Uim 125 class...

[G Stillwill]

The rod bearings for the OMC 15 A motor are NLA from OMC and that is also what we use in the Hotrods.

[win-or-flip]

Long Live the Two Stroke

I love two strokes as much as the next guy and i really doubt 3-4 lap heats are killing our environment. My interest is in using a method that is newly available per the rules, not to hurt the current method (two strokes). and if the almighty two stroke cant be beat then whats all the fuss about. i would like the rules to give people who want to explore new territory a fair chance. i agree that the Moto3 manufactures are out of their minds charging 35k us dollars for their 250cc racing replica but i think racers might find a cheaper route using what weve always used, easily availible motorcycle engines. there are tons of dirtbikes out there with fancy things like fuel injection that we could pick up for a couple thousand. also hondas little rumor is that their single cylinder 4-stroke NSF250R makes ~48 Hp at 13k RPM and weighs just 55 lbs. Compare that to a 15hp omc that weighs in at 70 some odd lbs. that should be our aim. as far as each rpm costing more, keep in mind that F1 cars revving to 18k RPMs use iron cranks and con rods.

[Smitty]

Again, the question is do you want to give 4-strokes a displacement advantage or other advantages that handicap the 2-strokes, guaranteeing that eventually you will have to have an expensive, harder-to-maintain 4-stroke to be competitive?

This is a tiny amateur sport in which there is hardly a ghost of prize money or sponsor money, and the expense is already hard to justify for an ordinary working stiff or family man with a thousand other expenses. People do this only for the love of it, including our engine manufacturers, and if it gets any more expensive, the whole thing might dry up completely.

[Rex Hall]

Engine manufacturers

The engine manufaturers are doing everything they can to keep the prices down and the technology high. To build a complete 125 motor with programmable ignition, hydraulic lift system, and tuned exhaust with silencer, that makes 48 hp and turns 14000 rpm for $5800.00 plus shipping is a big deal. Parts alone should keep you from wanting to go to a 4 cycle engine and the resulting cost to maintain it for a season.
Rex Hall, GRM

[Smitty]

Today's engines are expensive and yet are an incredible bargain. I don't know how they are able to do it, do build and sell them at those prices, given the tiny production numbers!! I think it would be shameful to undercut the efforts of today's engine manufacturers, with all of the time and effort and money they have invested, to give any artificial advantage to 4-strokes, just a dirty way to treat those fellows.

[bill van steenwyk]

Today's engines are expensive and yet are an incredible bargain. I don't know how they are able to do it, do build and sell them at those prices, given the tiny production numbers!! I think it would be shameful to undercut the efforts of today's engine manufacturers, with all of the time and effort and money they have invested, to give any artificial advantage to 4-strokes, just a dirty way to treat those fellows.

Smitty:

I completely agree with you regards 2 strokes in PRO Racing, and if (IMHO at least) the Mod and Stock guys are thinking ahead, they might want to consider the same unless future engine supply, or lack of same, would demand the Yamato 4 Stroke spoken about on this or another thread as run in the parimutuel racing in Japan as an alternate engine to the Yamato 80's, 102's, and 302's presently used her in the US when that supply of available engines dwindled to the point they were NLA and parts became a supply and cost problem.

As far as any artificial advantage to the 4 strokes, you may want to remember the reason this came about is there was a concerted effort by the bike folks, Harley-Davidson in particular, to have a "formula" for 2 strokes against 4 strokes, as in the world of flat track racing where the Harley's reigned supreme for a lot of years after WWII until the Japanese started importing the 2 strokes into the US, and even for some years after that during the time it took to get the larger displacement engines (2 strokes) competitive with the Harley's, HD (the AMA by another name) had it all their own way for a long time. When the time finally came they had to bow to public and racer demand to allow the 2 strokes to compete, they came with the 2 to 1 cubic inch formula. When that did not work and they still were not competitive any more (H-D) they just banned them altogether until just a few short years ago when the Japanese were starting to cease manufacturing them anymore, and consequently they were not a threat to win.

AT THIS TIME AT LEAST, the same situation does not exist in PRO racing as existed in bike racing, as there are no 4 strokes running now or to my knowledge the foreseeable future, and unless some type of ban of 2 strokes would occur, it really is a moot point with 4 stokes for all the reasons previously mentioned by you and others. That would seem to hold true for some time into the future unless something happened to the present suppliers of the PRO Category such as GRM and VRP.

I think what you might be seeing with the UIM rulings re this type of formula so as to allow 2 strokes to run against 4 strokes would be because a large part of boat racing that takes place on the World stage, and that they are the governing body for in other places that probably have not even heard of APBA, is to give the 4 stroke larger displacement motors in OPC type racing somewhat of an "even" starting point so as to allow them to compete against the existing 2 strokes for a time until the 2 strokes go the way of the dodo bird, like you and I in a few more years.

Like you, I am a "two stroke forever" type guy.

[Smitty]

I'm NOT against 4-strokes, just against giving them any special advantage. In the mid-'70s, one of the first street bikes with four valves per cylinder was a Yamaha 500 parallel twin, very high-tech for the day. I thought at the time that it would be an interesting project to try to make a 500cc outboard racemotor of one, although I speculated that it would be a shaker and a breaker of lower units and shearpins, even assuming you could get it to oil properly in the vertical position. But if the more daring experimenters in the game (Paramore?) want to do things like this, great! That kind of basement-built special is what makes the PRO division so interesting. Just hold them to all of the same displacement, fuel, induction, and other rules that the 2-strokes have to meet in each class.

[Yellowjacket]

Thoughts on 2 strokes

In reality a 4 stroke shouldn't get a displacement advantage. The idea was that a two stroke gets to fire on each revolution, while the 4 stroke fires every other time, so the 2 stroke has an advantage in that it's getting to work it's displacement every revolution where the 4 stroke only expands its displacement on every other revolution.

But...... a typical high performance 2 stroke has roughly 180 degrees of exhaust port opening duration, so in reality, about half of it's displacement is used for scavenging and filling the cylinder. So while a 2 stroke fires every revolution, it only has roughly half of its displacement for useful work. To me it seems that the reality is they should get equal displacement, but that doesn't happen.

As the engine speed gets very high it gets harder to scavenge the 2 stroke, where the 4 stroke, by virtue of the complete exhaust stroke does a better job of efficiently scavenging the cylinder.

2 strokes will always be lighter for a similar displacement, since there aren't any cams, cam drive and valve train, as well as a much more simple and lighter head, a shorter overall engine height and so on.

Bottom line is that a 2 stroke surely shouldn't be put at a disadvantage in displacement in a classing scheme.

One big disadvantage of a 2 stroke is the fact that a lot of unburned fuel goes out the exhaust if the system uses a conventional carb or some type of intake injection. That can be readily overcome (for a price) by using a form of direct injection (either the Orbital system or a high pressure fuel injection system). The other neat thing about a direct injection system is that there isn't any fuel in the crankcase and the oil is not diluted which results in less oil consumption and a lot better crankcase lubrication.

With the Optimax and other direct injection outboards in production, and direct injection coming into use in cars, I'm thinking that it won't be long before these parts start showing up on the used market, and that might make for some interesting mod motors....

[Smitty]

Just giving this thread a bump in order to hear whether there has been any actual movement on this (ill-advised) notion of giving 4-strokes a displacement advantage in the class rules . . . anybody?