Here's my take on the issue. I don't guarantee that everything is correct, but I haven't intentionally lied!!

I think you are right about using a traditional spraybar--it is up higher, so you would have more adjustment on an inverted engine setup, also, there would be less criticality if a profile installation where a uniflow vent is sometimes pretty far outside the spraybar. That being said, I have heard some people say that they see no effect of putting the spraybar more offset. I don't really understand why this would be so, but I haven't personally made such a test.

There is an article in an old issue of Stunt News where Scott Bair writes about our stunt engines. It is a really good article. One comment he makes is that he can get ~10-15% more power with a "true" spraybar-less venturi than with a spraybar type. I think what he is saying is that the air will flow with less turbulence and therefore with less pumping effort by the piston which has to pull in the air when it is moving up during the comperssion stroke. This costs some of the power from the engine.

Finally there is a question about the venturi pressure drop, which is what pulls the fuel in. Also in  Stunt News, Frank xxx (sorry--I have forgotten his last name--but he use to write the SN engine column and he posts time to time at SSW) showed (as I recall), that you can get a better fuel pull with a spigot version on the "true" venturi.  I can think of no reason not to want a good fuel pull, but I think it isn't supercritical with our 2 stoke Stunt engines, since we run relatively small venturi openings, so have a pretty good pull in most cases. We can compensate to some extent by opening up the needle. This may be different in 4-Stroke engines where you need to pull the air thru the intake manifold and the intake valve, so the air flow isn't completely limited by the venturi size.

Hope these ramblings make some sense.

Thanks guys. I thought about the RC NVA method after I had posted. That is the way I do my combat venturis, because it lets you angle them back out of the way. Angling is not so easy with a cowled engine though.

I think I'll try that on the engine with the big retainer pin hole and try the PA style on the other, which has a 4mm hole.

Here's another related question for Randy:

On stunt engines like your PAs, what diameter to you bore the crank gas passage to?
A post on the speed forum suggested that the venturi be about 90-95% of the crank bore diamteter, but obviously speed engines are a different animal to stunt. It occurred to me that your engines having shared heritage with hot Nelsons would need much smaller crank bores. Are they drilled smaller or sleeved down?

But a big hole does cost you crankcase compression and pumping efficiency. With small ports and stuffed boost transfers being all the rage in stunt, I'm wondering if the same approach is taken with the crank passage?

HI Adrian

Stunt and  speed motors  are  very differant, And  it would also be  very differant about which 90 to 96%  you are comparing, A 95% of a restrictor ( spray bar thu the center) is very differant than 95% of a venturie (spray bar off to the side)
ie:  .181   and  .300 is essentially  the same  size
I have ran many engines for stunt with a large and small hole in the crank, And of course the PA engines, It doesn't make much difference.
If you go from 10,000 RP Ms  up  to   22,000 with the same basic motor it does, so we are  talking about apples and  oranges  here

Regards
Randy