Effect of low octane gas?

[seanw]

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]

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]

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]

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]

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]

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]

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



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]

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]

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]

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]

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]

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]

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]

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]

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

[seanw]

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.

[NoRotor,NoMotor]

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]

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]

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]

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

[BlkZoomZoom]

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, 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.


What are you talking about? You answered your own question. A O2 sensor reads the oxygen content in the exhaust stream. Period, end of story. The Oxygen content, nothing else. It doesn't matter what sensor you use.
If it knows everything going in the motor, it knows what it should see coming out. By changing the octane rating of the fuel going in, it changes the oxygen content coming out. It's that simple. I did not come up with this crazy notion one day. It was all explained to us at the Mazdaspeed6 tech. training. On top of that if you went to Engine Management Level 2 class by Vw you would also learn this (New Vw's do the same thing).

Also I assume you mean you blew a MSP motor, totally different pcm's going on here.

And with the pcm's in the Mazda6 and Speed 6 you can determine the octane reading...sort of. Using a new pid called rear fuel trim. It gives you a value that can help you determine whether the correct fuel is being used.

You do not need a egt sensor to read the temperature of the exhaust.

[boostisgood]

What are you talking about? You answered your own question. A O2 sensor reads the oxygen content in the exhaust stream. Period, end of story. The Oxygen content, nothing else. It doesn't matter what sensor you use.
If it knows everything going in the motor, it knows what it should see coming out. By changing the octane rating of the fuel going in, it changes the oxygen content coming out. It's that simple. I did not come up with this crazy notion one day. It was all explained to us at the Mazdaspeed6 tech. training. On top of that if you went to Engine Management Level 2 class by Vw you would also learn this (New Vw's do the same thing).

Also I assume you mean you blew a MSP motor, totally different pcm's going on here.

And with the pcm's in the Mazda6 and Speed 6 you can determine the octane reading...sort of. Using a new pid called rear fuel trim. It gives you a value that can help you determine whether the correct fuel is being used.

You do not need a egt sensor to read the temperature of the exhaust.

dealer techs dont know shit except whats handed to them in a manual or learned at "new car coming this is whats wrong and how to fix on our campaigns"

Second, you NEED AN EGT gauge to know the EXHAUST GAS TEMPS. Oxygen content or lack there of does not give exact Gas temps.

GAS or the volitility does not control the ammount of O2 in the gas, AIR controls that. the actual combustion and ammount of fuel and air put into the engine controls the ammount of air coming out.

So you have now went from saying it KNOWS the octane rating to it SORT OF based on what the rear O2 sees ? you contradicted yourself there bro.

Dont argue with techs unless you are a tech yourself bro. :D

(I really am not trying to be a jerk man, just dont want wrong info put out to people is all)

Also, I dont know Vdubs so I cant answer ya there.

[terbow]

he IS a mazda tech. You shouldnt act so high and mighty. Its not like what he explained wasnt feasable.

[boostisgood]

I am not trying to act all high and mighty. I just hate wrong info put out to people. (BTW I am a tech myself and in ADVANCED ENGINE PERFORMANCE RIGHT NOW)

[BlkZoomZoom]

I am done arguing with you. You are right and I am wrong. The Head Mazda Trainers in the country know nothing and were feeding me bullshit just so you could prove me wrong by telling me two different types of oxygen sensors..


I am a Mazda Tech, Vw Master Tech, Ase Master certified.
Graduated almost top of my class at UTI, and re-built my first car at 15.

How does any car read the Cat. temperature without a Egt? Every ODB2 car reads it, but most do not have a Egt.

Octane controls the burn time of the combustion. The more delay you have in the combustion the more oxygen is going to make it out without burning.

[terbow]

it just sounds like you are. thats all. how is what he said wrong? your saying thers no calculation or amount of sensors that can guess at if you have the wrong octane? I dont buy that. he never said it tells you the octane, he said it tell if the octane isnt what its supposed to be. I dont think its an overly complicated thing for an oem ecu to do... but im not a tech.

[boostisgood]

BLKZM. Not gonna argue anymore either. No disrespect towards you at all man.

All Im trying to say is this, a PCM does not know the octane rating of the car based on sensor inputs from what I have learned and seen myself.

What you have learned from the guys that spread the word of the mazda engineering gods, I dont know. If something new has come out, I have not seen it yet.

as far as explaining the O2 sensors, that was without knowledge of ya bein a tech and felt that the explanation of the sensors would help.

[BlkZoomZoom]

Lets end it this way. The Mazdaspeed6 can determine the octane reading of the gas used. Now whether I convinced you how it determines it or not does not bother me. That is how it does it.

[boostisgood]

ok ok I will concede. I can see, based on what you explained, how it is possible. I just have not seen it for myself and will see what I can get on Monday.

Its always best to learn as much as possible in our field.

[RHAGEL]

So if you use Octane booster say 103; would it incresingly make the car quicker? Or is it programmed to only accept as high as 93 or 91? (depending on the state.)

[BlkZoomZoom]

Correct. It will only do what it is programed to do. Using a Higher octane fuel will slow it down, but not to the effect a lower octane will.

[BOOSTR]

Can you explain why using say Race gas of at least 100 octane will slow down performance? My car always felt like it performed better with it, but without data to support that, its just flapping. So I am curious to know why it has a negative effect.

[BlkZoomZoom]

It slows down the burn rate. If this is slowed down without need it is just a waste. The slower the combustion, the less oxygen is burned thus lower power will result. I do not know how much will be lost, it will be determined on the set-up. I have heard it is noticeable on a dyno though.
However, it depends on what the car is set up for. If you tune for the race gas then more power will result.

