Need help engine build
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Need help engine build

  1. #1
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    Default Need help engine build

    Starting off w/ Dart SHP 400 block new from factory. I'm thinking of using parts from my 383 (20 hrs.), such as forged crank, 6" rods, intake and alum. heads 65cc (AFR 195 Elim.). Builder says piston/ deck probably .020 w/o decking. Engine to be installed in 18' boat w/ Alpha SS drive.

    How much main/rod bearing clearance? What pistons and comp. ratio( 93 octane)? Ring composition and gap? The Bullet cam from old eng duration 230/238, lift .540/.525. Will try to att. cam card. Any help I can get would be great. I contacted a member who does eng. builds. He was very apologetic and said sorry, too busy. He suggested post on forum.

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  3. #2
    Some guy obnoxious001's Avatar
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    If that was your old combination, were you happy with performance? Do you know how much compression you had? What rpm do you plan to run with that Alpha drive?

    I would shoot for around 9.5-1 compression, depending on what fuel you will run. Out here in CA, 91 octane is the normal pump premium.

    Different people will tell you different things about bearing clearances, and your choice of oil and oil pump (std vs high volume) will factor in as well. A kind of old school rule of thumb might be .1% of the journal size (so for a 2.449" main, at least .0025" clearance)

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    The 383 pistons are dished 16cc and comp. calculator was 9.6- 1. Always run the 93 fuel and figure to use all that octane in a higher comp. Understand w/ alum. heads can up the comp. w/o detonating. Is that true? Would like to top out at 56-5800 rpm.

    Was not happy w/ 383. Leakdown on 2 cylinders were 35 and 40%. All leakage heard in crankcase.

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  6. #4
    Some guy obnoxious001's Avatar
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    Quote Originally Posted by Tombstone View Post
    The 383 pistons are dished 16cc and comp. calculator was 9.6- 1. Always run the 93 fuel and figure to use all that octane in a higher comp. Understand w/ alum. heads can up the comp. w/o detonating. Is that true? Would like to top out at 56-5800 rpm.

    Was not happy w/ 383. Leakdown on 2 cylinders were 35 and 40%. All leakage heard in crankcase.
    With the dome and chamber sizes you gave, you might have close to 9.6-1 with zero deck height. If the piston is in the hole .020", you drop about 4 tenths of a point by my calculations. I tend to try and not build something that will be "on the edge" of working, what if you get a bad load of fuel? Aluminum heads do tend to offer some extra safety margin, since they transfer heat more efficiently. There is another thread going about difference in horsepower between various compression ratios.

    The cylinder leakage is not the end of the world, depending on how valid your test was. What I meant was did you like the range of power with those heads and cam if you are trying to use them again. The new block won't really change the power.

    I didn't mention rings in the first post,, probably a plasma moly ring, using ring manufacture's recommended ring gaps, probably from "gasoline circle track racing" column, depending on your choices on their sheet. You are basically building a mild racing engine, piston to wall clearance needs to be opened up a bit too assuming freshwater marine cooling (not a closed system). See what the piston maker says about that.

  7. #5
    Senior Member EVILFORCE's Avatar
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    Ya. .002-.0025 clearence is cool. I myself would run close to 101/2 to 1 compression. With the right cam it will do it easy. Give Straub a call or look him up here to have you a cam ground. Build for max HP at around 5500-5600 6000 max rpm. And torgue around 3500. But I dont know chit. Ask anyone. In fact someone will be around shortly and say so. Have fun.
    No Fool Like A Old Fool is there?

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