Revving the engine with no load.

[rs2k]

What's the chance I will hurt something if I rev the Chevy 350 engine on my boat up to 4400 RPM? I'm getting some strange results while trying to reprop my cabin cruiser and I want to verify that my engines can do better than 3850 RPM. The engine is hooked up to an OMC stringer.

I only intend to rev the engine long enough to verify an RPM of at least 4400 RPM on the tach. I've revved the engine to 3000 to verfiy the tach was advancing correctly, but another 1400 RPM is a whole lot more force on the out drive.

 

[Don S]

Re: Revving the engine with no load.

Your engine will do 5000 rpm with half the plug wires off, It's a non test and will tell you nothing.

 

[Howard Sterndrive]

Re: Revving the engine with no load.

but another 1400 RPM is a whole lot more force on the out drive.

In neutral, at 4400 rpm, there is less than 5HP transmitted by the outdrive.

The issue is two fold:
1. lack of combustion pressures pushing the pistons down whilst valves are getting ready to float.
If you have a stack up of tolerances in the rod bearing, piston pin, piston etc. there is a risk of contact.
2. Water flow whilst on muffs

 

[rs2k]

Re: Revving the engine with no load.

What do you think about this:

I changed from a 3 blade 22' SST prop to a 4 blade 19' aluminum prop and the WOT RPM and top speed stayed the same. I then added 1000 lbs of weight to the same boat (but different day) and the WOT RPM and top speed still remained the same.

It's very odd to me that all three examples produced 3850 RPM WOT at 44 - 45 MPH. I'm trying to find a prop that will get the engines into the RPM range they need to go. I'm finding it hard to find what kind of prop I need to go with if I can't get the engines or boat to observe the laws of physics.

The tachs have been verified as accurate within 25 RPM using a hand held tach while underway at multiple points along the RPM range. The speed was observed using GPS.

Both engines are almost the same RPM at full throttle. You can hear they are almost in sync. They sync better than an airplane with both engines at full throttle.

 

[rs2k]

Re: Revving the engine with no load.

If you have a stack up of tolerances in the rod bearing, piston pin, piston etc. there is a risk of contact.

Yuck! This test is definitely not worth frying an engine over!

 

[Don S]

Re: Revving the engine with no load.

IS this boat new to you? Have you tried bringing the trim up??? You could be just plowing through the water and not actually planing. That alone would keep the rpms down on both engines, speed would be low too.

 

[rs2k]

Re: Revving the engine with no load.

Trimming makes about a 5 MPH difference. 40 MPH when full down or full up. 44-45 MPH when trimmed at about the halfway point of travel.

This is a big heavy boat. I'm very happy with 45 MPH. I just am looking to get the engines into the proper WOT range. I'm don't want to cause damage by lugging them. I noticed with the extra 1000 lbs of weight it took nearly twice the amount of time to do a hole shot, but what really makes me wonder is the WOT RPM and speed remain unchanged. I'm almost afraid of seeing the same speed and RPM when I try a 17" prop... Well at least the same RPM. I'm already showing 8% slip with the 4 blade 19" props.

 

[Howard Sterndrive]

Re: Revving the engine with no load.

IS this boat new to you?

needs to be answered.
Have you ever observed what you are aiming for?
Maybe your hull just really doesn't want to go over 45...

Did it come with the 22P SS props? That's bass boat steep.

I think std. 19P 3 blade aluminum would be the cheapest to try.
And just stick to standard, normal props....
Change pitch, but don't change:
manuf./pitch/material/#blades
all in one shot - that's changing 4 variables at once- you don't know what's making a difference one way and what's making a difference the other way