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Touring Motor Gliders Association (TMGA)
John Heeboell

GROB 2500 engine vs. unleaded fuel

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John Heeboell

We have a rather fierce debate running on whether or not our GROB2500 engine can accept unleaded fuel.  Service bulletin 817-46/2 completed. It includes acceptance of unleaded fuel.  There is a concern among some  of our technicians that unleaded fuel can damage valve seats , fouling the spark plugs and also cause ignition after engine has been turned off.  This again is supposed to cause mechanical damage to the engine. Any solid experience an viewpoints on non-leaded fuel and the GROB2500 in this group?

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Mark Calder

I think the bigger issue is if the fuel has ethanol. My partner and I have a Tiafun, it has a fiberglass fuel tank. Ethanol will turn a fiberglass fuel tank to Rubber. Because of this, I am only using 100 LL from our airport. Its very hard to find MOGAS without ethanol being added. Of course, if the GROB uses metal tanks, it doesn't matter.

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Sinus Flex Max

No specific experience on the Grob2500, but lots of experience with engines in general.

Many engines designed 'back in the day' relied on the lead in auto fuel to lubricate the valve/valve seat interface.  When lead was phased out, some engines experienced rapid wear of their valves and seats.  The 'fix' was simple: manufacturers produced valves and seats of harder steel which were installed in new engines and made available to retrofit to earlier engines.  So, unleaded fuel does not damage those softer valve seats, the lack of lubricating lead simply causes faster wear.  If you don't already know, presumably Grob could tell you whether or not the valves and seats in your specific engine are OK to use with unleaded fuel.

There is nothing in unleaded fuel which causes spark plug fouling, quite the opposite: witness the 15,000 mile plug change recommendations on typical pre 1970's autos v the sometimes 100,000 plug change intervals today.

"Dieseling" or continuing to run after the ignition has been turned off is caused by hot spots within the combustion chamber, typically hot lead or glowing carbon.  This is less likely to occur when using a cleaner burning unleaded fuel

Octane rating is important: using too low an octane fuel can melt piston crowns in just a few seconds.  Published European octane ratings are different to those published in the US, so a German specification of 98 octane is the same as a US rating of 93.

Airports selling non-ethanol mogas can be found in the AOPA flight planner, and also at flyunleaded.com.

Richard

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prg55100

with some late but for general information. I fly my Grob 109B, 2500 Grob motor,for more then 10 Years and 1200hrs and the previous owner did it for about the same time with unleaded fuel 98 octane EU standard. We just use a dedicated fuel additive (Brand Wynns for example) to compensate the lead lubrification features. This additive is commonly used for all "old" engines after suppression of leaded fuel. With the old engines used to burn leaded fuel it is likely that without this additive your engine valves or valve seats would not survive unleaded fuel.  

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