I recently purchased a 1967 L68 w/A.I.R. system intact. The engine is currently running full manifold vacuum advance and it runs very well. The car is going to be Flight Judged next month and I'm wondering if I should leave it as is and take a points hit or will I be opening a can of worms by going back to the ported vacuum? What will I have to adjust, if anything, to take it temporally back to ported vacuum and is it worth it? If anyone has a picture of the vacuum hose routing with ported vacuum and A.I.R. on a L68 I would appreciate it. Thank you.
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Re: Ported or Non ported for Top Flight judging
OK, Now I'm confused. Page 416 of Noland Adams C-2 book shows a K-19 Tri Power car with full manifold, non ported vacuum going to the distributor. I'm assuming this is what I want in this car from a NCRS viewpoint and a drivability viewpoint. It must be later Corvettes that have the dreaded ported vacuum but I thought I read somewhere that all A.I.R. cars had ported vacuum. -
Re: Ported or Non ported for Top Flight judging
Bill,
I have a 68 L79 small block and it has ported vacuum.
I was told by someone on this board that the reason for the ported vacuum was due to the new A.I.R system used in 1968.
I cannot attest to the validity of this but am only repeating what I was told.
My car has the A.I.R. system and the vacuum advance is connected to the base of the carburetor at the front of the engine as per my 68 AIM .
KurtComment
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Re: Ported or Non ported for Top Flight judging
This may or may not help. If you have the 67 service manual, look at the tune up section. If it says to plug the vacuum advance or disconnect the vacuum advance to check/adjust engine timing, this is to stop the ignition timing from being advanced by the vacuum advance unit at idle. A ported vacuum car will not have the ignition advanced at idle by the vacuum advance unit. My 68 L79 with air injection system has on the tune up lable to plug the vacuum advance hose to check/set ignition timing.Comment
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Re: Ported or Non ported for Top Flight judging
The nipple on the center carb for the vacuum advance is ported, and if the configuration is not as original you will get a deduction.
In order to run full time vacuum advance on a '67 L-68 or L-71 you have to tee the vacuum advance line into the choke vacuum break or power brake circuit. Both L-68 with manual trans and L-71 used the same center carb.
If you have an AIM it will show the proper OE configuration.
Converting back to ported vacuum advance for flight judging should be simple if the conversion to full time was done with some thought. I recommend teeing into the choke vacuum break hose, but do so by removing the OE choke vacuum break hose and making a new harness, so the OE hose can be reinstalled intact.
The loss of total idle advance will mean the car probably won't even idle with the idle screw in the current position, so idle speed will require adjustment (You might just start by tuning it up at least a quarter tune before you start the engine), and for best operation the idle mixture should be adjusted to optimum any time you swap from ported to full time or vice versa.
DukeComment
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Re: Ported or Non ported for Top Flight judging
Duke, I was under the mistaken impression that the upper vacuum nipple was ported and the lower nipple was manifold vacuum on the 3660A center Holley 2bbl. I wrongly figured the lower nipple would be below the throttle butterfly and subject to full manifold vacuum and the upper one would be above the butterfly and thus ported. Back to the books for me and thanks to everyone for their help in this matter. I have the car running so well now with the ported vacuum advance that I can't wait to try the manifold vacuum, but I'll wait until after I have it judged. I did have to go up to a .028 accelerator pump discharge nozzle (shooter) to get the stumble out of it though. Don't tell the judge, I'm hoping he won't notice.
-BillComment
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