Crab Distributor vacuum brake

[pete rose]

When you install the crab distributor on the engine and screw in the vacuum brake, how do you tell how far to screw in the top 7/16 screw? into the base lock nut? Is it by feel or is there a calibration ?
Most responses I've seen here say to back off the top screw until the engine pings then adjust it inward a bit
Need to know the starting point
Thanks to any helpful folks on the barn
Pete

[flatjack9]

Start with it backed all the way out.

[Flathead Fever]

There is a factory precise setting using "distributor" rpm and vacuum for a completely stock '46-'48 engine. I'm pretty sure that is for setting it up on a distributor machine. Your advance will differ depending on the compression ratio, bore & stroke, carb jetting, camshaft choice, fuel octane, altitude, air temperature... Unless your engine is an absolute stock '46-'48 the factory setting will not be correct. The stock setup will work okay but it won't be dialed in for your engine.

I dug out 1949 Motor's Manual.

It says the initial timing is 4 Degrees. That will get bumped up on a go-fast flathead

Initial Advance Crankshaft is a minimum of 21 and maximum 24 at 3450 rpm.

contact points spacing .014-.016

Dwell at idle, left point's 50, Right point's 45-55, combined dwell 78-85. You set each dwell separately by sticking a piece of cardboard in the opposite one. The combined dwell will be 78-85. We always adjust dwell first and then the timing. Dwell changes the timing; timing does not change the dwell. We adjust the carburetor last.

Then they have a separate chart for the advance. This gets a little complicated.

"Vacuum Brake released" Distributer RPM that is "not" crankshaft rpm.
At 200 0-1 advance
At 400 6 1/2-8 advance
At 600 10 1/2-12 advance

Vacuum Brake on
At 200 0-1 with 0 inches of vacuum
At 1000 4.5-6.0 advance with 1.6 inches of vacuum.


1.6 inches of vacuum means the throttle is open pretty far so the vacuum break is retarding the timing to prevent detonation. It looks like most of the time that vacuum break would not even be doing anything until you were under a load. That is exactly why everybody says, under a load like climbing a hill back it off until it pings and turn it until it stops. Then its dialed in for your engine setup.

It always good to install a vacuum gauge in your car so you learn how driving conditions effect vacuum readings.

You "do not" need a distributor machine to set up flathead distributors but you do need a timing mark, which the earlier ones do not have. Anytime you have the heads off a flathead without a timing mark you need to make a pointer and mark the pulley at TDC and if you want at 4 degrees. Rotate the engine one direction until #1 is on TDC using a dial indicator and mark the pulley by your pointer. Then rotate the engine the opposite direction until #1 comes up and mark the pulley. In between those two marks is true TDC. You can file a little notch in the pulley and paint it white. If you have that one mark you can do all your flathead timing adjustments with an "adjustable timing" light.


Buy an "adjustable" timing light, never-ever buy a timing light unless it is an adjustable one. It allows you to measure how much advance you have by rotating a dial on the back of the light. Next you need a tachometer and a handheld vacuum pump. You can convert the distributer rpm specs to crankshaft degrees. You can use the handheld vacuum pump to apply the correct amount of vacuum, in this case 1.6 inches of vacuum to the vacuum break at 1000 distributor rpms. You "do not" need a distributor machine or KRW timing fixture and its actually more accurate to make the adjustments on the actual engine, but you must have a timing mark on the pulley. If you can always put the pointer on the side where the timing adjustment is on the crab distributor so you can watch the timing mark as you move the breaker plate adjusting the timing.

[Tinker]

Back it all the way out. Bruce L used to say run it out all they way. The octane level today you will be hard pressed to get a "ping". Start out and work to the drag. I run a full vacuum gauge in my car. Just lets me know what's happening along the way. I thnk

If you have used a dizzy shop and they set it. Best to leave it alone or adjust slightly.

Fever has posted a lot of great stuff.

[Flathead Fever]

I'll toss this out there too about the elimination of the vacuum brake. This is all theory; I have no actual experience on crab distributors. I was just out in the garage, I have a '66 Shelby GT-350 with a factory mechanical advance only distributor which reminded me not all cars had vacuum advances and vacuum brakes. Lots of high-performance Ford engines came that way in the 1960s with around 9.5:1 to 10.5:1 compression. You just had to buy premium fuel. They sold thousands of those Mallory dual point, mechanical advance only distributors for those 1960s-1970s V8s. I just bought one for a 289 in a Falcon I have. It was at a very small Redlands automotive swap meet advertised right here on the Ford Barn. It was brand new for $10.00, got the 289 Falcon headers too for $40.00. The idea on those high performance engines is you don't want the distributor backing off the timing under full throttle and killing horsepower. When it retards the timing it lowers the pressure in the cylinders, less pressure, less down force on the pistons.

