Ignition advance? - Brindabella Motor Sport Club

Spac · 02-11-2005, 03:49 PM

OK, so my splinky new distributor for the Volvo turned up yesterday, and it has easily adjustable limits on how much mechanical (centrifugal) advance the distributor can provide.

I know it really needs to be worked out on the dyno, but the dizzy needs to be pulled apart to adjust it, it it'd be good to get a ball-park figure first.

Any suggestions on how much CHANGE in timing a typical twin-Webered, big cammed 2-litre rally motor would need?

There's no vacuum advance at all.

jamar · 02-11-2005, 04:01 PM

15 to 20 degrees is what i have been advised to run.. about 35 degrees on cam

Matt · 02-11-2005, 04:04 PM

What Jason said. About 15 degrees BTDC at idle, 35 degrees BTDC on song.

tortfeaser · 03-11-2005, 08:21 AM

Depends on the chamber shape, size, squish area, valves etc. (As you know.) The theory is that optimised chamber designs need less advance as the flame front travels quicker. Also, leaner mixtures need greater advance as they burn slower. The idea is to find the minimum timing for best torque (MBT). Advancing more means cylinder pressures are higher earlier, losing torque and risking detonation. If a turbo car use an EGT meter - you'll retard a lot to prevent pinging but too much will raise chamber temps to alloy melting potential - that's the boost limiting factor.

For starters with cars I've MS'd, I use the following basic info, then tune on the road and dyno from there. Its very conservative given most rallying installs will use 98 octane, (and the info is US sourced where RON in unleaded is 87 (regular), 89 (mid-grade) and 93 (premium). With Matt T's FJ20 (for example) it didn't require a whole lot of fine tuning once I added around 5 deg at total timing to take account of the difference in RON. (Ended up at 32deg once advance was all in at WOT, more didn't give more torque, less the torque dropped off.)

The basic principles are to determine a maximum advance for your engine and work backwards from there with heuristics:

- older engines (1960s up to 1990 or so) with two valves - max advance = 36°
- newer two-valve engines - max advance = 32°
- three or four valve engines - max advance = 30°

then adjust for bore size:

- under 3.5" (89mm) - subtract 3°
- between 3.5" and 4.000" (101.6mm) - no adjustment
- over 4.001" (+101.6mm) - add 3°

then adjust for the fuel:

- regular - subtract 2°
- mid-grade - subtract 1°
- premium - no adjustment

That gives a maximum advance figure. It you have an aftermarket combination with a good squish area and optimized quench, subtract another 2°. If you have a flathead, add 3° or 4° or more.

Use this to fill in the table at 100 kPa from 3000 rpm to the redline.

From idle to 3000 rpm, you want the advance (@100kPa) to increase fairly linearly from the idle advance to the maximum advance. idle advance is really a matter of tuning, but assume 8° to 16° in most cases, with stock engines being on the lower end, and 'hotter' engines being on the upper end.

So for a hot engine with 36° maximum advance and 16° idle advance (at 800rpm), the spark table might look like this for 100kPa:

100 16° 16° 18° 24° 28° 36°

rpm 600 800 1000 1500 2000 3000

Below 100 kPa, add 0.3° per 1 kPa drop. So for example, if total spark at 100kPa and 4000 rpm was 36°, the advance at 50 kPa would be:

36° + 0.3° x (100-50) = 51°

and the advance at 45 kPa and 800 rpm would be:

16° + 0.3° x (100-45) = 32.5°

However all of these would need to be tuned, and it often helps idle stability to limit the advance at idle to under 20°.

If you can, it might help to increase advance in the high MAP areas at idleish RPMs to improve off idle performance. Retard timing a lot at low MAP/high RPM for flames on overrun

Idle is usually tuned by looking for minimum MAP - this gives the greatest stability.

TKM · 03-11-2005, 09:00 AM

Yeh, 15 degrees is usually the norm on Datos. The lack of vacuum advance is not an issue, I've never run it on twin webbered cars, was once told that the twins set up a pulse in the vacumm so it doesn't work properly anyway.

guy · 03-11-2005, 09:18 AM

Yeah I reakon they are some very good start points Marc. Well done Mr Squirt! You will probably find ignition timing needs to come back (maybe a couple of degree's) right at the top end but that can't be done with a distributor, hence losing a couple of KW with the compromised figure. On the dyno with timing light 2valve ally head, 88mm bore 11.7:1 comp ratio final advance was set at 14deg initial 34 total. A couple of degree either way lost power. Note: I was only concerned with the 4000/7200rpm band.

KOS! · 03-11-2005, 09:55 AM

damn your a legend tort, got an answer for just about everything.

/copys and saves

Matt · 03-11-2005, 10:03 AM

Ummmm....yeah...I was just about to say what Tort said.

Where's that "Reputation" button?

tortfeaser · 03-11-2005, 10:11 AM

Shucks. I forgot to add that for turbo cars above 100 kPa you should subtract 0.3° per 1 kPa increase. (Its no coincidence this ratio is the same as for the reduction below atmospheric.) Because 101.3kPa=~14.7psi, this works out to ~2° per pound of boost. It is often the case that you want to limit the retard under boost as well (to limit EGTs), typically so that it takes out no more than about ½ of the maximum advance at 100 kPa. This won't get you a whole lot of boost though - in talking with TTR engine builders they're running down to 7deg BTDC at max torque, well below the 15deg the stuff above suggests. To tune a turbo you really need to know EGTs.