This is from the same article Jay refered to...

Because my logs show a max of 15 pulled that is where my info came from and it seems that this is changed in the INTENSE PCM as well. Stock PCM may very well be 25.5 degrees but I still maintain that any knock is bad, detected and accounted for or not. More from the same article...

The simple fact is that knock sensors and the pcms ability to adapt are LAST RESORTS. If for whatever reason the pcm hiccups, or a knock sensor goes bad while your driving 3ms is not a lot of time to do something before your engine goes. Why in the world would you put your engine at risk to save a few cents per gallon?

My "horrible" gas mileage is with 91 octane. I replaced the o2 sensor, and did all the usual things to try and improve it, but it stil can't top 17 city, which is what it was rated at, by the way. Just a couple months ago, I got 28.5 MPG on a highway trip, so I believe the car is performing as advertised. It'* just that my city mileage is a reflection of many short trips and stop and go driving with one big long hill I have to drive up everyday. That has a huge impact. When people say city driving, it really depends a lot on what city you're driving in. In areas with little hills, city MPG will be better. I also believe that California gas has a higher percentage of oxygenates which reduces the total energy of the fuel. And it'* more expensive. Cheap premium here is $2.25/gallon.

I also want to point out that if you are racing your car or going WOT on a regular basis, PLEASE use premium, because you definitely need the protection. You have to look at the conditions before you cite danger. It'* just like humans, if you are running constantly, you will wear out your ankles and knees quicker then if you just walk fast. Exercise is good, but strenuous exercise all the time will wear out the body whereas moderate exercise will make you live longer.

Well guys, I just got off the phone with the techs at Whipple. According to him Doc has it exactly right. "Gm has the computer programed so that the engine will survive on 87 octane" The difference (which we all know) is that the timing is retarded and the 91 octane will out perform the 87 in racing and economy. I guess that is why we are able to get more performance by changing the pcm curve. So I will say that Doc'* advice is only good for cars that have stock programing.

He did go on to tell me that if you run 87 with the Whipple programing and sc that you will have problems. The reason is that Whipple has a much more agressive timing curve. On a different note... He told me they are pulling 490 hp out of the 5.4 3 valve Ford. He said he runs his 5.3 on 100 octane and is pulling 467 hp out of it. Very cool toys.

and my point is very simply that a person can survive on macaroni and cheese and bologna for a year, but that doesn't mean it'* good for the person. 87 octane WILL hurt your engine, it'* just a matter of how much, and if your sensors protect her. I thought the point of this conversation was to determine if this was harmful for the engine, not if you would destroy it. we all know 87 won't destroy it right out - if that was the case, we'd have several dead cars here already...

I would say driving WOT on a regular basis with 91 will hurt it far faster than driving easy on 87, just my opinion. Anyway, I will know soon enough if there is any economical benefit to 87 in a couple weeks. I need to prove it to myself before I'm convinced. Keep the comments coming.

I went after it from every direction I could with the tech and he assured me that it doesn't matter how you drive it (WOT or not). GM is running a conservative timing set and as long as the pcm and knock sensor are properly operating you wont damage the engine running it. Does that mean the subject is closed? For me it is. Even with what he told me I would never run less then what the factory calls for or recomend it to others. Should the pcm or knock sensor fail even for a short period of time you could very well damage or destroy the engine. It seems that more then the knock sensor are looking for it. The pcm monitors all of the sensors looking for causes leading to it and retards timing accordingly. The guy I talked to was desinging a setup for the new mustangs so I will defer to his knowledge. Could he be completly wrong or did I not understand correctly. I would say it'* possible but not probable. Good luck fellas.

high performance engine = high octane gas




no way around it...


FAST
CHEAP
RELIABLE

you can only have two

The key is a PROPERLY operating PCM and knock sensor, along with any sensor that controls fuel flow. You can still blow up the engine with 91 or higher if any of the system either incorrectly advances or leans the fuel too much. Even limp home mode could set incorrect values if the PCM was faulty in that section. You are pretty much at the mercy of the computer and its sensors. If you're getting knocking at all, something'* wrong with one or more sensors, PCM or the wiring that connects them. A partially clogged injector could do it too.

Wow, I really appreciate you going out and calling them yourself to find out. For the record because the basis for the debate can't cover every possible custom PCM programming factory settings were assumed. I would hope this could put the debate to rest but I get the funny feeling it won't satisfy everyone despite the other evidence already posted. We'll see though

You also said, "Should the pcm or knock sensor fail even for a short period of time you could very well damage or destroy the engine." This can hold true for almost anything else on your engine. I know I'm being facetious but wouldn't this be the same as "You shouldn't use a MAF sensor because if it fails you'll run lean and burn a piston or valve" Or, "Fuel injectors are more prone to clogging which will make it run lean and burn a piston, you shouldn't use them." Or even "Superchargers are dumb, if even one sensor goes out with all that added heat and pressure you'll be sorry." I'm sure you get my point. I think its great that you take advantage of the extra power to be had by using premium fuel and I'm not going to try to convince you (or anyone else) to use a lesser grade. The point here is more for knowledge on the matter. You and others (and me!) know more now about fuel than when we started this. Now if you use lower octane because of it is your own choice.

