Mechanical advance

[adam05ford]

Anyone know much about mechanical advance on a distributor and how to go about what I would need to get it close? I just got a new distributor and it had this chart on the box but I'm not sure which would be best.

 

[AZ73]

It's all based on your cam.

 

[xcntrk]

Interesting so this distributor has adjustable mechanical advance separate from the centrifugal spring weight? Most conventional units require that you swap out the reluctor arm to change how much centrifugal advance the dizzy can generate.

Here's a real good read on mechanical timing for Fords:
http://www.fordmuscle.com/archives/2000/03/timing/index.shtml

"Dyno testing has shown that most small block Fords with 9:1 to 9.5:1 compression make peak HP with 38-42 degrees total advance."

If it were me, I would configure all my advance to come on as low as possible in the RPM range. After all this is a truck, not a drag car and you likely never see above 3k RPM. So from that chart, I would use the bottom right setup (2 silver springs) then run with the RED setup for 28 degrees of mechanical advance. Then you set your initial at 10 degrees (for a grand total of 38 degrees).

 

[adam05ford]

Yeah I heard it all depends on the cam I'm just not sure what is in mine. PO put trick flow heads and edelbrock intake, and I know the push rods are trick flow as well so I would think he stepped up the cam as well but I'm not sure. I was thinking on the same lines as well either using the bottom right or middle right chart. I feel like the bottom right might be advancing too soon at <1000 rpm.

 

[Quick & Dirty]

A performance engine should get full advance by 2500-3000 rpm, a heavily loaded tow-rig/RV should be delayed to about 3500.

 

[Seventee]

The middle right is closest to what I shoot for with my 400's and 460. I had a curve similar to the bottom right in one of my 400's that resulted in too much tendancy to ping between 2000-2500.

I have not yet tuned the advance on my 302, but plan to shoot for similar results.

 

[Broncobowsher]

More then just the cam, head design is a factor as well. Even air/fuel mix plays a roll.
Nice distributor. Looks like the limit stops (Red, silver, blue, black) are seperate from the advance rate (Blue and silver springs).

I like to see a little less initial advance. It helps with idle. Starts better. Less sensitive to loads (automatics put into gear, A/C turned on, initial loading of the clutch in a stick). With this distributor you should have no issue backing the static timing down to 4-6° and still get the total timing you desire. So how much total? The 28° red limit will allow plenty of advance while keeping the base plenty low. If it is too little initial to be happy I would then try the silver 25° advance. Remember that total mechanical advance is the limit plus the base. So 10° base with a 25° limit will be a total of 35°

I remember doing some dyno tuning. The rule for timing is run as little as possible without loosing power. With more timing power goes up. There is usually a platau where the power levels off. Then more timing will hurt power. The low end of the platau is generally the best target. Detonation can limit it even lower.

Lower base timing also cleans up the idle for emissions testing.

Now how fast (the springs) to run? Again based on the cam, heads, air/fuel mix. Most engines feel best with an aggressive curve (ramp up the advance quickly). This can cause wome detonation issues in the lower RPM band where the advance has ramped up too quick. Usually 3-4k RPM you want it to max out. I would start with the 2 blue springs.

 

[adam05ford]

So now after hearing that great explanation do you think I should run vacuum advance as well or is that just adding too much variable without being able to dyno.

 

[xcntrk]

I remember doing some dyno tuning. The rule for timing is run as little as possible without loosing power. With more timing power goes up. There is usually a platau where the power levels off. Then more timing will hurt power. The low end of the platau is generally the best target. Detonation can limit it even lower.

.

It's called Minimum Best Timing (MBT) or basically the point of diminished returns. Where adding more timing stops producing noteworthy performance gains and only results in diminished knock threshold.

 

[Broncobowsher]

Vacuum advance is yet another variable. Best thing it does is add fuel economy. The centrifical advance only factors engine RPM. Has no load correction factor in it. When driving at cruising speed you have manifold vacuum, which means the cylinders are not filling to capacity, limiting cylinder pressure, limiting power. Throttling back the available power, hence the throttle.

The key point is cylinder pressure. When it drops, the air/fuel doesn't burn as fast. When it increases it burns faster. With a slow flame speed and lower cylinder pressure the peak burn and peak cylinder pressure opertunity occur after top dead center. You are not getting the most bang for your drop of fuel. So a ported vacuum advance will bring the spark in sooner. This allows the peak cylinder pressure to happen at top dead center of the firing cycle. Well close to it, you can only get so close with mechanical controls and variable loads. But the vacuum advance is a great correcting factor. Now you are getting the best bang for the drop of fuel. For a given throttle level you will make more power. Since you are cruising you can back off the throttle a little bit and get the same power you would without a vacuum advance. Since you are making the same power with less throttle, you are using less gas.

On the dyno the typical scene is full throttle. No vacuum, vacuum advance tuning is not a variable. It is a light throttle cruising at speed tuning tool. If you get into detonation at light throttle and not full throttle (at the same RPM) you have too much vacuum advance. That really is about as far as you can shade tree tune a vacuum advance. Any more then that would take a nice high end R&D style engine dyno that you don't typically find in the performance world. I only know of a few and they are not available for public use.

 

[adam05ford]

Thank you everyone. That is more info than I could have ever hoped for. I will get everything back together this weekend and see what I can figure out.

 

[broncnaz]

Basically run as much advance as you can until the engine pings. As has been said a lower initial advance helps I like to run 10-12 initial the rest is mechanical. I really dont factor in vacuum advance but if yours is adjustable and you get pinging try backing off the vacuum advance first before backing down the initial or mechanical.

 

[Broncobowsher]

Basically run as much advance as you can until the engine pings. As has been said a lower initial advance helps I like to run 10-12 initial the rest is mechanical. I really dont factor in vacuum advance but if yours is adjustable and you get pinging try backing off the vacuum advance first before backing down the initial or mechanical.

That is part throttle detonation that you should back off the vacuum advance.
Full throttle detonation needs mechanical advance backed off (stiffer springs, reduced initial, or reduced limit stop) depending on where you get the detonation at.

Second, you should not run as much as you can. It should be as little as possible without hurting power. If it runs the same at 30° total as it does at 34°, 30° is the better setting.

 

[broncnaz]

If you actually check timing with vacuum advance hooked up you can see as much as 50 degrees advance. without detonation or pinging.
Of course the key words there are "without hurting power" which basically means run as much as you can. You should be making power until you start pinging. Of course each engine is different and timing needs are different. Some engines ping easliy others you cant make ping for anything.

But yes if you see no differance running 30 degrees than you do at 34 well there's no real benefit to running more.