View Single Post
spuggy spuggy is offline
Registered User
Join Date: Jul 2003
Location: Glorious Pac NW
Posts: 2,826
Originally Posted by Jim2 View Post
Spuggy, in practice, a pressure regulator hooked to the nipple pointing towards the distributer DID result in the advance plate turning, and did result in a measurable amount of retarded timing. I recall that I measured my timing being retarded by a max of 9 degrees at a peak of about 4 psi.

An excerpt from your first post:
"what I learned is that under boost both the advance and the retard produce pressure instead of vacuum"
"so how the hell is this distributor made to retard if blowing in the lines does nothing. I'm afraid I am driving around unprotected against detonation."
Actually, that was Eric asking the original question, but I wanted to know the answer too - good info, thanks!

By design, the vacuum side of the dashpot does not respond to positive pressure (boost pressure) even though it sees boost while the boost is acting on the boost retard side of the dashpot.

My car was a USA 1979. This is what I measured:
Mechanical advance - 9 degrees by 3500 rpm
Vacuum retard - 24 degrees by 6" mercury
Boost retard - 9 degrees at 4 psi
Great info, thanks - so your '79 had a two-nipple vacuum can when you got it? Was it a Cali model, do you know?

When I bought my car it was a dog off the line. A search showed a few threads discussing the problem. The grease under the advance plate was so hard you could not get the advance plate to move, and it was stuck in the full vacuum retard position. After I cleaned the rock hard grease and lubricated the advance plate my car drove far better. Something all 930 owners should look at to ensure that the advance plate is not stuck in an advance position under boost.
Yeh, they really don't like to run too retarded, it absolutely kills the off-idle response.
'77 S with '78 930 power and a few other things.
Old 09-12-2007, 08:04 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #18 (permalink)