Auto Fuel and Compression ratios

speyers · #1 12-05-2006, 09:31 PM

I have been looking into the relationship between compression ratios and the ties to auto fuel. It appears as though older engines originally certified for 80 octane have lower compression ratios (6-7.5:1) versus the engines certified for 100 octane fuel. They usually come in around 8:5-10:1 on the compression ratios. I understand the logic of needing a higher octane fuel but it seems as though the cut-off range is in the 8-9:1 range. Wondering if anyone has experience with auto fuel in a 0-320 or 0-360 with a CR or 8.5:1.
Reading Lycoming SL1070, they are (of course) very much opposed to the use of auto fuel. However, due to the amount of lead in the fuel, engines that were originally certified for 80 octane now are getting 4X the amount of lead they were originally designed for (0.5ml vs 2ml). I just finished helping a friend ream his exhause guide and polish a valve because of a sticking valve on his A-65 engine. Granted this is the opposite end of the spectrum with a CR of only 6.3:1. The point is this, can I run my 0-320 or 0-320 with a CR of 8.5:1 that was originally certified for 91 octane on auto fuel and still be OK? It seems as though it is right on the border with regards to the comprestion ratio scale...
Oppinions or personal experiences welcome.

Mel · #2 12-06-2006, 07:26 AM

Even though compression ratios are the primary reason for needing higher octane, it's not the only factor. Many autos use regular (87) octane with 9.5-10:1 compression. But, they are small bore higher rpm engines. The over 5" bore and low rpm of the Lycoming requires higher octane. Another factor is that the Lycoming is a very noisy engine. So if you WERE getting "clatter" you would not be able to hear it until it was too late. So you need a much wider safety margin. And because of this "noise" factor, knock sensors don't woprk on these engines. It has been tried.

hevansrv7a · #3 12-06-2006, 02:48 PM

Superior, with the 8.5 piston, allows the use of premium auto fuel at or above 91 octane. It may be as much about the lead for the valves as about the octane. The Precision FI from Superior is set up to use about 10% more gas at idle because of the auto gas provision (they told me yesterday).

JordanGrant · #4 12-06-2006, 04:53 PM

hevansrv7a:
I have a standard Superior XP-360 with Airflow Performance FI. What data do you have on running with auto gas - and where did you get it? I have considered trying to run autogas in one tank, but I wouldn't want to try it without some info from the company or someone that is smarter than me on the subject.

hevansrv7a · #5 12-06-2006, 05:40 PM

Quote:

Originally Posted by JordanGrant

hevansrv7a:
I have a standard Superior XP-360 with Airflow Performance FI. What data do you have on running with auto gas - and where did you get it? I have considered trying to run autogas in one tank, but I wouldn't want to try it without some info from the company or someone that is smarter than me on the subject.

The engine manual (the blue looseleaf binder) says it's OK to use 91 octane car gas. That is also an advertised feature of the engine and one of the reasons I chose it.

http://www.xp-360.com/faq.asp
2. What is the type and minimum grade of fuel?
The 180HP engine is approved for 91 octane unleaded auto fuel and a minimum of 100LL-aviation fuel. Superior recommends 100LL, although this is not a requirement. A 170HP, 87 octane version is also available with 7.2:1 compression ratio pistons. The 185HP version which incorporates 9.0:1 pistons requires the use of 100LL.
3. Does this mean I can use auto gas?
The XP-360 Engine can operate and perform at rated power using unleaded fuel. When operating on unleaded fuel, Superior recommends using fresh, premium auto fuel available at a major brand, reputable gas station. Acceptable fuels are specified per ASTM D-4814. NOTE: Use of auto fuel blended with ethanol or gasohol is forbidden. Ethanol (alcohol) mixed with unleaded fuel can cause vapor lock, carburetor ice, reduction in range, carburetor problems and damage to the fuel system. The use of an alcohol (and water) tester is recommended .

I have only run a blend of avgas and car gas. I'm not sure I noticed any difference except that the car gas does not leave blue stains and that can be a bad thing. I have no real experience with it.

BOBM · #6 12-06-2006, 09:08 PM

Petersen has numerous auto fuel STCs for 160 & 180hp Lycs in certificated aircraft dating back to 1983. They all require the use of premium auto fuel. All the info is on their website:

http://www.autofuelstc.com/autofuels...nAviation.html

For whatever reason, all the EAA STCs are for low compression engines that can run regular auto fuel.

BOB