exhaust

**river_ratj40** · 03-25-2007

i have read articles and books related to exhaust tunners and expansion chambers, and i have a pretty good idea how an expansion chamber on a 2 stroke motorcyle work. what i dont understand is how a mod 50 exhaust (that resembles a cowbell) works as an exhaust tunner. i understand that the bigger the tunner the more exhaust you can pull out of your cylinder. but it doesnt make since in my head that the soundwaves in an mod 50 exhaust tunner would keep fresh air and gas from exiting the exhaust port inside the cylinder. can anyone explain just how these tunners work in relationship to an expansion chamber. i know im hard headed and not looking at this correctly, but any help would be greatly appreciated.

**Mark75H** · 03-25-2007

Three cylinder two strokes have a unique feature that places one cylinder's exhaust port closing time just after the exhaust port opening time of an adjacent cylinder. This lets the blast from one cylinder force unburned overcharge just outside an adjacent cylinder back into the exhaust port that is closing - restoring the fuel air mixture that would have normally just been wasted out the exhaust port. This also increases the pressure in the cylinder where the piston is closing the exhaust port - so the motor has more torque from using fuel and fresh air that would have been wasted and from the increased pressure (same as higher compression ratio, but with less stress on the motor).

The degree time between cylinders on a three cylinder is 120 degrees, the exhaust open time is usually about 165 degrees, so there is about 45 degrees of exhaust port open overlap between adjacent cylinders. When the speed of the motor agrees with the lengths between the exhaust ports you get the best effect.

Engineers at Scott-Awater noticed this effect on their 63 ci triple back in the 1950's. The triple was an extension of their 40 hp twin, which barely made 40 hp ... adding a third cylinder increased the power to 60 hp. With a little work the 40 was bumped up to 45. When the same work was applied to the triple they expected 67.5 hp, but got 75.2 instead. The 7.7 bonus hp came from the triple exhaust feature.

**Roy Hodges** · 03-25-2007

so, McCulloch inherited a 75 h.p. motor , I had thought McCulloch developed it , glad you set the record straight. Thanks .

**Mark75H** · 03-26-2007

Originally Posted by Roy Hodges

so, McCulloch inherited a 75 h.p. motor , I had thought McCulloch developed it , glad you set the record straight. Thanks .

McCulloch bought Scott during this time, but no major personel changes at Scott were made, just change in ownership and name. Brewster Atwater Jr even stayed on to the end of outboard production in the late 1960's. The 75 was brought to market during the name change transition: Scott-Atwater, Scott, Scott by McCulloch, McCulloch

Many late year 1961 60 hp Scotts were actually 75 hp with leftover 60 hp covers.

**iwanajohnson** · 03-26-2007

Engineers at Scott-Awater noticed this effect on their 63 ci triple back in the 1950's.

so there was a 63ci 3 cylinder. I had a guy telling me about this the other night and I called him crazy. what years were they made? Another question with the exhaust, as river rat said, how does the cowbell shaped desighn help performance so much? lot of good information though, keep it coming.

**Fast Fred** · 03-27-2007

the Mod50 tuner is long, the funnel shape is not as pronounced as like SST60,

**Mark75H** · 03-27-2007

Originally Posted by iwanajohnson

Engineers at Scott-Awater noticed this effect on their 63 ci triple back in the 1950's.

so there was a 63ci 3 cylinder. I had a guy telling me about this the other night and I called him crazy. what years were they made? Another question with the exhaust, as river rat said, how does the cowbell shaped desighn help performance so much? lot of good information though, keep it coming.

They started in 1958 and ended just before 1970. It was a very popular motor, probably 200,000 or more of them made thru the years. In 1958 they were only 10 hp behind the Merc Mark78 (which was the highest horsepower outboard you could buy), by 1969 they were only half the power of Merc's biggest motor. They were also sold thru Sears under the Sears and Elgin brands.

The bell at the end probably adds 1 or 2 hp, it is not the important part

**river_ratj40** · 03-28-2007

is there any way to figure out how long and wide to make your tunner for a 3 holer, other than the try and fail method.

**iwanajohnson** · 03-29-2007

yea, like you told me buy a monkey!!!

**Fast Fred** · 04-04-2007

well, seein that nobodys takin a swing at this

. this is what i got,
thare is two styles, the Header, like 49 and the 56, and the troff, like the Mod50's, Yamaha, Hot-sue.

i'm thinkin that when one cyl Fires it's exhaust, the squese ( thinkin the turns in the header of the 49 make up most off the squese) holds it up just long enuff
to presureize the exhaust for the closin cyl above. and that helps it out more
on the scavagin than the reflection only.

stinger stock exhaust is dif', but for the rest, this gets ya some ware close.

under the exhaust addapter, the exit hole of the exhaust, what ever the dieamiter is X7 long (if rectangle or oval go with the longest mesurement, you can all way make it shorter, not easy to make it longer) with 2 1/2 X the exit hole area, about, goin out