'66 427/425 UPDATE for Duke Williams

[Duke W.]

Re: '66 427/425 UPDATE for Duke Williams

Yes, I agree, and that is my advice to all L-72 owners that I've repeated numerous times. Also, connect it to full manifold vacuum, either by teeing off the choke vacuum break or by installing a pipe thread adaptor with an eighth inch nipple in the threaded opening in the inlet manifold.

Duke

[Everett Ogilvie]

Some Mechanical Advance numbers for the 093 Dist.

Here are the mechanical advance numbers measured on my 093 distributor (vacuum disconnected);

RPM/degrees at crank
550/12
700/12
800/14
1000/16
1200/17
4000?/33

You can see that depending on the idle speed you have dialed in, you will get a different read of your initial timing. The numbers above show that my true initial timing without any component of centrifugal, is 12. The timing light showed 14 or 15 because my standard idle speed setting is about 900. The max degrees advance I can obtain is 33 - this is with 12 true initial, which indicates the dist centrifugal advance is contributing 21 (33 total minus 12 initial).

The 33 total number was when I rev'd the engine to about 4000 (I think). It is kind of hard when you are by yourself. The distributor is supposed to deliver about 24 degrees with the centrifugal advance, so either I did not rev it high enough, or the slot for the mech advance has been "limited". The old hot rod trick used to be to limit the travel of the slot (braze it up), and then dial in tons of initial (like 20), and end up with 40 total. I will be checking to see if the mech advance has been limited on my dist. If it has NOT, then I will dial in some more initial to get 36 - 38 total. This could mean I have to go to 17 degrees true, initial timing.

I buy TRICK racing fuel - partly for the octane (b/c I run so much advance and I don't want to detune the motor to perform with crappy gas), and partly for the lead b/c the heads are original and don't have hardened valve seats. You can cut the racing fuel in half with leaded fuel, but I often run it straight. Note that if you change from pump fuel to a racing type fuel, the specific gravity will change, requiring float level re-adjustment.

So far I am running the standard 360 vacuum can for the 093 dist on the L72 engine and it is plumbed to the stock vacuum source (a ported source). The car idles great, does not overheat and pulls like a freight train (4.11 gears). Note that I have lots of initial timing as Duke recommends, but most of mine is initial, rather than vacuum.

I have a 236 vacuum can and I will test it sometime in the future to study the differences, but I am so anal about originality that I will most likely keep everything stock, especially as I am not having idle or overheating problems.

[John Lokay]

Re: '66 427/425 UPDATE for Duke Williams

Thanks Duke, just one more thing I would like to clarify. As I mentioned in an earlier post, I hooked-up my vacuum line this past weekend to the port on the intake manifold and got a reading of 40 degrees of advance which includes 10 degrees of initial at 900rpm. When I did this the engine reved up. I think what happened was that I got the full 12 degrees out of the can, plus the 10 inital plus I must of gotten 18 of mechanical advance. Does this seem to make sense? Another thing, when I set the initial at 10 degrees I did it at around 900 rpm and now I know I should set initial at 500 to 600 rpm (I know that you suggest setting total advance and not to worry about initial advance) Thanks

[John L.]

Re: Some Mechanical Advance numbers for the 093 Di

Holy Cow Everett -- That is an awful lot of initial imho -- 14 @ 800 rpm. Do you have any problems re-starting when hot especially with a restoration type battery. I was running 12 earlier and that worried the heck out of me. Your numbers are interesting. Is it possible for you to get timing numbers with the can connected say up to 2800 rpm and at reasoable intervals ?? If I run that much initial with the B28 can then at idle (900 RPM) I would get 14 initial + 8 vac + 8 vac + 7.5 mech + 7.5 mech = 45 deg, at 1350 RPM I would 14 initial + 8 vac + 8 vac + 10 mech + 10 mech = 50 deg, at 2300 I project I would get 14 initial + 8 vac + 8 vac + 14 mech + 14 mech = 58 deg. I have a 3:36 rear in my L72 so the 2300 rpm represents my cruise. What do you all think of my numbers -- maybe too much timing ??? I am currently running 9 deg initial which would put my current timing numbers with the B28 can 5 degress lower in all three examples which equate to 40 at 900, 45 at 1350, and 53 at 2300. My distributor has been set up to the stock specs which I used in the examples.

thank you

john lolli

thanks

john lolli

[Duke W.]

