Fuel for a VF SV6 S1

[J_D 2.0]

The correct fuel for any engine is determined by the detonation resistance at the ignition advance producing maximum torque. GM are relying on knock sensors to reduce ignition timing and prevent detonation using a lower octane fuel. The LFX engine at a compression ratio of 11.5:1 would need at least 100 octane fuel as they detonate on 98 between 4750 and 5300 rpm. Manufacturer fuel recommendations take cost into account like extended oil change intervals for the perception of being cheaper to run and maintain, but it's not necessarily ideal for the engine.

Interesting. So that’s probably the case with a lot of modern cars. IIRC in the past it was difficult to get above 10:1 compression without causing detonation.

Modern high compression engines that are well above 10:1 are probably all relying on pulling timing out to run 91 octane, which sort of begs the question why some manufacturers insist on 95 octane fuel when they could just pull some timing out and allow their engines to run on any fuel?

Might be more to do with the higher sulphur content of our 91 octane which isn’t allowed in most other markets. Specifying 95 octane allows them to run the same fuel system as US and Euro markets without adjusting anything for our high sulphur fuel?

 

[Skylarking]

The correct optimum fuel for any engine is determined by the detonation resistance at the ignition advance producing maximum torque. GM are relying on knock sensors to reduce ignition timing and prevent detonation using a lower octane fuel. The LFX engine at a compression ratio of 11.5:1 would need at least 100 octane fuel as they detonate on 98 between 4750 and 5300 rpm. Manufacturer fuel recommendations take cost into account like extended oil change intervals for the perception of being cheaper to run and maintain, but it's not necessarily ideal for the engine.

Yes i understand it may not be ideal to rely on a knock sensor to keep things "safe" but there has been no fundamental failure of the LFX to speak due to the use of 91 RON (that I’ve heard of).

So if it was an issue, given the numbers of HFV6 in use down under and in the USA, we’d have heard something about it… but nada…

Contrast this to the valve chain saga that the HFV6 (can’t remember which one?) suffered and the internal clogging up due to PCV design screwups and too long oil change intervals and it’s obvious the fuel octane issue is a non issue…

So relying on Knock sensors seems a strategy many vehicle manufacturers have used successfully and it seems to work

But if one prefers to feel warm and fuzzy, by all means use a higher octane fuel, but from what you say it seems even 98 isn’t ideal on this HFV6

On a related note, i wonder what fuel was used in certifying the advertised 210kw of crank output of the LFX? Doubt it was 91 but was it 98 or E85?

Really, if we had proper regulators that protected buyers from deceiving manufacturers, those manufacturers would be forced to advertise power outputs and consumption comparison measures (those bullshite stickers on the windscreen) for all the fuel grades the engine can run on and such would be clearly stated in the specification section of their vehicle. brochures That way things would be much clearer to those wanting to buy a vehicles and porky pies wouldn’t be told… car uses 91 and engine produces 210kw is always associated wrongly…

(PS: I’d agree “optimum fuel” rather than “correct fuel“ so changed it above )

 

[Skylarking]

Interesting. So that’s probably the case with a lot of modern cars. IIRC in the past it was difficult to get above 10:1 compression without causing detonation.

Modern high compression engines that are well above 10:1 are probably all relying on pulling timing out to run 91 octane, which sort of begs the question why some manufacturers insist on 95 octane fuel when they could just pull some timing out and allow their engines to run on any fuel?

Might be more to do with the higher sulphur content of our 91 octane which isn’t allowed in most other markets. Specifying 95 octane allows them to run the same fuel system as US and Euro markets without adjusting anything for our high sulphur fuel?

What fuel a car can run safely on obviously depends on the quality of the fuel available to the owners and the tune the manufacturer chooses to provides. This means some manufacturers simply re-tune the ECU and take an hit on max power while others may demand a better grade of fuel be used.

But emissions, as they have been tightening particularly in EU, has played a role in this equation as sulfur does impact catalytic converter efficiency which complicates the choices previously made.

As is, down under, out fuels have been described as shite by a few manufacturers and our pollution laws means we get some engines no longer allowed to be sold in EU…

In the past Jap cars seem to work ok with 91 while German cars require 95… It’s was a choice and in part I’d guess a choice on how much one wants to lean on a knock sensor… though I suspect Euro6 and later makes the choices more difficult staying with dirty 91…

It’s all a problem above my pay grade… I’ll just enjoy the LS3 which I thing nearly made Euro 5 (not sure?).

 

[RevNev]

Interesting. So that’s probably the case with a lot of modern cars. IIRC in the past it was difficult to get above 10:1 compression without causing detonation.

Modern high compression engines that are well above 10:1 are probably all relying on pulling timing out to run 91 octane, which sort of begs the question why some manufacturers insist on 95 octane fuel when they could just pull some timing out and allow their engines to run on any fuel?

Might be more to do with the higher sulphur content of our 91 octane which isn’t allowed in most other markets. Specifying 95 octane allows them to run the same fuel system as US and Euro markets without adjusting anything for our high sulphur fuel?

