I bought a 2008 Rabbit 2dr/5spd as an urban driver, since I didn't want to beat up my Benz with 17 inch wheels, 45 series rubber and Bilstein HDs around town (correction - it was beating ME up!) I test-drove one on a whim and was blown away with how solid and competent the little bunny is. Anyway, some of the kids over at the Rabbit site are debating grades of gas, and other than increased detergents in some prem fuels, I have pretty much laughed at them for running 93 all the time ( I run Shell V-power every 4th tank simply for the detergent properties - this is also what I run in the Benz).

The car is designed for 87 octane. 2.5l 5cyl, 9.5:1 compression. HOWEVER, a VW tech pointed out the manual states "for maximum performance, use premium."

What gives?

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former: 83 300D, 97 C230, 93 400E

current: 08 C300 Luxury , 92 500SL

Most cars these days with EFI have knock sensors or such to retard timing as needed to compensate for increased volatility or faster burn of differing formulations of fuel. This allows them to run well and prevent engine damage with a wide variety of available fuel sources. However, these compensations - such as the retarded ignition timing - will reduce engine power efficiency if it is engineered to perform best with a higher octane fuel. The 9.5 compression ratio suggests this is likely the case.

An engine optimized for lower octane fuel (typically about 8 - 9 compression ratio) may not actually get any advantage from a higher octane, however.

Steve

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'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7

It's an economy car, don't waste money on premium gas for it. 'Taint no sports car.

I actually tried 89 octane in my 2000 Accord on this tank to see what would happen instead of 87 octane. The engine doesn't have as much kick and the mpg is probably going to amount to 22 mpg instead of 25 mpg around town.

I guess the ECU can't adapt the timing, in this case.

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1985 CA 300D Turbo , 213K mi

Put the word "Octane" in the search feature of this forum . . . been there, done that . . .

Then of course, there's the "dino versus synth oil" . . . "nitrogen versus air" . . . and the dreaded "K&N versus OEM" filter roundup.

With 9.5 compression you shure don't need a 93 octane fuel.
Even 91 is questionable.
And your Knocksensor is NOT going to have to retard timing.
Too bad you don't have access to a VAG tool ( VW diagnostic tool ), as you could actually watch your timing retard ( or NOT retard ).
As far as " The Kids " in the VW forums, hmmmmmmmm....we'll just leave this one as "No Comment ".

MTI
Are you into the spiked Coconut juice already ?

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2007 C 230 Sport.

Tis the season . . . and it's been raining all week.

[QUOTE=Ara T.;1696881]It's an economy car, don't waste money on premium gas for it. 'Taint no sports car.



Actually, it's a 170hp inline-5 with 170+ lb.ft of torque. That thing will smoke my C230! Also it is only rated at 22/30. The main reason I bought it was utility and urban parking/driving

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former: 83 300D, 97 C230, 93 400E

current: 08 C300 Luxury , 92 500SL

sorry if this is long.........ok, fuel octane numbers are the most misunderstood reason people use higher octane fuel.first off, the higher the number the SLOWER the burn of the flame front. 89 octane burns hotter than 91 octane in the same engine. thus the misconception if you use higher octane you get more power. not true. higher octane was developed for higher compression engines to control engine predetination, not to produce more power. thus using 89 will produce more power verse 91 in a lower compression engine. the reason Mercedes recommends 91 octane in some cars is because high compression and more ignition timing to produce the horse power they designed to to make. the higher the compression the more dense the fuel charge thus more power, but this also leads to predetination due to the the fuel charge squish wanting to burn sooner during the power cycle. compression ratio, timing and cam shaft all play into the factor of what fuel to burn in each engine. we found running 91 octane in the race engines made more power than 108 race fuel but created more engine temps during the long hard pulls. just a few thought before putting high test in the tank for no reason. if you want to go deeper into this pm me for more discussion. take this post as you will but i have burned up a few engines running the incorrect fuel in the application that required a cooler burn than what we built

Joe

As said above higher octane doesn't mean the fuel is "better" it just means its harder to burn. Some engines have higher compression, like most MB's and need the higher octane to prevent pre ignition, which on older cars you would hear as popping when you get on it.

On modern cars it doesn't matter the computer will adjust. However you will lose performance and milage as a result.

If the car is rated for 89 putting 93 in it will get you nothing other than a lighter wallet. But if it makes you feel good have fun.

As for different brands like Shell's "V power" that they promote the heck out of its all marketing hype. I got a chance to talk to an engineer that designed cleaner packages for gas awhile back. He said that every fuel will leave deposits. The key is to switch brands because each brand uses a slightly different package, which cleans out the deposits left by the others.

People make this so much harder than it needs to be. Run whatever octane your car is built for, and just buy from a bunch of different stations.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

Ya, that's what I say Hatteras, but it's just odd that the VW owner's manual would say to use premium for max performance. Weird!!!!

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former: 83 300D, 97 C230, 93 400E

current: 08 C300 Luxury , 92 500SL

I have been running regular in my '69 280 SL for years. It has 9.5 compression ratio and on a compression check it reads 195PSI. There is no knock sensor. The timing is adjusted to the book. It doesn't knock or ping even on hard acceleration. If it did, I'd use premium. My '95 E320 has 9.2 compression ratio. I'm also running regular fuel. But I never rev the engine much over 2500. At 60MPH it turns about 2400. Now, when I take the car to Germany on the autobahn, I'll buy BV Aral. "Freie Fahrt für ein freies Volk"

Its to keep at the kids that buy them happy.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

joseph_c, your explaination makes good sense and confirms what I have suspected for a long time. I wonder if you or anyone could clear up a similar question:

In my '77 280E owner manual, the recommendation for fuel is "Average octane and motor 87 (RON) 91." I am guessing the first part is a German measurement of octane and in brackets is US of 91. Would you agree?

It also seems a bit strange that the 110 engine in this car, which has a compression ratio of 8:1 would require 91 octane. I am able to, however, run regular unleaded with absolutely no pre-ignition and no need to retard the timing. Any ideas??????????

There are 2 ways of rating octane numbers.
RON = Research Octane Number.
MON = Motor Octane number.
These ratings and their " use ", can vary from country to country.
In North America we use the formula of RON + MON : 2.
In Europe they still use the single ( I believe it's RON ) rating.
An engine with 8:1 compression ratio sure does NOT require anything higher than 87 ( our rating ) fuel.

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2007 C 230 Sport.