Does anyone understand the ignition curves in the WSM?

[boosted79]

I'm trying to figure out the centrifugal advance curve on pgs. 28-4d in vol1 of the turbo supplement WSM.

On pg 28-4d it shows a centrifugal advance curve for USA, Cali and Japan from model 78. At 2000 rpm dist. speed it shows 8* advance of dist shaft. It also shows at 500 rpm dizzy speed it has a +-1* advance of dist shaft from TDC. So since dizzy turns 1/2 crank speed that means at 4k crank rpm the centrifugal advance is 16 crank*and at idle speed centrifugal advance effect is +- 2 so essentially zero effect at idle as expected. On pg 28-4c for USA (not Cali) it says timing must be 26*+- 4* BTDC at 4k rpm with vac hose disconnected. Ok so max total timing for USA version could be 30* at 4 k rpm. So since centrifugal only gives 16* advance that means the timing at idle with vac hose disconnected would be 14* (30 total less 16 centrifugal), correct?

Of course then the vac pot pulls out 20 crank* at idle so that gives timing at idle of 4* ATDC (14 BTDC - 10 = 4* ATDC). That agrees with the sticker on my engine lid that shows timing of 5* ATDC.

I'm assuming this retarded timing crap at low rpm was to keep the exhaust hot for the reactors to work?

[356911930]

Wouldn't 14* BTDC -10* = 4* BTDC? Engine type 930/61? or 930/63?

Yes, theoretically smog retarded. Idle advance can be 10* as long as total is still 26*... maybe up to 30* but that may be pushing it too far for safety on stockish set-up..


Rahl

[boosted79]

This is a 930/64, 1979. My mistake, vac pot pulls out 10* on the dizzy so 20* at the crank. So 14 BTDC - 20 = 6* ATDC with vac.
All the emission crap is MIA, SC cams, Garretson IC, K27, headers, MSD, Leask WUR, rpm switch.. This car was delivered to the original owner from a dealer in NJ so I'm assuming it is a rest of USA car, eg. not Cali, but then it says timing is 5* ATDC on the sticker which agrees with the WSM for a Cali car? Rest of USA is 10* ATDC according to WSM. Euro spec timing is 29* and we have 93 octane so I'm thinking I can go to 29. Anyway I have a programmable 6AL-2 which I will be utilizing after I get this broke in and get rid of this timing retard BS. Setting the AFR's right now.

[356911930]

Quote: "vac pot pulls out 10* on the dizzy so 20* at the crank."

Is this a correct relationship? I think it is just 10*

Rahl

[boosted79]

A pic is worth a million words. Retard curve shows 10* retard at dist. shaft so that is 20* at crank.

[T77911S]

i have tried looking at the charts and I cant figure them out.

one chart looks like it might be showing how much advance the dist provides, which is ok, but then another looks like it tries to show the actual setting for the timing, but its not correct. it shows full advance over 30degrees. I don't think any 930 went over 30 as far as I know.

if that is right 20 degrees retard is a LOT of retard

[356911930]

Where does the engine see max vac, RPM? If the vacuum is inverse to RPM and decreases as RPM increases that would make more sense.

Then at about 3,000 rpm the vacuum retard would be zero and mechanical advance would be 15* or 16*. Which sounds about right.

Rahl

[T77911S]

there is no RPM relation to vac retard.
vacuum is max at idle and reduces as RPM goes up.
if that is an idle vacuum retard then it is all out pretty low, like around 1000-1300 RPM

did the 78 have a vacuum retard and/or advance port?

you also have to look at what the WSM says about setting the timing.

to me, the timing setting and the charts never jibe.

if the 78 timing is set to 0 and has a vacuum advance port, then the lack of vacuum would reduce the timing to about 16* which is about right.

[Speedy Squirrel]

Here is an example of the charts using the EURO timing spec. Remember these rules:

1. The RPM shown is cam RPM. Engine RPM is twice that.
2. ATDC is After Top Dead Center. That means Retarded timing.
3. BTDC is Before Top Dead Center. That means Advanced timing.
4. Vacuum means the pressure is below atmospheric pressure. It is meant to be a measure of load. When the throttle is closed vacuum is high, that means a big number. When the throttle is at Wide Open Throttle (WOT), the vacuum is low, that means a small number.

