View Full Version : Smog test and octane

02-04-2009, 10:29 AM
I've STFA and seen mixed opinions on this. Is there an advantage to using higher octane gas for a smog test? I have up to 100 octane race fuel available to me at the pump.


02-04-2009, 12:34 PM
be sure to change the oil, get it good and hot on the way to the test.

If you know it's running rich, change the mixture.

02-04-2009, 01:49 PM
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Ethanol is one of the best fuel additives we have to fight air pollution from vehicles. Ethanol contains 35% oxygen. Adding oxygen to fuel results in more complete fuel combustion, thus reducing harmful tailpipe emissions. Ethanol also displaces the use of toxic gasoline components such as benzene, a carcinogen. Ethanol is non-toxic, water soluble and quickly biodegradable.

I had to run the "enhanced" test this year, an even more strict test with dramatically reduced "maximum limits" allowed on the dyno and comletely blew away my results from 2 years ago on the same test. And I mean by H-U-G-E margins of between 37-66% cleaner on EVERYTHING across-the-board! And the most recent test with stand alone ECU, high-lift/long duration cams and about 55 more HP than in 2006! 2006 no ethanol/2008 with ethanol.

02-04-2009, 02:01 PM
basically... the older fuel injection systems don't mix well with ethanol based fuels... and emissions go up.

While I've never had performance issues when running ethanol-blend fuels, I have seen (and witnessed with other cars) my cars fail for having Sunoco 94 with 10% (or is it 15%) ethanol content.

02-04-2009, 02:03 PM
I'm not sure if this is the same where you are, but like Doug mentioned, the fuel in Ontario (or maybe even all of Canada, not sure) has something like 10-15% Ethanol in it now, it's the law. So if Ethanol really helps, then bonus for us :)

02-04-2009, 02:05 PM
And yes, we have the same laws requiring oxygenated fuels (ethanol) where I'm at. It used to only be winter months Nov-thru March (i believe) but I don't know if that's become full-time now. Could be as I'm not all that attentive to those little decals on the gas pumps any more ;-)

02-04-2009, 02:08 PM
emissions-wise on lower octane but MUCH better and only slightly dirtier emissions-wise on higher octane. Something to consider. I no longer switch to lower octane fuel for emissions testing as I now know I can pass using premium or regular.

Ethanol is also a solvent that will dissolve gums and varnishes that gasoline leaves behind. I know there were LOTS of issues early on. But small amounts tend to dissolve those more slowly and over time. May be that some of those cars now have cleaner fuel systems and can more easily accept ethanol in small doses than in years past when this all started.

I don't like having blends put in my fuel but long as I can't feel a performance loss I'll tolerate it. And cleaner emissions with a cleaner planet is ok with me too. But leave my torque alone! ;-)

02-05-2009, 06:03 AM
But there far too many other variables to say what the cause is:

Lower octane fuel (87 v 93)
Warmer outside temps (40 v 25)
Idle (950 v 850)
New 02 sensor

02-05-2009, 06:58 AM
but I will give it the nod for 10-15% of the improvement this year over 2006. Everything I've read indicates if all other things are equal the ethanol blend percentage "going in" is par equal to the emissions reduction amount samplings taken "coming out" the tailpipe, ie; 10% blend is 10% cleaner/15% blend is 15% cleaner etc. That is to say "if" the same car is tested with an oxygenated fuel and then with a non-oxygenated fuel of the same octane rating the tailpipe emissions of the oxygenated fuel tailpipe sampling will be a similar/identical percentage cleaner based on amount of ethanol used emissions-wise than the same car using 100% gasoline of the same octane rating.

Least that's what California/Colorado Air Resources Boards claim. But even if it's not 100% accurate I do believe its VERY close to accurate... maybe within a percentage point or less of empirical.

02-05-2009, 10:36 AM
One of the downsides to having a fuel that's less susceptible to explosion is that it burns a bit slower. It's all about chemistry.

So, 100 octane, if you have a 12:1 motor, might burn just fine, and give great emissions results. But at 9.5:1, you might get super-high HC and CO. The relationship between octane number and fuel make-up is really very complicated, and it's super-hard to explain in a web forum.

A lifetime ago, I worked as a petroleum chemist. Things have changed A LOT since then, but the basic chemistry has not. For reference, when I blended gasolines for octane, I used tetraethyllead to increase octane number in finished product. Yeah, that long ago.

Lago Blue
02-06-2009, 11:50 AM
<center><img src=""></center><p>Octane, t-e lead, and ethanol (amongst others) can all be used to that end. Obviously the use of these different chemicals, varies widely. See "Regional Variations" in article below.

Ethanol's RON is (129 (MON 102, AKI 116)), high enough that it is used to raise gasoline's octane <i>rating</i>.

