Timing question

[Bruce Allert]

The last time I timed the 914(recently) I didn't pull the vacuum hoses off the distributor. When doing this do you plug the hoses or the ports on the distributor or both?
Also, why is this done? I haven't been able to find an explanation for this anywhere.
bruce

[pbanders]

Just pull them off of the dizzy, they don't need to be plugged. The issue is that you want to set only the mechanical advance during the timing process, and don't want any contribution from the vacuum mechanisims. For the most part, the vacuum units are pretty much inactive at 3500 rpm, so you'll probably find your timing is OK.

[seventythree]

As much as I hate to contradict Brad, in my experience, vacuum advance is very much active at high RPM. Bottom line, you should have both vacuum lines disconnected to properly set the mechanical advance.

http://www.pelicanparts.com/techarticles/914_timing/914_timing.htm

[Bleyseng]

If the vacuum advance is soo important why is it absent on most 2.0L cars. The retard is for idle only so it does nothing for timming at 3500.
The mechanical advance is all that matters on the djet distributors. I plug the retard hose when I time a car.
Geoff
(Wow, the big 500!)

[Andyrew]

Is it possible to get the vacume advance to work with carbs? can anyone clue me in? just installed a Vacume advance distributor with electronic ignition (got it at a swap meet for 20 bucks, looks new, crane 700 at that) and i hooked up the vacume, i think, but there isnt much vacume. i had a 009 in their before, and it still feels really weak, like its making 70 hp, maybe its the 36 webbers in their but still. anyone? thanks

[seventythree]

Vacuum advance is vacuum based and may or may not be present depending on the position of the throttle plate, as well as engine RPM. Mechanical advance is based on the rotational speed of the motor and should be present, in proportion to engine RPM, regardless of the throttle plate position.

In regards to setting the mechanical advance, the importance of vacuum advance is irrelevant; however, it is important to set the mechanical advance correctly without the presence of a vacuum related offset.

[pbanders]

73, I think you're right. Here are the charts from the Bosch Ignition Technical Guide:



OK, this is pretty complicated, so here we go...

The two lines on the bottom graph correspond to (1) part load (partially closed throttle) and (2) full load (WOT). Under full load, manifold vacuum is low at all engine speeds and never exceeds the vacuum threshold where the vacuum advance mechanism is activated. Therefore, the advance curve is solely determined by the mechanical advance (curve 2 on the top graph).

For the particular case of part load shown here, the throttle is partially closed and engine vacuum peaks at some particular engine speed, which depends on the part load level and the breathing characteristics of the motor. In this case, the peak is a bit over 2000 rpm. When the vacuum reaches the onset level for vacuum advance (first circle at about 1000 rpm), the vac advance mechanism is activated and begins to increase the advance as the vacuum builds. At the second circle, the advance mechanism reaches its mechanical stop, and the total advance is equal to the max vacuum advance plus the mechanical advance as a function of engine speed (line 1 on the top graph).

As engine speed increases at part load, vacuum begins to decrease because the throttle needs to be opened more. Eventually the vacuum drops back to the region where the advance mechanism isn't up on the limit stop, and it begins to decrease (third circle). Eventually, to get the engine to attain near maximum speed, the throttle is nearly full open, and vacuum drops below the onset level, and you end up back on the mechanical advance curve.

For a 2.0L 914, the onset level is about 100 mm Hg, and maximum advance of about 7 deg. is reached at about 150 mm Hg. As 73 says, these levels are likely to be reached during the setting of the advance at 3500 rpm, and the vac advance definitely should be disconnected.

Personally, I haven't seen much effect, but I'll look for it next time I set my timing.

[pbanders]

Originally posted by Bleyseng
If the vacuum advance is soo important why is it absent on most 2.0L cars. The retard is for idle only so it does nothing for timming at 3500.
The mechanical advance is all that matters on the djet distributors. I plug the retard hose when I time a car.
Geoff
(Wow, the big 500!)

I've wondered about this. The effect is significant, over 10 deg. on the 1.7L's and about 7 deg or so on the 2.0L, and active across a fairly large range of load and engine speeds. The Bosch Ignition guide states the need for vacuum advance is that the air-fuel mixture at a lower load (i.e. higher manifold vacuum) is less dense and burns more slowly, requiring additional advance for optimal combustion.

