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11-10-2009, 10:52 AM #16Member
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Jet boat factory cams as installed STOCK are typically 214/218 degrees at .050. Most stock gm cams are 192/204 at .050 so the car engines are down by about 30-40 hp just from that, add to the fact that boat engines typically run about 1 to 1.5 points more of compression and you can see why car engines or rebuilt car engines from a NON BOAT engine builder are poor performers. A normal "RV" cam runs 204/214 at .050 duration, so they are still smaller than a stock boat cam. For best performance and fuel mileage, an 850 is really the SMALLEST, best choice for even a stock engine. NO vacuum secondaries either, those won't fully open at normal jet RPMs. A Crosswind or RPM airgap intake is the best deal for all but the more radical engines. We have built 650 HP Reliably with those intakes quite often though. A small carb will not have enough airflow through the primarys to keep a jet on plane, and I know an HP 950 sounds too big, but it will work better than most anything else, even with log exhausts! We have done that carb/intake install many times, and always with good results. A stock Quadrajet carb flows 790-835 CFM depending on model, so don't use anything smaller than that. We have been building and specializing in BOAT engines for over 30 years. As a matter of fact, we do glass work and all types of pump work too. We are the Southwests complete ONE stop shop. Board constraints prevent me from posting contact info here, but I provide as much free info as time allows. I do have to take care of customers first. Thanks for being understanding. TIMINATOR
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11-10-2009, 11:18 AM #175000 RPM
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- Shell Knob MO 65747
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No plans for racing, or engine mods. I just want all I can get from stock. What would you suggest I run?
My favorite for that application is a 0-80537 Holley Marine Carburetor. Works very well.
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11-10-2009, 07:04 PM #18Junior Member
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- Fort Payne, AL
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Excellent info and thank you. I Have two carbs for this boat now a 600 and 750 Edelbrock vacuum secondary. I have a buyer for the 600 that is on the boat now and I am going to go ahead and put on the 750 so I will at least have a working unit. I am going to keep a lookout for a 750 manual secondary. When one comes around at a good deal it will be mine.
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11-11-2009, 05:31 AM #195000 RPM
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- Apr 2009
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- Orlando.FL
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The Edelbrock is pretty much a mechanical secondary.
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11-11-2009, 07:32 PM #2079 Southwind Tunnel Dragster 540ci BBC
UMPBA 926 Gas Jet
My Projects http://s200.photobucket.com/user/Dir...?sort=3&page=1
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11-13-2009, 04:40 AM #21New Member
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- usa
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Can you please give some information about the matter so i will try to answer.
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11-13-2009, 05:56 AM #225000 RPM
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Holley is easier to tune X 2 , my choice
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11-22-2009, 02:39 PM #23Member
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I have a stock 460 Ford (rated at 320 hp) in my 1978 Glastron CVX20 ... stock, they have a 600 Holley vac secondary carb on them. From this thread it sounds like a bigger mech secondary carb would work better ... right now it tops out at 62 mph on GPS at about 4400 rpm and accelerates strong and smooth for a stock motor. Would adding a larger mech secondary carb actually add speed over the 600 vac secondary?
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11-22-2009, 08:24 PM #24Member
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Any vacuum secondary carb will allow the secondary barrels to open as much as the primaries are. Most people don't realize that, but thats how they are setup. This causes inefficency in the mixture distribution. An 800 or 850 will cruise on the primaries alone, and that is better for fuel mileage. The bigger carb will allow a higher top speed and rpm too. We have installed hundreds of them with no complaints. Most customers after trying them wonder why the bigger carb wasn't installed from the factory. Faster speed, better acceleration,and better fuel economy are the plusses, cost at the factory is the only reason that I see them not installed. Be sure to check that your distributor advance is 12deg. mechanical (24deg at the crank plus about 10-13deg initial for about 34-37 total), and fully advanced at 3000 rpm. You can check that with a timing light, but any curve changes should be done by machine. TIMINATOR
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08-07-2012, 10:20 PM #25New Member
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Ok,I"M Building a BBC454 and I have a 427 steel crank,284 comp cam,hydraulic lifters,freshly built Dart aluminum heads with large springs,double roller timing chain,new forged rods with forged domed pistons,the bore is 30 over stock and i have a 850 d.p holly carb,holly mech. fuel pump and a Berkley upgraded pump. Im hoping i did not make a bad decision on such a big cam! What is your input and am I on the right track?