'65 Ign.Distrib's Mainshafts -- T.I. vs. Points

[Wayne M.]

Was comparing part #'s for the L76 (365hp) and L79 (350hp) mainshafts, and notice that they're different. eg. L76 1111060 with transistor ignition uses shaft # 1964051, whereas the points ditributor 1111069 calls for shaft # 1964337. Same situation with the L79 [1111088 distr. for K66 w/shaft # 1965299; points distr. 1111087 w/ # 1964925}

I suppose it's all in the shape of the auto-cam brazed (?) to the top of the shaft, but you'd think the same engine horsepower ratings would have the same spark map. All four distrib's above use the "quick" advance 1116236 vacuum can.

But when looking at the Delco Test Spec's (DR-324S-2), it appears that the T.I. distributors' centrifugal advance comes in quicker, like around 2300 RPM, versus 3000 RPM for the points versions. In fact, the centrif. adv. for the T.I units @ 3000 are LESS than at 2300. A Delco foot-note for K66 is as follows:

"After maximum centrifugal advance is reached and distributor speed is increased above that needed to obtain maximum centrifugal advance, it is characteristic of this unit for centrifugal advance to decrease approximately 0.5 of a degree for each increase of 500 distributor RPM"

So does this mean that @ 6500 RPM, the points unit maintains about 2 degrees more total advance than the Trans Ignition unit ? I tried to check if this is compensated by different initial timing numbers (they're in the AIM for K66) but got various values for points initial timing (6 to 10 for L79; 8 to 12 for L76), depending on the source.

And does this mean that if you strip your tach drive gears, you save the auto-cam and have it re-brazed onto another new shaft ? In the re-pop arena, there seems to be just 2 shafts, one for hi-horse and the other for also-rans.

Any thoughts appreciated

[Duke W.]

Re: '65 Ign.Distrib's Mainshafts -- T.I. vs. Point

The L-79 has a different centrifugal curve - not all in until 5000 versus 2350 for L-76/84 - see the appropriate shop manual or AMA specs.

AFAIK the curves were the same for each respective engine whether single point or TI, but differences in electronic switching characteristics of transistors versus a mechanical switch might cause some slight variations, which won't make much difference.

At the power peak retarding the spark three degrees from optimum for peak power only loses about one percent power, which is tough to consistently measure on a dyno due to all the other variables that can cause small run to run differences.

Duke

[Michael H.]

Re: '65 Ign.Distrib's Mainshafts -- T.I. vs. Point

Duke,

Bet you've seen this on your dyno results. The 2 to 3 degree retard at high RPM in the Delco TI unit worked out to be a positive thing. As engine speed passes about 6000 RPM, timing requirements are actually beginning to drop off slightly and the TI did exactly that. Not sure if intentionally, by design, or if it was just typical of the system, but it worked. The whole thing had to do with the charge swirl velocity in the chamber and at a certain point in RPM, the swirl was so great that flame travel speed reduced the need for the same amount of lead as was required for lower engine speeds.

Bet Clem remembers when guys used dual point distributors and would switch off the primary set of points for high RPM in 4th gear. That would drop off about 2 to 3 degrees of timing, depending on point setting.

I agree, I think you are correct on the built in lag in timing when using the Delco TI system, which would require a different curve and total than a conventional point system. Even with the lag, though, the TI system is still far superior and much more accurate than the point system.

[Joe L.]

Re: '65 Ign.Distrib's Mainshafts -- T.I. vs. Point

Wayne-----

I believe that the shaft, itself, is different for point-type and TI distributors. I've forgotten what the difference is and I can't bear the thought of going out and digging out one of each for comparison purposes.

[Wayne M.]

Could be - everything's different on the 1111263

I took apart that NOS ball-bearing distrib (in the interest of learning -- shot comparative pics) and found that NOTHING (shaft, stationary pole, rotating pole, cast housing) was interchangeable with a regular production K66 system.

[Joe L.]

Re: Could be - everything's different on the 11112

Wayne-----

Yes, that's how it is. Plus, very few internal components were ever available from GM to SERVICE this distributor.

However, the shaft difference between regular points distributors and PRODUCTION TI distributors is not as great as the difference between theses distributors and the "263".

[Duke W.]

Re: '65 Ign.Distrib's Mainshafts -- T.I. vs. Point

Didn't Smokey Yunick recommend 1 degree/1000 revs more advance above the torque peak? I've never seen much comparison data, but theoretically flame speed propagation slows at mixture density drops as it does past the torque peak because VE is lower, so this would imply the need for a little more advance.

