More distributor education for me!

[Donald H.]

I'm trying to educate myself on the distributor and understanding how parts of the distributor work in relation to timing.

I've studied most of the articles by Duke Williams, John Hinckley and Lars Grimsrud. So I have a decent understanding how Initial Timing, Vacuum Advance, and Centrifugal Advance interact to affect timing.

In fact I just finished building the distributor test machine from the article by Joe Randolph in the Spring 2019 Restorer to tune/test some distributors off the car (thanks Joe).

I need a little more clarification on a couple of things, namely the centrifugal advance springs and the distributor shaft cam 'football'.

I assume the purpose of the springs is to affect the rate timing is advanced. I.e., lighter springs allow the timing to advance at a faster rate (lower RPM) and vis-versa for heavier springs.

I assume at least one of the purposes of the cam/football is limit the total centrifugal advance. I.e., the shape of the cam will limit how far the weights can spread and thus limit the total advance.

So if my assumptions are correct, the only real difference between the 66 corvette small block distributors for the 300HP (#1111153) and the 350HP (#1111156) would be:

- the vacuum can (the 300hp and 350hp have different specs for vacuum advance)

- the shaft cam/football (the 300hp spec for top centrifugal advance of 26 degrees at 4100 and the 350 has a top of 30 degrees at 5100)

- both the 300hp and 350hp engines have a spec of 15 degrees at 1500 so I'm thinking they may have the same springs.

Do you see any big fault in my assumptions?

Thanks,
Don Harris

[Duke W.]

Re: More distributor education for me!

Total centrifugal is limited by the length of the slot in the cam assembly, and sometimes the pin has a bushing that limits centrifugal. In addition sometimes it's limited by the weights contacting each other before they retract all the way to the "football".

The shape of both the weights and football also affects the rate of advance. Some are linear and some are non-linear.

Are you seeking to optimize the spark advance map for a specific engine configuration? I assume you have read my 2012 San Diego National Convention tuning seminar. I give starting point recommendations for nearly all original engine configurations.

The 327/300 has several OE centrifugal and vacuum setups - '62-'63, '64, '65, and '66-'67. I think it was the '65 vacuum advance that caused so many problems GM wrote a TSB on the problem. The '66-'67 is the best starting point - they just need lighter springs to bring total centrifugal in earlier, and maybe a different VAC depending on transmission type and whether the car has A/C, so it only took GM four years to get it reasonably correct. Sometimes I wonder if they knew what they were doing.

Duke

[Donald H.]

Re: More distributor education for me!

Thanks Duke, yes I've read your San Diego tuning document.

My project in this case is non-corvette. I have a 65 El Camino and recently purchased a 327 out of a 67 Impala SS (327/275HP). It needs a rebuild and will need to be bored some, I'm hoping only 0.030 over. I want to add L79 pistons and cam but not planning to change the heads to the larger valves, I'll leave it with the stock 1.94/1.50 valves.

I also have a 65 base engine distributor (1111075), the correct distributor for a 65 L79 Chevelle is the 1111071, but those are impossible to find.

So my only issue now, and it's probably not worth spending an inordinate amount of time on. Is the top centrifugal advance specs for the 65 327 base engine is 26 degrees at 4,100. The top centrifugal advance specs for the L79 is 30 degrees at 5,100. Those were unchanged for the 66 specification. In 67 (if I'm reading the specs in the service manual correctly) the specs for the 300 and 350 HP engines were the same, i.e. 30 degrees at 5,100.

Testing my distributor and changing around the springs (Mr Gasket set) it is at the 26 degrees at 4,100. I can't seem to get any more centrifugal advance. Thus my question was why, and I think you have answered that.

Given how I will be driving the car, it's probably not anything I should worry to much about.

Thanks very much for you insight

Don

[Ed S.]

Re: More distributor education for me!

Don,
Don't know if you have read this - don't know if it will answer your question. It is on our chapter website -

http://www.ncrsmac.org/ed/Engine/L-79%20Distributor%20Overhaul.pdf

Ed

[Donald H.]

Re: More distributor education for me!

Don,
Don't know if you have read this - don't know if it will answer your question. It is on our chapter website -

http://www.ncrsmac.org/ed/Engine/L-79%20Distributor%20Overhaul.pdf

Ed

Thanks Ed, yes that is one of the articles in my collection.

