New engine, need more prop

[ramster]

Well I could not leave well enough alone, and over the winter I pulled a good running 5.0 GL from my Glastron 195 SX and replaced it with a much better running 5.7, now I have have about 350 HP to harness. I have a Stiletto 21P 3 blade that the old 5.0 would struggle to turn 4400 rpm. The new 5.7 spins it right past 5000 rpm @ 62 MPH with the wife kids and a full tank of fuel. Tried a Laser II 23P and it was no faster and slipped more than the Stiletto 21P. Then I tried a 23P PowerTech which was only slightly better than the Laser. PowerTech sent me a supposedly better prop with more cup in a 24P and it was slower 55@5000, and I had to throttle it back or it would hit 5500. So back to the drawing board, anyone use PowerTech props? Going for a 23 Stiletto next.

 

[craze1cars]

Re: New engine, need more prop

What kind of 350 did you put in this thing? If you now have a 350 that is producing 1 hp per cubic inch, then it sounds to me like your new engine's peak hp hits well past the 5,000 rpm mark, and all Mercruiser specs go right out the window...did you have it dyno'd before install? What does the curve look like? High HP small blocks simply can't make their peak hp unless they're spinning much faster than you're accustomed to. They don't make all that much torque at sub-5000 rpms, so you gotta wind them up fast.

What is the peak rpm you are trying to acheive? Depending on how it's cam'd, I wouldn't be surprised if your new target needs to be in the 5,500 range, if you're truly looking for max power and speed...I believe that's where many people who really build up a strong 350 for a boat will aim for.

You aren't sharing much specific RPM data from your runs, but that 21 Stilletto may actually be perfect for this new setup, thus the 62 mph top speed...it seems that all those 23's and 24's you're trying are probably lugging the motor below it's peak HP RPM level, thus they make the boat slower.

 

[Dhadley]

Re: New engine, need more prop

We'll assume you have a Volvo style drive. Are you using a 19 spline prop?

 

[ramster]

Re: New engine, need more prop

Yes is a Volvo SX, the engine is the ultimate marine small block, estimate 350 HP and 400+ ft lbs. from 2000-4500rpm. I used Al heads 10-1 comp. and a 214/218 roller cam, roller rockers, an Al dual plane manifold with a Q-jet. It makes tons of low end torque, and peak HP should be around 5000-5200. I used Al heads to save weight and fight detonation. The problem is not lugging the engine, I have good holeshot, and can over rev all the 23 and 24 props I have tried so far. PowerTech said the 24 may be mis labeled as it should have more cup than a Stiletto. I'm not just worried about top speed, i would like my cruise RPM's a bit lower also. So far nothing beat the 21 Stiletto, so I have ordered a 23 Stiletto, will let you know how that works.

 

[Dhadley]

Re: New engine, need more prop

Not really sure that a PT will have or should have more cup than a Stiletto, usually a Stiletto has more bite than a PT. Anyway, I know where there's a new 14 1/4 x 25 Stiletto already hubbed for SX if you find you may need one.

 

[craze1cars]

Re: New engine, need more prop

The problem is not lugging the engine, I have good holeshot, and can over rev all the 23 and 24 props I have tried so far.

Oh...I did not interpret your first post in that way...I thought you were only reaching 5,000 with the 23 and 24 props. Didn't realize that was partial throttle. About all you can do is keep going up in pitch until you find one that can handle a WOT run to get you the data you need....

I wouldn't be at all surprised if you need to go to 25 or even bigger. Most 350's in smaller runabouts push a 1.5 gear ratio...I'm guessing you kept your 1.6 ratio factory outdrive from the 5.0? If yes, that's a whole lot of motor in a light weight boat, with a comparitavely low gear ratio. Something tells me it's gonna need a pretty big prop to harness it.

 

[ramster]

Re: New engine, need more prop

I ran that 24P PowerTech on my friends boat, he currently runs a laserII 23P and that PT has to be labeled wrong. He runs 66MPH with a light load, and just hits the rev limiter, with the PT he hit the rev limit at only 55MPH. No way that's a 24, has to be 19, maybe 20P. So back it goes let them send me another.