The other issue is that if timing is retarded about 3 degrees from optimum, it will only cut power about one percent, which is within the random error range of back to back pulls.

IIRC when Dave McDufford dynoed his 10.35:1 CR L-79 rebuilt with LT-1 cam and nicely massaged 461 heads, peak power was about 360 gross@6500 and this best power run was achieved with about 42 degrees total WOT timing.

He used race gas for the dyno pulls.

Duke

[Michael H.]

Re: '65 Ign.Distrib's Mainshafts -- T.I. vs. Point

I'll try to dig out some old dyno sheets to get exact numbers as I don't recall the exact point where the slight drop in timing picked up the power but I seem to remember 6000 and up. I also remember that this was most effective on small blocks as the spark plug location was definitely less than desirable in the heads with straight plugs. The new (at the time) angled plug heads did SLIGHTLY reduce the need for the timing drop but there was still a substantial increase in power when the timing dropped off a few degrees, especially above 7000. As I'm sure most know, the small block probably required more timing than almost any other engine of the era. That was due to this poor location of the spark plug. It was just about as far off center in the combustion chamber as it could be. The new angle plug heads that placed the plug a little closer to center required less timing and ran best at around 36 degrees.

I suppose, if it were just a matter of time/RPM/flame travel, the timing curve would not taper off and eventually stop at 38-42 deg total above 4000 RPM. If it were purely a function of speed, the timing requirements would continue to increase as RPM increased, until the intake system was no longer able to supply the cylinder with a full charge of air/fuel, resulting in a lower C/R.

I remember all of the engineering paperwork that explained the theory and it kept refering to the incredible increase in turbulence and swirl/rotation of the air in the chamber at high RPM which would speed the flame front. According to these documents, there was little swirl at low RPM so the flame front had to cross the entire distance of the chamber but as the swirl velocity increased at high RPM, the bubble of burning gasses that was growing from the spark plug, was pulled around the chamber, reducing the required time for complete burn.

Not familiar with the Cosworth combustion chamber design/plug location but I would have to guess that the spark plug is very near center and your total timing is in the neighborhood of 34 deg? Reminds me of the old Hemi Dodge with their centrally located plugs. 32-34 deg of timing was all they liked. The turbulence and swirl had little effect on these engines because of this plug location. The bubble started at the plug in the center and worked it's way out so rotation wouldn't have had much to do with it.

I'll see if I can find the paperwork as it will explain it MUCH better than I can. Always easier for me to see it happening in my brain than it is to put it into words.

Michael

[Clem Z.]

the ball bearing dist were made for GM gear

drive cams which cause a "downward" thrust in the dist shaft not "upward" like the chain drive cams. i found this out the hard way back then when i just changed the bottom gear on the dist and found out that the downward pull cause the top dist bushing to wear down very fast and lower all the rotating parts in the dist. this was with a point dist so i just made a thrust bearing to fit between the point cam and the point mounting plate,this took all the downward thrust. i was too cheap to buy GM ball bearing one because it was for the TI ignition and this was a low budget operation. we were using points with double spring tension and a "dwell extender" at the time. this was on a drag race engine. we did try the switch to cut out one set of points on the dual point dist to back off the timing because of the extra load on the engine when shifted into 4 gear at the strip but the "dwell entender" worked better than anything. i looked the other night for the plans for the "dwell extender" but they were not were i though they would be.

[Duke W.]

Re: '65 Ign.Distrib's Mainshafts -- T.I. vs. Point

Yes the CV has a centrally located plug as do most four-valve pentroof chambers.It was Keith Duckworth who "invented" the modern narrow valve angle (40 degrees included) four valve/pentroof chamber. The first four-valve cylinder head was designed by a Swiss engineer for Peugeot circa 1905, but it and later designs until Duckworths seminal mid-sixties revision had much wider valve angles. The narrow angle makes for a very compact combustion chamber. With a 3.5" bore the normial head chamber volume is about 45 cc.

The Chev. Power Manual recommends 32-34 max. WOT timing for the CV engine, and since the small quench zones are cut away on the production pistons to reduce CR and quench area (lower HC) theory would say it needs a little more, but 32 seems to be about right by my SOTP.

The recommended initial is 12 and the distributor has 17, but mine is a hair high at 18, so I run 14 initial and the centrifugal is all in at 2000. I also run a very aggresive vacuum can (none in OE config) - 10@8". Some mid/late seventies L-82s used the same can - 1973577/D1356.

