What should I do lost RPM, but still same speed
 

I just rebuilt my 502 mag. Everything is stock other then exhaust, and I changed valve terrian over to roller. And also probably made a bad choice in cam selection. I put in a crane 139721.

After rebuild I changed my prop, Performance with the first prop was 60 mph (GPS) at 5000 rpm w/ a 21 pitch 3 blade. But with that prop it just felt like I was blowing it out (spinning in place). And forget about triming it out. No grab at all.

Now I have a Bravo 1 26 pitch 4 blade on it. What a difference, now it grabs with trim up in any position. But the kickers is the top speed is still 60 mph (GPS) but the max RPM I can get is 4100. I know I should probably get a smaller pitch prop. But is there anything else I can do like changing the cam to say something like a 139731. Will that give me some more RPM. Or can I get away labbing it. Or am I to far off a pitch for that?

The boat is a 1990 Carrera 27 ft V-Bottom Bravo 1 1:50 Stern

Re: What should I do lost RPM, but still same speed
 

It seems to be overpropped to me. Can you borrow a 22 pitch Bravo 1 from someone? This should give you less slipage than the 3 blade and get your rpm back up, given the couple of changes you made.

Re: What should I do lost RPM, but still same speed
 

Do you have access to a 23" 3 blade or a 24" bravo 1 for a test run??? Looks like your at the opposite ends of the spectrum with the two props your running, you might find something in between will work better and get you into the rpm range your looking for. Those 4 blades are really heavy and take the hp and torque to spin them.... I'd like to see what a 23" 3 blade will give you in rpm... if your still bumping 5 grand with it, then go to a 25" 3 blade....... though not knowing your boat or it's weight, a 25" might be to big for it.... I'd start with a 23 mirage and 24 bravo 1 or hydomotive...

Good luck!

Re: What should I do lost RPM, but still same speed
 

I live on the West Coast and Teague Marine does'nt offer Prop exchange anymore and I took a gamble and bought the 26 pitch Bravo 1. Labben the 26 pitch would'nt work I take it. I really dont want to go back to a 3 blade. I really like the way the boat handles with a 4 blade. Maybe a 3 blade will handle better with a bigger pitch. Ill try and get my boat weight. Thanks for the info..At least I know where to start.

Re: What should I do lost RPM, but still same speed
 

You're overpropped for sure and are not getting into the powerband of the engine. Changing to the 731 cam will only move the powerband up farther.
I think you will need to go down a 22" Bravo.

Re: What should I do lost RPM, but still same speed
 

Check with Throttle Up or other good prop shop and give them your engine specs and some info on your boat like X dim, dead rise, weight, and what ever else they ask for. They can take a lot of the guess work out of the prop selection process.

I agree with Griff....over propped.

Re: What should I do lost RPM, but still same speed
 

I agree w ECB.
Borrow a 23P and work up or down from there.
You may be able to sell the 26P on ScreamandFly.com
Outboards have the gearing to swing 26P props, you can buy and sell alot of used props on S&F and ebay without loosing too much money in the process.

I really lked the Michigan Wheel "Raptor" 22P on my Donzi and want to try it in a 23P.

Good Luck

Re: What should I do lost RPM, but still same speed
 

The 721 cam is the same cam that Merc used in the 420 and 525sc (both 454s) engines. It's not all that bad of a choice.

Re: What should I do lost RPM, but still same speed
 

Keep the prop & add more HP. Add a whipple, procharger or vortec. That and a new cam shaft should get you there.

Re: What should I do lost RPM, but still same speed
 

prop it right and you should be good to go wouldnt bother with more engine mods unless you have a big pocket then would make more changes but would first I would see what she will do with the right prop!!!!!

Re: What should I do lost RPM, but still same speed
 

Yea your right. I think I'll just hold on to the 26' just in case I decide to with a super charger. I just look out for a 23' pitch near my area.

Re: What should I do lost RPM, but still same speed
 

I'm having the same sort of problem. I've been through numerous props can't seem to find the right ones.
boat is very heavy 11500 dry probally about 15000 lbs loaded with fuel and water. trs drives
running built up 454's with prochargers at 5lbs of boost
1. started with orginal 21p 3 blade props hit rev limiters
2. then went with 27p 3 blade props hit about 3000 rpms then seemed to blow out, just over rev'ed (is it possible transmissions where slipping)
3. then went to 25p 3 blade cleaver props would not move boat at all
4. now running 23p 3 blade runs ok but still hits rev limiter near wot, Still running the same speed as as orginal 454 HO's with the 21 pitch props. 58 mph gps at 5000 rpms. doesn't make much sense but true. maybe getting more slippage don't know
going to try 24p 4 blade next week and I have a set of 25p 3 blade coming in. Was wondering if a large 32p cleaver prop would work, would it move a heavy boat, the 25's wouldn't do anything or is a 4 blade better for a heavy boat. I need something that will lift boat out of water and give more speed. awful lot of money wasted if I can't get more speed out of these motors.

Re: What should I do lost RPM, but still same speed
 

The cam used in the 420/525sc is a Crane #132561 and is a flat tappet.

The 139721 is a roller and is used in the HP500 or 500efi, can't remember which.

Re: What should I do lost RPM, but still same speed
 

I thought the 731 cam is what was used in the 500's?

Re: What should I do lost RPM, but still same speed
 

After a little research, its neither the 721 or 731. Its a grind in between them.

http://www.offshoreonly.com/forums/s...00hp+cam+specs

721 specs 214/222 .553/.576 on a 110 lobe sep

731 specs 226/234 .587/.610 on a 112 lobe sep

500hp cam crane #169611 specs 222/230 .576/.598 on a 110 lobe sep