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Discussion Starter #1
Does anyone know the highest (useful obviously) octane that our Elise can use?
I know it will be higher than our 91 R+M/2 grade :rolleyes:
 

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Is there such a thing as maximum octane? I thought it was "burn it if you got it". So long as it's unleaded and cat-friendly, it shouldn't burn a hole in anything other than your wallet.

Too low octane and you get predetonation. What might happen if it's too high?
 

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I don't think there's any upper octane *limit*, but the owners manual explicitly warns against the use of certain fuel additives. Just something to check when shopping for octane boosters (though i don't know why one would need it - less power, ya know). Likewise, we need to research what different gasoline companies add to their fuels. We're not supposed to use fuel with Alcohol or Methanol in it (Ethanol is OK).
 

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I assume you mean the lowest octane at which the ECU won't retard the ignition to save the engine at the cost of power? That will be a function of ambient temperature and humidity, but I would guess the average value is right at whatever the manual tells you not to put anything lower than it. (No I don't have a car or manual yet, so I don't know what that is)
 

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I don't think you would feel a difference by using any high octance fuels (94+) I don't think the ECU is capable of advancing timing enough to make it useful in anyway.
 

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The ECU may though, I don't think we know for sure. I think thats what he is asking, what is the max useful octane that the ECU can adjust beneficially for. Like maybe the ECU can advance enough for 93 to be beneficial. Since Lotus retuned the ECU the real answer may be difficult to get.
Many ECU's do not immediately adjust the timing when you put in higher octane because they don't know you put in higher octane. So you may have to go through few tanks to accurately measure the difference in hp you may have gained.
 

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The OBD-II port can help a little bit here. By watching the long-term and short-term fuel trim values and especially the timing advance, you can get some idea of whether or not the car is being agressive in timing, or backing off at the sound of knock.

The timing advance backing down quickly under heavy accelleration is a good clue you need better gas (or less boost).

Some cars are kind enough to put an actual knock count out on the OBD port. I'm very curious to see what Lotus shares with us via the CANBus.
 

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the higher the octane the harder the fuel is to ignite. If you run uber high octane fuels for no reason you're just slowing yourself down.

Yes, I do imagine there is a point where you reach octanes that will require a hotter spark to ignite. :)
 

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Discussion Starter #9
Geez, I didn't think I phrased it that badly :eek: Thanks z06fun, that is of course what I mean't . Not having an Elise yet I am sure the manual gives a recommended and a lower limit for the octane to use. I am interested in finding out what the upper boundary is where the ignition is no longer retarded. As an FYI, The highest pump octane here is 91. I know that my Boxster will improve in performance to 95 -97 octane. I have never tested this increase on a dyno, only with a Geez but this shows around a 10% (yes 10!) power increase.This has also been mentioned in Excellence Magazine. I tested this with 10 runs, same day same time. I have read all the stuff written here on the Lotus LCU and rough Idle. I know on the Boxster I can hear the idle change when the octane changes but it only seems to take 20 seconds or so to adjust. Living up here in SLC we lose a significant amount of performance due to altitude :)
Sometimes a TC is a good idea ;)
 

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Another thing about octane on small, high specific output lumps...

They become less temperature sensitive. Even if the dyno results max out with say 92-93 octane, a few more points might still help out.

High output lumps are sensitive to temps and if the intake air gets hot they will ping and the ECU winds up yanking out timing. This is especially noticeable if you are idling for a time and then floor it. After a few moments the car will recover a bit from the high underhood temps. For those who autocross this can even be a factor as the car will regain some go by the time you finish a run.

When you are on the roll in hot weather this can also be a factor...on say a 95-100F day.

The standard temperature for an SAE dyno test is 68F. Sure you can correct for the change in the air from higher temps but this does not correct for the increased tendency to have to pull out spark timing due to high air / water temps.

FWIW I'll be running 93 octane when I get my Elise chassis dynoed later today.
 

