C2 340 SB-Thanks Guys

[Duke W.]

Re: The infamous distributor drive gear dimple

You are correct, Kevin. The logic from my previous post (which I deleted) applies to gear ratios that are non-integer values. With a 13/26 tooth count or any other integer ratio, teeth on one gear will only mesh with certain teeth on the mating gear. This is why most gear ratios are non-integer, however, in the case of the distributor it must rotate at exactly one-half crankshaft speed.

Craig's question: Why is is the distributer drive gear and odd number of teeth remains unanswered. Installation of the drive gear could have been made "idiot proof" if the number of teeth was even and the roll pin hole was drilled symmetrically relative to the tooth pitch.

Duke

[Duke W.]

New theory

The relative position of the rotor tip and cap terminals is established by the initial timing. Centrifugal advance advances the cam/rotor and vacuum advance advances the breaker plate, so the rotor tip will always stay in the same alignment relative to the cap terminals regardless of the amount of centrifugal or vacuum advance; however, changing initial timing requires rotating the dist. housing relative to the shaft, so rotor alignment is affected.

The range of initial timing specs for early 327s is 4 to 14 BTC, and the difference between these two extremes would alter the rotor-cap relationship by 5 degees.If the rotor is in exact alignment with the cap terminals at 9 degrees then the extremes of 4 and 14 would "misalign" the rotor and cap terminal by 2.5 degrees and they are wide enough that this amount of misalignment should not result in a significant loss of ignition energy.

The early CA emission controlled engines are an interesting case because their initial timing was speced at 4 ATC, which would require the dist base to be rotated another four degrees from 4 BTC. This would result is and addition 4 degress of rotor misalignment, or 6.5 degrees in this example, which seems like quite a bit. The other issue is that at some some point you run out of housing rotation room because the vacuum can will interfere with something. However, if you were to reindex the drive gear half a tooth, which is what happens if you rotate the drive gear 180 degrees, the whole housing would have to be rotated about 14 degrees to achieve the correct timing. On a typical SB V8 in a Corvette this results in the vacuum can interfering with the manifold or coil bracket before you can achieve the correct initial timing, so the timing is either too far advacned or too far retarded from spec, but it's possible that some combinations of physical geometry and initial timing could require a different angular indexing of the base, so the possiblity exists that some applications might require the drive gear to be installed 180 out from the rotor tip.

Having an odd number of teeth on the drive gear provides more flexibility for indexing the dist. housing for a wider range of physical geometries and initial timing specs. Could be that the designers of the original SB were far sighted enough to see that an odd number of teeth on the drive gear would provide greater flexibility for future changes in engine/accessories geometry and initial timing requirements.

Duke

[Joe C.]

Re: Way Back When, In The Old Days..........

Clem:

I cain't rember.....much anymor. I do reckon its nineteen hundred and sixty four thow. Why, gol durn it I cain't spell much good, er reed too good ether. Besides, I kin only et vittles dat is soft or prechewed by the missus, on account a I lost me choppers when I powershifted the ol Lizzie with der big blok in 'er at 7 thou.....thats wen dee ol choppers done popped right outter me kisser an went aflyin out the side curtin. I think they done smacked Missus Halapenster in der noggin cause she went down like a rock..........aint never spoke to'er after , but she never been der same since.
Gotta go now an refill the nitrus bottle fer ol Lizzie.

Carl Sandberg,
Salinas, Ca.

[George Romano]

Re: Was that on the menu in Calrisle?

They were as good as the ones we had at Willy's.

Pasta

[John M.]

Re: New theory

Duke,
You are right on the mark! Tom Wander wanted to put a vaccuum advance distributor on his 270 horse 60 engine, and this is the only way that we could get to the proper timing. As you are aware, there is almost no room to rotate the distributor before the vac advance can hits the rear carb, but if you put the gear on backwards, you can get the timing perfect!

Regards, John McGraw

[Duke W.]

Re: New theory

One other point. The gear indexing and initial timing determine the angular indexing of the distributor housing relative to the crankshaft axis or some other datum, but one other device can affect it - the vacuum advance. As far as I know, all vacuum cans index the breaker plate at the same angular orientation to the housing at zero vacuum, but this may not be the case and may be the reason some vacuum cans have nearly identical specs, but different part numbers i.e. the "rest" position of the vacuum advance rod is a third parameter that can affect distributor housing indexing.

Duke

[John H.]

Re: The infamous distributor drive gear dimple

I have several Chevrolet machining blueprints for cams, and all of them have a transverse section through the drive gear which is VERY specific about the precise location of a point on a tooth relative to the dowel hole in the front of the cam; this establishes the direct relationship between the cam timing and the drive gear tooth location.

[Brian McHale]

Re: Was that on the menu in Calrisle?

Let's not forget the skinny Italian cheese sausage from the BRONX. And those peppers stuffed with prosuito and provolone cheese. MMMMMMM.

[Clem Z.]

Re: Was that on the menu in Calrisle?

i was a little leary of the sausages after i heard that al has a very large grinder that is powered by a 350 chevy.

[Chris D.]

Re: New theory

Good point on the "at rest" position. My vac control seemed to have couple degrees more throw than advertised. I noticed some of the older repair manual and P&A pictures showed a small sleeve on the vacuum contol rod. This would reduce the travel and change the at rest position. I used one of the plastic sleeves intended for the mechanical advance limit post and voila, vac can met specs. None of the units I've picked up at NAPA nor the one from GM had this sleeve installed. Is it supposed to be there?

This subject of rotor phasing was such a distraction a few months back that I took a hack saw to an old cap(relax guys it wasn't an AC Delco) to see what the heck was really going on in there. I had assumed that the rotor moved relative to the cap for both mechanical and vacuum driven changes. This proved only half right. Mechanical advance does not change the rotor / cap relationship at the spark event; Vacuum advance does. So the trick is to position the rotor so it does not wander too from a cap terminal. I was playing with various additions of static timing. Flipping the drive gear was a bad thing in my case. With only a couple iterations of moving over one tooth and/or reindexing the wires it was not possible perfectly center the rotor so I chose a combination that put me a bit closest at low vacuum; who wants a miss-fire when you jump on the gas. Best combination was (drum roll) leave it stock. But that should surprise no-one here.

[Duke W.]

Re: New theory

Off hand I don't know if my vacuum can has one or not. The '63 AMA specs have graphs for both the centrifugal and vacuum advance and I think the allowable variance is about +/- two degrees.

Duke

[Clem Z.]

Re: New theory

a lot of the orignal GM cans did have a sleeve on the rod to limit its travel. you can limit the travel by using the inner can hold down screw and a piece of metal the correct length under it to limit the rod travel. you can cut down the amount of advance these 20 degree cans give you and still have orignal can on the distributor for judging.