MEANGREEN231

So old props were 21p SS 4 blade, I was turning 4200rpms on twin sbc's....and having a little fueling issue.

Now I threw some coin at the motors and bumped up power by about 75 hp per side. Whole top end is brand new with high end aluminum heads and roller rockers, new intakes and carbs and so on. I am thinking that the old mercruiser recommended 4200-4600rpm stuff should be now about 5,000 rpms for WOT.

Who has a good starting point for props. I am thinking I want to go with rev 4's. But they are only in 2" increments. I am thinking that 23P wont be enough, will 25P be good or???

Thanks guys.

Griff

If you were turning the 21 4 blades at 4200, then the 23 Rev4's should be very close to 5000rpms with your power increase.

Rockfish71

The old props were 21p SS 4 blade and was turning 4200rpms on twin sbc's old props new Updates 21p SS 4 blade now maybe able to turn 4800/5000 rpm That's alot more rpm given you will loose 400 rpm just on pitch changing if you go up the 21's are a good place to start and more that likey what you have will work for the change

onesickpantera

iTrader: (1)

IMO 25s will be too much and 23s will be absolute max.

You actually may find the 21s perfect as spinning them from 4200 to 5000 will be a nice speed increase. i would use the props you have now and see what it does before spending money.

EDIT - Rockfish beat me to it.

dereknkathy

same boat. iron dart heads. performer (non-rpm) intakes. 600 cfm performer carbs. flat tappett LT1 (early non-smog) cams. alum GLM exhausts. the guy i sold it to 5 years ago who i just got it back from clains 63 mph on gps. 5300 rpm. 23 mirage props. not mirage plus. there are lots of stainless 23 and 25 merc props on ebay these days for about $100 each. buy a pair. see what they are like. resell them for same money if you don't like them. i would go back to 3 blades. you want speed. not towing a buncha skiers. although i am gonna get a pair of alum cheapy 15's for towing the hatteras around...also, the FIRST thing you do is run the 21's you already own and see what you got. remember, thunderbolt ignitions don't have rev limiters. be ready to pull off the power before you hit 6 grand.

mustangmark

Simple.....call Bblades! They are great, know there stuff and you won't be playing the guessing game....

Keith Atlanta

Even simpler, don't change a thing and run what you got. You just took over 150 lbs out of your stern. You may have increased slip and you wont know till you try it.

I just swapped 425 HP 496's for 600 HP 496's and aluminum heads. I thought there was NO WAY I would be running these 28's again and sold them. I bought 32's and 30's - My slip went up a little from loss of weight! And the extra 600 RPM puts 28's just ripe for BBlades tweaking. I am back where I started due to a lighter boat and a lot more horsepower spinning at a significantly higher RPM. Run what ya brung then make changes. Too many changes and you have ZERO consistency and no baseline to start from.

I am the guy that you see always selling new parts because "I thought" the package I put together would work and end up selling new stuff for 30% off because I was an idiot... save your money till you have that baseline.

MEANGREEN231

Yeah but the 4200 was I believe to be due to old nasty carbs(thats what sparked me to drop new top ends on the motors). From the factory this boat spun 23p cleavers at 4600 rpm. That is why I was leaning towards starting with 23 or more likely 25p, due to the power increase and weight loss.

Well I have a few weeks before all the college turds are gone before I take the boat out for a test run!

Griff

All props are not the same. Trying to compare completely different props based only on pitch does not work.
Your numbers are all over the place if you just look at pitch. If it spun 23's at 4600 and you were only spinning 21's at 4200, then the engines would have been down on power around 100hp each from when it was original or something else was wrong. Were they 3 or 4 blade cleavers...big difference.
Exactly what 21" props were you using???
You boat, being older probably has a deep X dim and will probably run the fastest with some Mirage plus props, but it may porpoise easily.

You did not say if you switched cams??? I hope so, because if not, I highly doubt you'll gain more than about 40hp.

Some box stock 23's should be a good starting point and will get you a baseline.

Going from 4200rpms to 5000rpms takes at least 100hp gain(per engine).......and that is with the same props.

MEANGREEN231

oe was 23p 3 blade cleavers. Newer props are stiletto bay pro3 4 blade.

The boat sat for 2.5 years, and was having some carb issues after that. Hard to start no matter if it was hot or cold, and the boat just did not have the punch that it used to. So instead of just replacing the carbs, I figured going with all new top ends.

I wanted to stick with four blades as i felt that overall performance was far superior with the four blades. Plus i have heard from many other boaters that the merc rev 4's are just as fast as a 3 blade, but have all the 4 blade benefits. Wether or not that is true, not sure. But i am more concerened with overall performance over flat out top speed as I spend way more time cruising now that fuel prices are sky high.