paul246

My Feedback: (9)

We plan on testing the .91 on a Delta Vortex running with no exhaust. The test will be conducted over private land well away from any population, so the noise will not be a concern. We will also be wearing ear plugs under ear defenders. The objective is not speed, but climbing ability. My question is, ... will there be any appreciable degradation in performance to be expected while using an open exhaust?

pe reivers

Two strokes and exhaust systems work together like bread and butter. No exhaust system will work, but not to the best of the engine's ability.
A mousse can type muffler like the black tuned MVVS muffler is slightly better than an open exhaust, but a tuned pipe will realy bring life into the beast.

David_Moen

I agree 100% with Pe. Configure the engine for rear exhaust and use the longer of the two MVVS pipes available for the engine, let me know if you need a header and pipe, I've got both in stock.

Still flying in Sask?

DarZeelon

Paul,


This engine and also the .61 and the .77 are designed with preserving flying fields in mind.
They are not designed for high RPM, in the interest of being less shrill sounding.

Their crankshaft is the same diameter of that of most .40-.53 engines, i.e. 15 mm.

So mid RPM torque is their strongest point.

Trying to make this engine go to over 12,000 RPM is counter-productive.

The #3250 tuned pipe will help the engine attain just that.

If still more power is desired and noise is not an issue, you can easily find a .90 sized non-muffled pipe, like those made for marine applications and to use a looooong header to get the effective RPM down to 11-12,000 RPM.

Exchanging the carburettor to the #3219Q 9 mm Quickie unit will help get several more RPM on any prop, or to spin a larger prop in this range.

paul246

My Feedback: (9)

Thanks for your assistance and advice, everyone.

We will not be trying to wring the life out of the engine, but would like to prop the engine in a way to best utilize its available torque. We want climb ability rather than fast speeds.

We may have to use an open exhaust in order to produce noise. Sounds counter-intuitive, I realize, but the mission is to use the aircraft and its associated noise level as part of a bird deterrent program on a military aerodrome.

So, we don't need high end horsepower or speed, but a good rate of climb on an 8 pound aircraft. I was curious if running with no exhaust would be detrimental to the overall performance of this engine.

I have already purchased a used .91 MVVS and it has been run as a diesel for less than a dozen flights. It comes with both glow and diesel heads and carbs. Any problems I can expect by running it as a glow engine from now on? I'm wondering if I should treat the engine as "new" and break it in as per the directions posted by DarZeelon.

pe reivers

since you want to use the engine as scarecrow, I suggest that you add a very short diverging exhaust stub. This hurts top end, but moves available torque to even lower values, so you might consider 15x6 props for the application.

DarZeelon

Paul,


If, as you were reported, this engine was run in the past, a complete break-in will not be necessary.

It is, however, very important that you get to know your 'new' engine and how it runs and behaves.

This is best done by running it on the bench (test-stand), for a while.


Most of my customers use this engine in their WM Tai-Ji .60 and similar F3A planes.

They use it in the rear-exhaust configuration, the #3250 tuned-pipe and an extended header, a #3219Q carburettor, a Perry pump with the tank on the C/G and a 14x8 APC prop.

They are seeing about 10,000 RPM.


The biggest detriment I foresee in not using an exhaust, is not having a pressurised fuel system.
If it is going to be a 'scarecrow', it should be reliable.


The 15x6 is close load-wise, so if you select what PĂ© suggested, you will see about the same RPM.

Using an older style, dual-cone, non-muffled pipe will give you a higher boost, fuel pressure (I suppose you will not be using a fuel-pump) and the high noise level you desire.

paul246

My Feedback: (9)

Thanks again everyone for the advice. I'm looking forward to this spring when we will be out experimenting and putting your suggestions to the test.