Premium vs Regular

[deerefeed]

I've noticed that the supercharged 3800 engines and Cadillac V-8s recommend using premium fuel. Why do they require premium and is there any problem associated with using regular 87 octane fuel?

[BNaylor]

Can't speak for the Caddy engines but for L67 engines, they require 91 octane premium gas or better per the GM owner's manual. However, 87 may be used only in emergency situations for a short period of time. The primary concern on a L67 engine is knock retard (KR) due to the supercharger, especially in high boost conditions. If KR is detected the PCM will retard igniton timing degrading performance.

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 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."