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Old 04-25-2004, 10:18 PM
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aluminum head question.

i was wondering if aluminum heads are worth getting for an engine with around 9.5:1 compression? they take alot of weight off the front end of the car, would that make it worth while?

thanks

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Old 04-25-2004, 10:35 PM
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Old 04-25-2004, 10:40 PM
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Old 04-25-2004, 10:51 PM
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cool, i just thought people would get aluminum heads if they had high compression. thanks for clearing that up. im not planning on getting them any time soon. what are the cooling characteristics of an engine with aluminum heads? do they tend to run hotter, cooler?

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Old 04-25-2004, 11:20 PM
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Well slow the "I love aluminum heads for everything because marketing ploys control my mind" train down a little bit here. Aluminum is not always the best choice, and it may hurt your performance if you're using a bigger than stock cam, because your effective compression ratio has effectively been reduced to about 8.5 to 1. This topic has been discussed at length on this board, and its generally held that aluminum acts as a "heat sink" and moves heat out of the combustion chamber much more quickly than iron. So given that, people bump their compression up TO COMPENSATE, not because aluminum ALLOWS more compression. Compression is what causes heat, and heat is what makes horsepower. If your compression is as mild as yours is to start, make sure you compensate accordingly if you're really going to go with aluminum heads; iron may be a better choice.

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Old 04-25-2004, 11:32 PM
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Killer, made some good points. There are a couple of reasons aluminum heads are the choice of many hot rodders/racers. 1. Aluminum is lighter, 2. Aluminum is easier to repair, 3. Aluminum is easier to port/modify. There are more aftermarket aluminum performance heads to choose from, when compared to iron

The only real down side is that aluminum heads cost more.

With all thing equal iron heads will usually make slightly more power due to keeping the heat in the chamber. This same benefit of iron can also be a negative, that extra heat can mean detonation if you are pushing the limits.

The general rule of thumb is you can run 1 point more compression with aluminum heads. You do not have to but you can get away with it.


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Old 04-26-2004, 12:33 AM
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well, i want to stay around the 89 octane area, so i dont really want to bump up my compression, so from what your are saying, iron heads are a better choice? the engine would be in the 350-400hp area and would be driven alot, maybe on a daily basis. i dont have any further specs, because i havent built this engine and i dont plan to do so for a bit. but i want to get may facts straight.

thanks for the replys
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Old 04-26-2004, 06:27 AM
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If you are building for 9.5 to 1 compression, then you will need to run premium gasoline with cast iron heads. If you put aluminum heads with the same size (volume) combustion chambers, you can run a lower grade gasoline. As far as motor running cooler, the aluminum will transfer heat quicker than iron. If the cooling system is up to the chore of getting rid of this heat quickly, then the heads will tend to be cooler in the combustion chamber area. This is why you can run about 1 point higher compression before predetonation with aluminum heads.
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Old 04-26-2004, 08:22 AM
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I agree with everything but the use of the word "can." Its not that you "can" use more compression with an aluminum head, but to get the same results as an IDENTICAL iron head you MUST use about a point more compression. This is because the only reason you want more compression in your engine is to create heat which makes the gasses expand more violently and rapidly, and thus makes more power. The problem with aluminum (and not with iron) is that as this heat is being created, a significant amount of it is absorbed and dissipated before it can create more turbulence in the combustion charge. So if you're already using 9.5 compression, and you go to aluminum, you're basically right back down to stock compression (about 8.5:1 effectively) and a cam requiring more compression (or heat) isn't going to run the way that it should.

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Old 04-26-2004, 10:41 AM
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wow great thread!
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Old 04-26-2004, 11:09 AM
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There was an article in a car mag (can't remember which one) a couple years ago that dyno tested a cast iron and aluminum head to see which made more power. It turns out there was a minute difference which skewed the results a little bit in the end, but they kept the remainder of the engine identical. The result was very very close. Looked to me if they had identical castings and only varied the material the results would have almost been equal. I don't know if I buy into the iron makes more hp at a given compression ratio theory due to heat transfer being quicker out of the aluminum. I think I would need to see some cold hard numbers to believe. Personally if I were building a street engine and had the money I would invest in set of aluminum heads. My .02. Good luck!
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Old 04-26-2004, 11:21 AM
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Killer is correct.

Chris
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Old 04-26-2004, 12:14 PM
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Hey killer and cstraub...can you point me anyplace that backs up the iron vs aluminum head theory. Any sites with dyno results or the like...I guess I just have to see to believe. Thanks.
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Old 04-26-2004, 12:40 PM
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68,
I have been apart of this type of testing. Thermally cast iron is more effiecent and will make more power when you take equal port flow and compression ratio. The negtive with iron is you can spend 8K in port work and the engine burbs and you have junk. Aluminum. . .throw the rod to it and fix it.

Same with blocks, and alumium block gives up around 30 to 40HP to an iron block, (this was some testing we did with 500CID stuff) due to the fact the aluminum block moves so much.

Chris
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Old 04-26-2004, 12:40 PM
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Below is a little blip from a CHP article. The test that they are talking about was kind of dumb because they didnt test comparable parts (it tells why in the article), but they contend in the end that the Iron heads will make slightly more hp if all the other varilables are the same. In the end of the article, it focuses on the real factor being the ability of the heads to flow. Thats where real hp is made. If it were me, I would buy a set of AFR heads. This is not really because they are aluminum, but because for the price, they outlfow eveything that is the same size at them. You can get a set of unported assembeled Dart Pro 1's for around $1100 a set, or get a set of CNC ported AFRs (that flow much better) for $1300 a set. I would recommend aluminum in your case but the only signifigant reason for that is that AFR only make aluminum heads.

Good luck
Adam

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