What I don't know and I have meaning to research is what was the fuel octane in the 1930s and 1940s I know that Gilmore had "red" gas and "blue/green" gas so that when it went through the site glass on the pump you knew you were getting the octane you paid for but I have never seen what those octanes were? It may be that 87 is higher than what was available back when engines were 6.5:1 and you would do not need a vacuum brake at all with 87 octane. I just checked, I guessed close on the compression ratio, my 1949 Motors Manual says that a '46-'48 Ford V8 has 6.75:1 and 100 Horsepower at 3800 rpm. That compression ratio is a joke by modern standards. It might not ping at all on 87 octane. Back it off all the way and see what happens.

I worked with a mechanic that had worked for a research company formulating gasoline during the lead years, he taught me some stuff. He said, "you are wasting your money putting premium in a regular engine, your paying extra to actually lose power". The higher octane is only to prevent pinging/detonation in a higher compression engine. It burns slower to allow the mixture to be compressed further than lower octane fuel can. The slower burning speed has the same effect as retarding the timing. Even though the higher octane fuel does not produce as much energy that extra compression more than makes up for it.

I'll tell you a secret since were discussing timing. I had a CA State Smog License because we smogged the phone company vehicles inhouse. I couldn't do my own vehicles, I was in the same boat as you guys which just didn't fair. I should have been able to get away with all kinds of stuff on my personal vehicles. The allowable timing is plus or minus 2 degrees from factory or it fails the functional test. A lot of the time you can get a borderline hydrocarbon failure to pass by retarding the timing, but you can only retard it 2 degrees. We did everything by the book at work, no funny stuff. Whatever it cost to make it right we did it. I could not have done this at work but on a personal vehicle if you put premium fuel in it that has the same effect as retarding the timing because of the fuel's slower burn time. When the technician checks the timing, it will be within that plus or minus 2-degree window but it will actually be retarded further than the timing mark shows, with the premium fuel. That is perfectly legal, no rule about what octane you put in the tank. That just shows you how the octane effects the timing.

[flatjack9]

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Originally Posted by Flathead Fever

I'll toss this out there too about the elimination of the vacuum brake. This is all theory; I have no actual experience on crab distributors. I was just out in the garage, I have a '66 Shelby GT-350 with a factory mechanical advance only distributor which reminded me not all cars had vacuum advances and vacuum brakes. Lots of high-performance Ford engines came that way in the 1960s with around 9.5:1 to 10.5:1 compression. You just had to buy premium fuel. They sold thousands of those Mallory dual point, mechanical advance only distributors for those 1960s-1970s V8s. I just bought one for a 289 in a Falcon I have. It was at a very small Redlands automotive swap meet advertised right here on the Ford Barn. It was brand new for $10.00, got the 289 Falcon headers too for $40.00. The idea on those high performance engines is you don't want the distributor backing off the timing under full throttle and killing horsepower. When it retards the timing it lowers the pressure in the cylinders, less pressure, less down force on the pistons.

What I don't know and I have meaning to research is what was the fuel octane in the 1930s and 1940s I know that Gilmore had "red" gas and "blue/green" gas so that when it went through the site glass on the pump you knew you were getting the octane you paid for but I have never seen what those octanes were? It may be that 87 is higher than what was available back when engines were 6.5:1 and you would do not need a vacuum brake at all with 87 octane. I just checked, I guessed close on the compression ratio, my 1949 Motors Manual says that a '46-'48 Ford V8 has 6.75:1 and 100 Horsepower at 3800 rpm. That compression ratio is a joke by modern standards. It might not ping at all on 87 octane. Back it off all the way and see what happens.

I worked with a mechanic that had worked for a research company formulating gasoline during the lead years, he taught me some stuff. He said, "you are wasting your money putting premium in a regular engine, your paying extra to actually lose power". The higher octane is only to prevent pinging/detonation in a higher compression engine. It burns slower to allow the mixture to be compressed further than lower octane fuel can. The slower burning speed has the same effect as retarding the timing. Even though the higher octane fuel does not produce as much energy that extra compression more than makes up for it.

I'll tell you a secret since were discussing timing. I had a CA State Smog License because we smogged the phone company vehicles inhouse. I couldn't do my own vehicles, I was in the same boat as you guys which just didn't fair. I should have been able to get away with all kinds of stuff on my personal vehicles. The allowable timing is plus or minus 2 degrees from factory or it fails the functional test. A lot of the time you can get a borderline hydrocarbon failure to pass by retarding the timing, but you can only retard it 2 degrees. We did everything by the book at work, no funny stuff. Whatever it cost to make it right we did it. I could not have done this at work but on a personal vehicle if you put premium fuel in it that has the same effect as retarding the timing because of the fuel's slower burn time. When the technician checks the timing, it will be within that plus or minus 2-degree window but it will actually be retarded further than the timing mark shows, with the premium fuel. That is perfectly legal, no rule about what octane you put in the tank. That just shows you how the octane effects the timing.

That is a total myth about high octane gas burning slower.Tests show that regular, premium, high octane produce the same power with the same timing when used in the same engine.

[pete rose]

Thanks everyone, especially Fever and Flatjack, great info
Tinker, I'll take your advice and run it all the way out to begin with
Thanks again, knew I'd get the answer here
Pete

[ford38v8]

Pete, with today's fuels, the brake need only be tight enough to prevent erratic bouncing of the timing disc.

