this maay be a stupid question but.. do i really have to put 91 octane in my car? gas is sooo expensive and my car is just stock, its a 78 280z. does the octane depend on what the engine compression is? what is the difference between 87 and 91? does it really make that big of a difference? i read somewhere that gas stations only have 2 tanks, one for 87 and one for 89.. and 91 is a mix between the two. is that true? thaanks for the info guys.

Hot Rod Magazine did an article on this a while back. In a nutshell; You are wasting your money on high octane fuel. The higher the octane rating the more anti-knock additives are present in the fuel. So If you have a high compression engine and you notice that the engine "knocks or pings" with 87 octane then move up to a higher octane.

And the myth that the higher octane fuel gives more power than the lower octane was up for debate. It all depends on your engine set-up.

I've never heard that stations only have 2 tanks and that they mix the fuel.

Z cars have low enough compression that you should be fine with 87, and unless you have been milling your head, or throwing in high comp pistons, you should still be able to run 87 (i know i can on my '77). in fact, running too high of octain can cause a buildup in carbon, as it is not burning right for your engine. If you are experiencing some preignition, it might be in your best interest to check your timing, and if that is fine, your engine has a LOT of carbon build up... get some ATF, and spray it into the intake (kinda hard with that AFM sensor in the way), or put a little bit in the last gallon of gas you have in your tank. It might run rough while your doing it, and it will smoke really bad, but when your done, your engine will have that minty fresh kind of clean.

My early 72 240Z with the high compression (early E88) cylinder head will not run properly on regular gasoline. I have to use 89 octane at least and even then it doesn't run well. My original 72 FSM reccomends 92 octane. With the price of gas now days I guess I should be glad that I'm not using Bambikiller as a daily driver.

I'm not glad, but I should be! At least I can use regular 87 octane in the 81 Dodge D50 P/U (2.0L). It's gutless, but it saves extra wear on the Z.

I've found that in Australia, the 'performance' L engines (ie stock 240Z L24, or other modified engines) only run 'right' on Premium Unleaded (98 RON). I also run a stock L20B on LRP (lead-replacement) which is 95(?)RON. My main problem is that all my cars are turbocharged (except the L20B) so I'm stuck with the expensive stuff....

BTW the 98RON stuff is over $1.10AUD per litre currently - that's $3USD per gallon!

now, maybe im just thinking about the US versions, but isnt the highest compression engine in a Z the 240's 9.0:1, with something like 8.8:1 on the 260 and 280. the ZX was even lower comp...

this really shouldnt be too high to run 87 or 89 on. My t-bird is 10:1 and runs fine on 89 unless i am drag racing and geting into the 5500rpm range.

FYI the following is a brief explanation of MON vs RON:

"

The motor octane rating, referred to as MON (motor octane number), is the best rating to use when selecting fuel for your race or high compression engine. When testing MON, the fuel is heated to 300° F and the intake air is heated to 100° F. The test engine is a single cylinder 4 cycle engine that is run at 900 rpm. Ignition timing is varied with compression ratio. Engine load is varied during test.

Research Octane Rating

Known as RON (research octane rating). Tested at 600 rpm with a fixed timing of 13° BTDC. The fuel temp is not controlled at all and the intake air temp is varied with barometric pressure. This is done to covert everything to a SAE standard day, which is 60° F, 0% Humidity, and 29.92 inches barometric pressure. The RON should not be used when selecting fuel for a race or high performance engine. The RON will always be higher than the MON.

"

Note that this was taken from a US site. In Australia & Japan the only rating used is RON. In Japan the high-perf fuel is 100 RON, in Australian it's 98 RON, so generally the cars are slightly de-tuned to run on our fuel. However there is talk of making 110 RON unleaded fuel available at the pumps in Australia.

"This is done to covert everything to a SAE standard day, which is 60° F, 0% Humidity, and 29.92 inches barometric pressure"

Where do they get 29.29 inches of barometric pressure ??

I thought / understood that natural bp is 14.7 . Do they increse it for testing??

- Jeff

14.7psi ~ 29 inHg ~ 1 bar ~ 1 atmosphere

Originally posted by Ben

14.7psi ~ 29 inHg ~ 1 bar ~ 1 atmosphere

& 1013. XXXmb.

As far as the mixing goes.When I was in high school working at a gas station a customer came in and put in half regular,half hi-test.On that day one of the suits from the main office was there and saw this.He's wasting his money he said.Different grades of gas are like oil and vinegar.They don't mix.I have always remembered that although I have no proof of it being fact.

Interesting reading http://www.seansa4page.com/resource/octane.html