Merc more speed

[xs651]

Hey guys, first question on this board, I just bought a 85 glastron carlson with a 6 cyl merc 115 and i am wondering what the differences are between the 115 and the 150 of the same year. The manual shows the same short block dimensions but diff. carbs, is there a porting difference as well?

[bigbore]

Might wanna have the rings top-pinned:D :D :D :cool: :cool: :cool:

[mattm24tx]

i think the tuner is differnt and the carbs

[xs651]

Thanks, that's useful....I appreciate it!

[RK Outlaw]

Pistons in a 115 are not power ported.

[Raceman]

Pistons in a 115 are not power ported.

That's not necessarily true, and just like the rest of the original question, it depends completely on the year model.

Merc made 115's beginning in 1970 and all the way through the inline 6 model runs. In most cases there's no difference in tuners, but even if there was there wasn't a noticeable result. People lose track of the fact that a tuner is just a VERY SMALL tuning aid and generally effects the way an engine responds at lower RPM's than at full throttle. Some of the 115's were in fact power ported and the after the change in horsepower ratings from gross to net, the 1150 was ESSENTIALLY the old 150. Give us the year model and we can give you a better answer.

The last year that Merc made an inline 6 that was actually rated at 150 HP was 1977. In '78 they had a V6 rated at 150 also, and since they didn't want to confuse the two they dropped the rating of the inline down to 140 with no changes in it from '77 except for the stickers.

All that having been said, most inlines don't respond well to performance upgrades. The reason I say "most" is, obviously if you had an early 70's 115 and changed it to a 150 powerhead you'd see a noticeable difference. If you're thinking of doing some mods to a pre-net rated 115, I wouldn't do it, although it's theoretically possible to turn it into a 150. If you're thinking of it on a post-net rated 115, don't even think about it. I've never seen an inline 150 modified successfully by anything but radical work to increase compression. Above all, don't let Bubba down at the bike shop use his magic porting formula that works so well on dirtbikes. Bubba's ruined more outboard powerheads than sorry gas and cheap oil.

You should either be satisfied with the engine as is, or upgrade to a V6. Inlines also have a distinct set of requirements for long happy life, but that's another topic.

[xs651]

Thanks raceman, that was a TON of valuable information. The motor in question is a 1985, I believe that that is one year before they went to prop shaft ratings. So what do I have here then?

[Prof. O/B]

Nice update, thanks for sharing your insight, thanks:)

[RK Outlaw]

Orginal post was comparing a 115 to a 150 in-line of the same year. 1984 115 having a different piston than a 90, could be power ported. Sorry about the mix up.

[xs651]

Hey Raceman, you mentioned certain requirements for longevity for this motor, what else do I need to know that is different from any other 2 stroke outboard? You talked me into just concentrating on the setup and keeping this motor stock....

[Raceman]

Good waterpump (changed AT LEAST every other year, more depending on amount of use & type of water used in), good gas (I run super unleaded in my inlines, some people disagree) good oil at 40:1 instead of 50. (I run mine at 32:1, but don't mind the smoke), timing no higher than 21º, and pitched where it can turn in upper 5K's at WOT. Lugging the inlines makes em burn pistons.

Actually I don't know what an '84 115 is exactly and don't have sales brochures that new to help. I haven't fooled with inlines much after the '78 1500XS, although I did rebuild an 1150 last year for my neighbor, which was actually a 140 like we were talkin' about with the new horsepower net rating. If yours is the older 115 with a bigger inline in the line that year, then the RPM range I was recommending was too high. It's not that it'll hurt the motor, but it just doesn't make decent power up there. Maybe somebody else can fill in the blanks on the '84 inline 6's.

[xs651]

Doesn't more oil in the mixture above what the oil manufacturer recomends effectivly lean out the air/fuel mixture? What is the weak link that you need so much oil? The speedometer will show if RPM's in the upper 5000's are helping during prop selection won't they? You are a veritable font of usefull info Raceman, thank you for your help.

[Mark75H]

No, the oil ratio is so lean there is no effective change in fuel/air ratio

Even if the oil was completely inert, the reduction in fuel by changing from 50:1 to 32:1 is 1 1/8% less fuel. Changes in temperature, humidity and local barometric pressure along with load on the boat (such as different prop or different tilt angle) will cause more fuel ratio change.

The only tool for rpm adjustments is a tach; much more important (and accurate) than a speedometer

[Raceman]

The best way to answer the lean out question is by reading Merc's old high perf brochures. The T2/T2X and early T3's, as well as some of the other old Merc race motors had recommended ratios of 18:1. The first T3 was basically a stock 175 powerhead with plastic reeds, locked timing, alum. flywheel, and tighter heads on a race mid & gearcase. When Merc recommended 50:1 in the old inlines like the 1500's I always ran "double oil" as we called it back then, or 24:1 mix. I never lost a powerhead in all these years with the exception of one I quickly stuck on a new boat (used, but new to me) and put in the water with the existing gas/oil that was already in it. I found out later that the fuel was over a year old, and regular on top of that with no telling what or how much oil.

The "upper 5000 range" is about durability, NOT helping the performance of the boat. Most cars now don't detonate or spark knock as it's commonly been called when going under load without downshifting, but that's essentially what happens when an inline, especially a 1400/1500 or later 1150 is lugged at not enough RPM at full throttle. Only difference is, lug an old car in high gear and you can hear the rattle, do it to the outboard and you can't.................. the pre-ignition just burns a piston.

[Mark75H]

Thanks raceman, that was a TON of valuable information. The motor in question is a 1985, I believe that that is one year before they went to prop shaft ratings. So what do I have here then?

They went to prop shaft rating in 1984, so the 115 you have is the equivalent of the drive shaft rated 150.

[Raceman]

They went to prop shaft rating in 1984, so the 115 you have is the equivalent of the prop shaft rated 150.

That being the case you need to turn that engine upper 5's minimum at full throttle. A little more would be better than a little less. Put too much pitch on where it can only turn lower 5's and you'll likely burn a piston.

[xs651]

Great info! I haven't got this boat wet yet,it is 29 degrees outside right now but I am trying to learn as much as possible during the down time. That being said, I intend on checking out every thing to make sure it is perfect as it can be, but I can't believe that the prop it came with is even close to being able to run in 5500 plus range, it is a 12.75x21p alum. and it looks to new to have been run long on it. The boat is an 85 Glastron Carlson cvx 16 that weighs 720lbs. dry with an 18 gal fuel tank mounted fwd of the dash [terrible location] and is a pad vee. Do you have a starting place for prop selection to allow these rpm's? anything you have is welcome.

[Raceman]

I would guess that prop would be a pretty good starting point. I'm sure you'll want to upgrade to a stainless when you find a good pitch, and in most cases an engine that would barely turn a given pitch in alum. won't turn the same in stainless. It would surprise me if you'd have to drop to a 19 in the alum, but I guess it's possible. (your engine has a 2:1 gear ratio assuming somebody hasn't swapped the lower for something really old, or one off a 4 cyl engine, so 5800 powerhead RPM only turns the propshaft 2900.

[xs651]

Great i'll start there, I am planning on stainless but I want experiment a little, it's nice to know that I can learn from this prop. Will changes in motor height affect prop selection much and if so do I want to concern myself with motor height first and prop around a height I like? The motor is at stock hieght for this boat, 6 inches between prop shaft center and keel.