Is high test really necessary?

Sh0velMan

To be fair, at higher altitudes, like over 7k feet, 89 octane is likely plenty.

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Jordo!

I mentioned it

Yes, and no. Higher compression piston heads tend to be more likely to have flame kernels dissipate irregularly, creating hot spots that may lead to knock by way of pre-ignition of the next combustion event.

This is one of the reasons that it is generally easier to tune a boosted car with more boost but lower compression pistons, rather than just assuming more pressure however its achieved is better. You still have to deal with knock and that's generally easier on a low CR high boost motor, at least using port injectors (in theory you can do more with less using direct injection).

In either case, the ignition advance tables were tuned based on the presence of 91 AKI, so to run optimally and safely, especially under load, going below the recommended octane is potentially risky.

Sort of. It will aggressively retard ignition timing and may also pump more fuel to control what is determined (correctly or incorrectly) to be knock -- HOWEVER, this state of affairs is far from ideal in terms of performance or fuel economy, and will not necessarily preserve the engine.

It's not really simply a secondary map, where you just make less power, as it almost certainly is for an engine where the manufacturer only "recommends" higher octane fuels, but claims it will neither damage the engine nor void the warranty if you use a lower octane -- if nothing else, assume that they don't want to have to keep replacing motors for customers, so they have some confidence the "low octane" spark maps are perfectly fine for normal driving.


Well, true, provided you are high enough that pressure levels are sufficiently lower in the cylinder -- but without a way to monitor what the motor is doing, it is still probably a bit of a crap shoot...

And... if something does break, evidence of the use of a lower than required octane may void the warranty...

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Red__Zed

You say "yes and no" but as far as I can tell you are agreeing with me. Plenty of cars run 11:1 fine on 87/85 octane gas, the Z simply wasn't designed for it.

Jordo!

91 is fine.

I'm saying that yes, it is possible to run a high CR piston on octanes < 91, provided the spark advance tables are properly tuned for it, but no, the Z was -- at least according to Nissan -- not tuned for it.

Also, there are indeed special considerations for octane needs unique to the design of higher compression pistons, and it is unknown (or at least not widely known) how the VQ37HR pistons behave in terms of hot spots under load.

So, I'm agreeing with you, but calling attention to the potential unique needs of higher CR pistons, in much the way that Sh0velman was pointing to potential exceptions associated with running motors at higher altitudes.

I don't know how much of a role the piston crown design and specific CR is playing a role in octane needs for the Z's motor relative to the tune alone, but it is indeed possible.

I do know this tho': In the owner's manual it says some light knock when chugging up hill is "normal"...



That alone tells me this motor prefers higher octane fuel.

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Red__Zed

Always helpful to have an echo.

Jordo!

Yeah... I see your point.

You know, the only thing more annoying than an "echo" post is someone who insists on having the last word in a thread.

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