understanding timing - someone set it straight

[87coupe50]

Wanted to restab the dizzy cause the edge of the TFI is real close to the water neck.

I have done a couple searches on timing but I'm still confused. I know the base timing is 10* BTDC and the rotor rotates counter-clockwise. I set the timing to 10* BTDC prior to getting the car tuned last Monday.

Before I pulled the dizzy I marked the #1 plug at the base of the cap, set HB on 0* with the #1 piston at the top. When I pulled the cap the rotor was farther counter-clockwise than the mark for the #1 plug. ???Wouldn't this be ATDC? I don't understand when people say "advance" the timing to 12* or 14* BTDC with the spout out. Isn't that actually retarding the timing? What am I missing? It never made any sense to me why you would spark the cylinder prior to it receiving full compression on the stroke. please help me understand!

 

[Foxfan88]

the reason the timing must be "advanced" (spark comes slightly before TDC)

you must remember that the spark itself occurs at say 10* BTDC, but it takes a slight amount of time for the flame front to reach out and hit all around the cylinder and chamber.

for best performance, you want the flame front to reach its maximum engery and press on the piston while the piston is at TDC just starting to go down.

since the flame front takes a small (real small) amount of time to move from the point of igntion and fill the whole chamber.

if you have too much timing advance. your piston pretty much tries to compress the explosion. which isnt good and its hard on parts. timing that is too retarded means the explosion chases the piston down and you get less combustion pressure and lose power.

at rpms increase so does the need for timing advance. the time it takes for the flame to reach the whole chamber remains the same, but at higher rpms the piston speed increases. so you have to further advance the timing, since things are moving faster at higher rpms. the additional advance will help it where the flame is expanding with its most force at TDC as the piston is going back down.

 

[87coupe50]

that definately makes sense.

But if I set the timing at 10* BTDC why would the rotor be farther counter-clockwise than the #1 postion on the cap? Isn't that after top dead center?

 

[Darkwriter77]

that definately makes sense.

But if I set the timing at 10* BTDC why would the rotor be farther counter-clockwise than the #1 postion on the cap? Isn't that after top dead center?

"Before TDC" epends on which way the rotor turns on a motor. If the rotor turns counter-clockwise, then having it farther counter-clockwise in the rotation would, in fact, be advancing the timing - the ignition is set to fire earlier in the piston stroke/crank rotation/cam rotation/distributor rotation sequence.

 

[87coupe50]

"Before TDC" epends on which way the rotor turns on a motor. If the rotor turns counter-clockwise, then having it farther counter-clockwise in the rotation would, in fact, be advancing the timing - the ignition is set to fire earlier in the piston stroke/crank rotation/cam rotation/distributor rotation sequence.

Exactly!

Previously I set the HB to 10* BTDC. Car runs great, just dynoed 413 RWHP. Today I lined up the HB to 0* TDC to restab the dizzy. Pulled the cap and marked the #1 position. The rotor travels counter-clockwise, and the rotor was already farther counter-clockwise than the #1 position! I really was expecting it to be 10* clockwise from the #1 position. I'm confused...

 

[Foxfan88]

the dizzy housing can move around if its loose. maybe you moved it around some.
doesnt really matter where it sits, when you pull the dizzy 9 times out of 10 you need to get a timing light and set the timing again. thats the best way to do it.

like Ron Popeil would say "set it and forget it" lol

 

[87coupe50]

the dizzy was tight. I just wanted to restab it cause the TFI module end was very close to rubbing the water neck. I had replaced the dizzy with a reman about 2 months ago cause the stator went out. Timing was set with a timing light.

I'm just trying to get a better understanding of "advanced" and "retarded" timing and why the rotor was "advanced" if it was set to BTDC.

Actually, I just had a thought. The rotor would be farther counter-clockwise than the #1 position at 0* on the HB because when it's set at 10*BTDC the cap #1 plug position would fire before it reaches 0* on the HB, therefore BTDC! Don't know if what i wrote actually makes sense to anyone else, but I think I got it now. I was focusing on why the rotor was past the #1 position not that the ignition would actually fire while contacting the #1 wire/plug position.

 

[67coupe]

the dizzy was tight. I just wanted to restab it cause the TFI module end was very close to rubbing the water neck. I had replaced the dizzy with a reman about 2 months ago cause the stator went out. Timing was set with a timing light.

I'm just trying to get a better understanding of "advanced" and "retarded" timing and why the rotor was "advanced" if it was set to BTDC.

Actually, I just had a thought. The rotor would be farther counter-clockwise than the #1 position at 0* on the HB because when it's set at 10*BTDC the cap #1 plug position would fire before it reaches 0* on the HB, therefore BTDC! Don't know if what i wrote actually makes sense to anyone else, but I think I got it now. I was focusing on why the rotor was past the #1 position not that the ignition would actually fire while contacting the #1 wire/plug position.

I was just about to reply. I think you figured it out after reading your last post.

The first time, if you set the timing to 10* BTDC. The rotor should be pointing at the #1 terminal. The second time, you moved the HB to 0*, since the rotor rotates counter-clockwise, it will be more CCW when compared to the position at 10 BTDC (i.e past the #1 terminal on the dizzy). So, what you are seeing is completely normal, nothing has changed. It should not unless the dizzy hold down bolt is loose, HB outer ring slipped, or the timing chain stretched.

Stab the dizzy back in according to your marks and comfirm timing with a light and you should be good to go.

 

[Fast63]

I think the issue is with rotor and cap positioning, not that anything is moving.

You're thinking of it too much in terms of rotor position, when it is easier to be thinking of it in terms of cap position. Instead of thinking of it as the rotor turning counter-clockwise, think of it as the cap turning clockwise. Then it is easier to understand that the farther "behind" the rotor is from the cap, the more timing BTDC you will get.

Does that make sense? Is that the issue you are trying to understand?