I'd suspect that the real reason the advance is disconnected on later motors (and that the vac advance port on the throttle body is missing) is that more advance results in higher NOx and HC emissions, so they removed it on the later motors that had stricter emissions requirements.

It would be interesting to see the dyno curves of a motor with and without vac advance active. I'd suspect that the one with advance would have fatter mid-speed torque and lower fuel consumption.

[Bruce Allert]

Well... thank you very much... all of you...Brad, 73, Bleyseng. I asked for the why and boy, did I get ti. Now I just gotta go eat, digest then sit here and read 73s & Brads answers till they make sense. BUT, I did go out & re-time with the hoses off & plugged. Had to move the dizzy ever so much but, none the less, I did have to move it. Now I just have to deal with the idle not dropping down. Probably that con found AAR. Was just reading up on how to clean it. I can take the hose off the decel and the rpms go right down to where they're supposed to be.
thanks again guys
bruce

[pbanders]

RE: '73's comments on contradicting me...

Guys, feel 100% free to contradict me any time you think what I'm saying is wrong. I'm not an expert on automotive engineering by any means, I'm just a 914 owner who has stuck my nose into what's going on with the FI system on the D-Jet cars. This forum is a great place for all of us to learn about our cars, me included.

[Zeke]

Originally posted by Andyrew
Is it possible to get the vacume advance to work with carbs? can anyone clue me in? just installed a Vacume advance distributor with electronic ignition (got it at a swap meet for 20 bucks, looks new, crane 700 at that) and i hooked up the vacume, i think, but there isnt much vacume. i had a 009 in their before, and it still feels really weak, like its making 70 hp, maybe its the 36 webbers in their but still. anyone? thanks

Well, your question seems to have gone unanswered so far. The way I see it is how you hook up your vacuum. The Weber setup doesn't have a plenum like a lot of engines where there is a common vacuum created by all cylinders. If you hook up a vacuum line to all four ports on your Webers with t's, you may pull enough continuous vacuum to sustain advance on your distributor. This has been discussed by Dave D. and others, you might do a search on *carburetor advance* before taking my word on the subject.

(Wow, that was post #914, I'll have to remember that one.)

[Bruce Allert]

Milt, congrats on the 914 realization! was there a revelation???
bruce

[Bleyseng]

Brad, It would be interesting to see the part load hp test on a dyno. All the chassis dyno work I have done has been done at WOT.
I had heard that Porsche dropped the vacuum adv because of emissions. Looking at the chart if I read it right, then you could be at 35 degrees of advance at 3500 rpms, part load. Then the adv drops back down to 27 degrees as the rpms go higher.
I run as much adv as I can get without pinging. I set my timing at 32 degrees at 3500 rpms but I have a later style TB. I also have made sure my A/F mix is right on so I don't run lean.
Geoff

[pbanders]

Originally posted by Bleyseng
Brad, It would be interesting to see the part load hp test on a dyno. All the chassis dyno work I have done has been done at WOT.
I had heard that Porsche dropped the vacuum adv because of emissions. Looking at the chart if I read it right, then you could be at 35 degrees of advance at 3500 rpms, part load. Then the adv drops back down to 27 degrees as the rpms go higher.
I run as much adv as I can get without pinging. I set my timing at 32 degrees at 3500 rpms but I have a later style TB. I also have made sure my A/F mix is right on so I don't run lean.
Geoff

Geoff, here's a URL to a .pdf of the Bosch Emission Control Technical Instruction guide (thanks to the nameless person who posted this):

http://www.wrenchead.ca/pub/Bosch-Emissions.pdf

Look on page 10 and you can see the problem. Advanced timing gives you the best fuel economy and torque, up to the point where you get predetonation (knock). However, more advance causes higher NOx and HC. Higher HC is caused by lack of secondary reactions due to lower exhaust gas temperatures, and higher NOx is caused by higher combution chamber temperatures - seems kind of contrary, but that's what the guide says (see p. 11).

If you have the later-style TB, you might gain part-load torque by converting to a '73 TB and using vacuum advance, at the expense of emissions.