Can't seem to dial in 502's
 

I'm a bit confused. We just added Vortech superchargers to my 31' Scarab with 502's. Before the upgrade I was running 28's on the stock motors and Bravo I's. They were turning about 4600 RPM's and running 72 to 73 MPH on GPS.

I have upgraded the drives to XR's (same 1.50 ratio) and added the Vortechs. Now I can turn 5000RPM with 30's and get 75MPH GPS.

The boost kicks in at about 4000 RPM and the numbers reflect a significant horsepower increase.

So, what's the deal? Could there be that much slip? It is a flat hull. Scarab didn't go stepped until the following year.

I think your speedometer is not holding against the higher water pressure. This is not uncommon. Check out this table. http://go-fast.com/Prop_Slip_Calculator.htm

Re: Can't seem to dial in 502's
 

I think GPS is a key word here...no speedo or water pressure involved involved;)

dont XR's use more HP???
correct me if Im wrong, you had stock bravos before?

Re: Can't seem to dial in 502's
 

How much boost are you runninng? My ProCharger kicked in around 3 grand.

Yeah XR's use a little more HP, but not that much. Maybe an additional 20-30hp.

He should have picked up an easy 10 mph.

Your prop slip is WAY high--like 25-30%.

I have a fixed mount Garmin GPS unit that was used to mark my speed prior to the upgrade. No wI am using a handhelp GPS to track the spped because the fixed mount unit is still in storage.

I understand that the XR's use more HP, but at 5lbs of boost, the Vortecs are supposed to provide between 175 to 200 HP per motor. In additionn I swapped the stock exhause systems for Stainless marine manifolds and risers.

Once thought that I had is that once the boost kicks in is where I am experiencing the prop slip issues. Maybe moving to 32's would reduce the slip.

I'm not sure that the Pitch/RPM rule of thumb applies in this case because of the slip factor.

Or, am I missing something obvious and it's not slip at all?

Do handhelp GPS units have a maximum display speed?

How are you midrange numbers? Before boost kicks in.

On a tubocharges engine I used to have, I had to have the prop cupped quite a bit or it would blowout when boost came on. It started to build boost at 3200rpms. I also had to change to a Mirage style prop (from a Hydromotive) to get more bow lift. Lifting the bow help my prop to bite better.

My midrange is great. I am just about wher eI was before the change. I was running labbed 28's before th eupgrade and now I have Labbed 30's. They are both 4 blade Bravo I's

It seems to me that most people jump at least 4 prop sizes when they s/c an engine.Mine was running a 24p and then went to a 30 4blade.

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?:D

Re: Can't seem to dial in 502's
 

Mike, were the ECUs reprogrammed by Vortech or did the SCs come with a different ECU? The reason I ask is I am using 2 remapped ECUs from vortech with my new rebuilt 502 MPIs this year and I am having a hell of a time getting them dialed in. The IACs just don't work well with their new program although they won't admit the speed of the IAC can not be controlled by a program so idling is always a challenge now. Also the pulse width at WOT does not seem suffiecent for the additional flow, plugs show this. They also fail to tell you that after installing the adjustable press reg you still need to cut off the vacuum to the press regulator or it will indeed pull down your new added fuel pressure, even during acceleration. Bottom line make sure you've got plenty of fuel at WOT by checking the plugs and install a fuel press gage.

Re: Can't seem to dial in 502's
 

Lester,

The motors seem to be running OK. I just can't get the right setup.

I moved from Mercury Labbed Bravo I's to Standard 'Boxed' Bravo I's and got to about 79MPH. I have heard from several others that the added torque at high RPM's are causing the extra slip. Mercury removed most of the cupping in their labbing process.

I have ordered a pair of 30" Props from Darin at Houston Prop. He believes that adding some additional cup, slightly trimming the diameter and a satin finish will cut down on the slip ratio. According to my calculations, if I can cut down some slip and get my RPMs to about 5200, I should get to the low 80's.

Considering the fact that I am not dealing with a stepped hull and it's a production line boat, that seems about all I can do for it.

