How do you acquire your total timing? And how do you adjust for it?

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91 Formula
305 TPI 5speed
1LE/G92/WS6
w/GTA gfx,rims,hood

Paxton 10lb blower, SLP airfoil, ported/polished plenum, March pulleys, Crane AFPR(43psi),Crane Gold 1.6rrs,MSD coil,MSD6AL, Holley 9mm wires, fastchip, Bosch O2sensor, SLP headers & catback,short shifter,3:73s w/Auburn posi,160* t-stat,JET 195* fan switch, Macewen white face gauges, Autometer gauges, Zoom hi-performance clutch.

 

i'll over simplifie it, but base is where the timing is at when the engien idles. you also have vacumne controled advance that changes the timing in relation to vacumne applied to the distributor and then you have mechanical advance that changes the timing by means of weights and springs inside the distributor. as the distributor spins it allows the weights to move changeing the timing.

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ICON Motorsports

1st & 3rd

 

Vaccum advance is only in effect when part throttle applications need it.

Total timing is usually refered to as the base and mechanical combined. If the base timing is 10 degrees and the mechanical is 26 then the total is 36 degrees. Vaccum advance can add another 10 degrees to that.

Now when you advance the distributor from 10 degrees to lets say 15, the total timing is now 41 which can be too high. You need to bring that back down to the original 36 (or whatever total works best). This is why you need to get distributors recurved. If you want to increase the base (initial) timing but keep the same total, the mechanical weights need to be changed to only offer as much mechanical advance as you need.

To find your total timing you either need an advance timing light or a balancer that is degreed. Bring the rpms up to about 3000 and locate the timing marks. Use the advance timing light to bring the mark on the balancer down to zero degrees. Read the timing on the light. It should be around 36-38 degrees. That's your total. Most cars are at full advance by 3000 rpm.

The ignition timing on my race car is locked out at 36 degrees. The ignition does a 20 degree retard for starting the engine then goes to the full 36 degrees. There is no ignition curve or advance since the engine mainly operates at WOT. If I find that 36 is too low or to high I just adjust the timing a couple of degrees either way until the engine finds the best total timing point.

Clear as mud?

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87 IROC-Z Pro ET Bracket Race Car
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87 IROC bracket car, 91 454SS daily driver, 95 Homebuilt Harley

 

Excellent description of how a mechanical distributor works. Believe it or not, your SD system works similar, except it is controlled by the computer. The base part is the same, on a TPI car, its 6*. Then the ecm uses the Spark Tables in the eprom based on specific RPM and MAP values. MAP is just the reverse of vacuum.

At maximum vacuum (29.5", you have 0" MAP or 0 KPA as GM measures it in metric). At WOT, your vacuum is near 0", or 29.5" MAP or 100 KPA as GM measures it in metric). The Spark table uses values from 20 KPA (deceleration mode) to 100 KPA (WOT) in 10 KPA increments and 200 RPM increments. The ecm uses the base timing, plus the Spark table (plus some minor adjustments for EGR, startup, coolant, and Power Enrichment) to compute your total timing that it needs.

This effectively works exactly like a sophisticated vacuum advance with the mechanical part all built in. But you can see the parallels of the computerized version and mechanical version are surprisingly similiar, except you have more control with the computer version and one other big difference, KNOCK RETARD.

Now there are other wrinkles and I have over simplified this the way the ecm works (it actually computes a relative spark advance and with the base timing that you preset, it gives you the "total timing"). But that will confuse you, so leave it at "base timing plus the computers additional or relative timing" will equal the total timing that you will see if you hook a timing light up to the engine and DO NOT disconnect the brown ESC wire...this is the wire that adds the computer's part of the timing and why you must disconnect it if you want to set the base timing of you SD car.

Muddier yet?