1977 OMC V4 Plenum Intake mod?

[Jason1955]

I've read on here a while back about there is a mod you can do to the intake to create a plenum style setup to where it pulls from both venturies instead of just one. My question is, has anybody ever done this and did it help? Also.....I see where you would mill out the section between the holes in the intake but how far down do you go? I didn't want to take too material out.

Thanks,
Jason

 

[Dhadley]

Re: 1977 OMC V4 Plenum Intake mod?

It will definately help if you're at or close to a max set up now. Which 77 V4 do you have? What's the rig? What's the top rpm?

 

[Jason1955]

Re: 1977 OMC V4 Plenum Intake mod?

Hey Dhadley,
It's an old 1973 115 that we built a 1977 powerhead for. I kept the 1 5/16 carbs and added the bubble exhaust with no block fillers in the exhaust. I also didn't mill out the section in the bottom of the block for the exhaust like I should have. I didn't know about it at the time. I'm also running the 135 heads part # 319943 and #319942. The motor is on a 1985 17' sunbird. I've raised the motor to where the cavitation plate is just about even with the bottom of the boat and turning a 19" S.S. omc prop. Right now with it trimed out I'm turning around 5300-5500 rpm's.

Back to the intake.......how far down into the intake do you mill out? I know you want to make it straight across but I didn't want to go too far down with it.

Thanks,
Jason

 

[Dhadley]

Re: 1977 OMC V4 Plenum Intake mod?

If you're using a 1977 115 block the restriction is going to be the port timing & port area.

Anyway we cut the rib out about 1/2 way down. It doesn't seem to make any difference after that. Make sure that center rib is at least a 1/4 to 3/8" thick.

You'll also need to epoxy the hole in the back of the carb that leads up and into atmosphere. And you'll need the old style carb base gaskets.

 

[Jason1955]

Re: 1977 OMC V4 Plenum Intake mod?

If you're using a 1977 115 block the restriction is going to be the port timing & port area.

Anyway we cut the rib out about 1/2 way down. It doesn't seem to make any difference after that. Make sure that center rib is at least a 1/4 to 3/8" thick.

You'll also need to epoxy the hole in the back of the carb that leads up and into atmosphere. And you'll need the old style carb base gaskets.

 

[Jason1955]

Re: 1977 OMC V4 Plenum Intake mod?

So......you don't think it's really going to help any due to the port timing? I don't want to waste my time if it's not really going to help.

I looked in the back of one of the carbs last night and there is a little small pin hole and then a oblong looking hole there too. So both of those need to be filled in?

Also,
Shouldn't I be turning more RPM's than 5300? I wonder if I would gain more from a prop change? What would be a good prop that can run on the surface and not slip? I can take my motor and keep triming it up till the rpm's reash 6000 but the prop has lost all of it's bite and the speed falls off drastically.

Thanks again,
Jason

 

[Dhadley]

Re: 1977 OMC V4 Plenum Intake mod?

I think you'll see some gain but you just have to keep in mind you still have the old 115 (same as new 90) porting. Just keep that in mind as you gain rpm.

The small pin hole is for the primer. If you're not using it, just cap it off. The other hole (almost like a milk bottle shape) has to be sealed perfectly flat with the rest of the carb base. That hole goes forward and up to air.

If you can use 90% of the trim range without the prop losing bite, you can probably go up some. As you go up and that prop loses bite sooner, you'll need a prop with more technology or bite.

Depending on what your slip is now and where you're at, you may be able to gain some but that older shorter gearcase isn't a good the 78 & up style.

 

[Jason1955]

Re: 1977 OMC V4 Plenum Intake mod?

You mentioned the gearcase, What is the difference in the 2 gearcases? Is it gear ratio or just design?

Which line of S.S. props are better, the Ballistic or the Raker?

Thanks again,
Jason

 

[Jason1955]

Re: 1977 OMC V4 Plenum Intake mod?

Also.....if I do decide to do this intake mod......is there a need to go with larger jets or just keep the same .067" I have in there now?

Thanks,
Jason

 

[Dhadley]

Re: 1977 OMC V4 Plenum Intake mod?

The jetting stays the same. If you get over 6000 we may want to look at it but for now it stays as produced.

The older gearcases are shorter so they have a lower blow-out speed. Nothing to worry about until you get aroound the hi 60's. Being shorter they are not a s streamlined as the newer ones. We might be sacrificaing a couple mph there.

Overall I think the Raker is a better prop than the stock Ballistic. Don't overlook the Turbo Lightning Jr if you can find one in the pitch range you have.

Only make one change at a time. If you feel the set up is maxed out, then go for engine mods. But don't do the intake and a prop change at the same time. Stick to your baseline info, make a change and record it. Then another change. One at a time.

 

[Jason1955]

Re: 1977 OMC V4 Plenum Intake mod?

10-4 Dhadley,
Thank you so much for your help!! I'll keep you informed on the changes.

BTW: If I'm running the 19" S.S. OMC prop now and I went with a raker....should I stay with same pitch?

Jason

 

[Dhadley]

Re: 1977 OMC V4 Plenum Intake mod?

Raker's and Lightning Jr's are in even numbers. Stick with the prop you have. Do any changes - ONE AT A TIME - and then worry about a prop. If you're going to raise the motor, do that and once you settle on a height go to the next change, the intake if you so choose. Then test.

ONE CHANGE AT A TIME! The prop will be the last thing you change.