Tom Rathjen

Hello again. I hope everyone is starting to find time away from work for the holidays (and finding more time to tune/tweak/fix/pamper their Porsches).

Now that I have my '82 928 recently made running and road legal, I'm transitioning to the task of trying to make it run GOOD. First on the list is to try to solve the "stumbling" problem at RPM over about 3K. I've been reading some other threads, and based on those I bought a timing light and took a look at my ignition timing. And I got strange results (so either I am confused...very possible....or there is something wrong).

When I remove and plug the two vacuum lines going to the distributor, and set the timing to 23 BTDC at 3K RPM, I observe the following: At idle, the timing is retarded behind 23 BTDC by about 13 degrees (it's at about 10 BTDC). Then, as the RPM is gradually increased, the timing advances until it "lands" at 23 BTDC a little before 3K RPM (where I set it), and stays there till full throttle (about 4K....which seems low).

When I re-attach the two vacuum lines to the distributor (which are going to ports at the front of the throttle body), I observe the following: at idle, the timing is advanced about 7 degrees from 23 BTDC (it's at about 30 BTDC). As the RPM is gradually increased to about 2.5K, it advances further to about 40 or 45 BTDC. But then it gets weird: When I continue to open the throttle beyond about 2.5 RPM, the timing abruptly retards to about 23 BTDC (and the RPM suddenly drops to about 2.1K). And it stays at about this spot till full throttle, which is about 4.2K (still seems low). Doesn't seem like it should be doing this. Bad distributor or vacuum advance? Or would this have something to do with the ignition controller (which I know nothing about how it functions or what it does).

Also....it doesn't seem to make any difference if the vacuum line that attaches to the lower part of the distributor is connected or not. It is only the top one that seems to do anything.

Also, for reference, the job before this was replacing the timing belt (twice...since I got the wrong round-tooth one the first time). So I know the cam timing is set correctly. I don't think this is timing behavior is something I just caused, but rather has been happening all the time I've had it since getting it running. (This car was purchased from ebay in May, and it was reported to have been sitting for 5 years prior.)

Thanks for any of your thoughts....

belgiumbarry

that's something i also have to figure out still...due my carb setup , and even with the Webers i don't have "ported" vacuum connection as on a Holley carb....

now what i know from reading racing prep's on chevy's is that they want all advance in at about 3000 revs... staying there till max revs. Which is about 36° advance. In the Porsche manual i also see that 36° number, as a max advance.

Now the OEM distributor gives between 13 and 15° mechanical advance form idle to 3000 revs... and hold this further steady.

So only using this ( no vacuum lines ) i should point for 21° at idle, giving me max 36° at 3000 and further.

The distributor has 2 vacuum lines : on top it's timing advance , on the back it's timing retard.

First : timing advance : this is all about emissions , mpg etc... i think. What this does is advancing the timing when the engine is at high vacuum, so with near closed throttle... giving a other fuel mixture which can be ignited sooner ( =more advance ) to reach max explosion force just after TDC... But with the pedal down , you don't have vacuum high enough to operate the vacuum advance , so this is doing nothing at that driving condition.

So for my "racing" application i can forget that vacuum thing ... again, i think.

Second : timing retard... to be honest it's the first time i see this on a engine...

i see the diagrams in the manual but noy explained what "vacuum" they take for it , nor do i know it's function... sorry.

"What vacuum ": you have the full engine vacuum as in the manifold , but also "ported" vacuum : vacuum that only gets trough on certain positions of the throttle blade... as on a Holley carb.. not using the injection stuff i didn't figure out (so far) what this retard function does and when...

blandis

Have asked the same for my 84, I get timing readings all over the place. I was trying to solve a gas ping problem, and normally timing being advanced can cause it.
Seems that few on here have ever timed their car. From what I found once its back together after the TB change, just snug the distributor down and go.
Sorry I cant be of help, but would love to see the answer and proper procedure.

