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Two stroke scavenging systems, what are the differences and benefits of either.

L

leachy

Contributing Member
Hi All,

I have a 4 cylinder, 75HP stroke 1986 model Merc. I was wondering what type of design that is and what are the benefits or problems between the types of scavenging systems.

Regards
Leachy
 
That is a crossflow design.----Piston has a deflector to direct incoming fresh fuel charge.-----A loop charged motor has a flat top piston.----Intake ports are positioned / aimed to scavenge exhaust.-----Benefit is that crossflow types are easier to inspect pistons via bypass covers.-----There really are no " problems " with scavenging systems !-----Why this question ?----Having issues with your motor ?
 
Thanks for the information. No issues with my motor, hopefully it will keep running for another 35 years. I was just reading that some people prefer one type over another for efficiency I think.

Regards
Leachy
 
leachy said:
Thanks for the information. No issues with my motor, hopefully it will keep running for another 35 years. I was just reading that some people prefer one type over another for efficiency I think.

Regards
Leachy
Click to expand...
High cubic inch OMC engines made back decades ago used cross flow designs with ignition points unlike todays engines with Capacitor Discharge Ignition Modules which are marvelous.....just plugging in a side note here and surely part of the MPG equation.

The cubic inches produced great torque at lower RPMs which powered many a fishing boat full of gear and folks. I mean a pair of 35 HP Gayle, Johnson, Evinrude and a few other spinoffs of their design would power an 18' plywood semi-V with 2-3 fishermen, and all their gear at probably a 30 MPH clip....estimated by wake and boat attitude in places like Galveston Bay (Tx.) where I spent a lot of time as a youngster. I learned to water ski behind my 18 HP rude on a 13' boat.....no game changer, and was with 2 skis, but it got me out of the hole and I had fun.

Problem was that they were thirsty. Since HP = (Torque x RPM)/5252, Mercury decided to go after the RPM side of the equation with low cubic inches and high RPMs. OMC patented the "Loop Charged" scavenging system racer mentioned back in '68 (model year) as I recall with the 55 HP 3 cylinder Evinrude Triump. It was a game changer and realizing that the Loop scavenging system produced superb MPG soon everybody wanted to be on the bandwagon.

Since OMC had a patent, Merc had to come up with a solution. They kept the crown on the piston but redirected the cylinder charging path, similar to the looper and called it Direct Charge. I had a 115 I6 built in that last production year of the I6 (1988) and it stood up to OMC in power and economy even though it was running a HP per cu. in, but at much higther RPM than the OMC engines.

By 1989 Merc had a totally different engine lineup and almost all were Loop charged (OMC patent had expired). I currently have a 2002 115 Merc 4 cylinder looper and It is strong on power, and low on fuel consumption. I love it.

Reference: My memory....at 81 you can guess just how accurate it is.....grin.
 
The Lawn boy power mowers ( OMC product ) had loop charging in the mid 1950's----So it was not a new concept in 1968.-----Casting techniques allowed it to be incorporated in multi cylinder motors.---Direct charging was just a marketing term.-----Nothing different on the pistons.-----But I think the crossflow engines benefit from easier to inspect / repair design.-----Modern motors run great, but difficult for the backyard / DIY individual to diagnose !!
 
Texasmark said:
High cubic inch OMC engines made back decades ago used cross flow designs with ignition points unlike todays engines with Capacitor Discharge Ignition Modules which are marvelous.....just plugging in a side note here and surely part of the MPG equation.

The cubic inches produced great torque at lower RPMs which powered many a fishing boat full of gear and folks. I mean a pair of 35 HP Gayle, Johnson, Evinrude and a few other spinoffs of their design would power an 18' plywood semi-V with 2-3 fishermen, and all their gear at probably a 30 MPH clip....estimated by wake and boat attitude in places like Galveston Bay (Tx.) where I spent a lot of time as a youngster. I learned to water ski behind my 18 HP rude on a 13' boat.....no game changer, and was with 2 skis, but it got me out of the hole and I had fun.

Problem was that they were thirsty. Since HP = (Torque x RPM)/5252, Mercury decided to go after the RPM side of the equation with low cubic inches and high RPMs. OMC patented the "Loop Charged" scavenging system racer mentioned back in '68 (model year) as I recall with the 55 HP 3 cylinder Evinrude Triump. It was a game changer and realizing that the Loop scavenging system produced superb MPG soon everybody wanted to be on the bandwagon.

