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.