Last year i had a very tired 502 with water getting into at least 2 cyl. and about 85% leak down rate on at least 1 of them,the motor was in my 25 checkmate, i was runing a 26 4 blade'er and turning about 5200 rpm at about 55 mph( i know but thats what the tach said) this year we rebuilt it and made it a 540,merlin heads,comp roller hyd. cam, now the tach says 5400 to 5600 and speedo says about 70,does this seem right, a 15 1/4 x 26 bravo 1 4 blade at 5400 and only doing 68 to 70.Also going from 5200 at 55mph to 5400 to 70 ????

kenny, boat speedo's suck. check your speed with a gps. your tack sounds off also. if your swinging the same prop as last year and only picked up a couple hundred rpm's, that wouldn't give you 15 mph.

Thats what i thought 2 to 4 hundred rpm is not 15 mph but im turning the same wheel, same boat, same tach just went up in hp.I was told bu checkmate that this boat with a stock 502 with 390 hp should run 68 to 70. This motor has close to 600 hp in it and im only going 68. But what gets me is the rpm only picked up about 2 to 4 hundred.My next step i guess is to check out the tach.

Kenny, Id try out a Hydromotive 4 blade prop on that Check also. Start with a 25 pitch. Your drive has 1.5 gears correct? If it's 1.36 that may be part of the problem. What year is your 25? I have an 89 251.

Eric

Hi Eric my checkmate is a 88 and the gear ratio is 1.50, but i just came back from testing the tach. If the tach i was using is right then at wide open the stock tach says 5400, the test tach said 46 to 4800.Now if the test tach is right man do i feel like a jerk.Now if it is right and i drop down to a 25 pitch to pick up rpm and get them into the 5000 range would i pick up speed.
The test tach was a autometer sport comp. that we took out of my brother's drag car. I should work fine with the stock thunder bolt ign. in the boat.

O.K. guys; First we need a starting point that we can trust. We can start with a tach that we trust or a GPS for speed. Then use Merc's "Speed/Power/Propeller Slip Calculator" (Merc pt.# C-90-86147) "V" hulls like most of ours, (and you cat guys blow us away here) are doing very well with 10 to 15% slip. For example: A big block turning an honest 5200 RPM through a 1.5/1 drive with a 27"Mirage+ with 12% slip (and that would be a REAL CLEAN running "V" hull") should show you a true 80 MPH on a GPS. But whatever, The slipfactor is the variable. Forget trying to get a direct proportion between HP and MPH. --- Jer