In the carby V8 days, 10:1 compression ratio needed Avgas, or 20% Toluene added to Super leaded petrol for optimum ignition advance in most cases. I think by memory, they lowered the compression ratio in VL V8's to 8.4:1 for use with unleaded fuel. Fuel injection raised the compression ratio threshold a bit and direct injection has stretched it further.

Where HSV recommend higher octane fuels in LS3, and we compare the 340 ignition mapping with the stock GM tune, there's a few areas where the 340 tune could reach 34 degrees of ignition advance in ideal circumstances compared to the stock tune locked at 18 degrees. With greater timing potential in the base ignition maps, there'd likely be a safeguard factored in where the knock sensors may not reduce the timing enough to eliminate detonation on lower octane fuels. The higher octane fuel will make more power with increased ignition advance too.

 

[RevNev]

Yes i understand it may not be ideal to rely on a knock sensor to keep things "safe" but there has been no fundamental failure of the LFX to speak due to the use of 91 RON (that I’ve heard of).

The only technical disadvantage with retarded ignition, is higher combustion temperatures over time can weaken piston ring tension and create blowby at lower km's in the engine life cycle. With GM's history of getting an engine through warranty and rejecting every warranty claim they can, then avoiding responsibility for known engine faults, their recommendations, fuel, oil etc don't inspire me much!

 

[keith reed]

The only time I have had trouble with fuel is when running 98 continuously on my VE. It didn't like cold starting otherwise was fine. I now use a mix of 91 and 98, problem solved. The VH has to run on 98 because of the tune. The ute gets the cheapest going though that can often mean using 98 (Costco).

 

[RevNev]

The only time I have had trouble with fuel is when running 98 continuously on my VE. It didn't like cold starting otherwise was fine. I now use a mix of 91 and 98, problem solved. The VH has to run on 98 because of the tune. The ute gets the cheapest going though that can often mean using 98 (Costco).

Yes, that can happen as lower octane fuel has a lower ignition point and takes less spark intensity to fire it. Reducing spark plug gaps or rectifying a weak ignition system can help engines cold start better on higher octane fuel. The highest octane or fuel with the greatest detonation resistance is Methanol and to fire methanol engines effectively, takes a magneto or CDI ignition. Some methanol race fuels have 5% acetone added for easier starting.

Thinking of methanol, the old Shell Racing Fuel A was 96% methanol, 3% Acetone and 1% Castor oil and was the most magical smelling fuel an engine could burn, pure nose candy!

 

[Skylarking]

The only technical disadvantage with retarded ignition, is higher combustion temperatures over time can weaken piston ring tension and create blowby at lower km's in the engine life cycle. With GM's history of getting an engine through warranty and rejecting every warranty claim they can, then avoiding responsibility for known engine faults, their recommendations, fuel, oil etc don't inspire me much!

Sadly, in this hypocritical and environmentally woke world we live in, many people simply don’t care about such minutia (but I find it interesting ). Many people simply flip their salary packaged cars every few years leaning on the taxpayers in the process while adding to the environmental damage in doing so. They have no care about the octane of their fuel and use only what is cheap and within the manufacturer minimum acceptable requirement… Their old car simply becomes S.E.P. and it’s a philosophy not too distinct from many a car manufacturers warranty shitfuckery.

Sad as I said. Sadder that it seems such minimums are contagious as we as a society slide back into the primordial ooze we emanated from in what seems like a chase to the bottom

So the minutia may indeed interesting to us and be as you say. I have no reason to doubt it though I haven’t seen a huge numbers of HFV6’s blowing plumes of smoke due to fubared rings pounded from excessive heat from retarded timing… maybe that’s the next big failure waiting us

I probably need to reevaluate using 91… I know my 4 banger daily runabout (not a Holden) is spec’d for 91 but doesn’t like it greatly. That’s especially true on hot days where oddly the light pinging stops (timing retarded even more I guess) and it seems it’s down on even more power as if one or two cylinders are switched off But it’s a POS daily so meh, it gets the cheap 91…

The LS3 commodore on the other hand… hmmm... guess I should give it the better juice more often

 

[Pollushon]

All of this matters if you're tracking your car or drive like you're tracking it, if you just drive it then minimum octane isn't really an issue. The power/torque ratings were measured using 91 on the Alloytec

 

[RevNev]

All of this matters if you're tracking your car or drive like you're tracking it, if you just drive it then minimum octane isn't really an issue. The power/torque ratings were measured using 91 on the Alloytec

Not really, a lot of people into cars who don't do much more than plonk around to the shops like the idea of using optimum products, fuel oil etc. Given the choice of a range of products, it's surprising how many people choose the fundamentally better and more expensive products even telling them they probably don't need it.

Speaking of fuel, another engine the knock sensors pull ignition timing out on 91 big time, is the BA to FGX 4.0 litre Ford Barra engine, but on 98 there's no knock sensor intervention.