[66sprint6]

Can someone post a pic and other details of how the PCV and vacuum are set up with a '68 intake? I have the intake and the air cleaner. I have a couple of different valley pans, and I see that the PCV hole is in the front on the old '74 motor that I'm removing. I have to hook up the vacuum for the various accessories, namely A/C and cruise. I'm not sure if it's drawn from the back of the carb or an intake tap. I realize that this won't be original to our '67 car.
We're picking up our engine this weekend (yahoo).

[chiphead]

I know the PCV was on the back of the valley pan on '67-older. I'm pretty sure the PCV was moved to the front of the valley pan by '69. The PCV valve should go from the valley pan to a large port on the base of the carb, to distribute the oil vapor across all 8 cylinders. The brake booster typically connected to a large vacuum port on a rear runner. Look on your 68 intake and see if it has a hole in the front that allows the PCV hose to route up from the valley pan. Use the valley pan that allows the PCV to fit with the 68 intake.

Vacuum advance goes to ported source on carb
Vacuum modulator (auto trans) goes to manifold vacuum
I would guess the AC ductwork and cruise would be sourced from a manifold vacuum source from a smaller port on the intake or the carb base.
The later engines had the thermal vacuum switches and special ports for emissions, but it doesn't sound like you're using those.

[Tom Vaught]

The first year of the "front mounted" MANIFOLD PCV was very unique and the same design from that point on.

When you remove the 90 degree fitting from the intake you will notice it is several inches long. The PCV fitting has a single downward tube that has an opening on both sides of it at different levels (one opening that intersects the upper manifold plenum and one opening that intersects the lower manifold plenum.

This was the crankcase gasses are distributed EVENLY to both runner plenums.
So each plenum on average gets about 2 cfm at max vacuum levels in the intake.

A very smart Pontiac Emissions Engineer and the Intake Manifold Engineer came up with
a very compact and functional design for those 1968+ engines.

Tom V.

[Kenth]

1968-76 PCV.

[Kenth]

1968 vacuum diagram.

[Tom Vaught]

Thanks for the GREAT pictures Kenth.

The Brass Fitting picture clearly shows the "lower plenum suction hole" on the bottom of the fitting
and the "upper plenum suction hole" on the side of the fitting.

Thanks again

Tom V.

[66sprint6]

Very helpful. Thank you..

[Joe's Garage]

There were an assortment of vacuum 'trees' used for various years.

Here are pic of a few. We like to use the last one with the line to the power brake booster attached to the large, angled nipple and other vacuum like AC or vacuum advance attached to the two smaller ones.

That one is reproduced and is sold by Year One among others.

If you have one of the other fittings already, you can 'T' the smaller line for both AC and cruise control.

And, just a reminder, you CANNOT use the PCV line for anything other than PCV.

Good luck!

[Tom Vaught]

Thanks Joe's Garage.

Tom V.

[Schurkey]

Originally Posted by Joe's Garage

And, just a reminder, you CANNOT use the PCV line for anything other than PCV.

Quoted for truth. Thank you.

[Kenth]

Originally Posted by Joe's Garage

And, just a reminder, you CANNOT use the PCV line for anything other than PCV.

Some Ford and GM applications uses the PCV for EVAP.

FWIW

[66sprint6]

Do I need that vacuum switch and lines?

[OG68]

Only if replicating 1968 emissions standards or building a factory appearing engine.
Otherwise just pick the vacuum source you require and run a single line to the advance

[bulletpruf]

Good thread. I'm re-doing the vacuum lines on my '68, and I plan to use this one for the power brake booster and vacuum modulator.

[Schurkey]

Originally Posted by bulletpruf

Good thread. I'm re-doing the vacuum lines on my '68, and I plan to use this one for the power brake booster and vacuum modulator.

Remember, the OEM vacuum plumbing for the trans modulator has a "hump" formed into the tubing, to keep fuel and condensation from draining downward and getting trapped in the modulator. The tubing bends upwards before curving down behind the engine to the trans modulator.

Lotsa guys forget the "hump", and as a result the modulator fills up with condensed fuel vapor, and water. This does not really affect modulator operation...until winter, when the water freezes.

You'd end up with a car that has harsh, late shifts when cold. Once the trans warms-up, everything is normal and the tech can't find a problem. Then when the thing sits overnight, the water in the modulator freezes, locking-up the modulator in the no-vacuum position again.

Happened to me on a '69 Impala with TH400.

For the record, I would not connect the trans modulator to any source of vacuum that has a "bleed". Modulator and vacuum advance connected together and sourcing manifold vacuum...sure. Modulator or vacuum advance, and PCV, or brake booster, or anything else that bleeds vacuum...no.

[bulletpruf]

Originally Posted by Schurkey

Remember, the OEM vacuum plumbing for the trans modulator has a "hump" formed into the tubing, to keep fuel and condensation from draining downward and getting trapped in the modulator. The tubing bends upwards before curving down behind the engine to the trans modulator.

Lotsa guys forget the "hump", and as a result the modulator fills up with condensed fuel vapor, and water. This does not really affect modulator operation...until winter, when the water freezes.

You'd end up with a car that has harsh, late shifts when cold. Once the trans warms-up, everything is normal and the tech can't find a problem. Then when the thing sits overnight, the water in the modulator freezes, locking-up the modulator in the no-vacuum position again.

Happened to me on a '69 Impala with TH400.

For the record, I would not connect the trans modulator to any source of vacuum that has a "bleed". Modulator and vacuum advance connected together and sourcing manifold vacuum...sure. Modulator or vacuum advance, and PCV, or brake booster, or anything else that bleeds vacuum...no.

Thanks for the tip.

Car is a 68 Lemans convt, pump gas 455, TH400, Q-Jet, Performer RPM intake.

I have the vacuum advance going to ported vacuum on the carb.

PCV by itself to the fitting on the back of the carb.

I changed my plan slightly; still planning to run both the trans modulator and the brake booster off the same source of vacuum, but now I'm going off the intake runner at the driver's side rear, using a fitting pictured below. What sort of problems would that cause?

[Schurkey]

MAYBE you'd see a reduction in vacuum to the modulator when you step on the brake. Your trans pressure would increase when the vacuum booster is releasing air.

OTOH, your foot would be off the gas, and the vehicle coasting, so manifold vacuum should be fairly high to begin with.

Will this cause problems? Probably not. You'd have to measure the vacuum to the modulator to know for sure.

As a general principle, I'd want the modulator on another vacuum port from any device that bleeds air. But the brake booster is WAY better than the PCV, for example.

[grandam1979]

The first picture Kenth posted tells the OP what they wanted to know.