Effect of low octane gas?

seanw · Dec 9, 2005

Day 2 and I take the wife and kidz out for a cruise in the new MS6. Prior to this I had been babying the engine, but now I wanted my wife to experience the rocket-like acceleration that caused me to dump a perfectly good, plenty quick Altima SE (albeit an automatic, and the stupid door look would relock 2 out of 3 times as my hand brushed by to reach for the latch, and the cruise control button broke and had to be repaired, and various plastic thingies broke, etc., but the engine was great!). So I stepped on the gas to pass a Chevy Galactica SUV or some such monstrousity. Naturally, I was expecting to zoom forward at what would be a suitably alarming rate for my wife. It didn't happen! I wanted to believe it was happening, I almost thought it was happening, but it wasn't . . . . (boom07)

Total weight in the car was no more than it was with a tall, burly salesman in the car during my test drive and we scooted down the road plenty quick at least once or twice. Anyway, I remember reading some post about how low octane gas causes the engine computer to open the waste gate early. Could someone remind where this info was or provide some other verification of this? I want the salesman to check with the clowns at the prep station to see if they fillled it with 87 in instead of 91 before the hand off, and I want to know what I'm talking about when I do. Kind of making me wish I paid the extra cash to buy from local dealer instead of going out-of-town. Then I could drive down there right now . . . wait, it's 6 AM. Well, as soon they open up, I could drive down there and . . . . Yikes! This car is wearing me out between the pre-purchase obsession, the stress of the actual purchase, and now this post-purchase issue.

BOOSTR · Dec 9, 2005

The PCM will detect lower octane and retard the timing and that will have an effect on the power output. Premium gas of at least 91 is needed for a turbo car. What gear were you in when you were passing? Perhaps you could of downshifted prior to passing. When my MSP was completely stock, it was a slug in 5th gear for passing. Now it passes with authority in 5th.

Da 6 · Dec 9, 2005

Yeah low octane in car tuned for high octane means you don't have the horsies you paid for. I had similar isues and once you rid it of that octane you will fill the power restored. Fun part is getting rid of that tank of gas.

seanw · Dec 9, 2005

Da 6 said:
Yeah low octane in car tuned for high octane means you don't have the horsies you paid for. I had similar isues and once you rid it of that octane you will fill the power restored. Fun part is getting rid of that tank of gas.

I've got my fingers crossed that's all it is. I think I'm going to go and buy a syphoning tool and put the 87 octane gas into the minivan where it belongs.

seanw · Dec 9, 2005

DAWIV said:
The PCM will detect lower octane and retard the timing and that will have an effect on the power output. Premium gas of at least 91 is needed for a turbo car. What gear were you in when you were passing? Perhaps you could of downshifted prior to passing. When my MSP was completely stock, it was a slug in 5th gear for passing. Now it passes with authority in 5th.

Thanks, I thought the earlier post said something about the waste gate, but timing is just as plausible.

seanw · Dec 9, 2005

seanw said:
Thanks, I thought the earlier post said something about the waste gate, but timing is just as plausible.

The dealer confirmed that they put low octane gas in by mistake. I just put half a tank of 93 in, so I guess I'm at 90 right now. It's getting quicker all the time.

bazooka joe · Dec 9, 2005

stupid stealerships!! they knew it but didn't tell until you asked them...what a bunch of dummies!

seanw said:
The dealer confirmed that they put low octane gas in by mistake. I just put half a tank of 93 in, so I guess I'm at 90 right now. It's getting quicker all the time.

BlkZoomZoom · Dec 9, 2005

DAWIV said:
The PCM will detect lower octane and retard the timing and that will have an effect on the power output. Premium gas of at least 91 is needed for a turbo car. What gear were you in when you were passing? Perhaps you could of downshifted prior to passing. When my MSP was completely stock, it was a slug in 5th gear for passing. Now it passes with authority in 5th.

Well you are almost, kinda right. The Ms6 will actually open the wastegate up at 4200rpm if it detects lower octane fuel is used. So you will lose all boost pressure and thus feel like a complete slug.

seanw · Dec 10, 2005

BlkZoomZoom said:
Well you are almost, kinda right. The Ms6 will actually open the wastegate up at 4200rpm if it detects lower octane fuel is used. So you will lose all boost pressure and thus feel like a complete slug.

