ScarabMike, I am curious about how it is running, like at idle and off idle. It sounds like there is something set wrong, like timing or bad plug. I just installed a procharger on a friends 28ft Nordic and had a similar problem. It ran a little faster, but didn't sound quit right. The boat kind of vibrated at an idle and didn't have a crisp sound. We checked plugs and found a cracked plug so we thought the problem was fixed. changed it and still had the same problem. Upon further exploring I discovered that an injector plug had worked loose because the wire hadn't locked on to the injector. Problem solved. It runs great now.
My point is that reading the plugs will show a lot about what is happening. Not just one plug, but all of them. They should all look the same, basically they should be mostly white and black around the threads with a heat line on the ground electrode. Start with the basics; plugs, plug wire order, cracked cap, timing and max timing. You probably won't notice an improvement with a 32 because the 502 should be able to run up around 5500rpm with out any problem, but once you find the problem I think you will need the 32P Bravos. Don't stop looking for a problem, because you should notice a Hugh increase and you will never be happy until it is running the way it supposed to. The XR drives are not your problem either unless there is something wrong with one of them. Are these MPI or carbed.

Before we talk about what might be wrong, could you please confirm your RPM and speed numbers. The 28 Bravo numbers work out to about 10% slip. The 30 Bravo numbers work out to about 20% slip.

That's a huge change. I would double check these results with the use same GPS to eliminate this variable. The pitch/RPM rule of thumb would indicate a speed of 85 mph using the same slip% as the 28 Bravo.

I don't see why the 30 Bravo would slip much more than the 28 Bravo, unless your XRs are shorter drives. And if they are shorter drives and the props are slipping you should be revving much higher. Are you hitting the rev limiter?

What is your boost at 4000 RPM and at 5000 RPM? What is the upper pulley diameter?

If these are carb engines, what was done to the carbs?

Mike, Sorry I missed the GPS numbers. Have you got a rev limiter? Could you have spun the hubs on the 30's? How much weight did you have on board while testing the 30's? Where was the trim set on your tests? Do you have shorty drives now?

ScarabMike,
If you don't mind me asking, How much did it cost you per side to upgrade to the XR Drives? I'm thinking about doing the same thing but haven't looked into the cost yet. Thanks

In my 502 mpi procharged. The guy I bought the boat from pulled the thermostat out. The computer was running in cold mode the entire time. Ran way to rich and lack of power. Cold mode from the factory shut off at 140. I did not see if yours was injected though.

28Eliminator: The motors sound good at idle and WOT. Although it could be a setup issue, I have twin 502's that both run the same. It would seem odd to have the same problem twice. The motors are MPI and the fule system has been modified according to specs. I'm only running 5lbs, so nothing major has been done. Only the additional fuel pump, etc. The drives were replaced with the same size XR's.

Tomcat: My actual numbers work out near 30% slip! Could the aLabbed Bravo props be that inefficient at top-end with the extra boost? It's possible that whenthe boost kicks in te slip rate goes through the roof. Maybe a different prop would perform better.

John B. The XR's use a solid hub, they can't spinout (as far as I know).

Fountaineer: I traded my working Bravo's in for the XR's through an ad in the OSO classifieds for Stern Drive Specialists in Canada. With trade-in it cost about $4,000.00 per drive. Much less expensive than replacing them outright or waiting until I blew a drive.

Like BLOWNBOAT stated, did they have you pull out the thermostat and if so did you have the ECU reprogrammed. If there is no thermostat and the ecu hasn't been reprogrammed, then it is burning way too rich robbing you of HP. Also the rev limiter is set to 5150 I believe from Merc. and maybe the you are hitting it, but I would think you would notice that.
Please take a look at the plugs and let us know how they look.
Are the props new, or is there maybe something wrong with them. Maybe trying the 28s would be a good place to start. Something just doesn't sound right. Even if you were running the 28 props and ran 5000rpms you would be getting 78mph assuming the same slip factor minus the loss of the XR drives, but at 5lb of boost you should go way faster than that. Something is wrong.

The ECU was re-programmed and the thermostat was changed as well. They raised th erev limiters to 5200, I think.

The props are brand new. I purchased them directly from Mercury last season along with the 28's. At that time, I wasn't sure which size I needed. The 28's worked better on the stock setup but I kept the 30's because I knew that in the offseason I was adding the SC's. My estimation was 32's also, but since I had the 30's, I had to try them.

I also wonder if the boost should kick in earlier than about 4000 RPMs. It's good for the drives, etc at 4000, but maybe the sudden burst of torque at high RPMs is causing the props to break loose causing the slip.

I will be near the boat again over the weekend and I'll do some more testing. I was really just trying to see if I may be missing something very obvious. The numbers just don't make sense.

I agree, more testing is needed. At 27% slip, something is wrong. At 12% slip you would be around 84mph, but I think you will need the 32p and being conservative at 15% slip you should be close to 90 and I know that's what you were looking for. Good luck and keep us posted!

If you bought the Vortech kit for the 502 Mag MPI (the black engine) then it is rated at 570 HP @ 5300 RPM. It generates about 5 psi boost with the stock exhaust manifolds. This is a low boost kit that allows you to keep the stock injectors and fuel system. The rev limit is raised to 5400 in the reprogrammed ECU. At 5000 RPM this kit produces 550 HP, about 130 HP more than the stock engine. This should be worth at least 10 mph. The 30 Bravo would be my pick for the right prop.

Did your boat gain 1000 lbs over the winter?