Converting a 20 HP Johnson to 35 HP

[fix_it]

Seems like there is a lot of conficting advice on the 20 HP through 35 HP motors of the late 70s and early 80s. Some say the only difference is the carb and intake. Others say there is much more to it, such as the block, crank, or exhaust tube. I have a 1988 20 HP johnson. after reading what I could on the net and looking at the parts catalog, I thought it was worth a shot. I ordered a 35 hp used carb from ebay. I then enlarged the hole in the intake manifold to match the carb. I put it all together no problems at all. The difference in power is amazing. Before I cold barely turn a 11" prop. I had a hard time getting on plane or staying there. My top speed was low 20 MPH. Now I can turn a 13" prop with ease (and could probably turn a 15" with a light load). I can plane at half throttle. My top end is now 38 MPH per the fishfider's speed probe.

So in conclusion, by changing only the carb and porting the intake to match, I can turn a 2" larger prop and picked up almost 8 MPH top end speed. The boat feels more resposive on acceleration now too. Some will argue that this can't be done and it won't work, but I am here to say ti can and it will. I'll do it again if the chance arises.

Am I actually performing the same as a 35 HP motor? I don't know anyone with a dyno to find out, but I can tell it is A LOT faster that a 20 HP.

 

[jbjennings]

Re: Converting a 20 HP Johnson to 35 HP

Was the 38 mph a typo? If not, then that would be a 18 mph increase from the 20mph mentioned earlier in your post.
Myself, I've read that on a couple of early 80's 20hp motors you could do what you describe and get 25hp, but I didn't think there was a way to get 35hp out of a 20hp OMC motor. Heck, that's almost double the hp! I wouldn't have thought you could have gotten 25hp on an '88 model but obviously you did your research on the parts diagrams! It's neat that you were able to get it like you want it. Do you have a GPS? It'd be interesting to see what it says on your top end. It's amazing that a 35hp carb would idle right on your motor, must less run top end!

Edit: I did a little research and the part #'s for the 20hp head, crank, pistons, rods, etc. in the block appeared to be the same for the 35hp of 1988. I was suprised! Heck, maybe you do have a 35hp now?! If so, that is really interesting. I would've thought the 20-25-30 were the same, but the 35 would have a bigger block or bigger cylinders for more cubic inches. Obviously, the 35 has the same block as the 20.
More power to ya'!!!!!!
Curious,
JBJ

 

[fix_it]

Re: Converting a 20 HP Johnson to 35 HP

The first speed I stated (low 20s with the stock parts) is just an estimate. I did not have a speedometer at that time. I do know how it ran and where the water spray was on the boat, ect. I feel that that is a pretty honest estimate. Maybe 25 MPH? On the 20, the intake hole is smaller than the carb outlet. I ported it out and put the 20 carb back on for a while. This should be a 25 hp combo. Anyway, this is when I got the fishfinder with the speedometer. I know they are notoriously wrong, but At the "25 HP" level, I could get 32 MPH. After I put on the 35 carb it went to 38 MPH. I wish I had a GPS to verify that. But, even if they are wrong, they prove that it is indeed faster with the 35 carb. As far as running right, it idles BETTER with the big carb than the little one. Don't ask me why cause I can't explain that one. The little one was clean as a whistle, So that wasn't it.

 

[wbeaton]

Re: Converting a 20 HP Johnson to 35 HP

Enlarging the intake port to the same size as the carb should gain you 5 hp on that particular motor. Changing the carb to the larger 30-35 hp may get you an additional 5 hp, but I think there is also something different in the exhaust, but I'm not sure. Its been awhile since I looked into this. There is a great webpage that had everything you needed to know about increasing the hp in the 1980's 20-35 hp OMC's. If I can find it I'll post it.

I replaced my 1987 20 hp with a 1988 30 hp and went from barely planing with two guys at around 28 kmh / 17 mph to 42 kmh / 26 mph with the 30 hp. I use GPS to determine speed. I wouldn't rely on a fishfinder speedo.

 

[rolmops]

Re: Converting a 20 HP Johnson to 35 HP

The problem you may encounter is the very high RPM that your engine is running now.
They were made to go at a WOT of 4500 to 5500.Where is yours at?

 

[fix_it]

Re: Converting a 20 HP Johnson to 35 HP

I don't know what the RPM is. I wish I had a tach to verify. I had a multimeter with the clamp on tach, but someone thought they needed it more than I did! Anyway, I did mention that I changed the prop for 11" to 13" at the same time I changed the carb. That is where the speed comes from. Before the motor could not turn the 13" prop. It would bog. Now it turns it with ease. The motor sounds the same at 38 MPH with the 13" as it did at 30 MPH with the 11". The motor does not sound like it is running any faster than any other similar motor. I am aware that the fishfinder may be inacurate, so maybe the speed reading is a little off, but comparing before and after numbers with the same speedometer should verify the increase. I will try to get my hands on a GPS to verify the speed reading.

All I know is that whatever the speed actually is, the performance is greatly improved. You would not need a speedometer to tell the difference.