Since OMC had a patent, Merc had to come up with a solution. They kept the crown on the piston but redirected the cylinder charging path, similar to the looper and called it Direct Charge. I had a 115 I6 built in that last production year of the I6 (1988) and it stood up to OMC in power and economy even though it was running a HP per cu. in, but at much higther RPM than the OMC engines.

By 1989 Merc had a totally different engine lineup and almost all were Loop charged (OMC patent had expired). I currently have a 2002 115 Merc 4 cylinder looper and It is strong on power, and low on fuel consumption. I love it.

Reference: My memory....at 81 you can guess just how accurate it is.....grin
Click to expand...
Here is a picture of a Holmes plywood boat like the ones described herein, built in Houston, Tx. Superb bay boats for Galveston bay because although the mornings were calm with slow rolling swells, in most afternoons in the summer the wind would pick up out of the South and it was a rough, wet ride....if you didn't have one of these with a canvas top....I didn't, rented boat, 10 hp engine, one hand on the tiller and the other on a can baling.....no brag just fact.....being a kid I just thought it was fun....not scary!

www.ladyben.com

Holmes

www.ladyben.com www.ladyben.com
 
Thanks for the pic of the piston Faztbullet, I think that is what is in my Merc, 1986 model motor. I actually haven't still had the opportunity to take my boat out yet with to know what sort of fuel consumption is like since I bought it. I think it should be OK as it seems a big motor for the size of the boat. Glastron V156 with 75Hp, so will only need half throttle to get along at a good pace, I think???

I know my father in law and uncle had the same boats, one with a V4 Evinrude 120Hp and the other a 90Hp Yamaha 3 cylinder. The Yamaha was so much more effecient compared to the V4 both being used in similar ways.

I appreciate the information.
 
racerone said:
The Lawn boy power mowers ( OMC product ) had loop charging in the mid 1950's----So it was not a new concept in 1968.-----Casting techniques allowed it to be incorporated in multi cylinder motors.---Direct charging was just a marketing term.-----Nothing different on the pistons.-----But I think the crossflow engines benefit from easier to inspect / repair design.-----Modern motors run great, but difficult for the backyard / DIY individual to diagnose !!
Click to expand...
I recall seeing pictures of what I said! That is why they called it "Direct" charging
 
Texasmark said:
I recall seeing pictures of what I said! That is why they called it "Direct" charging
Click to expand...
I had one and on my Ranger I had a 10 gallon gas tank and I could fish all day running large Texas lakes on a tank of fuel. It had much better fuel economy of my previous OMC cross flows. I had it 7 years and sold it to my SIL and only thing other than plugs was the rear water jacket cover jacket started leaking and I replaced it...impeller was original.
 
Direct charging was a " marketing " term.-----The OMC loopers ( 3 cylinder 55 / 60 HP ) were selling really well.-----Mercury had to come up with a term that sounded good when explained by a dealer trying to convince the buyers.
 
racerone said:
Direct charging was a " marketing " term.-----The OMC loopers ( 3 cylinder 55 / 60 HP ) were selling really well.-----Mercury had to come up with a term that sounded good when explained by a dealer trying to convince the buyers.
Click to expand...
Then why did I see a hole in the side of a Direct Charged Piston that cross flows don't have?
 
Repeat.------A marketing term called " direct charging "-----And direct charging still has the deflector on the piston.-----Loopers are generally a flat top piston.
 
racerone said:
Repeat.------A marketing term called " direct charging "-----And direct charging still has the deflector on the piston.-----Loopers are generally a flat top piston.
Click to expand...
Yes the crown was still on the piston just like cross flow pistons. Unless you have all the exhaust tuning that comes with the looping process you need the crown to keep the bang in the cylinder long enough to get power out of the fuel mix.

Oh on that subject, the original OMC looper mentioned herein was 3 cylinders, not 2 or 4 because at the time OMCengineers hadn't figured out how to get the reflected pulse from one firing cylinder timed properly to push the charge back into the cylinder getting ready to fire (that's published data and I read it....that's why I know). Seems Mercury engineers didn't have that problem because my 2002 Loop charged 115 4 cylinder is of the same basic design as their first loopers of that HP back in 1989 model year.......as I recall.
 
My thought is the machined piston crown diverts the mixture instead directional ports. Is it possible that Mercury had a hybrid system, modified crossflow? Apparently my boat originally had an Evinrude 1969 model (not sure of the size) but the replacement 1986 75Hp Mercury was alot better fuel economy.

Regards
Leachy
 
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