I would thing you would be at around 9:1 right now with the 76cc heads and flat top pistons. That will without a doubt run fine on premium, as far as 87 goes, I think it will work, all you can do is try it out.

Adam
How much compression I can have, when I use 95 or 98 octane gas? Iron heads.
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Old 04-21-2005, 06:29 AM
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Quote:
Originally Posted by firestone
Siggy, you are right about that, but here is something I was thinking about. What causes the iron heads to make more power is the additional heat of combustion if both situations have the same compression. Now think about the aluminum heads with higher compression.
Adam

Nope, siggy is right. THe heat sinking properties of aluminum carry the heat away from the combustion process, so you'll always need more with an aluminum head to keep up. THe 1 point rule is not an exact science. Some alloys might need more, some less. That's just a benchmark rule-

K
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Old 04-21-2005, 08:00 AM
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But when you increase the compression with the aluminum heads, you also increase the heat. So, the aluminum heads, at what ever compression brings them to the same heat of combustion as the iron heads will make more power than the iron heads because the 2 sets now have the same heat of combustion, but the aluminum heads have more compression. The aluminum heads are still dissapating heat faster, but when you add more heat by adding compression, the amount of heat in the aluminum heads would be the same as the iron heads. Am I right? I agree that with both heads at say 9.5:1, that the iron heads will make more hp. On the other hand I think at 10.5:1, or whatever the compression is where the iron heads will ping, and the aluminum heads are on the verge of pinging, but are not pinging, that the aluminum heads are going to make more hp for the above stated reasons. What do you think?

Adam
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Old 04-21-2005, 10:44 AM
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Quote:
Originally Posted by firestone
But when you increase the compression with the aluminum heads, you also increase the heat. So, the aluminum heads, at what ever compression brings them to the same heat of combustion as the iron heads will make more power than the iron heads because the 2 sets now have the same heat of combustion, but the aluminum heads have more compression. The aluminum heads are still dissapating heat faster, but when you add more heat by adding compression, the amount of heat in the aluminum heads would be the same as the iron heads. Am I right?
Adam

I think the important thing that you're overlooking is compression and heat are not two different things. Compression is heat. PV=NRT, remember? Pressure is increased by the heat.

K
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Old 04-21-2005, 10:54 AM
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Quote:
Originally Posted by 69novarodr
thanks for all the help and the other threads guys.

Basically, what I have figured out from most of the threads is that it depends on your application and preferance so I wanted to ask for your opinion on my application.

350 sbc bored .020
Stock type pistons, flat top
Comp Cams Magum 280 (Dur. @ .050 is 230/230, lift @ .050 is .480/.480)
Comp Cams Magnum Roller Rockers
Edelbrock Performer RPM Air-Gap
Edelbrock Perforer Series carb 600cfm
Hedman shorty headers
Stock '76 pickup heads 76cc 1.94/1.60

The motor is in a '69 Nova that is daily driven, but sees the track every once in awhile. I would like to be able to use pump gas 87octane. Any suggestions would be greatly appreciated.
Almost the exact setup I just built for my '65 Chevelle...just got it going last weekend so give me a few weeks and I'll let you know how it runs. Only major change I have is that I am running an XE268 cam.
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Old 04-21-2005, 12:21 PM
Chevy stuff makes a Ford Tough
 
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yeah Wickster, that would be awesome if you could get back to me.

About the heat dissipation and compression. I'm still in query about what Killerformula has been saying. Please help me if you can, but when you raise the compression of the aluminum heads you will be getting the same heat as the iron heads that have one point of compression less. Since they are now at the same heat, you can rule out that factor. What is left is one more compressio point for the aluminum heads which should create more horsepower, right? I am way far from an expert just looking for some guidance.
Another thing, what does the equation PV=nRT have to do with this? I know this deals with ideal gas laws and such, but please explain.
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