Compression tests on cylinders

[Tony Smith]

I wonder if the 100LL I put in my plane would make my airheads run better. I know the Jet A sumped from the Lears made my VW Rabbit feel special. 

What irritates me is many years ago, we spent a small fortune doing an auto gas STC on our Stinson. Except that with all the ethanol in car gas in the last few years, we can't use it. Have to find non ethanol fuel. Its cheaper than avgas, but a PITA to find.   :-/

The 100 Octane fuel would require some ignition modification to the R65. The lead content (at least in Australia) of 100LL had about twice the lead content of the old "super" petrol.

I used to have a friend in the aircraft refuelling game  and used to buy 200lt drums of fuel drain discard Jet-A1 from him to run in my Toyota Hilux. I used to add about 5lt of cheap engine oil to 200lt of Jet-A1/Av-Tur and it loved it. Almost as much as it used to like surplus heating oil cut with naptha.

I tried (quite illegally) Mo-gas (old "super" petrol) in the PA-28 I once owned. At the time there was no legal way to run auto-gas in an aircraft in Australia so I sort of lost interest in the project, in any event I parted with the PA-28 shortly after I stopped flying professionally and my wife expressed interest in building a home.....

[Bob_Roller]

I worked for a helicopter FBO ( Fixed Base Operator ) and when we got a helicopter with a recip ( piston) engine, we had to drain the fuel out of the tanks as part of the heavy maintenance check and we could not put that fuel back in the helicopter, it was considered ' contaminated ' .

I used 100LL in my '79 Yamaha XS400, then the '81 R65 .

I left this FBO in September , '79, but I still worked at the same airport, for an airline .

I kept getting calls from them after I left there for 14 years, they wanted me to take the 100LL from them, it cost them plenty to dispose of it .

I didn't purchase fuel for the R65 for until 1993, when I left for Phoenix  .

I never did anything to the R65 for use with avgas .

But the exhaust did smell different .

The use of a somewhat leaded fuel, also did not make any difference in the area of valve recession, my exhaust valves were almost at the failure point at 45,000 miles .

[Bob_Roller]

Tony, different countries, different regulations !!!!!!!!!!

It was a big deal when this came about here in the US .

I just got my drivers license, when this happened, so it is  a memorable episode in my life !!!!!!!!!!!!!!!

[Bob_Roller]

OK,this is unusual, I responded to a post from Tony, but it doesn't appear now !!!

I'm not drunk, or on recreational drugs at this time !!!!!!!!!!!!!!!!!

Oh . hell the Twightlight Zone has come to roost in this site !!!!!!!!!!!!

[mrclubike]

Leaded  fuel was not completely   banned in the US until 1996
But I remember   it not being readily available well be for 96
I do know you could buy it in the 80's here in the US
I  had a 1962 Land Rover and I worked on a fleet of Ford trucks that were having severe valve recession issues after it was removed completely
The problem usually didn't occur until after we ground the valve seats during a routine  valve job
We would then have to pull the heads off and install hardened seat and valves 

[Bob_Roller]

Tony, no apology necessary, different countries, different regulations .

[davidpdx]

Easy enough to pull the carbs off before you start your compression test. Makes a huge difference, I found out after thinking that I needed to rebuild my engine due to low compression.

[jamestnewsonr65]

So this weekend I did another compression test and the results were a little better.

I used another of the fittings to attach the tester to the cylinders as I don't think it was creating an air tight seal before.

The results I got were about 95 psi on each side. How far is this from normal?

I guess a leak down test would discover more.

[Bob_Roller]

145 psi is about the highest you'll get on a low use engine .

I think under around 120 psi, it's going to run well, but will be down on power .

What are you using as a compression gauge, is it a threaded fitting that screws into the spark plug hole, or are you holding the rubber end against the spark plug hole .

How many km's, or miles has the bike got on it

Another possibility, how accurate is the gauge ??

[jamestnewsonr65]

The tester has both the threaded fitting and the rubber ended fittings.

The lower readings were from the threaded fitting which indicates it was not sealing completely. I tried it with the rubber fitting pushing this in place whilst testing and that gave me the higher readings.

The bike has about 18000miles on it, and only really gets used by m in the summer months.

[Tony Smith]

I used 100LL in my '79 Yamaha XS400, then the '81 R65 .


I never did anything to the R65 for use with avgas .

But the exhaust did smell different .

The use of a somewhat leaded fuel, also did not make any difference in the area of valve recession, my exhaust valves were almost at the failure point at 45,000 miles .

Bob I should have explained myself better.
The obvious difference between 100LL and the stuff more usually sold at garages is the octane rating. The aviation stuff is as I understand it (and certainly in Australia) quoted accord to the RON method.

Also in Australia pump gas octane is also quoted according to the RON method, the most common varieties are:-

Unleaded Petrol (ULP) 91 octane

E10 - which contains up to 10% alcohol and is quoted at between 92 and 94 octane depending on which fuel company blended it and what they blended it with.

"Premium" Unleaded Petrol (PULP) - mostly 95 Octane, but some fuel companies blend slightly differently and quote 98 Octane.

By compression ratio and standard timing, the "best fit" for a stock R65 is 91 octane. You can of course (as you have done) run them on higher octane, but there is no point (unless of course the fuel is free as it was in your case) without altering the ignition to take advantage of the increased knock resistance of the higher octane fuel.

Now I have never done this to any BMW, but back in the early 80s when I was playing with a Kawasaki GPZ1100, running on Aviation fuel (in those days, particularly for military use some real "jungle juice" was available offering up to 120 Octane (and higher). On a dyno, with a knock sensor attached, I found that the ignition could be advanced quite significantly, with a measurable difference in power output. A white paint pen was used to mark the ignition plate with the positions for "super" and "avgas".

I agree that running the R65 on 100LL with no modifications would generally cause no problems, but I do ask if you found your plug life shortened due to lead fouling?

I'll also bet that on a long run, the ends of your exhaust pipes were a nice chalk white colour.


Something we have all forgotten (and young folks never knew) is that in an engine in otherwise good condition, lead used to eventually kill most spark plugs, we have become used to plugs lasting pretty much forever since the advent of unleaded.