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Your car won't benefit from higher octane unless it's tuned for it.
 

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What makes you say it'll run worse? I'd think that at worst case you'd get additional protection from preignition and detonation.

I know that a lot of the racing people here (like turbophil) have tuned for 93 but run Sunoco at the track and gotten as good or better power. I would think it could help supercharged cars avoid combustion issues on hot days when the IATs start to creep up.

For an N/A car I wouldn't bother. I've run some when I'm low and the only option (local track has only race gas) and it doesn't seem to make a difference.

Some race gas is supposedly less stable when stored than non-race gas. I don't know if that applies to that particular Sunoco, but I wouldn't leave it in the car over the winter.
 

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Gamera The Atomic Turtle
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turbophil told me atleast some race gas mixed with our regular fuel is advisable on the track - like 50% 93, 50% race gas. This is because pump gas is not necessarily "as advertised". I bought some bad gas on the way home from Kansas City in my just built Elise - and i was getting the "marbles in a can" sound briefly when I accelerated. So, I stopped hard acceration, and got new gas as soon as possible - problem gone. BUT - if you go out on the track with bad gas - your motor is BLOWN, at least if the car is supercharged.
 

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I ran 100 octane before but my car is tuned for 91 octane. I didn't feel any difference at all. In fact, I think I got worse mileage for some reason.
 

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What makes you say it'll run worse?.
Short answer: higher octane fuel burns slower than low octane fuel, which is not good for HP, most of the time! How much power does "too much" octane cost you? So little it would be difficult to measure.

Octane is like Vitamin C in your body. Too much or too little is not good.

The maximum octane that your motor needs is (loosely) based on total compression ratio, combustion chamber (heads & pistons) design.

The ecu in most modern cars can add to the rounding errors in the above by playing with the timing. (Lotus says you NEED 91 RON Octane, but you can drive it using just 87 octane, just not as efficiently (read fun), as if you use 91)



Some Google links for the very long answer

Sunoco Race Fuels | Beyond Octane

Fact or Fiction?: Premium Gasoline Delivers Premium Benefits to Your Car - Scientific American

https://answers.yahoo.com/question/index?qid=20061010181219AAM2aNa

https://mn.gov/commerce/weights-and-measures/images/OctaneFacts.pdf
 

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After reading that assertion I Googled around and the conclusion was that yes, higher octane fuel contains organics that will burn more slowly, but that the effect in actual power output was negligible.

As for running 87 and letting the ECU pull timing, I've always wondered how safe it really is - sure, the ECU will alter the timing (octane scaling), but only after it's sensed knock. And in a drive-by-cable supercharged car where the ECU doesn't control maximum boost pressure OR throttle position, I don't see how the ECU could compensate for preignition if you put lower octane fuel in.

Like I said in my post I certainly wouldn't bother running high-octane gas in an N/A car unless nothing else was available or you had known-low-octane gas you were trying to "bring up" to spec. For a supercharged car, though, I'd think it's good insurance against preignition caused by extra-high IATs and I don't see a drawback to mixing some in, especially in a hot day at the track.

Obviously in a modern DBW + turbocharged car with an ECU controlled wastegate things are quite different since the ECU can play with another variable (boost pressure) in addition to timing.
 

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As for running 87 and letting the ECU pull timing, I've always wondered how safe it really is - sure, the ECU will alter the timing (octane scaling), but only after it's sensed knock. And in a drive-by-cable supercharged car where the ECU doesn't control maximum boost pressure OR throttle position, I don't see how the ECU could compensate for preignition if you put lower octane fuel in.

Obviously in a modern DBW + turbocharged car with an ECU controlled wastegate things are quite different since the ECU can play with another variable (boost pressure) in addition to timing.
Still more control than a carbureted engine has to adjust to knock. Retarding the timing can still have a big effect on knock. I’m not sure if the ECU will alter the amount of fuel flow or not, the throttle cable only allows more air in. Does the ECU control the DBW position, meaning if I floor it and the computer disagrees I’m not getting full throttle?
 

