Regular Unleaded OK in 1999 PA Ultra?

[snakeman3116]

I bought my PAU at a lot I worked at. The car had been there for about 280 days and had only had reg. unleaded the whole time. I am no mechanic and I'd like to make sure that continued use of reg. unleaded won't kill my car.

[maxwedge]

Supercharged engine should run on at least 89-91 for maximum performance, regular is ok but will cut down that performance.

[Bassasasin]

87 minimum per the book...

I dont know or agree completely about performance effect with octanes. I would suspect the compression ratio controls that. If they say 87, they have figured in all that. If your getting too much compression with a octane it will detonate and back off timing from knocking. I would suppose the knock sensor would cause timing to back off..

Detonation is dieseling, a compression firing.. Pre ignition is from a hot spot in the engine firing the gas..

I used to hear often of a car supposidly dieseling when the ignition is turned off. My belief was the Idle is set too high and still getting gas and getting a pre-ignition effect on the old venturi fed gas carbs.
I think thats right? (:scratching my head

It would be interesting to hear if someone tested different octanes to see if it actually does help putting in a higher octane. I know performance fuels work but that's more bang for the buck.

Bass

[HotZ28]

Originally Posted by Bassasasin

It would be interesting to hear if someone tried different octanes to see if it actually does help putting in a higher octane.

Bass

Here is a good thread on that subject!

Happy New Year!

[BNaylor]

Originally Posted by Bassasasin

87 minimum per the book...

I dont know or agree completely about performance effect with octanes. I would suspect the compression ratio controls that. If they say 87, they have figured in all that. If your getting too much compression with a octane it will detonate and back off timing from knocking. I would suppose the knock sensor would cause timing to back off..

Detonation is dieseling, a compression firing.. Pre ignition is from a hot spot in the engine firing the gas..

I used to hear often of a car supposidly dieseling when the ignition is turned off. My belief was the Idle is set too high and still getting gas and getting a pre-ignition effect on the old venturi fed gas carbs.
I think thats right? (:scratching my head

It would be interesting to hear if someone tested different octanes to see if it actually does help putting in a higher octane. I know performance fuels work but that's more bang for the buck.

Bass

You left out a major factor in your theory and that is the supercharger (Eaton M90 Gen3) especially when boost kicks in. We are not talking about a normally aspirated engine here. It is an L67 SII 3800. The L67 has a lower compression ratio than a normally aspirated L36 but knock retard (KR) is the primary concern. The PCM module will react if it senses detonation/pre-ignition.

The GM owner's manual has always recommended 91 octane or better premium unleaded. Only in an emergency situation should you run a lower octane as low as 87 and only if knocking does not occur. Unless you have calibrated hearing or a rear end to sense detonation/pre-ignition which is very subtle unless you run proper tests with diagnostics equipment capable of sensing knock retard (KR) I would be careful recommending to anyone with the engine in question about it being OK to run 87 octane regular unleaded.

I have recorded KR as high as 4* to 6* degrees running regular versus 2*or less running premium. KR will destroy the engine performance and fuel economy not to mention the long term effect on the mechanical parts of the engine running only regular unleaded. The highest readings will be at rpms over 4000 and WOT. On a stock L67 in good tune with no problems the typical KR running 91 or better octane fuel will be around 1* or less which is acceptable. 4* - 6* is not acceptable...period!

Here are some interesting facts:

For every 1 degree on a L67 SII 3800 you will lose approximately 2 - 3 horsepower.

For every degree of knock retard it takes .8 seconds recovery time.

"Once the PCM has retarded timing sufficiently to reduce knock below the currently detected peak level, a changeable parameter in the PCM governs how quickly the overall ignition timing can be restored to normal levels. The engine could see a peak of 15 degrees of KR from which the originating detonation may immediately disappear. However, the PCM will not instantly restore timing to pre-detonation levels. Instead, the PCM cautiously and conservatively restores ignition timing at a rate of 0.8 degrees per second. In the event of a 15 degree KR event, it would take nearly 19 seconds for the ignition timing to be restored to pre-KR levels. By the time your car sees full power again, the damage has been done or the race is already over. This 'time' that the PCM takes to restore the ignition timing is called the Recovery Rate. The Recovery Rate will continue in this slow fashion until KR reaches zero, KR increases back above the current recovery value, or the throttle is released."


FAQ: Why is knock retard (KR) bad?

"Due to the retardation of the ignition timing, KR causes the vehicle to lose substantial power. More importantly, though, the flame front collisions are EXTREMELY harmful to the pistons. These highly volatile areas in the cylinder can cause stress cracks in your piston, which will eventually give way causing an entire CHUNK of your piston to lift right off and begin banging around inside the cylinder. This is why when the spark plug is removed after such an event, the plug end is bent all the way over. The broken piston can be VERY expensive to fix if you are not capable of doing the work yourself. DON'T EVER DISABLE YOUR KNOCK SENSORS. It takes less than 3ms to damage your engine due to knock."

Therefore it is not OK to use regular unleaded fuel! Lets get our facts straight. However, if you desire to run regular unleaded in a GM car with a L67 engine no problem it is your car and money $$$.