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About Ignition Timing

What does advancing the timing accomplish?

Comments by John Todd

By having the spark plug fire a couple of degrees sooner, you give the mixture more time to burn, thus burning more completely, thus releasing more energy, thus creating more power. One of the limiting factors in how much advance you can use is knock. This is one of the places octane comes into play. Lower octane fuels burn more quickly and limit the amount of advance you can dial in. But our motors are designed to run regular 87 octane, and since 17 degrees is within spec for the motor, you shouldn't need to go to higher octane, unless you've done something else to the motor like running higher compression.

What's the best timing?

Without the JWT ECU: 17 degrees if you want to run 89+ octane fuel and 15 degrees if you want to use standard 87 octane fuel.

With the JWT ECU:

Comments by Edward A Wolff

I called JWT on the best timing for the ECU 15 or 17.

Clark's comments: You will get slightly better performance at low temperatures, i.e. under 75 or so degrees. Slightly means very very little, How much air temp is bonused from being sucked in near the engine is unknown to myself. Based on the various inputs, like temperature, the ECU does many changes with the timing already so the 2 degrees would be above those changes.

At high temps you will begin to loose power. But because of the excellent design of the combustion chamber, you will not necessarily get pinging or knocking if you are overadvanced. You will however loose HP. So you could change for winter only, to get any potential benefits and change back each summer.

Additionally all the tuning and optimization was done at 15, so by changing the advance to 17 you are getting an unknown mix.

How is it done?

Comments by Wayne Cox

Here's the instructions from the factory manual on adjusting ignition timing. My (Wayne's) comments are in [square brackets], the rest is pretty much verbatim from the book.

  • Warm up engine to regular operating temp, and ensure idle speed stays below 1000 rpm.
  • Run engine at 2000 rpm for 2 minutes under no-load [in neutral, no accessories on]. Race engine 2 or 3 times under no-load, then run engine at idle speed. [this heats up the oxygen sensor so it is working]
  • Turn off engine and disconnect throttle position sensor (TPS) harness connector. [It's on the side of the throttle body facing the firewall - driver side of the engine, down low and past the brake master-cylinder]
  • Start engine. Race engine (2 - 3000 rpm) under no-load, the run at idle speed. [You may have trouble keeping it running with the throttle sensor disconnected. Depending on where your timing is set, it may stall. If so have someone sit in the car and press the gas *very slightly* to keep it running. Don't rev it up or you will not get an accurate timing reading/adjustment. Keep it at idle speed, about 800 rpm]
  • Check ignition timing with a timing light. It should be 15 degrees plus or minus 2 degrees Before Top Dead Center. [The timing mark at the extreme left (counter-clockwise) is zero degrees. There are lines at 5, 10 and 20 degrees to the right of it. There is a wide paint mark from 13 to 15 degrees, which the factory probably set yours to. Eyeball 17 degrees between the marks and that's what you want.]

    From the 1991 Technical Bulletins, Beginning with April 1991 production,
    the ignition timing mating mark has been changed as shown below.

    NEW marks at -5,0,5,10,13,white paint,15,20 B.T.D.C..
    OLD marks at 0,5,10,15,20 B.T.D.C..

    [Editoral note:  there has been a lot of dicussion about the timing marks and basically it comes down to the easiest way to find the 15 degree mark after the early '91 production is to look for the white paint or the two marks that appear very close to each other. ]

       -5    0    5    10 1315   20
        |    |    |    |  ###    |
  • If not, slightly loosen two bolts holding the distributor, and twist it [toward the front of the engine to advance, toward the cabin to retard] to obtain the desired timing. [Recheck the setting after tightening the bolts!].
  • Shut off engine and reconnect throttle sensor.

That's it. The trick is to set it to the extreme limit of the specification - 17 degrees. You'll get noticeably better throttle response there. Advanced timing helps power, but can cause knocking or pinging. The EFI computer *should* "hear" this and back off the timing to prevent engine damage, at which point it may run worse than if you had kept the timing conservative. Mine didn't benefit from going beyond 17. I always run 91 octane or better gas too, which lets you push the timing farther without trouble.


> When I disconnect the TPS, the idle speed jumps to around 1000rpm and the
> timing is stable but it is a little bit past the last mark (past 20 BTDC).

Mike Kojima writes: you need to rev the motor past 3000 rpm 3 times for the motor to lock the timing (after disconnecting the TPS)