383 head gasket steel or comp

Engine, Transmission & Driveline

  1. Stormrider

    Stormrider New Member

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    Gentlemen, as a novice I am looking for advice,

    Trying to figure out which head gasket to use, the stock .020 steel or the .040 Fel Pro comp

    The pistons are TRW 2315f -30 and are .005 down, (that’s the closest one to the top) with stock 906 heads and a mild cam -

    INT. 0 TDC 34 ABC 214 32 BTC 76 ATC 288 .295 .443 107 HYD.
    EXH. 49 BBC 5 BTC 224 81 BBC 37 ATC 298 .310 .465 117 HYD.

    I don’t know, if the steel gasket is an option that puts my clearance at 0.025 and what happens to the quench with open heads? I know the steel gasket would raise compression but is it too close?
     
  2. Davea Lux

    Davea Lux Old Man with a Hat FCBO Gold Member

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    The steel shim gasket will run the compression about a half a point higher than the comp gasket. For a street engine, if you plan to run pump gas you want to be running about a 9.5-1 compression ratio. The stock piston for the 906 head would, per blue print specs, be about .0025-.003 below the block deck for an advertised compression ratio on the performance engine of 10.0-1. As a practical matter, factory engines were all over the map for piston height, some running as much as .030 down the hole. This was the result of variations in the machining processes. If the heads or the block on you engine have been milled, the amount of milling also has to be taken into account. You will be pretty close to the correct quench with a .025 as .027 is generally considered to be ideal. My advice would be to start with the steel shim gasket. With the mild cam you are going to need to play with the timing and advance settings in any case, so try that approach first. You will probably also need richer jetting on the stock carb if that is what you are running. If you fail to control pre-ignition, you can always fall back to the thicker head gasket.

    Dave
     
  3. 70bigblockdodge

    70bigblockdodge Old Man with a Hat

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    Screenshot_20210805-170638.png
    9.58:1 according to summit compression calculator.
    Your quench is going to be minimal, above .060 it starts to diminish. Your at .025 not counting how far down the dead area is below the surface of the head. Straight edge and a stack of feeler gauges will give you a ball park.
    Still a decent compression, and better quench and air movent than a 8.5 2bbl 383. I would try to knock down any rough edges in that dead area across from the plug to eliminate hot spots, just the high stuff don't make it smooth, paying close attention to the edge by transition to gasket surface. Then bolt them on and call it good.
    The 86cc chamber is a ballpark that seems to be on the small side of normal as cast that I have seen. I know they list the open chamber heads as 81 cc, unless milled alot they ain't.
     
    Last edited: Aug 5, 2021
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  4. Power75

    Power75 New Member

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    One item in your favor for using the steel shim and it's slightly higher compression is the slightly bigger cam profile. My experience has shown that with a stock cam with little overlap directly impacts cylinder pressure. By using a longer duration cam you will lower the cylinder pressure which will help with detonation. 10 to1 with a stock cam usually means retarding timing to the point of poor performance. If you are in fact about 9.5 to 1 as shown in above post, and with the bigger cam I'm thinking the steel shim also would be ok.
    Pat
     
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  5. Stormrider

    Stormrider New Member

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    Thanks guys, the pistons are down .005 -.012 from a dial indicator years ago one side better then the other. The cam is mild, has a longer duration Melling 23204 specs are above, the carb right now is a holley 510-80508 750cfm and the dead area on the heads is big see pics, if you consider the dead area the flat area across from the spark plug. If I add the dead area to the approx .025 + I want to be under 60 right ? any concerns with valve clearance going to the steel gasket.
    John

    IMG_20210805_163223554_HDR.jpg

    IMG_20210805_163502792_HDR.jpg

    IMG_20210805_163742163_HDR.jpg
     
  6. 70bigblockdodge

    70bigblockdodge Old Man with a Hat

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    Valve clearance should not be any issue with that cam. Yes that big open expanse of detonation prone surface is not great. That plus gasket thickness, plus piston down in the hole gives you quench. The .020 gasket is almost .020 better than a .039 fel-pro,so your .019 closer to the magic .060, which no matter what is waaay better than a stock piston down in the hole .080 plus that dead area. So live with what you have, use the steel shim gaskets and enjoy a good running 383 with a appetite for 93 octane.
     
  7. Stormrider

    Stormrider New Member

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    Thanks for the help and education:thumbsup:
     
  8. 66 Monaco 500 365

    66 Monaco 500 365 New Member

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    Don't forget you have to re-torque shim gaskets after the motor is warmed up, but not perma-torque.
    You have to remove the exhaust manifolds to re-torque all the head bolts.
    With the crappy gas we get these days, lower compression would be better.
    I'm in California so we have exceptionally crappy gas that modifies my opinion.
    My $0.02