[Question] professional gear head advice (781) or (290) castings
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professional gear head advice (781) or (290) castings

  1. #1
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    Default professional gear head advice (781) or (290) castings

    heres the deal..looking for a mild 500hp in a 468 bbc i have choice of running 2 heads either the 781 casting or a closed chambered 290 cast. i hear the 781 is a decent flowing head etc etc,, but a friend of mine has some 290 castings sitting around i can get cheap. whats the pros and cons here....??? any professional motor head advice would be greatly appreciated. should i hold out and wait for these 781 castings to put money into them, or should i grab the 290 casts and punch them out for this build.i wanna stick with a pump gas engine..91-octane, so yeild around a 10.1 or so compression, or as high as i can get and still run pump gas. im building a 18ft 77 tahiti, i dont plan on going to any races thats for sure, itll be a show and cruise boat with the 500hp im after.

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    "its a shame for a man to spend his whole life, not knowing what his body is truly capable of" :Socrates:

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    Purple headed member Quickjet's Avatar
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    The hull probably isn't going to respond to the advantages of the better flowing 781 head. I would just do a cheap build and enjoy it. The 290 head will work fine for what you are trying to obtain.
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    Thanks for the input...so 500hp build it should be fine to use the 290? Would it not be easier to get that hp with the 781? I hear about detonation with the 290 or any close chamber for that matter..anyone have success with close chamber heads. I'm new to the jet boat world, I know this hull isn't meant for speed, but i would like to get all tge potential out if it i can. Im happy with 60-70mph really, im in it for the fun, not the racing, what do you think the top speed would be on this Tahiti hull with that hp, not knowing the pump.
    "its a shame for a man to spend his whole life, not knowing what his body is truly capable of" :Socrates:

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    Purple headed member Quickjet's Avatar
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    Its always easier to get power out of better parts (781 vs 290). The 781 head will be a better head all around for pump gas, power and investment.
    Not knowing the pump or condition of the hull I can only give you a comparison. Same hull as yours with a 455 Olds ran right at 68 mph. I'm not an olds guy but the motor had their version of a good head, single 850 on a torker intake, headers, JC Berkeley with a Place diverter. You should have no problem getting the same if not better results.
    Quote Originally Posted by thatguy View Post
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    Keith...
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    I had the same choice. I had a set of 781's and a set of 290's that were ported and polished with bigger valves and unshrouded. The builder I used grabbed the 290's because you can run flat top pistons and still get higher compression and take advantage of the quench area of the closed chambered heads.
    Both have their own advantages. Depends on what you want to do with it.

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    Thanks guys, all the advice is being taking in. This site has been helpful for sure. What's the compression on your bbc with the "290" on it. Do you still run pump gas 91? If you do t mind hows it run speed wise and has there been any problems running thise heads? Whats it mean when you unshroud the valve, I know they say the valves are shrouded but I can't really tell what that means exactly, what'd it cost to have then unshrouded? Defiently wanna stay pump gas friendly. It's for pleasure not race, no reason to run 110 octane in it at a high compression ratio if the boat can't use the power anyway. And the pump is a jacuzzi "the better of the 2" with a new energizer kit and jetovator..don't know what impeller, but I don't wanna turn my motor or 6k so I'll deal with that when I get it in the water, or is there a better way to do it, I don't wanna pull the pump apart myself not knowing.
    "its a shame for a man to spend his whole life, not knowing what his body is truly capable of" :Socrates:

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    Quote Originally Posted by Krazykracker View Post
    Thanks guys, all the advice is being taking in. This site has been helpful for sure. What's the compression on your bbc with the "290" on it. Do you still run pump gas 91? If you do t mind hows it run speed wise and has there been any problems running thise heads? Whats it mean when you unshroud the valve, I know they say the valves are shrouded but I can't really tell what that means exactly, what'd it cost to have then unshrouded? Defiently wanna stay pump gas friendly. It's for pleasure not race, no reason to run 110 octane in it at a high compression ratio if the boat can't use the power anyway. And the pump is a jacuzzi "the better of the 2" with a new energizer kit and jetovator..don't know what impeller, but I don't wanna turn my motor or 6k so I'll deal with that when I get it in the water, or is there a better way to do it, I don't wanna pull the pump apart myself not knowing.
    Definitely pump gas friendly. 9.75:1 compression. I could have gone a little more but since it's my wife's boat I want to keep it very user friendly and very reliable. Again, it takes a little more to get the 290's to flow like the 781's, so be prepared for that. The flat tops and larger quench area of the 290's help defeat detonation, so since I had a set of 290's that had been worked over I went with them. If they weren't as nice, (larger valves, port and polish) I might have gone with the 781's. I let the guy that did my machine work pick the heads. He's the same guy that does work for Loveless racing so they know boat engines. When you go to your engine builder take both sets of heads. Don't decide on pistons or cams until you talk with whomever is going to machine your block. Cam, pistons, heads, and intake all have to work together. Some say to pick your heads before anything.
    No numbers yet on speed for this engine. I'm still putting it together and breaking it in. Should be on the water soon.

    Oh, and to add a disclaimer, I'm just a garage hack, so talk to a number of people about this. My opinion is just that of what I've researched and what I came up with my machine guy. You may get a number of different opinions. This is good. Take the advice of those that really know what they're doing. There are a lot of them here.
    Last edited by wolfie; 08-15-2012 at 11:39 AM.

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    Thanks man.
    "its a shame for a man to spend his whole life, not knowing what his body is truly capable of" :Socrates:

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