Crab Style Distributor Help

third_edition

third_edition

Member
Hi,

I have a '48 Flathead which has a front mounted, crab style distributor. It has been retrofitted with a Pertronix Electronic Module (replacing the points).

My first question is regarding the Vacuum Brake - what does this do and do I still need it with the electronic module?

My second question is regard the ignition advance - what should the "base" timing be on this ignition system / engine and is there any timing added as the RPM increases?

All help / insights would be appreciated.

thanks,
George
 
Great setup. It works very well for me, even on 6v positive ground.

Yes, you need the vacuum brake. There’s instructions here on how to set it if you do a search.

To time this setup, you’ll have to find top dead center, make some marks with tape on the dist cap and crank pulley, and time it with a light. I timed it to 24 degrees at full advance.
 
Last edited:
third_edition said:
Hi,

I have a '48 Flathead which has a front mounted, crab style distributor. It has been retrofitted with a Pertronix Electronic Module (replacing the points).

My first question is regarding the Vacuum Brake - what does this do and do I still need it with the electronic module?
Click to expand...

"Re: Crab distrubtor ignition advance
The vacuum brake is unique to the old Ford/Mallory designs. Ford changed the advance unit to the 11A configuration in 1941 and that flyweight set was used till the end of production in 1948. It was retro fitted as a replacement to all the older ones as a service part. The Ford instructions for adjusting the brake was to turn it far enough to eliminate pinging but they rarely ping even with minimal setting due to the volatility of the modern gas we use. Flatheads don't need a lot of advance due to the way the fuel mixture charge burns in the cylinder. Compression ratio is limited and the flow path is hectic. A person rarely needs much movement of the vacuum brake so the spring tension doesn't need much adjustment at all unless the motor is in a heavy truck pulling max load.

Just make sure the flyweights work smoothly all the way through their arc with good spring return and the advance will work as intended. If a person advances it too far, they could have overheating of the engine. That was the reason for the load control of the vacuum brake was to eliminate detonation under load by slowing down the advance. Pre-ignition is not the same as detonation, true, but it is rather rare with it's own distinctive cause. Detonation is caused by too much advance under high load conditions. Detonation is a spontaneous full burn of a fuel air charge. Another word for explosion. Engines are designed to have a burn start at ignition with full burn flame just a bit after TDC. If it burns too fast then it's an explosion before the piston starts travelling down which is not only counter productive but tends to do some damage. The flame front should fully transition into downward push of the piston.
Last edited by rotorwrench; 01-13-2026 at 03:33 PM.

Check the leather washer part of the brake. If there is a heavy groove, rotate it 90° for a fresh portion or replace the leather. Unscrew the adjustment with your fingers, then screw back in 1 to 1-/2 turns as a start. Most times with today's good fuel, that should be correct.

third_edition said:
My second question is regard the ignition advance - what should the "base" timing be on this ignition system / engine and is there any timing added as the RPM increases?

All help / insights would be appreciated.

thanks,
George
Click to expand...

The base or static timing for the crab is 4° advanced. Vanpelt shows a method provided by Ford when a distributor test machine isn't available.

http://www.vanpeltsales.com/FH_web/flathead_engines_distr-timing.htm

566679


I do it as below. Note that the P sized drill bit is snug on one of my crabs but not another, that one is drilled for a 3/8: bit. Select a bit that is snug in the hole.

543565


Dynamic advance is controlled by flyweights and springs in the crab. Make sure that they are lubed and free to move. To check the tatal advance yoou would need a distributor test machine or by adding a TDC marker, wiith a timing light. A dial back light is most convenient.

573898
 
You must log in or register to reply here.
Back
Top