280SE 4.5 Ignition Timing

[Bezant]

Hi all,


I am planning on doing a valve adjustment and Pertronix upgrade this weekend. I know timing is a necessary step in both of those, so I figured I'd peek at it tonight and get familiar.



My idle, at temp, with vacuum was around 7 BTDC. Factory spec is 5ATDC as far as I can tell.


Car has always run pretty strong but low mileage (8-9mpg mostly town driving).



I set it back to 5 ATDC just to see how it runs. But I'm expecting this will make my mileage even worse?



Speculating that PO may have advanced it to try and fight the same MPG issue.


Hope some experts can weigh in and also give advice on best timing from a performance / mpg standpoint.

[Tomguy]

Retarding the timing by 12 degrees will hurt your MPG and your high-end power. Keep in mind that when those cylinders are firing at 6000 RPM, it takes the fuel just as long to burn as it does at 600, so it needs to be ignited much sooner.

Does your distributor's centrifugal advance seem functional? You should be able to turn the distributor rotor clockwise. It will then "snap" back to the original position. If one (or both) of those doesn't happen the mechanism is likely bound up, meaning you're not getting the 45 BTDC needed at 4500 RPM (If memory serves for both of those values) and as a result, you're going to be eating gas.

[Frank Reiner]

B:

Do the valve adjustment first, and preferably a compression check also. The compression check is to insure that there is not a very low cylinder.

This method of setting the ignition timing requires a vacuum gauge and a timing light:
1) Locate a source of manifold vacuum (not throttle body); it may be used for the distributor vacuum advance, and may pass through a solenoid valve. Connect vacuum gauge to manifold vacuum source.
2) With the engine warm and idling, and vacuum disconnected from the distributor, rotate the distributor small amounts in both directions to determine the point at which manifold vacuum is greatest. Tighten distributor clamp. With the timing light, observe total advance (w/o dist. vac.) at 3000-3500 RPM. Total advance should be ~35-37 deg.
3) Reconnect dist. vac. Test drive. There should not be any pinging on acceleration. If any pinging, retard timing ~2 deg.

[Bezant]

Originally Posted by Tomguy

Retarding the timing by 12 degrees will hurt your MPG and your high-end power. Keep in mind that when those cylinders are firing at 6000 RPM, it takes the fuel just as long to burn as it does at 600, so it needs to be ignited much sooner.

Does your distributor's centrifugal advance seem functional? You should be able to turn the distributor rotor clockwise. It will then "snap" back to the original position. If one (or both) of those doesn't happen the mechanism is likely bound up, meaning you're not getting the 45 BTDC needed at 4500 RPM (If memory serves for both of those values) and as a result, you're going to be eating gas.

Rotor seemed to work as you described.



I drove on the 5atdc this morning. Felt awful, extremely sluggish at low speeds. Got some coffee, drove it home and the drive home felt similar to 7btdc again.


(clocked my mpg for a 40 mile freeway drive with a little bit of town and got 12.75).


Got pertronix in tonight no problem, will do valves this weekend.

[Bezant]

Originally Posted by Frank Reiner

B:

Do the valve adjustment first, and preferably a compression check also. The compression check is to insure that there is not a very low cylinder.

This method of setting the ignition timing requires a vacuum gauge and a timing light:
1) Locate a source of manifold vacuum (not throttle body); it may be used for the distributor vacuum advance, and may pass through a solenoid valve. Connect vacuum gauge to manifold vacuum source.
2) With the engine warm and idling, and vacuum disconnected from the distributor, rotate the distributor small amounts in both directions to determine the point at which manifold vacuum is greatest. Tighten distributor clamp. With the timing light, observe total advance (w/o dist. vac.) at 3000-3500 RPM. Total advance should be ~35-37 deg.
3) Reconnect dist. vac. Test drive. There should not be any pinging on acceleration. If any pinging, retard timing ~2 deg.

Interesting..won't have access to a vacuum gauge until next week. Could you use the vacuum going to the door locks for this?

[Tomguy]

Yes, you can use the line right at the firewall, before the check valve to the door lock system. Any point after that, no. So essentially, disconnecting your vacuum locks to use that as your test line.

[ejboyd5]

Originally Posted by Tomguy

Keep in mind that when those cylinders are firing at 6000 RPM, it takes the fuel just as long to burn as it does at 600, so it needs to be ignited much sooner.

Excellent concise description as to the need for an advance curve in a distributor. Everyone with an interest in internal combustion engines should make this part of their understanding.

