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Discussion Starter · #1 ·
I'm getting ready to drop a gear swapped HEI into my 76 258. I've read some vague and conflicting opinions on what vac. advance to use. I read one guy say that the factory chevy unit allows too much total advance. Also, are a new set of weights/springs in order? I'm sure through trial and error I could figure this out, but I would much rather benefit from the experience of others. Thanks in advance. Tom


1,181 Posts
Just a note, summit racing sells an adjustable vacuum advance by Crane Cams for the GM HEI, PN: CRN-99600-1, $23
I had problems with my vacuum advance HEI in my chevy (cam was too big) and switched to Electronic Advance
on a Mallory Unilite 9000, an expensive fix to vacuum problems!

big ed


1,748 Posts
I bought the Crane Adjustable vac advance kit from
Summit, and had that on for a while, then I took it
off and went back to the stock unit - there were so
many adjustments I was overwhelmed. The only mod I
have on the HEI at this time is one of the heavy
springs from the Crane kit, and one of the light
springs, for the mechanical advance. I think this flattens
out the mechanical advance curve some, bringing some
of it in at a lower rpm (the light spring), and
reserving some for the higher rpms (the heavy spring).
The stock GM springs are very stiff (heavy). I have
an automatic tranny, and use a lot of the rpm band,
and with the stock springs, I found that if I set it for no
ping on the highway at higher RPMs, it was really
doggish around town (no advance in the lower rpm range).
This setup seems to work best for
me. If you have a standard tranny and don't ever see
2500+ rpm, you may be ok wiht the stock springs, or may
want a different combination. Other than the correct
gear, I have made no other changes. I also run it off
of manifold vacuum, not the port on my weber carburetor, and
the regular chevy vac can seems to work ok for me.

Good luck

88YJ, 4"susp, 33"BFGMT, 9k#winch, rear homemade swingout, reb.258, 999, 4.10, weber32/36, GMHEI.

6,870 Posts
Pete found through trial and error exactly what I verified on the distributor machine.
The stock centrifugal advance springs should be a performance boost right out of the box.
The advance curve is a lot faster in the HEI than in your old distributor.
If you want to bring it in faster, don't mess with the vacuum advance other than to verify it is working, and hook it up to ported vacuum on the carb.
If you want to try faster (lighter) springs, get one of the spring kits for the HEI, and have at it.
Just remember, it's the total spring pressure, not the balance of the springs.
You should start with a Heavy/ Heavy total spring arrangement, and progressively move to lighter springs, One at a time.
IE: Heavy/ Heavy, then Heavy/ Medium, then Medium/ Medium, then Medium/ Light, then Light/ Light.
When you start to get 'spark knock' or 'valve clatter' (all detonation) then back your timing off about 2 degrees.
If the timing retard doesn't stop the knock, then go back to the next heaviest spring arrangement, and move your timing back.

Hope this helps, Aaron.

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