If I were to put in Higher Octane gas, will the car get higher HP and torque, without changing the spark and fuel mapping?

 

If you're not getting any timing retard on what you're running now then you won't see any benefit whatsoever. If you ARE getting timing retard now then trying some higher octane might prevent that and you can get a significant increase.

Essentially the octane rating is a measure of how hard the fuel is to detonate. On high compression engines if you run too low an octane it will detonate prematurely robbing you horsepower. Sometimes the car can compensate by backing off the timing and you'll never hear it ping or knock but it will still hurt because you're not really firing at the optimum time.

The rule of thumb is to run the minimum octane that you can without getting any detonation to make peak power.

 

If you are getting detonation or spark retard due to low octane fuel or high compression, yes it would help. Other than that I doubt it would do much, if anything.

Nathan must type faster than I do! :D


[Modified by ANTI VENOM, 8:21 AM 2/4/2003]

 

Okay. Well if I made a chip specifically for a higher octane for track use..hmmmmm

 

That would work. Of course there is a limit to how far you can advance the timing and still do any good as well. It is certainly something to play with.

 

What octane are you using now?
What did you want to go to??

My 94 the recommended fuel is Premium so the next highest thing would be to stop by my local airport for some Av gas.

 

:iagree: Well said!

 

My car is tuned to run on 91 octane,the best gas in our area. I ran some Sunoco 104 unleaded in it, and the car actually lost 1 mph in the quarter.

 

not true, some stations (at least out here in OK) have 93 or 95 octane as opposed to the "standard" premium level of 91. I put 93 in last week for the first time, and I can feel (it may be a mental feeling) a difference :D

 

technically, no it wouldnt, but if your engine requires 94 and you put in 92, you could lose power because of retard

If your engine requires 92 and you put in 94, you dont gain anything.

Everything depends on your compression ratio.

 

There are a few caveats to this whole thing (went through this with a non forum person...actuall a Porsche guy...a few days ago).

There are two ways that you can gain power with higher octane:

1) if your ignition timing is detonation-limited with the base octane fuel in your comparison, thereby allowing the timing to return to optimum with the higher octane fuel

2) if the higher octane fuel contains oxygenates to achieve the higher octane rating. Though not commonly used, there are some race fuels which contain oxygenates. The oxygenates are supposed to be used for increasing the octane level of the fuel, but they also do something else: they carry more oxygen into the cylinder because the fuel molecule actually has oxygen in it. Nitrous oxide is the most commonly used oxygenate. Two other popular ones are nitromethane (which has enough oxygen in the molecule that it will burn in an inert gas environment), and propylene oxide, which also happens to be a level 1 carcinogen. It's funny...I know people who won't use Propylene Oxide because it's carcinogenic, and then they turn right around and light up a cigarette. :confused:

Anyhow, it doesn't matter one bit what you tune the chip for. Each piston, displacement, cylinder head combination has it's optimum timing point. A given combustion chamber, the optimum timing is roughly the same. You hear about "fast burn" heads. The faster the "head" will burn, the more your optimum ignition timing retarded in relation to stock. This is a GOOD thing, because the earlier the spark ignites, the higher the pressure in the cylinder as the piston is finishing it's rise to TDC.

The key is to develop peak in-cylinder pressure somewhere in the 8-10 degree ATDC range. Shooting for this range tends to minimize the losses due to the expanding gases as the piston is still coming up, while still maximizing the useful average in-cylinder pressure (pressure in-cylinder after the piston passes TDC).

Basically, if you're putting a brand new combination together and you're trying to tune the timing on the motor, you start your timing very retarded (around 25 degrees BTDC at full advance) and start advancing it. If you start detonating before the engine's power peaks, back the timing off just enough to stop the detonation and set it there until you can get higher octane fuel in the motor. If your power peaks and begins to fall off again before the engine starts to detonate, back your timing back to peak power and leave it, whether you put higher octane in it or not. That timing (where the engine makes maximum power without detonation) is your MBT timing, or maximum brake torque timing at minimum spark advance.

 

if you do not need it for compensation on ignition timing you will lose power due to a slower burn with the higher octane fuel put in mininum octane to eliminate detonation due to timing to make most power :cheers:

 

The only thing you will gain is a lighter wallet.

 

Interesting question. I can tell you this. I had a 91 Camaro RS that I used to run some fuel additive made by NOS. It boosted the octane to 107, and contained nitro methane. In my Camaro, I could definately FEEL a difference. :yesnod: In my vette, the same exact stuff yields no better performance. What gives?