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Still more control than a carbureted engine has to adjust to knock. Retarding the timing can still have a big effect on knock. I’m not sure if the ECU will alter the amount of fuel flow or not, the throttle cable only allows more air in. Does the ECU control the DBW position, meaning if I floor it and the computer disagrees I’m not getting full throttle?
I agree with you about carbs. It's obviously safer to run a car that can adjust dynamically on lower octane gas than one that can't. I'm just not convinced that it's totally safe or a good idea routinely, since the ECU doesn't know the octane upfront and thus is stuck being reactive, as in knock still has to happen before the ECU can start doing anything about it.

As for your DBW question, on the Lotus, I'm 99% sure this is how the optional "traction control" was implemented as a fuel cut wouldn't be safe for forced induction cars. While it theoretically could, I don't think the ECU will cut the throttle if it sees knock, though.

Of course the throttle cable only allows more air in, but on a supercharged car, that's all that's controlling boost. That means the ECU doesn't get much wiggle room to "fix" the knock, since it's given only timing and possibly fueling as options. On a turbocharged car an ECU can (and most factory tunes will) open up the wastegate, giving the ECU one more way to "fix" knock.

As for fuelling on the Lotus specifically, I'm not sure if the Lotus ECU starts dumping in more fuel if it detects knock. I know some cars/ECUs will, but based on the "octane scaling" thread here:

http://www.lotustalk.com/forums/f3/exige-s-koldfire-270-down-power-220041/

it looks like the Lotus ECU only plays with timing when it detects knock, as that dyno run where octane scaling was in effect doesn't have the pig-rich AFRs you'd expect if it were trying to use fuel to control for knock as well.
 

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That paper's interesting - if I'm reading the abstract right, it's actually encouraging gasoline companies to start selling higher octane gas at altitude with the advent of electronic ECUs, presumably because an electronically controlled engine won't be running massively rich at altitude like an old carb engine that wasn't rejetted would.

But yet we still only get 91 here!

I'm always amazed at how much faster N/A cars are at sea level. I sort of jumped the gun without thinking about that when I got my Elise, and now I find myself wanting a supercharger a lot more than I first anticipated. Turns out air is an important part of combustion :facepalm
 

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re: Fuel Octane

The way to get more power from higher octane fuel is to run more timing or boost. In other words, you need to tune for it.

If your car is knocking on stick fuel and the ECU is pulling timing, which drops power, then higher octane may help and improve power.

Different fuel will have different density. It maybe higher or lower than what the car was tuned for. If the car will make more power, if it ran richer then higher density fuel would make more power. Usually higher octane contains heavier hydrocarbons, so it is more dense. (this is a generalization, as in the summer, oil companies put a lot of lighter butane and pentane into the pump gas.) High end dragsters have a very close tolerance on the fuel to make the most power. They measure both air density (pressure and humidity) and fuel density to get the mixture just right!

Therefore, given the price of 100 octane, unless you have a problem to fix at the track, it is a total waste of $$. It will not harm your motor, though.

BTW. ECU can sense knock quickly, but still only after it happens and starts damaging the engine. It slowly returns the timing back and knock and damage happens again! and again! So using 87 when the car was designed/tuned for 91 is possible, but not very good.

Anton
 

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I'm just not convinced that it's totally safe or a good idea routinely, since the ECU doesn't know the octane upfront and thus is stuck being reactive, as in knock still has to happen before the ECU can start doing anything about it.
This might not be true. It depends on the sensor technology used.

If the ECU (electronic control unit) is using acoustic sensors to detect knock, then I would agree that is reactive.

If the ECU is measuring the ionization current across the spark plug gap to measure the time that the flame front passes, then it can adjust before knock occurs. (Saabs have been doing it this way for years.)

