Dick Cabasa

There's a thread about dyno testing 100 octane gas floating around.

It FEELS more responsive, but the dyno isn't showing more power than 91 octane. Even with 2.9 KR.

Go figure...

TheZman

High octane DOES burn slower!! If your engine doesn't need higher octane it will hurt performance. 100 octane is not available at the pump in my part of the country, but 94 sunocoULTRA is, and it seems to work perfect!

BIGBOS

The main reason I did it was to get rid of the carbon build up...I was reading up on someone having problems with the build up driving around in 6th gear at 40 all the time...so he put a tank of high octane in there and it solved the problem...

GeorgeC

I've been caught without my fuel jug at the track twice, and had to buy a few gallons of Sunoco 100 unleaded. Both times, my slowed down just a tad. Like TheZman said, it will hurt performance if you don't need the extra octane. My compression ratio is around 11:1.

Colonel

Too much octane will cost you power.

Yes, higher octane burns slower.

SCZ28

100 Ul is too expensive to drive around on. I run 110 leaded at the track and have run 100 UL and didn't notice a difference in my performance.

Colonel

If you run leaded gas you will be buying some new O2 sensors before long. Been there, done that. Doesn't take much.

ChrisB

Octane rating and flame speed are totally different factors. they are not related at all. A higher octane fuel may just as easily burn faster than a lower octane fuel.

Octane is simply a measure of a fuels oxidation activation energy - the amount of energy it takes to ignite the fuel. If conditions in they cylinder reach this threshold before it is intended (e.g. before spark event) you have pre-ignition. This isn't related to how fast the mixture burns - rather how hard it is to ignite.


If the flame speed is slower then you will loose power (if you don't increase timing) - and depending on how much lead you have and the geometry of your engine this may or may not give you additional power.

If the flame speed is faster then you will have to decrease timing - which again, may or may not give you power (a smaller r/s ratio will tend to like less ignition lead, and vice versa).

If flame speed is the same and you have enough energy to ignite the mixture (you should) then there would be no difference in power, assuming no ignition anomolies with either fuel; it would simply cost more.

Chris

So Slow

iTrader: (2)

[quote]Originally posted by Colonel:
<strong>If you run leaded gas you will be buying some new O2 sensors before long. Been there, done that. Doesn't take much.</strong><hr></blockquote>

A couple of weekends ago I bracket raced the car and decided to run half a tank of 100 octane LL with a half a tank of 93 octane..Didn't notice any performance difference and come Monday morning the cars SES light came on..

Scanned the car and it showed codes 1133,1153 which are the right and left bank 02 sensors on the manifolds...Needless to say the car goes in the shop this Monday for warranty work..

Colonel

"If flame speed is the same and you have enough energy to ignite the mixture (you should) then there would be no difference in power"

...which is exactly why I think the high octane unleaded racing fuels that I've tried burn slower. When NA with low compression, I definitely lost some power with the higher octane just as others have. From what you've said I guess it's incorrect to say that higher octane means slower burn but maybe it's common for higher octane fuels to happen to have a slower burn to decrease the chances of detonation as well as the pre-ignition that you described.

BIGBOS

I did it for the first time last night....a full tank...the car seemed like it was more responsive...do any of you guys do this...??

It actually smelled good, I wouldn't think 100 would be that much higher...

I think I'm going to do this 1 time a month now to clean out all the carbon...

And another question does higher octane burn slower??

SCZ28

[quote]Originally posted by Colonel:
<strong>If you run leaded gas you will be buying some new O2 sensors before long. Been there, done that. Doesn't take much.</strong><hr></blockquote>

I got about a season out of this last set of O2s. I have a blower car and the extra cost of injectors once a yeat or so is more than worth the risk. The cost is much less than a motor. Is it overkill...might be but I run more timing and I think it is worth it. Highest UL gas I have found in SoCal is 100 and you can get 118 for the same price.

99 Black Bird T/A

iTrader: (4)

FWIW:

dyno w/93 Octane: 323rwhp & 335rwtq...the dyno operator said the car ping'd but I couldn't hear/tell that it did. He said I'd make more power with the ping gone... <img src="images/icons/grin.gif" border="0">

Changes

dyno w/100 Octane, 160 stat & TR6 @45: 317rwhp 329rwtq...dyno operator said the car didn't ping... <img src="images/icons/rolleyes.gif" border="0">

I leaned the car out 4% @ WOT with MAFT hp returned to: 323rwhp & 335rwtq...dyno operator said the car didn't ping <img src="images/icons/shocked.gif" border="0">

Even leaning out I don't think 100 octane helped much. It was a cool day.

...and I don't think the car ping'd it was just the noisey valve train.

ChrisB

[quote]Originally posted by Colonel:
<strong> From what you've said I guess it's incorrect to say that higher octane means slower burn but maybe it's common for higher octane fuels to happen to have a slower burn to decrease the chances of detonation as well as the pre-ignition that you described.</strong><hr></blockquote>


yep, you got the idea! A higher octane fuel may very well burn slower - especially if it is a turbo mixture (c-16, etc.) Conversely some of the fuels the stock eliminators use (c-3x, c-40) will burn just as fast or faster than 93 octane. - and they are around 115-125. It really depends on the specific mixture you use.


Chris

Colonel

Kinda like adding Propylene Oxide. It's an ignition accelerator. You could add it to race fuel to speed the burn while still having excellent pre-ignition properties. I don't know how this would be different than advancing the timing though.

[ November 23, 2001: Message edited by: Colonel ]</p>

ChrisB

Propylene Oxide is some nasty stuff though - I wouldn't recommend using it.

but as to what the difference is - in regards to before TDC - not much, as you would adjust your timing accordingly. But after TDC if you have a faster flams speed then you are going to reach peak pressure sooner, and have your pressure distributed over a smaller percentage of the power stroke.

If you have a large r/s ratio then your angularity just past TDC will be low, so the component vector of the piston force will be small, and you will not be harnessing all your force.

If you have a smaller r/s ratio your angularity will be greater, so you will be harnessing the more of the force.

Ideally you want the integral vs. crankshaft degrees of pressure * component vector to be the greatest. In practice there are so many other factors that effect this it is generally easier to experimentally determine it.


Chris

Colonel

So basically, propylene oxide would more than likely cost you power assuming that your timing advance was correct before?

cantdrv65

iTrader: (3)

[quote]Originally posted by Colonel:
<strong>So basically, propylene oxide would more than likely cost you power assuming that your timing advance was correct before?</strong><hr></blockquote>
Add some Ethylene Oxide and really make it interesting... <img src="images/icons/wink.gif" border="0"> BOOM.