My Good friend wants to know if a Ryobi 31cc engine needs a certain amout of backpressure in the exhaust system? I guess he is thinking of a pitts style muffler...instead of tin one with one outlet that came on engine. What do you think? Thanks Captinjohn

I got the best RPM's from my Ryobi with the muffler completely removed.

Most 2 strokes(gas) require some back pressure
RJH the reason you got better R's (higher) was because the stock muffler has a restricter in it(flame screen)

The muffler I am using has the baffle cut out larger than the inlet and a 3/4" outlet pipe.

RJH, well now you are saying something different. first it was no muffler now it is a modified muffler, so what is it????

I'll try to make it clear this time. I can get about 300 more RPM when I remove this highly modified muffler.

stalspin: Can you explain in more detail....you say most 2 strokes require back-pressure. What happens with no back-pressure?? Thanks Captinjohn

CapnJohn

nope, won't give any lecture here. besides too many technical books on the subject.
But a simple way to prove my ststement is to set up an engine with a muffler tune it properly, start it and run it, then as it is running pull the muffler off. See what happens.

Generally, 2 stroke engines are sensitive to back pressure. The less back pressure, the better it runs. This especially applies to the typical gasser engine. (Ryobi) However, to run nothing attached to the head, will burn the top corner of the piston and perhaps the ring? You need some kind of short header on it at least.

With a glow engine, most use muffler back pressure to help force the fuel up to the carb. This makes them run consistantly when level or vertical.

Also Schnurle scavenged 2 stroke glow engines do work better with just a little back pressure, fuel pressure or not. But it doesn't need hardly any, just a little.

With loop scavenged glow engines, the less pressure the better as long as your getting fuel to it. This would be like the old engines from the 60's or early 70's.

There was a big move to mufflers in the very early 70's and it was discovered that a muffler didn't restrict the new schnurle type engines as it did the old loop or baffle piston engines. So there was a trend where almost all model glow engines today are schnurle scavenged. It was mainly because of the interest in quieter engines, not so much the greater horsepower with sport engines.


Enjoy,

Jim

Funny characteristic on old 250cc Rotax snowmobile engine. No way would would it start, not even pop, without the muffler on it. Slip the muffler on, without any bolts in it,would start right up. Take the muffler off while it was running, it would run fine. Strange animals these 2 strokes!!

RCIGN1 How about your views on this subject.....or your fellow engine guru TKG. What is the pros and cons of back pressure on GAS 2 strokes. Thanks Captinjohn

Some 2 strokes idle funny without a muffler..In general no muffler at all gives the best rpm, but not as good as a tuned pipe..

Two stroke motorcycles have what's called a power valve in the exhaust...It works...It would require some linkage and controls, and would weigh more..The K.I.S.S principle applies to airplane engines.....

Why couldn't you just put a spring loaded cover on exhaust exit like on a semi truck so when there is not much pressure it alows it to block the exhaust and when you rev it up it opens under pressure of course you would have to experiment with spring tension. maybe?

Darin

someone needs to try this that has a single exhaust tube

Rotax has RAV valve in the ex port. Slides down over port at idle. A diaphram pulls it open by intake manifold vacuum. Supposed to make more midrange torque also.

many years ago there was a contraption on I believe FOX glow engines. It was a means to control throttle by blocking the exhaust with a plate coupeled to a linkage. Noting new under the sun HUH this was used in control line. course you R/Cers wouldn't know bout that.

Exactly what will the engine do with variable exhaust timing?
It won't run any better for an airplane engine.
Increasing the low end torque is of zero value because the prop load is exponential with rpm. I don't need my engine to idle any lower than it already does - lands fine now, with a Menz prop nearly stops dead in the air when it throttles back.

Early RC engines had the throttle baffle linked to the carb. Without a muffler it kept the cyl hotter to keep the glow plug from cooling off. Made for a lower more reliable idle speed.

captinjohn:
Diabo has a very good point though. You are all talking about modifications that makes the engine make more torque and/or power around idle and right off idle, WHY? Except for taxying or that 30 lbs plane with a 200cc gas engine hovering, you never have the engine in that band and needing to make power. There is almost no load on the engine at idle and low rpm when turning a prop. If your engine doesn't idle low enough, or responds well right off idle, it is probably a tuning questions and fixable.
DKjens