cloudmaster_321

tomorrow is the day i am going to test out my two props. I have a 23 and a 25. I have a gps and a engine analyzer for accurate tach readings, since i know my teleflex tach is a pos. Now the mercruiser sticker says 4400-4600 for max wot. I'm pretty sure i was running 4800-4850 with the 23 last year but i will find out for sure tomorrow. Just wanted to get opinions if i should keep my wot rpm's in the mercruiser range or if the actual world revs can be at the 4800 mark. I have a 90 7.4L non-mag. No hour meter installed from factory, so no idea on hours. thanks

FOUNTAINATLAST

In my opinion they are playing it pretty conservative with those low RPM's. I had a 90 454 that I did some mods to (bottom was stock from the factory) and was pulling around 5300 for two seasons with no problems. Sold the boat and have still not heard of any problems with the motor. Maybe luck but alot depends on how you abuse the motor, I only used it here and there not all day long. Just my 02

WETTE VETTE

You can turn a few more RPM's than what merc recommends. It will not hurt anything to turn 5000 or even 5300 RPM. The question how fast the boat is going. A 330 powered boat will be faster propped for 4600 than propped for 4800.

Craig

Gary Anderson

Be carefull of valves floating. I think the 7.4 with the 4200rpm redline use about 90lbs on the seat. The 365 motors use over 120 lbs with a 5200rpm, redline. I would switch to heavier springs before spinning it too high.
Gary

cloudmaster_321

well, here are the results. About a foot chop or less, 66.6mph @ approx. 4850rpm's using 23 mirage that has been repaired a million times and has blades that are super thin. 65.9mph @ approx. 4300 rpm's using newer 25 mirage, with no work done to prop. Seems like an awful big spread for rpm's but you can really here the difference in engine rpm's as well, sounds good though. Looks to me that i should be running a prop somewhere inbetween the two. Maybe get the 25 labbed or something. I think i like the way the engine pulls and tops out with the 23 for now, until i get prop work done to the 25 for next year maybe. So the question is, will i be safe running at 2850 rpm's for possible a minute or more at a shot, or even sustained periods of time, or is this too much wear and tear on the motor?

Gary Anderson

Your prop slip number are under 5%, probably unrealistic, but good I'm sure.
Go with the higher pitch prop and keep the engine reliable with the lower rpm.

Whiteknuckle

I think your RPM or Speed Indicators are not giving you an accurate reading. The math just does not support that. I think you would be better off with the 23 inch and not load that engine down as much. Twisting it to 5000+ once in a while is not going to hurt anything either. Merc is very conservative.

cloudmaster_321

well, the speed is for sure, used a gps. the tach is the unsure part, i bought a engine analyzer from sears, anolog meter. I noticed with the 23p after i hit 4200 rpms the meter would go down in revs for whatever reason. It's supposed to go up to 6500. I may have to borrow my buddies timing light that has the rpm readout again. I'm pretty sure of the 4800 with the 23p, just not sure about the 25p rpm's. Very noticeable gains in rpm in sound comparing the two. Ran all last summer with the 23 and owner before me as well. I was told to run the 25 to get more speed, and that my boat came stock but i'm not so sure about it. I like the way the 23 runs, just don't want to throw a rod or blow. Guess i will have to give bblades a call on getting my 25 to work the way i want.