Last summer it pulled 122 RWHP @ 7000 on the OE 8.5:1 CR (8:1 actual) with regular unleaded in "track trim with open exhaust and no fan. The head has been mildly pocket ported, but doesn't do much since it's so free flowing. The biggest improvement was reindexting the cams to get rid of most of the excessive overlap for NOx control, and the valve timing is now essentially identical to the L-79 cam. Eighty percent torque bandwidth runs from 1900 to an estimated 7200, as we only revved it to 7000, which is my normal observed rev limit, The power curve is very flat between 6500 and 7000 and probably doesn't drop significantly until 7500.

The valvegear is good to 9000, but my concern is the bottom end and particularly the oiling system at over 7000.

Duke

[Michael H.]

Re: '65 Ign.Distrib's Mainshafts -- T.I. vs. Point

About two weeks after the new Cosworth Vega became available, I had the opportunity to drive a brand new one for three days. I was surprised at the power that four cyl produced. Fun car to drive.

That engine was probably the first that GM ever sold that had a decent combustion chamber surface to area ratio. Unfortunately, they couldn't claim it as their own design but it certainly was the beginning of a new trend in chamber design for the competition small block. Today, combustion chambers are almost non existant and the spark plug keeps moving closer and closer to the center of the chamber. That's where a lot of the horsepower is coming from on todays race engines. Much more efficient.

I vaguely remember reading that the initial design Cosworth piston/chamber used a "squish band" around the entire circ of the piston dome but it was removed because of emissions. Is there a quench area on both sides of the piston or is the chamber the same size/shape as the cyl?

[Duke W.]

Re: '65 Ign.Distrib's Mainshafts -- T.I. vs. Point

I think we're talking about the same thing. Quench area and squish are basically the same thing. With a flat piston crown (with either just valve clearance notches or nothes and a done) there are small quench areas on either side of the CVchamber, which would be along the two sides of the engine when viewed from the exterior. The piston in these quench zones was beveled to effectively eliminate these quench areas, for lower "engine out" HC emissions.

After the 1974 disaster, when the 50K mile emission durability CVs failed due to burned valves, Chevrolet completely redesigned the emission system. The non-catalyst '74 version actually had a "vacuum retard" system on the TI ignition, and this is why the valves burned. Adoption of the HEI and catalyst allowed for a more conventional spark advance map, though they escewed a vacuum advance system altogether. It was the cleanest car the EPA had ever certified up until that time, and before I reindexed my cams, my CV was below the test limits for a BRAND NEW CAR. Retarding the inlet cam 8 degrees and advancing the exhaust a like amount results in more conventional valve timing, and the reductrion in overlap does increase NOx, but I am now only at half the limit, versus one quarter the limit with the OE cam indexing.

I drove a '75 CV and passed. It was slower - at least to 40 MPH - than my tuned '72 Vega GT. The gearing was just not appropriate for a heavy car with a torque shy 2-liter engine. I bought a '76 five-speed without even driving it as some calculations showed that the gearing was ideal, and the rest was just some judicious tuning.

Combustion chamber design has come a long way since the original quench chamber of the forties that I think can be attributed to Ed Cole. It's amazing what relatively small changes can do to both thermal efficiency and emissions. The less spark advance a design requires for peak power, the more thermally efficient. I think modern LS-series engines barely need 30 degrees for peak power, which is quite a testiment to the efficiency of the combustion chamber design.

Duke

[Mark #28455]

Vortec big block heads need 28 to 30 degrees

In case you ever get a chance, the vortec big block heads are quite something too. Despite the tiny port size, they actually flow really well on the flow bench with the stock valve sizes. In fact, on a 502 cu in engine comparing the stock open chamber big port iron head with stock 450 HP 502 cam to the pocket ported vortec head with stock small valve size and a stock vortec pickup truck cam, they were only down by 10 HP and equal on torque with only 28 to 30 degrees max timing on the vortec heads and 36 to 38 for the big port heads.

Who would have ever expected that!

They also make about 10:1 compression in a 489 (4.25 stroker 454) with a flat top piston.

Mark

[John H.]

Re: '65 Ign.Distrib's Mainshafts -- T.I. vs. Point

When I ran my AA/FD back in '61 (6-71 blown 392 Hemi), it needed 48*-52* advance in the Joe Hunt Vertex mag; nitro is a lousy fuel and burns slow, but it carries LOTS of its own oxygen. Wouldn't even turn over with that much advance - shut off the mag until the push-start got moving, then dumped the clutch and switched it on to fire it up. Electric jet engine starters and bottles of alcohol for starting with the car stationary didn't come along until a few years later (still had flagmen at most strips in those days)