Mercedes-Benz Pontons
and Fuel Octane Ratings

By Stu Ritter / / Denver, Colorado

Doug Broome wrote:

Note the item on page 88 of the September/October, 2003 issue of The Star magazine. There, find a discussion by Stu Ritter regarding octane ratings based on RON (Research Octane Number) and MON (Motor Octane Number).  Ritter writes, The old 91 RON is equal to today's regular gasoline, with an octane number between 85 and 87 (RON+ MON divided by 2). "This is more than ample for an engine with as low a compression ratio as 8:1."

What does that mean for our Pontons with the 6.8:1 to 8.7:1 compression ratios?  The 220S manual specifies 93-95 octane fuel rating according to the Research Method (ROZ).  How does 93-95 ROZ equate to today's octane expression?

Stu Ritter answers:

ROZ stands for Reguläre Octan-Zahl which translates to Regulation Octane Number.  ROZ is the German equivalent of RON.  93 - 95 RON (ROZ) fuel was fairly low octane in its day.  If you remember, or if you don't remember, octane up to 104 and 106 were available at the pumps.  An 8.7:1 compression ratio is considered low.  There would be no need for premium fuel in such an engine.  In the early 1960s, American engines were running as high as 10:1 or 10.5:1 compression ratios for street use.  These engines did not have knock sensors.  While the cylinder head design of the Ponton engine is fairly archaic and can cause pre-ignition from hot spots, modern fuels have excellent knock prevention.  If I owned a Ponton, I would probably run it on middle grade fuel. At altitude, here in Denver, Colorado, I would even think about running it on regular.  You are not going to get an 8.7:1 engine to ping on today's fuels.

Recommended Fuel Octane for Mercedes-Benz Pontons

Model Compression Ratio Recommended octane (R+M/2) from the pumps
180a 6.8:1 89
180b 7.0:1 89
180c 7.1:1 89
190 7.5:1 89
190b 8.5:1 89
220a 7.6:1  89
219 7.6:1 or 8.7:1 89
220S 8.7:1  89
220SE 8.7:1  89
190SL 8.8:1 89
All engines in the Ponton series will run on regular, except for the vapor pressure problem. I would put them all on middle grade fuel.  There is absolutely no need for premium in any of these cars.  The 190SL might be the only exception and that is because they were over carburated with little control, and they can hot spot pretty easily.  I would still run the Type 190SL on middle grade fuel unless or until they showed signs of pre-ignition.

- Stu Ritter / October 1, 2003

Created: September 30, 2003 / Jeff Miller
Thanks to Stu Ritter / Doug Broome / Len Sokoloff

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