PontiacBonnevilleClub.com

anyone have thoughts on whether i need to use mid grade 89 octane or premium in my 04 gxp? it seems to run fine on mid grade but just wondered? thanks robb in va

You can run regular. There have been a couple of us that have tried running premium vs regular and nobody has found enough of a difference to overcome the difference in price. I'm sure some of them will chime in today as well.

I run mid-grade 89 octane. Never saw the need to run premium and have never run regular in any vehicle I've owned. Just personal preference here.

The best way to tell is really to scan it with a scanner and monitor both timing and knock retard and do a couple of WOT runs. Typically if you get several degrees of KR during that WOT run, you might want to consider running a premium grade fuel for it's higher octane, which the higher octane is meant to resist pre-ignition (I.E. knock). However, as is always the case, sometimes a few degrees KR is actually good, if it's scaling back your timing into the engine's sweet spot. This is why I said above to monitor your timing also.

For example (these numbers are just for example purposes, not factual), if you're engine likes 25* of overall timing at WOT, there are two ways it can get it;
1) 25* of initial timing - 0* of KR = 25* of overall timing
2) 27* of initial timing - 2* of KR = 25* of overall timing

When it's bad is if you're getting 20* of initial timing and -4* of KR for 16* of overall timing.

So really, you need to find out what works best for the engine and your setup. The only way to do this is by scanning. No one here has a finely calibrated butt-o-meter to be able to tell you the difference. It could be very much the case, like Donky mentioned, where yours works just fine with 89 and it's not worth the money spent. Each engine, it's tune, outside variables, etc, etc, all make things slightly different. What works for mine, most likely won't work for yours. On the other hand, my engine likes the 93. I also don't sweat about paying the extra $.20/gallon for it since I don't spend money on other things like the latest CD/MP3, or latest electronic gizmo. I don't need to go over to Starbucks after filling up and dumping $10 for a fake coffee disguised as a mocha, choca, cocoa, frappa, wappa, dappa, crappa chino on grande size! I can get along perfectly fine with a large, BLACK, cup of regular coffee.

Otherwise, most premium fuels only contain extra additives over lesser grades.

What is so bad about excessive KR? Is it just a build up of carbon deposits?

The requirement of premium gasoline is one reason I won't be considering a supercharged 3.8 in the future. In Vancouver the cost of gasoline is pretty much the highest in Canada, perhaps in all of North America.

KR is knock retard. It is the engine pulling timing due to detonation, pre-ignition, whatever you want to call it. The fuel is burning before the plug ignites, causing "knock" which can severely damage pistons. I think that an engine should be at 0* KR with the optimal timing, not using KR to make optimal timing. KR is a reaction to lessen the effect of the event, but the event is still taking place. An event that can pop a piston so quick if out of control, it'll make your head spin.

I realize that KR is knock retard, and I know that pre-ignition knock can be very damaging for an engine. My question is, why is the knock retard bad for the engine, besides lost performance?

Knock means that the fuel is igniting while the piston is still going up in the compression stroke, before it is supposed to. Those expanding gasses have to go somewhere, and eventually, they blow your piston out. That's why it's bad.

Pretty sure he wants to know the meaning behind KR itself, Ed

Basically, the idea is that if KR readings are present, the knock has already happened and the computer is now compensating. The potential for long-term, or if severe enough, immediate damage is there. See CMNTMXR57's post for the rest

I can tell you that the owners manual for my Caddy recommends 89 octane.

I guess I'm not stating my question clearly, my apologies for that.

When KR is present, it retards the timing to reduce the knock. So, what are the effects of retarded timing? Is it a less complete burn of the fuel/air mixture which might increase carbon deposits?

I hadn't considered that from Jerry's post that some knock has already occurred if KR is commanded by the computer. The knock that preceded the KR could have already done some engine damage.

As engine RPM increases, you need to have the ignition system advance the timing to match. For example, on an old school car where you could physically set the timing. At idle, typically 6* BTDC was a standard time. That means, the plug fired, starting the flame kernel moving, 6* before the piston was at Top Dead Center. This way, by the time the physical movement of the piston was at TDC, the ignition of the Air/Fuel mixture in the combustion chamber would be initiated, and that explosion from ignition of the A/F would now force the piston down on the power stroke.

Apply that same principal to an engine at 5,500 rpm now, and you have to advance the timing so that it maintains that ignition of the plug in enough time so as that by the time the piston was at TDC, the ignition process had fully started, and the expansion from the explosion, forces the piston down.

Iginiting it too soon, could result in knock from an starting the ignition of the A/F mixture to soon before the physical movement of the piston was at the right point. Too late and knock can result from not complete combustion. There are other reasons knock can be present too, but we can open up that pandora box in another topic. Right now, let's just concentrate this on pure timing related knock.

Cliff Notes;
So as can be seen, any engine likes an optimum level of advance timing as the RPM's increase. If there is bad knock present, it is scaling back that overall timing, and therfore reducing optimal power. Remember, any KR you see on the scanner, is a reactionary measure, and it's teh PCM's way to quell it, and keep from self destructing itself. However, as I also mentioned, sometimes there can be "good" KR too! If your overall timing is too much, the systems check/balance of KR, may actually push it back into the optimal range. Both of these points is what I was going for in my original post.

Once again, this is just a Cliff's Notes explanation. THere are many other variables/issues involved. Just purely a timing explanation.

I live just outside of Vancouver and pay even more for fuel.
I don't mind paying the extra $5-10 for a fill up with 94 Octane every 2 weeks.

94 Octane is the only grade here that doesn't have 10-15% CornAlcohol added to it.

If you don't have a SC running more than 5 psi of boost then use regular 89 octane.

I've always noticed a lower temperature in the summertime when using 93 octane. For the minimal difference in the cost of a tank of gas, it's worth it.

July 1st the fuel just went up 15 Cents a liter !

...being glad I don't have a S/C... I'll settle for regular at these prices.

Another advantage of the GXP - regular gas!

I'm getting spoiled. The last couple weeks I've been drivin the Charger and it takes regular. Over 600 kms on 3/4 tank!