porting 70 johnson

[Cut n' Jump]

I have a spare powerhead in need of a ring job. It's a 1977 70hp johnson. I have some experience with porting 2 strokes and I'm an auto mechanic for a living. I'm just wanting some guidelines on port height/shape to increase power in the midrange. Durability is important and I don't want to rev it to the moon! My boat is a 15 1/2 foot trihull and 40 mph is probably as fast as the hull could handle without glass water. Right now it runs 35 and pulls a slalom ok. I still need the 'holeshot' and a faster pitch for more top speed. Also I thought about Boyeson reeds since they help dirtbikes so much on bottom end. Any opinions on those? [btw I posted here last year about a locked up rusty engine and thats the powerhead I'm running right now. That sucker runs perfect!]

 

[Dhadley]

Re: porting 70 johnson

Whats your target rpm?<br /><br />Do yourself a favor. Get the Boyesens. Kit #133.<br /><br />Good luck!

 

[Cut n' Jump]

Re: porting 70 johnson

bump- What I mean is this motor is almost like a half a v6- right? I'm sure there are people here who port v6's? The manifolds have almost no room for improvement. I would want to just clean it up and maybe alter the port height. Port height scares me because it's trial and error. I can't afford to make an error. A little more rpm's is what I'm after as long as the low end is the same or better. Sure I could make a peaky big hp gain by hogging it out and overlapping the ports but I want power all around. Does anyone have experience with this kind of mod?

 

[Dhadley]

Re: porting 70 johnson

Yes we do.<br /><br />Whats your target rpm?

 

[boating brad]

Re: porting 70 johnson

dhadley; those boyesens really make much difference? im asking because i just saw an article on em and i was wondering about them myself.

 

[Chinewalker]

Re: porting 70 johnson

You 70 has absolutely nothing in common with the V6s, save for using some of the electronics. The V6s of the era your 70 came from are cross-flow motors, while your 70 is a loop-charged motor...<br />If you want to hop up a 3-banger, start with a 75 Stinger block - much better high-end porting already cast in...<br />- Scott

 

[Dhadley]

Re: porting 70 johnson

Catfish -<br /><br />You'll like the Boyesens. Just make sure the motor does not sneeze at idle. Thats about the only thing that will kill 'em.<br /><br />Good luck!