Iron vs Aluminum heads.
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Rookie
Joined: Jan 2011
Posts: 58
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From: British Columbia, Canada
Iron vs Aluminum heads.
Just for a point of discussion as many of us have or are considering going to aluminum heads. I once read a long article on aluminum vs iron and in the end the writer (who's name I can't recall and was highly qualified) stated the following. If three engines were exactly the same in all aspects of compression,cubic inches,head design, valve size etc. etc. the dyno results would be:
All iron engine...most power
Iron block, aluminum heads.....less power
All aluminum engine...least amount of power
So in order to get the full benefit of aluminum heads you would have to take advantage of their ability to dissapate heat and go to a higher compression ratio on your build.
Another question would then be..How far can we push our compression with aluminum heads and still run pump gas?
All iron engine...most power
Iron block, aluminum heads.....less power
All aluminum engine...least amount of power
So in order to get the full benefit of aluminum heads you would have to take advantage of their ability to dissapate heat and go to a higher compression ratio on your build.
Another question would then be..How far can we push our compression with aluminum heads and still run pump gas?
Administrator
Joined: Apr 2010
Posts: 87,362
Likes: 4,209
From: Clayton MI
Anything over 11:1 is not exactly streetable. Even at that high of a compression, you are going to have to run high octane fuel, and possible octane boosters as well.
Aluminum heads allow you to run higher compression, on the same octane fuel, with less tendency for predetonation. (spark knock) If there is a power difference between a cast iron engine, and an identical aluminum engine, I would be surprised if it was noticeable...... (except on a dyno...)
Aluminum heads allow you to run higher compression, on the same octane fuel, with less tendency for predetonation. (spark knock) If there is a power difference between a cast iron engine, and an identical aluminum engine, I would be surprised if it was noticeable...... (except on a dyno...)
Captain
Joined: May 2011
Posts: 652
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From: Central Florida
My BBC is 10.2:1 cr running 93 octane with No problems. My 95 trans am is 10.5:1 on 93 octane. It's kind of it's own animal though because Lt1's have reverse flow cooling so they can get away with quite a bit more than even 10.5:1. I would say with the dodge and aluminum heads around 10.5 would be my limit.
Legend
Joined: Jul 2007
Posts: 7,843
Likes: 13
From: DFW, Texas
10.5-11.0 compression ratio based on quality of gas, octane rating available (like California only runs 91 octane, some places I've seen 94), and the skill of the tuner that writes the tune for that vehicle.
Most aluminum heads have smaller combustion chamber recesses which raise the compression ratio the amount that the aluminum material offsets detonation.
Most aluminum heads have smaller combustion chamber recesses which raise the compression ratio the amount that the aluminum material offsets detonation.
Professional
Joined: Nov 2011
Posts: 189
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From: Canada
If you really want to explore the compression ratio limits of an engine, you should be looking at Dynamic Compression Ratio rather than Static Compression Ratio.
Because the intake valve is off the seat from bottom dead centre to some point in the compression stroke, the engine compression ratio during this period is irrelevant ... no actual compression of the charge is taking place.
Once the intake valve closes, you start compressing the Air/Fuel mixture.
There are some fairly well defined guidelines for DCR and Fuel Octane. If you punch in the numbers for the 5.2 Magnum in my truck, which has 9.1 CR pistons installed, you get 7.1 DCR. This makes good sense since a DCR of roughly 7.25~7.5 is considered safe for 87 octane pump gas.
If you are an OEM configuring a stock engine that will probably have some combustion chamber deposits reduce the static CR over the life of the engine, 7.1 seems like a decent number to build to for a factory go-anywhere-do-anything motor.
Because intake valve timing is a critical factor in DCR calculations, you can alter the effective compression (ie the DCR) with cam timing alone. Alternately, you can choose the cam first and then choose pistons that provide the target DCR for the fuel octane you are willing to pay for.
Broadly speaking, an aluminum head will allow you to run about 1 point higher CR than an iron head.
Compression is pretty much "free power" ... it doesn't cost more to buy moderately higher compression pistons, and a cam is a cam; prices don't really vary from the same supplier with similar profiles. You get more work from the same amount of fuel, and the amount of work you get from a gallon is the real-world cost of fuel to you.
That is why the stuff you were reading doesn't tell the whole story ... sure, at the same CR maybe the iron head and aluminum heads differ slightly, but the whole point of aluminum heads is to run higher CR, and the whole point of an all-aluminum engine is to save weight ... 100 pounds less weight means more power applied to moving the vehicle and less to overcoming inertia, which should result in higher ETs, faster acceleration, and better fuel consumption numbers. In a truck, there's a payload advantage.
Because the intake valve is off the seat from bottom dead centre to some point in the compression stroke, the engine compression ratio during this period is irrelevant ... no actual compression of the charge is taking place.
Once the intake valve closes, you start compressing the Air/Fuel mixture.
There are some fairly well defined guidelines for DCR and Fuel Octane. If you punch in the numbers for the 5.2 Magnum in my truck, which has 9.1 CR pistons installed, you get 7.1 DCR. This makes good sense since a DCR of roughly 7.25~7.5 is considered safe for 87 octane pump gas.
If you are an OEM configuring a stock engine that will probably have some combustion chamber deposits reduce the static CR over the life of the engine, 7.1 seems like a decent number to build to for a factory go-anywhere-do-anything motor.
Because intake valve timing is a critical factor in DCR calculations, you can alter the effective compression (ie the DCR) with cam timing alone. Alternately, you can choose the cam first and then choose pistons that provide the target DCR for the fuel octane you are willing to pay for.
Broadly speaking, an aluminum head will allow you to run about 1 point higher CR than an iron head.
Compression is pretty much "free power" ... it doesn't cost more to buy moderately higher compression pistons, and a cam is a cam; prices don't really vary from the same supplier with similar profiles. You get more work from the same amount of fuel, and the amount of work you get from a gallon is the real-world cost of fuel to you.
That is why the stuff you were reading doesn't tell the whole story ... sure, at the same CR maybe the iron head and aluminum heads differ slightly, but the whole point of aluminum heads is to run higher CR, and the whole point of an all-aluminum engine is to save weight ... 100 pounds less weight means more power applied to moving the vehicle and less to overcoming inertia, which should result in higher ETs, faster acceleration, and better fuel consumption numbers. In a truck, there's a payload advantage.