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  1. #1
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    OMC 2.6 V6 Crossflow 235, performance increase going to 638/639 heads?

    Ok,
    So Iíve searched and searched and haven't been able to find an answer to a question that's been bouncing around in my head.
    I'm about to purchase locally a set of 638/639 heads that have apparently been shaved ".020 under the advice of Mr. Al Stoker and then have been confirmed to have the minimum .040" clearance between the head and piston while at TDC.

    My question is:
    Why is it that my motor which makes "235 HP" at the crank and can do so with the ****ty low compression "bathtub" heads while as other motors do it with these high comp but end up at the "same" power level? There is a sticker on my motor that says to run 30 degrees of timing, however I have reduced this to 26 degrees under the advice of racer, forkin and some other knowledgeable people on this site. Is it that my 235 is on the lower spectrum of the allowable variance of power ratings while as a motor with higher comp heads would be nearer the top? For the record my current heads are 327124 and 327125.

    More importantly what type of HP increase can I expect to gain with the head swap?

    My motor already has CCM reeds, smooth exhaust passages, a drilled lower unit and will run at 26 degrees of timing at 6200 RPM to be safe. Everything Iíve ever owned has been modified to the point where it requires premium fuel so this boat will be no different and will live on 94 octane fuel (it already does). It has ".030 over pistons and currently has a compression reading of 105 PSI across the board.<O></O>

    Your knowledge and insight is appreciated!<O></O>

  2. #2
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    JMO. I suppose it shows that even cylinder comp. importance. Sounds like your engine is running well at 6200, higer comp. in itself should make things a little more responsive maybe and should use a little less fuel as a result. Some may say a jet change is necessary [i don,t agree with the theory] and raising it may compliment the slight mod,s you made so far thannks.
    free spirit / ONTARIO, CANADA.

  3. #3
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    Quote Originally Posted by terry taylor View Post
    JMO. I suppose it shows that even cylinder comp. importance. Sounds like your engine is running well at 6200, higer comp. in itself should make things a little more responsive maybe and should use a little less fuel as a result. Some may say a jet change is necessary [i don,t agree with the theory] and raising it may compliment the slight mod,s you made so far thannks.
    I can understand why you might think a jet change is not necessary; the air/fuel ratio would still be the same with the higher compression ratio so at a glance there would be no need for a jet change. However it I believe it is often necessary to go with slightly larger jets in order to keep the combustion chamber temperatures down as the fresh fuel has a cooling effect as it is drawn in by the descending cylinder. There are a lot of people on here that know more than I about hot-rodding these motors and I think most say you need to go up a few sizes on the mains to be safe.

    Still wondering if anyone knows the answers to my first questions???

  4. #4
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    My reply was to help draw more input partialy, I won,t address the jetting it,s my opinion given the old cross-flo. system. Let me say I would leave the timming at 30 and run premium fuel and synthetic oil, with surface gap spark plugs. The result may be close to the same as raising comp. and retarding the timing . I hope this helps and let others answer your questions thanks.
    free spirit / ONTARIO, CANADA.

  5. #5
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    There is some variance in the HP ratings. I don't think raising the compression will raise the HP that much, but more so on the torque and mid range punch.

    Let me say I would leave the timming at 30 and run premium fuel and synthetic oil, with surface gap spark plugs. The result may be close to the same as raising comp. and retarding the timing .
    I can't say I agree with that statement.
    I may have been born at night, but it wasn't last night.

  6. #6
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    What size main jet are you running now? I run 67's in hot weather and jet up in colder temps. Its always a good idea to run a little fat when it comes to cross flows.

    Your motor will run much better with QL77JC4 plugs than the surface gap plugs.

    You will notice a significant difference with those heads.


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  7. #7
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    Quote Originally Posted by John J 80`viking View Post
    What size main jet are you running now? I run 67's in hot weather and jet up in colder temps. Its always a good idea to run a little fat when it comes to cross flows.

    Your motor will run much better with QL77JC4 plugs than the surface gap plugs.

    You will notice a significant difference with those heads.
    Hi John J,
    When i got the boat last year it ran like ****! I rebuilt the carbs with the stock 62C mains and picked up 20MPH! It turned out someone had slightly drilled out the jets.
    What do you have done to your motor? Our setups arn't that far off ('86 Hydrostream Valero YT) and I'd like to be in the 80's like yourself this summer. Do you have these better heads? It sounds like you have experienced the difference between the two styles first hand.

    Oh and my motor has the Champion UL77V plugs

  8. #8
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    Those heads made a nice difference on my GT175. I would come off the timing as you have been told, run only premium fuel, and good oil. If you can find gas with no ethanol, then that is better.

    I think I ran 24 degrees of timing at WOT, Racer told me to go up 4 sizes on the high speed jet. I had boyesens, and the intake filler blocks and the punch the motor had was great.

    Hope you enjoy it.

  9. #9
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    Quote Originally Posted by Fullthrottleguy View Post
    Hi John J,
    When i got the boat last year it ran like ****! I rebuilt the carbs with the stock 62C mains and picked up 20MPH! It turned out someone had slightly drilled out the jets.
    What do you have done to your motor? Our setups arn't that far off ('86 Hydrostream Valero YT) and I'd like to be in the 80's like yourself this summer. Do you have these better heads? It sounds like you have experienced the difference between the two styles first hand.

    Oh and my motor has the Champion UL77V plugs
    Our motors are about the same except I still have the metal reeds in mine.

    Yes about the heads, I ran the stock bathtub heads for a while the changed to the tighter heads so I could hang w/ my friends who all run mercs.

    If I were you I would buy jet sizes 67 thru 71.

    You should have no problem reaching the mid 80's if your boat is set up properly.

    John
    Last edited by john j 80 viking; 04-23-2012 at 11:03 AM.


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    "I said I didn't have much use for one. Didn't say I didn't know how to use it."- Mathew Quigley

  10. #10
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    Don't take the timing over 26 degrees. The only thing you will gain by leaving it at 30 will be a pile of blown pistons. The jetting will depend on your carbs. There were many different styles that ran different jetting.


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  11. #11
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    Thanks for the info guys!

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