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Why do car magazines always suggest hooking your vac advance to port vacuum? Then you drill holes in the carb primary throttle blades to get the car to idle.

It seems like spring has sprung and there has been a rash of timing and idle questions on the forums lately.
 

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Why do car magazines always suggest hooking your vac advance to port vacuum? Then you drill holes in the carb primary throttle blades to get the car to idle.
because they don't know what the heck they are talking about! :D
I have been playing around with these cars along time and I have yet to drill the throttle plates to get something to idle.
 

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because they don't know what the heck they are talking about! :D
I have been playing around with these cars along time and I have yet to drill the throttle plates to get something to idle.
Must not be large cams ! Install a 283/291 @.050 .680/.632 106° in a 327 or 350 and you will find out how big of a hole you need
 

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you should never need to drill the plates in a Holley carb.
if it don't idle,something else is wrong
 

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you should never need to drill the plates in a Holley carb.
if it don't idle,something else is wrong
Maybe true for Holley, but Oldsmobile drilled the throttle plates on it's early seventies carbs. Sure, you could probably find a way around drilling throttle plates, but it's not an "unsound" practice.

Getting back to the original post, I'm not sure why the magazines use ported. Manifold vacuum always worked fine in my cars. If I had to pass an emissions test though, I'd go with ported.

Mike
 

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If I had to pass an emissions test though, I'd go with ported.

Mike
That's probably the answer. Most of the aftermarket stuff is for street cars and is C.A.R.B. certified. The installation instructions would default to the low emissions setup.
 

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onovakind67,

I'm sure you are right. The problem comes in when someone tries to run ported, but the rest of their setup is in line with running manifold vacuum. It's one of those newby (and sometimes not so newby) snags.

Mike
 

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No opinion on why magazines say what they say...

Giving an engine the fuel and timing that it needs under every condition it must run is real tough with conventional controls.
And as the cam gets more radical, and the engine's ability to run at low RPM gets more and more compromised, giving it what it needs get's tougher.

The classic scenario is a motor that pulls 10" of vacuum at idle, but the vacuum drops to 7" when the automatic trans is put into gear. It's virtually impossible to find a vac can that can handle that. Vacuum drops 3", can pulls 5 degrees of advance, motor stalls.

Sometimes ported vacuum can be better even tho it's not.

'Course we could all go to electronic engine management.
My Holley system does not have these limitations.
 

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Must not be large cams ! Install a 283/291 @.050 .680/.632 106° in a 327 or 350 and you will find out how big of a hole you need
Don't need cams that big in a mid/high 11 second street car..big block or small block.
I am talking street/strip stuff and you are mentioning max. effort small block strip stuff :)

you should never need to drill the plates in a Holley carb.
if it don't idle,something else is wrong
yup!!
 

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My Pro systems carb came with them drilled. I assume Patrick knows what he is doing :yes:
Jerry,no offense with what I am about to say...you have a strong running,impressive combo IMHO but a 108 MPH 406 SBC probably really doesn't need drilled primary throttle plates.
A few guys I know run 400 SBC's in the 108-110 MPH range in the 1/4 and they are using either a 3310 or a 4779 (cams are in the 240ish-250ish @ .050)
I personally don't have experience with Patrick James and don't need to for the stuff I work on.
If he built you a nice custom calibrated carb and you are happy with it thats all that matters :beers:
 

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I agrree with Chris,for most of the mild to fairly hot sbc/bbc street street/strip combo's we are running here in t/chevelle that there isnt a need for drilled prim throttle plates in non max effort setups like was stated.

I think the reason why hi perf carb mfgs like BG or Holley's hi perf line of carbs drill prim plates is they are expecting most of their cambs to be used mostly on hot strip type setups with very large cams and it helps eliviate some problems/tuning issues for the mfg & customers.

I too have never hit a time in past 37 yrs with some prettty hot setups where i couldnt get it to idle without drilling the prim throttle plates,thats usially required for max effort stuff only unless you dont have it dialed in/tuned correctly,then drillin gplates helps to mask real problem.

But you need full vac for vac adv that matche dproperly to cams idle/lowe speed intake vacuum on most all perf setups for more timing at idle with longer duration cams,ported is NG and the mag's are simply wrong on that one.
 

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Don't need cams that big in a mid/high 11 second street car..big block or small block.
I am talking street/strip stuff and you are mentioning max. effort small block strip stuff :)



yup!!
MP ran cams that big in 327's and 283's running 11's, not on the street.

My cam on the street requires a hole in two butterflys to idle, its a 260/268 @.050 .630/.630 110° lobe center

Before the carburetor didnt have holes in the butterfly you couldnt gain control of the air/fuel mix because the transfer slots were open too far, closed the tranfer slots down to .020/.020 and drilled two butterfly's and it will idle all day long

Here is a video of my car idling last year at the Nostalgia event open headers, It idled for a while and will, its under trmnatr
http://chevelledotcom.com//video/
 

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I used to run manifold vacuum advance exclusively but recently switched to ported vac. For my combo ported has worked better.

The problem with manifold vac is that as soon as you put the car in gear or put a load on the engine such as letting the clutch out at low engine speed the advance you had at idle goes away, causing rpm drop and a possible stumble.

When I swithched to ported vac my throttle response pulling away lightly from a dead stop improved.

The downside is that you need more throttle opening at idle, but this wasn't a problem for me. I just added more idle bypass air to my q-jet to keep rpm up and the transfer slots in the right spot.

When ported is set up right it will kick in as soon as the throttle is opened. Other than no vacuum at idle ported behaves the same as manifold vacuum.

Many people think that a ported source supplies vacuum anytime the throttle is opened off idle but it is identical to manifold vacuum- it goes away under heavy acceleration just like manifold vac.

I suppose in the end it depends on your combo, driving habits or personal preference, but in my case ported worked better.
 

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The problem with manifold vac is that as soon as you put the car in gear or put a load on the engine such as letting the clutch out at low engine speed the advance you had at idle goes away, causing rpm drop and a possible stumble.
This can be fixed by installing the correct vacuum advance canister.



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When ported is set up right it will kick in as soon as the throttle is opened.
This is correct and tells you that you will have the same amount of timing when you release the clutch and open the throttle to accelerate whether you are using ported or manifold.
 
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