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  1. #1
    thewhitewonder86's Avatar
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    engine internals: boost vs. compression

    I have been wondering this for a while about our stock engine parts like the pistons, rods, cranks, ect. Has anyone replaced any of these for an after market set up? Is there a point when people have gotten to such a high boost level they need to replace these or do the stock parts stand up to it all? I know the rule of thumb here is more boost equals more fun but how does boost equate to compression in the cylinder? I read a couple posts where Jerry said if you have the RR cam it is dialed in when your compression is at 135 but then saw another post where Shibby had his head milled and a rude cam at 140 psi. Is there any real gain from the 5 psi? Just was wondering about some technical questions.


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    Quote Originally Posted by thewhitewonder86 View Post
    I have been wondering this for a while about our stock engine parts like the pistons, rods, cranks, ect. Has anyone replaced any of these for an after market set up? Is there a point when people have gotten to such a high boost level they need to replace these or do the stock parts stand up to it all? I know the rule of thumb here is more boost equals more fun but how does boost equate to compression in the cylinder? I read a couple posts where Jerry said if you have the RR cam it is dialed in when your compression is at 135 but then saw another post where Shibby had his head milled and a rude cam at 140 psi. Is there any real gain from the 5 psi? Just was wondering about some technical questions.
    if you want to go fast eg 81mph on an rxp just put bolt ons on, the standard engine can cope with that , regards bill

  3. #3
    thewhitewonder86's Avatar
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    Thats pretty much how it seems everyone is doing things, i mean if you dont have to replace them then why do it? But i wanted to see if anyone had and for what reason. Also wanted to know about how compression and boost relate whether its a direct relation or how they increase together.

  4. #4
    HORSEPOWER JUNKIE 9secZO6's Avatar
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    boost and high comp. have an effect on total cyl pressure under load at max torque output. Higher compression will scavenge more air on the intake stroke and put this higher volume under higher pressure to be lit off in the combustion chamber. Boost, on the other hand does just that, at higher rpm the intake tract is boosted getting more air in that way. U can not have both. If u run 10.5 to1 comp. ratio u are limited to about 6 psi and that better be intercooled. 7.5 to 1 u could go say 18-20, given street gas. Too much static compression and a lot of boost = a ton of detonation, heat, and melted parts. Have a nice day

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    It sortof annoys me that all the Sea-Doo hop-up folks think that 13 psi is alot when stock is 8ish. Stock Buick GN's were running 12 psi with an inferior 2v iron cylinder head. My LT1 Camaro is rock-solid reliable at anything below 15 psi. Stock WRX STi's are running around 22 psi. The Buick folks regularly go up to 20 on 93 octane. All these engines are around 8.5/1 compression just like our RXP's.

    I just don't see what the big deal is, especially when these little engines were built for boost. The biggest road-block seems to be the supercharger drive system (gears, bearings, and washers) then the ECU selection (don't get me on that rant again).

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    I tried an experiment; One half an extra head gasket... guess the result?

  7. #7
    home of the dislexic fingers breon's Avatar
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    i agree with mike 13 psi on 8.5 to 1 is nothing ,,,,i see 25 to 40 psi all the time in my shop ....if it were a car you would barely need an intercooler ..at 13 ...at 8 i probably wouldnt run 1 at all ...since the amount of heat generated by compressing the air 8 psi neglagible factor in the resistance of an intercooler and there wouldnt be much difference ....i feel that probably if you just removed the stock restrictive intercooler you would probably see a slight performance gain on a stock ski,, mike would probably know better than i ...we need gears to spin the chargers faster ....seadoo needs to make them so the stock is 70... coast guard and insurance companes are happy ....but change a gear and bigger injectors and poof 80 + ...anyone know of jackson racing??? ...they sell a supercharger for the miatas in the us that comes stock from mazda in europe <<<the insurance company's and bunnie huggers rule what we get ....

  8. #8
    HORSEPOWER JUNKIE 9secZO6's Avatar
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    If what u say is true (from what I can see it is) then it's the in-ability of the s/c clutch to spin a high-boost impeller w/o cooking itself. This would make a water-jacketed turbo the only way to go. The crank is good to go as-is and the pistons are nice and coated, but use full-length pins and the rods are ok, but for the money some billet 4340 or titanium. When do we get to work and who can handle the bal. shaft and crank issues due to diff. bobweights?

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    Yea, I think turbo is the way to go for 15+ psi. That said, I believe that these engines are capable of about 350 hp at 17 psi reliably, assuming the intercooler is properly sized and the tuning is right. I think it could do more, but holding at full boost for minutes at a time might become a problem at 20 psi.

  10. #10
    HORSEPOWER JUNKIE 9secZO6's Avatar
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    Quote Originally Posted by engineermike View Post
    Yea, I think turbo is the way to go for 15+ psi. That said, I believe that these engines are capable of about 350 hp at 17 psi reliably, assuming the intercooler is properly sized and the tuning is right. I think it could do more, but holding at full boost for minutes at a time might become a problem at 20 psi.
    That's why I stated the pistons and rods be changed and bal., then there will be the issue of the heat in the exh. manifold outlet, but then a water-jacketed header would compliment a turbo nicely

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