steel shim head gaskets for compression/HP boost

Discussion in 'Technical' started by scooper77515, Mar 6, 2005.

  1. scooper77515

    scooper77515 No current projects.

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    My 74 with stock heads has been reported to have either 7.5:1 or 8.0:1 compression. If I replace the "stock" (meaning these are what I put on last time I took off the heads) FelPro gasket with steel shim gasket, how much compression increase would I see, and how much Horsepower?

    I saw a chart on someone's website where the old 302 had 9:1 with 210 HP. THen they dropped to 8.5:1 -140HP, then 8:1 with 138 HP. I don't see a curve in these numbers that lets me estimate what I would have with 9.5 or 10:1 (not that I could get these higher numbers with a gasket replacement.

    Anyone have real numbers?
     
  2. CometGT1974

    CometGT1974 Gearhead

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    Just because you use a "steel shim" head gasket doesn't mean that it will automatically raise your compression. It's the thickness that changes compression and it changes so little with a head gasket change that you will not notice a difference. Steel shim head gaskets are for high performance applications where lots of boost or nitrous is used, they are an alternative for o-ringing the block and heads.
     
  3. igo1090

    igo1090 Member

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    i was told that shaving the heads would raise compression about .5 for each .010 removed. ie shave .010 and go from 8.5:1 to 9:1. just tried some figures in a compression ratio program i have for a stock bore and stroke 302 with 60 cc heads. each .010 reduction in the compressed gasket thickness was about a .4 change. so if you took .030 out of the gasket thickness or cut the heads, you should gain about 1.3 on the compression ratio (estimated). remember that steel shims can be a bi**h to seal. both heads and block must be very even or you may have problems. if you milled the heads you would get similar results. remember that the more material you remove the more intake misalignment you create which may require milling the intake and egging the intake bolt holes. there is also a question of head deck thickness if you are using stock heads. aftermarket heads generally have much thicker decks.
     
  4. Grabber5.0

    Grabber5.0 Gear-head wannabe

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    Keep in mind, the 210 hp in 71 were rated at the flywheel, they were really about 140 net hp, same as the later ones.
     
  5. bmcdaniel

    bmcdaniel Senile Member

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    The 74 - 76(?) blocks had .023" taller deck height than earlier and later 302s. The best way to raise the compression is to have the block's deck milled or put in higher compression pistons. Second best way is mill the heads. Just remember to also have the intake surfaces milled too, or the ports won't line up. I've never seen anyone have luck with shim head gaskets on a street engine. BTW, the amount of compression gained by milling will vary according to which heads you have. Chamber volume and shape figure in, so there there is no rule of thumb that "if I mill .010" I'll gain 1/2 point".
     
  6. Maverick73

    Maverick73 Senior Member

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    Where is the net rated at?
     
  7. stmanser

    stmanser Looking for a Maverick

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    210 at flywheel
    140 at the rear wheels at the ground with OPTIMAL traction and conditions
     
  8. 74merc

    74merc computer nerd

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    According to Chilton's, which I know to be wrong on occasion, Net is at the rear of the tranny with all accessories installed and operating. Gross is at the flywheel with nothing but the water pump.
     

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