AudiUberMan

I'm not a motor head, so not really sure, but it seems to me that I heard this from a mechanic:

Even though a high compression motor may have been designed for high octane (higher combustion pressure) in very high milage engines, you can run lower octane no problem.

Is this true, or would you be better off continuing to run high premium even in an older Audi inline 5, with 220k on the clock?

thanks guys!

audithepeanut

you feed it. My 84' only gets premium (unless I'm low on cash)

AudiUberMan

thanks for the post.

Opposite from what the mechanic told me, however, it is very possible he could be wrong. What you posted does make sense.

There is a real continental divide between PRO premium and ANTI premium gas on older cars. I've also heard, from a gas exective (smile), that the older the car - higher the milage, the more you need premium!

Maybe some more will chime in.
Thanks again!

audithepeanut

longer, though if you were asking about a geo or something similar i'd say go regular without oil in the engine...

squidix9

One of the difference between premium and regular grade gas is the octane level. The addition of iso-octane and other compounds raise the octane level. This causes the gas to burn slower. The faster the fuel burns limits show far you can advance the timingm otherwise the pressure pulse occurs too soon and actually opposes the momentum of the piston causing it to "knock".

A knock is detected by the ECU and it then pulls back the timing till it detects the knock has stopped.In a really old engine prior to knock sensors and the ability to control the timing you'd audibly hear the knocking and you'd feel the lack of power.

Having the timing advanced as far possible gives the most efficiency and power.

Premium gas also typically also contains more detergents to help clean combustion chamber and injector deposits.

An older engine may have more deposits in the combustion chamber which may create hot spots which could cause detonation which is where the fuel prematurely ignites causing a ping which could be detected by the knock sensor also.

Do you need premium gas? Does your engine have knock sensor(s)? Can your ECU control timing? Is your motor knocking? If you run premium do you notice better fuel economy and performance?

AudiUberMan

Thanks Squid.
I knew about the difference between regular and premium re: burn rates. Didn't realize, however, that this was combned with an advance timing issue. Interesting.
OK, well I have no idea if my 91 Audi 100 inline 5 has timing control via ECU or knock sensors. I did run a full tank of regular last year and, personally, did not "notice" any difference.... however, who knows. I might have just not noticed pinging, or knocking, or loss of power/milage, etc.

Good post. Thanks.

squidix9

The spark occurs BTDC but by the time it burns enough to produce the force necessary to push the piston down it it either at TDC or slightly after. If the timing is too far advanced or the burn rate ir to fast then it occurs too far BTDC and causes a knock.

Hence the higher the octane(slow burn rate) the more you can advance the timing. Also need to start thinking aout the kinetic energy and the force delivered over time and...

The '91 is CIS E III if I remember?

Mister Bally

My 1987 turbo Inline 5 had 286,000 miles on the clock before I donated it to charity. The engine never had to be opened up and compression was good at 250k when I last checked it. I never had to replace any fuel components other than the fuel pump (three of them).

Other than premium when I first bought it new, I always burned the cheapest junk I could buy. Withing reason........... I usually used brand name regular unleaded. Exxon, Chevron, Shell etc.

Sometimes on I-5 north of LA on the big hills (the warning to turn off A/C hills) the engine light would come on so I would back off of the pedal which shut the light right off. That was the knock situation cited by others. I have put premium in on trips over there and Northern Nevada & Colorado to prevent the light from turning on.

That car served me well for 18 years.

squidix9

Sounds about right

Extreme20V

What do you think is the absolute maximum compression ratio that can be built into a 20V, normally aspirated 7A 5 cyl to run of California's 91 octane gasoline?

I'm running a rebuilt head milled 0.015 inch and have NO problems. More CR = more torque!