hi im new here. just wondering if this car can run on regular fuel. or needs to be 91
thanks

 

SLK 430? Do you mean CLK430? I would stick to the highest octain you can find. You can run them on lower octain you will just see a differance in performance. stick with the 91 oct.

 

could be eather or. its my sisters car. were just in a argument casue shes afraid to put lower grade incause it will hurt it.im telling her its fine. see im old school car guy and i know that octain on lets say a chevy really only matters on timing and compression. and im sure the compression isnt high enough to need 91

 

Hi there,

Fuel does indeed impact compression and ignition timing.
In the old days compression was relatively low and ignition timing was fixed in order to give the desired performance / fuel economy.
You needed to use the fuel recommended by the manufacturer otherwise you risked wrecking your engine.

Today things are much more complex.
Engines run (in general) at much higher compression ratios and timing is determined dynamically by the engine ECU.
Due to the demands of increased performance, gas milage and legislative requirements the engine management intelligently responds to a number of inputs like engine temp, power requested requirements etc and adjusts the timing to give the best compromise whilst ensuring emissions remains within the legal envelope.

Now fuel impacts this whole process.
As the fuel RON (Research Octane Number), or just octane to most people, reduces the propensity for the engine to "Pink" / "Ping" / "Knock" increases.
This can destroy an engine (burns holes in pistons etc) especially if it occurs at high speed / engine output where it may not even be audible to the driver.

As a consequence the engine listens for this condition and retards the ignitions to avoid it occurring.
In retarding the ignition you loose performance and suffer worsened gas milage.

So modern engines are designed to run on a specific RON to give optimum output.
If however poorer fuel is used the engine will protect itself at the cost of reduced performance.

So you CAN use a lower RON fuel (if the recommended grade is not available) BUT IF THIS IS DONE TO SAVE MONEY AT THE PUMP IT IS A FALSE ECONOMY.
The reduction in performance MORE THAN OFFSETS any saving.

Stick with the manufacturers recommended octane rating.

Hope that gives you a better understanding of the situation.

Stuart

 

that helps kinda thanks. i guess im still just cant belivethat a factory car needs 91. (besides it goes against what my auto teach taught us) lol oh well thanks again for all the info

 

In the UK we use 95 RON (regular) and 98 RON (premium)
Nearly all modern UK cars use 95 but the gas companies promote 98 in order to raise additional revenue.

Usual marketing BS!!
Gives extra smooth running, increased gas mileage, easier starting etc.
98 does give some of these benefits but the improvements do not warrant the additional cost unless cost isn't important to you.
...and don't forget gas is in the order of twice as expensive in the UK vs USA.
Was £1.30 / litre. Now dropped back to £0.98.
£/USD was over 2.0. Now something like 1.40

Stuart

 

is the rating system the same as over here or is your 98 like our 91

 

I have read that the dealer can make an adjustment to allow the slk engine to run on lower octane. Otherwise, the engine will not be efficient at lower octane. Others have stated that when you use octane lower than the manufacturer recommendation, you get lower gas mileage with the lower performance. Therefore, you can actually spend more on fuel and hurt your engine.

 

You can safely run the car on lower grade fuel for a period of time BUT YOU WILL NOT SAVE MONEY.

The dealer cannot overcome this situation.

Incidentally RON is an international Standard. i.e. USA fuel 95 RON is the same as UK 95 RON. This doesn't mean that the fuel is from the same source / crude etc just that when in an engine it behaves in the same way regarding "knock" propagation. This is what limits ignition timing / compression which in turn limits available performance.

OK? More info available at e.g. Wiki if you Google RON Octane.

Stuart

 

In North Carolina, USA the octane at the pumps is 89, 91, or 93. 93 is generally known as premium. 89 is regular. This discussion got me to check my owner’s manual. It recommends 91. The gas flap tag just recommends premium. Yippee, I am dropping down from 93 to 91.
Why do European models need 95?