[Flathead Fever]

Originally Posted by flatjack9

That is a total myth about high octane gas burning slower.Tests show that regular, premium, high octane produce the same power with the same timing when used in the same engine.

I don't buy that. The burn rates are different so if everything was identical in two engines but the fuel octane they would not produce the same power. You would have to adjust the timing to make the peak pressures of the different octanes happen at the same time ,but why buy premium when you can do it with regular. I have to go by what I was taught and tested on, to become a licensed smog mechanic and ASE Master Mechanic with 17 certifications. There is plenty I don't know, I learn new things everyday. Here is a Motor Trend article, and I did not look for an article that agreed with me. If you can find one of those studies show it to me, I'm open for different theories.

Fueling Basics: Explaining Octane Ratings

Octane ratings are plastered all over every gas pump, and, contrary to marketing terminology, "premium" does not necessarily indicate a fuel is "better" for your car as much as whether it is "right" for your car. Fuel with an 87 octane rating burns more quickly while higher-octane fuels burn more slowly. In engines designed for standard unleaded fuel, efficiency and performance is optimized for 87 octane and could actually perform worse with higher-octane fuel since the burn rate is slower. By contrast, a higher-performing engine, which includes engines with higher compression ratios and/or forced induction, requires the slower burn rate of higher-octane fuels to defend against engine knock. Knocking or preignition occurs when the air/fuel mixture ignites before intended. This happens because higher compression ratios and cylinder pressures cause the air/fuel mixture to heat up, and, in some cases, ignite prematurely. Knocking that goes unchecked inevitably leads to engine damage. Higher-octane fuels help to mitigate preignition events while ensuring proper combustion and higher output for performance engines.

[69a]

X2 fever

[vincent]

All the fast men in the last 100 years of motorsports were using the lowest possible octane ratings without pinging to get the fastest burn. My friend Marty Dickerson held a couple records with his Vincent motorcycle on the salt flats in 1949 and further on in the 8 to 1 category and told me that when using higher octane fuel the timing slip was showing - slower speed!

[flatjack9]

Originally Posted by Flathead Fever

I don't buy that. The burn rates are different so if everything was identical in two engines but the fuel octane they would not produce the same power. You would have to adjust the timing to make the peak pressures of the different octanes happen at the same time ,but why buy premium when you can do it with regular. I have to go by what I was taught and tested on, to become a licensed smog mechanic and ASE Master Mechanic with 17 certifications. There is plenty I don't know, I learn new things everyday. Here is a Motor Trend article, and I did not look for an article that agreed with me. If you can find one of those studies show it to me, I'm open for different theories.

Fueling Basics: Explaining Octane Ratings

Octane ratings are plastered all over every gas pump, and, contrary to marketing terminology, "premium" does not necessarily indicate a fuel is "better" for your car as much as whether it is "right" for your car. Fuel with an 87 octane rating burns more quickly while higher-octane fuels burn more slowly. In engines designed for standard unleaded fuel, efficiency and performance is optimized for 87 octane and could actually perform worse with higher-octane fuel since the burn rate is slower. By contrast, a higher-performing engine, which includes engines with higher compression ratios and/or forced induction, requires the slower burn rate of higher-octane fuels to defend against engine knock. Knocking or preignition occurs when the air/fuel mixture ignites before intended. This happens because higher compression ratios and cylinder pressures cause the air/fuel mixture to heat up, and, in some cases, ignite prematurely. Knocking that goes unchecked inevitably leads to engine damage. Higher-octane fuels help to mitigate preignition events while ensuring proper combustion and higher output for performance engines.

In a recent Engine Engine Masters episode, they tested all the fuels and found absolutely no change. You can believe what you want but tests totally disprove any need for changing timing.

[ford38v8]

Flatjack9 watched a TV show that proved all the experts wrong. Imagine that!

[tubman]

Originally Posted by ford38v8

Flatjack9 watched a TV show that proved all the experts wrong. Imagine that!

I have learned to like "Engine Masters". I know that a couple of the guys (Freiberger and Dulchich) come off as kind of goofballs, but deep down they really seem to know what they are doing. And the old guy runnin' the dyno? Well, I'm impressed. I wonder how many of the "experts" referred to here have as much time on the dyno as these guys? Not many, I'd venture.

[flatjack9]

Originally Posted by ford38v8

Flatjack9 watched a TV show that proved all the experts wrong. Imagine that!

What experts? As I said believe what you want. As Tubman has said, those guys know what they are doing and the dyno proves their point. Pooh, Pooh if you will.

[flatjack9]

Originally Posted by tubman

I have learned to like "Engine Masters". I know that a couple of the guys (Freiberger and Dulchich) come off as kind of goofballs, but deep down they really seem to know what they are doing. And the old guy runnin' the dyno? Well, I'm impressed. I wonder how many of the "experts" referred to here have as much time on the dyno as these guys? Not many, I'd venture.

Amen!