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Thread: LT1 aluminum heads swap

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    LT1 aluminum heads swap

    Hi all,
    I am currently rebuilding my LT1 out of 94 Cadillac Fleetwood. I have a pair of aluminum heads and a cam out of 95 Firebird that I wanted to use to save some weight.
    Can anyone advise if the ECU will have to be reprogrammed to account for different heads / cam?

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    Quote Originally Posted by superfragl View Post
    I am currently rebuilding my LT1 out of 94 Cadillac Fleetwood. I have a pair of aluminum heads and a cam out of 95 Firebird that I wanted to use to save some weight.
    Can anyone advise if the ECU will have to be reprogrammed to account for different heads / cam?
    'Have to be' is a strong phrase.
    If you use the aluminum heads and cam, you'd be better off using the '95 F- or Y-car spark tables.

    If I were not going to port the heads, I'd use the iron heads, as they flow a lil bit better than the aluminum heads.
    Aluminum heads are much easier to port, however.

    Sell the iron heads on the Impala SS Forum, someone will be grateful.
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    Quote Originally Posted by LeMarky Dissod View Post
    'Have to be' is a strong phrase.
    If you use the aluminum heads and cam, you'd be better off using the '95 F- or Y-car spark tables.

    If I were not going to port the heads, I'd use the iron heads, as they flow a lil bit better than the aluminum heads.
    Aluminum heads are much easier to port, however.

    Sell the iron heads on the Impala SS Forum, someone will be grateful.
    The added compression of the aluminum heads more than make up for the slightly less flow. That being said the F/Y-car timing tables are not a good bet for 1,000+ lbs more car. The car will always be under more load and detonation will be likely with all the timing. He would be better adding small incremental amounts to the B-car tables. The F/Y car VE tables should be a better starting point though.
    Last edited by Fast355; 03-14-2023 at 05:18 AM.

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    Quote Originally Posted by Fast355 View Post
    The added compression of the aluminum heads more than make up for the slightly less flow.
    That being said the F/Y-car timing tables are not a good bet for 1,000+ lbs more car. The car will always be under more load and detonation will be likely with all the timing.
    He would be better adding small incremental amounts to the B-car tables. The F/Y car VE tables should be a better starting point though.
    OE IronHead LT1 is 10.5to1 compression.
    If he uses the gasket that goes with the iron heads, but with the aluminum heads instead, what will his compression be?
    I'm not a lawyer, I'm genuinely asking.

    That said, the B-car tables were written for 87 octane, whereas the F- & Y-car tables were written for 91 octane.
    So if he uses F-car spark tables, he should definitely use 91 octane.
    PLEASE feel free to correct me if I'm wrong about any of this.
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    Quote Originally Posted by LeMarky Dissod View Post
    OE IronHead LT1 is 10.5to1 compression.
    If he uses the gasket that goes with the iron heads, but with the aluminum heads instead, what will his compression be?
    I'm not a lawyer, I'm genuinely asking.

    That said, the B-car tables were written for 87 octane, whereas the F- & Y-car tables were written for 91 octane.
    So if he uses F-car spark tables, he should definitely use 91 octane.
    PLEASE feel free to correct me if I'm wrong about any of this.
    I can say 100% without a doubt the iron head chambers are much larger. More of a CC increase than the thinner head gasket recovers. From memory an aluminum head with the iron head head gasket is just under 11:1. The iron head engine I cc'd out years ago was only around 9.8-9.9:1 and it was an OEM GM build. Not sure where you are getting your 10.5:1 number. The iron heads are like 62-64cc, the head gasket is like 0.028" compressed and the 6cc worth of valve reliefs in the pistons 0.025" down the bore are right at 9.8-10:1. I just built a L31 with LT1 pistons and the GM 0.028" gaskets and it ended up right at that number as well. The factory aluminum head engines are a 1/2 point higher than the iron heads even with the thick head gaskets. IIRC the head gaskets on the aluminum engines are 0.053" or in that ball park. I forget because I never use them. The LT4 was 10.8:1.