Re: '66 427/425 UPDATE for Duke Williams

Yes, if you increase the idle advance, by whatever means, engine speed will increase and this may cause more centrifugal advance. This is why you must isolate the advance systems when testing. According to the OEM specs the L-72 centrifugal starts just over 900 and is maxed out at 30@5000, so you will have to free rev the engine to at least 5000 to check centrifugal.

Also, the L-72 vacuum advance spec is 0@6" and 15 (not 12) @ 12". I may have previously stated 15.5", but that applies to the '67 L-71. This is why the 8" can is appropriate as the engine will likely never generate over 12" of idle vacuum to keep the vacuum can plunger pulled to the limit at idle, unless you idle it at well over 1000 RPM.

As Everett said, reduce engine speed to an absolute minimum to set initial, then bring the revs up slowly and observe where the centrifugal starts. Make a mental note of this for future timing checks to be sure you set the speed below this level whenever you check initial timing.

Duke

[Duke W.]

Re: Some Mechanical Advance numbers for the 093 Di

The L-72 max centrifugal spec is 30@5000. Have a helper slowly rev up the engine while you check the centrifugal advance. I always rev to at least 6000-6500 to look for any signs of overadvance. The Cosworth Vega sounds a lot scarrier with my head bent into the engine compartment than my SWC at maximum revs.

My '63 L-76 is set up to '65 SHP/FI specs. At idle the total advance is about 32-34 degrees - 12-14 initial, 16 vacuum, and about 4 centrifugal at 900.

Since all the centrifugal is in at 2350, at freeway cruise total advance is 12-14 inital plus 16 vacuum, plus 24 centrifugal for a total of 52-54, and it gets 22 MPG at a steady 65-70.

Big blocks like a little more advacne than SBs.

As far as originality and judging is concerned, it appear that NCRS judged ony look at the geometry and finish on the cans. Since they do not deduct for the "wrong" numbers, you can run the NAPA/Echlin V1810 can and not lose judging points.

Duke

[John L.]

Oops - I screwed up -- Revised Numbers

Sorry folks -- I screwed up. The RPM I was referencing was Distributor RPM. Therefore here are corrected numbers.

If I run that much initial (14) with the B28 can then
at idle (900 RPM) I would get 14 initial + 8 vac + 8 vac + 0 mech + 0 mech = 30 deg,
at 1800 RPM I would get 14 initial + 8 vac + 8 vac + 7.5 mech + 7.5 mech = 45 deg,
at 2700 RPM I would 14 initial + 8 vac + 8 vac + 10 mech + 10 mech = 50 deg,
at 4600 I project I would get 14 initial + 0 vac + 0 vac + 14 mech + 14 mech = 42 deg.

I have a 3:36 rear in my L72 so 2300 rpm represents my cruise at approx 50 mph. What do you all think of my numbers -- maybe this is too much timing ???

I am currently running 9 deg initial which would put my current timing numbers with the B28 can 5 degress lower in all examples which equate to 25 at 900, 40 at 1800, 45 at 2700, and 42 at 4600. This now looks like too little timing to me.

My distributor has been set up to the stock specs which I used in the examples.

thank you

john lolli

[Everett Ogilvie]

Re: Some Mechanical Advance numbers for the 093 Di

The Chevrolet spec for initial advance on the L72 is 8-14. At my idle speed of 900-1000, there is already a few degrees of centrifugal coming in, and the true initial is 12, which should be "safe". It does not start hard, even when hot. I am also at relatively high altitude - almost 6000 feet, which allows for running more advance. As I mentioned, all I can get for mechanical advance is 33 so far, and that is either b/c I did not rev it high enough, or my mech advance has been artificially limited, which I will determine. When I connect vacuum up to the std port, the 33 becomes about 44 degrees, so the can is bringing in another 11 degrees (the 360 can is rated at 12 degrees, so the vacuum contribution appears in the ballpark). Admittedly, the 44 is probably with the engine only rev'd to about 4000, so it may not be all in. I may see if anyone around here has a distributor machine - the absolute best way to see what you've got.