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

[Mark75H]

How do the designers determine what makes the best combustion chamber for what kind of loop charged engine. Is there some kind of set of rules governing a given overall engine design or??

No there is no rule ... just like props ... test, test and test some more

[John (Taylor) Gabrowski]

Mark 75H:

That is too simplistic an answer no one can really accept. When putting together an NOS complete class B 348cc stock racing British Anzani, its original head installed at the factory was bell shaped no different than the OMC in basic design. The Alky head on the Anzani though is different with offset combustion chambers just like that copied in principle later by Quincy Flatheads. There must be engineering motivations to make those kinds of choices and decisions when engineering any kind of 2 stroke loop charged engine? I am sure testing over and over again with design variants helps prove designs but there must have been some very basic technical rules involved that helped to make decisions and not just but also included hunches?

Anyone else have some more insight? There are serious engineering concerns here at play.

[Mark75H]

There are squish band considerations that can be used to arrive at the best turbulence and spark plug placement guesses to reduce overheating of the piston.

These issues are all covered in the standard texts for 2 stroke design: Gordon Jennings' Two Stroke Tuner's Handbook, etc ... but they all come back saying there is no substitute for testing ... as many configurations that seem to be "right according to theory" are just plain wrong for any particular motor.

[Fast Fred]

hey 850,
nice finger ports, the blending work looks good as well. that head is nice, who made it? what happened, can't see the cyl' walls good, looks like a hone and reload with fresh rings from hear.

i'll put up that map, tell me if you can see it .

[Sam La Banco]

John if it were me, I would start with the high compression factory head dimensions. Stop development at that point, and measure cc of combustion chamber and squish band dimensions. Run the motor, Optimize timming, jetting
and record running temp.

Then I would say your ready to start a development program. (if you want to do it with the sound engineering princples you mentioned).

As Sam said, then it's all testing after that,.......... well except for a whole lot of compromise between motor survial and power.

The Jennings book is an excellent ref. on the subject.

[Fast Fred]

i would want to see how the rotary valve set up worked on the outboard. no clutch, sleds gots a clutch, bike ya it's got one too. was a dude that ran a Rotax, i think, i was not thare, but any way i was told it did not have the torque it need low end.

i'm feelin ok about sayin that it's not guna make a gain in hp that would make it worth all the work.

[Fast Fred]

the more meat you take out of the crank case the lower the transfer psi.

[Bill Gohr]

You're not gonna start about squish area's, ratio's, and angles are you? Too many variables to relate to to make a "blanket" statement regarding this topic, and if there was a given "standard" alot of engineers would be out of a job. Read the Jennings book it will either make you think or totally confuse you, you can try numerous configurations on the same engine and 2 or 3 out of 6 will have the same outcome, which is correct????????????

[John (Taylor) Gabrowski]

The interesting thing about looking at engines with 2 types of heads is evident and that is from the late 1950s and into the 1960s. Some took the Anzani bell bead and machined it down to where it could produce high compression for Alky racing. Some used the gasoline bell head to use more nitro in the methanol fuel. Still others made the high compression Alky head higher compression still and still put more nitro in the fuel cutting back ignition timing to get power another way. All that took a lot of configuring way back then by some real knowlegable people. What were they thinking?

Back to the OMCs, to make them cooler other than ditching thermostats if present what other cooling tricks can be used to specifically cool them down and keep them that way? Any old Deflector Alky Merc tricks of any use?

[Bill Van Steenwyk]

Fred:

I have read many of your posts about the 3 cyl OMC and other engines you have worked on with much interest. You seem to have the knowledge and capability of doing a lot, if not most of your own work, and don't have to depend on second hand info to find results. I enjoy your posts and pictures, and hope you continue with them.

Regards your comments about rotary valves: Since the late 60's until his death Dieter Konig built a large number of disk type rotary valve engines that for a 25 year plus period were the dominant engine in the "alky" racing community. His basic design with porting improvements is still manufactured today and marketed under the "Konny" brand. If you would use the word "Konig" on the search function of this site, you would find much information that would probably keep you busy reading for several weeks. These engines used neither clutch nor gearbox, merely the same type of lower unit used by many other outboard racing engines, one single gear ratio. Wayne Baldwins thread on this site entitled "An Amazing Story" has many stories about the Konig engines and their performance. I understand your thoughts about narrow powerbands, but the Konig rotary valve "cut" was such, along with porting, that overcame a lot of those obstacles. Along with that was the way the motor was designed, i.e, two cylinders firing together to enhance the torque of the engine. I am sure you would find their design interesting. They were made in quite a few different displacements, from 250CC up to 1000CC and 3, 4, 6, and 8 cylinder. There was even a 12 cyl designed, but I don't know whether it was ever built. There were also a large number of 2 cyl's built, but to my knowledge all those were reed valve. The other models must have been built in the hundreds as far as quantity was concerned.

Regards your comment about crankcase volume versus pressure:

I think I understand you to mean the tighter the crankcase volume, the more pressure to force fresh fuel/air mixture into the combustion chamber. This has always been "conventional wisdom" but their are some cases where more is not always better, in the case of an engine restricted carb venturi wise for example. For more info in this you can also see some of John Taylors threads where posts relating how increasing the crankcase volume increased the performance of the engine.

Theory is great, but nothing beats "doing it" and then putting it on the boat and testing. I am sure you have experienced this yourself. As I mentioned, I really enjoy your posts and pictures. Hope you are able to continue.