Timing advance curve question

[Steve Critchlow]

I have been reading a few articles with conflicting specs, and I cant quite get everything straight. I have a bone stock 327/300 and am trying to figure out the proper timing curve.
My specs as of right now are as follows:

16 degrees base timing
17 degrees of mechanical advance
19 degrees of vacuum advance

for a total of 36 degrees of advance and 52 degrees of total timing.

Like I said, some articles say this is correct, while others say 36 degrees of total timing is correct.

you knowledge in this subject is greatly appreciated.

[Joe L.]

Re: Timing advance curve question

Steve----

The 327 300 hp was installed for a total of 7 model years in Corvettes. You didn't mention which model year that you're talking about and whether or not the engine is AIR-equipped. It all makes a difference.

[Duke W.]

Re: Timing advance curve question

Ditto what Joe said, and, depending on year, the max centrifugal varies from 24-30 and is not all in until as high as 5000 revs, so you will have to rev it that high to check.

Vacuum adv. specs vary by year, too, but reading the numbers on the can bracket helps ID what you have.

The actual specs are published in the appropriate shop manual for your year and the AMA specs that come with the "restoration package" from GM, available at no charge from GM Cust. Service 800-222-1020. They will want your VIN.

Duke

[William C.]

Re: Timing advance curve question

A generic answer to what seems to be your question is that the engine wants more advance at light load, and that is provided by the vacuum advance. The rpm at which full mechanical advance is reached on a 300 hp engine generally requires enough throttle opening and engine load that the vacuum advance is less than fully energized, if at all(wide open throttle for instance) The 36 degree number is for total advance under high load, so no vacuum advance would be seen under that condition. Under cruising, light throttle operation a number in the area of 45+ degrees total from all sources is not unusual.

[Steve Critchlow]

(Message Deleted by Poster)

Message Deleted by Poster

[Duke W.]

Re: Timing advance curve question

Do you have a CSM or AMA specs to see what the timing map should be?

Duke

[Steve Critchlow]

Re: Timing advance curve question

My appologies.
The car is a 66 convertible, all #s match.

no air equipped.

The main reason I am inquiring about the situation is that 16 seems extreme for a base timing and is undoubtably affecting my idle.

-steve

[Steve Critchlow]

Re: Timing advance curve question

I have service manuals for a 65 (my previous car) but it does not list my distributer number.
I also have a more broad year range manual with only base timing specs. (10 degrees).

I will take the advice to get the "restorers package" from GM

[Joe L.]

Re: Timing advance curve question

Steve-----

For 1966 300 hp (non-K-19) the INITIAL advance setting is 6 degrees BTDC. The total centrifugal advance is 26 crankshaft degrees at 4100 rpm. The maximum vacuum advance is 14 crankshaft degrees at 12" of vacuum. As Bill explained in his post, the vacuum advance degrees are not necessarily additive.

There is no Chevrolet V-8 engine that I am aware of that ever had an initial advance setting of 16 degrees. The L-88 was 12 degrees.

[Steve Critchlow]

Re: Timing advance curve question

Joe,
thanks very much for your post. I will give those settings a shot and see how she runs. It definately looks like I am lacking in the centrifugal advance and am compensating by BTDC.

Now what would you recommend in weight springs, because I know the rate at which the weights spread is significant.

[Joe L.]

Re: Timing advance curve question

Steve-----

For centrifugal advance springs all I could recommend to you are GM #1951743. These are the original springs. Unfortunately, they were GM discontinued in May, 1980. I don't know of any replacement springs that meet the original specs. So, for springs it becomes a question of buying a "generic" kit from a source like Mr. Gasket and using "trial-and-error" to get it right.

Another method to get it right is to take or send the distributor to someone that knows how to set up one of these old distributors. They can set up the advance characteristics to match the original specs.

[Duke W.]

Re: Timing advance curve question

Joe - those look like '65 specs. The specs I have for both '66 and '67 300 HP from Corvette News:

Initial 6, 6-12 range
Centrifugal 0@900, 15@1500, 30@5100
Vacuum 0@6", 15@12"

Steve - There are probably thousands of vintage Corvette distributors that have never received a good overhaul. It's one of the most overlooked components on the car. It's an easy job and is covered in the '63 Corvette Shop Manual, and '66-up COM.

As long as the shaft bushings are okay (shaft should be snug to just barely perceptable side play.) it's an easy job for a competent DIYer.

Duke

[Mark #28455]

A common racing modification

Was to braze or weld to shorten the mechanical advance limiting slot in the rotating pole piece (for TI dist) or the rotating piece with the points rubbing block and then use a lot more initial advance.

Example:

Stock 435 HP curve - 4 degrees at idle, 30 degrees mechanical advance for total 34 degrees

Racing - 16 degrees initial at idle, 20 degrees mechanical for total of 36 degrees.

Why? Race engines frequently didn't have enough vacuum to adequately work the vac advance so you compensated with initial so the engine would idle better. Slightly over-advanced in the midrange but you rarely race from idle to 3000 RPM.

So, check the slot, you can grind it back to stock, or if points ignition, get any old donor distributor from 60's to 70's and swap in an unmodified piece.

Also note, that same piece should move freely on the distributor main shaft, if not, it needs to be disassembled, cleaned and re-lubed. Test by removing the cap and grabbing the rotor - you should be able to rotate the rotor 20 degrees by hand WITHOUT the main shaft turning.

Hope this helps,
Mark

[Duke W.]

Re: A common racing modification

In this case the amount of rotor rotation should be 15 degress,which is 30 at the crankshaft. Advance at the dist. if one-half the amount at the crank.

Duke

[William C.]

Re: Timing advance curve question

One thing to check is that the "racing" weights available in many aftermarket packages will often limit the total advance to less than spec as they tend to have a different position "at rest" from the factory weights. As recommended above, find someone in your area with a Distributor machine to get the timing map back to the stock specs and go from there. I've seen this problem many times in distributors I have worked on.