[bridrive55]

We've already shown that 100 octane will help it.....
You'd lose power in a car running 87 octane that should be running 91 octane, knock sensor or no. Why? Because the knock is incomplete combustion..... The typical engine only uses 33% of the energy contained in a given amount of fuel. When improper octane levels cause knock, that means far less is used.
Knock is improper flamefront propegation in the combustion chamber. Typical knock can be best explained as too fast of such propegation, instead of uniting as a single "wave" and creating the powerful push on the piston, it splits up, "echoing" off the walls of the combustion chamber and charging the middle, where one or more meet and cause a reverberation effect. All of this is supersonic and takes place in milliseconds, your brain registers it as a high frequncy tapping sound.......
Anyway, what I'm trying to say , is that you would lose power whether the knock sensor heard that noise or not. Knock sensors are designed to hear that certain frequency of where 90% of all knock occurs. However, it can happen at a much lower or higher frequency, especially if the car has any modifications that affect airflow. Air density affects knock frequency. It's possible to have knock the the sensor will not even notice, especially in older vehicles.

And also, conversely, noise that the knock sensor recognizes as knock that is really harmless engine noise. Subaru's have huge problems with overzealous knock sensors.
I see what they were saying above though. Running C107 race fuel in a 350Z will not increase power, because all that a higher octane rating does is increase an engine's knock tolerance threshold. With a stock or even slightly modified VQ35DE, this isn't a consideration, because it doesn't knock with 91. 91 octane will deter 100% of knock just as well as 107 octane will. 87 octane however, might make the engine knock, which would activate the knock sensor, which triggers the ECU to retard the timing.

 

[Zen31ZR]

And also, conversely, noise that the knock sensor recognizes as knock that is really harmless engine noise. Subaru's have huge problems with overzealous knock sensors.
I see what they were saying above though. Running C107 race fuel in a 350Z will not increase power, because all that a higher octane rating does is increase an engine's knock tolerance threshold. With a stock or even slightly modified VQ35DE, this isn't a consideration, because it doesn't knock with 91. 91 octane will deter 100% of knock just as well as 107 octane will. 87 octane however, might make the engine knock, which would activate the knock sensor, which triggers the ECU to retard the timing.

Actually the Subarus (the STi) are having problems because the EJ25 was designed to run on 95+ octane gas.... The 91 octane salad oil we have completely freaks out the ECU. This problem is solved with an ECU reflash, but then the STi is not longer 300 Hp.... The 350Z, as JamesZ has already said, responds better to 100 octane fuel. Engines are specified to run the octane they are to balance fuel economy, Horsepower, and warranty. Also, cars that require higher octane can be offputting to potential buyers, especially with the prices we pay for gas these days. At 10.5:1 compression ratio, I would say that 91 octane was barely adequate to control knock in the 350Z. The 91 ZR1 Corvette, at a 10.1 ratio, specified 91 octane. Half a point higher, it makes sense that more octane would be required.
Something else you don't realize, almost all engines experience knock at some point. Could be one out of every 30 power cycles. Could be one out of 100. Knock sensors start to wake up when a threshold point is reached, which is about 1 out of every 10-20 power cycles. At 5000 rpm, you do the math. What higher octane does, is create smoother more useable power for the engine. Higher octane of itself is not more powerful, higher octane actually slows down combustion, makes it more useable by the engine. It lowers the chance of having knock in the threshold range of the sensor, and lowers knock in general, which means the engine can make power on every stroke, not just most of them......

 

[Zen31ZR]

I'll put this into perspective for some of you. What do a 79 308 GT Ferrari, an 86 Porsche 911 Turbo, a 94 ZR1 Corvette, and a 2004 350Z have in common? They all require 91 octane gas.... Even with such disparate power applications. What I'm trying to say is, factory octane requirement is a generalization. Especially for modified vehicles, factory octane requirement goes right out the window. Octane requirement is also specified the same for all altitudes, which is hugely incorrect in a place like Colorado.....

