Questions on ignition timing for vanagon

[reluctantartist]

I have an 82 California L-jet vanagon. I have always set the timing to 5 degrees ATDC at ~950rpm per the Bentley. I am interested in doing the timing adjustment where you go to 3400 rpm and check the timing. I have looked around and have found inconsistent data. Some say to set timing at 28 degrees at 3500 rpm. Some say 28 degrees at 3400 rpm. And, if I understand correctly, the Bentley states centrifugal advance should be 21-25 at 3400 rpm. Centrifugal advance is the same thing as setting the timing without vacuum hoses right? So what is the correct timing specifications?

[SlowLane]

First of all, the most comprehensive information about VW distributor specifications is the Old Volks Home distributor page. A link to their information for your distributor is here: http://www.oldvolkshome.com/ignition.htm#V8083CA

Setting your timing to 5 degrees ATDC with vacuum hoses on and the idle stabilizer bypassed is the right way to go for a CA-spec vehicle. As a check after you've done that, try disconnecting (and plugging) the vacuum retard line. The ignition should jump to 7.5 BTDC at idle speed (you'll probably have to turn the idle speed screw in a couple of turns to slow the engine down). This confirms that the vacuum retard is working.

The idea behind setting the total mechanical advance at 3400+ RPM is to confirm that you're getting the full mechanical advance (no more, no less) at the specified speed. This should be the maximum mechanical advance where the distributor weights are hitting the internal stops. Increasing RPM beyond the prescribed speed should not increase the advance any further.

For your distributor, the Old Volks Home page has this data:

Advance/Retard Range: Vacuum: 9-12deg Adv @ 8.2 In. Hg, 11-13deg Ret @ 8.2 In. Hg; Centrifugal: 9-13deg @ 1600 rpm, 12-16deg @ 2800 rpm, 21-25deg @ 3600 rpm

Note that there are three inflection points for the centrifugal advance. This means that the advance curve changes slope over the full range. This is achieved with clever use of different spring rates coming in at different RPMs, and the setting of one weight stop at another speed to flatten out the curve.

Also note the wide range of acceptable advances at the three inflection points. This is a reflection of the primitive and barely predictable behavior of the weights-and-springs arrangement used for the centrifugal advance mechanism, accounting for manufacturing variations and age. Modern electronic systems have the ability to be far more precise, of course.

So, to check the max advance of your distributor, take off and plug the vacuum hoses, confirm that you have 7.5 degrees BTDC at idle, then rev it up to 3600 RPM and note the advance there. It should be 7.5 + (21 to 25) = 28.5 to 32.5. Then rev it up some more and make sure that you don't get more advance. If you're feeling pedantic, and can fix the revs at the other two inflection points (1600 and 2800 RPM) you can check those too. Just add 7.5 degrees to the acceptable ranges for those speeds. I don't know that they actually mean much though.

I'm sure there will be some disagreement about whether the 7.5 + (advance range) is truly correct, as there is a bit of centrifugal advance introduced even at idle. I believe, however, that distributors are designed to have negligible mechanical advance at idle, in order to allow predictable adjustment.

[Amskeptic]

First of all, the most comprehensive information about VW distributor specifications is the Old Volks Home distributor page. A link to their information for your distributor is here: http://www.oldvolkshome.com/ignition.htm#V8083CA

Setting your timing to 5 degrees ATDC with vacuum hoses on and the idle stabilizer bypassed is the right way to go for a CA-spec vehicle. As a check after you've done that, try disconnecting (and plugging) the vacuum retard line. The ignition should jump to 7.5 BTDC at idle speed (you'll probably have to turn the idle speed screw in a couple of turns to slow the engine down). This confirms that the vacuum retard is working.

The idea behind setting the total mechanical advance at 3400+ RPM is to confirm that you're getting the full mechanical advance (no more, no less) at the specified speed. This should be the maximum mechanical advance where the distributor weights are hitting the internal stops. Increasing RPM beyond the prescribed speed should not increase the advance any further.

For your distributor, the Old Volks Home page has this data:

Advance/Retard Range: Vacuum: 9-12deg Adv @ 8.2 In. Hg, 11-13deg Ret @ 8.2 In. Hg; Centrifugal: 9-13deg @ 1600 rpm, 12-16deg @ 2800 rpm, 21-25deg @ 3600 rpm

Note that there are three inflection points for the centrifugal advance. This means that the advance curve changes slope over the full range. This is achieved with clever use of different spring rates coming in at different RPMs, and the setting of one weight stop at another speed to flatten out the curve.

Also note the wide range of acceptable advances at the three inflection points. This is a reflection of the primitive and barely predictable behavior of the weights-and-springs arrangement used for the centrifugal advance mechanism, accounting for manufacturing variations and age. Modern electronic systems have the ability to be far more precise, of course.