[Da 6]

What he said again. If mazda didn't have the wds you could tune it yourself with the help of programers and a Xcal2. also not to fuel the already lit fire but I got cels from my intake but when my engine threw 2 pistons I didn't even get a cel and the car was running for a good 9 seconds afterwards. Moral of the story is Low grade octane in Hi octane rated/Hi perfomance vechicle=Bad don't do it :)

[seanw]

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 :)

Don't need any pix. I've known it needed premium since I read the first review way back when. Thanks to info in a previous thread, I suspected that the clowns at the dealer prep station DIDN'T know and put in regular before handing the car off to me, DESPITE the label inside the fuel door. I started this thread to get more info re: the wastegate for when I talked to the dealer.

Anyway, I'd like to thank everyone for the full and complete edyookayshin on PCM vs. wastegate. (deadhorse

[Da 6]

most dealerships do that and wonder why people won't buy the Hipo cars when they fail to perform better than the non Hi po version at a cheaper price. Neon R/T was one of those cars that couldn't sell cause they put 87 in it. At least the dealership admitted to it :)

[seanw]

most dealerships do that and wonder why people won't buy the Hipo cars when they fail to perform better than the non Hi po version at a cheaper price. Neon R/T was one of those cars that couldn't sell cause they put 87 in it. At least the dealership admitted to it :)

Yeah, I was kinda shocked that they admitted it so quickly. Best in the long run for them, I think, since they lose trust and reputation when they try to hide/deny/etc.

[neit_jnf]

about using higher octane fuels and power:

As mentioned earlier, higher octane fuel burns slower that's why it's more resistant to pre-ignition / detonation. To extract more power out of it the ignition has to be advanced to provide a longer burning time before the exhaust stroke. If the PCM is programmed to maximum safe ignition advance with 93 octane, then using higher octane may slightly reduce power because it will burn slower and less power will be extracted. At the same time it will provide a bigger margin of safety against detonation; that's why even though my RX-7's ECU is programmed for 91, I always use 93 or 94.

Now, some cars will continue to advance timing until detonation is detected, in these cases 100 or 110 octane race gas will provide more power.

[RHAGEL]

Here in CA we only get 91, so I don't even have an option of 93.

[AzMz6]

yup turbo car= must run 91...;)

[FX-MAN]

I have the same problem as the guy above. At about 4000rpm my car is having a total loss of power (like the turbo is just not working).

However, I always fill up with 92-94 octane (whatever the highest octane is at the station). Could brand make a difference? I did go to a cheaper non-national-chain gas station...

This is the second time this week that it's happened though, and its really annoying me.

[Da 6]

not all gas stations follow the strict octane ratings. Some have bad gas in general. I remember in texas shell was the don't go to stations cause people were missing like crazy and V8 owners were at one point only running 5-6 cylinders. Try a difrent brand of gas. Whats your location? If you are in the 48 try using cheveron with techron from gas station or in the 20 gallon concentrated formula.

[seanw]

I have the same problem as the guy above. At about 4000rpm my car is having a total loss of power (like the turbo is just not working).

However, I always fill up with 92-94 octane (whatever the highest octane is at the station). Could brand make a difference? I did go to a cheaper non-national-chain gas station...

This is the second time this week that it's happened though, and its really annoying me.

I only had this problem b/c the dealer apparently filled up the tank with 87 octane before handing it over to me. I checked with them and they confirmed this. Now that I've filled it with 93 a few times, it is running normally (ie., great!). I have heard that non-nationals stations are more likely to have old tanks which allow water to get into the gas somehow. These days with the strict environmental requirements, it seems unlikely that this is true, but you never know.

[Nando 6i]

about using higher octane fuels and power:

As mentioned earlier, higher octane fuel burns slower that's why it's more resistant to pre-ignition / detonation. To extract more power out of it the ignition has to be advanced to provide a longer burning time before the exhaust stroke. If the PCM is programmed to maximum safe ignition advance with 93 octane, then using higher octane may slightly reduce power because it will burn slower and less power will be extracted. At the same time it will provide a bigger margin of safety against detonation; that's why even though my RX-7's ECU is programmed for 91, I always use 93 or 94.

Now, some cars will continue to advance timing until detonation is detected, in these cases 100 or 110 octane race gas will provide more power.

You guys got a little bit to technical for me (boom07) but I think from what I read is that higher octane WILL reduce power on for example a Mazda 6i (No Turbo) but not on a Mazda 6 with Turbo...correct ?

Finally, since 91 Octane is a more of a controlled burning fuel. Would it be safe to say that if you want to get more miles out of for example, My Mazda 6i can i put 91 Octane (note: not caring about loss of power)? But in the future (say 2 years from now) switching to 87 Octane (due to the fact that I will be modding my 6i)?

Your inputs would truly be appreciated?
Thank you.

First Mazda
1990 MX 6 (No Turbo) - switched to a 2005 Mazda 6i Hatchback November.

[Da 6]

I have a tuner who emails me the tune (SCT Xcal2 Tuner for the other car) and thats how it temporarly runs race gas. Other than that timing/boost never changes untill you drop below the reccomended octane level IIRC.

Anything above 87 in N/A 6 will result in bad gas milage, washed down cylinders, depostis, and not so Zoom-Zoom acceleration.

[MS6freek]

i live in montreal and im putting 94 octane daily drive, very good for the car, instant response and all the power is there. (thumb) (first)