What octane rating is required for both motors?

TIA.

87 octane

Thanks for the quick reply.

If you're going to get the V8, I'd run high test in it. For the extra twenty cents a gallon, it's worth it.

If you're going to get the V8, I'd run high test in it. For the extra twenty cents a gallon, it's worth it.

??? worth what?

If you're going to get the V8, I'd run high test in it. For the extra twenty cents a gallon, it's worth it.

This has been discussed before. Putting in 93 in a car turned for 87 will net you nothing. Don't waste your money. Gas is high enough as it is.

If you're going to get the V8, I'd run high test in it. For the extra twenty cents a gallon, it's worth it.

Only if high test prevents spark retard. If you're not close to the point that the ECM is reverting to the spark retard timing table then premium is doing nothing but draining your wallet faster.

The ambient conditions that accelerate this are hot and dry.

I choose my octane level based on driving conditions. I run regular in cold rainy weather. On a long stretch of normal interstate driving I run regular. However if it's a hot summer day and I want to do some spirited driving I'll use premium.

I hope to be tuned before the hot summer days are here. :)

I hope to be tuned before the hot summer days are here. :)
It's already hot here. :coffee:

For the extra twenty cents a gallon, it's worth it.

For me it's $0.40 more a gallon. :banghead:

So, let me try to understand this octane situation.

The engine will run fine on 87 unless it's either hot outside, or under a big load, or both, under which circumstances the ECU will adjust the timing to prevent knock, therefore reducing the power available from the engine.

However, if you are running steady-state with low or moderate load the ECU won't be retarding the timing, but in these circumstances you don't need much power from the engine anyway.

Is it a correct assumption that 89 or 91 octane gas will make the engine less prone to knocking, particularly under high-heat or high-load situations, and that the ECU would not need to adjust the timing as much to compensate, thereby allowing the engine's available power output to remain higher in these circumstances?

So, let me try to understand this octane situation.

The engine will run fine on 87 unless it's either hot outside, or under a big load, or both, under which circumstances the ECU will adjust the timing to prevent knock, therefore reducing the power available from the engine.

However, if you are running steady-state with low or moderate load the ECU won't be retarding the timing, but in these circumstances you don't need much power from the engine anyway.

Is it a correct assumption that 89 or 91 octane gas will make the engine less prone to knocking, particularly under high-heat or high-load situations, and that the ECU would not need to adjust the timing as much to compensate, thereby allowing the engine's available power output to remain higher in these circumstances?

Yes, that's a very good summary. The only other factor is that once the ECM decides to use the retarded timing table it tends to continue using this conservative table for some time. I'm not sure if it's a certain number of drive cycles of if it resets immediately at the next startup. Maybe a tuner guru can clarify this.

A lot of people mistakenly think that high octane = more energy or a hotter fuel. It's just the opposite. Regular fuel has more energy and ignites faster. High octane has a more controlled (ie slower) burn which does nothing more then prevent predetonation.

So...other than during the summer it would be best to run 87 octane in the GTs?

You can run 87 any time. That's what the car is designed to run on. If you get bad gas and the car begins to knock the ECM will automatically retard the timing so that it's safe.

You can run 87 any time. That's what the car is designed to run on. If you get bad gas and the car begins to knock the ECM will automatically retard the timing so that it's safe.

But if you would like the benefit of having access to whatever additional power is available when the timing is not retarded, you may see some benefit from running 89 or 91, if I understand this correctly.

But if you would like the benefit of having access to whatever additional power is available when the timing is not retarded, you may see some benefit from running 89 or 91, if I understand this correctly.

When I first got on this site I said the exact same thing. I got into a discussion with sandog on this. I believed that if you ran 93 octane you would get the full 361hp. If you ran 87 octane you get a map with a different amount of hp.

Unfortunately, no one has done back to back dyno's to prove that there is any benefit from running higher octanes. ( At least that I am aware of ) Most likely no one ever will.

I am just saying people don't have to go spend extra money at the pump if they do not want to. The car is designed to run on 87 so even if it's hot in the summer there is no real need to run 93.

I have the VMS tune so I have to run 93. Doesn't really matter to me. Just don't think it's necessary for everyone to read this and assume that it's hot so I need to put a higher octane in.

When I first got on this site I said the exact same thing. I got into a discussion with sandog on this. I believed that if you ran 93 octane you would get the full 361hp. If you ran 87 octane you get a map with a different amount of hp.

Unfortunately, no one has done back to back dyno's to prove that there is any benefit from running higher octanes. Most likely no one ever will.

I am just saying people don't have to go spend extra money at the pump if they do not want to. The car is designed to run on 87 so even if it's hot in the summer there is no real need to run 93.

I have the VMS tune so I have to run 93. Doesn't really matter to me. Just don't think it's necessary for everyone to read this and assume that it's hot so I need to put a higher octane in.
Agreed 100%, dodson. I'm just trying to get my head around the whole issue, and separate truth from fiction.

When I first got on this site I said the exact same thing. I got into a discussion with sandog on this. I believed that if you ran 93 octane you would get the full 361hp. If you ran 87 octane you get a map with a different amount of hp.

Only the ECM and it's knock sensors will determine the map. Octane in itself won't. It will however give you more margin before knock occurs.

If you don't go into the spark retard map then HP will not vary between 87 octane or 93 octane.

Unfortunately, no one has done back to back dyno's to prove that there is any benefit from running higher octanes. ( At least that I am aware of ) Most likely no one ever will.

Back to back dynos would also have to be performed at various ambient conditions. I would be willing to bet that back to back results done in Seattle in the winter would show very little difference. Do the same back to back dynos in Denver in the summer and I bet you'd see a significant difference and I bet all numbers would be significantly ower then Seattle.

Hot, high and dry are worst case for HP.

For me it's $0.40 more a gallon. :banghead:

Yeah, finding premium for 20 cents more than regular is pretty hard... sometimes its close though.

One station here has

87: $3.799
89: $3.929
92: $4.059

So its close... but still really sucks with these high prices. All my spare change is going right in the tank lately :banghead: