| I don't know about as far back as '82 Evinrude 85's, but recent 90/115
are in fact identical block assemblies. But I believe that there are
a lot of little things that make up the difference between 90 and 115
including carbs/jetting (and there are two carbs), reed and intake
port sizes/styles, exhaust system differences, etc. The 115 is more
highly tuned - timing, jet adjustment, carb synchronization, etc are
all more critical. To make all the necessary changes to make a 90 into
a 115 will amount to a lot of effort and more cost than the book value
difference between the two engines which is why no one does these
things.
What is hard to believe that the differences between the engines cost
OMC the $600-$800 price hike. As someone mentioned in another note,
it would seem that OMC profit is in fact the biggest difference betweem
the multiple engines built on the same block :-)
|
| Thanks for the input guys. I did a little research at the library
last night. I did some reading on carburetion, 2 stroke vs. 4 stroke
engines and outboard engine design.
To make a long story short, 2 stroke outboard engines have what is
called a "scavenger" which in essence is a vein with valves between
the top of the cylinder and the bottom. This vein is enabled during the
compression stroke. It allows the air/gas intake volume to increase
beyond what would be possible normally (80% of cylinder volume, 99.6
cu. in. on the Evinrude V4's) by using the pressure differential
between the top/bottom of the cylinder. To increase HP with the same
block, you put more air/gas in the cylinder. That's why the 90 vs. 115
has different carburetor orifices. This implies that the blocks are
somewhat different (maybe the size of the scavenger vein, or the amount
of the time the valve stays open before/during the change to the
power cycle). So, I may be able to get a bit more HP by changing the
carb orifices, but probably not much, without changing the scavenger
performance also. I'd be getting more gas, but the same amount of air.
I can't figure out why I thought the blocks were identical, they can't
be.
So it's just time for a new SST prop. That should get a little better
performance. Later.
|
| RE: Note 1019.3
>> I can't figure out why I thought the blocks were identical, they can't
>> be.
For the most part. they *are* the same. They're cast from the same mold
and have the same bore and stroke. What they do to get 90hp from one model
and 115hp from another seems to be a pretty well kept secret. It involves
lots of little things like changing piston domes and compression, port size
and location, carb size and jetting, and ignition timing. I'm sure it's
possible to boost your 90 up to 115hp *if* you know what you're doing and
what every one of the modifications are. It's also possible to turn your
90 into a 60, with the fuel mileage of a 150 if you screw it up.
Back in the 1950's, boat mags used to regularly show you how to extract
a few extra ponies from your outboard. You rarely see articles like that
any more. Partly because few people are interested. But mainly I suspect
it's because of advertising dollars. If Trailer Boats showed you how to
spend an afternoon converting your 90 to a 115, everyone would buy the
90 and convert it. OMC wouldn't get their extra $1000 free profit on
the 115, and would not be happy campers. No magazine wants to Pi$$ off
OMC or Brunswick, who besides selling motors, build and advertise the
majority of boats sold.
re: Boyeson reeds
Reed valves are simply a one way flapper valve which allow air and fuel to
flow into the crankcase when there's a vaccuum, but prevent them from
blowing back out through the carburetor when the crankcase is pressurized
(the crankcase goes through cycles of pressure and vacuum on each stroke).
Like any thing else, the stock reeds are a compromise. Ideally, you want
a reed with very little tension for low speeds and starting, when the
amount of flow and vacuum is low. For high speed, you need a stiff reed
valve so that it can close quickly between each pressure/vacuum cycle.
The stock reeds don't work perfectly in either situation. Boyeson reeds
are a "2 stage" reed, meaning they actually have 2 flappers. One has very
low tension for easy flow of fuel and air at low speeds, and they fo allow
much better starting and idling. The other flapper is very stiff to allow
for quick closing at high speed, which makes the motor run cleaner and
perform better. Even Boyson reeds are a compromise, however. They give up
the one thing the stock manufacturers can't afford to compromise on, which
is reliability. Their reed flaps are made of fiberglass, which as you can
imagine break more easily than the stock metal ones.
For the average outboard owner, converting your 90 to a true 115 is probably
out of the question. But by adding a set of Boyeson reeds, making sure the
motor is properly tuned and jetted, and playing around with props and motor
height, you can improve performance drastically. I have a friend with a
17' Wahoo (Whaler clone) with a 1989 Mercury 90hp in-line six. He's added
Boyeson reeds, a Quicksilver 5 blade prop, and a "Land and Sea" electric
jacking plate. The boat does 62 mph.
Rick
|
| The Boyeson reeds are available for 2 stroke motorcycles, too, and used
in racing, cuz if one breaks, it won't frag the whole engine.
In normal use, breaking a reed is not very likely.
Carl
|