My 1973 450sl has a M117 motor and in checking the timing I ran across the attached page in my manual. Can anyone tell me the difference between the "Adjusting Value" and the "Installation Value"? There's no way I can set the timing to 5deg. ATDC as shown in the "Adjusting Value" column. The engine quits right at about TDC. So I assume I should be aiming for the "Installation Value" 7deg. BTDC. Frankly the car seems to run best when I set the idle timing at about 12-15 BTDC.

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The recommended timing values are merely to meet emissions requirements - they really ruin your performance. When a friend and I did the timing on my '79, we both agreed that the "suggested" timing (O degrees) was so absurd that the car wouldn't even run properly at idle!!!

Here's how you do it. Since the MB balancer has degrees on it, this is easy.... You want the "all in" timing to be about 36 degrees BTDC. By "all in" we mean the timing value when the engine is at higher rpm (generally about 3500 rpm), the point at which the timing advance is no longer advancing. (Be sure the advance mechanism is actually working.) I know this sounds radical to some, but it's how hot rodders have done it for decades!

Get a helper to rev the engine while you work the timing light. Have them increase revs to the point where the timing mark stops moving. Dial in 36 degrees BTDC. (Obviously, you should do this rather quickly. Don't take several minutes to figure it out! Repeat as necessary.)

Once you've dialed in this timing value, you need to drive the car and check carefully for any pinging. If there is pinging, back the timing off a couple of degrees and try again. Repeat until satisfied.

Need I mention that my 450SL goes like a scalded cat running from the bathub?

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'01 SLK320, '79 450SL & '01 C320 -- What? 3 Mercedes? I am DEFINITELY crazy!!!

An interesting theory. Since there's no vac advance, I guess I could get the same results setting the timing at idle or say maybe 1500. Then drive around and listen to see if I need to back off. Too bad this page from the "book" is so damn cryptic.

Thanks!

Not a theory! This method was originally taught to me by a GM engineer (now a rocket scientist for Martin Marietta).

There may be no vac advance on the '73, but there should be mechanical advance. I seriously doubt the D-Jet computer was sophisticated enough to handle engine timing. Try the above suggested method first. You should see an advance in timing as rpms increase.

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'01 SLK320, '79 450SL & '01 C320 -- What? 3 Mercedes? I am DEFINITELY crazy!!!

To do the “instillation value” without vacuum. Does that mean you pull and plug the vacuum hose connected to the vacuum advance? Then adjust timing to 7 degrees?

I’ve got a 1974 450SL and the chart shows 7 degrees btdc.

When I pull the vacuum from the vacuum advance on the distributor the timing mark changes to about 20 degrees btdc? Is this correct?

Or should I set the timing to 5 degrees atdc with vacuum hose attached THEN unplug the vacuum line from the vacuum advance and plug the line. Then adjust to 7 degrees?

What is the best timing for this model. It runs at 5 degrees atdc. But like people say it runs better at btdc.

Also when I use my multimeter turned to dwell reading it just shows 45 degrees.. it doesn’t move. Even my timing light has the ability to read dwell but also says 45 degrees. It doesn’t change. Am I doing something wrong?

Thanks
Josh