Tune up help - dwell

[tonyk72]

I just got a digital tach & dwell meter, never used one before but it seems simple enough. The emissions sticker on my 72 LT-1 says the dwell should be at 30 degrees.
My question is at what RPM's should I be measuring the dwell?
These are the readings I got:
at 980 RPM = 31.2 degrees
at 1500 RPM = 30.7 degrees
at 1600 RPM = 30.2 degrees
Am I OK? Are these readings close enough to spec?
I also made sure to plug the vac hose when I took the readings...

 

[clar2001]

Your pretty much on the money dwell is just your point gap it should remane about the same at any rpm a little difference is fine ,remember this is a menechical dwell open closed so there will be no perfect setting . Some time you might want to consder a HEI Distrubetor it's dwell works off a electronic chip so there is no gap.......
You set your dwell at idle I would let the car warm if your choke is kicking in then set .

 

[JohnZ]

Dwell is essentially constant at any rpm; if it varies up and down more than a degree or two at steady high rpm (4000-5000), that's a clue that your distributor needs some attention. You don't need to disconnect the distributor vacuum hose to check dwell - only for setting initial timing.

 

[ChuckG]

The dwell is the point gap, which you can set with a feeler gauge, BUT, the meter is MUCH more accurate. The dwell shouldn't change. As John Z says, if it changes, your distributor needs some work...new bushings to keep the shaft from wobbling, maybe the shaft is bent, maybe the points are loose, etc.

If you're timing the engine, always remember that DWELL will change the timing, but TIMING will not change the dwell. SO, always set the dwell first, before putting a timing light on it. You have to disconnect the vacuum line before setting the timing. The vacuum line has no effect on dwell (aka point gap) Chuck

 

[pws69]

Although the point gap DIRECTLY influences the dwell, DWELL is a FUNCTION of the point gap in relation to the height and length of the lobes on the distributor cam - and it is essentially an average reading across all 8 lobes.

Adjusting the point gap is a "close enough" setting - adjusting by dwell is much more precise, as Chuck has stated.

As John Z said, if dwell varies more than a degree or 2 at a steady RPM, you should consider a bushing replacement/rebuild.

Your dwell should be fine the way it is.

 

[AKRAY4PLAY]

Mallory ignitions makes a conversion kit to eliminate the points. Mallory part number MAA-051 should work for you. they run about $100 in Summit Racing. i hate playing with points, so i always convert points dists. or replace with HEI. other manufacturers make the same thing, so if your interested shop around and you may find a better deal, Brian

 

[Stallion]

I didn't really understand this dwell concept, so I went to an automotive dictionary and I found this and I was wondering, are our C3 Vettes a contact breaker ignition system or a breakerless ignition system?

 

[JohnZ]

Corvettes used breaker-point ignition systems until 1975, when the HEI (High Energy Ignition) system replaced the point systems (except for the small number of T.I. - Transistor Ignition Corvettes built beginning with the 1965 396 big-block and continuing through about 1972).

 

[LIRacer]

There isnt much I can except, dump the points in favor of a Petronix system. It uses your stock distributor.

 

[Stallion]

Corvettes used breaker-point ignition systems until 1975, when the HEI (High Energy Ignition) system replaced the point systems (except for the small number of T.I. - Transistor Ignition Corvettes built beginning with the 1965 396 big-block and continuing through about 1972).

John, what exactly is the idea behind this breaker-point ignition system? And why did they stop using it in 1975 when they can out with HEI?

 

[JohnZ]

Having the points driven by the 8-sided cam on the distributor mainshaft does two things - when the points are closed, current flows in the coil primary windings, and when they're opened by the cam, the coil generates a high-voltage spike that goes into the top of the distributor cap, through the rotor tip, and into the correct plug wire to fire each cylinder at the correct time and in the correct order; cheap and effective, but they require occasional maintenance (dwell adjustment) and replacement when the contact faces of the points become burned/pitted. The HEI system generates a higher-energy spark and requires no owner maintenance, both of which were important for emission system performance.

 

[Stallion]

I see what you mean. John, a little side-note. Now that we are on ignition, how does the starter work? When you turn the key, the engine turns, but how? How exactly do we get this whole timing and combustion going? Where does the starter send its spark? Through specific plugs? :confused

Thanks.

TR