jasonpg1300

i have a magnum 27 sport with two 383 strokers with trs out drives turning 4,000 rpm which props should i go with to get the rpm up on the motor like to see around 5,000 i have 141/2 x 21 mirage props on there now

A.O. Razor

Do you have some info on gps speed and gear ratio?

If your numbers, slip ect. with the 21* are good, then to go from 4000 to 5000 you will need a 17" prop, if it's a Mirage+ you put on.

jasonpg1300

iam new at this gps is aroung 58 to 59 i do not now the gear ratio how do you find that out what is a good prop to go with thanks for the reply

Biggus

iTrader: (1)

All TRS are 1.5:1 ratio. rpm sounds too low for 21 pitch props with 383's. Could both motors be down on power? Have you checked total timing? Throttle blades fully open at wot?

A.O. Razor

If the gear ratio is 1.5, somethings way off. Either it's your tach, your prop or gps.

58-59 is impossible with a 21" prop @ 4000 rpms. It gives you a negative slip between 9% and 11%. A negative slip does not exist. My guess is that, either your props are not 21", or your tachs are not working right.

bbladesprops

If the props were 25's this would make more sense.

Brett

jasonpg1300

hi thanks for the reply the motor has been timed and the carbs are opening all the way the motors run good it takes off good just don't do anything past 4,000 rpm i have two tachs for each motors i tried two different gps

A.O. Razor

Then the next step will be to bolt on a set of new 21" stock props and get a baseline number. As stated before, 58-59 with 21" props, 1,5 gears @ 4000 rpms, can never be done. It would simply defy the laws of physics. With 10% slip, 21" props will run about 48 mph @ 4000 rpms. You are dead sure your tachs are right? They are not wired wrong in any way?

MagMark

I have 400's in my 27 and am turning 5200-5400 with 25" mirages.
When we installed the engines my "buddy" crossed 6&8 plug wires on BOTH motors. We couldn't turn over 4500 with 24" mirages.

We pulled both motors and pulled one head and discovered that 6&8 never fired.

Look for something stupid and obvious.

Boat was running 80+ gps @ 5400 until we toasted a lower
Now saving up for konrads!