What would be the resultant effects if I ran my '72 450SL without the ignition retard vacuum connected to the distributor ?

According to the manual the timing retard was installed in an meager effort by MB to control some of exhaust emisions with little real reduction in emissions.

So it seems to me that removal of the retard vacuum should not be harmful to environment or the engine, and would improve driveability and MPG.
The car is in fine tune, and runs much better without the vac retard.

The ignition timing with retard vac is: 5deg ATDC or 7deg BTDC wo/vac.

Greatly appreciate getting facts on pros and cons. Thanks

IS the vacuum retard on the side of the distributor side of the pot? I'll have to try that and see what happens....

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RIP: 80 300SD
RIP: 79 450SEL
2002 E430 4matic (212,000km)
2002 ML500 'sport'

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Hello Panzer,
The retard vac is hosed from the bottom of throttle body to a 2 way valve and ends up at the distributor vacuum canister. The vac is constantly applied, and is off on my year car only when the AC goes on or the coolant temp is at or above 212F or 100C.

Here in the USA, disconnecting emission control devices is technically illegal for vehicles registered and operated on public roads. That being said, so long as you are not experiencing detonation or pinging and the car runs fine without it, go for it. Hook it up for any emission inspections though.

The vacuum line running to the distributor advances the timing when you get up under load. I am fairly positive it is a vacuum advance. Most engines of the day have a vacuum adance.
When automobiles went to the computer then the computer advanced the timing from the crankshaft position sensor.
I hope someone can follow up and pitch in. I am assuming the D jet has never advanced the timing. It doesn't control timing through electronic or even vacuum.
It controls start up and delivery of fuel.
I happen to like the D jet system because they are easy to figure out. I am understanding the D jet as simple enough.

It's up to you, a lot of people remove the vacuum line and plug it, and set timing to 7° BTDC. I kept it connected as I could keep the idle speed lower without having to choke the AAV screw too low, as I'd suck up ATF when letting off the gas due to a ruptured modulator diaphragm!

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Current:
2021 Charger Scat Pack Widebody "Sinabee"
2018 Durango R/T

Previous:
1972 280SE 4.5
2014 Jeep Grand Cherokee Limited "Hefe", 1992 Jeep Cherokee Laredo "Jeepy", 2006 Charger R/T "Hemi"
1999 Chrysler 300M - RIP @ 221k

Thanks for your response.

Ive asked the question because I really do not see any need for the retard timing on the MB cars such as mine.

Did I miss an important piece of information ?
I have found no info. that saids the distributor vac advances the timing.

Would you point me to where you got your info.

From all that I have read and experienced the distributor vac. line on the 450SLs are to retard the ignition timing.
The vac line runs from the throttle port to a 2 way valve and then goes to the distributor.

In my '72 450SL, and many other models and years, the retard vacuum is canceled when the 2 way valve is energized. The distributor Vac unit is then vented to the atmosphere without vacuum. The timing advances when you disconnect the vacuum line from the distributor.
In the '72 450SL, the 2 way valve is energized only when the AC is turned on or when the coolant temp is equal to or greater than 100 C or 212 F.

At these events the mechanical advance is allowed to advance the timing without being held back by the retard vacuum.

So if I'm correct in my understanding, the vacuum being applied is constant and therefore, retard timing is always present as long as the 2 way valve is NOT energized.

The only explanation I can come up with is that the mechanical advance mechanism cancels out the vac timing, and takes over at higher than idle RPMs because the vacuum used for the retard is very weak ??

Hey Tomguy,

The reason for me asking the question is because I see no real need for the vacuum retard. Seems the retard timing is a slight coke hold on the getting the full potential out of the engine.
The Car seems to run better with just the mechanical advance, and of course be tuned correctly.

Ya but, it seems to me that the vac retard force is constant throughout the entire distributor curve. the vac is only canceled and distributor vac unit is vent to atmosphere. when the 2 way valve is energized.
( So, if you disconnect the distributor vac line the dizzy vac unit should not be plugged up, but opened to the atmosphere for unrestricted movement to occur.)