For the Euro example below, the procedure states to set the ignition timing at 0 degrees crank with the vacuum hose connected and at 1000 RPM. For the centrifugal diagram we divide the crank RPM by 2 to get cam RPM. At 500 cam RPM we see on the chart that there is 0 degrees of advance. We are setting the timing at idle, so the vacuum is high, and is taking out 10 cam degrees, or 10 x 2 = 20 crank degrees. In other words, if we pulled the hose at idle, we sould see 20 crank degrees of advance.

Now, if we button everything back up and go out and stomp the throttle, at 6000 crank RPM (or 3000 cam RPM) we will get the 10 crank degrees (5 cam degrees) of centrifugal advance, and 20 crank degrees as the vacuum decreases due to the open throttle. 10 + 20 = 30 crank degrees of nominal timing at 6000 RPM and WOT.

[356911930]

I'm not sure that is quiet right.

At idle you have 0* centrifugal and 10* ATDC due to vacuum retard.

For Speedy's data we have full throttle max centrifugal 2 x 5* = 10* and no vacuum retard.

For T77911S data we have full throttle max centrifugal 2 x 8* = 16* and no vacuum retard.

I'm not sure a Porsche turbo can take 30* advance (BTDC) at full throttle?

Rahl

[Speedy Squirrel]

The USA curves are slightly different.

The USA cars have the timing set at 4000RPM (2000 cam RPM). Also, the vacuum hose is disconnected on the outside.

At 4000RPM (2000 cam RPM) there is 16 crank degrees of centrifugal advance, with no vacuum advance. The timing is set to 31 crank degrees at 4000 RPM with no vacuum, so at idle with the vacuum hose disconnected you would see 31(total) - 16(centrifugal) = 15 crankshaft degrees of static timing.

With the vacuum hose connected at idle, the vacuum pulls out 20 crank degrees (or 10 cam degrees), so you idle at 5 degrees retard with the USA timing (15 static + 0 centrifugal - 20 vacuum = -5).

When you button everything up, and you go out and punch it:

- before the boost builds up you have the same ~30 degrees as the EURO setup

- when the boost is at the 0.8 bar maximum the boost retard side has taken out 4 crank degrees (2 cam degrees), so you are running ~27 crank degrees under boost.

The Euro cars are set up to run 98 RON. The USA cars are set up to run 91 R+M/2 (roughly 94 RON).

[Speedy Squirrel]

Also, I need to point out that the vacuum retard curve posted by boosted79 is for the single hose USA setup, which is relatively uncommon. In this case there is no boost retard. The centrifugal advance at 4000 RPM (2000 RPM cam) is the same 16 crank degrees (8 cam degrees). The vacuum advance is disconnected and the timing is set to 26 crank degrees at 4000 RPM. That means there is 10 degrees of static timing (26 - 16 = 10) With the vacuum hose connected the timing at idle is 10 static + 0 centrifugal - 20 = 10 degrees retard at idle. Note that this setup gives about the same ful boost timing as the dual diaphragm distributor.

[boosted79]

I agree. I have the two hose dizzy so the Cali retard curve applies even though this car was delivered in NJ. Engine lid sticker says set it at 5* retard at idle. But for the single hose "USA" dizzy curves that I posted they give total timing of 35* at 6k crank rpm, 15* of centrifugal plus 20* of advance from the retard curve since there is no boost pot that pulls out timing when on boost, right? The engines are all the same internally, 7:1 CR, cams etc right? Was the WG set at the same .8 B for the "USA" versions? IOW, how can the "USA" engines run 35* but the Cali engines have to run 26-29*??? And Euro's run a max of 30 (10 centrifugal + 20 from retard). But Euros were set up for 98 RON and USA cars for 94 RON?? I guess it depends on what octane they were assuming for the "USA" cars, "Cali" cars and EURO cars?? Anyway I have my car running on the stock dizzy now at -5* at idle and it runs like a raped ape.

EDIT - forget the above, max timing is still 25 or so for "usa" since it is set a -10 at idle so -10 + 15+20 = 25 total.

[Speedy Squirrel]

Originally Posted by boosted79

EDIT - forget the above, max timing is still 25 or so for "usa" since it is set a -10 at idle so -10 + 15+20 = 25 total.

Yep, you got it down. The curves make a lot more sense on a Bosch distributor machine, which is the environment they were intended for.