56 ci Johnson 70 Optimal Max RPM Stock Porting

[abdrews013]

I have a Johnson 70 that has been lightly modified with Chris Carson reeds and crankcase stuffers, and the head was machined down 80 thousandths of an inch. What is the optimal maximum RPM for peak power when porting is left at stock spec?

[108]

55-6000, port it to sst 60 specs they will go well into the 7's

[powerabout]

Sst60 porting (with its custom megaphone) was for tunnel boats.
Its not great on a mono less top end.

[abdrews013]

Very interesting. I thought I could go somewhere between suggested max (6000) and rev limiter (6700) for some more power. Am I equal or losing at 6300-6400 compared to 6000? Looking to get my prop labbed, so I want to have it configured to turn an rpm level that achieves peak power.

Thanks!

[108]

Whatever it will turn your best prop at is how fast it is going to go.

We did a 56 to sst 60 spec on my buddies tin boat. Doesn't idle worth a sh*t (expected) but it turns a 21 or 22 well into the 7's

[racer]

If everything is correct meaning timing for the added compression, jetting and set up 6500

[wolfgangb]

If everything is correct meaning timing for the added compression, jetting and set up 6500

Even with these low revs, the lifespan of the top crank brg may become critical at 25 to 30 racing hours with fuel goo messing up everything under the flywheel.
wolfgang b