[cth350]

another spec to watch is the dwell at different RPMs. The book says if it wanders more than 3 degrees, your distributor shaft is toast.

-CTH

[Bezant]

Originally Posted by cth350

another spec to watch is the dwell at different RPMs. The book says if it wanders more than 3 degrees, your distributor shaft is toast.

-CTH

put pertronix in so no dwell lol.

[Bezant]

I used the vacuum method and ended up going to -10 (-20 with the distributor vacuum disconnected). No pinging so far on 87.

My mechanical advance was only going up to -30 though.

It's in the bottom of the distributor right? Is it most likely just gummed up? Any advice on that job?

[Frank Reiner]

Originally Posted by Bezant

I used the vacuum method and ended up going to -10 (-20 with the distributor vacuum disconnected). No pinging so far on 87.

My mechanical advance was only going up to -30 though.

It's in the bottom of the distributor right? Is it most likely just gummed up? Any advice on that job?

B:

The use of a sign(-) is a bit unconventional. The ignition firing point is usually designated as being Before Top Center (BTC), or After Top Center (ATC). We could assign a + sign to firing BTC, and a - sign to firing ATC.
Since it is not likely that the firing point retarded (movement ATC or -), your result of movement from 20 to 30 with increasing RPM should be understood as advancing from 20 degrees BTC to 30 degrees BTC.

A vacuum peak at 20 deg BTC is not a surprise; failure of the centrifugal mechanism to advance past 30 deg BTC @3500 RPM is a surprise. Did you check for free movement of the rotor as was suggested in a prior post? The rotor should rotate smoothly about 10-15 degrees CW with finger pressure, and then snap back when released.

It would be helpful if you could post a couple of pics of the distributor from different angles.

[Tomguy]

My thought is that if you're only getting 10-20 degrees of mechanical advance (I think 10 as the manifold vac that retards the distributor is bypassed off idle but may be wrong), that there are two possible causes:
1) The springs are weak, so it's already advancing too much at idle,
2) The mechanism isn't clean and thus isn't able to advance far enough.

If you can't get enough mechanical advance you may need to remove the distributor to clean it well. I had to do that on mine, and once I did, higher RPM driving was much livelier than beforehand.

[vwnate1]

So far all the comments are good -but- you really should check the FSM as some old Mercs use the ATDC timing point, it's important as the ones that do use manifold vacuum to the advance biscuit not ported vacuum like most American cars.....

This threw me for a loop with my 1975 350SLC graymarket but it worked out quite well in performance and cool running engine .

I never passed 15MPG's in it but it never pinged, knocked nor failed to easily spin the rear wheels on demand .

Late ignition timing helps make a dead smooth idle, as long as it begins to advance when you open the throttle it'll be right, *do* get an advance timing light so you can check the full advance, all in timing too .

[Bezant]

Originally Posted by Frank Reiner

B:

The use of a sign(-) is a bit unconventional. The ignition firing point is usually designated as being Before Top Center (BTC), or After Top Center (ATC). We could assign a + sign to firing BTC, and a - sign to firing ATC.
Since it is not likely that the firing point retarded (movement ATC or -), your result of movement from 20 to 30 with increasing RPM should be understood as advancing from 20 degrees BTC to 30 degrees BTC.

A vacuum peak at 20 deg BTC is not a surprise; failure of the centrifugal mechanism to advance past 30 deg BTC @3500 RPM is a surprise. Did you check for free movement of the rotor as was suggested in a prior post? The rotor should rotate smoothly about 10-15 degrees CW with finger pressure, and then snap back when released.

It would be helpful if you could post a couple of pics of the distributor from different angles.

I'll get a video of the rotor movement and a couple pics.


I've never really felt a lack of power at high rpms, but you never know it could always get better.


I roughly timed my 0-60 at 9.5 seconds with the new timing 10btdc.

[Benz Dr.]

Originally Posted by Bezant

I'll get a video of the rotor movement and a couple pics.


I've never really felt a lack of power at high rpms, but you never know it could always get better.


I roughly timed my 0-60 at 9.5 seconds with the new timing 10btdc.

I set most 4.5's at 10 degrees BTDC or they won't make good power. I would also check your ignition wires and make sure there isn't any carbon core stuff in your system.

After rebuilding distributors for the past 25 years, I can say that even small things can have big effects. The ones that used CDI units tend to be worn the worst due to a lack of servicing. The points in that type of system last a really long time and the 4.5 was often used with CDI.