Engine timing for 92 oct. vs 100LL problem

[joeboisselle]

I didn't want to hijack the thread on 100ll so I thought I'd ask in a new one. So riddle me this... I've got 1 hour smoh on my lyc o-360. I purchased 10 gal of 100ll at the local airport for nearly $6 a gallon (could be slightly old) and filled it into the right tank. I then went to the local Texaco and bought 10 gallons of 92 octane and filled it into the left tank. When running the engine on the 92 oct. I get a mag drop of around 75 rpm, both sides. On the 100ll I get a mag drop of about 300 on the right and 400 on the left!! I'm using the original slick mags set dead on at 25 deg btc as per the overhaul manual. Any thoughts? I'd like the option of being able to fly somewhere and purchase 100ll without worrying about it.

[N941WR]

Joe,

I'm not sure about the timing but I have been told not to run auto fuel until after the engine is broken in, say 10 to 15 hours.

Just what I've heard, so take it with a grain of sand.

[DanH]

Well, I'l take a wild stab. The 100LL has a lower Reid vapor pressure (less easily atomized) and a high lead content/octane rating (slower flame front travel, something that becomes noticable with only one plug operating). The 92 octane car gas would atomize better (less intake wall wetting, and uniform burn of what makes it to the chamber) and burn across the wide chamber more quickly.

A guess, no study. It would be interesting to hear from someone with more combustion research experience.

[jonbakerok]

You shouldn't get a 300 rpm drop on a mag check regardless of what gas you're using.

The fact that the mag check on cargas from the other tank was normal merely proves that something was wrong with the gas in the other tank and not the engine. Or maybe you aren't getting full fuel flow from the bad tank, causing it to run lean.

[rv6ejguy]

Ran many auto engines on the dyno with 100LL and 92 mogas. In most cases, both required about the same ignition advance (barring detonation) to make max power. 100LL does not burn slower or we'd have to advance timing to achieve PCP (peak cylinder pressure) at the optimal crankpin angle.

Sounds like you may have some old fuel there. I've seen major variations with this aspect, even to the point where the engine won't start at all or having serious detonation under load. Mogas seems to degrade faster than avgas in hot climates in my experience. This generally takes years however. Is the avgas that old?

[joeboisselle]

Originally Posted by N941WR

Joe,

I'm not sure about the timing but I have been told not to run auto fuel until after the engine is broken in, say 10 to 15 hours.

Just what I've heard, so take it with a grain of sand.

Grain of sand taken! This is kinda what I was planning to do, but after spending around $60 on the 10 gallons, I opted for 92 oct.

Originally Posted by jonbakerok

You shouldn't get a 300 rpm drop on a mag check regardless of what gas you're using.

The fact that the mag check on cargas from the other tank was normal merely proves that something was wrong with the gas in the other tank and not the engine. Or maybe you aren't getting full fuel flow from the bad tank, causing it to run lean.

I hadn't thought of fuel flow, I'll be sure to check the fuel pressure guage next time. Thanks.

Originally Posted by rv6ejguy

Ran many auto engines on the dyno with 100LL and 92 mogas. In most cases, both required about the same ignition advance (barring detonation) to make max power. 100LL does not burn slower or we'd have to advance timing to achieve PCP (peak cylinder pressure) at the optimal crankpin angle.

Sounds like you may have some old fuel there. I've seen major variations with this aspect, even to the point where the engine won't start at all or having serious detonation under load. Mogas seems to degrade faster than avgas in hot climates in my experience. This generally takes years however. Is the avgas that old?

I bought the 100ll out of 10,000 gallon tank that probably gets filled every 12 to 18 months. I might have just hit the bottom of the tank. I'll have to try the other FBO on the field, see if there's any difference. At least it's not a mechanical issue, thanks guys!

[roadrunner20]

Originally Posted by joeboisselle;

I'm using the original slick mags set dead on at 25 deg btc as per the overhaul manual. Any thoughts? I'd like the option of being able to fly somewhere and purchase 100ll without worrying about it.

I'm going from memory here, but I believe Superior recommends 28 deg btc for autofuel exclusive.

I'll have to check when I get to the hangar tomorrow.

I use 93 UL local, & 100LL when enroute so I never reset.

[az_gila]

Originally Posted by DanLandry

I'm going from memory here, but I believe Superior recommends 28 deg btc for autofuel exclusive.

Dan... doesn't seem to be the case for the certified Vantage 360 engine.

This engine is certified for both auto fuel and 100LL, and the Type Certificate Data Sheet give 25 degrees BTDC for all fuels.

http://www.superiorairparts.com/vantageManuals.asp#

gil A

[roadrunner20]

Originally Posted by DanLandry

I'm going from memory here, but I believe Superior recommends 28 deg btc for autofuel exclusive.

I'll have to check when I get to the hangar tomorrow.

I use 93 UL local, & 100LL when enroute so I never reset.

Checked the manual today.
You actually retard timing to 22 for autofuel vs 25 LL.

I will also add that I have not changed the timing from 25 BTDC. When local I always use 93UL.
Cross country, I use the 100LL. Superior states you can combine octane as long as the combo mix is at least 91.

[frankh]

I seriously doubt they have tested it on a dyno for peak performance with both fuels.

I have 26Degree max timing advance for either fuel and there is no runaway detonation or any difference in the engine running.

The E/Pmag also allow a max advance (with altitude) of 34 degrees and it runs just fine.

Remember there is one guy running a standard Lyc (25BTDC and 8.5:1 CR) and he is using 87 octane mogas...I.e regular not premium and has been for over a thousand hours I believe.

No signs of detonation on my engine when borescoped either.

I think the evidence suggests there is ample detonation margin running a standard motor with premium mogas.

Frank