What to do

[gchaney]

I have a 1979 20 foot formula thunderbird with a pre alpha mercruiser drive. When I got it there was a seized up chevy 305 so I went up to a mercruiser 260 or chevy 350. The prop that came with the boat is a 17 pitch prop and it lets the engine spike the tach well over 6000. Yesterday we went and got a 21 pitch cupped prop and while we picked up about five miles an hours it still just barely pegs the tach out right at 6000. My goal is 4400 to 4800 am I going to be able to accomplish this or am I talking gear ratio change?

 

[Philster]

Re: What to do

Certify tach is correct.

If it is, you would a need an oddly large prop to knock the RPM down.... or a gear change.

 

[gchaney]

Re: What to do

Yeah I was going to recheck it today but I had a digital tach on it when I set idle and timing and it seamed pretty close.

 

[steelespike]

Re: What to do

It appears props up to 28"
are available; here at boats it is a $529 stainless.

 

[Bondo]

Re: What to do

Yeah I was going to recheck it today but I had a digital tach on it when I set idle and timing and it seamed pretty close.

Ayuh,... Bring it with ya, 'n be Sure...

Commonly found props go up to 23",... 25"ers have been offered, 'n are stocked, but not Commonly...

Any bigger, 'n it's Hi-priced SSteel specialty props...

There is the possibility yer spinnin' through 1.65:1 gears,... I doubt it's 1.84:1s, but maybe...

My 4.3/ Alpha, I'm spinnin' through a 1.65:1 drive, at 20" of prop for 4800 rpms,...
I threw on a 1.98:1 drive for S&Gs, it ran straight to Redline,+ in a heart beat, at 26" of prop....
That's both SSteel, cupped...