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Baffling for cylinder cooling

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b.carl@sympatico.ca

Joined: 20 May 2008
Posts: 77

Posted: Mon May 26, 2008 7:23 am Post subject: Baffling for cylinder cooling

I built my 701 from plans in 1994/95. It is powered by a Rotax 912 covered 

with a Zenair cowl without any internal baffling as per the Zenair 

demonstrator and recommendations at the time.

The engine now has 490 hrs and is still running smooth and strong; I see no 

difference in static RPM since first flight. I am concerned about two things 

however:

1. Front Cylinders head temp is much cooler than the rear;

2. Compression on the front cylinder is 20 lbs less than the rear. 115 vs. 

135. This has existed and remained steady since first annual in Nov 1995.

I suspect a cause and affect between these to parameters.

My questions to the list is:

    I know of two other 701s that have the same cowl set-up and have never 

seen a 701/912 with baffles. Is this the norm?

    I'm considering investigating the reason for the lower compression in 

the front (vales or cylinders) and designing some baffling to equalize the 

cooling. If I just correct the cooling could this destabilize an engine that 

is running well?

Any comments appreciated.

Carl

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Roger Lee

Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.

Posted: Mon May 26, 2008 3:07 pm Post subject: Re: Baffling for cylinder cooling

Hi Carl,



Just so we are on the same page this is a 912ULS, correct?

I wouldn't play with the air flow. Zenair spent good money to make sure you had decent airflow under the cowling. Number one cause of Rotax 2 and 4 stroke failures is owner induced. Leave your engines stock would be the best advise I could give anyone. I don't personally know of a 701 that has any special cowl for cooling for the engine. If you start to play with it you better have an extremely good working knowledge of baffling and air flow patterns under your cowling. Many people have done this only to cause worse problems. Thinking they are redirecting flow for the better and only hindering it. A CHT difference from the front to the back is normal under a cowling for the 912. You did not say what the difference was? 20-30F is quite normal. I see about 12F on my CT CHT's between front and back. 



I take it 115-135psi is static compression test. If this is static then something is really wrong. Did you have the engine at operating temp. Sounds like a cold engine check. What is the differential pressures. This would be more informative. At operating temps and at top dead center you should not hear any air coming from the air intake or the exhaust. If you remove the oil tank cap you will hear a small amount of air here and that is normal. Make sure the orifice size for the differential tester is .040. It should be marked right on the front of the test gauge.



The static compression should be between 174-130 and no more than a 29psi difference between cyl.



The differential should be between 87 to 65 psi. Make sure the engine is at operating temps or your readings [b]will be wrong.[/b]

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_________________
Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056

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