Salutations:

I find that I get better mileage using what passes for high-test here in Canada (94) and see a slight increase on top of that with a carefully measured octane booster. I'm using a 1993 Aero Trionic 5 speed with a home built single stage regulator type boost controller set slightly down from max potential pressure (limits overshoots - but doesn't normally go into the glory zone unless you know about double clutching into lower gears).

I experimented a bit this summer by adding a substantial amount of legitimate test formula and found that the system couldn't improve timing much above 42.4mpg (smaller Imperial Gal) on the highway - it did start to run a little hotter than norm as the mix was uprated above that mileage figure. Around town was around 25mpg - and again - the engine ran a little warmer.

I got much more heat by using some decidedly low octane (some very stale pump regular I had in storage) for a couple of tankfuls. But saw considerably lower highway mileage - about 36mpg hwy and 19mpg city. I wonder now if the lower temp engine thermastat and fan controller modification is dictated by lower overall North American octane ratings.

Anyway - I finally got it to correct average temp and about 40.8mpg hwy - 24mpg city on what should be about 115 octane fuel based on some pretty loose mixture estimates. As to cost savings - you sort of have to do the math as you go along to account for the higher prices of fuel and legitimate octane additive. It is unwise to try and mix regular octane to high octane using only octane boosters because octane enhancement isn't the only component of good higher octane fuel.

It is a common misconception that higher octane is somehow more explosive or forceful than pump regular - it actually has a less explosive nature at normal compression and will soot up an engine if not matched to use. Higher octane provides a slightly more complete (longer) burn on the down stroke and delays ignition appropiate to higher compression engines. This provides both more power per stroke and makes better use of the explosive content of the fuel overall. However - I'm now pretty sure it requires an engine timed across the whole system to get a mileage advantage from silly octane fuel.

As we know, the Trionic system particularly, is made so it will adjust to varying octanes based on what it reads as happening in the actual combustion chamber - which allows considerable ignition latitude. However, based on my experiences this summer, Trionic must only work within the given range for better octane given the extra heat and occasional false O2 sensor faults provided with the Chateu de Storage-Shed and the silly octane stuff I mixed up later on.

Personally, I think this ranging has much to do with cam timing - in that it comes a point at both ends of the range where ignition timing and injection pulse alone cannot adjust for fuels and compression and you end up pre-igniting or dumping more flame down the exhaust post-ignition. Trionic could theoretically be able to burn anything from Bunker C through to nitro fuel otherwise.

As to the pre-digital Porsche/SAAB - you *must* stick to the rated fuel that was used when it was last correctly timed because the envelope is much more narrow given that it cannot make substantial adjustments across the octane range on the fly. It is why you can't run the turbo full out as long as you can on a Trionic SAAB without eroding the impeller and/or burping the exhaust valves.

The knock sensor system is usually only telling you are knocking - the Trionic system does what it can to cope with the knock - then tells you are dumping something past the O2 sensor it didn't expect when you are really outside the limits.

I now think that the best set-up would have to be some sort of variable valve technology with a Trionic type ignition and fuel controller matched with a more advanced APC controller.

At the end of the day though, if anyone uses my cfpm (Cubic Feet Per Mile Calculation) - the 9000 is still the very best on the road on almost any fuel.

Cheers.

--

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I've always felt that my 1993 9000 2.0 lpt ran better on premium, but since
this thread came up, I've been more careful with my observations. I'm
now convinced that premium fuel prevents the fuel cut-off at the end of a
maximum acceleration run on a particular stretch of road. This tends to
happen when using regular fuel, presumably by the anti-knock mechanism in
combination with a reluctant wastegate. But much less of a problem with
premium.

....you could of course suggest to your wife that if she really wants to
save money she could sell the 911....but let's not go there...   :o)