distributor timing

bowlsby · 12-08-1999, 09:30 PM

I have what I think is a simple question and I am hoping that someone would be kind enough to validate my assumptions.
I need to know what it means to *advance* and *retard* the timing, and what the effects are. Please indulge my ignorance as I have looked everywhere for the answers to these simple questions without success.

1. My 74 2.0L needs timing to 27 deg BTDC @ 3500rpm with the vac hoses removed. I assume that means both vac hoses on the distributor advance/retard diaphragm and not other hoses. Correct?

2. I can see on the fan housing the 0deg white mark and the 27deg red mark. I assume that the 27deg mark means that when timed correctly the spark fires at 27deg BTDC. Correct?

3. To *advance* the timing then, I would want to set the timing to like 30deg BTDC, (so the spark occurs earlier in the cycle), and not to say, 25deg BTDC and vice-versa to retard the timing. Correct?

4. What are the effects of timing advance/retard, and to what extremes can they safely be taken? Is there any benefit to adjusting to anything different than the 27deg BTDC for my car?

Thanks to all and especially to Wayne and Tom for this incredible forum.

wjdunn3 · 12-09-1999, 05:27 AM

Your reasoning is correct for questions 1,2, and 3.
Now for question #4:
When the spark plug fires and sets off the cumbustion in the cylinder, there is a set amount of time that it takes for the combustion to occur fully enough to produce the needed force. This amount of time is the same regardless of engine speed. (just because the engine is spinning faster rpms does not mean that the explosion in the cylinder is occuring quicker).
If you set the spark timing to occur at TDC, then by the time the full force of the combustion is realized, the piston is already on its way down, thus not able to use all the force of the combustion. As you advance the timing more into the Before TDC area, this gives time for the combustion to occur closer to TDC. Ideally you want the combustion to occur fully at the the point when the cylinder is at TDC. For this to occur you have to ignite the fuel some amount of time before then. But as the engine speed increases, the sooner you need to ignite the fuel, thus the need to more spark advance as engine speed increases. Now understand that the combustion time is still the same, but the momentum of the receprocating mass increases with the engine speed.
If you advance the timeing to far BTDC, then the force of the combustion starts acting against the piston while it is still travelinig upward. This has a "braking" effect and puts stress on the piston, rod, crank, and bearings. It also raises the temp. in the combustion chamber to the point that the fuel ignites before the spark plug fires, causing "ping". This also can damage the pistons and cylinder heads (it can actually melt holes into the alum.)
A mechanical advance distributor such as the 009 will advance the spark timing as the engine rpms increase. The problem with this is that it does not take into affect loads on the engine (hills, passing, etc.). A vacuum advance dist. can. When crusing along at a steady speed, the vacuum to the dist. is relativly high which causes the advance mech to provide more spark advance. When you speed up by opening the throttle more, the vacuum drops temporarily thus causing the advance mech to "back off" the timing (not provide as much advance) so that "ping" can be avoided. As the the engine rpm increases in response to the open throttle the vacuum returns. So to recap, the vacuum varies with the load of the engine which causes the timing to vary with the load on the engine.
Timing retard came about due to stricter emissions requirements. the retard mech will cause the timing to back off to TDC or even ATDC when the throttle plate(s)/butterfly is closed (ie you are completly off the gas pedal and coasting). This has the affect of raising combustion chamber temps, but without the combustion force stress of too much BTDC spark. The higher temps cause a more complete combustion which reduces emissions. Spark retard is strictly an emissions thing and does not provide any performance advantage that I know of.

Whew! this is getting long. If you still need more then email me and I can go into more detail. Hope all this helps.

[This message has been edited by wjdunn3 (edited 12-09-1999).]

[This message has been edited by wjdunn3 (edited 12-09-1999).]

bowlsby · 12-09-1999, 02:02 PM

WJ Dunne - thank you for the detailed response, I have been studying it carefully.

My '74 2.0L has the 205 distributor, vac advance/retard diaphragm and original FI. The throttle body has a stub for vacuum, but it is permanently plugged, I read somewhere else that the 74's are like this from the factory and the 73's have a functional vacuum advance por on the TB. I think this means that I have no vacuum advance, although the vacuum retard mechanisms seem to be in place. Could not having advance be a reason for hesitation in an increasing rpm condition (rev-up and acceleration)?

To what extremes can timing advance settings safely be taken?

Dave at Pelican Parts · Dave at Pelican Parts's Garage

The "74" throttle body has a single fitting on it, which is used for the vaccuum retard. This throttle body is used on many (maybe all?) of the 74 2.0 engines, and at least some of the later 2.0s as well.

The 73 2.0s had throttle bodies with two fittings coming out of the sides. One (pointing toward the distributor) was for vaccuum advance, the other was for vaccuum retard.

Both used a distributor dashpot (the flying-saucer thingy) with two fittings. On the single-action 74-style, the advance fitting was left unconnected. Or a hose connected there that simply went under the manifold and was not connected to anything.

There are 74s that have throttle bodies with both advance and retard fittings. But it is not clear if those were added by someone after the car left the factory or are original.

I have one friend (Josh2 from the Rennlist) who installed a 73-style TB on his 74. He said that it made a noticeable difference in low- to mid-range power, particularly in transition from closed to opening up the throttle. I have, however, also heard that the advance was mainly for lowering emissions.

If you look at where the advance port is in the throttle body, you can see that it is very very close (but above) where the throttle plate sits when closed. When the throttle plate starts to open, air goes through the small opening right at this port. It travels very quickly, and this acts as a venturi. The air is at very low pressure there, and that "vaccuum" (high vaccuum == low pressure) is what is used to activate the vaccuum advance.

I'd like to amplify or clarify WJDunn's explanation of centrifugal advance. The concept is that the mixture needs a certain time to develop maximum pressure, which we want to occur at TDC. The amount of time is very nearly constant. We don't have that convenient a way to measure absolute time.

We *do* have a convenient way to measure the angle of the crankshaft. At higher engine speeds, the crankshaft goes through a larger angle in that specific amount of time. So we need to start the spark at an "earlier" angle of rotation in order to have the combustion pressures reach maximum right around TDC.

I think this is what WJ meant, but I wanted to re-state it.

Incidentally, I will be submitting a tech article to Wayne on this very subject in the next week or so.

--DD

wjdunn3 · 12-09-1999, 04:14 PM

Well said (typed) Dave!

bowlsby · 12-09-1999, 04:48 PM

Great information as always.

I have a very early VIN on my 74 and my TB does in fact have 2 vac ports. The one on the left rear (drivers side) is active and is for the retard function per the DD hose diagram.

There is also another port 180 degrees from that position (right front), nearly facing the distributor diaphragm which is solid...actually it appears to have been originally hollow (thereby functional) and soldered full to close it off.

Does anyone know if this port will just screw out, or is somnhow removable so I can simply replace it with a hollow port and regain functional vacuum advance?

Alpine · 12-10-1999, 07:49 AM

I think that I can add something to the vacuum advance topic.

A book that I have states that the high vacuum that exists at part throttle operation draws in a lesser amount of air/fuel mixture which is compressed less and burns slower. To get the most out of this air/fuel mixture it is necessary to advance the timing beyond that provided by the centrifugal advance alone. Any benefit from the additional advance applies only to the part throttle operation. During acceleration or full throttle operation, no vacuum advance is applied.

Alpine