Going to crank fire...how do you lock the distributor?

Avery · 09-18-2008, 08:47 PM

I've installed a crankfire sensor on my SC race motor....but sticking with the SC distributor. What the best way to lock out the mechanical advance in these units?

cmonref · 09-19-2008, 03:46 AM

If you are going to use the distributor to fire the plugs based on an ECU's computations, then you do NOT want to lock out the advance on the distributor. The advance allows the rotor to align with the cap's terminal. You will get misfires when the rotor cannot properly align because the distributor is locked. Pelican tbitz found this in his application just like yours. His thread on the subject is titled "CIS to EFI & Ignition."

JohnJL · 09-19-2008, 06:27 PM

If you are using crankfire sensing why are you using the distributor? You are almost all the way there! Summit and others have lots of modern, high-power, durable coil choices, depending on what ECU you are using and whether you are twin-plugged or not you dont need a heck of a lot of work do finish off a trick application. Smart Racing and I think Clewett have distributor block-offs.

ischmitz · ischmitz's Garage

I think you need to give some more details as to what you are planning to do. As others said the distributor in an SC generates the advance and triggers the CDI box accordingly. It is the trigger signal that defines timing. The mechanic advance of the rotor just follows that advance and so happens to sit on the same shaft at the trigger wheel in a stock SC.

If you use a crankfire sensor to drive the CDI box directly you'd have no advance (even if the stock distributor advance mechanism is still moving around). So you'll need to use some sort of brain (aftermarket ECU) that generates a modified trigger signal based on RPM and MAP to drive the CDI box. As others said instead of driving the CDI box you could go all the way to coil packs (individual coils for each spark plug) and loose the distributor. This has the advantage that no moving part in the ignition circuit wear out (cap, rotor).

If you want to retain the CDI box and use the stock distributor you want it to advance. You have to hope that its advance curve will be similar enough to the advance created by the aftermarket brain. If not you won't be able to cover a wide dynamic range without causing miss-fires at either end of the spectrum. Locking the advance mechanism will be a bad thing. It will limit your dynamic range to the angular section as wide as the contact on the rotor plus the lenght of the contact pin in the cap.

Ingo

Avery · 09-22-2008, 11:28 PM

Thanks for all the replies....I should have provided more info. I am using the SDS (Simply Digital Systems) EFI which uses a crank mounted timing sensor. The SDS has integrated ignition timing control so one can set timing advance relative to engine speed. I was thinking that you had to lock the SC distributor mechanical advance since the SDS EFI took care of the advance. My reasoning was that the mechanical advance built into the distributor would mess up the programmable advance that you set in the EFI. Based on what Ingo is saying, I now get that you don't have to lock the distributor...

As far as using a coil pack vs distributor...decision is based on economics and aesthetics. The SDS unit will drive an MDS box and coil. The motor is in a 914GT clone that I built and I wanted to keep a certain level of vintage look to it.

JohnJL · 09-23-2008, 02:26 AM

In that case you can just remove the springs in the distributor and replace with safety wire. Be sure to check your timing throughout the rev range against your expected results.

j

304065 · 09-23-2008, 06:20 AM

Don't lock it out.

The advance curve determines when the spark occurs but it also phases the rotor.

Since your ECU determines when the spark occurs you no longer need any triggering device in the distributor. Such as points or a hall sensor. But you DO need to phase the rotor, assuming of course that your ECU is generating an advance curve that's fairly normal e.g. up and to the right with RPM.

Misfires will result if it's locked.

Avery · 09-24-2008, 10:09 PM

I did not lock the distrbutor as suggested. I had tested the motor on my dyno and it ran perfect but somewhere in the back of mind, I thought I had to lock the distributor. I was aware that the 964 distributor had a mechnical advance to maintain proper phasing but I never bothered to try to understand why that was needed. Now I know.

Thanks all...motor went into my 914 GT clone this past weekend. Still need to fab up hard oil lines to the front cooler before I fire it up.