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Old 10-20-2009, 08:56 AM
TRBenj TRBenj is offline
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Quote:
Originally Posted by OLNOLAN
Hey TRB, Nice ski boat. I'm a HotBoat guy too, 20ft Eliminator, hotrod sbc, mercruiser. I've always wanted to get my hands on a v-drive ski boat and modify it for high speed rather than its low speed pulling power. Alot can be done, gear ratio changes, prop size and pitch, engine hop ups, etc. But all these modifications are a trade off, reducing pulling power for higher top speed. In your case, you can have your cake and eat it too. There are several torque boosting modifications that will increase your top speed without losing low end grunt to get skiers up on the water.
About your engine, its likely 8.5:1 CR so it will run on any junk marina gas available. Most had 224/224@.050 .500/.500 lift cams. It likely has forged pistons. As far as reverse rotation thats a reverse ground camshaft & distributor gear. This is normally reserved for twin engine boats, I'm not so sure your engine is reverse rotation, got a feeling its just a reverse rotation propeller because of the v-drive gear box. The rectangle port heads are generally too large for operation in the 5000rpm range. The marine manufacturers had to use what GM had available in the Gen 4 era. There were the peanut port truck heads, the medium port pass car heads(781,049) and the big rect. port heads. When GM quit producing the medium port heads, marine manufacturers chose the still available rect. port heads vs. the peanut port heads that were weak on the upper end.
I think that stroking your engine is way overkill. You can probably get what you want with some torque boosting. Aftermarket high performance exhaust manifolds, change the rectangle port heads out to medium port with big valves, maybe a cam change, but in that order. Likely the exhaust manifolds will get you what you want.
You should get this book; Big Block Chevy Marine Performance by Dennis Moore. It is loaded with information specific to marine performance engines, tons of part numbers, specific camshaft recommendations, etc. Ok, there's food for thought, hope it helps. olnolan

EDIT- I had to leave the computer for a while, now I see it's a direct drive, not a v-drive. The torque boosting approach is still valid though.olnolan
Olnolan, thanks for the reply!

Concerning the reverse rotation engine, it definitely is. Correct Craft primarily installed RH engines in all of their direct drives from the early 60's through 1989, at which point they introduced a transmission that reversed the direction of engine rotation. This allowed them to keep the RH prop that offsets the weight of the driver and keeps the boat running level with just one person aboard (important for the ski wake). I havent torn it down yet, but everything Ive read indicates that the reverse rotation 454's (and possibly all RR Chevies) run gear to gear timing sets that have the cam spinning the same as a LH engine, albeit with a reverse firing order. This keeps the dist gear the same as a LH engine. (My RR Fords do it as you mentioned with reverse cut dist gears and chain drive timing sets.) If a RH BBC chain drive cam exists, Id love to know about it!

With all the reading Ive done, I know the 781/049's are the way Id go if I was starting with a regular peanut port headed engine... but this thing already runs so strong once its on plane that I am very hesitant to swap out the 990's. It is easily among the fastest boats in our nationwide community of Correct Craft nuts, and Id like to keep it that way! Id like to keep the 990's and either boost the cubic inches if necessary, or just bump the CR and cam if that would be considered adequate. I dont plan on pulling the engine out of the boat more than once- so now is the time to get it right (thats why Im here). Like I said, the holeshot isnt bad as it sits, and the top end is amazing.

New exhaust manifolds are certainly in order, but I was hoping to go with a classic set of aluminum logs and do some port matching, rather than go with a high dollar set of new-tech shorty header type manifolds. I was already planning on increasing the exhaust size from dual 3" to either 3.5" or 4" (these are wet exhaust pipes, my 351w spinning to 5200rpm loves the 3.5" Ive got on it). Maybe I need to rethink this if the exhaust is a major BBC bottleneck.

Like you caught, its a direct drive with a 1:1 gearbox. I like the speed vs. RPM's its pulling now, so I dont see major prop changes in our future. It runs pretty good with both the 13x14 and 12.5x15 we've tried, I can play with the cup to get the WOT RPM's in check if necessary.
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