I thought I read that somewhere. Just curious, is this info out of a tech manual or something?

boostisgood · Dec 10, 2005

The PCM has no way of detecting your octane raiting. I dont know who where or how you read or learned this but it does not do this. Sorry, but it dont. There is absolutly no sensor in a vehicle that says "ohh we got 87 octane" You need a lab to do the test to determine the octane raiting.

I was unaware that they have developed a system that will control the wastegate also. Last I have seen it was controlled ONLY by Vacuum and what the WG was set to open at for + pressure.

daedalus · Dec 10, 2005

no... it doesnt, but it probably will detect sub-audiable knock from the lower octane and probably retard timnig based off of that...

Da 6 · Dec 10, 2005

what he said...can't detect an exact octane rating but can detect wrong fuel and then lets you know you using the wrong fuel. As for not telling you it's marked somewhere. My car has it under the fuel gauges"Remium Fuel Onlt" but other companies put it on the inner gas cover. not shure where they put it on the speed 6. If someone could, please snap a pict

boostisgood · Dec 10, 2005

Da 6 said:
what he said...can't detect an exact octane rating but can detect wrong fuel and then lets you know you using the wrong fuel. As for not telling you it's marked somewhere. My car has it under the fuel gauges"Remium Fuel Onlt" but other companies put it on the inner gas cover. not shure where they put it on the speed 6. If someone could, please snap a pict

A car in no way shape of form can detect wht type of gas your using. IF you run a low octane raiting gas, which has a lower combustable rating then higher octane, you can have pre ignition (Misfire) and that will retard timing. If the issue happens 2 times in the same drive cycle you will get a check engine light.

Dont know what could have caused this issue for ya bro, I would take it back to the dealer and explain to them what happened and get a tech to MONITOR ALL DATA FROM THE SENSORS via a Mazda WDS or other scan tool.

BlkZoomZoom · Dec 10, 2005

boostisgood said:
The PCM has no way of detecting your octane raiting. I dont know who where or how you read or learned this but it does not do this. Sorry, but it dont. There is absolutly no sensor in a vehicle that says "ohh we got 87 octane" You need a lab to do the test to determine the octane raiting.

I was unaware that they have developed a system that will control the wastegate also. Last I have seen it was controlled ONLY by Vacuum and what the WG was set to open at for + pressure.

Why does everyone question me? Every modern pcm has the ability to detect the octane readings of gas. Whether they actually use it or not is their choice. It does not need to have a sample of the gas to do it either, imagine that.
I really don't feel I need to educate you guys so I will just dabble into this. Octane is the resistance to burn (yes very general). Engineers know that, and use that fact to determine what octane gas is being used in the motor. OMG. How do they do this? It's very ingenious actually...
If you know the ambient temperature, the intake air temperature, the atmospheric pressure, coolant temperature, fuel pressure, compression, Cht, Egt, etc. etc. etc. And you also know what the burn rate of a certain fuel is. Combine the things you know and its all a calculation...
Knowing the burn rate of certain octane fuels allows the engineers to tailor the pcm to want to see certain things. Higher Octane fuel burns slower. Burning slower allows more oxygen into the exhaust. This is read by the front o2 sensor. By monitioring oxygen levels and exhaust temperature and knowing all the above information it can determine what fuel is in the engine. And before you say " but intake air temp. can change what the exhaust temperature is", you are right, and that is one of the reasons they monitior it. The pcm also monitiors what the temperature of the air that is coming out of the turbocharger. It's all a really big calculation using known parameters, and a ton of different sensors. This is just one way it knows, there are a few other ones.
If you want to know more than that or still don't believe me you can pm me or do your own research.

As far as the "controlled wastegate" thing. Vw's been doing it for years. The wastegate is activated by boost pressure (not Vacuum) that overcomes the internal spring and allows the wastegate to open. They simply install a fast acting solenoid in the line to the wastegate. Depending on several factors (including the calculated octane rating) it will control the wastegate via the solenoid by a duty cycle signal. This is basically a EBC with a really big brain. By doing it this way they got a much faster spool time, with little to no spike, and will have the 15.5psi no matter the outside temperature or what altitude you are at.

daedalus · Dec 10, 2005

BlkZoomZoom knows more than me and most people on this forum... I'd take his information as correct fact.

seanw · Dec 10, 2005

boostisgood said:
A car in no way shape of form can detect wht type of gas your using. IF you run a low octane raiting gas, which has a lower combustable rating then higher octane, you can have pre ignition (Misfire) and that will retard timing. If the issue happens 2 times in the same drive cycle you will get a check engine light.