So, to check the max advance of your distributor, take off and plug the vacuum hoses, confirm that you have 7.5 degrees BTDC at idle, then rev it up to 3600 RPM and note the advance there. It should be 7.5 + (21 to 25) = 28.5 to 32.5. Then rev it up some more and make sure that you don't get more advance. If you're feeling pedantic, and can fix the revs at the other two inflection points (1600 and 2800 RPM) you can check those too. Just add 7.5 degrees to the acceptable ranges for those speeds. I don't know that they actually mean much though.

I'm sure there will be some disagreement about whether the 7.5 + (advance range) is truly correct, as there is a bit of centrifugal advance introduced even at idle. I believe, however, that distributors are designed to have negligible mechanical advance at idle, in order to allow predictable adjustment.

There is not supposed to be any advance at idle. The hoses-off idle timing number you suggest above on retard distributors is merely for the curious. Set at 3,400 rpm, check for no additional advance at 4,000, let idle, plug in retard hose, if you see 5* ATDC, that's great, if you see 10*ATDC, still no worries, plug in vacuum advance and rev the engine to build up vacuum on throttle tip-off, if it hits 38*-40*, yer good.

Set idle with stabilizer unplugged to 850-900, that will give it the best range of control when you plug it back in.

pe·dan·tic   [puh-dan-tik] Show IPA
adjective
1.
ostentatious in one's learning.
2.
overly concerned with minute details or formalisms, especially in teaching.

Colin

[SlowLane]

There is not supposed to be any contribution by the centrifugal (actually, centripetal) advance mechanism to the ignition advance at idle.

Fixed that for ya. Glad we're in agreement.

Set at 3,400 rpm,

Umm, set to what, exactly? Does one assume that the advance mechanism provides 21 or 25 degrees @ 3400 RPM? So set to 28 or 32 total advance at that speed? Or split the difference and settle for 30?

if you see 5* ATDC, that's great, if you see 10*ATDC, still no worries,

Fine, unless you happen to have to bring your bus into a smog referee who will check that the idle timing is within factory spec. Still no worries?

pe·dan·tic   [puh-dan-tik] Show IPA
adjective
1.
ostentatious in one's learning.
2.
overly concerned with minute details or formalisms, especially in teaching.

Pedantic? Moi? You bet yer bippie, sunshine. Pretty much have to be in my line of work.

[SlowLane]

Some say to set timing at 28 degrees at 3500 rpm. Some say 28 degrees at 3400 rpm. And, if I understand correctly, the Bentley states centrifugal advance should be 21-25 at 3400 rpm.

[pedantic]
Sorry, I should have made this distinction clearer: The first two things that "some say" are essentially identical. They are referring to total advance, which is the intial timing at idle, plus the contribution of the centrifugal advance mechanism.

The value you quote from Bentley is simply the amount of advance that the centrifugal mechanism should contribute to the total advance at 3400 RPM. It doesn't take into account the initial timing.

So there's no contradiction between what "some say" and what Bentley says in this regard. One is referring to total advance, the other is talking about centrifugal advance in isolation.
[/pedantic]

[reluctantartist]

Ok please forgive my ignorance here, the Bentley amount would be 21-25* BTDC with the timing light at 3400 rpm which would equal 28-32* total advance from my 5* ATDC ? And then if I rev it to 4000 rpm the advance should not change? And when it is all connected if I rev it up I will see 38-40* BTDC?

Thanks

[reluctantartist]

Ok please forgive my ignorance here, the Bentley amount would be 21-25* BTDC with the timing light at 3400 rpm which would equal 28-32* total advance from my 5* ATDC ? And then if I rev it to 4000 rpm the advance should not change? And when it is all connected if I rev it up I will see 38-40* BTDC?

Thanks

[Randy in Maine]

Disreguard the retard function in calculating the total advance.

With the hoses off, idle stabilizer out of the loop, and with all of the centrifical advance it has offer is all in 28º BTDC. Exact RPM doesn't mater. You will be wound up to about 3500 RPMs though.

When it drops back to about correct idle speed (when the retard line is on, the idle speed will go a bit lower so don't worry too much about it), that initial timing will be right around 7.5º BTDC. Since your timing light is still on there, check it. If we do the math.... 7.5º + 21-25º = in the range of 28.5-32.5º. Right where you want to be.

When you plug in the retard line, that initial timing should drop to around 5º ATDC. Retard line off back to around 7.5º BTDC with slightly more (~ 200 RPMs) idle speed.

[SlowLane]

Ok please forgive my ignorance here, the Bentley amount would be 21-25* BTDC with the timing light at 3400 rpm which would equal 28-32* total advance from my 5* ATDC ?

Randy's got it right, but perhaps I need to expand my pedanticity a bit more.

