I get full mechanical advance at 3000rpm?

tbitz · 08-19-2005, 12:21 PM

I noticed that my the mechanical timing advance on my 1978 dizzy has a range of about 20 degrees and that it is fully advanced at 3000rpm. I checked this with the vacuum advance hose disconnected.

I'm wondering does this seem right? There is no change in timing from 3000rpm and up.

Does anyone have an timing advance curve for the 1978 dizzy?

Perhaps there is supposed to be two mechanical advances and the upper rpm one is locked?

Cheers,

tbitz · 08-19-2005, 12:37 PM

I found this graph:

Now the graph axis are in respect to the dizzy, so the rpm and advance axis need to be multiplied by 2 to be in relation to the crank shaft.

So the mechanical advance from the graph shows one stage maxing at 1300 dizzy rpm (2600rpm at crank) and the other at 3200 dizzy rpm (6400rpm at crank).

It seems like I am missing the upper mechanical advance?

Note my total mechanical advance is 20 crank degrees which is the same at the graph (10 dizzy degrees is 20 crank degrees).

So how can this be explained? Perhaps the springs for both mechanical advances stages are the same and they both kick in at the same rpm range.

Something is not right.

tbitz · 08-19-2005, 01:02 PM

I also found this curve, which is exactly what I get on my car. Problem is this curve is for a 1978 TURBO. It only shows one stage of mechanical advance.

tbitz · 08-19-2005, 01:05 PM

Question remains, should the 1978 911SC have one or two stages of mechanical advance?

Bill Verburg · Bill Verburg's Garage

Spec for US '78 SC is given w/ vacuum disconnected
5&deg BTDC @ 900 &#177 50
24&deg - 28&deg BTDC @ 6000

ROW used
24&deg - 31&deg BTDC @ 6000

Ca & Japan 930
used
5&deg &#177 1&deg ATDC @ 1000 &#177 50 vac connected
31&deg &#177 4&deg BTDC @ 4000 vac disconnected

tbitz · 08-19-2005, 01:49 PM

Thanks Bill.

Is there any graph for the ROW that would indicate when the dizzy if fully advanced?

My car is ROW and I get 20 total, so it looks like I am missing some mechanical advance.

jpnovak · 08-19-2005, 02:07 PM

Don't forget there is a vacuum component to this. I think the two stages are: one mechanical and one vacuum. that is what the big canister on the side is for. Teh canister rotates the coils on the magnetic pickup. Mine was frozen last year. I bet your's is the same meaning you have lost part of your advance.

When fixed, I set my timing to 28 BTDC at 6K and let it idle whereever it wants.

Bill Verburg · Bill Verburg's Garage

It would help to know what version of motor you have

The US version and California version used the same dist but w/ different specs and strategies to get the advance all in.

If your engine compartment instructions say 0&deg &#177 2&deg @ 950 w/ vac. attached it should be a US version

If it is the Ca version it has 14&deg mechanical advance at the crankshaft, but the vacuum retards the timing so that only a max of ~21&deg is seen at the crankshaft., then they use the other side of the vacuum can to advance an additional 14&deg for a total of ~35&deg

no diagram handy

Bill Verburg · Bill Verburg's Garage

Im agree w/ Jamie only of the gas permits I would go to ~31&eg @6k, if there is any vac retard plug it.

tbitz · 08-19-2005, 02:41 PM

my car is 1978 911SC 3.0L ROW (not US).

My vacuum advance works. It applies 5 degrees advance (yes advance not retard).

Still the part that bothers me a bit is I get full advance at 3000rpm.

Looking at the first graph (80SC) there is 5 degrees (crank) of advance that is added from 5000 - 6400 rpm. I don't get this.

Bill Verburg · Bill Verburg's Garage

Quote:

Looking at the first graph (80SC) there is 5 degrees (crank) of advance that is added from 5000 - 6400 rpm. I don't get this.

They did a lot of weird things(see the part about Ca. above) for emissions reasons on the US and Ca. cars in particular.

Is that a max. 5&deg of vac. adv the?

the part throttle vac. advance is good for fuel economy and throttle response

max. performance comes from the mechanical adv.

tbitz · 08-19-2005, 03:10 PM

In my dizzy the vacuum contributes to 5 degrees of advance that is added to the mechanical advance of 20 degrees.

The first graphs above says "centrifugal advance curve", so there is no vacuum adv/ret here. It clearly looks like there are two stages of centrifugal advance.

one stage is 1200-2600 crank rpm
the second is 5000-6400 crank rpm

Bill Verburg · Bill Verburg's Garage

You are missing some mechanical advance then, at 6k there should be no vac. it is all mech. you apparantly need another 4 - 11&deg of mechanical(ROW spec)

the dual slopes must come for using 2 different springs.

open the dist. clean and lube it. re test, if you need more advance hot rod shops used to have different springs and/ weights, th e weights probably won't fit but the springs might.

or send it to a shop that specializes in recurving a dist.

