Help Modifying OMC 31.8 cid

[JohnsonM50]

I agree with Joe on porting not making as much difference as I'd hoped, My 1st attempt was to square the tops of all the ports & I raised the exhaust a little. I re ran the re-asemble as it was before & it was slightly faster, alot louder & pretty darn thirsty. I figured the intakes were good as were so the next one I did about the same to the exhaust only & thats a little better yet but by no means great. The tuner that you saw curved is mostly to fit but also is in a shortened tower. I did a little sheetmetal work to protect the waterpump in that too. I got lucky & found a 2nd effort head on a motor I traded some junque for. The catch is this motor was so badly blown its surprising it got that bad w/o stopping lomg before. [musta been wail-n hot]. Gotta wonder if the head was a contribting factor for some reason like timing or jetting.
Good & bad news.. On a reed experiment I foung a motor to run significantly faster till a reed failed & that is probably the best reason to used composite reeds. I tried Boyesen 2 stage & decided to take them out before I broke em. No great improvement noticed. I have a single stage set from Carlson to try but haven't yet. Reeds are where some potential is & Ive been thinking about how to v reed one for some time as this would decrease crank case volume as well as flow better.

[Smokin' Joe]

I agree with Joe on porting not making as much difference as I'd hoped, My 1st attempt was to square the tops of all the ports & I raised the exhaust a little. I re ran the re-asemble as it was before & it was slightly faster, alot louder & pretty darn thirsty. I figured the intakes were good as were so the next one I did about the same to the exhaust only & thats a little better yet but by no means great. The tuner that you saw curved is mostly to fit but also is in a shortened tower. I did a little sheetmetal work to protect the waterpump in that too. I got lucky & found a 2nd effort head on a motor I traded some junque for. The catch is this motor was so badly blown its surprising it got that bad w/o stopping lomg before. [musta been wail-n hot]. Gotta wonder if the head was a contribting factor for some reason like timing or jetting.
Good & bad news.. On a reed experiment I foung a motor to run significantly faster till a reed failed & that is probably the best reason to used composite reeds. I tried Boyesen 2 stage & decided to take them out before I broke em. No great improvement noticed. I have a single stage set from Carlson to try but haven't yet. Reeds are where some potential is & Ive been thinking about how to v reed one for some time as this would decrease crank case volume as well as flow better.

Jim Nerstrom told me that the only hope with that (fishing) motor is reeds and exhaust tuning. Carlton Callahan once fabricated a
plastic manifold for v-block reeds on a river racer, and used a 2 barrel carb from an old 85. I've never experienced a broken reed on any motor. In SST45 and SST60
we replace plastic reeds with steel ones because the plastic reeds leak and kill the acceleration. We're turning 7500-8000 RPM with steel reeds. And we do accelerate.

[JohnsonM50]

The rig: In many cases someone will put more pitch on or a bigger motor to go faster to find the motor could harly spin the prop & the bigger motor mostly only accelerated better. So having not done that yet its a good time to look at what you do have. Is the boat bottom straight? meaning no 'hook' or 'rocker' condition. Is it smooth? aluminum can drag pretty hard so these things as well as good paint or epoxy type coating can help. The weight sometimes is not as important as its distribution. Moving the gas & battery [if used] are usually all that is needed if not optimal already.
Set up.. as important as thr prop [almost] The prop should be as near to the surface as it can be & still work & the boat handle adequetly. The tilt should be so it has a lazy porpoise slow & almost goes away at speed. Pounding is of course no good for several reasons. If your using the standart pin tilt you can sleeve the pin for finer adjustments as the difference between adjust holes can be too much.
The GT pros last I saw were about to break 50 & are not super light so the potential is there. The motor Mike Petty built is to go Yamato hunting.. Hope he does well, theres alot of work in it but it is a stock block aside from the open exhaust. It should get well into the 70's. My fastest yet did 65. its above water exhaust but is in the midsection so is closed. The motor is pretty modified but a little on the cheep & back-yardish. I'm going to spend a bit on making it a more professinal build soon.

[Smokin' Joe]

The only good head is from 1982, the others have only a 180 degree squish band.
A stock 1982 head might be better than a milled head of too small squish band.
I had a powerhead with light rods and pistons and 1982 stock head, it
was slightly faster than my 1983 powerhead with heavy rods and pistons and
.060" milled head. Milling the head really picks up the acceleration. also, be aware that the
only good gearcase is 1980-1983. 1976-1979 is not streamlined and will not run. 1984 and later has a lower than
1:2 gear ratio. 1980-1983 is streamlined and has a .53 gear ratio.

