hblair

I've got an old Scarab 300 I just basically restored. I bought the boat last September. At that time the boat would run 53-55mph at 3900 rpm.

Between then and now, while the boat was restored the bottom was refinished (had rough bottom paint), drives repaired & refinished (skegs were bent up somewhat).

The boat has the old 330hp motors. We replaced the cams with lunati 10110702 and went with heavier valve springs, heads reworked and several valves replaced(oval port heads) added some edlelbrock air gap intakes, and took off the quadrajets and added edelbrock 750 carbs. Boat has the original exhaust manifolds.(I took out the captains call and ran stright thru hull) I plan to upgrade more later.

The boat has the quicksilver mirage 21 pitch props.

We took the boat out and it now does the same top end performance, 53-55mph @ 3900rpm.

We swapped the quadrajets back and the performance is the same top end wise.

The total timing is supposed to be 36 degrees. The engine builder set them to 14 degrees and the distributors are supposed to advance 20-22 mechanically.

If my boat is right I know I need to go with a little less pitch.

I would have thought I'd get a little more top end speed out of this deal.


Thoughts? Thanks for any input.

(My bad, 21 pitch props not 27 )

SFOcean

iTrader: (1)

My guess is that you need to upgrade your exhaust.

markt111

need to run a 19* prop to get the RPMs up into the power range of the motors, the engines most likely make similar power at 3900 as the old, the power comes in up higher

hblair

Thanks for the input SFOcean. I plan to upgrade the exhaust, prolly this winter. But I wonder what the top end rpm was on these boats originally. I would guess 55mph would be about top end speed, but surely 3900 wasn't top end rpm? What do you think? I wonder what degree of pitch prop these came with new, and if maybe someone "upgraded" the prop with one too agressive for the engines...

hblair

Thanks markt111, you replied while I was typing mine, That's kind of what I was thinking.

anotherbane

good, the engines most likely make similar power at 3900 as the old, the power comes in up higher thanks

hblair

Is there a particular prop brand or prop exchange company you guys recommend?

sabo

iTrader: (2)

what markt111 said, and stock 330 hp enignes are labbeled max rpm @ w.o.t. 4200-4400 rpm, you need to be at least 4,500 and up with that cam.

SB

iTrader: (1)

Yup, 4600-4800rpm should do it with those cams.

Beg/Borrow to use someone's props that are next size down and go from there.

Make sure your fuel supply rigging is not capping your rpms either. Fuel tank pick-ups/anti siphon valves, fuel pumps, etc.

dereknkathy

my 30 IMP twin 454 330 hp. pushed 25 in merc props to 4500 and near 70 mph. with 1000 hours on the engines. (FWC). the IMP is essentially a stretched 24-7 pantera-ish hull and pretty fast in it's own right, but I would think you could get better than that out of those motors in the scarab. I would verify what total timing is, not what it is supposed to be. a 19 inch prop is a 50 mph prop.