Ignition Timing for 2.5SS?

[Scott Clarke]

I'm realizing that I have no ignition timing spec for my short stroke 2.5. The 2.2E I started with had a spec of 29-31 degrees BTDC @ 6000 RPM. I see that the 2.4 motors, as well as the 2.7RS, had a spec of 38 degrees BTDC @ 6000 RPM. So, what is right for my motor? A brief list of specs: 90mm bore, 9.7 measured CR, 2.4E distributor, MFI, early E cams.

[911pcars]

Where did you obtain those specs? Here's a chart from the factory repair manual:



The chart shows advance in distributor degrees (1/2 engine). Thus, 13º advance max. on the chart is 26º at the crank. To obtain 38º advance, your initial, static timing would have to be set at 12º and/or the distributor is advancing more than you think and perhaps a little on the borderline. I suggest pulling it back to about 30-32º so there's less risk of detonation damage. Better yet, since your engine is modified, optimize the setting on a chassis dyno.

Sherwood

[Peterfrans]

Go one page back in factory repair manual and it shows a 38 degree advance from 5800 rpm for the carrera 2.7 engine.

[Peterfrans]

[Peterfrans]

[Scott Clarke]

Sherwood-
The specs came from the back of Wayne's book. The 2.4 distributors, unlike the 2.2 versions, have vacuum retard. Does the RS distributor have vacuum retard? Wayne's book lists an idle spec of 5 degrees ATDC @ 900 RPM. This leads me to think that the vacuum retard amounts to 12 degrees. The centrifugal advance for both the 2.2 and 2.4E distributors seems to be the same @ 29 degrees, with S and RS @ 31 degrees. Should I plumb for the retard? Of course, my 2.2 injection does not have a provision for this.

I think that the idea of adjusting timing on the dyno seems sound, but I'd like to understand the difference in specs for 2.2 and 2.4/7 motors.

Thanks for your responses,
-Scott

[911pcars]

Scott,
The charts you saw and repro'd by Peterfrans are advance curve charts for 2.4 and 2.7 liter USA engines. In both cases, the advance mechanism totals 24-26º.

In the first chart (Vol. VI, page 9.3-2/6), the distributor advance begins at around 10º BTDC and ends at around 34º (total 24º). However, static ignition timing is already at 10º before the distributor has advanced.

The second chart (Vol. VI, 9.3-2/1 for 2.4/2.7 except Carrera) shows an advance curve for a 2.4/2.7 that begins at around 6º BTDC and ends around 34º (total 28º).

According to the manual, In either case, total advance for those engines must not exceed 38º BTDC. I wouldn't recommend using these curves for your engine.

The chart I posted earlier is another for the 2.7 Carrera engine, perhaps one w/o a vacuum retard unit. My mistake.

The late E and S dist. advance chart follows (Vol II, page L40a):



You'll notice the chart divisions are in distributor degrees so double the values to compare. Total advance using either 006 or 007 distributor should be 30º BTDC. Static timing ends up to be around 4º ATDC - 2º BTDC.

In each case, the distributor advances around 34º. However, the total advance might only be 30º due to setting the static timing ATDC. I'd say the short answer is to suggest you adjust the total ignition advance similar to the late E or S engine spec; more if local gas octane, ambient air temps. and other variables allow it, less if it produces detonation (although difficult to hear at 5000 rpm).

You could even try a little more aggressive timing by duplicating the early S curve (002 distributor, Vol II, page L39). Although not accurately depicted in this chart, the manual suggests a spec of 30º @ 6000 rpm and min. 5º BTDC at idle. More ign. advance at the bottom end will either produce more torque or more detonation. YMMV:



Hope this helps and not confuses,
Sherwood