help with 383 chevy thunder 383 vortec
 

just purchased a 1988 donzi 227 with that engine in it. says it a one year old or so summitt gm crat engine. runs great but only turn 4700 4800 rpms thru a cobra outdive and 21 michigan 4 blade prop.speedo says 53mph....this seem low on both the rpm level and mph...has aftermarket exhaust manifolds thru transom 4" exhaust. i can't find a whole lot of infoonthis donzi model...supposedly a pretty rare model....but previous owner said boat weighs close to 4000 lbs.....any info on this engine? can't find much on summitt website or donzi's for that matter. shouldn't it turn 5200 to 5500 and at least reach to mid 60's?

If you can find a three blade OMC Viper 21p i think you and that stroker will be much happier . Dump the Michigan Wheel , your trying to get performance out of a stone .

I'm rebuilding one of those engines now for a friend who flattened lobes on the cam in an hr or two run time. His engine has:

18cc dish pistons kb hypers
64cc vortec heads
cast rods and crank
9.5:1 compression
The heads were modified to be able to take a .507 lift cam w/ 1.5 rockers
His bore is actually 4.04 so its a 385
34* timing
Supposed to be 400hp

Make sure you are running good high zddp oil and/or additive to make the cam live. That is if yours is flat tappet too.

Its a roller motor. With roller cam. Is 5200 or so rpm about right?

It would depend on what the cam is. The one I'm building is 57-6400rpm but it is for street use. You need to reprop to get the rpms to around 5500 I'd guess to get to the max hp. Maybe try a 3 blade prop too. I'd probably hunt for a 19p laser prop. You will gain 400rpm with the pitch change and probably another 100rpm going to 3 blade which would put you right where you should be rpm wise.

Describe the carburetor, intake and exhaust. I agree...should be in the 5000~5500 RPM range at WOT. i am thinking a 3: blade 21" but the 19" will be a good start. Two tons with that horsepower/torque should put you at 60+ if it's dialed in properly.

I disagree I'm running a 350 with lt4 hot cam and vortec heads. Boat is between 2500-3000 lbs and I'm breaking into the low 60's with a 21p laser at 5200 rpm I'm at 3000 feet elevation but I would say 22ft and 4000 lbs calls for a BBC

Might try posting this question on Donzi.org, theres a wealth of knowledge on all and rare Donzi's. I'm sure someone over there can tell ya what the boat should run with your power

fixx
 

is this the same style donzi as you have?
http://tampa.craigslist.org/psc/boa/2949436495.html

if it is be glad your getting that kind of speed out of it..that boat has a wide beam for its size..

I think that is THE boat, he has another post on here looking for a cobra drive because the one on the boat he bought is busted. Not a bad looking boat tho, just not the fastest thing on the water, but doesn't appear to be designed for speed.