    My 97 Express specd 87 at 9.4:1 but always had severe knock retard on 87. I put 91+ in it and was able to sneak some more timing into it. Enough that it was cheaper to run on 91. The 10.65:1 aluminum head 383 I replaced the 350 with has a good bit more timing in it than the iron head 350 was able to take. I built the 383 0.028" gaskets and at 11:1 with 0.041" quench but ended up putting Felpro 1003s on it when I had the crappy Jegs head bolt fiasco which dropped the compression but hurt the quench. On E85 the 383 loves timing. On the traditional small blocks the Felpro performance gaskets like the 1003s have redesigned water passageway openings to help cool the cylinder heads where they run hotest. They seem to work extremely well.
    Last edited by Fast355; 03-14-2023 at 05:55 AM.

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    LT1 specialist steveo's Avatar
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    the only place iron is better in an engine is the part the piston slides up and down in. the aluminum heads are just straight up better for an n/a car with the correct timing tables and quality fuel. having consistent combustion temperatures from reverse flow cooling and aluminum heads were a huge advantage in this era engine to how much timing advance you could run and how much power you could make. if the b-body rig was better it would have made more power than the aluminum head version -- and it did not.

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    I should have mentioned that the engine is used in a 53 GMC truck, which is roughly 3000 LBS or even a bit less.
    I am definitely going with aluminum heads - because they are lighter and well.. I just want to use them.
    I would not want to lose low end torque - maybe I should stick with original cam then?
    Smaller combustion chambers on aluminum heads will dictate the use of higher octane gas - fine with me.
    I can try using existing timing tables and increase the numbers slightly. Is that the best way?

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    Quote Originally Posted by superfragl View Post
    I should have mentioned that the engine is used in a 53 GMC truck, which is roughly 3000 LBS or even a bit less.
    I am definitely going with aluminum heads - because they are lighter and well.. I just want to use them.
    I would not want to lose low end torque - maybe I should stick with original cam then?
    Smaller combustion chambers on aluminum heads will dictate the use of higher octane gas - fine with me.
    I can try using existing timing tables and increase the numbers slightly. Is that the best way?
    I had a 99 Suburban with a LT1 cam in it and had a LT4 cam in my 97 Express conversion van. No loss of low-speed torque. You are only talking 201-203* @ 0.050. That is smaller than the old 204/214 "RV" cams.

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    LT1 specialist steveo's Avatar
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    i would start with the f-body tables and decrease advance slightly in higher load areas because truck
    actually might just run the whole stock fbody bin to start
    then bring advance back in to see what it likes

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    Thank you all for good advise.

    I compared the bins for a 94 Firebird ECU I had as a spare and original Fleetwood ECU and found that the Firebird timing tables are more conservative at higher RPM, usually by 2-3 degrees, even by 5-6 degrees in some cells. Other multipliers and routines that affect timing are pretty close.

    Given all that I think it is safe to install the 95 cam (a bit more duration and lift) and heads (less weight and higher compression) and leave the ECU bin as is and see how the engine runs.
    Last edited by superfragl; 03-16-2023 at 04:32 PM.

  11. #11
    LT1 specialist steveo's Avatar
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    oh it'll 'run' fine but you'll leave a ton of power on the table. the aluminum heads straight up need more timing and less fuel. you can definitely run the b-body timing tables and advance to suit. eehack has a function to temporarily advance timing across the board with that ecm just via the datastream, so you can test in real time or tune by feel without re-flashing. try adding 1-2 degrees at a time to find knock then back it off a bit.

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    LT1 specialist steveo's Avatar
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    now here is a real comparison between a b-body iron head timing table and an f-body aluminum head one.

    this is in degrees of timing added going from iron to aluminum head. the difference is dramatic.

    i question your original comparison

    Untitledtiming.jpg

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    I agree, quite a difference. Perhaps the bin I have is just different, or 94 was was different, idk..

    Is there a way to monitor the knock? I have never experienced a knock on this engine and I am afraid to miss it by feel..

    Speaking about the fuel, I noticed that the flow rate numbers are lower for f-body bin.
    Last edited by superfragl; 03-17-2023 at 06:14 AM.

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    LT1 specialist steveo's Avatar
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    yes the injectors are a different size
    but you'll be okay
    the datalog has knock retard and knock count
    actually the knock system is another consideration, the bbody and fbody sensors and knock module are different, probably because of the heads

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    Finally put the engine back together and ready to modify the timing tables. What I noticed that the bin for f-body has spark advance table bias zero, and b-body has it set at 20.
    Do I just leave it at 20 and simply modify the main spark advance table?
    There is also Minimum spark advance bias, which is zero for for f-body and 10 for b- body.
    Thanks
    Last edited by superfragl; 04-22-2023 at 06:59 AM.

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