Have the rules for premium grade gas usage changed?

[Jeremy]

I ask because my mom is currently looking for a new vehicle. Looking at the engine specs for a few, they have a 10:1 compression motor, but specify 87 unleaded. I know 87 is usable with 9:1 as my 528e and my Wrangler motors both ran quite contendly on 87. Do modern electronics make this feat possible with 10:1 as well? Logically, I would think you could use more agressive timing curves (and thus better power/ fuel economy) with 87 octane and 9:1 than 10:1, but I could be wrong.

I also read that Honda's new ultra high fuel economy vehicle which isn't here quite yet but will be soon (the Honda Fit) uses an 11:1 compression engine but runs on regular??

This all has me kinda confused, as the m30b34 high compression motor is 10:1 but likes the highest octane gasoline you can get for it. Is it just in the timing curves?? Do you get more power running modern engine management with higher compression and less aggressive timing maps than you would with slightly lesser compression and more agressive timing? This all assumes the use of 87 octane.

Inquiring minds (namely mine) want to know!

Jeremy

[IceHaus Dave]

I'm no where near an expert, but I think your last paragraph just about answered your own questions. The trick seems to be the engine management systems with some pretty sophisticated knock sensors. Just my 02 cents.

Dave


[Edit by IceHaus Dave on [TIME]1109525524[/TIME]]

[Guest]

It's not simply a matter of timing maps and knock sensors. The new engine management systems can analyze what's going on with each individual cylinder by looking at feedback coming from the coilpacks themselves. The system used in my Bride's E46 325Ci is very sophisticated and knows when a cylinder isn't firing correctly. When it sees this happening you don't have to worry about killing the catalytic convertors with raw gas, the engine management just shuts down the fuel to that cylinder.

If you combine the latest in engine management technology from Siemens, with variable cams, both lift and duration, along with sequncial fuel injection and spark at each cylinder, it's reallly pretty easy to understand how the engines can run 11:1 on pump regular. This is the ideal thing to do anyway. Think about what the Octane rating really does to the fuel, it slows down combustion. This is why one is lossing performance and wasting $$$s when running older engines like our's on premium fuel. If the engine isn't pinging under laod, it doesn't need premium fuel.

The flip side is that a car company can still spec premium fuel, which is the case for our E46 325Ci. This allows the engine management system to extract maximum performance from the 10.5:1 compression ratio. But if we can't get premium for some odd reason, we can fill up with regular unleaded and the engine will run just fine, but with somewhat lower overall performance.

Rich

[Threeshifter]

During the firing of the spark plug there is some kind of damped wave plot of resistance across the spark gap that is a function of variations of octane. This creates a resistance feedback that the computers can read to make the proper adjustments to accommodate variations in fuel. That is about all I know about it and it might not be accurate.

Someone mentioned to me once that Saab had been one of the early automakers to employ this technology. I do not know if that is true either but I think there is some writing on this in a Saab Bentley that I have- maybe I should go check it.

[fastpat]

An excellent reply, Rich. Most folks don't know that 91 or 93 octane fuel runs colder, not hotter, than 87 octane and can actually reduce power all things being fixed and equal like in our Motronics 1.0 engines. ~0

That's the main reason for my interest in MegaSquirt, particularly the UltraMegaSquirt due out late this year or early 2006. UMS will control injectors individually, run individual coils per cylinder, and manage all other engine parameters.



[Edit by fastpat on [TIME]1109527974[/TIME]]

[ej in NorCal]

I have a stupid question. I remember the lower octane fuel was
recommended when our cars were new.
I have a JC chip and isn't higher octane required?
I have not tested lower octane due to this perceived requirement.
I know some board members run chipped cars on lower octane. I assumed
at some risk.
So, am I wasting money on the higher octane?

Thanks,
Erik

[Guest]

Erik,

Here's a simple test.

Next time you're getting close to needing a fillup, go ahed and run the tank way down to the tattletale light coming on. This way you only have about 1-1.5 gallons left in the tank. Then only do a partial fill maybe 1/4 tank with regular unleaded. Take the car out for a spin on your new fuel and purposely lug the engine. I'd do this test in 3rd gear by shifting at a much lower rpm than normal and then apply full throttle load. If the engine rattles or pings you need higher octane, if it doesn't regular is OK.

If you ping on regular, you still might be able to run the mid-grade fuel. So you'd have to do the test all oover after you've filled up with SUper and run that tank down the same way as before.

HTH

Rich

[Jeremy]

Wow, ok, I guess I didn't realize exactly how complex modern engine management had become. The variable lift and cam timing stuff I knew about, but using the resistance on the plug as an engine feedback tool is pretty remarkable. Do the aftermarket management systems (TEC3, Megasquirt, Autronics) use the same technology in their systems that have one coil pack per cylinder? Is that part of the "auto-learn" mode that systems like TEC3 and Autronics have?

What would that feature be called if the system did have it? I looked through the specs and didn't see anything that would indicate that the TEC3 does it, and it would be the most likely to have that feature I would think.

Thanks for all the responses, this is very interesting stuff.

Jeremy

[spinedocab]

Fastpat is correct. Higher octane burns cooler and is more resistant to combustion and thus pre-ignition or detonation or pinging/knocking. But this characteristic becomes necessary under certain circumstances. In the case of a very carboned-up combustion chamber/piston/valve, the carbon retains heat and actually becomes a superheated focal point. When fuel comes in contact with these little "glow-spots" it can ignite from that heat source alone rather than waiting for that precisely timed spark, and this is usually while the piston is too early in its ascent toward TDC. The knock is the explosion in the chamber while the piston is coming up.

In the case of a pressurized manifold (turbo/supercharged) the coolness and resistance to spontaneous combustion of the higher octane fuel becomes vital, because turbos run a bit hotter anyway, and any detonation in a pressurized chamber is a much more serious blow to the mechanicals of the head/gasket/piston etc.

So, in my chipped 535 I run regular fuel and have yet to hear of any detonation. But I did a head gasket last summer and cleaned all the piston crowns and valves/combustion domes/ of carbon.

In my way-over-boosted saab turbo, I cleaned things similarly when I changed the head gasket about 4 years ago, AND I run premium fuel ALWAYS. I also adjusted the AMM to a very slightly richer mixture at high/WO throttle settings, which helps keep things a bit cooler too.

But I like the advice to test your motor by a mild lugging under load/throttle. This seems to be the soundest advice. If YOUR engine needs it then use it and if not then don't.