Re: Can't seem to dial in 502's
 

Here is a long shot for you. Do you know what size your fuel pick up tubes are in your fuel tanks? I have procharged 502s on my 31 Sonic spinning 32 bravo 1s to 5300 rpm. I had to replace the fuel tanks and after that I could only spin to 4800 rpm. Motors sounded fine. I would hit 4800, push the sticks to WOT and no gain. After 3 trips back to the mechanic we switched fuel pick ups and it was back up to 5300 rpm. My point being you may not be getting enough fuel. You need a true 1/2" tube and fittings. Hope this helps.

What yr Scarab? I would think you should be mid 80s +

Re: Can't seem to dial in 502's
 

Pure Energy,

I'll check that out. I seem to have that last bit of throttle left that doesn't seem to do anything. I've been going under the assumption that it is slip.

I should have my props soon. I'll see if there's any difference.

BTW, it's a 1997 31 Scarab. No Step. They went AVS Step with the 33 the following year.

Re: Can't seem to dial in 502's
 

What was the RPM with the standard 30? If you were still at about 5000 RPM then you should still be going 84 mph, if the slip was the same as the 28. What happens when you try the original props? Do you still stop at 5000 RPM?

These props can absorb a lot more HP than you are giving them. I don't think the extra HP is making them slip. You never said whether you went to a shorter XR drive.

Re: Can't seem to dial in 502's
 

I would check your tubes first!!! If your not getting enough fuel you may damage the motor. You should have an access panel right above the tanks and the pick ups are right there. Unscrew them and measure the tubes AND the fittings. You may have to drill the fittings larger or replace them.

Also that last little bit of throttle holds ALOT of power when you are supercharged!!!

Re: Can't seem to dial in 502's
 

The stock 30's give me 4900 RPM. The setup is rated to run at 5200 with the rev limiters bumped to 5400.

I agree, mid 80's was my target number. I am running 5lb pullys but only making 4lbs of boost at 4900 RPMs. My first instinct is that I won't see the 5 lbs until I can get the RPMs up a bit.

Darrin at Houston Prop is going to cut down the diameter of a standard Bravo 30 just a bit in an effort to get the RPM's up by about 300.

I will pull the access panel off of the tank and check that size this weekend.

Thanks for the info.

Re: Can't seem to dial in 502's
 

Good luck and post the results

Re: Can't seem to dial in 502's
 

Can you spin the 28s against the rev limiter? If you could that would be 85 mph. At the higher RPM the blower is spinning faster too and would move more air. The 28 was really too much prop before. Good luck!

Re: Can't seem to dial in 502's
 

first of all the prop is to big. second of all changing the drives automatically lost you 1.8 mph, and even more if you have low water pickup. but tomcat is right. sound like a 28 would do the job. if you wanted a bigger increase go with shorties and a pair of extender boxes. not having a stepped hull hurts so you need the props as far back as you can get them and a higher x dimension. that with the chargers would get you your 10 mph your looking for.
Treadwell

Re: Can't seem to dial in 502's
 

I haven't tried the 28's because they are also labbed from Mercury. I always suspected that the lack of cupping would just slip too much. Since I have them, there is no harm in trying them.

I have also ordered a pair of 30's from HoustonProp. He is going to cut the diameter slightly to increase the RPM's on the 30's

Re: Can't seem to dial in 502's
 

that's a start. let us know what happens.

Treadwell

Re: Can't seem to dial in 502's
 

Well, I ran with the 28's this weekend and I believe that they are too small after all. I got them up to 5200 RPMs with a full tank of gas and a crew of 3 and I still had an inch or so of throttle left. To be honest, even if 5200 was maxed out on throttles, that's too close to the 5400 RPM limiters for comfort anyway. If I break loose once or twice, I could be at those limiters way too fast.

The interesting (although not surprising) thing is that once I got to 5000RPMs, I got all the way to 5lbs of boost. The extra boost (remeber that I only had 4 lbs before) could also account for the additional RPMs.

My guess is that the 30's that Darrin is making for me should do the trick. He is going to cut the diameter down slightly in order to gain the missing 300 RPMs. If I can get the 30's to spin to 5200, which should give me the full 5lbs of boost, I should be right on target for 83 - 85 MPH.

BTW, at 5200 RPM,s the 28's ran 79.3 MPM GPS.