I asked this question on the OS engine support forum but got no reply:

I have an OS91FX that I want to fit a tune pipe to. I have never done this so I am doing some home work first. I have tried to measure the exhaust port timing of the engine and according to my measurement it’s about 140 degrees. Can you give me the exact timing? Some sources say that an engine with a port timing of 140 or less does not do well on a pipe, has this been your experience?

OS91FX runs very nice on a tuned pipe and you will gains a whole lot of power about +1000rpm more compared to the stock muffler. Calculating "tuned" pipe lenght is only good as a starting point but you can not rely on the numbers, it is better to start with a long "tuned lenght".
So the exact timing is not necessary as all you need to do is to fit an good 91-sized pipe and a extra long header and run the engine. Fit a slightly larger prop, one size larger diameter or one more pitch, than you intend to use for normal operation and run it at full throttle and wright down the peak rpm. Then cut the header about 10-15mm and try again. The peak rpm should be slightly higher now. Repeat this cuting and taching the peak rpm until you will not gain anymore rpm at peak. Fix the lenght there and fit the slightly smaller prop size you wanna use for normal operation. You can read more about pipe tuning and "in better english" at Jettengineering home page.

Piet,


A tuned pipe's boost depends on the 'blow down' period; not on exhaust timing degrees.

The degrees (with engine displacement and total system length) will determine the RPM at which the boost will be realized.

The blow down period is the period of time between the closing of the intake bypasses and the closing of the exhaust port.
It is this time when the fresh mixture ingested into the header, is blown back into the cylinder, creating a supercharging effect.

If the exhaust timing of that OS is 140º and the intake bypasses are open for 115º (boost port), the 'blow down' period is 12.5º.

This number is modest and is considered 'mini-pipe' timing. This is why this engine responds well to a tuned muffler, like the Nelson UltraThrust, or the JettStream.


It is not likely to gain very much more, from a full length pipe system.

Piet, check with the pattern boys in their forum - they may be able to point you in the right direction. The OS FX .91 (some with full length pipes) is a popular choice for semi-pattern aircraft such as the Excelleron, Quest 3D and the like.

Thanks everyone for the feed back. Firstly Dar, if I understand you correct the “blow down” time determine how much gain the engine can get from a tune pipe and not just the exhaust timing. Yes Harry I have been looking at the pattern pages because this 91 will end up in a 91 size pattern plane, our limit for sportsmen is a 91 two-stroke and a 120 four-stroke. From what I read it seems that most people get about 1000 rpm more with a tune pipe if you let it rev between 11000 and 12000 rpm. Most pattern boys, it seems, is trying to turn their 91 into a 140 with a very long tune pipe and a big prop of about 14X10. I think “jet style” pattern is a thing of the past and you will never get maximum power from a 91 at 9500 rpm no matter how long the pipe is that you put on it. But remember I do not have the experience and I am only thinking about the matter.

Sorry

Yes, Piet.

The 'blow down' period and it duration determines the ability of a tuned exhaust to provide supercharging boost.
The exhaust duration determines the RPM range, at which the boost could be realized.

An engine designed to run with high power around 16,000 RPM would have about 160º of exhaust duration.
180º would work around 24,000 RPM...


If the 'blow down' period is 0º, as it is with some Cox .049 engines, no pipe boost is possible, but power can be gained if no muffler is used... Good Q500 engines have ~20º of blow down.


140º of exhaust duration, is a good number between 10K and 13K RPM.

Piet Le Roux,
There is no need for the engine to turn 11000-12000 to produce 1000rpm more when using a fullength tuned pipe as you can tune it at any rpm you want. Many people "want" to think that way so they dont have to use a long pipe
91-sized twostroke engines produce maximum thrust and torque if fullength pipe is used and they are proped and "tuned" to turn about 8500-9500rpm static.
My 91FX turned the APC12x14 @8700rpm with the stock muffler, with a pipe it turns now 9700rpm.
The 120AX turned the APC 14x12@8900rpm on the stock muffler and it does 10000rpm on a slightly larger "high volume" fourchamber quiet pipe.

These two was just an example as all the twostrokes I have used will gain a whole lot of torque and rpm when a pipe is used.

The OS 40/46 VF's have an exhaust timing of 140 degrees and a blow down of about 10-12 degrees (they vary slightly between engines) and respond extremely well to full length tuned pipes especially when set for around max torque revs (~11,000) where they're then about equivalent to an unpiped 60. I see no reason why your 91FX shouldn't respond equally well with a full length pipe.

Well that’s what I don’t get. A 12X14 tuning at 8700 is producing 1.365 Hp, a 12X14 tuning at 9700 is producing 1.893 Hp. My OS91FX with standard muffler turn a13X8 at 10800 RPM, that ‘s 2Hp with a tune pipe it should turn at11800 that 2.68 Hp I know a slow spinning prop is more efficient than one spinning at hi revs, but I think my 13X8 is producing more pulling power at a lower top speed than your 12X14.

Piet,


It is in reality only 1.79 HP (Reivers PropPower) and this number is achieved on this prop by the MVVS .61 with the MVVS tuned pipe.

The OS.91FX with the Nelson UltraThrust, on 5% nitro, can spin an APC 12x10 at 11,800 RPM (adjusted a bit rich for flight), which is 2.11 HP. With the normal muffler it couldn't better 10,500 on this prop.

my 91 fx with the Tower muffler turned the 12.5 -12 APC at 10000 rpm and the 14-10 apc at 9300 rpm. My impression is that the 91 fx responds well to tuning

APC 13x8 is about equal load as APC12x11.
I forgotten to mention the main reason for me to use a APC12x14 is to lower the noise level and still have a good top speed as I use it on a speed plane an not a pattern.
To have a correct comparison we must compare tha same engine "before" and "after" installing a tuned pipe as many other things as fuel, engine condition, ............ will affect how much it will rev with a certain prop.


My supertiger 90 on a quiet pipe turns a APC 13x8 @12000 and APC 16x8@8400rpm when the tuned pipe lenght is readjusted. One would think the APC 13x8 @12000 pulls harder because of the higher HP but in reallity the larger 16x8 pulled my plane with much more authority and the engine was much more quiet.

One more thing
All APC sizes are not equal as the 13" diameters are narrow bladed and 12" dia. has wider blades in comparison. And 14" diameter are also wide.

And some wide blades rev up like there is no tomorrow Can you believe my 15x4W turned up the same rpm as my APC13x8 which has narrow blades.

For this reasons the thrust HP or PPC can never be used for comparison[]