Premium vs. regular unleaded?

I've been wondering this for a while, but I didn't want to enter that Australian-argument thread, so here's a Saab-only question:

What would happen if I ran my 2002 9-5 Aero on regular unleaded, instead of premium?

My wife thinks it will save us money. I'm concerned it could cause damage to the engine, or at the very least cause enough power drop that you'd lose the cost savings.

Then let's extend the question to her car: she has a 1984 Porsche 911. It's supposed to have premium, and it doesn't have the fancy electronics that my 9-5 has. What happens if she feeds HER car lower-octane fuel?

Thanks! Gary

Reply to
Gary Fritz
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I run my 9000 Aero on 95 RON all the time. I don't drive it hard all the time, in fact rarely give it full welly for prolonged duration so there's not much benefit of burning tanks worth of anti-knocking fuel while pootling about!

I don't believe you'll notice any difference to be honest, only perhaps in the cost at the pump.

With a manually set ignition system, it can't retard when knocking is detected so it's not a good situation.

David.

Reply to
David Taylor

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.

Reply to
Dexter J

I did check this on a 1997 9000 (2.0 lpt) and found the consumption to have been better by 8% or by 4-5% (I guess it might have depended on what I did with each tankful). From memory my calculation at the time was that at 4-5% better consumption it evened(?) out the price difference .... etc.

I stayed with premium fuel because I "honestly" thought the car run smoother at around the 3rd tank. (Could it be that I felt what I wanted to feel? Who knows who cares.)

Best not to make comment, I can see what Ausies would say :-( ... but given the age of the car the more octanes (and thus less preignition if I am not mistaken) the better.

:-) Charles

Reply to
Charles C.

Your 9-5 will cope with the lower octane without damage as the trionic engine management will manage knock.

My 9000T works happily with 91RON (the owners manual says it requires

98RON). The only change I can detect is that I can't boost into the red zone. Still plenty of boost for spirited driving however. I would probably use 98RON on a very hot day if the trip entailed climbing steep grades.

Another family member drives a V6 NG900. This car also runs happily on

91RON, indeed it gets worse MPG on 98RON.

Does the 911 have a knock sensor? If not then it is possible to damage the engine. Perhaps you could have the static engine timing changed and get the distributor advance curve re-graphed.

Reply to
ShazWozza

....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)

Reply to
Pidgeonpost

Your car will be fine, power and fuel economy will drop some, whether it will offset the savings isn't possible to say but it won't hurt you to try it. Her car may well ping, you could back off the timing to prevent that but again that will kill the power and fuel economy and very likely more than offset the savings.

Reply to
James Sweet

electronics

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).

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.

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.

It's not really lower octane, it's just measured differently.

Reply to
James Sweet

I thought that to - but as I said, mileage and heat levels improve as I mix to what should be close to 115. Over that I get a extra heat and no extra mpg - when I went to what is probably about 70 - I get *a lot of extra heat* and significantly lower mileage

After the fact, I inquired with my pro-mechanic (a European rally circuit transplant) who is also of the opinion that pump formula 94 here in Canada is a close match to Euro 94. Perhaps it's just Canada, because to be frank - what little non-partisan info I could find on this side of the border was pretty dodgy.

Depending on how you look at - it could also be I getting false stats given I haven't been able to get back to installing a properly variable BPC - which should provide even greater range as regards octane if the system operates as described. However, what information I have passed along would have to be based strictly on Trionic timing and injector variables given boost is now fixed to a set curve.

You can take it as read that I am first person to assume it's all snake oil to begin with and that I was more surprised than anyone that I got the results I did. But I am passing along what I recorded given I did see relative improvements.

I will be repeating the tests this winter because they adjust the octane formula here in Canada as the country freezes over.

As a final note, I still think that the best thing anyone can do for maximum efficiencies from their fuel is to keep the plugs fresh and properly gapped (I run to the wider end of the range), install a fresh set of high quality air, fuel and oil filters as well as replace all the fluids with a race grade Synthetics (if your seals are up to it). It usually takes a couple of passes at things like the steering rack - but I think all these maintenances together really reduce inefficiencies in the power train.

Does it all end up being a 'true' cost savings? Given the price of quality supplies and fuel, it is *very* debatable that anyone is ever ahead of the game. However, I have to think that it does reduce overall stress on the engine and thus should increase the overall ROI before the rust really sets in in earnest.

Reply to
Dexter J

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.

Reply to
Johannes

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