I don't know which method Lotus uses. I just enjoy the drive! :shift:
 

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If the ECU is measuring the ionization current across the spark plug gap to measure the time that the flame front passes, then it can adjust before knock occurs. (Saabs have been doing it this way for years.)
That is SWEET and a really cool idea. I had no idea anyone was actually doing that, and from some reading around online I'm not sure anyone other than Saab actually does. They always seemed ahead in engine management - if I recall correctly they also had the first wastegate management / electronic boost control system that would scale boost when it detected knock.

There are a few other reactive methods of detecting detonation as well: cam/crank velocity timing comes to mind.

At any rate, Lotus use the oldschool, bog-standard acoustic sensor variety.
 

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Normal gasoline has ~115k BTU/gallon LHV. That race fuel has 109k, so all things the same you will make less power. (Assuming your car is NA. If it is super or turbo charged it is a whole different story)

That said, depending on how far the ECU advances the timing, you could gain enough efficiency to still make more power.

In short, there is no way to tell without testing it. Fortunately, the difference will be at most only a couple of horsepower, so it doesn't matter anyway. It is a waste of money to run race fuel on a naturally aspirated car, but as long as it is O2 sensor safe, it it makes you feel good, go for it!
 

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Race Gas from my view...

My 2 cents and take this advice for what you paid for it....

ALWAYS run at least 50% 100-104 race gas in a forced induction car on the race track. Always. Many tunes are not perfectly designed for the tortures of the track in addition to a lot of pump gas octane issues, the cheapest insurance you'll find for your motor is a 1/2 tank or more of race gas.

Additionally. 100 octane has very little negative power impact on a tune that was designed for 91. So run 100octane even if 100% mix. Anyone that says they can feel a power drop from 100octane, I call horsepucky. It barely shows up on the dyno let alone the SOYP. Any power degradation you feel on the track is due to the add'l heat not higher octane fuel.

Now, some race gas carries oxygen by weight. For instance, my FAVORITE fuel is Sunoco GT260 Plus 104 octane. It's another one of those magical liquids that's permeated with glorious track pixie dust... similar to redline shockproof oils... straight from the heavens....

anyway...We all know that between the two inputs that go into a motor, fuel and air, air is the only one of the two that is scarce, as fuel supply is infinite. Well, what a nice thing that your infinite fuel supply carries some air with it!!! The only catch is that your tune needs to be adequately "rich" if you're going to run oxygenated fuels, since air is taking the place of some fuel for a given unit of the fuel...

Do your best to avoid leaded fuels. The lead will foul a narrow band in half a shake of donkey's tail. that said, if in a pinch and it's between running straight pump gas or a mix of pump and leaded race gas, RUN THE MIX OF LEADED RACE GAS. If you end up running 110 octane leaded like a lot of tracks have available, and you end up with a full tank of it, you will lose some power and likely foul the 02 sensor... still cheap compared to a motor. 110 octane is just too much octane for good power for all but the most extreme builds in Lotus 2ZZ-Ge land...

Regarding using the ECU to sense knock and pull timing like running 87 octane or chancing pump gas at the track... Well, let's chalk that idea right up there with "New Coke" and "sub prime mortgages"!:facepalm:D The knock routine on this ECU/firmware is quite crude and ill tuned from Lotus. Don't trust it. It's bad. Run the right fuel...

-Phil
 

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BTW. ECU can sense knock quickly, but still only after it happens and starts damaging the engine. It slowly returns the timing back and knock and damage happens again! and again! So using 87 when the car was designed/tuned for 91 is possible, but not very good.

Anton
This is NOT true!
While it is true that the ECU does not retard timing until it detects knock, it is not true that this knock is damaging the engine.

