At least with sunoco, you can get up to 100 octane unleaded street-legal gas or up to 104 octane "off-road use only" unleaded gas (which is what I used). Everything I've seen over 104 has lead in it and, yes - that's bad for your cat and your O2 sensors.

 

CoralDoc,

As texan pointed out, higher octane doesn't necessarily mean slower burning, it just means more detonation or knock resistant. I use the terms interchangably, but a chemist would view detonation as a superacoustic wave in the combustion gasses while knock could be something else. That gets way complex because even the experts can't fully agree how it happens.

For a particular car/engine/fuel combination, there will be an optimum ignition timing advance for maximum power. This optimum may occur before the onset of detonation, at the onset of detonation, or after the onset of detonation (assuming standardized conditions for all three test cases).

Personally, I've never hit upon the second case, but I've tuned cars (usually my own) that have fit the end points. IOW, I've had a car where power kept going up with timing advance, but I was getting detonation. I also own a car where advancing timing past a certain point causes me to stop gaining, or lose power, but detonation was not a factor.

The reason we run ignition advance is that the air/fuel mixture takes time to burn. We want to build up peak pressure about 15 degrees after top dead center (ATDC) on the power stroke. This gives us maximum conversion of cylinder pressure to mechanical force. In order to achieve this, particularly at high rpms where the cycle time is small, we have to start the burn long before that, usually on the order of 30 degrees before top dead center (BTDC) on the typical production engine.

When we start the burn BTDC, we are by nature building up some combustion pressure BTDC as well. And that pressure is resisting the force of the piston coming up to TDC on the compression stroke. So, the force is actually working against us. Think of it as "negative work". While one of our engine's other pistons is finishing it's power stroke, the new piston is actually pushing in the opposite direction.

So, if there is a way to speed up the burn, we can actually start the ignition process later. This means we can do less "negative work". And that means more power for a given power stroke.

There are other ways to improve combustion speed, or reduce negative work. In particular, the combustion chamber design is highly critical.

In the case of what high octane unleaded race gas will allow you to do, you have to experiment. I don't know if our cars are detonation limited or power limited in terms of ignition advance. I know the old B18C1/5 engines would continue making power up to about 5 degrees past factory recommended advance, and then would start to fall off a little bit, without detonating. If we are highly detonation limited, then we could run a lot more advance and pick-up power. If we are not, we could retard timing and take advantage of the quicker burn. Even if we are detonation limited, the quicker burn would mean we were closer to the optimum peak pressure timing than before even without changing ignition advance.

Hope that helped.

UL

 

I have discussed the octane question with my mechanic friend, and he says that there is no way an 11:1 compression motor will give you max performance with 92-93 octane. He says 96 minimum maybe 98 (doesn't mean the car will run badly with 93, you just won't get peak performance). 100 could be a little overkill, but not by much.

 

Alex, there really isn't any way for your mechanic to know that without experimenting. If you don't know how the combustion chamber dynamics work, you really don't know what the octane requirement is.

For example, I had an 11.5:1 engine that ran on 92 octane pump gas. We made peak power with distributor timing set at 16 BTDC. Running more advance did not change the power at all. If we changed to a higher octane gas with the same characteristics, power gains would have been nil. Unfortunately, we couldn't change to such a gas because all the higher octane stuff available also has a different burn rate.

Its entirely possible that we could run more advance with higher octane and gain power, but _only_ experimentation will tell. Honda makes very good combustion chamber designs.

UL

 

This can be a confusing discussion because octane is measured in more than one way.

When you pull up to the pumps at the track you are faced with signs that can be misleading. Street gas is labeled with a mandated index, which is the average of the MON and RON octane. The racing gas is sometimes labeled with the RON measurement that would make it appear to be higher even if it wasn

 

Good point cdelena,

I believe, but I'll have to confirm, that the Unocal 100 octane sold at the pumps around here is rated using the average method, mainly because it is sold at the pumps.

UL

 

Here is SUNOCO's Gasoline Grades the one I use at least once a month is the GT100..

I don't have enough technical skills to understand some of this crap but maybe someone else is interested..

IMO filling my baby up with 100,makes her feel much better...


Sunoco Gasolines

 

That is a pretty good chart from Sunoco. I called someone that is knowledgeable who assured me that the Unocal 100 sold on the street is subject to normal regulations so the posted number is the AKI, or average of MON and RON.

For a little more info on the various properties of gasoline look at: http://www.tgill.com/racing/faq.htm

 

Yes, I believe that all gasolines sold at pumps must show octane readings with the MON + RON / 2 method (called PON, Pump Octane Number). And yes, MON is the more telling of the two tests for high performance engines, inlet air temp is held at 148.9 degrees celcius and the engine is tested at 900 RPM (RON uses much lower intake temps and only 600 RPM).

As a side note, anything with over 100 octane is not technically rated using any of these methods. Octane ratings are basically a symbol of how a particular fuel behaves in the test when compared to two reference fuels, iso-octane (with a rating of 100) and heptane (with a rating of 0). A gasoline with a MON of 90 is a fuel that will have the same knock resistance as a 90/10 blend of iso-octane and heptane, respectively. So anything over 100 octane (pure iso-octane no longer being it's equivalent) is not directly available from this testing method, one must arrive at these numbers figuring how much better the gasoline is at each test than pure iso-octane. Here's a link for anyone who wants to learn more... http://www.r-t-o-l.com/laboratory/learning/faq1.htm