Real Story on need Premium gas please

[Mac]

Do you like this logic?

If your MB engine was designed to run on regular, it would be less than 10:1 compession, e.g., 8.7; and, it wouldn't make any difference in performance to burn higher octane gas.

Since your engine has intelligent electronics, it can compensate for lower octane gas but the electronics cannot decrease your compression except by choosing a less than optimum timing arrangement so you get the worst of all worlds when you choose a lower octane: an engine that is capable of superior performance, performing less well than if it had been designed to run on regular to begin with.

Moreover, you risk engine damage when operating the vehicle in severe conditions such as at altitude, towing, and/or running in high heat with the a/c on full blast, etc.

Plus, in addition to sacrificing performance, you get less mpg operating at less than design spec so that decreases any "savings" due to a lower price--probably by 8% or less at current prices (usually there's not more than a 20-cent difference between low and high octane prices) so if you ordinarily would get about 19 mpg on premium, if you only get 18 mpg on regular, instead of saving 8% you're actually saving less than 3%, e.g., about $60 a year.[8D]

[tcp_ML500]

Mac - 5/17/2004 10:47 PM

Do you like this logic?

If your MB engine was designed to run on regular, it would be less than 10:1 compession, e.g., 8.7;
...

the electronics cannot decrease your compression except by choosing a less than optimum timing arrangement

As far as logic goes, I like the first statement.

The second is very questionable in what it implies since the electronics cannot increase of decrease the compression ratio.

The compression ration is calculated using the volume swept by the pistons and the volume of the cylinder heads cylinder heads. The complex surface of the piston head (domes, troughs) also comes into consideration, so does the volume (yes volume since it has thickness!) of the head gasket (usually measured with the bolts torqued at 20 lb.ft.

[jpdp.au]

But only on the highway did I notice better mileage. Around town, there is no difference

[Mac]

If a car is designed to most efficiently operate at tdc, for instance, and the engine managementment delays the firing until a few degrees after tdc, then vuala! the detonation takes place at less compression; the volume would be bigger.

[8D]

[tcp_ML500]

[Mac]

If you're saying than burning a higher octane fuel than the car was designed for is a simple waste of money, then I think that's true as earlier in the thread: my understanding is that if it's designed to burn regular, then burning premium doesn't provide more power; and, if it's designed to run on 91 octane, using jet fuel probably isn't going to make much difference either.

jpdp.au - 5/18/2004 7:45 AM

But only on the highway did I notice better mileage. Around town, there is no difference

[tcp_ML500]

In the US, we use the anti knocking index (AKI) which is (MON + RON) / 2

When you fuel up with 88/98 in Europe, that is equivalent to (88 + 98) / 2 = 93 AKI.

So your 98 may be equivalent to our 93, be aware of that and stick to 98 (unless it's 98 AKI which then would be a waste in your ML)

[jpdp.au]

On the highway it did seem better in terms of mileage.
It is so hard to compare one tank to another, unless you do the exact same trip and drive the same way.
The variation might be in driving style (eg: Rapid take-off's etc) rather than Octane.

[tcp_ML500]

95 RON might be OK, but you'll find that in your owners manual. I would personnally stick with 98 RON, which is what I get when in Europe when I drive in my family's other MBs (except on the CDIs for which I use diesel [:)]).

I would think that the variation you see is due to driving style, highway VS city and nothing else! It makes perfect sense to explain the variation that way, excluding all other factors, (assuming you re not at 5000 RPM (top gear) on the highway)

[timf]

seems this topic comes up every 6 months or so and results in two factions those who do and those who don't use premium.
My views are well known on this matter, but just for fun here they are again.
FYI I spent a good number of years working for Ford designing major parts of their ECU (EECIV) which was (is?) used on both their road cars and F1 racers.
One of the major factors affecting engine efficiency is ignition timing - but clearly other factors come into play such as compression ratio and design of the combustion chamber etc etc.
But with regard to ignition timing (which happened to be my speciality) the key is to always have the engine on the brink of detonation. This is without doubt the optimum point and where road engines normally run.
To make this happen the knock sensors are always sensing the onset of detonation and the ECU adjusts timing to keep the engine on this threshold. Thus there is always a very small amount of residual detonation - not enough to hear or be damaging. Occasionally - under extreme heavy loads at sea level or lower altitudes some detonation can be heard for a few seconds.
More sophisticated ECU's have individual knock sensors for each cylinder and can adjust individual timing to avoid this problem.
So what does this mean with regard to premium versus regular ? It means that the engine is experiencing the same amount of detonationation regardless of octane. (Closed loop control theory proves this and is backed up by data collection).
Yes using regular means slighly less power and slighty worse fuel consumption - but these are not a significant factor in MPG as driving style far out weighs any lost efficiency.
In any event the lost efficiency is only more apparent at higher output levels and not many drivers use that level of power on a sustanined basis. (Too many tickets !!)

Also at higher altitudes you can't get premium as there's no market for it - the compression ratio effectively drops because of the thinner air.

Premium only makes sense for very high performance high revving engines ie those running in excess of 10,000 RPM when the MIPs in the ECU might not have sufficient time to calculate the optimum firing point.

As a data point my ML430 runs on 89 octane as a compromise and has been returning 15.7MPG around town driving with no ill effects - it's now at 61,000 miles.

Indeed I have also run it on 87 octane for extended periods and not noticed / recorded any significant drop off in power or MPG.

As for my 911TT that always gets premium as its turbo charged and runs high boost levels - but even then I would have no qualms about regular gas should the need arise as I have confidence in the ECU to back off accordingly. I just couldn't tolerate the more noticable drop off in power - don't want to get beat off the line by a ML500 ;-)