AV Gas Blues (AV vs 91 octane)

[drmclaws]

I have a BPE IO-360 with a FP Sensenich. The compression ratio is 9:1. As with most of us the price of fuel is becoming a pain in the wallet. I was wondering if I could hear from those of you who are using 91 octane and those of you who have some knowledge regarding the pros and cons of its use. Many may not want to admit that they have gone that route but there are some real safety concerns I have such as vapor lock and damage to the engine. Lets assume we are not using blended fuels with alcohol. I mean there are many STC's out there for the use of MOGas and I used it with an STC on my 172 years ago. Can the lubricative (I don't know if thats a word) properties of lead be provided by some other means ie. Marvel Mystery Oil or are we doomed to 100 LL. What are the considerations when using an engine with higher compression ratios etc. I think this could be a real hot topic. One that has been around for years and which I'm sure is understood by some but not well comprehended by the rest of us.

Hope to hear from you.

[Ron Lee]

What does the engine maker recommend?

Personally I am not going to experiment with 91 octane just to save a little.

[Ironflight]

Originally Posted by drmclaws

I think this could be a real hot topic. One that has been around for years and which I'm sure is understood by some but not well comprehended by the rest of us..

Uh...yeah - a hot topic for a long time! Try searchign the forums for "MOGAS", "Auto Gas", and similar search terms...then settle back for a few evening's reading....

Paul (currently paying almost the same for 100LL as for regular auto gas....)

[John Clark]

Originally Posted by Ironflight

Paul (currently paying almost the same for 100LL as for regular auto gas....)

Uh huh! At my current usage and calculating the spread between 100LL and 91 Octane Mogas, the cost difference is less than $30.00 a month. At the moment, I can't justify the hassle of mogas.

John Clark
RV8 N18U "Sunshine"
KSBA

[frankh]

1) Not one shred of evidence the lead provides any benefit whatsoever....A lot of tribal knowledge but no evidence. Think about it...Its an engine...As long as it has the hard valve seats lead free gas will be perfectly OK as it has been for every other engine for the last 30 years.

2) Detonation...This is a function of combustion temperature (CT) and ignition adavance (which really is related back to temperature). There is an argument that says that as the motor is air cooled the (CT) will be higher..and therefore any grade lower than 100 oct is cutting the detonation margin too fine... Well, in that case when Peterson Engineering did its extensive testing using specially blended 89 octane fuel it should have seen detonation...They found none!

Now all of the above holds true for 8.5:1 CR motors...You have 9:1...Will that make Much difference?..Probably not, and indeed there is more than one person running 87 octane in an 8.5:1 engine and they seem to be OK....But your situation is untested as far as I know.

3) Vapour lock...Probably the biggest argument against using mogas...In fact that is why my fuel pumps are in the wingroots..But there are many RV's running around with standard fuel systems and no VL issues...As a precaution you could run a tank of 100LL in really hot WX in one tank and 91 in the other. In my system there is no need to do this however.

Locally 100LL is a buck a gallon more than 91 octane (a local airport has just put a tank/pump of ETOH free in..Planes line up for miles....).

Over 2000 hours the savings will roughly pay for the engine rebuild.

I been using mogas for 240hours (out of 270TT) without problem.

Frank

[frankh]

If you wanted to drop your CR to 8.5:1 i wonder how hard it would be to add a shim under the cylinders?...Should be easy to calculate the thickness (minus the thickness of the extra compressed gasket). You could probably do that on a Saturday morning you can borrow the tools.

Frank

[zilik]

Originally Posted by frankh

If you wanted to drop your CR to 8.5:1 i wonder how hard it would be to add a shim under the cylinders?...Should be easy to calculate the thickness (minus the thickness of the extra compressed gasket). You could probably do that on a Saturday morning you can borrow the tools.

Frank

I personally would rather experiment with 91 mogas than with cylinder shims.

[deene]

Originally Posted by frankh

If you wanted to drop your CR to 8.5:1 i wonder how hard it would be to add a shim under the cylinders?...Should be easy to calculate the thickness (minus the thickness of the extra compressed gasket). You could probably do that on a Saturday morning you can borrow the tools.

Frank

Sorry, can't easily change deck height due to pushrods (fixed length, thus valve clearance goes out of spec quickly). Usual method for CR change is to replace pistons, fairly easy to do...pull cylinders, replace pistons/rings, have cylinder honed and reassemble. Probably doable in 2 weeks with knowledgable mechanic plus some $$$.

[Mark Burns]

Originally Posted by deene

Sorry, can't easily change deck height due to pushrods (fixed length, thus valve clearance goes out of spec quickly). Usual method for CR change is to replace pistons, fairly easy to do...pull cylinders, replace pistons/rings, have cylinder honed and reassemble. Probably doable in 2 weeks with knowledgable mechanic plus some $$$.

Deene,
A good mechanic could do this in one (long) day. You can hone the cylinders easily on site.

Mark

[RScott]

For a thorough discussion by experts, go to EAA's website: http://www.aviationfuel.org/autogas/articles/

And, FWIF, Bart from Aerosport told me I could run premium with no problems.

Gas is blended for each market and the blends are changed throughout the year for changing weather conditions. In other words, the vapor pressure of mogas in Arizona in the summer is lower than it is in Oregon, and lower than it is in the winter anywhere.

Lastly, FWIW, I have flown mogas in my Cadet (with a Continental, and lower compression) since 1995 with no problems. All the guys at our airport with whom I have discussed gas used mogas until the state mandated ethanol.