Converting a 20 HP Johnson to 35 HP

[fish56]

Re: Converting a 20 HP Johnson to 35 HP

This is a very interesting post. Kind of reminds me of hopping up an old Mopar car engine, match porting the intake to the heads, adding headers, bigger carb, etc...

Great replies!

 

[fix_it]

Re: Converting a 20 HP Johnson to 35 HP

To G DANE:

There may well be differences in the older 20s. I do not know. Mine was a through hub like the 1986 you just worked on. As far as the clamps on the transom, that would make sense, but I did not know that. It doesn't matter much in my case as I bolt the motor to the boat. Almost lost it in the river once. Don't want that to happen again. I probably would have dropped it if there wasn't a passenger to help me. I tried to turn left and the motor twisted off the transom, pulling the throttle wide open in my hand at the same time. When it did this it also lifted the prop out of the water. I couldn't turn loose of the throttle as that was the only thing keeping the motor on the boat. Luckily I was able to push the kill button (overboard switch was disconnected by previous owner) before it blew up.

Also as far as the prop, I tried a lower pitch prop and it helped greatly, but you could tell that I was still lacking. When I would get on plane, the motor sounded like it was turning close to max RPM. But it was a great struggle to get on plane, as you would need to move weight around and it you hit some small chop it would fall off plane very easily. I didn't want to risk going lower on pitch, and besides that would trade speed for planing ability. I wanted both.


To tmcalavy:

RE: enlarging the intake hole.
I used a 1 1/4" (I think) hole saw to get the rough opening, and the cleaned it up with a die grinder. As there was nothing to center the hole saw, I had to make a jig. I placed a piece of 3/8" plywood over the carb studs and tapped the wood with a hammer, leaving the impression of the studs. I then drilled holes for the studs and measured the exact center and cut a hole with the hole saw. The jig was then bolted to the intake in place of the carb and use to center the hole saw. The cut was not really smooth, so I cleaned it up with a burr in a die grinder. Of course the intake was removed from the motor while the work was being done.



I will have a GPS next spring, so I will verify my speeds then. Still, the difference in the way the boat handled and got on plane (and stayed there) was tremendous.

 

[Tolly family]

Re: Converting a 20 HP Johnson to 35 HP

Thanks Chinewalker, I shouldn't be thinking about more power anyway as the 28 is on an 11' Whaler.

Dan

 

[ryanmitton]

Re: Converting a 20 HP Johnson to 35 HP

Hi there. So if I am reading your post correctly...I should be able to take my 1981 johnson 20, get a 35 hp carb, use the hole enlargement method with your wood jig etc, and it was recommended to swap to a 15 inch prop to keep the rpms reasonable.

That sounds very easy, and cheap, for a whole lot of extra power.

Fix it, can you verify your wot speed, and also your trolling speed?

Does doing this make trolling at reasonable speeds out of the question?

Best Regards, and this is a totally awsome thread. I'm going to do this to my 20. ryan

 

[ryanmitton]

Re: Converting a 20 HP Johnson to 35 HP

Just to confirm, this is the carb you bought for yours right?

http://cgi.ebay.com/ebaymotors/Johnson-Evinrude-35-HP-Carburator-and-Cover-Carb_W0QQcmdZViewItemQQ_trkparmsZ72Q3a1205Q7c66Q3a2Q7c65Q3a12Q7c39Q3a1Q7c240Q3a1318Q7c301Q3a1Q7c293Q3a1Q7c294Q3a50QQ_trksidZp3286Q2ec0Q2em14QQhashZitem4a97da4e50QQitemZ320375246416QQptZBoatQ5fPartsQ5fAccessoriesQ5fGear

 

[insanclown]

Re: Converting a 20 HP Johnson to 35 HP

ryan, that carb would work fine. I have the same one sitting on my desk. it's from the early 80's because after '84 they went to carbs with primers instead of butterfly chokes. good luck with the swap.

 

[larlarsa]

Re: Converting a 20 HP Johnson to 35 HP

Ryanmitton,can you post some picture when you do the converting,i wnat to know how to enlarge the intake port,i have the evinrude 81 20hp E20RCIM too,as G DANE said,i think 20 hp is aprt from a 25 hp,(because i only can find parts on 25hp model),is it realy work on this model,(safe?)what prob sized and pitch will be use ?

 

[Sandhammaren05]

Re: Converting a 20 HP Johnson to 35 HP

Exactly as I said.

What might lead to a problem is to do what I'm about to do, namely, put a 35 carb on a rebuilt 25 powerhead that came with the old style small gearcase (13 spline prop shaft, gearcase looks otherwise like the 1972 shearpin model). That gearcase is smaller and higher geared, is therefore faster, but will wear out (or crap out) quicker with 35 hp than the more modern ('sprint'-type) gearcase on the 20.

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.

