Vacuum retard range

[magnograil]

I am building a programmable ignition but so far have not been able to find what the range of retard and vacuum are typical. My Alfa V6 manual gives 250 mbar = 0 deg and 380 mbar = 10 deg which does not make sense (250 mbar = 3.626 psi, 380 mbar = 5.511 psi). Intake vacuum ranges from around 14 psi at idle, closed throttle, to 2-3 psi at high RPM, full throttle.
The circuit will measure the manifold vacuum with a pressure trandsucer. Once the unloaded vacuum curve is known, the difference between the calculated and measured vacuum can be used to add in a retard value to the RPM advance curve.
I intend to use this on various vehicles so general information would be appreciated.
Thanks.

 

[icrman]

Most of the old vaccum advance units would be connected to venturi, if you wanted a combined total advance.
And some applications where connected to the inlet manifold to retard on acceleration and a large load.
Venturi vaccum is much lower than inlet manifold vaccum.
And is inverse. Wide throttle opening means lower intake vaccum, and higher venturi vaccum.

 

[1Bluegrass]

There are ignition controls on the market that are adjustable for nearly and combination of advance you would want and doesnot depend in any vacuum for operation.
We run one on a sprint car engine.
We can dial in any starting timing, have it advance for idle and low rpm, set mid rpm rates and hi rpm advance.
The rates of advance is also adjustable.
This unit has the option of sensor inputs for other special application such as retard for blower or juice operation.
You would just be duplicating devices already on the market.

 

[magnograil]

Thanks, I found a web site where someone measured the centrifugal and vacuum ranges of Lucas distributors.
1Bluegrass - I know Crane builds an electronic distributor where you can select from several centrifugal and vacuum curves. However, you have to use the curves they provide. $450 for $30 (non-quantity retail) worth of electronics.
Mine will allow the user to set the curves and ranges and load them into the EEPROM. It also is a distributorless system. The coils are turn on only when needed.
No one makes ignitions for my applications. First is for my BSA triple motorcycle. The Boyer and Lucas do not work with stock coils, below 10 volts or have vacuum retard. Second is for my Lotus Europa using dual output coils, third is for my 911 with dual plugs.
The two systems in use are vacuum advance and vacuum retard. The advance system works off a ported manifold vacuum whereas the retard works directly off the manifold vacuum. The only difference is the high RPM high load centrifugal setting. Either 35 degrees centrifugal with 0 degrees vacuum advance or 45 degrees centrifugal with 10 degrees vacuum retard.
Vacuum advance/retard is for better economy at part throttle. For all out racing it is not necessary where the engine never runs below the centrifugal curve except for starting.

 

[1Bluegrass]

We have an MSD contoller on the sprint car that is fully adjustable. The dist. has no part in timing adjustments.
The controller is adjustable in three rpm segments for advance rate and amount.
We also tried a DIS system with crank trigger that was also fully adjustable but the problem was the trigger reference rpm at starting was to slow to start system timing, with the starter/battry/compression combination, so we had to abandon it.

 

[magnograil]

That could be a problem with using a reluctance type pickup. If the pole does not move fast enough across the face, not enough voltage gets induced to trigger the input. Making the input more sensitive can cause problems with noise. One of the reasons for using the optical IR pickups. A drawback is the sensitivity to dirt.
The pickups could be placed on the crank for a little better accuracy but not enough to justify the installation problems it causes.
This system also allows decreasing advance with RPM. At the RPM limit, it goes to full retard for a soft limiter and cutout at a predefined RPM above that.

 

[Flatjwl]

I don't remember if "Bluegrass" mentioned the high speed retard chip included in the computer timing system. Or at least the MSD system we use on the dyno has that useful feature. It is interesting to experiment with how timing changes can affect WOT performance. We also use an adjustable timing control so "on the fly" timing adjstments are at the operators fingertips. Another interesting feature available with systems already available.

 

[Tmoose]

"250 mbar = 0 deg and 380 mbar = 10 deg which does not make sense (250 mbar = 3.626 psi, 380 mbar = 5.511 psi). Intake vacuum ranges from around 14 psi at idle, closed throttle, to 2-3 psi at high RPM, full throttle."

In most non-supercharged cases (original VW beetle being one, dark days of 70s emission control being another) I think you will find the typical car uses centrifugal advance and vacuum advance.

IF your Alfa is non-supercharged, and the full throttle 2-3 psi is >>manifold<< vacuum, then I think the carpet has bunched up under the accelerator pedal preventing the throttle from opening fully. Should be much closer to zero.

 

[1Bluegrass]

Intake vacuum is a function of carb throat area provided there is not a restriction in the airflow path to the valve that alters this. (small runners in a large displacment and high rpm)
The idle level vacuum is closely related to cam shaft timing and duration.
A 5L Ford has intake vacuum levels in the 18" of water range.
The cam in these engines have negitive valve overlap and a wide lobe seperation angle.
A near zero vacuum at WOT is normally what is sought in a performance engine.
An interesting thing that happens in a 2 barrel racing class on a meduim to large engine is the power valve in the carb will open as per normal off the turn but as the engine winds, vacuum is again built up closeing the power valve and leaning out the engine. Under this case if the intake vacuum were to be monitored you might find as much as 8" +/- at peak rpm.
In a vac advance system this would certainly alter desired timing for max power.
That answer is to use centrifagal advance only.
This is getting away from the original post but to illistrate some of the different operating situations that need to be considered when designing a ignition advance control system.

 

[magnograil]

1Blugrass, That would be easy enough to correct for by cutting out the vacuum effect at some RPM. An advantage you do not have with mechanical systems without the help of Rube Goldberg. Since I am writing the program I can make whatever changes I want without being confined to some manufacturers idea of what I want.
If you measure manifold vacuum instead of intake tract then the cam timing has less effect. The diameter and length of the connection to the sensor can also be used to dampen the pulses. You would not want to measure vacuum on the intake tract as the vacuum timing will be different on that cylinder from the rest.
All the distributors that were measured only operate over a narrow vacuum range (~200 mbar) and nowhere near idle vacuum (~400 mbar maximum).
Tmoose, I did not measure the full throttle vacuum, only estimated it. The throttle plate is a long way downstream from the air filter and air meter through a convoluted hose so it will have some pressure drop. Since a running engine obviously has a wider range of vacuum than 130 mbar, I erroneously assumed that the manual was incorrect.
Most modern engines use vacuum advance over vacuum retard but that requires ported vacuum over strictly manifold vacuum.
There is nothing wrong with the accelerator pedal or carpet. Being an '82, some of the accessories are emissions related and not strictly performance driven.