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Old May 29th, 09, 09:38 AM
Big Dave Big Dave is offline
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Default Timing

Ignition timing isn't a black art. You connect the timing light to the number one cylinder spark plug wire (preferably by way of a magnetic induction pick-up connector), and point it at the balancer to read your timing value at idle. This is called your initial timing setting; if (and only if) you remembered to unplug the vacuum line going to the distributor and plugged the vacuum line with a small Philips screw driver. It should be steady not flickering and not changing the value on the damper as your motor purrs at idle.

If your car isn't exactly purring but growling and bucking because you have a longer than stock duration value with a tight LSA (which enhances the effect of the overlap) then your timing numbers will be bouncing around to reflect the changes in RPM. This is because in addition to the vacuum advance you have a set of weights spinning inside the distributor cap held in check by two springs. At idle the springs are not greatly stretched so a small increase in RPM can cause a change in the timing.

When (how much timing for a given RPM) the timing is fully advanced by the mechanical part of the equation depends upon the springs and their condition (they sag with age just like wheel springs do); and the amount of weight involved. There is a third variable called friction from corrosion and wear on bushings.

I have no idea why people will spend two hundred dollars on valve covers but balk at replacing a distributor. The factory built these devices to last 7 years or 100,000 miles (designed into the distributor by their choice of materials and how it was designed). A new forged steel shaft distributor riding on full roller bearings with a new weight stage and springs using a built in magnetic pick up seems like a no brainier on the things to replace in my opinion. The factory brass bushings wear continuously from being put into service, the mild steel stampings used to fabricate the parts start to rust before they were ever installed at the factory, the springs have been through hundreds of thousands of heat cycles and corrosion and have lost most of their spring.

So what is timing? It is a point in time at which the spark plug initiates combustion in the Otto cycle. It is based upon the crankshaft's rotation and measured in degrees. When it happens is obviously important, but it is not fixed (which is why we time the car). Things like quench, combustion chamber design, piston design, compression ratio, gas octane rating, moisture in the air, and cam timing (which is half of the crank's rotation) all have an affect on the engines performance. The spark defines the Otto cycle and separates us from the Diesel cycle. If you get into detonation that distinction blurs.

Our hot rods want more advance than stock cars because we strive for higher compression and more cam timing. Our new 'fast burn' heads require less timing than the older stock wedge shaped heads but still more than stock. Because gasoline doesn't explode but burns it takes a finite period of time to burn. This is were quench, piston design and combustion chamber design are important. Quench (if controlled) contributes to flame propagation by mixing the fuel and air just before ignition takes place. The combustion chamber design with a new fast burn chamber has no corners and is heart shaped to deshroud the valves. Finally a piston can not get in the way (domes) of the flame front by blocking or separating one side of the chamber from the other.

The gas quality is a no brainier but you only want enough octane to keep you out of detonation. High octane fuel doesn't have more power than regular gas it just doesn't burn as fast. This is to resist being lit off by a glowing spark plug electrode or thread in the head. Moisture (humidity) effects the octane as well (water doesn't burn) and raises the octane of the gas mixture (which saves my butt living in Florida with high compression ratios).

You want the timing set (wherever it lands) so that you do not get into detonation with your octane gas (it can not change from tank to tank so I buy only one name brand gas to get consistency) at idle and at full tip of the mechanical advance. A knock sensor really helps with this, but only one company sells one (Autometer) and it isn't cheap.

Big Dave

Last edited by Big Dave; May 29th, 09 at 07:50 PM. Reason: A lot of bad spleling
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Old Jun 4th, 09, 09:52 AM
glenns68 glenns68 is offline
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Default Re: Timing

Hi Big Dave, I still have an issue with timing on my 68. HEI distributor-accel plug wires-autolite plugs#26-carb rebuilt-clean gas and tank. When the timing is set, I still get alot of mis and backfire. I'm more or less shoping around for the correct number of degrees because there is no info anywhere that I can find on my motor (1 M O 428 7VP)
Spark gap set at 40 (correct?)
I'm sure it's a Mexican re-pop GM crate motor but can't find any specs. 4 bar intake Holly 650 and headers. I'm following a chiltons manual that shows 350 auto with HEI ignition and it shows the rotor pointing at the #1 plug at tdc. Does this sound correct or am I missing the boat? Sorry for my lack of knowledge and abillity to give a clear discription of my issues, as this is new to me but I love it and am determined to get this car out on the road. Any suggestions? Is this motor worth the trouble? Thanks, Glenn.
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Old Jun 4th, 09, 10:26 AM
Big Dave Big Dave is offline
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Default Re: Timing

Back firing is most likely due to the carb running lean, unless we are talking about backfiring and missing at idle. In that case it sounds more like crossed wires. Check your firing order again following every plug wire by hand to the plug. Look at the wire for breaks (silicone rubber tears or cuts very easily). Once you have assured yourself that the wires are firing 18436572 and that the plugs are 1,3,5,7 on the driver's side starting from the front and 2,4,6,8 on the passenger's side again starting from the front; then it is time to throw in the towel and go watch TV till it gets good and dark. At night (not in your well lit garage) you can see any arcing of a short in the ignition system. Mater of fact even a good well running engine will put on a light show for you if it is damp enough to have high humidity (water molecules being polar help to ionize the air around them with high voltage looking for a ground path).

If your plugs are wired correctly with no shorts, then it has to be inside the rotor cap. MSD and Accel do not use black caps for more than just marketing reasons. Black hides the carbon tracks caused by oil fumes making little short circuits inside the cap. Beige or white works best, but they show up by contrast with a red cap nearly as well. They will look like little black spider webs or cracks in the cap and they act like a bunch of fine strands of copper wire bleeding off voltage or misdirecting it to the path of least resistance. Which in this case is going to be a cylinder with no pressure in it: such as one with the intake valve open (to get a back fire); or a miss if the exhaust valve is open.

I forgot to mention above that mechanical timing is controlled by weights and springs up to a point; and that point is the stop. You can adjust how much mechanical advance you want by moving the stop on the slot on the advancing timing plate. I also mentioned friction so a dab of white Lithium grease on the shafts helps the weights to move freely. Personally if you have any doubts about your distributor take it to a hot rod shop that has a Sun distributor machine that allows your timing to be checked and set dynamically (most roundy-round speed shops have one, because they are more interested in helping their customers than they are in selling parts I have noticed).

Big Dave
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