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How Much Manifold Vacuum?

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How Much Manifold Vacuum?

beertracker beertracker
New User | Posts: 23 | Joined: 03/11
Posted: 01/23/13
02:45 PM

I hope this is the right section to post in.  I have a Pontiac 400 with Summit 2802 cam:  

Duration at 050 inch Lift: 224 int./234 exh.
Valve Lift with Factory Rocker Arm Ratio: 0.466 int./0.488 exh.
Lobe Separation (degrees): 114

How much manifold vacuum should I measure with this combo at idle?

What idle speed in drive should I set the engine for?  Has TH350 trans & 2200 rpm stall converter.


My71 My71
Guru | Posts: 1261 | Joined: 02/10
Posted: 01/23/13
03:48 PM

Beertracker.. Gotta love the name!!
Welcome to the forum..
This is my 2 cents and I'm sure others will chime in..
I don't have a Pontiac w/an auto trans so I'm of little help there but i would think something between 500 and 750 RPM in drive would be close.
As far as manifold vacuum, you can set that yourself, realizing that vacuum is a direct result of cam shaft over lap, lobe separation, RPM, and timing (presuming your pistons and cylinders are in good condition). Looking at the specs you gave, it's pretty close to some of the performance Pontiac cams. I woudl think you would see at least 12" of vacuum, but the links below would point you in the right direction.
You can practically tune your motor using a vacuum gauge.
This link has some good info in it.

And this one has more good info in it.  

My71 My71
Guru | Posts: 1261 | Joined: 02/10
Posted: 01/23/13
03:59 PM

what year is your motor? CHecking my Chiltons manual it lists anything from 550RPM to 750 RPM from years 1970 to 1977.  

tuffnuff tuffnuff
Moderator | Posts: 2567 | Joined: 12/09
Posted: 01/23/13
04:34 PM

With the dual pattern cam and 114 LSA, I would say that 12" to 13" of vacuum could be expected at 550 to 750 RPM.
This largely depends on the condition of the engine.

When The Flag Drops.,.


The Bull ***t Stops.,.

P. Engineer, Engine Builder

Pontiacman8 Pontiacman8
Moderator | Posts: 5779 | Joined: 02/08
Posted: 01/23/13
07:09 PM

Best thing to do is get a vacuum gauge and test it.  
Engine builder,self taught auto body guy.
Horsepower sells engines and torque wins races


chrisaustria chrisaustria
User | Posts: 204 | Joined: 01/13
Posted: 01/23/13
07:34 PM

Test it with a vacuum gauge.
My calculator says even 16" idle vacuum and 190psi crank compression
Because of the high lobe separation I think it will be more than 12-13" if the engine is in good condition. Please tell us after checking it with the gauge, would be interesting how accurate my calculator works.  

pepsi1 pepsi1
Enthusiast | Posts: 348 | Joined: 09/11
Posted: 01/23/13
08:51 PM

There are some things that affect idle quality vacuum. Best way as Pman suggests use a good quality vacuum gauge. Also check a couple different vacuum sources on the intake. If you just replaced the cam and lifters, go over the intake bolts.

1.Leaking intake or exhaust valves will affect idle vacuum. (It will be erratic) Bouncing around.

2.Make sure the lifters are adjustaed properly. (if you have Zero Lashed)* the lifters it can affect idle vacuum. *Like we sometime do for Hi Performance use.

3.Timing will affect idle vacuum.

4.If there is a problem with manifold vacuum, if the valve springs weren't replaced with the cam that could be a problem. (An intake valve may not be opening all the way, or closing all the way)

5.Or a combination of all the above....Hopefully their isn't any problem.

+1 Tuff. chrisaustria suggests the wide lobe seperation and sometime that .230* degree duration @ .050 its like a magical duration number that can cause an erratic vacuum. Especially on a dual pattern cam.  Remember that old overlap cycle....

6.I don't see where you should have a problem with your disc brakes. Unless the engine has an internal problem. Were the valve springs replaced that would be my real concern, after installing a new cam and lifters? Grin  Cool  


beertracker beertracker
New User | Posts: 23 | Joined: 03/11
Posted: 01/24/13
01:46 PM

Thanks for the welcome & links.  The cam I am using is close to Pontiac's 744. Let me tell a little more about my engine.  It's a 1975 Pontiac 400 & has the thin saddle so I did not build it as a race engine.  Just close to stock in terms of hp\torque.  Based on a post below 750 rpm idle in drive should be my goal.