In my '72 the 2 way valve is energized when the AC is on and when the coolant temp is 212F or more.

Thanks

errrm the PC term for this is now "Special needs vacuum".

My recollection is that the vacuum retard is only activated at idle and only with the AC off, so it really doesn't have any effect on down the road running or economy.

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Berfinroy in CT
Present vehicles:
1973 300 SEL 4.5
1959 Rolls Royce Silver Cloud I
1959 Ford Thunderbird convertible/430
Past vehicles;
1958 Bentley S 1
1976 ex-Max Hoffman 6.9
1970 300SEL 2.8
1958 Jaguar MK IX
1961 Jaguar MK IX
1963 Jaguar E-type factory special roadster
1948 Plymouth woody
1955 Morgan plus 4
1966 Shelby GT350H Mustang

l have always wanted to take the spare distributor to the hot rod shop and have it re-curved / set for full mechanical operation. Greater retard / lighter springs in order to obtain better mileage and performance.
Has anyone tried?

Hi Berfinroy:

So what cancels the retard timing vacuum as you go down the road ?

Isn't the retard timing vacuum present as long as the 2 way valve is not energized? and therefore present through out the range of the distributor trying to advance the timing.

The vacuum to produce the timing retard is, at idle very small, so it would seem that the engine, as it gets to higher RPMs, would easily maintain enough vacuum to keep the retard timing as a negative force against the total advance timing the engine is trying to achieve ?

But the real questions are:

What is the purpose of the ignition timing retard? and

Would the engine suffer or benefit if the timing retard is not used ?

Seems like a black hole in the MB manual, the timing retard function is there with very little vague explanation for its presence.

scia63 & I have been exchanging PM's, but I though seeing this subject often comes up, that it might be just as well to post them here too:

The retard only is functional at idle and only then if the A/C is off, so it can't do any harm to not use it. Retarding does cause engine to run hotter, so it might be beneficial during warmup.

The retard doesn't do any harm and seeing it has nothing to do with car performance once above idle, I would just leave it there. If you have a leak in the vac tubing or the vac actuator, then you might have a small problem, but you can fix that.

I don't see mpg being affected much unless you are sitting at idle for long periods. And, I don't know what driveability is

What the manual refers to, is at idle.

As soon as you open the throttle, the idle switch in the throttle position switch signals to the ECU that the car is no longer idling and the retard is canceled. (BTW, when you take foot off pedal and coast with A/C off, the engine if under 100C, will also retard)

I have my car currently set at 5deg BTDC at idle without vacuum. I had it at 8deg, but it did not run quite as well. I checked vacuum while adjusting advance and found that backing off slightly from 8deg (the most I could get), gave a higher vacuum. But still only 16" HG.

So what happens when you pull off, is that as soon as you touch pedal, the retard is canceled. Then as you accelerate, the mechanical advance kicks in and eventually you have 27-30 deg of advance at about 3000rpm if you use the euro specs as many of us do, or 18-22deg if you follow the US specs.

By the way, it is just as well to check that the idle switch on the TPS is operating properly (test is in manual) Besides affecting the retard, it also affects the idle mixture controls.

Graham

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

Frank - You have this backwards. Try putting a vacuum gauge on your car.

BTW - You have three threads running on this subject plus the PMs. Why not try and keep it in one place so everyone can see the responses?

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

There are three engine signals that cancel the power to the retard solenoid. The throttle position sensor cancels the power when the throttle is opened past idle speed. The AC cancels the retard solenoid power when AC is switched on. The 100C engine temperature sensor cancels the retard solenoid power when engine temperature exceeds 100C. The retard solenoid only receives power to retard the spark at idle speed and only idle speed.

If you want your engine to run better, set the timing at 30 degrees advance at 3,000 rpm without the vacuum line connected. Re-connect the vacuum line and reset your idle speed to the book value. You will then have a smooth even idle and also when the AC kicks on or the temperature exceeds 100C your spark will still advance slightly to raise the idle speed and compensate for the higher AC drive loads or to lower the rising temperature. If you fail to use the retard feature then your engine will slow down and may stall at idle when the AC is turned on or it gets hot.