Porsche Head · Porsche Head's Garage

I am having the same problem, I see almost all of my advance in by 4k, I forget exaclty how much advance, I'll recheck.

One thing to note is that I have a vac and retard on the dist for a 930/06 (CA) 1979 3.0, so I think the dist was swaped out at some point.....maybe the dist is from a diff year/model?

I tried a million settings, the best is 0deg (no vac) and vac connected to the vac side of the dist (way better throttle resp). I tried 30-35deg max and it was rougher with less power. I have SSI's and a Dansk dual out.

911pcars · 08-19-2005, 04:38 PM

"In my dizzy the vacuum contributes to 5 degrees of advance that is added to the mechanical advance of 20 degrees.

The first graphs above says "centrifugal advance curve", so there is no vacuum adv/ret here. It clearly looks like there are two stages of centrifugal advance.

one stage is 1200-2600 crank rpm
the second is 5000-6400 crank rpm"

Tony,
The chart is a curve with a ramp to the final advance. And yes, the chart should show centrifugal advance only, which it does.

You report you have "only" 20 degrees total. That's within spec according to the chart you provided. What are you setting the static timing at? (vacuum advance should be disconnected and plugged). If it's 5º BTDC, you'll have 25º total which is on the lower range of specs for your engine.

Bill's spec:
ROW used
24° - 31° BTDC @ 6000

If you're getting full advance by 1500 rpm distributor (3000 eng.), then it's advancing too quickly. Perhaps weak advance springs.

If you want more total advance (and your initial timing is within spec), I'd try a little more initial advance (e.g. 8-10º BTDC) or open up the distributor and modify it to provide more centrifugal advance (allow more weight swing).

I've always interpreted the double lines of the advance charts as describing an upper and lower range. If they used a single line, techs and owners would compare strictly to the "curve" and think it was out of spec. It's probably not achievable given spring/mechanical weight tolerances. Correct me on this if I'm wrong.

Sherwood

tbitz · 08-19-2005, 04:48 PM

I agree my centrifugal range is OK (ie: 20 crank degrees), but it's just coming in too early. I've opened up my dizzy. Pics to follow...

tbitz · 08-19-2005, 04:57 PM

Here is what I did to check the dizzy advance curve. I took it apart and placed the dizzy shaft on my lathe. The lathe can turn at 650rpm, 1100rpm, 1800rpm, and 2800rpm.

This corresponds to a crank rpm of 1300rpm, 2200rpm, 3600rpm, 5600rpm. I took pictures of the dizzy at each rpm to see how the springs are opening.

first pic is at 0 crank RPM as a reference:

Here is at 1300 crank RPM. Doesn't look like any opening of the weights:

Here is at 2200 crank RPM. You can see the weights have opened about 50%:

Here is at 3600 crank RPM. Looks like it's full open. Note the weight with the heavier gauge spring has hit the stop (11:00 clock on pic).

Here is 5600 crank RPM. I see no difference from 3600 crank RPM:

911pcars · 08-19-2005, 05:08 PM

Tony,
Nice technique, btw.

I'm sure you figured it out; bend the anchor on one or both springs to increase the tension so it advances at a higher rpm. Since each spring has a different tension, the spring you choose to modify will change the curve somewhere in the rpm range.

Bend the stop to limit or increase total advance.

You can plot the curve more accurately if you have a distributor machine. Yeah, I have one too. Save that, use an advance-type timing light to plot the advance curve as you increase engine speed.

Sherwood

tbitz · 08-19-2005, 05:27 PM

I can bend the stops out more, but that will only increase the range of mechanical advance. It won't make the advance come in at a higher rpm.

I'm wondering if the springs are reversed? ie: the springs need to swap locations?

tbitz · 08-19-2005, 05:47 PM

I think I am figuring this out.

There are two stops on the dizzy. One for the heavy weight and one for the lighter weight. Currently the heavy weight hits the stop first. Using some trig I've measure this as 20 degrees of crank advance. The lighter weight still has some room left to move. Now if the lighter weight is pushed to the end, I measure 26 degrees of crank advance. I never see this on my car. It looks like the last 6 degrees isn't kicking in. In fact looking at the pics above it doesn't look like the lighter weight is hitting the stop even at 5600rpm.

Swapping the spring locations does nothing. I checked this out, but swapping the weight locations does. Currently the lighter weight is in the location where you'd get the last 6 degrees of advance. It seems like it just isn't heavy enought to make it happen. If I put the heavier weight here then the last 6 degrees should kick in earlier due to the extra mass.

I'll have to try this out.