MOD VP Exhaust for OMC, Worth Effort?

**adsit** · 08-13-2006, 08:46 AM

1988 70 hp Johnson. Is it worth the effort to port the exhaust. Will this gain anything besides sound on this engine. So far accomplished was by suggestions; engine raised, 2 inch spacer plates. Boat is near perfect in water on pad. Running 44 mph at 54 hundred rpm. I am looking for another prop. Current 20 pitch raker, works well just low r's. Any sugestions on props? PS/Carbs a little rich on bottom end. Have to clear its throat after idling. Thanks for everyone who has responded in the past. Got me hooked now.

**ROLF** · 08-13-2006, 09:20 AM

What hull/prop and prop shaft hight?I have never seen any gains in sport or recreational outboards with loud exhust and you could loose some bottom end torque.When the propshaft is even with the pad the exhaust is out of the water anyway.Full race boats may be a different story.

**adsit** · 08-14-2006, 06:12 AM

15 1/5 pro-gator 3. Large gear case, 13 1/2 raker, 22 pitch, repitched to 20 pitch. Hight is 1/4 inch above top water intake hole. Have motor to highest point with set-up. 1" spacer plate with 1" 5 degree angle plate. Had no negative trim before. Running pad correct. Still blows out without a stingray tail coming out of hole.. Max trim produces tail at mid to lower cowling high.

**dry3** · 08-14-2006, 05:48 PM

http://forums.screamandfly.com/forum...hlight=exhaust

**adsit** · 08-15-2006, 02:00 PM

Thanks, will try with current. Any suggestions on prop?

**dry3** · 08-15-2006, 02:38 PM

I would try a 19 pitch, you are currently running a 20 at 5400 with a 2.1 ratio large 3 cylinder case small barrel at 43 mph gps, which is reflecting a 15% slip which is kind of high, 10% would be good goal. If you went to a 19 pitch and spun 5600 that would reduce your slip to approximately 9-10% and raise your speed to approximately 46 mph. Just my 02 worth

**dry3** · 08-15-2006, 02:45 PM

Here is a 19 pitch you could try for cheap http://bbcboards.zeroforum.com/zerothread?id=125303

**adsit** · 08-15-2006, 06:37 PM

Had tip on 6 holes at 19/32 size each. 3 down each side. Completed. Prop sounds like a gamble from a red gambler to me.

**adsit** · 08-15-2006, 08:14 PM

Definatly made a snappy response on the muffs. Water test next. Props? Anything comprable to lift and etc. in a 3 or four blade. If four blade what changes in pitch, diameter, brand and etc. would any one suggest. Really could use a big hand at this one. Thanks dry3 for the tip. PS 20 pitch raker currently.

**dry3** · 08-16-2006, 11:34 AM

Originally Posted by dry3

I would try a 19 pitch, you are currently running a 20 at 5400 with a 2.1 ratio large 3 cylinder case small barrel at 43 mph gps, which is reflecting a 15% slip which is kind of high, 10% would be good goal. If you went to a 19 pitch and spun 5600 that would reduce your slip to approximately 9-10% and raise your speed to approximately 46 mph. Just my 02 worth

Look at these numbers this should be a safe target to shoot for with a raker, for which you have your boat set up. A 4 blade small hub is still going to require the roughly the same pitch. you should be looking for high rake 4 blade in 18-19 pitch, here is OMC'S 4 Blade high rake equivalent to the raker ( Bass Renegade: Is an "Over/Thru-hub" 4-blade polished propeller.