Re: above illustration: "Up to 10% ethanol enhanced since 1996! I believe that that limit is only there, to protect some plastic components in some fuel systems in some old cars. See your owner's manual, this may also be Audi's reco'd limit.

We want a deftly controlled ignition by spark alone, not by compression, nor glowing deposit, because we want max. compression, because that gives the best power and economy. The highest OEM reco'd fuel is going to allow the ECM to give us that. This is because that <i>proper</i> fuel allows max. design ignition advance, <i>soonest</i>.

Running today's high-test where it is not required is wasteful, yes, but not because the high-test is slow-burning and therefore dirtier. It is because to get the same power delivered, you have to put your foot down farther, and that extra consumption trumps the lower cost of regular.


"higher octane fuels have higher activation energies, it is less likely that a given compression will cause detonation.

It might seem odd that fuels with higher octane ratings are used in more powerful engines, since such fuels 'explode' less easily. However, an explosion is not desired in an internal combustion engine. An explosion will cause the pressure in the cylinder to rise far beyond the cylinder's design limits, before the force of the expanding gases can be absorbed by the piston traveling downward. This actually reduces power output, because much of the energy of combustion is absorbed as strain and heat in parts of the engine, rather than being converted to torque at the crankshaft.

A fuel with a higher octane rating can be run at a higher compression ratio without detonating. Compression is directly related to power"

As to emmisions, your 3-way cat has to work less, will last longer, and works best at delivering the cleanest power at stoichiometric, which is what your ECM is trying to give you, and what your O2 sensor is dithering about. Now-a-days, that system authority given to the O2 sensor in closed loop, its performance is what gives you the lowest and least harmful combo of unburnt HC, CO and the NoX's.

A really clean pass of an E-test requires those components to be working properly. A high HC result <i>also</i> means your throwing extra money <i>out</i> the tail-pipe unnecessarily.

Using a higher octane than specified for peak performance means your throwing extra money <i>in</i> the tank needlessly. If it where also dirtier for that reason alone, I'm sure you wouldn't be allowed to do it.

Notwithstanding the following:

"When the throttle is partially open, only a small fraction of the total available power is produced because the manifold is operating at pressures far below atmospheric. In this case, the octane requirement is far lower than when the throttle is opened fully and the manifold pressure increases to atmospheric pressure"

...the use of the highest OEM reco'd fuel for peak performance is probably going to <i>also</i>; give you peak mileage.

If your Audi 90 produces higher HC on todays higher octane rated fuel, (and frankly I do not yet understand why it would) I would think it requires, other attention.

There is no significant short-cut to a clean tail-pipe that would not also be good, every other day.

It just doesn't make sense that on one hand you want the engine fully warmed-up prior to the E-test, yet also say: put in a lesser (or a greater) than the recommended fuel.

I think that today's max. 10% ethanol blend fuels can't help but be cleaner than non E-blends. Ethanol is drinkable; gas or lead, not so much.

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Look (above) at the extremely narrow band within which the HC, CO, &amp; NOx emmissions are minimized in. Note also how this corresponds to the best spots on the torque and fuel consumption curves. That alone, ought to be reason enough to get you motivated to want to be able to run clean every day. Regular or Premiium, regardless; if your car can't deliver those above conditions, it shouldn't pass!

IMHO, our ability to continue hot-rodding is probably quite contingent on how clean a fashion we can do it. Please note all the (currently voluntary) government programs whereby they will pay you to crush your &gt;10 yr. old car and subsidize your buying of a new one.

From my cold dead hands.

02-06-2009, 12:24 PM
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exhaust tips inspection how my A/F ratios are and to a lesser, albeit closer-than-before determination of emissions related crappola I'm putting out into the environment. Lean and clean does NOT in and of itself represent imminent threat, danger or even less power. Done correctly it simply wipes the floor power-wise with any/all black, sooty, puffy tailpipe deposits on an otherwise identical car. Don't believe it? Take a walk around the pits of ANY ALMS or SCCA national event and look closely at the exhaust tips of those cars in the pits. Yes, you can have your cake (more power) and eat it too (cleaner emissions). They don't "have" to be mutually exclusive!

Basically long as my exhaust tips look like the above pic when day-to-day commuting as well as after running a full day on the road course I'm happy. And getting those tips to look like that in ALL driving conditions I participate in may be the most expensive, challenging, complex and difficult thing I've EVER attempted with an Audi! Let alone an Audi with 104k miles on it!

Lago Blue
02-06-2009, 01:23 PM
is possible, is one of the most interesting things I've learned here. Previously I just wanted to pass the damn test. Now I know exactly why I want the tester fellow to think his machine may not be working correctly, and that my car couldn't possibly be that clean.

IMHO on any given day lately, gas has historically never been so cheap, and what we are free to technically do with these old cars never more full of possibilities, yet for politics etc., that might not always be the case. We lucky few.

02-10-2009, 09:49 AM