Stock exhaust performance

[jumjum01]

Hello.
Does anyone know which of these stock RD350 exhausts makes the best performance on relative stock 84 RD350?

- All black exhaust 31W-14711
- Black with Silver/chrome muffler 1UA-14721

31W-14711 seem to rev higher to about 10000rpm, where 1UA-14721 refuses to rev past 9000rpm

regards Kristian

[MK]

Define "best performance".

The older one usually revs higher, the newer 1UA has better low/midrange.
The rpm issue can be improved by cutting off the end cans and widen the stinger. Something like 20-22 mm inner diameter.

[(F5)]

Also 1UA ignition will call time early. I swapped mine with a 250 and it gave same curve but revved on 500rpm rather than dramatic cut. Of course these days you'd just Ignitech.

[jumjum01]

Well if the 1UA made more HP in the entire rpm range over 31K, I would go with that one. I have cut end cans off the 1UA and are in the process of widening the stinger. For now I run the 31K exhaust and found more HP after 8K rpm by using the microsquirt to control the ignition timing. Since I had problems with the stock YPVS power valve controller, I made en adruino based controller myself and are now able to program the YPVS valve opening with my laptop. I would also like to build my own expansion chamber exhaust system in stainless using MKs pipe guide but its a looooot of work. Next step might be buying a used cylinder head and mill to set proper squish.

[jumjum01]

Did a dyno run today with the stock 31K exhaust pipes. Bike is stock other than EFI conversion

[jumjum01]

Since the "real" expansion chamber exhausts are crazy expensive I have modded my old 1UA exhausts in an attempt to make my own. I cut open the chambers and removed the mesh plates inside and opend up the stinger diameter like MK suggested. I made a muffer similar to the Jolly Moto end can with 24mm inner diameter and exhaust wool. Then I welded a flange to the stinger to bolt on the muffler. A lot of the aftermarket expansion chamber exhausts has a longer small diameter stinger between the chamber and muffler, I dont know if this makes a performance difference?
Anyway this project didnt cost a lot other than time, so I thought it was worth a shot and then do a back to back dyno vs. the cigar exhausts next time I visit the dyno shop.

[kpke]

That's cool. Seems like a lot of work but it came out really nice.

[jumjum01]

Yes it is a lot of work, but I dont mind. I am learning to use lathe and mill and made the aluminium muffler parts on the lathe. We have a nice selection of shop tools at my new workplace, so any chance to get better I go for it. I just finished anodizing the muffler end flanges and drilling holes for final assembly

[RD4ME]

Looks great - Nice skills.
I have the 1UA exhaust and I find them quiet - and they don't have the "ring-ting-ting" sound of a set of aftermarket expansion chambers.
I wonder if your new end cans will make it sound snappier!

[(F5)]

If you can make them removable without compromising strength and plan to dyno test; removable inserts at the Stinger interface perhaps 20mm long with a bit of taper can be used to tune the Stinger restriction size. As you get more power or capacity from a given engine they need to be a little bigger. However if you make them too big you are giving hp away.
Maybe temporary and weld up what you find works best?

[RZtuner]

I'm a big fan of the 1UA pipes. Performance wise they dyno within 1 hp of a Loma$$$ pipe and most of that is due to the restrictive muffler. So far 129.5 mph on absolutely stock pipe and carbs.

132 mph on same bike, no fairing, 34mm carbs and 1UA pipes with a 1 inch OD open stinger welded in place of the muffler.

If you grind off that last outer cone that Yamaha weld on top of the last tapered cone to mount the heavy muffler and then weld in a stinger, it gives you a much cleaner look. I've done a set like that with Aprilia RS250 cup mufflers. Looks very sharp and nice and quiet on the road so you don't get a headache!

[jumjum01]

For now I have only finished LH exhaust and tested. But the sound is way more crisp compared to the cigar exhaust. I was worried it would be too loud, but fortunatly not the case. I can make up some 20mm long stinger sections and bolt in between flange and muffler with long bolts, which would make the easy to swap out during dyno. Mabye 20mm and 22mm diameter cone shaped and 24mm (bolted directly to flange). I always thought bigger diameter would aid top end HP (more free flowing) and smaller diameter aid low end HP due to increased backpressure?

[MK]

Nope.
Smaller stinger dia increases top performance.
Just until the point where it decreases it very suddenly to zero emitting a very disturbing noise...

For stock'ish RDs some 20-22 mm is in the ballpark. Higher tuning levels with more performance will require a bigger diameter.
I prefer being on the save side over quenching the last hp from the engine and run stuff like 24.5-27.5 mm.
Only exception: My roadbike uses Aprilia RS250 road legal cans with just 22 mm which results in running pretty hot.

Frits Overmars has published a rule of thumb to get that diameter:
https://4.bp.blogspot.com/-MgZiKNYsFws/ ... s_ex10.gif

[(F5)]

I'm with this guy. Test is best, don't give away power with the bigger must be better theology.

[RZtuner]

Smaller diameter stinger increases piston crown temperature, go too small and you will get deto and a meltdown situation. Martin is dead on with his comment about engines with higher tuning levels that produce more heat requiring larger stingers.