C3... TI Distributor specs

[bruce11495]

I'm looking for the curve specs for a 70 and 71 LT1 TI ignition. Does one of them use ported vacuum versus manifold vacuum or are they both the same? I had seen a website that had posted a bunch of dist part #s and the setup figures, but I can't find it now.
Thanx in advance

[Warren F.]

Re: C3... TI Distributor specs

Bruce:

There is a phone # you can call at GM customer service, they will send you a package that gives a trememdous amount of detail for the year car you ask for. This package contains Corvette specifications from several sources including Engineering Product information department. It is quite detailed as far as the engine mechanical section goes.

[Mark #28455]

1 800 222 1020 *NM*

[Kent K.]

Re: So what's the phone number?

Warren ----- Do you have the phone number at GM customer service we may call to obtain the information you wrote about? Kent #6201

[Kent K.]

Re: 1 800 222 1020 --- Thanks *NM*

[bruce11495]

Re: C3... TI Distributor specs

Warren...I already have that package. I was looking for the curve specs for the distributor. I found them in Dave Fiedlers website, all I need to know now is if the vacuum is ported or manifold.

[John H.]

Re: C3... TI Distributor specs

Bruce -

LT-1's had A.I.R. originally, so I'd guess they had "ported" vacuum, which was the key to making the A.I.R. system effective (retarded idle timing, high EGT for effective afterburning in the exhaust manifolds). Virtually all '68-up into the mid-70's Corvettes used "ported" vacuum for emissions reasons, and many had TCS piled on top of it so the distributor never saw manifold vacuum except in high gear, not warmed up, or overheating.

[bruce11495]

Re: C3... TI Distributor specs

John....That's the answer I was looking for. I couldn't remember if the 70 LT1 had ported or manifold vacuum. Never thought about the pollution stuff. Did the earlier 365 HP engine use ported too, or they weren't required to install pollution pumps so they used manifold vacuum?

[Duke W.]

Re: C3... TI Distributor specs

The first tailpipe emission standards went into effect for MY 1966 in California and 1968 nationwide, so the mechanical lifter 327s were never subjected to emission controls.

Keep in mind that emission control engines have VERY DIFFERENT timing maps - more than just ported vacuum advance. As a general rule centrifugal advance was slowed down, initial timing and/or total centrifugal was reduced and vacuum advance increased but at much higher manifold vacuums.

If you switch from ported to full vacuum advance on an emission controlled engine you should backdate the ignition map to the closest antecedent non-emission controlled engine. For the LT-1 this would be the 327/365, the early 327/300 for base engines, the L-72 for mechanical lifter BBs, and L-36 for hydraulic lifter BBs.

This is not a major modification as it will usually only involve substituting a different ten dollar vacuum can (and bypassing TCS) along with switching to full time vacuum advance and increasing the initial timing to achieve the proper total WOT timing (sum of initial and full centrifugal). Also, in some cases the centrifugal can be quickened with lighter springs, but the rate of centrifugal may be limited by fuel octane quality as measured by low rev detonation.

Duke