[Modified by 95AquaC4, 9:37 PM 2/4/2003]

 

Thanks gain guys, just curious. I use 92 Octane here in Hawaii. Nothing special, they have race unleaded here, and some guys use it. It is expensive, 8 dollars a gallon. I run around 10.6 to 1 on my 396, but my timing goes up to 41 degrees with no detonation. Just wanted to maximise my last run till roll bar, as I am putting long tubes, and a Vigilante from my 87 Covnerter. Running 12.1 at 115.5 mph now, and 1.85 60' times. I figure if there going to kick me off with the changes I make, might as well get the most of it.

 

"if you do not need it for compensation on ignition timing you will lose power due to a slower burn with the higher octane fuel put in mininum octane to eliminate detonation due to timing to make most power "

This is a common misconception. Octane raises only the flashpoint of the fuel, it does not change the rate of burn of the fuel. The burn rate (actual rate at which chemical reaction occurs) is a function of the fuel itself, not the octane content.

In answer to the question about the guy running race gas with some nitro in it, it must not have much nitro at all in it if it didn't make a difference...if any nitro at all.

Question for Steve40th....why in the world do you need 41 degrees of total timing?!?!?! That's an absurd amount of timing! I have a feeling that you're way in advance of MBT timing at 41 degrees. I should think that your MBT timing should be around 35 degrees or so. Just wondering, because in my experience the should never be that advanced.

 

CorvetteZ51.., I know it looks like allot, but the tuner got my RWHP to go from 381 to 407rwhp. The timing was 29 degrees with up to 8 dgrees retard, now 0 degrees knock retard. I am happy though. No knocking

 

???? He advanced the timing and got RID of knock???? Something's up there...that's not right.

 

Why is that not right? Do you know the pistons, dome shape, head work, camn , duration, pulse widths? I dont know much , but I ran a Big Block with 42 degrees and Nitrous, with 10 to 1. The O2 sensors are right at 940mv. I ahve a friend that got his car up to 48 degreees on a 383 LT-1. I have no idea, but the car seems to be running fine, and cooler.

 

More advance will increase the pressure/temperature gradient after the beginning of combustion in the combustion chamber, especially before TDC (you'd be surprised how little effect it has after TDC) Raising the pressure/temperature gradient too high (after the beginning of combustion) is one of the causes of knock, therefore increasing the timing advance would increase the engine's propensity to knock, not reduce it. Quite frankly, I can't think of any reason why advancing the timing would reduce knock at all, as the temperature/pressure gradient doesn't change in relative location much after TDC if you increase the advance.

The only thing I can think of right now is....at what rpm did the "knock" occur? Was it only at high rpm? If so, do you have an aftermarket ignition similar to MSD or something along those lines? If memory serves, the LT1's do not have a capacitive discharge ignition system. If that's the case, and you're only "knocking" at high rpm, my bet is that the ignition can not ionize the air/fuel mixture consistently enough at the more retarded timing because it can't keep up. The higher the temp and pressure in the cylinder when the ignition is trying to fire (i.e. the closer the piston is to TDC), the harder it is to ionize the gas, or the more spark energy you need. A typical transistorized coil ignition can not recharge the magnetic field around the ignition coil fast enough at high rpm to generate the spark energy needed to ionize the gas...to spark. It also doesn't really matter all that much how many volts your coil is in this case, as the problem is the system's inability to get the current to the coil fast enough after each discharge.

Long and short, if all of the above is actually happening, then I would think your engine was having occasional misfires with the more retarded timing, which the knock sensor mistook for knock, cutting back on the timing. Again, IF all of the above is happening, you'd gain more power by installing an MSD-type ignition system (capacitive discharge system which is capable of keeping up with the higher rpms) and retarding your timing.

Before any goes off on this post, let me clarify a few things.
1) I do not remember if the LT1 has a CDI ignition...if it does, then this is all moot
2) Understand that the propensity to knock is due to excessive temperature/pressure after ignition
3) Understand that excessive temperature/pressure BEFORE ignition is one cause of misfires
4) This is me brainstorming, and by no means saying this is for sure what's going on...I'm trying to understand why in the world advancing the timing would reduce knock, and this is all I can think of...I've seen these symptoms before, but it's rare to see someone try to fix it by advancing the timing.