Ladyflyer flat knows what she is talking about!
By the way, my first dirt bike was a B-50 motocrosser, and I worked for a fellow who used to race a Matchless 600cc Typhoon - this was at a Triumph-BSA- Norton shop in IL.

Exhaust tuning works on 4-stroke engines and we used it to boost the power of the B-50. For an engine that peaks around 6500 rpm, we used a straight pipe with a length of around 31 inches (from memory). You can extend the range of the boost effect by using a tapering megaphone. You can also minimize the "off-tune" phase or megaphonitis by using a small reverse cone on the end of the megaphone.

For model engines turning much higher rpm you would need a shorter pipe - a guess of 15-18 inches should put the boost in the 12,000 rpm range.

I think it would be great to get a thread going on this,Lady flyer just so i know were you are coming from do you think Harlys are fast.

Oh ya lets talk about whats being built now and winning and not antiques that they don't make for good reasons.

Say Wildthng:
Last time I checked, the laws of physics were still the same.
The power boosting methods of the 1970s apply even in the year 2002. And if you'll look at any normally aspirated high performance 4-stroke engine, you will see careful attention paid to intake runner lengths and diameters and exhaust lengths and diameters. Some of the formulas to determine these lengths were developed by Chrysler engineers more than half a century ago.

Diablo a lot was learned years ago that still works today but there have been a lot of advances over the years that make our engines faster,and more efficient than in the 60s or 70s.I don,t profess to know alot about cars but i do know a bit about motorcycles and model airplane engines and in the offroad circuit 2 cycles rule for very good reasons.

I love the 4 Strokes but I am considering getting into some pattern competition via the SPA and they don't allow YS engines!
I have a ThunderTiger 4 Stroke and love it, but it has a huge flat spot from just above half throttle to full throttle. As you can imagine it is hard to do consecutive loops with this flat spot. I understand this is a problem with all 4-Strokes except the YS, though I suspect it is worse on the ThunderTiger.

I have considered taking the choke adapter off the TT and using an ocilllating pump to make up for the loss of suction. Anybody try this?

Saitos do not have this flat spot, their throttle response is very linear unless not set up correctly. If your TT has a two needle carb, it should not have a flat spot either. If your needles are set correctly you may have a fuel or a glow plug problem. My Enya .46 is the only four stroke I have that had a flat spot, I blamed it on the air bleed carb but found out I was running too large a prop.

Maybe not the one's you have but some do see thread by Hawkeye58:

http://rconline.community.everyone.n...rtOrder=descpl

Actually it's a problem with most 4 strokes but I suspect it is much worse on the TT 91. The carb is way oversized and will not have enough suction, the choke adapter is much smaller than the carb and without it you will deadstick after half a tank. But there is absolutely no RPM increase from 90% open to 100% open! This I know for sure cause I made the problem better by limiting the wide open throttle to 90%, but not enough. From about 60% to 90% there is only about 600 RPM decrease. Instant throttle response, just you can't notice the power decreasing until you move the stick down about 30% or more! Then it starts decreasing rapidly. Great power, in fact I don't think there is a more powerful 91 except YS.

It sounds like you need a prop that will let the engine rev a little more, if it will not speed up from 90 to 100% throttle opening it is because it simply does not have enough power to rev any faster. That is not a flat spot. A flat spot is when an engine is accelerating and the torque curve flattens out briefly or even decreases some, then resumes building torque. My brothers race car engine has a flat spot at 4,400 rpm where it drops 19.6 foot lbs. of torque with a corresponding 100 degree drop in exhaust temp. Obviously a carburetor problem. Couldn't be in a worse place because it's right where he hits the straight. I believe a slightly less load would cure your TT.

What I mean by it is that the throttle is not at all linear. That is not enough control etc. I think 10,600 RPM on a 14-6 prop is revving plenty enough! A carb that is oversized does this, butterfly valves do this unless staged with a secondary set, or the rate of opening exponentially decreased. Not sure about our rotating barrel carbs, but would appear to be better than butterfly valves. The TT problem is that the restriction of the choke adaptor is making the last part of the throttle ineffective.

I'm making an assumption that your TT is a .91, if so and it turns a 14x6 at 10,600 rpm, you have nothing to complain about, I'd consider that a great engine.

Great sport engine, but maybe not a good pattern engine. If I cannot control speed on downlines I guess I'll put it on the shelf and get a 2 Stroke.

Took my RCM 40 to the field this AM and flew for two hours, just getting current after winter layoff. Would love to have a Harley but not here where I live, not that brave. The RCM is powered by a Saito .40, will do 25 minutes on a tank. Cannot make it hurry, just like a Harley. I was a big time Cal Rayborn fan as a kid.

That wont help you any, 4-stroke downlines are far superior, unless you want to run 3 or 4 blade props on the 2 stroke. But why a 2-stroke when the 140DZ from YS takes on the OS 140 with ease and has the beauty of 4-stroke operation

SPA is about flying older design pattern ships from the 60's and 70's. Here is their site:

http://www.rcpattern.net/spa

As you can see the YS engines are not legal. Only 91 normally aspirated 4 strokes or 60 sized 2 strokes are allowed.

I will be flying novice so the only long downline will be from a stall turn. The highest scored maneuver are 3 consecutive loops. It's also the hardest which is why it is the highest score. With the TT .91 acting the way it is it is even more difficult. I said I would buy a 2-stroke. Actually I won't because I have a good one or two now.

YS produced fantastic 60 size 2 strokes. 45s as well.

Very legal

But they don't sell them anymore, or do they? Besides he was talking about the four strokes. The .91 four strokes are popular in SPA, mostly the OS for some reason. You would think the .91 Saito would be more popular.