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1989 Johnson 20 hp to 30 hp conversion

G

geezer1

Contributing Member
I've decided to convert my 1989 Johnson 20 hp to 30 hp by swapping out the intake manifold and carburetor. The 30 hp intake manifold is missing the hose nipple, which should be substantially larger than the existing 20 hp nipple so I can't use that one. I've researched and can't find a part number for the 30 hp nipple anywhere. I suppose I can drill and tap the intake manifold if I know what size nipple I need. Anyone have any ideas as to what I should do?
 
Pappy is just trying to save you some trouble, brother. Your motor won't perform with enough improvement to make your efforts worthwhile. You asked "what you should do"........and that was his suggestion. You will end up dumping too much fuel and air into a crank chamber that cannot effectively turn it into horsepower. You could look into a used block.....I might have one.
 
Yeah, I get all that. I opened myself up to that by putting all that conversion information out there instead of just asking about the nipple size. The "what should I do" regarded the missing hose nipple, not the entire project. But because the thread has gone this direction, I'll probably never get the answer to my original question. That's what frustrated me.
 
When you finish the project, please let us know what you did and how it performs. I am not against making certain modifications to improve function based on personal needs. I've been doing that for 50 years as a mechanic and body man.
 
Haven't actually decided whether I'll do this mod or not. But if I do, I will post the results here. Thx to you and faztbullet for your inputs.
 
Okay, good luck and take care. We are always learning, but the big problem is that when we've accumulated all the information and experience and we reach that "golden age".........we usually can't do anything but "talk" about it.
 
Now here's an oddity... The 1989 omc 20 hp motor (J20ECEM:. my motor) cyl & c'case is P/N 432540. The 30 hp of that same ilk is P/N 396165. That pretty well validated, in my mind, what faztbullet said about porting. But, on further review, the same year 30 hp with tiller uses the same cyl & c'case as the 20 hp remote motor (J20ECEM). I wonder if the only difference is block bosses?
 
The conversion to a higher hp?

I remember how it was done with the automobile in the late 1930's, early 40's.... jacking the radiator cap up and...... ............... Who else remembers that?
 
joereeves said:
The conversion to a higher hp?

I remember how it was done with the automobile in the late 1930's, early 40's.... jacking the radiator cap up and...... ............... Who else remembers that?
Click to expand...

Not that far back, but... In 1967 I dropped a 327 in a '55 Chevy. Loved that car.
 
Love 'em those 327's, big bore, short stroke, you can't blow them up. I still build them here. Just put a 307 crank in a 350 block. Good thing I collected all those 307's when I could. The earlier 327's were small journal cranks, you can't build a small journal anymore.......unless you find one. They were steel cranks too, not iron cranks, way stronger for their smaller journal size. LOVE those Chevy "shoe boxes", brother.
Very interesting on the 20/30's. I never looked into them like you did. Let us know what you find, eh?
 
timguy said:
Love 'em those 327's, big bore, short stroke, you can't blow them up. I still build them here. Just put a 307 crank in a 350 block. Good thing I collected all those 307's when I could. The earlier 327's were small journal cranks, you can't build a small journal anymore.......unless you find one. They were steel cranks too, not iron cranks, way stronger for their smaller journal size. LOVE those Chevy "shoe boxes", brother.
Very interesting on the 20/30's. I never looked into them like you did. Let us know what you find, eh?
Click to expand...
Ahh, to be young again... I sent you a PM timguy.
 
Likely just a different carb advance curve to iron out a tiny flat spot in acceleration/operation. I have noticed this on many 9.9 to 15 conversions, besides just the larger carb.
 
Look up the block part # on BRP parts site. If the numbers are the same for 20/30 block it will work. If part number is different the block has differences to make it a 30(porting)
 
faztbullet said:
Look up the block part # on BRP parts site. If the numbers are the same for 20/30 block it will work. If part number is different the block has differences to make it a 30(porting)
Click to expand...
By "block" are you referring to cyl & crankcase? If so, please see my earlier post (#10).
 
That 30 hp intake manifold with a missing hose nipple turned out to be a 20 hp intake manifold. Returned it and got my $ back, but now I'm on the hunt for a 30 hp intake manifold, specifically, P/N 396790. Hope I can find one before too long, I'd like to get this project going.
 
Hi Geezer1. Wondering what came of your conversion project? I have an ‘89 evinrude 20hp which I have modded to increase hp. Sharing my experience and how I went about it and if you are still entertaining the idea I hope this may help you and others looking for more hp out of these power heads.

There are a few modifications I have come across that can be made individually or in unison to increase the hp of these engines; upgrading the carburetor (increase in throat size), increasing jet sizes (more fuel/air) and upgrading the intake manifold (better flow/greater volume). The most telling information out there is a comparison of part numbers between the different hp motors of the same year. In my case I made one major modification which immediately increased my hp. Boring and porting the intake manifold.

Its been my observation with my motor that Evinrude/Johnson accomplished 20hp out of a power head designed for up to 35 hp by integrating a restricter plate in the intake manifold. Restricting air to zap power but with the same fuel consumption as a greater hp motor. Yes the plate accomplished 20 hp but my motor always ran rich regardless of fuel adjustments. I decided my first phase of mods would be to remove the plate.

Using the gasket between the carb and manifold as a template I made a jig to remove the plate with a hole saw. With the intake bored I then used a dremal to open up and smooth out the “inside” of the intake. (I haven’t fully increased the volume of the intake… yet). Once thoroughly cleaned, bolted back together and some minor fuel adjustments made my 20 runs much smoother, leaner, has more power and increased rpm at WOT! On my boat (according to GPS SOG) 1 hp is about equal to 1 mph. Before I made this mod I was running between 18 and 21 mph. After the mod I am running 23 to 26 mph. Definitely worth the time and pucker factor in terms of fuel economy, speed and enjoyment on the water.

Currently I am satisfied with my motors performance but in the future when time allows and my heart desires more power I plan on increasing jet sizes, increasing the volume of the intake manifolds, upgrading the carburetor and finally play around with props.

I have pics of the process but can’t seem to upload them.

I hope this helps!
 
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The conversion works like a charm. Had a lot of fun with this...once I was able to procure the right parts. Ended up finding a 30hp intake manifold and carb. Rebuilt the carb with recommended jets, and replaced the throttle control cam (all as per the p/n's listed for J30RCED at marineengine.com). With a 13P prop I was running 29 mph wide open. Because I never ran this motor as a 20hp on this boat, I had no real base to compare with. However...I sold the motor 2 months ago and bought a Tohatsu 30hp 4 stroke. With a 13P prop this motor runs 30 mph (speed is at recommended max RPM for each motor). I was impressed with the converted 30 performance anyway, but this told me it was justifiable. BTW, I didn't sell the old 30 because I didn't like it, I wanted to upgrade to a 4 stroke. Part of me wanted to keep the old girl, but I had nothing to put her on, so...
 
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