Coil and vacumn advance

[Vetteophile]

Can anyone here set me straight on 327 340hp heavy cam engine on the following?

Coil - I read that coil should be 1.7 ohms resistance on Primary.. and not less than 11 on Secondary. All aftermarkets seem to be around 9 on Secondary. This a big deal?? or should I just get a stock coil and not worry about these numbers?

Vacumn Advance - lots of good articles on this forum about base....centrifugal...and vacumn advance. Since my vac at idle is low at about 7 (idling at 500), they indicate I should get a control unit that starts about 5" Hg, and is all in at 7. Problem is... where do I find a vac like that? Apparently Echlin used to offer one called a VC1810, but its discontinued. Any ideas on an alternate?

 

[JohnZ]

Can anyone here set me straight on 327 340hp heavy cam engine on the following?

Coil - I read that coil should be 1.7 ohms resistance on Primary.. and not less than 11 on Secondary. All aftermarkets seem to be around 9 on Secondary. This a big deal?? or should I just get a stock coil and not worry about these numbers?

Vacumn Advance - lots of good articles on this forum about base....centrifugal...and vacumn advance. Since my vac at idle is low at about 7 (idling at 500), they indicate I should get a control unit that starts about 5" Hg, and is all in at 7. Problem is... where do I find a vac like that? Apparently Echlin used to offer one called a VC1810, but its discontinued. Any ideas on an alternate?

A stock replacement coil will work just fine - there's nothing exotic about the ignition system.

500 rpm is an unusually low idle speed - with the stock "097" Duntov solid-lifter cam, 700-800 rpm is what I'd expect to see; you should verify your mechanical tach reading with an electronic one. That may also explain your very low vacuum at idle, which should be 12"-14" Hg. at 700-800 rpm, which calls for a VC-1765 vacuum advance unit (stamped B20 or B26).

The VC-1810 advance unit is only required for the later '64-'65 "30-30" cam, which produces only 9"-10" Hg. vacuum at 900 rpm. They are still available from GM.

 

[Vetteophile]

Thnx JohnZ.. this is beginning to make sense and actually lines up pretty good with my results.

The engine builder told me he installed 'a little extra cam' in mine, and mentioned 30-30. I didnt know what that was at the time, and my vac is running ~10 at 800 rpms, which matches up quite well with your above reference. If correct, the VC1810 should work well, especially if I can get the idle adjusted up. Hope to try it out this weekend.

Thnx again for the commentary.. all is very helpful and much appreciated.

 

[JohnZ]

The VC-1810 (stamped B28) vacuum advance unit is available from GMPartsDirect.com under GM part number 88924985 and AC-Delco part number D1312C.

 

[Vetteophile]

Well.. installed the VC1810 (B28) this weekend. Good news... this made a big improvement.

Started by changing out my existing B1 vac can for the new B28. Set dwell to 30* and base timing to 14*. With 24* from centrifugal, plus 16* from Vac should have a total of 54*. Initial look showed maybe 55/56* total. That resulted in a pretty good amount of surge/jerk at cruise.

Dropped base timing to 12*. That improved things a bit. So dropped base timing to 10*. Now it runs great. No surge.. though did get a cough/backfire once when ran rpms down to 400 with too much clutch vs gas. But overall, this is very much improved.

Thanks for all the help, guys. This forum is a big help.