Inline 6 performance questions, late vs earlier models…

[Pete Keppler]

Thanks everyone for the advice on getting my inline-6 running. After a good cleanup of the fuel system and carbs, and installing a water-separating fuel filter, the engine is now running very smoothly up to wot.

Now I’m back to the question I had a couple of months ago when I first bought the engine, before I found out the engine was ingesting water through the exhaust plate and had it rebuilt.

The performance just doesn’t seem to be what I expected. The 1985 115 Merc (prop shaft rated) engine is mounted on a 1985 Glastron Carlson CVX-16 (16”7” v-botton with pad, 725# dry, maybe about #1600 loaded with one person) with the anti-cavitation plate mounted even with the pad and turning a 21” SS cupped thru-hub Merc prop. I’m turning 5400 rpm at 48-49 mph (gps) so that works out to 9% slip, which makes sense.

I expected to turn higher rpm with this setup since in 1980 I used to own this same model of boat with a 115 Merc (crank shaft rated) at the same height and with the same model prop. This setup also ran 5400 rpm (about 51 mph, but that was with a water-pressure speedo), so I expected the higher hp six to spin quicker.

One thing that I’m wondering: I have read here that the late model IL6s with ADI ignition were engineered to have much more low end power, which this engine does. It hauls out of the hole pulling a skier with this 21” prop way faster than my old 115 did with a 19” prop also pulling a skier. The acceleration is outstanding (something the earlier IL6s were not known for). Could it be that they ported these late model 6s differently for low-end power and that these models just don’t spin as fast and generate high rpm hp like the earlier ones do? I have several old Merc catalogs, and I noticed that the 1980 catalog specifies the 140 should turn between 5300-5800 rpm and my 1985 catalog specifies that the 115 should turn 5000-5500 rpm.

I’m planning on raising engine height a couple of inches to see what happens, and if the boat can keep its hole shot for skiing and excellent handling characteristics, then I’ll consider trying a jack plate with a Trophy prop. What I’m wondering is, is this worth the money and effort… will this engine respond with higher hp and revs, or am I hitting a design wall?

Your comments and opinions are appreciated!

BTW: I had the engine rebuilt with “low-dome” Merc pistons, so I realize that this is not going to have the power of one of the earlier model 150s.

Pete K.

[Laker]

Your motor is too deep.
You need to get it up out of the water and the only way to get the best of both worlds is to install a hydrolic jack plate so you can lower it when going slow and raise it when you want to go fast. It will handle better too when you jack that motor up high durring high speed runs.
That motor will pull a 24-26p chopper at 5800-6000 Rpms if you rase it 5 inches....... However. You need Dual cable steering, solid mounts on an I6 for sure (Stockers are very soft) water pressure guage and low water pickups. If you dont install LWP nose cone Install a Pressure guage then raise the motor one inch at a time and keep a VERY close eye on the guage. A good guide line will be start 2 inches higher than you are now and keep going up. But to get water ski power and Top end you need a Hydrolic Plate to adjust on the fly.

[skydog]

Skydog

[tombrown]

Pete, Al Weeks gained 4 mph by raising his OMC V4 two notches on the transom clamp. I raised my old 1500 about an inch and a quarter and found a few mph and way better handling.

It's too bad I never had a chance to try the old 1500 with my jack plate but I'm confident it would have worked very well. With the jack plate, I'm sure I could have gotten into the 60s with a chopper. My best speed with the cavitation plate of my 1500 about 1/2 inch above the pad and a 24p small ear chopper was 56 mph.

With the XR-4, jacking makes a BIG difference. I'd probably ditch the Trophy, though. They are too expensive. I would try to find a 22p chopper for speed and cruising and just use a 21p aluminum for skiing.


Regards,

Tom Brown

[Jeff_G]

The 140 had low domes with power porting, it became the 115. The 115 had high domes with no power porting it became the 90. Later 115 had different port timing, late 90's had small ports. When the 140 became the 115 it was the same engine now prop shaft rated, the WOT recommendations were reduced, probably from fuel concerns. The other differences were internal in the carbs.

With your weight you need a prop with good bow lift. You won't have to raise your motor too much, keep in mind any raising of the motor will hurt low end pulling. I would agree witha water pressure guage, but wouldn't worry about low water pick ups. Playing with the weight distribution and props will make all the difference.