WallyP

The vacuum advance (front connection) should start at about 300 mbar (about 9" Hg) and go to a max of about 10 crankshaft degrees at about 450 mbar (13" Hg).

The vacuum retard (rear connection) should start at about 160 mbar (5" Hg) and go to a max of about 8 crankshaft degrees at about 200 mbar (6" Hg).

The centrifugal advance should start at about 600 RPM, and should max out at 23 crankshaft degrees at about 3100 RPM.

The retard should have vacuum at idle, and should retard the timing at idle by 4 to 8 degrees. The vacuum advance should have little or no vacuum at idle, and when it gets vacuum will overpower the retard, giving 8 to 12 degrees advance.

At idle, the centrifigual advance should be started, with about 8 to 10 degrees.

This complicated mess means, if I haven't made a mistake, that you should see perhaps 4-8 degrees advance at idle with everything connected. At 3100 RPM, the retard should be gone, the centrifigual should have added 23 degrees, and (with no engine load) the vacuum should have added 8 to 12 degrees, so you should see about 35 degrees advance. This is a very common advance figure for all performance engines.

You should never have more than 36-38 degrees advance.

If you are getting weird numbers, one thing to check is the positioning of the crank dampener ring. Pull the #1 spark plug, and use a long wooden dowel or similar tool to check for TDC. You should be at the TDC mark on the dampener when the piston is actually at TDC...

Glenn M

Wally,
Nice post as usual!

Glenn

belgiumbarry

yes, as compared to chevy or most cars , in fact we are starting the 928 engine ( no vacuum ) with a bit more advance then it gets once idling... should that be the reason for the retard function ?

Tom Rathjen

Thank you for the posts.

Sounds like my mechanical advance is working correctly.

I suspect my vacuum line routing is messed up, which is screwing up the vacuum advance. It wasn't right when I got the car (lines missing), so I tried to follow a diagram that was enclosed in another post. But maybe it wasn't right or I didn't follow it correctly.

I can experiment to see which ports on the TB have vacuum at idle, and which has vacuum at higher RPM ("ported"?). But can anyone describe which TB port should be for the distributor retard and which is for the distributor advance?

Thanks!

belgiumbarry

if they are right here, it's easy : the line with vacuum on idle should go to the back fitting ( retard) and the one with little or no vacuum to the front ( advance ) ... on a idling engine !

SharkSkin

Wally explained it pretty well. Not much to add beyond stating things in a slightly different way to help you understand.

The retard line that must have vacuum at idle goes on the fitting closest to the distributor body.

The advance line that must get port vacuum, or vacuum that appears as you open the throttle, is the outer connection furthest from the distributor body.

Your first observation indicates that the centrifugal advance is working fine, per Wally's description and the first chart below. It couldn't hurt to get in there and put a touch of lubriplate on the pivots and cams inside the distributor(don't overdo it) but that's just maintenance; it won't fix anything.

Since you don't know this car's history I should mention the possibility that you don't have the proper vacuum advance unit on the distributor -- it could have been changed out for the earlier version(second chart below). This would result in the advance/retard crossing over at the wrong point.

Yours(assuming it's a US car) should behave as in the third pic below. I've marked it with numbers converted to inHg to match common vacuum gauges in case yours doesn't have a millibar scale. You can verify with a mityvac, watching the movement of the advance plate with the cap, rotor and the shield underneath the rotor removed. The lower band illustrates the behavior as vacuum is applied to the inner, retard fitting. The upper band illustrates the behavior as vacuum is applied to the outer, advance fitting. If you wanted to get fancy you could attach a mini plastic protractor to the shaft and set up a pointer, but IMHO eyeballing it should be enough to spot a problem. Note that there is no precise advance/retard specified at a given vacuum, only a range of acceptable values.