 

[Zac]

Might as well also add, when fuel is reformulated (oxygenated), I have always been told to consider bumping up octane as it can cause pinging or premature burn, especially vehicles with excess carbon build up. I know that MD is reforumlated in winter months, and PA is not. Premium is generally 93 here, and 94-98 is readily available. I would imagine Cali is oxygenated. The STi may simply not be able to take reformulated 91 octane without having a serious reduction in timing.

 

[Zen31ZR]

Might as well also add, when fuel is reformulated (oxygenated), I have always been told to consider bumping up octane as it can cause pinging or premature burn, especially vehicles with excess carbon build up. I know that MD is reforumlated in winter months, and PA is not. Premium is generally 93 here, and 94-98 is readily available. I would imagine Cali is oxygenated. The STi may simply not be able to take reformulated 91 octane without having a serious reduction in timing.

Aside from a few gas stations catering to the racers around here, the best we get is 91 octane. Kind of odd, since at higher altitude, we should get the higher octane stuff.

 

[JAMESZ]

We are talking about the Z33, right? The ECU from the factory is tuned to 91 octane and the Z33 compression is 10.3:1 (fact) not 9:1. Have you noticed that it is very hard to make power on these cars? The reason is that Nissan has tuned the Z33 to it's optimal and why it's important to put in high octane gas. Anything less will drop your HP considerably and I'm not talking 5 hps (there is a reason the variable timing is used in this car). You won't find a dyno comparisions of octane difference because it's a waste of money and know one in there right mind would subject a $30+K Z to that kind of abuse.

Originally Posted by IreiMember
"Higher Octane fuels in low compresson engines will not produce more power PERIOD!"

I hate to say it but this statement puts you in the John.. A Z32 has a compression of 8.5:1 and when this car came out the octane rating in Cali was 92 and when they switched over to 91 octane people with JWT ECU's went into safety boost. What does that tell you?? If you have some knowledge of how a car works this is telling you octane does count.

If you do have data please post it I do have a open mind?

Very good post. You put it in words better then I did.

I didn't point out the knock sensor Zen good point, the EJ25 has more problems then the knock sensor, the ecu is just a pain and it is hard to keep the pre 05 STis away from knock. In 05 they redid the ecu so now that is no longer a problem.

 

[Zen31ZR]

Most cars have multiple spark and fuel maps stored in the ECU. Most cars run fairly conservative timing advance and injector pulse width depending on what the knock sensor and 02 sensor sees. Most cars also run a touch rich from the factory, turbo cars definitely do. This is to protect engine warranties, for no other reason. What happens when the knock sensor hears less knock, is that it lets the ECU advance timing until it once again hears threshold knock, within the fuel maps capabilities to do so..... If timing can not be retarded enough to combat knock, a lower value fuel/spark map is switched to. When less knock is heard, the ECU advances the timing til it gets threshold knock, or if it comes up on the out of range of a low end map, switches to a more aggressive one. This is why putting higher octane gas in a modern vehicle can have big results. Older vehicles with less advanced ECUs, or even no ECUs at all, don't really benefit from using higher octane gas, except to combat piston deposits, as was mentioned before.

 

[Spongerider]

ok idiot, run 87 in your car, but dont tell other peole to, because you are too ignorant to realize IF THE KNOCK SENSOR FAILS A 350Z WILL BECOME SERIOUSLY FUCKED UP WITH 87 OCTANE GAS! and like i said, you lose power with lower octane BECAUSE of the sensor, is that what you are disputing? or are you disputing my statement you WILL NOT gain power from higher octane?

its a high performance car, use high performance gas... god i went through with this shit in the qr section with spec V's, some dumb ass is gonna have 100 people fuck up thier cars to save 30 cents at the pump...

in the end, if your car says 91, run 91, 87 will hurt it, 100 wont help it, use what is recomended...

At first, I was wondering why you were talking to me but then I realized I responded to you thinking you were IreiMember. So please disregard my comment to you?

On a side note, please be mature enough to respond like an adult? You can disagree so long as we agree to be civil, ok?

 

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