The Jalopy Journal
Discussion in 'The Hokey Ass Message Board' started by 325w, Mar 27, 2020.
which port do you use to hook up your vacuum advance. 4 bbl.
Ports "above" the throttle plates is "partial" vacuum.
Ports "below" the throttle plates is "full" vacuum.
Full vacuum has been used since the dawn of man. This advances the timing at high vacuum. That is idle and cruise rpm's. The vacuum drops during acceleration.
Ported vacuum started in the "smog" era. This has no vacuum advance during high vacuum, during idle, or cruise rpm's, but advances during acceleration, then slowly drops off as the rpm levels to a steady state.
A lot depends on the way that you drive. Either can be of benefit, as can no vacuum. BUT, you'll need to vary the initial (crank) vacuum and experiment with each to find the one that "you" like best.
You can't just swap from one to the other without altering the crank timing AND get the best performance and mileage. As I said, a little time and experimenting is in order to get the best out of either way that you end up with.
All three choices will work, you just need to experiment and pick one that works best for you and your engine.
Drivers side manifold vacuum passenger side ported.
WRONG WRONG, I would love to know how this rumor got started.1969 Mopar shop manual Holley
1969 Mopar shop manual AFB
1954 Chevy truck shop manual
1939 Chevy shop manual
You are right that you can try them all, but you can't make the blanket statement that full vacuum at idle is the way it has always been done. Full vacuum was used for smog era but was usually controlled by a thermal switch to not allow vacuum advance until the engine warmed up.
Use whichever works best on your car. Try them both.
With a stock or a mild cam, I have experienced more torque with manifold vacuum. With high compression and an agressive cam profile I have had good luck with the ported vacuum and more inital timing. But that really depends on what your engine likes best. Just play with it until it won´t improve anymore
You'll get better gas mileage and better performance with the vacuum advance hooked up to the full time vacuum port.....
Only if you idle excessively. Off idle, there is no difference.
Mine is on the driver's side, (650 cfm AVS2 - goes to distributor vacuum advance). The directions are bit ambiguous, but if you read it very carefully, the driver's side is correct. I originally had it on the other side, but feedback from an expert corrected that.
Which one you run is up to you , the reason that ported vacuum even exists is a result of emission regulations ,driveability and performance got thrown under the bus in favor of lower NOx levels beginning in the 60's
I've not had good luck running over 6-8 BTDC with manifold vacuum for the advance, always got a off idle ping, if setting up too much initial.
I could get away with more initial using ported vacuum.
As been said, try both ways, but keep in mind that you need to tinker with initial for both cases.
This article may help. A little long but very informative.
Not to say you would do it, but a lot of people after they get done adjusting on the edlebrock carburetor the ported vacuum port has vacuum all the time.
And it just goes on and on..
SB Mopars dont follow tbe same rules as SBC's. I own both.
And this is correct!
What’s the best deer rifle? What brand of beer tastes best?
Sent from my iPhone using The H.A.M.B. mobile app
And that article just perpetuates the myth...
Why pre-64 Model 70's, ofcourse...I prefer a .270...flat shooter, for Elk I step up to my .06, or put the Winchesters away and use my Browning 300 Winmag.
Hey, why I have you here and we are waaaay off topic, do you ever see any small cam Cummings anymore?
I've a set of Jakes I'd like to send to someone if they could be used.
Perhaps you like to enlighten us with your DOCUMENTED wisdom ??
I prefer my Model 94 30/30. Man!, I haven’t seen a Small Cam Cummins in probably 12 or 13 years. Great offer though!
Sent from my iPhone using The H.A.M.B. mobile app
Aw yes, I have a few But only use them where a scope is a hindrance.
PS...I haven't seen a small cam in 30 years...lol
There is one next door to me in a needle-nose KW, the only one I have ever knowingly seen.
Scroll up and read post #4. Egads has it right...
I'm not to sure engines know what brand they are.
But those that do, can be rebellious at times....
This... from Motor Service's automotive encyclopedia 1965 edition. Unless you were in California, this was prior to many changes being made due to emissions.
Thank you ...the theory espoused in the 2nd paragraph seems a bit sketchy , but good job on the research !
Different combustion chambers and cam combinations have different characteristics. You can put damn near all the early advance you want to on a SBC but if you do that to a SBM, it will ping when you try to accelerate under load. That has been my experience. YMMV.
There are a lot of SBC combustion chamber designs over the years and they act different.
Separate names with a comma.