1980's Johnson/Evinrude 20/25hp conversion to 30hp with pics

[Edstrossner]

We had a beautiful day here in Georgia and I took the Lowe out for a spin . I just finished modifying the head so it was time to see if the mod did anything. The answer is a definite Yes!
Starting with the 1990 model, the cylinder head was changed to a low compression head. Apparently OMC was selling these motors to areas of the world that have poor quality gas. Their manual states that the minimum octane for the 9.9,15,25 and 30hp motors is 67 octane.
So, the stock head compression is around 100 psi. After my mod, it's at 145 psi.
Stock head with an 11 pitch prop and a 70hp carb......5800rpm and 25mph gps.
Modified head with the same prop and carb hit the pack limit of 6100 rpm so I switched to a 4 blade, 14 pitch prop.....5400rpm and 29mph.
I then tried a run with the hood removed. Hit 5800 rpm and 30mph. I suspected that the hood does not have enough air intake space.
I'll be adding some air holes to the hood next. The space just below the grab handle is a good place to start. After that it will be time to raise the motor.

 

[thill]

We had a beautiful day here in Georgia and I took the Lowe out for a spin . I just finished modifying the head so it was time to see if the mod did anything. The answer is a definite Yes!
Starting with the 1990 model, the cylinder head was changed to a low compression head. Apparently OMC was selling these motors to areas of the world that have poor quality gas. Their manual states that the minimum octane for the 9.9,15,25 and 30hp motors is 67 octane.
So, the stock head compression is around 100 psi. After my mod, it's at 145 psi.
Stock head with an 11 pitch prop and a 70hp carb......5800rpm and 25mph gps.
Modified head with the same prop and carb hit the pack limit of 6100 rpm so I switched to a 4 blade, 14 pitch prop.....5400rpm and 29mph.
I then tried a run with the hood removed. Hit 5800 rpm and 30mph. I suspected that the hood does not have enough air intake space.
I'll be adding some air holes to the hood next. The space just below the grab handle is a good place to start. After that it will be time to raise the motor.

You might want to go up a size on the main jet to compensate and take advantage of the extra air you are giving it. Also, to avoid a lean condition.

When I ran my 1994 Evinrude 25 with the hood off, she gained a lot of pep, and I gained a couple of MPH at WOT. But when I slowed down, she didn't want to idle. I wondered about running lean, so I popped the choke a couple of times, and that kept her going. After a while she would idle without issue. When I ran at WOT again, AGAIN, she wouldn't idle afterward without hitting the enrichener. But after a while, she would idle perfectly.

I believe that without the cowl, I was running lean at WOT, which "dried out" the crankcase. Pushing the key fixed it until the fuel distributed back into the crankcase. Obviously, running lean is NOT something I want to do for long!

So I had a choice: Open up some more air inlets and go up a jet size, or leave the cowl on. I left it alone.

But if you make more air intake and go up a jet size, I bet you will have a beast on your hands!

-TH

 

[Edstrossner]

Thanks for the info!

I did check the plugs when I came in. Since I started using the 70hp carb I have been increasing the main jet size. It was at 67 but it appears to be lean still so I just when up to 70.
I also cut a hole in the hood in the area behind the grab handle, then I shimmed up the grab handle a bit to act as a scoop. It's not a huge opening but as the boat speed picks up, more air will be forced under the hood. I did this to a 70hp a couple of years ago. It works great.

 

[thill]

Oh, that sounds good.

I need to stop reading this thread. You've got me wondering what would happen if I go up a jet size or two and increase the air intake.

HMMMMMMM.....

I'm gonna need a steeper prop. :LOL2: :LOL2: :LOL2:

-TH

 

[Edstrossner]

Has anyone tried using different cylinder heads for the 25hp?
There are 4 different part numbers.
1980-81---324319
1982-------327672
1983-89---327671
1990-------335655
The 1982 head is supposed to be the best.
1990 and up is the low compression head.
But, how good are the other 2 heads?

 

[vahunter]

I'm curious why omc used a low compression head for the '90+ 25hp outboards. Aren't they practically the same as the '86-89 outboards? I also thought the oem compression were the same. Am I making sense?

 

[thill]

I know that somewhere before 1990 they increased the HP output of all their motors, to rate them at the prop, instead of at the powerhead.

Because of this, my 1995 is WAY stronger than my 1981, although the compression is lower. It makes almost a 10 MPH difference. The older motor runs excellent, but it's just not as stong at the prop. I've found this to be the same across the board, especially with my 70 HP motors. The older ones run flawlessly, but just don't have the same grunt as the 90's motors.

It depends on how they achieved the higher HP, as to how you proceed. If they increased the bore and stroke, increased the carburetor and intake volume, increased the max RPM, or whatever, it changes how you take advantage of the differences.

Any idea of WHY the 1982 heads are supposed to be the best?

Of course, this is all theory to me, as I have not actually looked at the specs, and I'm not very experienced with making serious modifications to motors. My job is to get them running well, and keep them as close to stock as possible, so my opinion is not worth too much in this discussion.

-TH

 

[vahunter]

I believe it was somewhere around 85-86 is when they started rating at the prop. That's what I meant.

 

[thill]

Hey, I notice that you have a very similar motor and setup as I do.

Curious questions:
Is yours a tiller or remote steer?
What speeds do you reach with your 25?
Is your boat loaded heavily or light?

