ScarabMike

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.

John B

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

mopower

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

wwwTOPDJcom

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

KCHOTBOAT

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

Griff

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%.

ScarabMike

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?

Payton

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.

ScarabMike

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

kennyo

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.