[Smokin' Joe]

The rig: In many cases someone will put more pitch on or a bigger motor to go faster to find the motor could harly spin the prop & the bigger motor mostly only accelerated better. So having not done that yet its a good time to look at what you do have. Is the boat bottom straight? meaning no 'hook' or 'rocker' condition. Is it smooth? aluminum can drag pretty hard so these things as well as good paint or epoxy type coating can help. The weight sometimes is not as important as its distribution. Moving the gas & battery [if used] are usually all that is needed if not optimal already.
Set up.. as important as thr prop [almost] The prop should be as near to the surface as it can be & still work & the boat handle adequetly. The tilt should be so it has a lazy porpoise slow & almost goes away at speed. Pounding is of course no good for several reasons. If your using the standart pin tilt you can sleeve the pin for finer adjustments as the difference between adjust holes can be too much.
The GT pros last I saw were about to break 50 & are not super light so the potential is there. The motor Mike Petty built is to go Yamato hunting.. Hope he does well, theres alot of work in it but it is a stock block aside from the open exhaust. It should get well into the 70's. My fastest yet did 65. its above water exhaust but is in the midsection so is closed. The motor is pretty modified but a little on the cheep & back-yardish. I'm going to spend a bit on making it a more professinal build soon.

Minor correction. I built the (chopper) props for the fastest two GT-Pros and they haven't broken 48 mph yet to my knowledge. I
think they may be running the post-1983 gearcases. Them that has a 1980-83 gearcase ain't telling,
but I informed them of the difference ca. a year ago.

[JohnsonM50]

Joe, here's the head I got:


Its the one on the left. I really admire the work done in GT Pro, & what's been learned about how to 'go' bone stock. Its nowhere near here tho, so I'm just playing with these for fun, no rules. The 1st intent was to have a practice motor to put time on instead of wear & tear on the Yamato. It came down to this:

Same transome height, 80#'s 1:1's. A very similar ride to my 102 in the same place. I think the OMC has more pull & slightly less top end. I do intend to break 70 with this or the next 31.8 RaT.

[Smokin' Joe]

Joe, here's the head I got:


Its the one on the left. I really admire the work done in GT Pro, & what's been learned about how to 'go' bone stock. Its nowhere near here tho, so I'm just playing with these for fun, no rules. The 1st intent was to have a practice motor to put time on instead of wear & tear on the Yamato. It came down to this:

Same transome height, 80#'s 1:1's. A very similar ride to my 102 in the same place. I think the OMC has more pull & slightly less top end. I do intend to break 70 with this or the next 31.8 RaT.

On the left looks like nearly 180 deg. squish band. That would be 1982 according to my limited experience. Amazing how much
has been gotten out of that motor in spite of the flat reed plate. I assume you're running the light 1976-79 rods and pistons.

[Emustard]

JohnsonM50 what kind of compression do you get with that performance head?

The aluminum sticks a little to the water but not bad. I have played around with the weight in the boat and trimming it and still think I need it up higher but will have to wait till the ice goes away. I use this engine almost every weekend for fishing so would like to keep the stock lower unit.

So I replace the OMC steel reed with boyesen reeds do you think I should switch back to the steel reeds? I heard some one say you should take out your water pump and you will gain 3-7 hp what are your thoughts on this, is it true if I can still cool the engine.

I’m just looking to get into the 40s with her and need help getting there.

[Emustard]

Smokin' Joe i tryed to PM you but said your message box was full or somthing so.
I would be interested in the stiletto prop how much $$ are you looking to get for it and is it for the 14 splined shaft . Have you ever ran an OMC prop or something else to compare your prop, if so what speeds did you see with the other props.
The transom is about half an inch below the bottom of the boat and the motor is a short shaft. How did you modify or exhaust pipe?

[Smokin' Joe]

Smokin' Joe i tryed to PM you but said your message box was full or somthing so.
I would be interested in the stiletto prop how much $$ are you looking to get for it and is it for the 14 splined shaft . Have you ever ran an OMC prop or something else to compare your prop, if so what speeds did you see with the other props.
The transom is about half an inch below the bottom of the boat and the motor is a short shaft. How did you modify or exhaust pipe?

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