Vacuum for 1979 GMC

[Quik9r]

350 GMC is the distributor ported vacuum or direct vacuum(constant ). Any routing images for the lines?

 

[Goldie Driver]

350 GMC is the distributor ported vacuum or direct vacuum(constant ). Any routing images for the lines?

Factory ported on a 79 IIRC.

Don't know how good this pic is from the phone, but the emissions routing is probably on this site in the tech section. For sure For a LD 80 K1500 Sub since mine is there.

 

[75gmck25]

You can probably use either vacuum port, and it originally used the timed port. However, I find it best to time and tune with distributor vacuum disconnected and plugged, and then reconnect to manifold vacuum when I’m done. This bumps up idle timing a little and makes it run smoother.

 

[78 Jimmy 2WD]

I could be wrong but I believe 74 still used manifold as opposed to the later years used ported. That said it comes down to understanding the difference and your particular engine setup. Manifold vacuum is greatest at idle and goes away as "you stick your foot in it", this good for economy if your distributor is properly setup. This means all mechanical advance in by 2500-3000 and vacuum limited to about 10 degrees. Ported vacuum increases w/engine speed and means vacuum advance is increased along w/mechanical. Personally I never understood the switch to ported other than another way to tune as vacuum advance was originally used to improve fuel economy and I believe ported was used to improve emissions. If I am wrong one more time, I am sure someone here will correct me. Best bet is to find a reputable local performance shop to properly set up your distributor for your application and if you haven't already done so, upgrade to a HEI. This is based on your profile indicating your truck is a 74.

 

[Quik9r]

I could be wrong but I believe 74 still used manifold as opposed to the later years used ported. That said it comes down to understanding the difference and your particular engine setup. Manifold vacuum is greatest at idle and goes away as "you stick your foot in it", this good for economy if your distributor is properly setup. This means all mechanical advance in by 2500-3000 and vacuum limited to about 10 degrees. Ported vacuum increases w/engine speed and means vacuum advance is increased along w/mechanical. Personally I never understood the switch to ported other than another way to tune as vacuum advance was originally used to improve fuel economy and I believe ported was used to improve emissions. If I am wrong one more time, I am sure someone here will correct me. Best bet is to find a reputable local performance shop to properly set up your distributor for your application and if you haven't already done so, upgrade to a HEI. This is based on your profile indicating your truck is a 74.

I was inquiring about 1979 350 which is another truck

 

[Rusty Nail]

/shout "FIGHT!"

Hay everybody come quick! There's a Junior fight in tech discussion!

Git em Junior!



This topic starts fights EVERY. TIME.