exhaust relief hole modification

[phatmanmike]

i just cant leave good enough alone.<br /><br />ive seen pics and heard of drillingor making hole in the bottom of the exhaust leg so that , when on plain, the exhaust exits through the holes and ABOVE the waterline, instead of through the prop.<br /><br />im always one for getting the mostest from the leastest.<br /><br />is it as simple as drilling a series of holes in the back of the leg. ive seen kits online for this, but they were specifiaclly for v6 mercs.<br /><br />i have a 1990 merc 90hp triple.<br /><br />any ideas are apreciated.

 

[Laddies]

Re: exhaust relief hole modification

mike the reason they drill V/6 is for better idle not for top end I think if it would help top end you would see a lot of torque tubes drilled or cut on bass boats

 

[KCLOST]

Re: exhaust relief hole modification

Don't do it, You may sacrifice hole shot, especially if you are using a prop with vent holes...

 

[Chinewalker]

Re: exhaust relief hole modification

Also, most motors these days have inner exhaust tubes that route the exhaust directly into the gearcase housing. Simply drilling holes in the motor leg won't do anything at all for the exhaust.... And, if you were to drill in through the inner exhaust tuner, you could conceiveably ruin the tuning and scavenging capabilities of the tuner.<br />- Scott

 

[phatmanmike]

Re: exhaust relief hole modification

thanks, consider it NOT HAPPENING!!!

 

[phillnjack2]

well above stated drilling the gearbox above cav plate no good for a vented prop...!!!!!!!!
if you had a vented prop you would not be looking for better exhaust relief anyway..
the exhaust holes in the side of the leg (not the tuner) are put in on the omc high performance engines from factory, so they must have a valid reason for that.
Now anyone using a over hub prop will see a big benefit in the holes above the cav plate on the holeshot, that is pretty obvious.
99% of outboard engines sold are mainly for average jo and his recreation, so the engine manufacturer caters for this.
But if you decide you want a touch more performance there are tons of things you can do to improve the performance on an outboard and the exhaust relief is
just one simple thing to do.
the only downside to drilling holes in the gearbox above the cav plate is the noise factor when slow running. if you are worried about noise then go buy anelectric boat or sail boat and leave the real fun to others.
once the engine is on plane the difference will be zero as if you have your engine at correct height the exhaust will be coming from the prop that is at water level and with the swell from under the gearbox and prop will leave a void that the exhaust note will be heard of anyway.

Once the exhaust is out of the tuner, the faster it can escape the better, the tuner has done its work already and the exhaust from it needs be got rid off asap.
it will not effect any engine back pressure, that has already been taken care of 5 or 6 inches above the cav plate..

I personally have just drilled out 2 large holes in my spare gearbox as i want to run a over hub prop for just fast flat out stuff, with exhaust venting in the thru hub my take off was terrible to the point of almost zero using a over hub prop that was just blowing out on holeshot..
If i want to slow cruise rivers and not wake anyone up then will probably run the standard gearbox, it only takes 10 mins to change boxes so not a problem for me.