C2 advance curve for today's fuel?

[Joe R.]

The July 2005 issue of Corvette Enthusiast has an interesting article about optimizing the performance of a 1967 327/300. The author (Henry P. Olsen) makes the argument that both the ignition advance curve and the air/fuel mixture settings designed by the factory in 1967 are no longer optimal for today's gas.

For the ignition timing, he recommends quite a few changes. These include more initial advance while keeping total mechanical advance almost unchanged, bringing full mechanical advance in at a lower RPM, and reducing the maximum amount of vacuum advance. Carburetor modifications will be addressed in a future article.

I'd like to hear from some of our experienced Tech Board participants regarding the assertion that these types of advance curve changes will make the stock 327/300 better suited to today's gas.

I'm planning to have my 327/300 distributor rebuilt next winter, and my intent had been to have the rebuilder use the factory settings. I'm interested to know if there is a consensus that certain changes would be appropriate.

[Timothy B.]

Re: C2 advance curve for today's fuel?

Joe,

I have 63 and 67 vettes and both are 300 HP cars. In the last year I have gone through both distributors to verify the correct internal parts and check the advance curves to make sure they are in spec. The 63 distributor has 24 degrees centrifugal all in by 4600 RPM and inital is set at 12 degrees. The 67 has 30 degrees all in by 5100 RPM and 6 degrees inital. Both cars run great but I believe the 63 is more responsive than the 67. I wanted to make sure the distributors are in spec so I had a starting point before I changed anything. I decided not to change because they run fine for me but if I were to change the 67, the most I would do is go to the 63 spec which would be a 24 degree distributor so you can set inital at 12 which gives a total of 36.

These parts are available at swap meets and the job is easy once you understand how the parts work together. Keep in mind that GM engineered the cars very well so a slight change may be all your looking for but my cars run fine with everything at spec. There is alot of discussion in the archives about this and there are many on the board that helped me understand distributors and I was advised not to stray far from the original specs.

[William C.]

Re: C2 advance curve for today's fuel?

The authors suggestions would take you pretty close to the '65 365hp curve that has been recomended on this board by several as a good starting point for a "performance" curve. The newer the model after that the more the curves from the factory were driven by emissions considerations.

[Duke W.]

Re: C2 advance curve for today's fuel?

Tell us what the OE advance map is and that recommeneded by the author. Without specifics I can't offer an opinion.

As a general rule, you want the centrifugal in as fast as possible, but today's lower octane fuels may preclude a quicker centrifugal advance due to low rev detonation, and high rev detonation may limit the total WOT advance that the engine can handle from the 36-38 that is considered "ideal".

If anything the later 300 HP WOT advance curve is probably more detonation resistant than the early 300 HP curve because the former features more centrifugal, so it needs less initial timing to achieve the desired total WOT timing. Vacuum advance specs also, changed, but both are generally okay.

The ignition maps for 300 HP engines are different pre-'65 and '65-up, so you can't make a blanket statement on modifying the ignition map for all year 300 HP engines.

Duke

[Joe R.]

Re: C2 advance curve for today's fuel?

Hi Duke:

You have asked me to provide specifics on the OEM specs and the recommended changes, and I will attempt to provide that info below.

However, my primary question had to do with the basic assertion in the article that the 1967 advance curve is not optimal. As a starting point, I just wanted to get some opinions from experienced people like yourself regarding whether changes should even be considered. If the consensus is yes, then perhaps we could discuss the specifics.

Regarding the specs you asked for, the article is not clearly written. For example, the distinctions between initial advance and centrifugal advance are not always clear when "total mechanical advance" is specified. Following is my interpretation of what the article says (I can't vouch for the stated OEM specs):

Initial advance: 8 degrees OEM, 12 degrees recommended
Centrifugal advance: 30 degrees OEM, 24 degrees recommended
RPM for max centrifugal advance: 5100 OEM, 3600 recommended
Vacuum advance: 16 degrees OEM, 10 degrees recommended

Note that the total of initial plus centrifugal advance is relatively unchanged (38 degrees OEM, 36 degrees recommended), but the initial advance is greater and the total advance comes in at a lower RPM. I'm not sure how to interpret the effect of the changes in vacuum advance.

[Duke W.]

Re: C2 advance curve for today's fuel?

What he seems to be saying is that the '65-up 300 HP centrifugal curve should be back dated to the pre-'65 curve. I don't disagree with that, HOWEVER, as I said previously the pre-'65 curve is more likely to cause detonation on today's fuels because it yields more low rev WOT advance. The '65-up WOT timing curve is a little less aggresive, but does achieve 38 degrees with 8 initial near peak revs. As a rule the timing can be advanced slightly above the torque peak and one degree per thousand revs is a generally accepted recommendation. On the '65-up 300 HP engines, most of the centrifugal is in at 3500, and only advances slightly beyond that point, so I think it's a pretty good curve and should not require the initial timing to be retarded significantly from the "ideal" 8 degrees to keep out of detonation.

Beyond this I will disagree with his recommendation to reduce vacuum advance. I believe that 16 maximum vacuum advance is about right, but it must be matched to the engine's manifold vacuum characteristics, i.e, full vacuum advance must be achieved at not less than 2" less than typical idle speed vacuum with about 24-28 degrees total idle timing. The OE map will yield 28 degrees with 12 initial and 24 degrees with eight degrees initial Maximum vacuum advance of about 16 degrees is achieved at about 16" and the idle vacuum should be about 18" at 500 RPM

Keep in mind that retarding the total WOT timing three degrees from what is optimum for peak WOT torque at every point in the rev range will only reduce output about one percent, which can only be measured on a research type dynamometer setup, so don't obsess over a couple of degrees anywhere in the map. As a general rule advance the initial timing until you achieve about 38 degrees and if it doesn't detonate, this is probably near ideal. Then quicken the curve until you reach the detonation limit. At this point you've done about all you can do with the relatively simple and crude (but simple and easy to modify) ignition advance controls on vintage engines.

I believe there are also some changes in the vacuum can specs after '65, so one must know the OE specs for their specific model year/engine option via the CSM or AMA specs, and then verify how the car is set up before contemplating any changes.

Duke