What Octane fuel do you feed your Z?

[FastWoman]

For the first time EVER (and I've been driving 280Z cars since the 1980's), I've fueled up my 280 with 87 octane. It runs fine. Why 87? It turns out I can now buy REAL gasoline (no alcohol) at a gasoline distribution depot near me. It cost me about $3.80/gal, about $.65 higher than the gasohol. Unfortunately they only have the 87 grade without alcohol.

[Jeff G 78]

I'm sure that all of the issues I had with my race car were due to ethanol. In endurance racing, we are stuck with whatever gas the local station sells or paying huge money for high octane track gas. In a 25+ hour long race, we burned over 150 gallons of gas.

What really sucks is the EPA's new ruling to allow E15. There is no way a carbed car will run on that crap.

[Wade Nelson]

Alcohol added to pure gasoline will actually RAISE the octane rating, while lowering the amount of energy delivered per gallon. You WILL get measurably worse mpg's with gasoline containing alcohol.

My 4-Runner, which has a knock detector, runs perceptibly better on Premium fuel. My 280ZX, I've never been able to tell any difference in performance. I live at high altitude (6000') and that reduces octane requirements as well.

[Jeff G 78]

True Wade, but the big thing that makes E10 bad for us is that ethanol is much more prone to vapor locking. High pressure fuel systems keep this in check for the most part, but low pressure carbed applications really suffer. I laugh when I see E85 at the local stations for ~10% less per gallon than regular 87. E85 will deliver close to 30% worse economy for the modest 10% price break.

[5thhorsemann]

Man, the damn vaporlock is a PITA. I've been trying everything to fix my problem with this. I was heading up the highway yesterday and couldn't go past 60mph untill I popped the hood latch, then the problem stopped. I think the chin spoiler made it worse. Since the summer blend switch the economy went down and I have to run 93 oct.

[Jeff G 78]

Man, the damn vaporlock is a PITA. I've been trying everything to fix my problem with this. I was heading up the highway yesterday and couldn't go past 60mph untill I popped the hood latch, then the problem stopped. I think the chin spoiler made it worse. Since the summer blend switch the economy went down and I have to run 93 oct.

On the track, all is fine until I slow down. I have big issues at anything below 20mph. I plan to ditch the steel fuel rail. I think the heat from the head is being conducted through the rail brackets into the fuel. I'll also install a fan to keep air moving across the carbs and I'll cut a hole in the hood above the carbs similar to the later vented hood. If those things don't help, I'll have to try to run only premium fuel even though my low compression motor doesn't need it.

[LeonV]

One must also realize that diluting gasoline with alcohol will drop the stoichometric mixture point. "Pure" gasoline is 14.7, while E10 is ~14.2. As alcohol content increases, AFR must be enriched. The chemical makeup of ethanol has a hydroxyl group (-OH) which oxygenates the fuel, thus requiring you to dump in more fuel when compared to using "pure" gasoline.

[SportBikeMike]

Exactly. If the manual recommended 91 RON then 87 at the pump (AKI) will be fine.

All the octane rating is, is a measure of knock resistance, meaning resistance to self-ignition under high temperature and pressure. It has nothing to do with burn rate, energy, etc.

Thanks for explaining that. I've been putting premium in my 280ZX-T because I was under the impression the manual recommends 91 octane or higher. Stopped at a gas station the other day and they were out of premium, so I put 89 in it......and it runs great, maybe even slightly better! I'll try 87 next time.

[LeonV]

Thanks for explaining that. I've been putting premium in my 280ZX-T because I was under the impression the manual recommends 91 octane or higher. Stopped at a gas station the other day and they were out of premium, so I put 89 in it......and it runs great, maybe even slightly better! I'll try 87 next time.

No problem!

Just make sure you know which octane rating the manual is giving you! It can be RON (Research Octane Number), MON (Motor Octane Number), or AKI (Anti-Knock Index). RON and MON are experimentally determined using test engines, with MON being a more stringent test. Pumps in the US use the AKI rating which is the average between the two, whereas Australia and Europe use RON. RON will always be higher than MON, and what you see at US pumps is the average of the two.

Just make sure you know which rating the manual is going by. FWIW, Turbocharged engines typically require higher octane fuel, but if the manual says 91 RON then 87 AKI is just fine.

[SportBikeMike]

No problem!

Just make sure you know which octane rating the manual is giving you! It can be RON (Research Octane Number), MON (Motor Octane Number), or AKI (Anti-Knock Index). RON and MON are experimentally determined using test engines, with MON being a more stringent test. Pumps in the US use the AKI rating which is the average between the two, whereas Australia and Europe use RON. RON will always be higher than MON, and what you see at US pumps is the average of the two.

Just make sure you know which rating the manual is going by. FWIW, Turbocharged engines typically require higher octane fuel, but if the manual says 91 RON then 87 AKI is just fine.

Just checked the owners manual and it does say 91 RON. It also says 87 (R+M)/2. Looks like I'll be switching to 87 next time....why not save a few more bucks.