Hi to all,
can anyone please suggest his point of view to the subject?

98 Scarab 22 big block engine (7.4 GI Volvo Pento 800hours) + with CMI headers + antifouling paint + 25 BBLade (4 blade high rake propller) shows poor performance (67 mph with gaffrig water pickup speedometer).

Engine compression ok.
Fuel injection system ok.

What kind of performance should i expect from this boat?
What would be the best idea of upgrade or ardjust?

Thanks in advance

My gaffrig water pickup speedo is in sync with my GPS. If you getting close to 67 I believe your doing great, especially with bottom paint. what are you expecting?

I thought 75 or something after putting the headers....
because it was doing 70mph with stock engine and without any paint bottom...

which propeller are you using?

I would say you are right on par with a 7.4.

I had a 96 22 Scarab. With a 7.4MPI LX (330 hp) I hit a best of 70.6 GPS. Put in a 502 MPI (415hp) and hit a best of 74.1 GPS. No bottom paint and running a labbed 25P Mirage Plus. The boat was as clean as they come and ran it for many years. I learned a lot driving that boat faster than I should of in rough water. A blast to drive.

I think that is pretty good, esp with bottom paint.. With out the paint I wouldn't doubt a 69-70 mph pass from it...
What rpm are you turning with that prop...??


John jr

Thanks for all the comments,

now it is turning 4250 rpm, if i try to trim up more, the propeller starts spraying but no speed up at all...
do you think i should try on a mirage 25?

What is the optimum rpm of my 7.4 gi engine at top speed?

THanks

Just adding headers without anything to the ineternals ie. cam and heads you will be getting minimal gains. The headers will help when and if you upgrade your power.

I'm running 25" Mirage, but my boat is a totally different animal compared to yours.

I believe the main problem with the boat is what ever i do the boat`s hull has too much friction in the mid part (it does not matter how much trim up the boats bow does not come up as much as it should have) it goes more like a train in the water,
what could fix this problem?

More horsepower?
Different Prop? if yes which one?
to modify the hull?
to change the transom location?

thanks in advance

I would use GPS speed as a baseline. If you are getting 67 thats pretty good. If I remember right, I got low 60's out of a 7.4. Thats with a 25 mirage+. Doug