c/r & jetboats
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c/r & jetboats

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    Senior Member rogerroost's Avatar
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    Default c/r & jetboats

    I was planning on 11:1 c/r in my 460, already spent some money on my heads ,when it was suggested to me that with my cam(.562 in.lift& .565 exh lift 274 intake dur & 236 exh dur),94 octane gas, the load of a jet, cool block, I run a good risk of piston failure due to detonation.
    perhaps I should stick with my 9.5:1. The guy I was talking to (who I respect)was stressing the load of a jet as being the big threat as oppose to a car.
    The question is whats too much compression with jets? I don,t want to buy race gas . Thanks

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    My guess is 11:1 on pump gas will be too high. Best thing to do is calculate dynamic compression ratio. There are calculating programs for free on the web. A rule of thumb is a dynamic compression ratio of 8.5:1 is generally abut the highest you can use with pump gas and iron heads. If you are using the boat for cruising the benefit of the 11:1 compression ratio may not be worth it. My guess is the net benefit in top speed would be about 3-5 Mph.

    Boat Vs Car. A 300 Hp engne in a car will only need to generate 15-20 hp to maintain 60Mph. Your boat will have to generate and maintain around 350Hp depending on your hull to run 60Mph. More continuous power demand leads to higher cylinder temps and higher cylinder temps require higher fuel octane or lower compression to prevent detonation givien the same timing settings.

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    Quote Originally Posted by rogerroost View Post
    I was planning on 11:1 c/r in my 460, already spent some money on my heads ,when it was suggested to me that with my cam(.562 in.lift& .565 exh lift 274 intake dur & 236 exh dur),94 octane gas, the load of a jet, cool block, I run a good risk of piston failure due to detonation.
    perhaps I should stick with my 9.5:1. The guy I was talking to (who I respect)was stressing the load of a jet as being the big threat as oppose to a car.
    The question is whats too much compression with jets? I don,t want to buy race gas . Thanks
    Paging Paul Kane......Paging Paul Kane.

    What heads ? Dish piston or flat-top and what or who's piston?

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    gn7
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    I know this much. The Ford chamber is worth a full point in compression over the POS chevy chamber. Same for same on the head material, the Ford is way out front in detonation resistance. Smaller dome, or no dome on the piston, shallower chamber. The Ford chamber is the shiznit for pump gas. 11 to 1 pump gas in an alum headed Ford is a walk in the park.



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    Senior Member CK7684's Avatar
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    Just going by general rule of thumb, 11:1 seems high for pump gas, even in a car...but the more knowledgeable folks here can answer better than i can

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    Senior Member rogerroost's Avatar
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    I had d3 heads,picked up some c9's cheap , have flat tops w/one cutout(trw). by the book c9's should be between 72 -76 cc, i thought. putting me @ 11:1. I cc'd the c9's last nite & got 85 cc's ,3cc cutout ,.022.piston to deck clearence ,.041 gasket, that puts me @ about 10.2:1,great!. still wondering what the max ,safely,I can go with my cam(xe 247 h) is? I can squeeze 10.5 out with a thinner gasket

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    11 to 1 d3 headed 460 on a 150hp shot, never saw nothing but 91octane in it unless i went to the 200hp shot, then i'd run 110. total timing off nitrous 31*, on nitrous 150hp 28*, on 110 and the 200hp i'd run same 28*. i built the motor, put at least a 100hrs on it myself before selling, its been thru 4 diff owners since i sold it and still on the same engine i built 8yrs ago 11 to 1 d3 headed is easy, dont be stupid with timing tho
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    Quote Originally Posted by gn7 View Post
    I know this much. The Ford chamber is worth a full point in compression over the POS chevy chamber. Same for same on the head material, the Ford is way out front in detonation resistance. Smaller dome, or no dome on the piston, shallower chamber. The Ford chamber is the shiznit for pump gas. 11 to 1 pump gas in an alum headed Ford is a walk in the park.
    I believe you are pushing it but should be ok if the chamber has been smoothed. You probably need to stay at 34 degrees of timing max. Paul Kane will know best.
    Last edited by cp562; 11-30-2010 at 10:57 AM.

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    Quote Originally Posted by rogerroost View Post
    I had d3 heads,picked up some c9's cheap , have flat tops w/one cutout(trw). by the book c9's should be between 72 -76 cc, i thought. putting me @ 11:1. I cc'd the c9's last nite & got 85 cc's ,3cc cutout ,.022.piston to deck clearence ,.041 gasket, that puts me @ about 10.2:1,great!. still wondering what the max ,safely,I can go with my cam(xe 247 h) is? I can squeeze 10.5 out with a thinner gasket

    Sounds great at 10.2:1

    Some of the stock heads can handle a bit more compression than others, best I recall you can run 11:1 with the right cam and the factory iron heads with-out issue.

    You'll have no problem with the 10.2:1 compression. FYI Fel-Pro also makes a .051 gasket than can help lower the compression by .5:1. Cometic (sp) can make what ever gasket you need ( with-in reason)

    S CP

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