Detonation Margin 8.5 vs 9 Pistons?
 

Question for engine gurus.

I am having an engine built for RV10 and trying to decide on 8.5 vs 9 pistons. I plan to run mogas as much as possible and trying to understand how much more detonation margin I may have with 8.5 pistons with everything else being equal?

I would appreciate input from those more knowledgable than me... which should be just about everyonen

Steve

Mahlon's Advice
 

http://www.vansairforce.com/communit...ad.php?t=35765


If you split hairs on this - and say 100LL is needed for 9:1 then you could switch to mogas at altitude where the cylinder pressures are lower. Where ?, well that is another question. Above 2000 ft at standard temps, so at 6000ft cruise for most conditions? Hard to say precisely. Compression temps could be calculated using a polytropic process at .8 fill factor and get a better idea of where not to operate based on ambient temps. One would have to take some effort to define that envelope. Otherwise just use 100LL.

Maybe others haves some real experience.

Don't think you will get a definitive answer; it all depends. I have an IO-360 parallel valve with 9.5:1 pistons and run mogas from the right tank only. Take off and land left tank only. Always have to remember there are limitations on the right tank.
I read through the thread Bill linked, just now and years ago. No one mentioned timing. 20 degrees btdc my CHTs run cool, 25 degrees they were hot if I attempted to run ROP. Lean of peak was okay. I have settled on 23 degrees btdc. That's with mags. If you intend on electronic ignition I would advise to test carefully as the hotter spark effectively increases the timing.

Dave,


Thanks for the tip on electronic igntion's hotter spark effectively advancing the timing. I do plan to run dual electronic ignition via SDS system, so this is very pertinent.

After reading the referenced links to earlier posts, I may lean toward 8.5 as I want to enjoy the $ save of Mogas vs 100LL. I don't think the 10 extra HP for 9:1 vs 8.5:1 would give as much satisfaction as $20\hour less flying cost using Mogas.

Ross at SDS would certainly be willing to give you advice on your hope to run mogas using that system. Please post what you find.

For what it’s worth, I wish I had put in 9:1 pistons 10 years ago when I bought my engine. There isn’t enough savings to justify Mogas and all of the problems I’ve experienced with it. Especially since i run LOP everywhere.

You can always throttle back and lean out to get the same economy you might perceive with mogas, but you will also have the flexibility to bury your foot in it when you need to get a higher climb rate or faster speed.

There is no substitute for more power.

A spark is a spark and it's strength has no impact on timing. It either ignite the compressed air fuel mixture or it doesn't. Yes, a hotter spark will help light a mixture that doesn't want to be lit (i.e. too rich or too lean). However, this only avoids a misfire and does not impact timing, effective or actual.

Larry

If you're going with SDS, you can retard the timing slightly at high MAP (takeoff, climb near sea level) and still have it optimally advanced at typical cruise MAPs.

9 to 1 will give you a few more ponies and economy up high where most people cruise and spend most of their flight time.

If I was running mags or non-adjustable EI, I'd use 8.5 pistons with mogas. With programmable EI I'd opt for 9 to 1. unless I did almost all my flying below 4,000 MSL. This advice assumes 91 octane.

I ran 2000 or so gal of non ethenol 93 through the RV10 several years ago with 9.1 compression. Detonation was not a problem even with high ambient temp and high MAP. If I remember correctly I set the mag to 20 deg timing and adjusted the maximum available advance from 39 to about 35 which changed the entire advance curce on the electronic ignition. The problem I found was the lower reed vapor pressure even with "summer blend" was enough lower than av gas to cause vapor lock issue. Had to depend on my boost pump to keep the engine running in climb. If you design the fuel system so that there is less system restrictions it should work fine.

Just make sure you are realistic in the realities of Mog gas usage. I can run either, and think I have maybe run 2 tanks of Mog gas in 5 years.
I don't have fuel at my home airport, so if I want Mog gas I have to take seven 5 gal cans to the gas station and fill them - a major pain in the rear.
I have landed at just 1 or 2 airports that offer it at the pump.
If you are building a 10 then I assume your intention is cross country. Some areas of the country have plenty of it at the airport. Most do not, and flight planning to hit the airports that do have it takes away most of your advantages.
If your home field has it, then it might make a lot of sense. If not, I personally would not spend a lot of energy building around it.