During normal combustion, the fuel/air mixture in the cylinder burns at a relatively constant rate from the spark plug out to the cylinder walls. When the timing is advanced too far for the fuel quality/pressure/temperature, then larger portions of the mixture burn simultaneously, which causes a fast rise in cylinder pressure. In some cases this fast rise can cause peak pressure to rise above 'normal' levels. This 'spike' in cylinder pressure is what we refer to as knock. It also causes the cylinder walls to expand quickly and 'ring'. This ring is what the knock sensor is tuned to detect.

This knock does not cause any damage at all unless cylinder pressure gets high enough to exceed the yield strength of one of the components, or pushes down on the conrod hard enough to exceed the film strength of the oil in the bearing. In other words, you have to knock HARD to hurt anything.

The whole point of a knock sensor system is that it detects knock LONG before it gets bad enough to damage anything. In fact, many ECUs are set up to advance the timing until it sees a little bit of knock, regardless of what the base timing is or what fuel you use, so it won't knock any more on 87 than it would 93, it will just run more retarded and make a little less power. Obviously there are limits to how far it can go, which is why it probably can't advance the timing far enough to make use of 100 octane fuel.

That said, I don't know much about the Lotus ECU specifically and how its knock routine is set up, so the easiest thing to do is just run the fuel the car is designed for and call it a day. As mentioned above on an NA car the power difference is negligible anyway.

Pat


EDIT: regarding oxygen content, the linked GT260 says it has 3.7% oxygen by weight. Pretty much the whole country is stuck with E10 at the pump now, which is ~3.5% oxygen by weight, so no real difference there. The GT260+ that Phil mentioned above is 4.5%. Without going into the math, that 1% extra oxygen means about 1HP on a 250HP engine, so not much advantage. Every little bit helps though....
 

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This is NOT true!
While it is true that the ECU does not retard timing until it detects knock, it is not true that this knock is damaging the engine.

During normal combustion, the fuel/air mixture in the cylinder burns at a relatively constant rate from the spark plug out to the cylinder walls. When the timing is advanced too far for the fuel quality/pressure/temperature, then larger portions of the mixture burn simultaneously, which causes a fast rise in cylinder pressure. In some cases this fast rise can cause peak pressure to rise above 'normal' levels. This 'spike' in cylinder pressure is what we refer to as knock. It also causes the cylinder walls to expand quickly and 'ring'. This ring is what the knock sensor is tuned to detect.

This knock does not cause any damage at all unless cylinder pressure gets high enough to exceed the yield strength of one of the components, or pushes down on the conrod hard enough to exceed the film strength of the oil in the bearing. In other words, you have to knock HARD to hurt anything.

The whole point of a knock sensor system is that it detects knock LONG before it gets bad enough to damage anything. In fact, many ECUs are set up to advance the timing until it sees a little bit of knock, regardless of what the base timing is or what fuel you use, so it won't knock any more on 87 than it would 93, it will just run more retarded and make a little less power. Obviously there are limits to how far it can go, which is why it probably can't advance the timing far enough to make use of 100 octane fuel.

That said, I don't know much about the Lotus ECU specifically and how its knock routine is set up, so the easiest thing to do is just run the fuel the car is designed for and call it a day. As mentioned above on an NA car the power difference is negligible anyway.

Pat


EDIT: regarding oxygen content, the linked GT260 says it has 3.7% oxygen by weight. Pretty much the whole country is stuck with E10 at the pump now, which is ~3.5% oxygen by weight, so no real difference there. The GT260+ that Phil mentioned above is 4.5%. Without going into the math, that 1% extra oxygen means about 1HP on a 250HP engine, so not much advantage. Every little bit helps though....
Pat,

Good stuff!

The 260+, when tuned, really makes the motors sing when compared to VP's 100 (no 02) gas. There's certainly something to be said about the extra 4 points of octane as well, but tune unchanged, the 260+ agrees with these motors, IME. Like I said, pixie dust;)

As for the knock strategy, like I said; it's terrible. We've taken steps to improve it and will continue to refine it over time, but it is what it is for the time being and that's not real good.

thx,

Phil
 
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