Converting ported to full time vacuum advance

[John H.]

Re: Converting ported to full time vacuum advance

Also, my fan only has 5 blades (17" fan), shouldn't it have 7?

Mike -

Nope - all 67's had 5-blade fans.

[Duke W.]

Re: Converting ported to full time vacuum advance

OK, the VC1765 is on order now

If it really only idles at 10" it needs a VC1810 or equivalent (B28), which are getting scarce. Search the archives for "Two Inch Rule".

Duke

[Michael J.]

Re: Converting ported to full time vacuum advance

If it really only idles at 10" it needs a VC1810 or equivalent (B28), which are getting scarce. Search the archives for "Two Inch Rule".

Duke

I reset the idle advance to 10 degrees BTDC (from 5 degrees), and now I get 13 inches of vacuum, and it doesn't heat up as fast either, I can idle for 10 minutes now before it gets north of 210. So making some progress, will install fan clutch and new VAC soon.

[Michael H.]

Re: Converting ported to full time vacuum advance

I reset the idle advance to 10 degrees BTDC (from 5 degrees), and now I get 13 inches of vacuum, and it doesn't heat up as fast either, I can idle for 10 minutes now before it gets north of 210. So making some progress, will install fan clutch and new VAC soon.

There ya go. The timing on my 66 425 HP (basically the same engine as your 435) is set at 12* initial and it doesn't overheat and it doesn't even have a functioning vacuum advance. (and, I live in HOT Florida)

For 67, the initial timing was to be set at a much lower setting than 65 and 66 (4* ?) and that came about because of Fed emission standards.
The only issue you may have is that the distributor for 67 had more centrifugal advance built in to compensate for the lower initial 4* setting. That may give you too much timing at high RPM.

It's difficult to get a 425/435 to produce enough idle vacuum to make a vacuum advance unit function properly for both idle and part throttle conditions.

[Nick L.]

Re: Converting ported to full time vacuum advance

Here is the conversion on my 427/400:




I am also using the B26 cannister which is listed as a replacement for B20 (B20 no longer available through NAPA). I have verified its specifications against the B20 and found it it to be the same. Installed in the car, I have observed the B26 can actually produces 18 degrees of crank advance (probably within tolerance, but more than I wanted). I limited it to 16 degrees with a simple sleeve on the rod.

[Michael J.]

Re: Converting ported to full time vacuum advance

Thanks to all who have helped solve the problem. As I posted on another thread today:
Yep, now I have finished the fixes. Hooked ported to full time vacumm advance, $0. New VAC unit that pulls more advance at lower vacuum, $10. New fan clutch, $147. Radiator cleaned and rodded, $120. Advance idle timing to 10 degrees BTDC, $0. Check all shroud fit and seals, $0. Driving today for 3 hours in stop and go traffic and the temp stays beow 185 the whole time, PRICELESS.

[Duke W.]

Re: Converting ported to full time vacuum advance

It's difficult to get a 425/435 to produce enough idle vacuum to make a vacuum advance unit function properly for both idle and part throttle conditions.

WHAT....? It's a matter of selecting the correct specification VAC for the installed cam./idle vacuum. Using the Two-Inch Rule, for an engine that idles at about 14" (like L-79 and SHP big blocks), the B20/B26 is appropriate. If 12" or less, the B28.

According to the specs I have the maximum centrifugal is 30 @ 3800, which is the same total centrifugal as L-72 (30 @ 5000), but it comes in quicker. The nominal intial timing for L-72 is 8* with a range of 8-14. So...

I'd set the initial at the maximum it will tolerate without detonation on premium unleaded gasoline, which is a matter of experimentation. If you have to set it below 10 to ward off detonation, I'd leave it there and slow the centrifugal to not chop off the top end with too little WOT total advance.

With 10-12 intial, a proper full time 16* VAC, and a properly functioning fan clutch it should idle forever in 100 degree temperatures without getting over about 210F.

Duke

[Duke W.]

Re: Converting ported to full time vacuum advance

Thanks to all who have helped solve the problem. As I posted on another thread today:
Yep, now I have finished the fixes. Hooked ported to full time vacumm advance, $0. New VAC unit that pulls more advance at lower vacuum, $10. New fan clutch, $147. Radiator cleaned and rodded, $120. Advance idle timing to 10 degrees BTDC, $0. Check all shroud fit and seals, $0. Driving today for 3 hours in stop and go traffic and the temp stays beow 185 the whole time, PRICELESS.

Like I always say, if the cooling system components perform to OE spec the engine should not overheat.

The goofy ported vacuum advance and low initial timing was to meet CA tailpipe emission standards, and since L-71 was a low volume engine option, they didn't use different specs for the 49 states that didn't have tailpipe emission standards in 1967.

That reduced cooling margin, but with a new fan clutch, rodded out radiator, and full time vacuum advance, you shouldn't have any problems for the next 30 years as long as you use a low silicate antifreeze, like Zerex G-05, and change it every 2-4 years.

You got all the work done pretty quick, too!

Duke

[Joe C.]

Re: Converting ported to full time vacuum advance

And if you want more heat rejection margin, then make sure to check and/or upgrade the poor factory sealing system between the radiator and the fan shroud. If you don't have a tight seal here, the airflow will find the path of least resistance by bypassing the radiator and flowing through the gaps. This situation becomes particularly important if the car is static.

(edit) AHA......I see that you have already addressed the seals...................excellent!

[Michael J.]

Re: Converting ported to full time vacuum advance

Like I always say, if the cooling system components perform to OE spec the engine should not overheat.

The goofy ported vacuum advance and low initial timing was to meet CA tailpipe emission standards, and since L-71 was a low volume engine option, they didn't use different specs for the 49 states that didn't have tailpipe emission standards in 1967.

That reduced cooling margin, but with a new fan clutch, rodded out radiator, and full time vacuum advance, you shouldn't have any problems for the next 30 years as long as you use a low silicate antifreeze, like Zerex G-05, and change it every 2-4 years.

You got all the work done pretty quick, too!

Duke

Thanks, since I am retired I can spend all day (and night) working on my cars.