New Boat- WOT speed, RPM and water temp questions

[Bartnich]

Finally was able to run it full out with a gps today and it’s doing 44/45 at 1000 feet elevation and tach showing around 4400. I’m going to call that good. . So it was altitude and probably prop.

[Miss Rita]

There was a comment about "needing more revs". My boat with the Indmar 351/285 horsepower will run a GPS-verified 43 mph at 4900 feet, but the tach only reads about 4200 rpm. I have the Acme 543 11.5" prop. When you do the math, there should be no way that prop can push the boat that speed at that RPM. I'm pretty certain that my tach is showing lower RPM than actual.

Just one more thing to check.

[Ski-me]

You bet man. Just remind me after the 4th so I don't forget! Will need to find a prop puller.....

[Bartnich]

i was a part of this thread last year and I still think you need to at least borrow a 13x12 or 13.11.5 ACME prop and try it.

Yes, I may take Jeff up on his generous offer to lend me a prop to see how much of a difference it makes.

[SkiDad]

i was a part of this thread last year and I still think you need to at least borrow a 13x12 or 13.11.5 ACME prop and try it.

[Bartnich]

I will definitely check the timing at some point but given no symptoms such as pinging, hard to start, sluggishness or backfiring, it seems like that might not be an issue.

Im going to check however.....

[88 PS190]

No mechanical advance the ignition module computes advance off the RPM signal.

Check that the base timing is set properly with a timing shunt to -10 at 800 rpm and then remove shunt restart engine and it should achieve appropriate peak advance.

[tryathlete]

THIS;

One thing that could make a difference is total timing advance- if you haven't checked this with the boat in gear and on the water, you may never find the cause.[/QUOTE]

I assume there is mechanical advance on your distributor. I am pretty positive you’re not getting mechanical advance due to rust in the weights and springs preventing smooth and full mechanical advance. Your distributor is very rusty. If it were mine to fix,
I’d buy a new distributor.

As mentioned above: check total advance.

[Ski-me]

^^^ I think mine hits a similar speed in Idaho around 2500' elevation.

[88 PS190]

Last 205 LT1 1:1 I was on at <1000 ft pulled a solid 46 mph.

I suspect they breath slightly better at elevation than the base TBI boats do.

[Ski-me]

Sounds cool. I'll be on my boat the after the 4th and then pull it out of the water again. Maybe I can send you my prop to try out?? At least an idea.

[Bartnich]

Actually my original was a 13 x 12-1/2. My HP is 310 and yours is 285 (I believe). But we're similar elevations.

Here is what OJ spec'd for me: 13 X 11 LC 1" XMP 3-blade is $434.00, and this is what I run today.

He said I originally had a 13 x 12, and was then re-cupped to 12-1/2.

I think it is altitude and prop pitch.

Just for fun lets do the math that may or may not be even close to reality. Pretty sure my boat is only 275 HP so the difference is about 13%. Altitude difference is about 1300 feet so assuming 3%/1000 feet power loss, that is another 4% loss.

Theoretical 17% power difference between the two scenarios. If the equation were linear I would only be able to do about 35mph but am doing 38mph (GPS) but only spinning 3800 rpm. But obviously the higher the speed, the greater amount of HP/mph needed to increase speed.

I think that may be in the ballpark. In a week I will let you know for sure....

[Ski-me]

Actually my original was a 13 x 12-1/2. My HP is 310 and yours is 285 (I believe). But we're similar elevations.

Here is what OJ spec'd for me: 13 X 11 LC 1" XMP 3-blade is $434.00, and this is what I run today.

He said I originally had a 13 x 12, and was then re-cupped to 12-1/2.

[Bartnich]

I think your RPM's need to go higher too....

Yes, two differences that may make this an apples to oranges comparison. The LT1 is a fair sized HP difference and my prop is a 13x13 and yours is a 13x12 it looks like. That should give you higher RPM, right?

[Ski-me]

I think your RPM's need to go higher too....