Dont know what could have caused this issue for ya bro, I would take it back to the dealer and explain to them what happened and get a tech to MONITOR ALL DATA FROM THE SENSORS via a Mazda WDS or other scan tool.

Yeah, the dealer said to bring it in if I got a check engine light. But it's running great now that it has some high octane fuel in it.

NoRotorNoMotor · Dec 10, 2005

seanw said:
I've got my fingers crossed that's all it is. I think I'm going to go and buy a syphoning tool and put the 87 octane gas into the minivan where it belongs.

couldnt you have just added some octane booster to it until you ran it down?

BlkZoomZoom · Dec 10, 2005

seanw said:
Yeah, the dealer said to bring it in if I got a check engine light. But it's running great now that it has some high octane fuel in it.

Amazing isn't it.

boostisgood · Dec 10, 2005

BlkZoomZoom said:
Why does everyone question me? Every modern pcm has the ability to detect the octane readings of gas. Whether they actually use it or not is their choice. It does not need to have a sample of the gas to do it either, imagine that.
I really don't feel I need to educate you guys so I will just dabble into this. Octane is the resistance to burn (yes very general). Engineers know that, and use that fact to determine what octane gas is being used in the motor. OMG. How do they do this? It's very ingenious actually...
If you know the ambient temperature, the intake air temperature, the atmospheric pressure, coolant temperature, fuel pressure, compression, Cht, Egt, etc. etc. etc. And you also know what the burn rate of a certain fuel is. Combine the things you know and its all a calculation...
Knowing the burn rate of certain octane fuels allows the engineers to tailor the pcm to want to see certain things. Higher Octane fuel burns slower. Burning slower allows more oxygen into the exhaust. This is read by the front o2 sensor. By monitioring oxygen levels and exhaust temperature and knowing all the above information it can determine what fuel is in the engine. And before you say " but intake air temp. can change what the exhaust temperature is", you are right, and that is one of the reasons they monitior it. The pcm also monitiors what the temperature of the air that is coming out of the turbocharger. It's all a really big calculation using known parameters, and a ton of different sensors. This is just one way it knows, there are a few other ones.
If you want to know more than that or still don't believe me you can pm me or do your own research.

As far as the "controlled wastegate" thing. Vw's been doing it for years. The wastegate is activated by boost pressure (not Vacuum) that overcomes the internal spring and allows the wastegate to open. They simply install a fast acting solenoid in the line to the wastegate. Depending on several factors (including the calculated octane rating) it will control the wastegate via the solenoid by a duty cycle signal. This is basically a EBC with a really big brain. By doing it this way they got a much faster spool time, with little to no spike, and will have the 15.5psi no matter the outside temperature or what altitude you are at.

Ohhh educate me please. Or how about I educate you.

First, you have 2 types of Oxygen sensors in ODBII cars.

1. Zirconia sensors, which produce a reference voltage on thier one (Between 0mV to 1000mV with 450 mv being Stoichiometric) it compares the oxygen content of outside air which is 21% to the content of the exhaust. This is to allow the PCM to go rich/lean rich/lean so the Catalyst will operate properly and burn off the NoX and produce the water and C2O for emissions.

2. Titania sensors. They do not produce a voltage but act as a normal sensor in your vehicle and return a reference voltage back to the PCM (resistance type sensor). This sensor ACTUALLY detects the ammount of air in the exhaust and returns the signal voltage to the PCM. It does not use outside air to compare.

What your front oxygen sensors do is called cross count. It will go above and below .450 V (again that is stoich) and feed that info to the pcm so it can ad and take away fuel as needed for emissions.

now with that brief explanation of Oxygen sensors (and toyota uses a 5v in some cars) I will now ask you these questions.

1. Are you a tech or an engineer of PCM programs?
2. If you are and use a scan tool (Genysys Snap on red brick or OEM like the ford/mazda WDS) do they show what the octane rating is along with the data streams of the sensors? Ill actually answer that one for you NO THEY DONT. Now if the octane rating is that important and the PCM knew what the rating was, wouldnt you think that if the PCM was capable of determining the rating via what the sensors are sending it, it would ? Case in point, when my engine blew, Mazda could not gather the info on the type of gas in my car based on PCM/ECU data provided nor from the freezframe info from the CEL. If the PCM could determine it, should it not have been displayed ?

I could go on forever, but I would rather go play with my kids now.

boostisgood · Dec 10, 2005

ohh and before I go, what cars under 40k have EGT sensors in them ?

Effect of low octane gas?

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