Airport Speedway does have an allowance for a single cylinder 4 stroke with a maximum displacement being 636cc and a weight of 600 pounds.

No one has taken advantage of this lower weight allowance simply because the single would have to produce roughly 80 Hp in order to be competitive.
Most singles weigh at least 60 pounds less than their 4 cylinder counterparts.

A twin cylinder might be competitive if allowed to weigh 600 pounds, but everybody seems focused on 4 cylinders since they produce the most power.

The AMA allowed larger displacement singles and twins to run with 600cc multi cylinders that produced near the same Hp.

I wonder if the universal engine rules would also apply to single cylinder 4 strokes with up to 636cc displacement?

Not quite, it's pretty clear...
"2. Overall Engine
• Must be a Production Motorcycle Engine, 4 cylinder, 4 stroke"

Not quite, it's pretty clear...
"2. Overall Engine
• Must be a Production Motorcycle Engine, 4 cylinder, 4 stroke"

That means whoever wants to run a single cylinder 4 stroke 600cc will have to race at Airport Speedway.

Lanco does also allow single 4 strokes but they are of a much small cc displacement like 250cc.

Some tracks do allow 450cc displacement 4 strokes to run with the 250/270cc 2 cycles too!

Airport Speedway rules seem "more forward thinking" by allowing two, three, and four cylinder engines for the 600cc class.

Page 31 ! Section 12 Clearly states

http://www.airportspeedway.com/BMMRC%202009%20ByLaws%20Rules%20and%20Regulations. pdf

31
Section Twelve
600cc Micro Sprint Universal Engine Rules
Item 1. Terms and Conditions
a) Internal Stock parts – must use original manufacturer parts for the particular year and make engine except where noted. No mixing of parts from different year and make engines, even if made by the same manufacturer. Nonstock external bolt on parts may be used (example: billet oil pan, starter, water pump cover, oil cooler, pipes, etc.)
Item 2. Overall Engine
a) Must be a Production Motorcycle Engine, 4 cylinder, 4 stroke
b) Current year engines allowed
c) No Titanium anywhere in or on the engine, unless it comes stock from manufacturer of engine on production bike
d) Must have engine, clutch, and transmission all in one unit
e) Must be Chain Drive

Airport Speedway rules seem "more forward thinking" by allowing two, three, and four cylinder engines for the 600cc class.

But if there were teams that were running this set-up, they probably would not have been 'outlawed'. Obviously, it wasn't a good combo. I don't see how that is 'foward thinking', and thus making the U6SA 'backwards thinking' based on your analysis. I think quite the opposite is true...

I am not making a judgement on the universal engine rules at all. Airport Speedway made an allowance for other possible engine combinations involving single, twin, and three cylinder 4 stroke motorcycle engines. Now if you could get a competitive single cylinder Rotax and keep the weight close to 600 pounds that would be a challenge. It is still possible a future twin might make competitive Hp against the 4 cylinders was my point. They will not be allowed to compete under the universal engine rules. We have seen 450cc 4 strokes now making more Hp than ever too!

Well apparently there was an oversight since page 27, Section 9, Table C still has provisions for single cylinder 600cc. Obviously their adoption of the universal engine rules for the 600cc 4 stroke class found later down the 2009 rules makes no such allowance. My mistake by now fully reading Airport Speedway's 2009 rules further down. Ops!!!

we used to have a fully built rotax 675cc single and it got smoked by stock f3's without flatslides, now the cars are nearly 2 seconds faster...it would be bad and a waste of money.

Well Dave you are right about the 600cc singles, but there have been advances with twins. If they allowed twins in the 1000cc sidewinder class, there are a couple that would give the 4 cylinder 1000cc engines a run for their money. However, these engines are expensive even used
(e.i.-Ducatti). Would have liked to see a Harley-Davidson class as there would no doubt be a huge following. It would take an 800cc single to come close to matching the latest 600cc 4 cylinders.:rolleyes:

I don't think they should allow totaly differnet engine combinations to race. that would just get were it would be one having alot more power than the other and people trying to make it fair with weight and it become a big mess. Also what if you were the only one with this totaly new style engine and you were winning by a straight would you realy feel that happy with that win? I would be more proud of my self If i knew I had the same engine as other people and won. I also think bringing in different engine styles would be very expensive trying to make new parts to make the engines work and be competative

I am not making a judgement on the universal engine rules at all. Airport Speedway made an allowance for other possible engine combinations involving single, twin, and three cylinder 4 stroke motorcycle engines. Now if you could get a competitive single cylinder Rotax and keep the weight close to 600 pounds that would be a challenge. It is still possible a future twin might make competitive Hp against the 4 cylinders was my point. They will not be allowed to compete under the universal engine rules. We have seen 450cc 4 strokes now making more Hp than ever too!

jr culler did it years ago at airport with a rotax.....was neat but not all there