You mentioned that the vac hoses were connected to the same side of the throttle body. That doesn't sound right. The hose diagrams I've seen show the manifold vacuum and port vacuum at opposite sides of the TB. Verify the following for the lines at the distributor:

Idle: ~15inHg vacuum at retard fitting, ~1-3inHg at advance fitting.
1/8-1/4 throttle: falling(less vacuum) reading at retard fitting, rising(more vacuum) reading at advance fitting.

The abrupt retard above 2.5K makes me think you are pulling one or both of your vac lines from the wrong source.

The final pic is from a diagram for a 78 throttle body. The manifold is up, so vacuum is above the butterfly. The 78 US cars only have a single line to the distributor but it works the same as your advance line; it has manifold vacuum until the throttle is opened(4th pic, green line). As the throttle plate moves past the port, the port sees atmospheric pressure(more or less). The fourth pic shows a port vacuum connection as well(red line). I'm including this so you can get an idea where to look for port vacuum. It will be a port that is on the atmosphere side of the throttle plate, but the throttle plate will cause that port to be in the vacuum zone shortly after the throttle is opened. Note that the uppermost fittings, the ones high above the throttle plate and to the right of the red line, will always see manifold vacuum and are not the ports you want. They will show reduced vacuum as the throttle is opened but not to the desired degree.

For reference, get a a true manifold vacuum source, like the line to the brake booster, and if the vac behavior at the distributor matches that you've got the wrong port. The correct port will exhibit a much more rapid reduction of vacuum(fewer inHg) than what you see with manifold vacuum as the throttle is opened.

Tom Rathjen

Thanks for the great technical data.

I haven't had a lot of time to work on it, but I did verify that indeed the ported vacuum connection is on the backside of the TB, so I am now connecting that to the advance port on the distributor. The tming behavior still seems odd, though. I have not had time to fully characterize it, but it seems like the retard function is doing nothing (and I have verified vacuum at idle to this retard port). And it seems to be advancing more than the posts above suggest that it should when rev'ing. Anyway...I need to spend more time seeing what is happening. (And I also want to double check the TDC position by pulling the #1 plug as suggested above.) I will report more later. (Hard to spend much time on it with family visiting for Christmas! Plus it is freaking cold in the garage.)

belgiumbarry

hehe, that's what delays now my racer project... damn freaking cold !

Landseer

Just curious, what effect, if any, might a defective Temp II sensor have on timing?

Tom Rathjen

I don't believe the temp sensor would have any affect on timing (at least for my 82, since that is pre-electronic ignition). If you're talking about the temp sensor that connects to the DME, I believe that just affects the fuel delivery. And the other one is for the guage in the dash. (Not sure which is I and which is II.)

Regarding the earlier post that suggested checking the "crank dampener ring"....what exactly is a crank dampener ring? I assume that suggestion means I should check that the TDC mark on the crankshaft pulley actually corresponds to piston #1 being at top position. But I'm curious what this dampener ring is. Is that something that could somehow cause the TDC mark on the pulley to be in the wrong position? Thanks.

Arena996

Here is a direct photo scanned from the WSM. I verified the connections of my 1982 car and they are the same. Number 3 is hard to make out in the scan but it is below #2 in the picture, it has a t fitting in the line splitting the vaccum up with the advance on the distributor and the check valve for the charcoal filter lines. The front of the engine is to the right in the photo if you did not notice that.

SharkSkin

Temp II doesn't apply to the timing on this car.

The dampener ring is the disc behind the pulleys that spins with the crank where you read off the timing against the pointer that sticks out from the front cover. The only way that could be in the wrong position(correction -- unless the rubber has separated, also unlikely) is if the woodruff key that locates it in the correct angular position relative to the crank is missing. The key is the rectangular part on the crank surface shown just to the right of the dial indicator on the pic below.

It's doubtful this is your problem. Note that if you use a wooden dowel to check piston position, you should be careful. The spark plug hole is not parallel to the direction of piston movement and you could snap the dowel off in there as the piston comes up.