 

[kynolan2183]

Re: Converting a 20 HP Johnson to 35 HP

Completely possible. Nothing different in the exaust. I did it on a 1984 20hp to a 30hp. On my 13 pitch prop I went from 5300 rpm to over 6300. Had to move to a 15 pitch to get rpm to 6k.

Boat topped out at 37 MPH by GPS Max theoretical speed was 41
The boat would scoot. I ran off and left many heavily load runabouts on lake Degrey. But very careful and always wear your kill switch.

My 14x36 ran basically on the prop.
Sold it to a young man from Rodgers. Be very careful.




Kyle

 

[zpd307]

Re: Converting a 20 HP Johnson to 35 HP

the powerhead is the exact same for 20-35 hps. intake and carb are pretty much the only difference. even the exhaust tube was the same. the only other part with a different # was the carb throttle cam, but when i eyeballed them, they looked pretty much the same. when i rebuilt my 88 25, i did the same research. look at brp.com and compare part #s.

 

[UK Vintage]

Re: Converting a 20 HP Johnson to 35 HP

Yeah I once changed the carb on my 1 litre car to that of a 1.1 litre and it went faster!

 

[tmcalavy]

Re: Converting a 20 HP Johnson to 35 HP

Would be a very good idea to get a tach on there, so you can monitor rpms.

 

[Chinewalker]

Re: Converting a 20 HP Johnson to 35 HP

One other significant difference between them is the clamp brackets. The 30/35s have heavier clamp assemblies. The 30/35 clamps run deeper on the transom and sit wider.

Keep in mind, 1986 and later, they did away with the 31-cube 35 and renamed it the 30, as they were now rating at the prop, rather than the powerhead. Same motor, just rated differently. (The 1986 35hp, was actually a de-tuned 44-cube looper...)
- Scott

 

[G DANE]

Re: Converting a 20 HP Johnson to 35 HP

Well there are differences. The 20 is apart from the 25 by a different cylinderhead, lowering compression by 10-15% and a carb with a non-removable nylon restrictor, and a different intake. The 25 is apart from the 35, by again a different carb on some years, a different exhausttuning and thru-hub exhaust. The easiest conversion is 20-25, which is very easy. You have to get a trashed 35 to get the lower parts reasonable. Be aware that doubling speed normally takes 4 times the horse-power. The reason you see a significant improval is if you run a boat that is close to planning start at max speed. If it gets well boyond planing, it will take off.

 

[G DANE]

Re: Converting a 20 HP Johnson to 35 HP

Besides from that, when the boat is planing, the motor gets up in the ideal end of the powerband, and puts out the horse-power it really has. A better pitched prob would have given you more of the 20, though it is RPM restricted in the carb.

 

[Rosco 11]

Re: Converting a 20 HP Johnson to 35 HP

An interesting topic indeed.
I do not wish to hijack the thread, but I would like to say G'day to G Dane.
Dane, it's Rosco here from Australia. Rosco of the 35hp '76 Johnson with the dodgy condensors. .
How are you mate ? . . . I see you are still helping folks out on the forum. Good on you.
I hope you are well. Take care, hope to chat sometime soon.

 

[G DANE]

Re: Converting a 20 HP Johnson to 35 HP

Hi Rosco good to hear you are fine. I've actually been down under twice since we met first time. In match 07, to see how my son was doing, after he moved to Melbourne i 06. And believe it, when we were there, we found out he got him self a girlfriend. They married in march this year, so we had to go once again. We went all the way up to Frasier Island in an autocamper on the first visit, man you have a beautifull country.

 

[Tolly family]

Re: Converting a 20 HP Johnson to 35 HP

So where does my 1988 28 SPL fit in here? Is it really a 30? Can I just add a 35 carb?

Thanks,

Dan

 

[tmcalavy]

Re: Converting a 20 HP Johnson to 35 HP

How did you enlarge the intake for the bigger carb throat?

 

[Tinman57]

Re: Converting a 20 HP Johnson to 35 HP

I had a 1986 Evinrude 20, which a dealer converted to a 25 for me. The only change was the intake manifold and carb. The increase in power was dramatic, and I ran it as a 25 for 12 years. That motor was bulletproof, and served me well for nearly 2000 hours of saltwater use. The guy I sold it to is still using it 3 times a week. Truly an outstanding motor.

 

[G DANE]

Re: Converting a 20 HP Johnson to 35 HP

I stand corrected here. I just had a 1986 apart and the only difference between 25 and 30 was the carb. And the clamps to the transom, of coarse.

 

[Chinewalker]

Re: Converting a 20 HP Johnson to 35 HP

The 28SPL was kind of an odd duck of parts. It had the 30hp (same as older 35) powerhead and the early 25hp lower unit (exhaust above the prop, not through-hub). The lower unit acts as a bit of a restriction compared to the through-hub, which is why it was difficult to make the older 25hp into a 35hp. Then they standardized the lower units with the through-hub on all of the 33 cubers (except for the 28SPL) then it made it easier to convert within the family. To get the most out of a 28SPL you'd have to convert it to a through-hub unit, which is impractical as you'd be basically putting the 28SPL powerhead into a different chassis - not much of a conversion, more like a complete transplant....
- Scott