Distributor tuning

[Yadkin]

I am experiencing rough running under load and recently, some knocking at about 3000 rpm. My newly rebuilt engine has about 9.7:1 compression ratio with the stock iron cylinder heads.

Cam is a Comp Roller 33-000-9. This is one step up from an OE or "towing" cam. Idle quality is good, 650 RPM. I measured valve lift and they are all identical.

I did a dry compression test and found 7 cylinders at 170psi, 1 at 180. Wet compression all at 180.

Initial timing is 10 degrees BTDC. Using that as my zero, I measure centrifugal timing to be as follows:

RPM OE Rec'd This setup
800 .5-1.5 2
1000 3.5-4.5 8
1600 5.75-7 19
2800 8.25-9.5 24
4000 10.75-12.75 24(max)

Vacuum advance at 1000 RPM

In Hg OE Rec'd This setup
8 2-5 0
10 4-7 3
15 5.5-8.5 4
20 5.5-8.5 4

[Yadkin]

I run 93 Octane pump gas, E10. I have a FAST 1.0 fuel injection kit.

I did nothing to my mechanical advance from it's factory setup, except of course for putting 90,000 miles on it. It's obviously putting in too much advance, and too soon. The vacuum advance is adjustable so that's not a big deal, except no reason to adjust it until I have the mechanical dialed in.

Fordmuscle.com suggests using a total advance of 38-40 for peak power. Mustang 360 recommends no more than 34-36. I have 10+24=34.

[Yadkin]

I also have a Crane XR-i points conversion module in the distributor. I marked my crankshaft damper at 45 degree increments, tested each plug wire with my timing light and am within a degree at each.

Do I just need to re-curve the distributor to be more like factory specs? If so, how much lower should I make the total advance?

[Yadkin]

I'm going to set my initial at 8 and see what happens.

[simplyconnected]

...Cam is a Comp Roller 33-000-9...

Steve, this is a race roller cam core. It is a custom grind into whatever duration you choose.

Consequently, I have no idea what cam you actually have. It could very well be that your duration will produce a lumpy idle. Let's see the cam card.

Your compression spread looks ok. Your compression ratio demands premium fuel (93 is good but don't go lower).

You can always dial back ignition timing and it will work to a degree, depending on your cam timing. I've discussed my way of timing the cam but yours is quite different.

At what rpm is your cam timing 'all in'? - Dave

[Yadkin]

Thanks for your input Dave.

Here is a photo of the cam card.

[Yadkin]

I found out last night that I had my vacuum advance plugged in too low on the throttle body- oops. I was getting a signal below the throttle plates instead of on top. It's a rain day so I can't test it out.

[scumdog]

I found out last night that I had my vacuum advance plugged in too low on the throttle body- oops. I was getting a signal below the throttle plates instead of on top. It's a rain day so I can't test it out.

The vacuum advance SHOULD be hooked up to manifold vacuum, i.e. Below the throttle plates.

[Yadkin]

The vacuum advance SHOULD be hooked up to manifold vacuum, i.e. Below the throttle plates.

That's not what I read yesterday. And it makes sense to me; here's why.

Under the plate(s): At idle, manifold vacuum is high, so the vacuum advance would increase to it's maximum then decrease under load.

Over the plate(s): At idle, vacuum is zero, then increases with increased throttle opening.

I've been taught to disconnect the vacuum advance and plug it temporarily while setting initial (at idle) timing. If you then add in advance with vacuum, you lost that setting at idle.

[Yadkin]

I just came back from a 30 minute test drive. She's definitely running better. No matter how hard I mashed the throttle, no knocking. Idle quality is better; so is top end power. I couldn't even see any smoke while full throttle, where as before there was a big blue cloud back there. Still, though, not as smooth as I expect while cruising.

Could it be the cam? I figured for a bit of "lope" at low engine speed, but not cruising speed.

[scumdog]

That's not what I read yesterday. And it makes sense to me; here's why.

Under the plate(s): At idle, manifold vacuum is high, so the vacuum advance would increase to it's maximum then decrease under load.

Over the plate(s): At idle, vacuum is zero, then increases with increased throttle opening.

I've been taught to disconnect the vacuum advance and plug it temporarily while setting initial (at idle) timing. If you then add in advance with vacuum, you lost that setting at idle.

Your call BUT with manifold vacuum you get the following: lots of advance when manifold vacuum is high i.e. at light throttle setting/light load on engine and you get best economy, engine efficiency.

However when you stab the gas pedal and the manifold vacuum drops your timing retards so you don't get pinging/detonation, ease off on the gas pedal and you're back to optimum timing for smooth running and economy.

BTW: Ported vacuum control of timing was introduced by the car manufacturers as a 'crutch' to get through emission issues when smog control arrived.

[Yadkin]

Interesting perspective. Connected to the manifold directly, vacuum advance would be highest at idle. You'd have to keep that conected to set initial advance? Wouldn't you then get a decrease in advance at cruising? It seems to me that the mechanical and vacuum advance curves would be working against each other.

[OX1]

Interesting perspective. Connected to the manifold directly, vacuum advance would be highest at idle. You'd have to keep that conected to set initial advance? Wouldn't you then get a decrease in advance at cruising? It seems to me that the mechanical and vacuum advance curves would be working against each other.

Would think you have to either bump static up considerably or re-curve distr to get the total timing you had before (unless the theory is to have less total timing at high load, higher RPM).

[Yadkin]

Would think you have to either bump static up considerably or re-curve distr to get the total timing you had before (unless the theory is to have less total timing at high load, higher RPM).

By connecting to manifold vacuum you'd have to reduce static timing to get the factory recommended 8 to 10 degrees BTDC at idle speed.

Maybe I'm looking at this the wrong way but everything I've read is that ignition timing should be increased with RPM. There is a certain amount of time between spark and enough pressure building where it begins to push on the piston.

I look at it like pushing a child on a swing. If you push to late, that is while the swing is going away from you, you won't get power into it. If you push to early, that is the swing seat coming at you, there's going to be a shock, or impact. If the spark is too late you lose efficiency and power, too early and the engine knocks.

The time required to achieve pressure should be (I'm guessing here) a constant. So the RPM/ advance curve should be linear. But in a mechanical advance, the force resisted by springs is a linear function and the force developed by centrifugal weights is a squared function. I thought the vacuum added to that is to make the curve more linear.

[simplyconnected]

I'm not taking sides here but you need to consider air-to-fuel mixtures. I grew up with GM cars that use manifold vacuum. Yes, I know this is a Ford but hear this out:

For initial timing, GM pulls the distributor hose off and plugs it. Then they time at ~6 degrees BTDC. What does this do... When your engine is stopped there is no vacuum so timing is at 6 degrees. As soon as it starts, vacuum advance.

If you punch it at a light vacuum immediately drops off and so does the advance. As rpms increase the distributor weights advance timing even at WOT (wide open throttle). Ease off the pedal and the vacuum adds more advance, as it should.

Punch it again at high speed and vacuum immediately drops off and so does vacuum advance, as it should. Mechanical weights still control HS advance. Lifting the pedal again advances more with vacuum. There is no lag.

Remember, when vacuum is high, air is choked off. In affect, this makes your engine displacement smaller because the air is thin and fuel is reduced. This is where the air/fuel mixture tries to maintain 14.7:1 for the best economy. When vacuum is low, air is more dense and the mixture is rich. This is the time to back off on spark advance. - Dave