[Duke W.]

Re: More distributor education for me!

Thanks Duke, yes I've read your San Diego tuning document.

My project in this case is non-corvette. I have a 65 El Camino and recently purchased a 327 out of a 67 Impala SS (327/275HP). It needs a rebuild and will need to be bored some, I'm hoping only 0.030 over. I want to add L79 pistons and cam but not planning to change the heads to the larger valves, I'll leave it with the stock 1.94/1.50 valves.

I also have a 65 base engine distributor (1111075), the correct distributor for a 65 L79 Chevelle is the 1111071, but those are impossible to find.

So my only issue now, and it's probably not worth spending an inordinate amount of time on. Is the top centrifugal advance specs for the 65 327 base engine is 26 degrees at 4,100. The top centrifugal advance specs for the L79 is 30 degrees at 5,100. Those were unchanged for the 66 specification. In 67 (if I'm reading the specs in the service manual correctly) the specs for the 300 and 350 HP engines were the same, i.e. 30 degrees at 5,100.

Testing my distributor and changing around the springs (Mr Gasket set) it is at the 26 degrees at 4,100. I can't seem to get any more centrifugal advance. Thus my question was why, and I think you have answered that.

Given how I will be driving the car, it's probably not anything I should worry to much about.

Thanks very much for you insight

Don

The L-79 OE spark advance map is strange - the least aggressive centrifugal curve and the MOST aggressive VAC. It never made sense to me.

Base engines don't need as much total idle advance as SHP engines, which is why the 365/375 HP engines have 24 degrees centrifugal, and the late 327/300 HP engines have 30. With 16 degrees vacuum advance the 365/375 HP engines need more initial to achieve the 36-40 optimum range than a base engine. The base engines need low to mid-twenties total idle advance, and the high overlap mechanical lifter cam engines need high twenties to low thirties. The different total centrifugal gets total idle advance into the correct range for each engine.

IMO the optimal setup for the L-79 cam is 26-27 centrifugal all in at 2500-3000 with a 12" B26 VAC (manual trans, no A/C) and initial timing to get total WOT advance as high in the 36-40 range as the engine will tolerate without detonation. So the 26 @ 4100 distributor is the best choice other than you need to get full advance in earlier. The black springs in the Mr. Gasket 928G are the ones to start with.

Regarding pistons, the OE Replacement L-79 forged, domed pistons are not a good choice, especially since the heads don't have the inlet valve relieve that increases chamber volume by about 2 cc with the large valve set. The CR will be too high without use of a very thick head gasket. A better choice would be the KB157 hypereutectic that has a net 0.5 cc dome. You need to read the compression ratio article I co-wrote with John McRae. It's probably somewhere on this Web site, and it's also on the CF in a thread started by me, SWCDuke.

If you have 93 PON fuel I suggest a not-to-exceed true, measured CR of 10.3 and reduce by 0.1 of every octane point below 93, so for CA 91 PON fuel the NTE level is 10.1.

Also, I suggest you use the L-46/82 camshaft, which his nearly identical to the L-79 cam (except the IPOML) but has better lobe dynamics. On a 327 it should be installed with the Cloyes-manufactured OE truck replacement roller chain with the two additional indexing holes that allow 4 degrees advance or retard - about 25 bucks for the set at NAPA. In the case of the above cam it should be installed four degrees advanced to bring the as-ground IPOML of 114 deg. ATC back to 110, same as the L-79 cam.

If you massage the heads top end power will increase by about ten percent over stock heads with a valve job, and with careful OE valve spring setup (.090-.100 coil bind clearance) it should make useable power to about 6500 with a lifter pump-up speed of 67-6800.

Lastly for durability I recommend high strength conn. rods - Eagle SIR5700 (about $250 a set) or better, and the combined cost of the KB pistons and Eagle rods is likely no more than the cost of the OE replacement forged pistons, which are very expensive!

To assure the best outcome, it's very important to thoroughly plan your project and do a forensic teardown of the donor engine looking for internal problems that may need to be addressed, and part of this is measuring deck clearance of all eight cylinders. It's absolutely necessary to have this data to achieve the target compression ratio for the completed engine.

If you want to discuss in more detail send me an email (not a PM) and I'll send you my phone number. It sounds like an interesting project.

Duke