100 Octane

[dci4me]

Can a 2007 classic 1500 with a 5.3 flex fuel engine run high octane fuel? There is a VP by my house with 100 octane at the pump. The owners manual doesn't specify a range of octane, just that unleaded and E-85 can be run. If it could run on it, would it be a noticeable difference in performance without a PCM? With a PCM?

[Ranger]

Very little gains without a PCM reprogram to run on high octane fuel.

 

And your truck won't run without a PCM.

[cmoore74]

Even with a PCM upgrade (aka tune) you will get nothing out of 100 octane (except for a lighter wallet) as the stock compression ratio would not retard the timing even at 93 octane.

[Lances07Z71]

As stated above,it would be a huge waste of money and would only hurt performance.

[BigJayDogg3]

That is what I never did get. I can understand not gaining anything, but how would you lose power?

[Ranger]

The way I understand it is that over a certain octane point there's a lot of excess carbon and waste from the "unburned" portion of 87+ octanes starts clogging up the cylinders and injectors. Probably not the exact case but that was the best description that I could understand of it...Course, some higher octane fuels have some extra cleaners mixed in them to offset that but offers no real gain, just a cleaner engine.

[svensk]

It's racing gas. Unless your car is putting out 500 or more hp it's probably a waste of money.

[dci4me]

Does it make a difference that I run 91 rather than 87 or 89? Im not so much concerned with performance, just overall quality of fuel for the engine. My original thought was that 91 would be cleaner and therefore better. Was this assumption correct?

[ZZ327]

Does it sayif theres ethanol in that 100 octane?

 

E-85 is actually 105 octane

[Nytemare]

That is what I never did get. I can understand not gaining anything, but how would you lose power?

 

 

Many people get confused with octane numbers. Its higher octane so it will burn better and hotter, Right? WRONG.

 

The higher the octane number the harder it is to burn that fuel. This is why high compression engines need higher octane numbers. The octane in a fuel is what resists pre-ignition, or pinging. Its where the fuel gets compressed to the point that it self ignites before the spark plug sparks. This results in all sorts of bad news.

 

In order to stop pre-ignition you use a higher octane fuel, this means you can run a higher compression ratio, which compresses the fuel more for a more explosive reaction and therefore produce more power. If you run too high a number in a low compression engine, the opposite will happen, it wont be compressed to the point it needs to be, and you will get an incomplete combustion reaction. This results in Carbon Monoxide, carbon build up, clogged cats, fouled plugs...you get the picture.

 

Advancing your timing has a similar effect on this reaction. Advancing the timing makes the plug fire before maximum compression is achieved, This helps make more power by igniting the fuel early, since a higher octane fuel is harder to burn under compression, and even harder again when not fully compressed, you ignite a portion of the mixture, it starts to burn slowly and as the piston travels up the cylinder to full compression, the mixture, which was ignited earlier in the cycle, is compressed and allowed to burn completely. Its kind of like using the spark to ignite a portion of the fuel like kindling to burn a bigger fire.

 

Sorry for the combustion engine lesson....just thought you'd like to know how it actually works.

 

AND NOW YOU KNOW....CAUSE KNOWLEDGE IS POWER

[C & A s Dad]

Although I would not agree with running 100 octane, I strongly believe there are some slight advantages to running 91 or 93 in our engines. The compression is 9.0 to 1 or in some cases 9.5 to 1 and I believe this is high enough to run 93. I have been running 93 Chevron for over 2 years now and do it primarily because my truck seems stronger when pulling hills when towing my camper. I also am getting slighlty better mileage with the 93.

 

I have set my scan gauge to read knock retard and have tried my best to see if the computer would reduce some timing. I have lugged the engine and run at redline with no timing retard at all. What I do not know is would it have pulled some timing if I were running 87. I am having to guess it would but have never tried it.

 

Of course the theory is that our computer will allow the engine to run with more timing because the knock sensor is not detecting any ping/detonation. As long as no detonation is detected, the computer allows max timing resulting in more power.

 

I will also add that with 3 oil analysis being done, I have had 0 fuel in my oil leading me to believe I am not leaving unburned fuel in my comb chambers.

 

Aiden, your explanation of why carbon can build in an engine running too much octane is the only time I have seen anyone actually answer this question. This makes perfectly good sense and I would agree that if you were running more octane than your engine would handle, you could be leaving behind some nasty stuff.

 

I did try 93 in my Caravan for about 12 to 15 tanks with no gain in MPG at all. In fact, it was actually a little worse. The comp ratio in the little 3.3 is likely about 8.0 to 1 and there is no need at all for higher octane.

 

I guess my thought is if you could run a modern 5.3 without any electronic assistance, how far would you have to retard the timing to make it run on 87 without pinging it's butt off.

 

Again, I would not run 100 if I had it avail. In fact, if I had 91 avail I would try it instead of the 93. Here we have only 87, 89 and 93.

[Nytemare]

definitely....9:1 or 9.5:1 is up in the 91 range...I would say that the only reason we can run 87 is the knock sensor.

 

I believe 11.5:1 is the cut off for pump gas, if run with sufficient timing retard. I could be wrong though...Its been known to happen.

[Norseman]

In fact, if I had 91 avail I would try it instead of the 93. Here we have only 87, 89 and 93.

 

 

I might be wrong and please correct me if I am, but this is what I've been doing lately (because of the price of gas). For every gallon of 87 I put in, I add 1.5 gallons of 93 in netting me about 91 (actually 90.6 but to keep the numbers simple at the pump I call it 91). It would be interesting to see if your performance changed at all with that mix.

 

I know, I know, I don't save a whole bunch of $$ but I do save a few bucks at every fill up.

[C & A s Dad]

Have not thought about doing that. I have considered running 89 through the winter because I typically do not hook up the camper from Nov to March.

 

I sent you an e mail earlier. Did you see it?

[Norseman]

Have not thought about doing that. I have considered running 89 through the winter because I typically do not hook up the camper from Nov to March.

 

I sent you an e mail earlier. Did you see it?

 

 

Just got it. Sending reply, now.