1974 Distributor types

[Johan]

I'm looking to buy a new distributor main shaft for my '74 with L-82 engine. Some vendors has just one type of shaft, while others have two; one for standard, and one for "high-perferformance" engines.
What is the meaning of high-perf here, is it the BB or the L-82?
(i.e. is there a difference between the distributor in the L-48 and L-82 engines??)

 

[69MyWay]

Does it still have the cable tach drive? I can't remember if 74 had those or not. It was right around these year models that they switched out.

If it does, then you are pretty much stuck getting a direct replacment. Big block and small block distributors will interchange as they are exactly the same.

My only other thought here is the dristributor drive gear. It may be in the high performance application, that they are running a bronze gear as they are more friendly to the cam shaft on high end cam shaft applications.

Are you replacing, or starting from scratch?

 

[Johan]

Yes it has a cable tach drive (it changed in '75). I was replacing the tach gear, when i realized that the shaft itself also had some damage, and i must replace that one aswell.
Ecklers have just one type of shaft, while Zip for example have two types, standard and high-perf (same price), and this makes me a bit confused.
If bronze gear is better, then why have two models if they are the same price?

If BB and SB distributors are the same, then I cannot imagine that there is any difference between L-48 and L-82 distributors??

 

[69MyWay]

Yes it has a cable tach drive (it changed in '75). I was replacing the tach gear, when i realized that the shaft itself also had some damage, and i must replace that one aswell.
Ecklers have just one type of shaft, while Zip for example have two types, standard and high-perf (same price), and this makes me a bit confused.
If bronze gear is better, then why have two models if they are the same price?

If BB and SB distributors are the same, then I cannot imagine that there is any difference between L-48 and L-82 distributors??

The brass gear is softer and won't last as long as the steel gear. That is why you would not want a brass gear in all standard production units.

One other HUGE difference that I forgot about is the weights/springs for the curve. The hi performance unit (if it is a complete assembly) may have a different weight spring combo, as well as it may NOT have a location/attachment for the vacuum advance.

I think the vacuum advance module is a very important component of a mostly street driven car, and a heavy consideration.

Also, switching to the pertronix style electronic ignition is a major plus, as no more points to adjust. Oh! I think I got it as well. The hi performance models had dual points, and that may be a difference as well in the two models you are looking at.

If I was you. I would see about getting your existing shaft repaired, then installing a $80 Pertronix kit, reconnecting your cable tach drive, and hitting the street with more performance, more reliability, and less $$$$$ to be spent.

 

[Johan]

Well, I guess I'll go for a standard shaft then. I do have vacuum and single points (what is the purpose of dual points, by the way, are they used every secon time each? Because I guess you still use a single cable to trig the coil?), but I have considered to swithch to electronic ignition. The thing is that I haven't really had any problems with the points, but if there was an improvement in performance, that would be another thing...

 

[Jack]

difference is: Hi-Po = TI (no points)

I believe the difference between shafts lay here:
The TI (Transistorized Ignition without points) found on some late 60's-to-early 70's Hi-Po vettes (& other Hi-Po chevys) used a slightly different (from points-type) shaft to accomodate the TI internals. Don't confuse TI with HEI ... two different critters.

Since yours has points, order the standard shaft.

No difference in mainshaft between L-48 & L-82 points dists ... though possibly differences between their vac adv cans/points spring tensions/mech adv spring tensions.

Pertronix is a good choice ... so long as you accept that replacement parts for it (like any mod) aren't available just anywhere/anytime.

Dual points simply increases the dwell time ... charges the coil a little longer ... results in a bit hotter spark. BUT ... Dual points are more trouble to set gap/dwell & two pairs cost more than one to replace ... Do Not consider changing to a dual-point plate setup.
JACK:gap

 

[Tom73]

... is there a difference between the distributor in the L-48 and L-82 engines??...

Yes. The distributor is different for all Chevy engines. Other then different advance weights and advance rates, the shafts are different. They are all interchangable but the cam that bumps the points open is different. Each engine had its own special setting for the ramp on the cam.

The L82 would get the high performance version whild the L48 would get the "standard" version. The LS4 would also have it own distributor shaft.

tom...

 

[Johan]

Huh, three different opinions...

Placed an order yesterday On Corvette Central for a "standard" main shaft, maybe time to change it then, if you are certain in this matter Tom?

But Tom, how come you never see three different shafts in the catalogs? Ecklers and Mid America have one type (and say nothing about different engines), and Zip, Dr. Rebuild and Corvette Central have two types...

 

[Tom73]

But Tom, how come you never see three different shafts in the catalogs? Ecklers and Mid America have one type (and say nothing about different engines), and Zip, Dr. Rebuild and Corvette Central have two types...

My guess would be that the differences is so minor that you could not tell the differences.

Also, the differences are for the factory installed distributors that were installed on the assembly line. I am not sure what Chevy did for "service" parts. They may have just used one or two versions to cut down on parts inventory.

To correct myself, I believe that the 73 and 74 L48 and L82 used the same shaft. Check out the bottom of this page on my site, The '73 Spot http://www.corvetteforum.net/c3/vette73/S_ELECT.htm . The "Cam Angle" for the L48/L82 is 29-31 and for the LS4 it is 28-30. That is the very small differences we are talking.

I had to replace the shaft on my 73 L82 and just used the GM generic. I could not tell any difference.

But back to the question of if there are any differences, yes there are. But do the differences make any significent difference, I think that they are so small that you cannot tell one from the other (out side of the advance rates).

tom...

 

[Johan]

Ok, thanks for all the help!!

/Johan

 

[Jack]

http://www.corvetteforum.net/c3/vette73/S_ELECT.htm . The "Cam Angle" for the L48/L82 is 29-31 and for the LS4 it is 28-30. That is the very small differences we are talking.
tom...

Nice site Tom! I believe the "cam angle" reference is simply the dwell angle in degrees ... that is what you'd see on a dwell meter ... it's another way to measure point-gap ... most dwell meter needles will float 2-3 degrees. See how the LS4 has stiffer points spring tension.
JACK:gap

 

[Jack]

I stand corrected ... kinda

I wish to thank Dave Fiedler at TI Specialty :BOW http://www.tispecialty.com/ for shedding further light on this.

The differences in distributor main shafts lay in the various CAMS that're BRAZED onto the TOP END of shafts ... that's the cam that articulates with mechanical advance weights (not the cam that opens points). The brazed cams' various shapes, in conjunction with the weights' various shapes and various ma spring tensions, are what determines curve of mechanical advance. Dave indicated that reproduction shafts have but two cam options (std or Hi-Po) ... and that either is merely a compromise ... since GM made some 50 different cam profiles. Dave indicates he does not use repro shafts. I can well imagine how each engine option from each year could use a different cam ... matching to customers' expectations, engine power curve requirements & emission constraints. FYI, I've reviewed Lars' reference papers on different vac cans ... but I don't know of a reference describing ALL various ma cams ... I've seen one or two ... but not one showing ALL. Similarly, I agree with Tom that such differences wouldn't make much difference.

BTW, I don't think any of the cross gears themselves are brass-bronze ... but the large threaded bushing that the gear rides in is available in either steel (stainless)... or brass-bronze.

Here's another source for shafts/gears/rebuild kits http://www.tld-corvette.com/index.htm
JACK:gap