Ignition timing experts please help. Modified 2.7L
 

I have a 1975 911 Carrera rebuilt with the following engine modifications and I need to find the proper timing for this set up. Factory says set on Z1 @ 950RPM +/- for 75 US carrera.

ENGINE:
Crane optical ignition
CIS
RS pistons
SC Cam Profile
Fly cut heads 9.5 compression
SSI / Dansk exhaust
No emissions


Do you think I should use a 1978 ignition timing spec. 5degrees BTDC?

Thanks
Eric

bump

Eric,

The critical issue is the timing at 6000 RPM and how the curve advances in the 2000-4000 range. Only after you have resolved those issues does the advance at idle and below idle get addressed.

What did you do with the vacuum advance/retard?
I assume the centrifugal advance still works OK so the rotor stays better aligned with the cap terminals.

The spec maximum advance at 6000 is 38 degrees for a stock ’75 engine. With your bump in compression ratio, I would be concerned that is too much.

A good test would be to measure the actual advance curve at 500 RPM intervals. What does the timing do when you drop below idle (when you ease out the cutch, in gear with the parking brake on)?

Best,
Grady

Grady, Thanks for your response.

The vacum advance is connected and NOT plugged. I belive it is working but I have doubts that it is not a smooth action when the car heats up all the way. I do get a bit of bucking under 3K, and every so often I get a small hesitation when I take off from full stop when car is warm (ONLY). I just bought a timing light from sears with inductive advance and I am trying to figure out how to use the advance feature guage.

Yeterday I did set the timing at 5degrees BTDC and CO @3.5% and it seems to peform well. I did notice my overall temps drop.

As you suggested I will see what the car does when when I drop the below idle releasing the clutch with brake on.

Can you advise on using the the inductive advance feature on the timing light and what to look for on the advane curve between 2000 - 4000 rpm?

Thanks
Eric

Eric,

For now just concentrate on what the ignition advance/retard is doing. Collect data.

Your new timing light should have instructions. You can scan them and post for our interpretation and clarification. Most Craftsman stuff is pretty good.

Your crank pulley should have marks at 5 degrees ATDC, TDC (Z1), 30 BTDC and 35 BTDC. It is useful to fill the notches with white paint so they are more easily visible. You can paint yourself more lines as intermediate reference. You can sacrifice a school protractor to help with accurate marks, just find one the same size or smaller than your pulley diameter.

You want to collect the data both with the vacuum connected and with it disconnected. Remember to plug the vacuum line to the throttle body when disconnected. The port at the distributor can remain open to the air.

Another useful tool is a small, hand operated, vacuum pump with a vacuum gauge. You use this to measure the operation of the distributor vacuum advance/retard. It is also useful to tee a regular vacuum gauge into the hose during measurements.

I would take measurements every 500 rpm from idle to 6000 RPM both with and without the vacuum connected. Additionally you can add the measurement below idle.

Remember, it is not permitted to get your Christmas tie in the fan.

Best,
Grady

Here are the marks.

"
http://forums.pelicanparts.com/uploa...1103905315.jpg "
(C) Dr. Ing. h.c. F. Porsche A.G.

Best,
Grady

Grady has given you great advice. The advance dial on the timing light gives you an idea of what the advance is as the rpm's go up. You simply twist the dial and when your timing marks line up again the dial will read the current advance for that rpm. (Now I am not sure all timing lights are completely accurate but it will be close). If you have Bruce Andersons book look at the advance curve for one of the RS spec engines and yours should come close as you graph out the every 500 rpm curve. That said you may end up wanting to get your distributor recurved to ensure it is correct, especially with the higher compresion pistons.

Grady,

Thanks for your help.

I was able to do my timing last weekend. I also installed a brand new distributor (euro spec rs replacement). Car is running great with incredible pull.

Set the timing to z1 @ 950rpm with vac plugged and 3.5% co..
Readings with vac unplugged:
1500 rpm - 12 deg.
2000 rpm - 20 deg
4000 rpm - 35deg

Full advance occured way before 6000 rpm. Would that be normal? Also when I timed the car at 5 deg BTC it seemed to be a bit quicker. However I left it at Z1. Do you think 5 deg BTC may be the best timing for my car?

Eric

Eric,

5 degrees BTC on the idle setting will yield 40 degrees at 4000 RPM by your own measurements. Since the maximum allowed is 38 degrees (or less as Grady has mentioned) you should be able to see that 5 degrees BTC would be dangerous. I'd leave it at Z1.

-Andy

"....you should be able to see that 5 degrees BTC would be dangerous. I'd leave it at Z1."

...or attempt to replicate the S spec and limit the total to 30-35º but set the static timing at 5ºBTDC. However, to limit the total advance, you must modify the stop that limits how far the advance plate moves. A distributor machine comes in handy at this point.

Sherwood

Thanks all. I'll leave it at Z1

"Thanks all. I'll leave it at Z1. "

That's fine, but you might be leaving some low end torque on the table, FWIW.

Sherwood
http://members.rennlist.org/911pcars

Engines with 9.8:1 compression used 31° - 35° max advance at 6000 rpm as their limit ... back when 100+ octane gas was readily available! Even though your compression is somewhat lower ... I suspect 30° - 31° advance at 6000 rpm -- NOT measured or set at idle ... is how you should set the timing as a starting point.

Per the spec books ... in general, the timing was set at high revs and checked at idle. Obviously, the timing curve should be verified, but timing has always been set with full centrifugal advance 'in' and with vacuum retard diaphragm disconnected and line plugged -- for 2.4 and 2.7 engines with vac. modules. You shouldn't use 911SC specs as a guide, since the distributors are quite different from your 2.7 distributor!!!

"Engines with 9.8:1 compression used 31° - 35° max advance at 6000 rpm as their limit ... back when 100+ octane gas was readily available! Even though your compression is somewhat lower ... I suspect 30° - 31° advance at 6000 rpm -- NOT measured or set at idle ... is how you should set the timing as a starting point"

- Early_S_Man -

He's right-on with his statement which basically applies to even the later Porsches
with & without EFI, e.g. 911SC, 911 Carreras, 964/993s. Many think that a continual
timing advance mod, even minor, is unlimited without potential problems.

Glad you raised the issue Warren. Thanks.

Hmmmmm... must be the "good" Loren responding today. Let's see how long he can keep the charade up.

While everyone is here: Not to hijack, but to add some education.
What's the purpose of the second vac connection on later SC's?
Retard?
Ok, so wht's the benifit of Retarding? Is it for emissions at idle only?

My car is a bastard. It was originally a ROW 930/10 but now has a 930/16 long block. (Recently rebuilt/reringed) So I don't know the compression. It now has a US 83 distributer, but the CIS is the 83 ROW with the larger intake runners.

I have a dist curve graph for the US version, but not the ROW. What's the difference between the 2. Also the graph only shows the base timing, but not the timing with the vac advance and retard. The dist graphs are also done at Dist shaft speed, 1/2 engine speed.

My last shot at timing (I's been awhile) was at 4000 with everything connected. It may have been 35 or 38deg can't remember at the moment. There was a term floating arround "JW method"

Emissions is no longer a concern. Should disable the retard?
Would the euro distributer give me back something that is missing?

BTW does anyone know what marks should be on the ROW crank pully? I believe the sticker states to time at 4000 rpm then check at 950. But this was the ROW sticker.

See why I'm cornfused:confused: