'73 johnson 135 questions about producing more HP

[monk-monk]

I have some questions about the '73 135 Johnson. Where did Johnson measure the HP from, the crankshaft or at the prop?
Next, I was reading some forums about getting extra HP from the 135, one topic mentioned about cutting out the exhaust divider in the bottom of the block, then it went on to say about adding filler blocks in the intake manifold and to install carbs from a 140 hp motor. The whole point here: can a person squeeze more HP out of this motor?

 

[metriccrescentwrench]

Re: '73 johnson 135 questions about producing more HP

On a '73 the power was most definitely measured at the crank and not the prop. I know there's never exact agreement on how much power is lost between the crankshaft and prop, but I asked some experts that question and they said it's usually around 7% (which is less than most people claim).

As far as getting more power from an engine, in general the easiest way is to sell your engine and use the funds to buy the engine you really want. There are a handful of instances where there's an easy swap that will give you a reasonable gain, but I haven't heard of all that many.

Did that guy that told you about cutting out the exhaust divider actually do it and measure power on a dyno both before and after, or was it just hearsay?

Also, I'd bet that it's going to be tough to find some parts for an engine that old, even on an OMC.

 

[racerone]

Re: '73 johnson 135 questions about producing more HP

The 1973 motor is " unique "--------What that means is no internal parts from a 1972 motor fit and none of the internal parts from a 1974 fit.-----------Your best bet is to look for a V-6 150 HP.

 

[monk-monk]

Re: '73 johnson 135 questions about producing more HP

Thanks for the replies! Actually, I just wanted to "run this up the flagpole" to see what others opinions are. Me personally, I wouldn't cut that section of the block out even if it meant gaining 20hp...that rib is there for structural integrity/strength for the bottom lip of the block. As for finding parts for the '73 135, I have no problems with that, I have good connections for both NOS and used parts at very reasonable prices! I have come to learn, with 2 stroke outboards, there is very little you can do to make more HP....

 

[Chinewalker]

Re: '73 johnson 135 questions about producing more HP

The 135 is already pretty well maxed out and already has the divider cut and exhaust mods done. That's what they did to up it from 125 from the year before...

 

[emdsapmgr]

Re: '73 johnson 135 questions about producing more HP

You have been given some good advice. While true, the 135 is pretty much maxed out on factory hp-those additions you mentioned in your first post will add some hp. Just not much. The V4 140 made 5 hp more to a large extent because of the exhaust cutout at the bottom of the exhaust chest. See pic on this modified 135 block. You probably will get 3-5 more hp by adding the rubber intake filler blocks from a later 115/140. Also, why not consider squaring off (enlarging) the intake/exhaust ports. Your engine uses the 1 5/16" carbs. Yes, some 140's did use 1 3/8" carbs and you could try them. Your hole shot will suffer, but you may get it to breathe better at 5500. You'll probably have to spend some time rejetting them to get them to work to your satisfaction. It's already got some really tight heads, so keep them.