Fast Fred's Secrets of the OMC MOD50's Thread #1

[Danny Pigott]

lets just say its for the same reason that certain mod 50s were run reverse rotation, if i remember corectly

So you are saying that some Mod 50's were run reverse rotation. Jimbo told me once, that there are Mod 50's an then there are Mod 50's.This way he gave away no secrets. I asked him why my Mod 50 would only run 85 to 90 mph. he said that is what most Mod 50's will run.He did not mean his.

[MN1]

I think it has something to do with the exhaust tuning. (The pipe at the top and or the left hand rotation).
Mark N

[Tim Kurcz]

Sorry to keep you all in suspense today, and many thanks to Ron for his head scratching response regarding my teaser of an exhaust positioning explanation. All of you forwarded various pieces of the puzzle; I was waiting for one of you take me to task for leaving one out a BIG one. I'll try to "plug the hole" now with a better explanation.

You all know the piston needs clearance in a cylinder bore to move. In engines which rotate clockwise (viewed from the top down), the piston is forced to the starboard side of the cylinder by the connecting rod during the power stroke. Sure enough, there will be leakage of exhaust gas past the ring and piston on the port side which will cause some contamination of intake gasses below the ring for the next stroke. But,

There's a much more critical event which happens beforehand. Consider the piston is tight against the starboard cylinder wall. This means the clearance is on the port side. So what happens as the piston begins its downward stroke before the exhaust port opens? Crankcase gasses under increasing compression travel past the piston skirt and right out the exhaust port!

So, by reversing port locations in a right rotation engine (as viewed from the top), you effectively use the piston to "plug the hole" that gasses would otherwise escape through. The result is more power! The opposite is true for those down under..... if you're spinning left hand rotation engines - LOL!

Unfortunately for FE builders, we have no choice. Or, do we? How many of you have "low ported" a cylinder thinking you'd get better exhaust scavenging? Or run larger piston to wall clearances to loosen up the engine? In either case you've actually improved the ability for crankcase gasses to escape.

Does this make more sense now?

Tim

[Fast Fred]

with starboard exhaust the piston rides the exhaust side of the bore on the power stroke for the better seal, it's also the hot side, works it harder. works the same on a V6.

most likely had a newer Mod50 powerhead, each one made about 10 more HP than the model before it.

[Tim Kurcz]

Only one comment Fred: Because the exhaust runs down the middle, it works for only 1/2 of the V-6.

[Danny Pigott]

Sorry to keep you all in suspense today, and many thanks to Ron for his head scratching response regarding my teaser of an exhaust positioning explanation. All of you forwarded various pieces of the puzzle; I was waiting for one of you take me to task for leaving one out a BIG one. I'll try to "plug the hole" now with a better explanation.

You all know the piston needs clearance in a cylinder bore to move. In engines which rotate clockwise (viewed from the top down), the piston is forced to the starboard side of the cylinder by the connecting rod during the power stroke. Sure enough, there will be leakage of exhaust gas past the ring and piston on the port side which will cause some contamination of intake gasses below the ring for the next stroke. But,

There's a much more critical event which happens beforehand. Consider the piston is tight against the starboard cylinder wall. This means the clearance is on the port side. So what happens as the piston begins its downward stroke before the exhaust port opens? Crankcase gasses under increasing compression travel past the piston skirt and right out the exhaust port!

So, by reversing port locations in a right rotation engine (as viewed from the top), you effectively use the piston to "plug the hole" that gasses would otherwise escape through. The result is more power! The opposite is true for those down under..... if you're spinning left hand rotation engines - LOL!

Unfortunately for FE builders, we have no choice. Or, do we? How many of you have "low ported" a cylinder thinking you'd get better exhaust scavenging? Or run larger piston to wall clearances to loosen up the engine? In either case you've actually improved the ability for crankcase gasses to escape.

Does this make more sense now?

Tim

Tim I can see what you mean.But why was the engine not built this way in the first place. An why are all inline motors built with the ex. on the right side.The Mark 58 merc. had in line wrist pins, they cause piston rattle. so merc off set them to stop this.I think the in line pistons were better because of what you said about the f-3 ex. piston seal.Would not reverse rotation do the same thing move the clearance to the other side of the cyl.

[Fast Fred]

ya, only works on one bank of a V6.

the intake is the cool side of the bore, it's easier on the parts to run the cool side.

thinkin with a 1 to 1 lower unit with two or more pinion gears, spinnin backward
may or may not be better. don't think it's guna work out with the Nitro foot.

[seaward]

Tim
Do you run a single pulse line to both pumps or do you use 2 different cylinders for pulse pressure.
Hal

[Tim Kurcz]

Run one each from the #2 & #3 cylinders. Be sure to use high strength Loctite in the fitting in #2 as the casting is pretty thin.

Tim

[John (Taylor) Gabrowski]

Tim: I keep hearing stories that the OMCs of this era have very thin castings of what some term poor aluminum that its so porous it leaks? The 3 Holer here does not look or feel light weight so what is behind the stories and what can one do to make things better if so?