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OS 91 VR-DF square and roud head difference

Old 03-12-2014, 06:13 PM
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Default OS 91 VR-DF square and roud head difference

Hey Bax...

I got a gift from a friend a new old stock OS 91 VR-DF (square head) complete with tuned pipe with a round head parts.

I would like to ask what is the difference and the utility of the two types of head. as I read the manual is not explained in detail usability.

According to OS manufacturing timeline by two types of head is made ​​in different years about 15 years.
I had a break-in and flew with very satisfactory results, good idle, good transition and good top rpm ( is more than enough for my plane.)
I was flying conditions
Fuel oil 22% and 0% nitromethane
Glow plug OS 8
Propeller 11 x 11 groupner (13400 rpm max)
MACS tuned pipe
Temperature when maximum rpm on the ground 83 deg Celsius

Best regards,
Old 03-13-2014, 06:30 AM
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O.S. never said why. Likely, the engines really didn't need the cooling of the larger cylinder heads. Also, the smaller-head engines likely ran on fuels with lower nitro, which would make the engine run cooler overall.

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