Green - I agree that driving harsh on 91 will hurt more than driving normally on 87. I appreciate the input you've had so far.

Foghorn - Yeah I had more to say its just that I had a limited amount of time to respond and with my average of 45min per post I had to cut responses short ...

"Fact: Knock happens first, which the Knock Sensors hear to which the PCM responds by retarding the timing. " You're absolutely right but the important thing here is the LEVEL of detonation that occurs before it is corrected. If you hear a loud BANG BANG before timing is retarded, hey you've got a problem, but thats not the case.

"Opinion: This is a bad thing because knock happens first and then the timing is 'corrected'. Using Knock Sensors is a reative method to deal with a destructive problem, using higher octane fuel where required is a proactive method the dealing with knock. " Well again this is correct but the point here is that BOTH are capable of correcting the problem. If a problem is corrected it is no longer a problem..heh.

"Opinion: Part throttle conditions can sometimes have ignition advance settings of 35 degrees, can the PCM pull sufficient timing to eleviate any knock?" Sure it can, why not? It pulls timing down to 11deg at WOT, I've seen it at 0, and it can still pull 25.5deg. This should be fine.

"Opinion: Under normal operating conditions the pistons are fine. When subjected to very high heat from lean conditions or extra load from pre-ignition or detonation, the weakness in the piston around the dish near the upper ring land is subject to fail. The upper ring land is very high up on the piston for emissions purposes but is a weak point from a high performance perspective. " Correct again! But remember, if there is NO detonation there is NO problem.

It seems you have the right thoughts but you're not taking a good enough look at the whole picture. This isn't just "detonation = bad" but an entire look at how detonation happens, how bad it really is, and how the computer can compensate.

Rogue -

"If for whatever reason the pcm hiccups, or a knock sensor goes bad while your driving 3ms is not a lot of time to do something before your engine goes. Why in the world would you put your engine at risk to save a few cents per gallon?"

Detonation is not an on/off switch, it has varying levels of intensity. Hence different amounts of KR.

Here'* a quote by the Aeronautics Education Enterprise on detonation, "One thing to understand is that detonation is not necessarily destructive. Many engines run under light levels of detonation, even moderate levels. Some engines can sustain very long periods of heavy detonation without incurring any damage. If you've driven a car that has a lot of spark advance on the freeway, you'll hear it pinging. It can run that way for thousands and thousands of miles. Detonation is not necessarily destructive. It'* not an optimum situation but it is not a guaranteed instant failure."

Whats really interesting is that while you're preaching the damaging effects of detonation you're paying people to limit the computers capability to correct it while at the same time adding parts that make it more prone to detonation...hmm..

"The simple fact is that knock sensors and the pcms ability to adapt are LAST RESORTS."
Maybe we should look at why cars were given knock sensors in the first place. Knock sensors were added so that cars could run higher degrees of advance and higher boost pressures thus getting more effeciency and performance. Before this manufacturers had to dial down the ignition so that you weren't likely to incur any damaging detonation. Now we can ride on the brim of detonation and if it happens, the computer will compensate. So its not so much a "last resort" as it is a capability to run more advance and pressure.


Mkaake - "and my point is very simply that a person can survive on macaroni and cheese and bologna for a year, but that doesn't mean it'* good for the person. 87 octane WILL hurt your engine, it'* just a matter of how much, and if your sensors protect her. I thought the point of this conversation was to determine if this was harmful for the engine, not if you would destroy it. we all know 87 won't destroy it right out - if that was the case, we'd have several dead cars here already..."

Great point with the macaroni, I did it for about a year....I love mac-n-cheese haha..
In light of all the evidence you still say it will hurt your engine. Ok, fair enough but here'* the tough part: Prove it. No I don't mean by trying it in your engine or getting me to do it in mine. Find ANYONE respectable that agrees with you. Aparently Whipple doesn't, Pontiac doesn't, and a whole host of other links to professionals and experts I've posted don't either. With all due respect, who are you to disagree?

Just for ***** and giggles, lets assume my car is detonating. 1-4deg of detonation each time. How many times can it detonate before I surely get damage? I mean noticeable damage like blown ring landings and such. I realize there are a lot of variables but give me the mid and top end range of what you think... Do you think 20 times? 30? 50? 100? Remember the 3ms remark earlier. Just kinda seeing what you guys think about the effects of it.