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Stan said:

FWIW I'll be running 93 octane when I get my Elise chassis dynoed later today.
I'd love to see the curves. Will you be posting them here?
 

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NMRJock said:
I'd love to see the curves. Will you be posting them here?
And now I see that you will be. Ignore my ramblings...
 

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>>>I'd love to see the curves. Will you be posting them here?<<<

Sure thing...stock plus Stage 2 exhaust on a Mustang Dyno (which tends to give lower numbers than Dynojets so it can be hard to directly compare.)

I'll post whatever they give me (including AFRs) over here.
 

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If anyone gets dyno'd on a Dynojet please post up. I may do that soon.


No offense to mustang dyno users, but there are just too many human controlled variables to make those readings good to hand out to others. They're always low and depending on the operator readings can be different each pull. IMHO a mustang dyno is only good if you use it for your baseline, dont change ANY parameters on the dyno each session, and keep coming back to it as you mod the car. Nothing more.

Any dynojet within the US will read within 1 hp of each other and are usually fairly correct ( you dont have to use compensations or formula's) which is why I use them.

As far as load tuning, if you have standard software you can easily tune at set rpm's you dont have to sit there to do it.

As far as Air Fuel readings....are they just using a tail pipe sniffer? If so, then those are subject to scrutiny too because of the repeated use they see and how they inherently work. If you want correct AF readings you'll need to use a seperate machine plugged into the O2 sensor bung.
 

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So one may ask...is the Mustang "low" or is the Dynojet "high". LOL!

If you get a baseline on a good dyno that is setup right and operated correctly and you find 5% MORE in some useful RPM range, that shows up on *either* dyno and the car will be faster.

But the Dynojet is not so hot for tuning because you can't load the car. What if you find a flat spot at 5000 RPMS at about 1/2 throttle? Very simple to tune this on a dyno that can hold the car at that RPM under a load sufficient to find the flat spot. On a Dynojet the RPM always changes as the drums build speed. You can't hold them at some RPM so you can set the car up. I used a loading dyno to tune my M3...you can work your way through parts of the engine map and concentrate on trouble spots. I believe that good tuning needs more than just an emphasis on WOT.
 

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Stan said:
I believe that good tuning needs more than just an emphasis on WOT.
I do too. My miata is tuned from 900rpm up to redline incrementally and every aspect. Any good software allows for this. You dont have to sit at load to tune.
 

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>>> You dont have to sit at load to tune.<<<

Sure..and you can tune on the road too. But when you tune using a loading dyno you can be more efficient and effective. Some things don't show up as well when your speed through a particular RPM and load range. Later on...you can throw the car onto a Dynojet so you can get a bigger number and extra forum bragging rights. (J/K)
 

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Dunno, bragging rights are fun, but Im more interested in a peforming car. 4 of our cars have intense tuning and collectively over two thousand horsepower that have all been tuned time and time and time again on dynojets without fail. I've probably got about 100 hours on dynojets, and only about 8 hours on mustang. We do have a mustang dyno here in town, I have friends who use it because they have AWD cars. Im familiar, I think its great in theory, Im just not smitten with it.

To each their own. :)
 

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PS, 93 octane is pump gas here, may be where you are too. Do you have access to VP fuels? If so, you could put all this high octane stuff to rest by picking up a few gallons of C16. That would be cool!

116 octane unleaded should be just about the most anyone will be running here.

Do some 93 runs and then 116 runs, would answer anyones questions. I wouldnt be suprised if she wasnt a little slower wtih 116. Inquiring minds want to know! :)
 

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Well read some books on engine tuning to see what the experts say...and check out what kind of dynos the car manufacturers themselves use for tuning. Part of the popularity of Dynojets stems from the fact that they can cost 1/3 to 1/2 (or less) of other dynamometers. Like you said before in effect, Dynojets give higher numbers than Mustangs too. So when you run on a Dynojet you tend to get feel-good numbers more often than for some other dynos, some say. Personally I don't care about the number, I care about getting good baselines and the before / after.
 
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