- - 30p vs 28p ware on drives
(https://www.offshoreonly.com/forums/general-boating-discussion/327172-30p-vs-28p-ware-drives.html)
chris21hope
06-10-2015 10:09 PM
Originally Posted by Tinkerer
(Post 4316006)
I find it hard to believe that a 575 Merc would make peak HP at only 5000 RPM. I would think it would be at least 5400. Were these engines built using 575 parts or something close? If it was me I would raise the limiter to 5700 and see what it does.at 5400.
I spin my 509's 6000 RPM and they love it. I wouldn't worry about 575 HP going through a Bravo drive. I was using 28 Bravo props on my 34 SCARAB and the only part I broke on the drives was a design failure of the IMCO vertical shaft in the -2 shorty's.
It isn't a 575 Merc. We reused the HP500 block, but pretty much everything else is fresh. Dyno showed peak HP at 5,000. I've thought about raising the rev limiter, but that would require me replacing the distributors (still Merc). If I spend another dime on this project, my wife is going to kill me, so I think I should wait a while....
MILD THUNDER
06-10-2015 10:18 PM
These still 502's ?
Griff
06-11-2015 01:04 AM
575hp out of a NA 502ci with peak HP near 5000rpms??? That is some kind of recipe and must be a stump puller with a ton of torque.
The 28 vs 30 pitch prop won't matter significantly. The engine torque will have a lot more effect.
chris21hope
06-11-2015 06:49 AM
Sorry, guys....forgot to mention that they are now 540s. Peak torque is 650 at 3900 RPMs.
Tinkerer
06-11-2015 07:01 AM
Sounds like you could have used a bigger cam.
Keith Atlanta
06-11-2015 07:53 AM
Originally Posted by chris21hope
(Post 4315327)
Guys, I just had Wesco Racing Engines build me a pair of 575s out of my old hp500 blocks. The powerplants rock, but I'm now trying to dial in. Just ordered a set of 30 pitch four blade props to replace my 28s (need to lower the RPMs), but am concerned that this will put added stress on my stock bravo drives. How much harder will 30s be than 28s?
Before you answer – yes, I know that I am putting too much horsepower through these drives. You don't need to rag on me for that! I'm just interested in knowing the difference in ware and tear between the two.
I had this problem with almost the same horsepower gains. I went from a warmed up 496 with a cam and heads to Raylar 600's. I too started off with 30P but ended back at 28 but labbed them. The performance on to plane was much better with 28's and the acceleration was better. But, in my Bravo breaking abilities I found it better to spin them a bit more than use the torque. Just make sure you are at the top MAX RPM when wide open. If you are close to what you need with the 28's send them to BBlades and have them tweak them and put some cup and or possibly turn them up to 29's so it drops the RPM but gives you gains on the top end. I ended up with a faster prop that felt way bettter out of the hole too. Since you have a similar sized straight bottom V you will benefit from a decent labbing anyway. If one or the other is faster or better, they also might lab the 30 to be smaller too. Its a win/win you just need to pick the better of the two and have BBlades tweak them.
