Exhaust work on the tuner(megaphone) of v4 crossflows

**keefallan** · 09-16-2020, 03:36 PM

Ok gentlemen. I just got my 4 cyl block back from Chris. I am replacing the p-head on my boat. Its the motor I have been talking about that has a broken ring(and a bad head too, I'm sure). I got 6-7 years out of it(there's a longer story here), and it has had a bad hole for 3 years now. Its still running right now. But the block is junk. Its got an overbore of .064 in one hole, and its got compression around 160 in that hole.....crazy. The other healthy pistons are 145. The bad hole..........90. When I build this other powerhead, I was wanting to maybe open up the exhaust in the tuner (megaphone) area. I know the v4 is much smaller than the v6 in that area. I dont want to do any race mods or anything like that. But is there some way to free up some power down there? I am not asking for race secrets, just pointed in a direction to free up some air to moveit better thru the bottom half of the motor. I am trying to bring it closer to a real 140 crossflow(prop rated.....even tho it never existed).

I am using 135 heads.

I know the 77 megaphone is a unicorn...........but is there other things to do without getting radical. I am just looking to free up a few more hp......

I am sure it will force me to rejet, but I dont care about that. I am looking for free power(jets not withstanding).

**Gordon02** · 09-16-2020, 09:51 PM

John Tiger says on light hulls, that cutting the stock tuner to 8" total length will result in about 200 more rpm because it will exhale a little easier. I found me a 1977 model last month. It will be a future project. I want to see how this 115 to 140 build runs and performs before I plan my next one. I've got to finish a Merc 2.0L fatblock project that I started 2 years ago and 2 Chrysler 90's sitting on my bench before I touch another OMC...Gordon

**nitro_rat** · 09-16-2020, 10:14 PM

Yeah, for the TX tri-hulls the tuner length was well kept secret. I happened to be at a friend of mine's shop one Friday when they were scrambling to replace a broke mid before a race weekend. I'm a good hand when it comes to removing broken and frozen bolts so it was "all hands on deck." One task was chopping the tuner on the new plate and my friend came over and made a mark for me to cut on. I casually cast my eye around the shop looking for a tape measure and popped one on it before we zipped her up. I don't remember exactly what it was (it's probably been 10 years ago now and I haven't built anymore V4's since then) but it was somewhere between 7-1/2" and less than 8" for sure. This combo went on to win some championships it worked. I think I posted the exact number here in the past if you search for crossflow hot rod stuff...

**Gordon02** · 09-16-2020, 10:18 PM

Don't forget that you'll need the mid-section adapter from the same 1977 donor outboard, not just the tuner. I don't know when I'll get this one apart, but will take pics of the differences. I've gotta say this too - What a "pain in a butt" the OMC V4 crossflow lower pan and cowl is to fit on the bubblebacked motors. I sanded and spent lots of time getting my single stage urethane paint near perfect. Then I tried to fit the lower cowl onto the adapter.....all without scratching the work. You have to install the cowl/pan before the engine, but the cowl/pan won't fit over the thermostats cover and you must put the thermostats cover must go on before the powerhead on bubblebacks, but you can't have the cowl/pan tight, it must be loose and floppy so you can manage the strong springs and long bolts of that thermostats cover - really a pain. I'll never, ever criticize the Mercury "clamshell covers that hinge apart from the rear. Those are now looking brilliant! Gordon

Gordon

**keefallan** · 09-17-2020, 04:35 AM

Originally Posted by Gordon02

Don't forget that you'll need the mid-section adapter from the same 1977 donor outboard, not just the tuner. I don't know when I'll get this one apart, but will take pics of the differences. I've gotta say this too - What a "pain in a butt" the OMC V4 crossflow lower pan and cowl is to fit on the bubblebacked motors. I sanded and spent lots of time getting my single stage urethane paint near perfect. Then I tried to fit the lower cowl onto the adapter.....all without scratching the work. You have to install the cowl/pan before the engine, but the cowl/pan won't fit over the thermostats cover and you must put the thermostats cover must go on before the powerhead on bubblebacks, but you can't have the cowl/pan tight, it must be loose and floppy so you can manage the strong springs and long bolts of that thermostats cover - really a pain. I'll never, ever criticize the Mercury "clamshell covers that hinge apart from the rear. Those are now looking brilliant! Gordon

Gordon

I was chuckling when you brought up the difficulty of the belly pan. The v6 bubble back is worse!!!...........if you are trying to not scratch things up that is. You are trying to align the stars!! As far as 77 adapter, I actually have a 78 140 on my bench..........cutting bolts and belly pan off. I will check numbers, but that adapter should work.

So, shortening the tuner it is. Thanks Josh and Gordon for your input.

Keith

**keefallan** · 09-17-2020, 04:51 AM

Well, i see that the 77 used a similar setup to the v6 motors......a two piece design. The 78 and newer use a one piece megaphone that runs all the way down to the lower unit. What a difference. So, I will just have to just cut the megaphone and remove the connecter at the bottom that mates it to the lower unit. But if I do this, will the heat mess up(melt) the water pump housing? Maybe thats not a good idea. Thoughts or experiences with this?

Maybe I just port the aluminum in the adapter and at the top of the tuner for more volume up there if I cant cut the tuner.

