Affect of timing on engine vacuum - D-Jet V-8 - PeachParts Mercedes-Benz Forum

Graham · #1 09-15-2009, 08:03 PM

I tried this in the technical forum, but no answers. Maybe it should have been here.

Technical question about timing vs vacuum on '72 350SL with the M117 D-Jet V-8.

I set timing to max advance which seems to work for this engine - about 5degBTDC at idle with vac.

I did not take meticulous readings, but I think obtained maximum vacuum at the above timing settings. I saw as much as 20" briefly, but I think it settled at about 18".

But, when I backed off on advance, the vacuum dropped off. I must re do this, but it seemed to me that with timing at TDC (with vac) at idle, I had low vacuum - maybe 10-12". If I had set it to 5degATDC, I suspect I would have seen less.

Does anyone know what the vacuum readings for this type of engine should be with different degrees of advance/retard?

Tomguy · #2 09-16-2009, 07:46 AM

I have no idea, I don't know if anyone does. I am pretty sure it isn't covered in the manual as vacuum isn't ever a critical or measured setting really... depending how open your AAV is, how full your door lock system is, and whether or not your AT modulator and brake booster are drawing or not are all factors that will affect vacuum.

You seem to be focusing on this a lot, are you sure your throttle plate is closing 100% at idle? Remember, any adjustment on that AAV air screw will make that vacuum number change. The system also opens the valve more or less depending on current engine speed/vacuum as well as engine temperature. You cant really get above 18" vac steady with the AAV in the system, it'll open under such force to keep the engine from stalling AFAIK.

Graham · #3 09-16-2009, 08:18 AM

Quote:

Originally Posted by Tomguy

I have no idea, I don't know if anyone does. I am pretty sure it isn't covered in the manual as vacuum isn't ever a critical or measured setting really... depending how open your AAV is, how full your door lock system is, and whether or not your AT modulator and brake booster are drawing or not are all factors that will affect vacuum.

You seem to be focusing on this a lot, are you sure your throttle plate is closing 100% at idle? Remember, any adjustment on that AAV air screw will make that vacuum number change. The system also opens the valve more or less depending on current engine speed/vacuum as well as engine temperature. You cant really get above 18" vac steady with the AAV in the system, it'll open under such force to keep the engine from stalling AFAIK.

I was interested in how engine timing affected vacuum because of a couple of reasons.

One was that it could be a measure of the CAM timing. I was concerned that vacuum dropped off very quickly when I retarded slightly. But a more careful test showed it be more or less normal.

Other is that at one time, some mechanics tuned cars using the vacuum instead of a timing light. They would advance timing until they got a maximum vacuum, then retard about 0.5-1psi.

My engine seems to follow this. Seems to run best with timing backed off just a little from maximum vacuum (and advance) that distr slot will allow. This turned out to be 5deg BTDC with vacuum.

Thanks for responding.

Tomguy · #4 09-16-2009, 09:38 AM

I would think that'd work well with a carbed vehicle - after all, letting it suck harder on the carbs will result in a better misting - but I don't think it'd be the case on an MPEFI car. I can see where it's coming from on doing that, but I wouldn't do it on the 4.5 myself. If it's running well like that, it's a good sign as long as you aren't getting any knock!

250 Coupe · #5 09-16-2009, 01:10 PM

My K jet car ran like poo until I cranked the timing up to 10-14 degrees advanced. It would barely idle, occasionally backfire and diesel when shut off. That's all gone now. I had 13in of vacuum at idle with accessories plugged and now can get as high as 16in.

There maybe something wrong with cam timing but more likely a hidden vacuum leak somewhere on my car.

Michael

Bill Adkins · #6 09-17-2009, 10:14 PM

Ok here's the deal. You are working on an early EFI car and manifold vacum is quite important as it effects the fuel mixture directly. The timing is set at a specific point and should be left there. 5 degrees advances isn't a problem providing the engine doesn't ping during hot hard acceleration....... very important! The manifold vacum is important as the pressure sensor, the device that controls fuel delivery senses the vacum directly and changes delivery accordingly. Now if the timing is set at factory TDC then 5 before at idle should be OK, the more advance means the more time the fire has to burn....... to a degree. You can crack a piston with too much, trust me on this one. Now after the timing is set and the throttle air horn is cleaned and the plate gap is set you can consider fuel mixtures. It will take an exhaust analyzer to do this part, remove the throttle switch plug and count the pins. If 4 are present then a seperate adjustable idle circuit isn't present.... early engines. If 5 are present then it is adjustable and the switch needs to be checked. Using an appropriate connector check the center pin to either the next up or down with a vom. I had a plug made, still have it after all these years. Anyway you want the idle signal to dissapear after about 2-3 degrees rotation. That being done you are ready for the next step. With engine running, warm and stable with crankase vent line disconnected at manifold you will unplug the throttle switch and check the CO reading. Factory setting was at about .4 percent.You can increase it to 1.0 without catalyst damage in the later cars. Now with that set you will plug the switch back in and check the CO and adjust to match values/ hence no off idle stumble. To set the value of the unplugged reading is another story. More in the future..... I ran a chassis dyno for 11 years, these are things I came up with and I may not have shared them before now. It's not easy getting old.
Bill

Graham · #7 09-17-2009, 10:38 PM

Quote:

Originally Posted by Bill Adkins

Ok here's the deal. You are working on an early EFI car and manifold vacum is quite important as it effects the fuel mixture directly. The timing is set at a specific point and should be left there. 5 degrees advances isn't a problem providing the engine doesn't ping during hot hard acceleration....... very important! The manifold vacum is important as the pressure sensor, the device that controls fuel delivery senses the vacum directly and changes delivery accordingly. Now if the timing is set at factory TDC then 5 before at idle should be OK, the more advance means the more time the fire has to burn....... to a degree. You can crack a piston with too much, trust me on this one. Now after the timing is set and the throttle air horn is cleaned and the plate gap is set you can consider fuel mixtures. It will take an exhaust analyzer to do this part, remove the throttle switch plug and count the pins. If 4 are present then a seperate adjustable idle circuit isn't present.... early engines. If 5 are present then it is adjustable and the switch needs to be checked. Using an appropriate connector check the center pin to either the next up or down with a vom. I had a plug made, still have it after all these years. Anyway you want the idle signal to dissapear after about 2-3 degrees rotation. That being done you are ready for the next step. With engine running, warm and stable with crankase vent line disconnected at manifold you will unplug the throttle switch and check the CO reading. Factory setting was at about .4 percent.You can increase it to 1.0 without catalyst damage in the later cars. Now with that set you will plug the switch back in and check the CO and adjust to match values/ hence no off idle stumble. To set the value of the unplugged reading is another story. More in the future..... I ran a chassis dyno for 11 years, these are things I came up with and I may not have shared them before now. It's not easy getting old.
Bill

Many good points there Bill!

One is that each time we adjust the timing, we could also be messing up the mixture

. So I guess we set to factory or maybe a bit more advanced if we think that is good, but then leave it alone! I have taken to connecting vac gauge while adjusting timing - It's not easy to read exact amount of advance you have, but with gauge it is easy to see that vac is OK (16-17" seems about right on my '72 car)

Disconnecting crankcase vent threw me a bit! How does that affect things? On my old car, I think it just directs fumes from fuel tank to crankcase and eventually via breather hose to throttle housing.

I have followed everything else you said but my meter was not too sophisticated. So I don't know where I am exactly CO wise. But I have it in shop right now - will be run in morning. Plugs say I am not far off, but we will see!

Thanks for your input Bill. Please keep us posted with more of your experience. This stuff stays here for years and everyone benefits.