Octane question

[77 Z]

Why is it that the inline fuel injected 6 cyl engine in Z cars needs a min of 91 octane fuel when 8 cyl as well as 4 cyls will run well on 87 octane.

My Z owners manual says use a min of 91 octane, but our sons Mits Eclipse runs great on 87 , our Caddy runs great on 87 and has an 8 cyl engine..but the Z is supposed to have 91...

This is not about the cost of fuel , its a question about why the Z has to have the higher octane

77 Z

[MikeW]

Octane requirements have nothing to do with the number of cylinders in the engine. Octane is a measure the fuel's resistance to detonation. The higher the octane the harder it is to ignite the fuel. Pre-detonation (or knocking) is when the fuel ignites too soon. This is more likely to happen in engines with higher compression ratios and therefore those are the engines that require higher ocatane. If your car runs fine on 87 octane then you're wasting your money on the more expensive stuff. If your car pre-detonantes with the grade you're using you'll ruin the engine if you don't switch to a higher grade.

[Ed]

Well put Mike.

[Bambikiller240]

MikeW's answer is 100% correct.

Also keep in mind that back in the 70's (when Z's were new)Premium Gas was 98 or 96 octane (instead of the 93,92 or 91 that passes for premium now days), and Regular grade was even lower like maybe 89 I think (I don't remember exact # as I seldom used it).

I believe that they specified what was available at the time the cars were new and I doubt that 87 Octane was even available.

[77 Z]

Around here (Columbia SC ) the difference is 2.00 per tank full give or take a few cents in using 87 vs 93 octane. This car seems to do fine on 89 and as mentioned when the car was new 87 was not around or if so it was not all that prevelent, so I would imagine that what was regular fuel at that time was something in order of what is now 89 octane. But now you are looking at a difference of 1.00 give or take per tank full...think I will just stay with the 93 and not worry about a 100-150.00 per year difference...cheaper than a new engine....

Thanks for the information

77 Z

[beandip]

Here in Portland Ore. 91 is selling for $2.52 per. and going up. As the man said lower compression engines do not require the higher octane fuel , as the engine wears the compression lowers . A highly tuned engine with high compression requires the higher octane fuel and some require 100 + . It all comes back to the old saying , If you want to go fast how much do you want to spend. Or something like that. At the dyno they ran an engine on high octane and then on the lower grade, an the engine made more HP on the lower octane fuel . It was a basicly a stock engine with only slight mods.

[TomoHawk]

Isn't the compression od the stock Z engine (L28) about 8:1? I'd say that was a 'low' compression ratio, so go with the lower octane.

'High' compression would be around 10:1 and higher.

It should also help to prevent predetonation if your cylinders/heads were squeaky clean- with no carbon builup.

[77 Z]

One more thing..is the compression ratio listed in the owners manual. If not then how will I determine what it is.

77 Z

[TomoHawk]

It might be in your owner's book, but after so many miles/years, it should be different (less).

You'd determine compression with a tester in the spark plug hole. They probably have those in the auto parts stores. Somebody should have a good link to post.

[77 Z]

In the original owners manual for the Datsun 280Z 1977 Owners manual model S30 it indicates the following

Compression ratio: 8.3

Engine model and design:

L28, 6cyl inline OHC Eletronic Fuel Injection

Displacement: 168.0

[Bambikiller240]

With 8.3:1 compression, there should be no reason to use Premium gas. You should be fine using 87 octane.

FWIW, the Factory Service Manual for my 72 240Z states 9.8:1 (or 9.6:1, I forget which) compression ratio and recommends 96 octane gas. The highest octane I can get is 91 and the engine runs fine on either 89 or 91.

[mdbrandy]

Originally posted by Bambikiller240

With 8.3:1 compression, there should be no reason to use Premium gas. You should be fine using 87 octane.

I run regular, 87 octane in my '78 280Z, and it seems quite happy.