On the Lowe, which was bare, I hit 29 MPH
With the Crestliner, loaded with gear and batteries, only 23 MPH.

When I took the hood of the 25 off, as I mentioned above, the Crestliner was hitting 27, but it was not sounding good screaming like that. I won't do that again, unless I re-jet the carb to a larger size.

-TH

 

[Edstrossner]

Omc lowered the minimum octane required for all the 2 cylinder cross flow motors down to 67.
They where selling a lot of these motors to areas of the world that had poor gas.
They may have opened up the ports to keep the hp rating the same.
I have seen pics of the high compression 25/35hp head on boatracingfacts web site. They say that it is from a 1982 model year.

 

[vahunter]

Edstrossner, can you benefit from skimming a '90s head or is '82 head a different design in general? I thought about skimming mine but I also wanted to keep my motor dependable. I also swapped intake manifolds to the bigger cavity 30hp one and swear I lost a little power than with my 25hp manifold and 28hp carb.

Thill, mines a tiller. No PT but a manual jackplate. I'm running 6100rpm at 31mph, if I remember right, on light load. Trolling motor, 2 batteries and myself. Medium load (1 passenger and fishing gear) about 29-30mph. Heavy load (popup duck blind, 2 passengers, dog, 5dozen decoys, guns, etc.) I never checked mph but probably 25ish and forever to plane. I run a 10x13 omc SS prop. I could run a 15p prop in normal conditions but this is a good all around prop for me.

Btw in the summer, due to lighter loads (higher rpm) I run 40:1 fuel-oil mix.

 

[Lil' Blue Rude]

Omc lowered the minimum octane required for all the 2 cylinder cross flow motors down to 67.
They where selling a lot of these motors to areas of the world that had poor gas.
They may have opened up the ports to keep the hp rating the same.
I have seen pics of the high compression 25/35hp head on boatracingfacts web site. They say that it is from a 1982 model year.

Didn't the head from the 82' on boatracingfacts turn out to be a second effort high compression racing head? I'm pretty sure second effort or seaway made a high compression head for these motors.

 

[vahunter]

This flyer was shared with me on BRF a while back.

 

[Edstrossner]

I measured the piston dome at .625 from the deck to the dome at TDC.
The head measures .650. The gasket adds .050 so at TDC the dome to head squish clearance is at .075.
I like to run a squish clearance at .030 to .035.
So from a stock head you can mill .040 off the head without any additional milling inside the combustion chamber.
These numbers are from my 2000 Johnson 25hp.
I did a lot more work on my head to bring the compression up to 145 psi .

I don't know first hand about the 1982 head. I would love to see one.

 

[Lil' Blue Rude]

Yeah that's the one I seen on boat racing facts. I'm guessing you had to weld up the one side of the head and shape to match the piston to get compression up that high?

 

[Edstrossner]

I finally found a1982 35hp head. It looks like the one from the flyer.
So, I bolted it up to my 2000 25 hp and I was a little disappointed. This motor with the stock head ran at 105psi. The 1982 head ran at 125psi. I had read that guys were getting 150psi with this head. Port timing on these motors must be a bit different . So, I milled the 1982 head and brought the compression up to 145psi and I ran the boat last Wednesday. There was very little difference from the milled stock head verses the milled 1982 head.
Well that experiment only cost $65, so not much lost, not much gained.
Next up I have an old friend sending me some 1-1/2" carbs. I'll be trying those and possibly adding some cone spacers to the reed plate.
Has anyone compared an aluminum prop vs an ss prop?

 

[thill]

I've been playing with various props on mine, and YES, it can make a substantial difference. I found a lower pitch stainless prop, with a very noticeable cup gave me much better hole shot, AND a couple more MPH at the top end, also. Both of them running right up to 5500 RPM. Very happy with it!

I believe I was running a 13P aluminum, and this is a 12p stainless. Both 3-blade.

Unfortunately, I got the prop used, from an old parts motor, and I cannot find any marks on it to tell me who it's made by, unfortunately. Otherwise, I'd let you know.

And it's not a polished SS prop, but one that looks like it's been wire-brushed. A polished one might give even better performance? Or is that just for looks? I don't really know, actually. Is the brushing like the dimples on a golf-ball, actually raising performance??? I really don't know, but imagine the high performance bass props would all be brushed, if that were the case.

But now I'm just rambling about props....

 

[Edstrossner]

Thanks for the info. I new that an ss prop really helps on the larger hp motors but I have not tried one on the smaller motors. I'm running at 5900 rpm with a 4 blade, 14 pitch aluminum and with a few more mods I'll be at the pack limit of 6100 rpm. I know where I can get a 3 blade, 17 pitch ss but before I drop $200 it's good to know that it will make a difference.

 

[thill]

If buying new, a lot of shops will let you try the prop, and return it if it doesn't work for you. provided that you bring it back in perfect condition.

Places like West Marine will too, but don't tell them you ran the prop, or they may give you a hard time. Just clean it up perfectly before putting it back in the box, IF it doesn't work for you.

-TH

 

[vahunter]

I firmly agree about running stainless. I lost my rpm info for my prop tests but I was running an aluminum 10.5x11 omc which wasn't in great shape and got about 28mph and switched to 10x13(I think) omc stainless and running 30mph about 6000rpm or something like that. I also have a better hole shot and load handling. I could probably add more cup to jack the motor up and maintain rpm.