Low Octane Fuel and Knocking

[meestapeta]

I've got a 78 280Z, stock except for a K&N air filter, and the other day I noticed that premium (91 or 93 octane depending on the gas station) gas is getting to be almost $2 a gallon. So I thought I'd try the mid grade stuff, 89. I had maybe 1.5 gallons of premium left in there and added about 6 gallons of 89. The engine's been running fine and I haven't heard any knocking, but I was wondering...

Is it easy to distinguish knocking from the valve and timing chain noise of early Zs? The descriptions I've heard of knocking are a clickety noise on acceleration, which my car has always made. Just want to make sure I'm not doing engine damage to save a couple of bucks.

[meestapeta]

So I've just finished up my first full tank of mid grade and it's been running fine. Actually, I even got better gas mileage, 20.5 instead of 20 mpg. But that might have been other factors, because it's pretty small. Anyway, just trying to keep the post current. Anybody have any thoughts?

[tanny]

I've been using mid grade(88 rating) since I've been running my 240 with no knocking. Even mixed in a about 10 gal. of regular(86 octane) with maybe 2 gal. of mid grade with no problem. Maybe I'm too "retarded"(my wife said that's intuitively obvious to the most casual observer)? Definitely not very "advanced". Victor.

[dozer171]

i used to run high test when i first run my car (i listened to the manual... what do engineers know anyway?) last summer. but, then to save money, i decided to try 89, worked great, no loss in performance. i even ran regular for a while (stock 78 with EFI) with no weird noises. when i went to carbs, it didn't like the low test, so i went back to 89. a stock fuel injected Z runs around 8.3:1 compression with the aluminum head. you have to be trying REAL hard to get her to knock or ping with that sort of third world compression ratio. my timing was set at about 10* or so, but even advanced a little more, didn't knock, just had a nice little backfire on downshifts from higher revs. not bad ones, but, they still sounded pretty cool!

[LanceM]

A knock sounds like someone wacking the block with a ball peen hammer, nothing like valve noise. Usually only heard under hard acceleration or high load like lugging it in high gear.

[meestapeta]

Thanks for the advice. I definitely have not been hearing knocking then. Too bad now the mid grade is up to $2 a gallon.

[Gav240z]

Yeah pinging is quite a distinguishable noise you'll hear it going up hill in a tall gear as lance says.

It's similar to the noise you hear when your about to stall the car.

[beandip]

pinging sounds like a couple of rocks in a garbage desposer. There was a post recently from a Zdriver that had his car on a dino and was running plus fuel , so he tried the same testing with higher octaine fuel . Measured LESS HORSEPOWER. Dident have high enough compression to use all the bang for his buck! So it depends on the engine. I usually run plus, and no sounds of pinging in my 240, now that engine was a stock L-24. Will see with the "new " engine . now all bets are off.

[venom42]

I know I am resurrecting an old thread here, but I figured it was better to reply to search results than ask a question that's already been had. Anyhow - The previous owner of my Z ('75 280) said he ran regular gas (87) and got about 22-25 mpg. I think I got like 20-21 on the first tank I put in her. Then, I read in the manual that it requires premium, so that is what I have been running ever since. However, I only get 17-18 mpg. Would I be ok to run the regular gas? My Talon HAD to have premium to not grenade the engine (turbo), but being n/a, it would seem that it would be ok??? My car has 92,000 miles on it if that makes any difference.

[landmizzle]

Hi all,

The bottom line on what fuel to run is this; run the lowest octane that you can where pinging does not occur under load (i.e., hard acceleration, going up hill in the highest gear, etc.).

High octane fuel is not "better" or "high performance" nor is "gentler and kinder" to your engine. It is simply fuel that has additves that raise the octane so that it ignites "slower" (in simplistic terms). This allows it to run in engines that generally are higher compression without pinging. Pinging is the sound you hear as you engine is being slowly distroyed (or perhaps not so slowly.).

Irrigardless, of whether your engine is normally aspirated, turbo, super charged or whatever, run the lowest octane you can where pinging does not occur. Doing anything else, is simply wasting money.

BTW, two engines can appear identical, yet have different octane requirements. There are all kinds of things that can effect the engine's octane requirement; age, outside temperature, state of tune, elevation, and much more.

Finally, again, don't be fooled by the marketing hype. High octane fuel, if not needed, does not make your engine run "better". It actually contains less energy per given volume compared to lower octane fuels. That is why you sometimes hear people say that get better mileage or more zip with lower octane fuels ( surprising to many because it goes against the common misconception and hype.)

Hope this helps.

Best Regards,

Landmizzle '72 240Z

[240ZX]

Landmizzle is right on the money with his explaination of premium fuels. As he said, the additives (antiknock agents) prevent the fuel/air mixture from igniting prematurely. And yes, premium fuel has less BTUs per give amount of fuel. So, as it has been said many times before....is it's not pinging and your using regular grade fuel, everything is fine! Using a higher grade fuel is a waiste of $$$

[shoudt]

Maybe this is the wrong place to post this, but here goes.

I am using 87 octane and when I drive my 240z for about 45 minutes to work and then turn it off, the car continues to run for a little while. Every once in a while it gives a rather loud backfire then finally comes to a stop. Should I be using highedr octane?