**powerabout** · 09-17-2020, 07:20 AM

Originally Posted by keefallan

Well, i see that the 77 used a similar setup to the v6 motors......a two piece design. The 78 and newer use a one piece megaphone that runs all the way down to the lower unit. What a difference. So, I will just have to just cut the megaphone and remove the connecter at the bottom that mates it to the lower unit. But if I do this, will the heat mess up(melt) the water pump housing? Maybe thats not a good idea. Thoughts or experiences with this?

Maybe I just port the aluminum in the adapter and at the top of the tuner for more volume up there if I cant cut the tuner.

the 77 140 was in a can that why the adapter is different to allow for it.
You might get away with it with some carefully directed exit water spray?
Its going to be loud...

**keefallan** · 09-17-2020, 01:27 PM

Originally Posted by powerabout

the 77 140 was in a can that why the adapter is different to allow for it.
You might get away with it with some carefully directed exit water spray?
Its going to be loud...

Thanks Powerabout for the input. I think I will have to carefully look at things when I get the bad powerhead off. I need to take the tuner/adapter off to inspect things anyways. Its been 6-7 years, and I dont want the bolts getting stuck any worse by ignoring them. When I do that, I will see what I can do open up the exhaust......and maybe figure out how to flow some water towards the water pump housing. I am thinking using the pee stream? Maybe using a 90 degree fitting into the midsection(from the water outlet) to aim water from the pee stream directly onto the water pump?

**powerabout** · 09-17-2020, 07:29 PM

Originally Posted by keefallan

Thanks Powerabout for the input. I think I will have to carefully look at things when I get the bad powerhead off. I need to take the tuner/adapter off to inspect things anyways. Its been 6-7 years, and I dont want the bolts getting stuck any worse by ignoring them. When I do that, I will see what I can do open up the exhaust......and maybe figure out how to flow some water towards the water pump housing. I am thinking using the pee stream? Maybe using a 90 degree fitting into the midsection(from the water outlet) to aim water from the pee stream directly onto the water pump?

You need to have control of the water exiting the adapter after the thermos

**Gordon02** · 09-18-2020, 06:46 AM

Guys, I don't have those waterpump concerns. The 1978-up V4 crossflow exhaust housing keeps the driveshaft/waterpump "almost" separated. Have a look at photos 1 and 2 below and you'll note the driveshaft/waterpump are not aligned with the exhaust tuner and there is a cast aluminum divider separating the two areas. Yes, there is a slot where the copper water-feed tube routes water up to the midsection and engine, but the tuner is several inches back and aligns with the larger, rear most gearcase openings.

I own and run a 1999 Promax SS that I bought new. The short shaft has a short tuner that dumps on the water pump and I've never had a problem. I originally asked the same questions, but after looking at the V4's exhaust housing and the tuner's alignment - I'm not concerned. Gordon

**Gordon02** · 09-18-2020, 07:18 AM

The ugly stuff on the #2 photo is JB Weld. One of the previous owners had drilled 2 holes through this flange area as a means of venting the exhaust and making more noise...I welded those holes up with aluminum plugs and then filled the repair and dimples with the JB Weld. Since I wasn't planning to use the full exhaust tuner, I stopped with what you see here.

**powerabout** · 09-18-2020, 07:58 AM

Originally Posted by nitro_rat

Yeah, for the TX tri-hulls the tuner length was well kept secret. I happened to be at a friend of mine's shop one Friday when they were scrambling to replace a broke mid before a race weekend. I'm a good hand when it comes to removing broken and frozen bolts so it was "all hands on deck." One task was chopping the tuner on the new plate and my friend came over and made a mark for me to cut on. I casually cast my eye around the shop looking for a tape measure and popped one on it before we zipped her up. I don't remember exactly what it was (it's probably been 10 years ago now and I haven't built anymore V4's since then) but it was somewhere between 7-1/2" and less than 8" for sure. This combo went on to win some championships it worked. I think I posted the exact number here in the past if you search for crossflow hot rod stuff...

I went to a Port Neches race once, the leading V4's certainly sounded better than the rest..
I think you know whos dad won...again

**nitro_rat** · 09-18-2020, 08:12 AM

Originally Posted by powerabout

I went to a Port Neches race once, the leading V4's certainly sounded better than the rest..
I think you know whos dad won...again

Some of those have an elbow welded into the leg. the tuner dumps into the elbow which directs the exhaust straight out the back. I think it's for sound more than power. Lots of the tri hull mids are painted black to "camouflage" the exhaust mods. It looks like just a big hole drilled in the back but it's not...

**powerabout** · 09-18-2020, 08:38 AM

Originally Posted by Gordon02

Guys, I don't have those waterpump concerns. The 1978-up V4 crossflow exhaust housing keeps the driveshaft/waterpump "almost" separated. Have a look at photos 1 and 2 below and you'll note the driveshaft/waterpump are not aligned with the exhaust tuner and there is a cast aluminum divider separating the two areas. Yes, there is a slot where the copper water-feed tube routes water up to the midsection and engine, but the tuner is several inches back and aligns with the larger, rear most gearcase openings.

I own and run a 1999 Promax SS that I bought new. The short shaft has a short tuner that dumps on the water pump and I've never had a problem. I originally asked the same questions, but after looking at the V4's exhaust housing and the tuner's alignment - I'm not concerned. Gordon

I think that the middle photo is of a 77 140 or is that a 78 with the tube cut?