100 octane

[ssp2019]

Just filled my '73 tank with 100 octane gas. Wondering if it's overkill now that I think about it. Car has 38/38 weber, 264 cam, SuperSprint exhaust but otherwise stock internals. Any benefits of higher octane on non-modern fuel injected engine? It does feel a little more powerful but it could be due to placebo or density altitude as it changes day to day. Cheers

[Preyupy]

With a stock compression ratio and that camshaft you probably don't need 93 octane.  Higher octane does not produce more power unless you were experiencing detonation with the lower octane.  If you optimized your jetting and ignition timing on a DYNO with the lower octane then did it again with the higher octane you might see a 1-3 ft/lb or HP change but I doubt it.  If you just poured in the fuel and made no changes to the engine it didn't make any difference.  I don't trust the calibration of your SOTP (seat of the pants) DYNO.  

[02Pilot]

Octane is a measure of resistance to combustion. As Preyupy noted, unless you were experiencing specific issues associated with pre-ignition on a lower octane fuel, more doesn't help.

[jimk]

Here is a discussion about fuels and octane.

 

 

[Dudeland]

In short.  No. 

 

You should only look for something ethanol-free, which is typically the higher grades (at least here in Canada).   Petrocanada 95 is ethanol free,  or at least they post that.  The formula changes from winter to summer seasons. 

 

Regards

[Mike Self]

Ever the contrarian...

 

I've been using the "normal" (for these days at least) 91 (or 93) octane "up to 10% ethanol" premium gas in my 02 (9.5 pistons, 284 cam) and also in my 318is and it seems to work OK--no pinging on either car unless I lug it. 

 

And over the years I've not experienced any deterioration in the fuel system--hoses, carb components, fuel pump etc, using the 10% ethanol.  Admittedly I did replace all that old OEM braided cover fuel hose, primarily because it was cracking, but otherwise...as the factory (BMW and Weber) made the parts...

 

mike

[Preyupy]

 I try and run the ethanol-free fuel is on cars that don't get driven much (don't go through a tank of fuel at least once every 2 months).  The ethanol absorbs water out of the atmosphere and you can have an increased problem with Rust in the fuel tank as well some deposits in the bottom of the float bowls that build up over time.   The 2002 Turbo is fine on the 92 octane 10% ethanol fuel we get here.     I don't have any problem with the 10% ethanol fuels, they work just fine in my 325whp Mazda wagon at 19psi of boost. 

 

 I have heard of some vintage cars that have had a problem with vapor lock using the ethanol fuels but that is rare and I have never seen it for myself.  

[jimk]

There are fuel additive products that will provide treatment against fuel phase separation and also contain a rust inhibitor, good for up to a year storage.

[visionaut]

4 hours ago, Mike Self said:

Ever the contrarian...

Yep. Me too. And too much time on my hands obvs…🫠

 

 

Octane is the main hydrocarbon molecule found in gasoline that gives it the ability to resist combusting prematurely.  It’s eight carbon-molecule long chains are very stable under compression (unlike heptane, with seven carbons, and also found in gasoline, which combusts spontaneously under compression).

 

Non-hydrocarbon gasoline additives can also be used to deter detonation (and increase the gasoline’s anti-knock “octane rating”). Tetra-ethyl lead was quite good - very small amounts provided significant reductions in detonation - but a neurotoxin... It’s been replaced in today’s unleaded fuels by additives like MMT and MTBE.

 

So higher octane fuels provide the ability to run higher compression without detonating the fuel before the plug sparks.  In general, higher compression also means increased engine efficiency and power potential. The increased timing advance run in high compression engines takes into account the typically slower burn speed of their required higher octane fuel. 

 

But many modern racing gas formulations, like 100 or 114, actually violate the ‘rule’ that higher octane fuel burns slower, because they’re not like normal street pump gasoline formulations.

 

Many of these high octane racing fuels actually burn faster, so max hp/tq gains can be achieved with similar advance timing as for gas-station premium. They can still obviously tolerate a ton of advance without detonation, but these specialty high octane fuels don’t all burn slower anymore. (And they also still produce slower burning variants, like what’s needed for boosted applications).

 

Additionally most modern racing fuels are also now oxygenated.  Oxygen potent additives such as propylene oxide have been shown to increase power by almost 8% (with no jetting changes!) - just by making more oxygen available for combustion. The higher the oxygen content coming along with the fuel, the greater the potential power.

 

 

So I’d say it’s entirely plausible, with a pricey tankful of modern faster-burning, high octane, oxygenated racing fuel you could produce a bit more power with no other changes than the fill-up…

 

 

[Son of Marty]

48 minutes ago, visionaut said:

Tetra-ethyl lead was quite good - very small amounts provided significant reductions in detonation - but a neurotoxin... 

When I was a young lad I remember driving through the rail fright yard looking for a lost car load of cantaloupe and passing a tank car clearly labeled Tetra-ethyl additive and leaking a stream from the bottom valve onto the ground, you Mom was right don't play on the R/R tracks.

[visionaut]

2 hours ago, Son of Marty said:

passing a tank car clearly labeled Tetra-ethyl additive and leaking a stream from the bottom valve onto the ground

…but you still turned out alright, right? Don’t look at me with your third eye like that man… 🫣

 

I used to pump that heavy gas in high school and inhaled many times the minimum, if there even was one… You know I haven’t had any issues with my third eye since the LASIK.

 

….

ba-da-boom

😁

[Son of Marty]

[M3M3]

The benefit of NOT using 100oct is to your wallet.   $16-20 a gallon around here.  I use about $350 of it every race weekend. 

 

Unless you are fighting for the podium in a race with an engine designed and tuned to use it,  there is no reason to run 100.

[TobyB]

low- lead 100 from the airport isn't much more than 93 octane at the pump.

 

Then I can run up against 12:1 and keep timing at 36ish and mixtures in the low 13's and not worry so much.

 

t