 

[ryanmitton]

Re: Converting a 20 HP Johnson to 35 HP

@ Sandhammer: OK, are you saying the lower unit gearcase on a 20 is smaller and will spin faster it's possible that Fix It's 35 hp carb mod and will (most likely) wear out quicker?

Makes sense. I just don't know if the bottom ends are actually different on the 20's, 25's, 30's and 35's. Mines a short shaft.

Anybody who has access to the manuals care to look at part numbers to see if they are the same.


@ Larlarsa: Yes I will post photos. I also cannot find parts under the 20 HP for my motor. The Sierra parts catalog only shows 25 hp. That makes me think the 20 has same lower end as a 25, and by modifying the prop size and degree, we could keep the rpm lower as has been suggested but we'd be moving way faster as Fix It is experiencing.

 

[Sandhammaren05]

Re: Converting a 20 HP Johnson to 35 HP

Just mark the intake manifold hole with the carb gasket, and then take a rat tail file to it. Be careful that the manifold's completely free of filings afterward.

Hi there. So if I am reading your post correctly...I should be able to take my 1981 johnson 20, get a 35 hp carb, use the hole enlargement method with your wood jig etc, and it was recommended to swap to a 15 inch prop to keep the rpms reasonable.

That sounds very easy, and cheap, for a whole lot of extra power.

Fix it, can you verify your wot speed, and also your trolling speed?

Does doing this make trolling at reasonable speeds out of the question?

Best Regards, and this is a totally awsome thread. I'm going to do this to my 20. ryan

 

[Sandhammaren05]

Re: Converting a 20 HP Johnson to 35 HP

Any 20 made in the 1980s can be converted to 35 hp by opening the intake mfld. and bolting on a 35 carb. A carb from a 3 cyl. 75 (1975-1987) will also work, but you have to open the high speed jet a lot with a drill, and the throat's not quite as large.

To G DANE:

There may well be differences in the older 20s. I do not know. Mine was a through hub like the 1986 you just worked on. As far as the clamps on the transom, that would make sense, but I did not know that. It doesn't matter much in my case as I bolt the motor to the boat. Almost lost it in the river once. Don't want that to happen again. I probably would have dropped it if there wasn't a passenger to help me. I tried to turn left and the motor twisted off the transom, pulling the throttle wide open in my hand at the same time. When it did this it also lifted the prop out of the water. I couldn't turn loose of the throttle as that was the only thing keeping the motor on the boat. Luckily I was able to push the kill button (overboard switch was disconnected by previous owner) before it blew up.

Also as far as the prop, I tried a lower pitch prop and it helped greatly, but you could tell that I was still lacking. When I would get on plane, the motor sounded like it was turning close to max RPM. But it was a great struggle to get on plane, as you would need to move weight around and it you hit some small chop it would fall off plane very easily. I didn't want to risk going lower on pitch, and besides that would trade speed for planing ability. I wanted both.


To tmcalavy:

RE: enlarging the intake hole.
I used a 1 1/4" (I think) hole saw to get the rough opening, and the cleaned it up with a die grinder. As there was nothing to center the hole saw, I had to make a jig. I placed a piece of 3/8" plywood over the carb studs and tapped the wood with a hammer, leaving the impression of the studs. I then drilled holes for the studs and measured the exact center and cut a hole with the hole saw. The jig was then bolted to the intake in place of the carb and use to center the hole saw. The cut was not really smooth, so I cleaned it up with a burr in a die grinder. Of course the intake was removed from the motor while the work was being done.



I will have a GPS next spring, so I will verify my speeds then. Still, the difference in the way the boat handled and got on plane (and stayed there) was tremendous.

 

[ryanmitton]

Re: Converting a 20 HP Johnson to 35 HP

You mean to hold up the 35 carb gasket right? Then I'd see where to file out towards.

Wouldn't the hole saw idea of Fix_It while also sucking with a shop vac be the best way to get the increase in size without shavings in the manifold?


I have to beleive that the gears and bottom end is very different on a 20 than on the 35 though. It's like doubling the HP so isn't it a big risk to the integrity of the lower end?

Course if the bottom end is the same stuff...then no risk at all and this is amazing I'll be blowing my brothers 20 out of the water and he's going to have no idea why.

 

[larlarsa]

Re: Converting a 20 HP Johnson to 35 HP

i have check the parts list on the gearcase on 20hp and 35 hp,they are totally different,will the extra power safe on the smaller gearcase(i always mention on safe),or use the other prob to offset the hight rpm.

 

[ryanmitton]

Re: Converting a 20 HP Johnson to 35 HP

Sounds like Fix_it just put a new prop on it to counter the uber high rpm.

Fix_it are you out there? Did you test the speeds?

mmmmm. pancakes.

 

[Sandhammaren05]

Re: Converting a 20 HP Johnson to 35 HP

There is so far no known power/speed advantage to any short block, all are used competitively in APBA GT-Pro Class racing (1976- 1995 OMC 35). The intake/exhaust port differences on the different year models seems not to matter. the following are all competitive with each other:

1977 and later 25 block with any 35 carb
1976 and later 35 block
1985 and later 20 and 30 blocks with any 35 carb and 25-30-35 intake mfld.

