John B

Eric, Here is a front view. Should we fill the outer holes or continue the slots ( 2 outer or all four) toward the prop?

mmwalters

Eric are you saying that a GAIN of 5mph is possible by filling in nose pickups and adding transom pickups? I know I would be able to raise my motors. Please explain more about sanding the lower to merc specs.
Thanks Michael

mmwalters

Eric???

John B

How about this? Half as much water going in at speed and relief for excess pressure. Two extra holes if you you get into the weeds!

Boy, it looks ugly......Any thoughts?

Maybe a test after the hurricane goes by.

BenPerfected

John,
Have you had a chance to test your modified water pick-ups?
Sprague

John B

Was going today but it's raining! And rain forecast tomorrow, 55 degrees, Geezz.

Adivanman

John,
What is the point of trying to establish top speed based upon changes you have made to your drives when you have 20% leakdown on half of the cylinders in both engines? Have you considered the possibility that your loss in speed has more to do with the fact that your engines are smoked than it does with the drives? Just asking.

John B

Adivanman, It is a good question.

I had a single engine turboed boat that had a leak between two cylinders (head gasket), 30 & 60 psi compression. Top end was still there?? but it idled on 6 cylinders.

Another weird one: 38 Top Gun, ran great top end, also blown (sc), fuel restriction in tank pickup, cooked the piston #7. I had both engines replaced with modified crate 502's. The engine shop got leak down numbers from 30 to 60% on both the old motors. It took me two years to get it to run as fast as the (old leaky) 454's.



My conclusion, at 5400 rpm there is not enough time to leak enough compression to loose noticeable power! (within limits of course). What do you think?

I don't run an engine shop, but I wonder about a leak down test numbers for determining when to rebuild. And yes I certainly wish mine were better. I believe any possible loss has been gradual.

John B

Adivanman

John,

I am sorry but I do not follow your analogies and I am really not sure where you are coming from.

In your the other post, you were of the opinion that there was something wrong with the 500's that was out of the ordinary - 20% leakdown past the rings - on the center four cylinders of both motors. You were concerned about your plugs which appeared fouled and, in the cylinders with the leakdown issues, contaminated with oil. At the time, this was quite a major issue for you. Now, the leakdown issue is a non-issue - you've had worse anyway.

Good luck with your testing, hopefully you can find some of the speed you have lost. By the way, did you try a top speed run with the lowers before you modified the skegs?

John B

Adivanman,

In the two examples, the motors were rebuilt. In both cases there was no speed gain. My question is: how much horsepower may be lost with this much leakdown? My experience has been (minimal).

When I replaced the stock lowers with the same length Sportmasters, we lost 3 mph from the last test. The motors had about 2 more engine hours between the tests. On the first sportmaster test the extra two inches of skeg length had been removed. We put another 10 hours on the boat with varying loads and weather conditions, and the loss was verified.

Then we serviced the motors and found the plug problem and ring seal deal. Black spark plugs are not as expensive a problem as worn out piston rings.

You seem to be knowledgeable on motors, so I'm trying to learn cause and prevention. I didn't expect this type of problem on factory stock packages.

A couple weeks ago we reshaped and sharpened the skegs and modified the water p/u holes. We tested once and gained 2 mph back (apparently).

Sometimes I wonder why I GAF.


Thanks for your interest.