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911st 911st is offline
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Join Date: Jan 2004
Location: Sacramento
Posts: 7,269
Disconnecting Vacuum Retard

The Vac-Retard connection to both style distributors works only at idle. With acceleration off idle it quickly goes away and timing jumps about 8 to 10 deg advance. So same same to this point for both distributors.

Vac Retard at idle seems to be mostly a smog function thought it can also help with making a motor easier to start. Running with it disconnected should help reduce operating temps at idle as the fuel burns more in the cylinder and less in the exhaust tract.

If it is disconnected, idle RPM will increase as the motor will be operating more efficiently but this can be brought back in line by adjusting the idle air bypass screw.

As the motor quickly bumps timing with loss of intake vac and acceleration anyway, running with this disconnected should not be a big issue. I also would not expect it to improve off idle response much though it might add a little crispness.

Performance is gained by timing being set at more ideal settings. Many have reported increased low end response with more initial advance. Disconnecting the Vac-Ret dose not really get any real added advance off idle.

Which distributor is a better for performance.

Both can be worked to provide equal performance under max acceleration and boost.

However, because the Double Pressure Box distributor has a boost retard function I believe it has the most potential to be a better distributor. I think even the later Euro 3.3's may have had a double connection pot.

If we agree peak safe acceleration is found near -26 deg on US fuel, the US dist can be set to run at that on boost. However, with this will come the ability to run a more desirable ignition advance of about -34 through -36 at cruse knowing the boost retard will pull timing back 8-10 deg when needed. (The C2 Turbos pull about 40 deg on cruse.)

This should lower cruse operating temps and increase gas mileage as the motor is operating at a more efficient and ideal point and getting more power out of the same amount of fuel.

What happens at idle if we just set total timing with 10 deg more advance?

It will advance idle timing the same 10 deg. With the Vac Retard working, timing with acceleration and loss of Vac-Retard would then quickly move from about -10 to about -20.

This may or may not be to aggressive. AFR will have some effect. If CO is set at 3.5% or an AFR of about 13/1 it might be ok. Some have reported being able to run this much timing off idle or by about 1500rpm. If this is to much initial advance then we would have to increase the range the mechanical advance operates over. If we add about 5 deg to the mechanical advance range (mod the stop) this would bring idle timing to about -5 at idle and quickly jump to -15 or such with acceleration. This should improve off idle response a noticeable amount and work for most.

Again, both can be tuned for peak HP and acceleration and will be equals in these areas. However, the dual pot because it has 'boost retard' capability can also run a more favorable advance at curse for better mileage and a cooler running motor.


Please be careful.

A motor that can run 12.2/1 AFR and -26 deg advance on boost is going to make more HP than a motor at a more factory spec of say 11.5/1 AFR and -17 deg advance on boost.

However, if the fuel system goes a little out of spec and one or more cylinders should go lean, one gets a tank of poor gas, or should one's motor become heat soaked from sitting at a light with the A/C on on a hot day-- the factory spec motor will be less likely to sustain any damage.

What we do not know is how much advance most of us can run to improve off idle response and on boost power. This will vary by motor depending on its actual compression, cams, intercooler, boost level, out door temps, and other variables.

JMO
Old 04-02-2010, 09:28 AM
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