New EFI engine Need to match prop

[miktwalker]

Hello out there. Need some advice for correct prop. New 5.7 300 HP (240HP here at 5000') with an Alpha One gen 2. Currently have a 14" 19P Stiletto 3 blade Stainless prop. With the new motor and Holley HP EFI system WOT average after 4 WOT trimmed runs yesterday is 5000 RPM. Measured by my laptop plugged into the ECU. Factory dash tack read 5200RPM . Original Mercruiser 5.7 210 HP 2 BBL manual says max WOT is 4200-4600. Prop shop recommends Solas 14" 19P 4 blade stainless should bring down RPM 200. Says 4700 RPM OK even though over max manual 4600 RPM. Engine builder says motor can take 6300 RPM. Questions are:

1. What is the limiting rpm factor Mercruiser publishes in their manuals? Engine? Drive? or combination considering the factory prop specs?

2. If I switch to a 4 blade prop (from the 3 I have now) should I go up in pitch to a 14 20 to bring the RPM down to 4600 (extra blade worth 200 RPM 1 inch pitch worth 200 RPM)?

3. With the extra HP upgrade, and going to a 4 blade 14 20P, would I be placing too much torque on the drive?

Thanks for reading my post. Looking for input.

 

[Maclin]

Question #1, Both! but yes the drive is not meant to stay at 5 or above.

Question#2, I think an increase in pitch will be needed even with going to more blades.

Question#3, Alphas are ok to 300HP I think is the consensus I have seen here. Going higher in pitch always puts more torque load on the drive at the lower RPM's when hammering the throttle, but your build is probably still at or just under what it can take. Big blocks can hit it pretty hard with their inherent low end torque even with a plug wire off, but I think you are ok.

Do you ever boat in lower elevations?

 

[miktwalker]

Maclin, Thanks for your thoughts. Yes we do occasionally get to 3000 MSL locations so I would expect a little more RPM there. Ill just have to keep it within limits. I can change the rev limits easily with my laptop. I think ill get a 4 blade 14 20P and set the rev limiter at 4900 RPM.