The vacuum retard doesn't really come into play when making these measurements, and it only serves as a point of confusion because that's what we're told by VW to set the initial advance with. The retard side of the vacuum can has a very weak influence. It will only have an effect when the advance side of the can sees no vacuum at all, which is pretty much the case only at idle. When you rev it up to 3400-3500 RPM, the retard side of the can has no affect at all.

So when we talk about the 21-25 degree centrifugal advance adding to the initial advance, we're really talking about adding to the ~7.5 BTDC initial advance that we get with the vacuum retard disconnected.

And then if I rev it to 4000 rpm the advance should not change?

Correct.

And when it is all connected if I rev it up I will see 38-40* BTDC?

Only if you're vacuum advance is getting vacuum, which isn't necessarily the case if you just "rev it up". See Colin's comment re "rev the engine to build up vacuum on throttle tip-off". I'm not completely sure what he means, but I think it means to rev the engine up, then let the throttle shut by itself (like lifting your foot off the gas pedal). This will momentarily create a high vacuum at your vacuum advance port and give you maximum ignition advance with both the centrifugal mechanism contributing and the vacuum advance contributing their full amounts. You need to be quick when reading this, because it only lasts for a second or so, but it's easy to repeat.

Getting back to the original question of where to set the centrifugal timing. The following discussion assumes there are no vacuum advance mechanisms in the mix to confuse things.
Basically there are only two points in the centrifugal advance's mechanism where you can reliably and repeatably set the ignition timing: at either end of the mechanism's travel where it runs into its "hard" stops. That's either at idle or at the maximum speed (let's say 3400 RPM for now). The high-speed hard stop is the "harder" of the two, as the advance mechanism is physically blocked from advancing any further. At the idle speed end, the hard stop is a bit softer, since it consists of the advance mechanism shaking out any free-play it has and coming up against the resistance of the first of the two springs that come into play.

VW says to time the engine at the "softer" idle-speed stop to 7.5 degrees BTDC and then publishes that the high-speed advance should fall within +/- 2 degrees of 30.5 degrees (ie 7.5 + (23 +/- 2)).

Colin's approach is to use the high-speed hard stop to time the engine to the advance he likes and let the idle timing fall where it may. He has used his experience to arrive at the total advance which he believes is best for our engines. So if we decide to firmly set the high-speed timing at 30.5 degrees, then the idle timing can end up being 30.5 - (23 +/- 2) = 5.5 to 9.5 degrees.

All this is assuming your distributor falls within the published specifications. And I think this is really where the benefit of the whole exercise comes in. It gives you insight into what your distributor is doing. If you make these measurements/ adjustments and discover that you're getting either too much or too little ignition advance, then that's a yellow flag to find out what's up with your distributor. If it hasn't been opened up or services since 1982, then there's a good chance that the centrifugal advance mechanism is gummed up with ancient grease and gunk.

[reluctantartist]

Thanks for the info. I don't think my distributor has been serviced but everything seems to run well at this point. I am just doting all the i's and crossing all the t's. Onne day I will do a megasquirt set up and get ride of the distributor and the rest of the L-jet setup but for now it works well and it is at least electronic.

[Amskeptic]

If you look at the scale for the Type 4 engine, it has dots for your idle timing, and a rectangle for the known bit of scatter that occurs at higher rpm. This rectangle is between 25-28* on the scale, total advance. Period. Then when you go back down to idle, you find that fresh new factory engines are often right near the 7.5 BTDC, whereas older engines definitely start to expand the range due to slop, and are more likely to be closer to 5-4-2- 0 even.

I do not offer timing numbers to answer smog stations. That is something you do on smog test morning if the idle timing number is indeed checked at idle which I personally have not heard of, what a pain in the ass for many older cars without crank sensors to find the scale or the mark on the crankshaft.

Being an idiot savant when it comes to what makes my engines happy ( I so desperately need them to be happy when I have 40 customers in front of me and the need for 20,000 totally reliable highway miles no questions, ifs, ands, or buts), I have reached a point of auditory sensitivity that can sense the bearings getting a bit more beaten at the higher numbers. I trade in performance every time for nice mellow hits to the crankshaft and bearings and crankcases. Try it some time while you adjusting your timing, run it up to 40* centrifugal centripetal centripedipedapetoo, and rev the engine smartly with your other hand on the fan housing. Listen for the noise that comes with the vibration. Now bring it down to 25*.

The Type 4s seem to like 25 to 25 total just fine in the realm of smoothness and a couple of degrees of lower head temps (big deal), the Type 1s interestingly enough, I have found like 28-30* and get annoyed very quickly in either direction from there (centrifugal equipped distributors), the vacuum only distributors are quite interesting, they really haul in the advance at light throttle and then you watch it drop as you hit it harder, perfect.
Colin