It's the same motor, modulo carb (and on the 20 intake mfld.).

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.

 

[Sandhammaren05]

Re: Converting a 20 HP Johnson to 35 HP

The differences are cosmetic compared with the differences between Danes, Swedes, and Norwegians. Er du dansk, eller bor du i Danmark som amerikaner? Kjenner du Sandhammaren strand i n?rheten til Ystad i Sverig?

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.

 

[Sandhammaren05]

Re: Converting a 20 HP Johnson to 35 HP

No, the 1985 and later 20 & 30 gearcases are identical with the 1977 and later 35 gearcase. Likewise for the 1985 and later 25 gearcase. However, the
1976-1984 25 gearcase is much smaller (bullet is bolted together from 2 halves) and us geared higher, making it faster (less underwater drag from both gearcase and prop). The problem is the only props that fit the 7/8" dia. 13 spline propshaft are fishing props. For the 'sprint' gearcase (the standard 20,30,35 gearcase) you can adapt 10x14-10x17 Yamaha cleavers (but without rubber hub). The term 'sprint gearcase' comes from the 1975 shortshaft 75, which dominated its class in racing. The 35 gearcase is a smaller version of the old 75 gearcase. Actually, my favorite would be the 1976 35 gearcase because of smaller nonsweptback skeg (less drag), that one's like the original 1975 75 gearcase.

Here's the parts manual:

http://shop2.evinrude.com/ext/index.aspx?s1=963e48574baf7086d29fb7982ad90a1c

@ Sandhammer: OK, are you saying the lower unit gearcase on a 20 is smaller and will spin faster it's possible that Fix It's 35 hp carb mod and will (most likely) wear out quicker?

Makes sense. I just don't know if the bottom ends are actually different on the 20's, 25's, 30's and 35's. Mines a short shaft.

Anybody who has access to the manuals care to look at part numbers to see if they are the same.


@ Larlarsa: Yes I will post photos. I also cannot find parts under the 20 HP for my motor. The Sierra parts catalog only shows 25 hp. That makes me think the 20 has same lower end as a 25, and by modifying the prop size and degree, we could keep the rpm lower as has been suggested but we'd be moving way faster as Fix It is experiencing.

 

[Sandhammaren05]

Re: Converting a 20 HP Johnson to 35 HP

You can get a big acceleration boost by milling the head .060" (150 psi plus compression compared with 130 psi stock). Setting the spark advance at 34 degrees will give another 100 RPM or so on top end. However, we don't have the leaded high octane gas of 1985, so you're also in danger of sticking a piston with spark set above 30 degrees unless you run a fraction of high octane racing gas mixed with the pump fuel (no fisherman is likely to go to that trouble ....). I would in any case use Evinrude synthetic oil (XD-100) mixed 50:1 in OMC motors built for 50:1, or increase the oil mixture for normal oil if turning 6000-7000 RPM (which my 35s do).

This is a very interesting post. Kind of reminds me of hopping up an old Mopar car engine, match porting the intake to the heads, adding headers, bigger carb, etc...

Great replies!

 

[Sandhammaren05]

Re: Converting a 20 HP Johnson to 35 HP

I mean take a pencil and use the carb gasket as template to show you the necessary hole size. I always take parts that have been filed, ground, or honed to the car was and wash them with soapy high pressure water to get rid of metal particles that you can't see (and that will damage your motor).
Gearing on 20,30,35 is same as 35, gearing on 1985 and later 25 is same too. No problem with gearing. Be sure to leave the 20 cowl on the motor and don't say what you did ..... . I recommend Stilleto or OMC (cupped) prop. What boat, transom height, boat weight are you running? Be sure the cavitation plate is above the boat bottom, most drag comes from gearcase and prop if boat bottom is nice and straignt (no hook).

You mean to hold up the 35 carb gasket right? Then I'd see where to file out towards.

Wouldn't the hole saw idea of Fix_It while also sucking with a shop vac be the best way to get the increase in size without shavings in the manifold?


I have to beleive that the gears and bottom end is very different on a 20 than on the 35 though. It's like doubling the HP so isn't it a big risk to the integrity of the lower end?

Course if the bottom end is the same stuff...then no risk at all and this is amazing I'll be blowing my brothers 20 out of the water and he's going to have no idea why.

 

[Sandhammaren05]

Re: Converting a 20 HP Johnson to 35 HP

20, 30, 35 have same gearcase, gearing:

http://shop2.evinrude.com/ext/index.aspx?s1=963e48574baf7086d29fb7982ad90a1c

i have check the parts list on the gearcase on 20hp and 35 hp,they are totally different,will the extra power safe on the smaller gearcase(i always mention on safe),or use the other prob to offset the hight rpm.