I am trying to adjust the Q-jet and now the engine idles at 1375 rpm, throttle plates fully closed against the barrels, 0 degrees distributor timing, 14"Hg of manifold vacuum steady dist. vacuum advance cannister disconnected and plugged at carb source.  I am trying to get the idle speed down & hopefully this is just my in-experience at adjusting Q-jets. I have Ruggles book on Q-jets but this is my first time working on one.   I have also installed a rebuild and bushing kit.

I installed new hydraulic lifters with the Summit 2802 cam along with BBC 7/16 studs and poly locks. I used the old valve springs.  The lifters were zero lashed using this method;

Heads are 6x-4 very mild porting, milled .040", using shorter push rods, 3 angle valve job, valve train geometry is perfect.  Block is not zero decked.

The engine has a new oil pump, timing gears & chain.  It runs good, revs fine, no blue smoke.  I just have to get the idle speed down but that is another problem.

Thanks again for the info.  


My71 My71
Guru | Posts: 1261 | Joined: 02/10
Posted: 01/24/13
02:26 PM

If adjusting the idle mixture screws and idle speed adjustment screw doesn't lower the RPM, you may have some choke linkage issues.  There are some good threads on here around Quadrajet adjustments. Wayne is very well versed in Quadrajet carbs.
You mentioned your heads were milled .040". Was your intake milled to match? Generally anything more than .020" milled off the heads means the intake needs milled also.
You may have a vacuum leak around the manifold or posibly at the carb base. If you take some carb cleaner and aim it at the carb mounting base and again at the intake/head mounting line and spray it along those locations and find you have a noticable drop in RPM, then that is indicative of a vacuum leak.  

barnett468 barnett468
User | Posts: 223 | Joined: 12/12
Posted: 01/24/13
06:09 PM

hello beerman yes!

you have some very good suggestions above. i'm going to go with my71's suggestion first [because it is the quickest and easiest] however i will expand on it a bit and offer my technique. others techniques will vary so just use the one that sounds most reasonable to you. i have found and fixed many many air leaks with my method.

if your vacuum gauge reads steady then your valves are sealing fine so now just put it in the trunk or give it to little johnny to stick in his sisters ear.

0. if you hear a whistle at anytime now or during this process it is an air leak. you can listen with a small vacuum hose to your ear to find it.

2. make sure your jet rods are properly closed.

1. the outer [inertia] ring on your crank damper may have slipped providing a false timing mark. unlikely but possible. if it idled fine before the rebuild at 6-10 degrees than it is fine. if not then check it. my71 said in his post above, first be 100% positive front butterflies are closed [not stuck on choke] or if you loosened and retightened them they could possibly not be even and hold themselves open slightly. make sure the secondaries have return spring tension on them and are not getting sucked open at idle. you may have to remove the carb to look at them more closely to insure they are both closed.

3. cap off the vacuum including to pcv, distributor, smog and power brakes etc and try it again.

4. this may seem useless but i do it anyway. close the choke butterfly and cover the top of the carb with your hands. if the idle only changes slightly its definitely a big air leak. if it drops a lot it is probably a small air leak providing the items mentioned above are ok.

5. with choke butterfly open spray carb or brake cleaner at carb base and intake. make sure you are no more than 2" away. try not to push the nozzle all the way down or it may splatter in carb and/or on paint. VERY IMPORTANT - always use the red nozzle the can comes with otherwise when using flammable products the vapor can travel into the top of the carb which will give a false reading by increasing engine rpm. it will also help narrow down the exact location of the leak because the nozzle confines the spray to a much smaller area. i prefer using flammable liquids [other do not] because the non flammable liquids will not always change idle speed when an air leak is present. therefore you can still have one.

6. if you find no air leak it DOES NOT mean there isn't one. it means that it is likely on the valley side of the motor therefore flammable liquids or a stethoscope [vacuum hose] won't find it.

7. you must then remove the intake hopefully you will see an impression on the gasket where it was leaking.

8. clean the gasket off lay the intake on the motor and look at the space in front between the block and where the cork or rubber seal would go. if it's more than around .020 or if the cork seal slips between it easily then you definitely need to resurface your intake.

9. if the gap is ok you're ok but it's very marginal because you surfaced your heads so much.

10. reassembly - DO NOT USE INTAKE GASKETS WITH METAL IN THEM [stiff non bendable type]. get some thick [white] paper intake gaskets or black ones if you can't find white ones however white is best because they are thicker. felpro, mr gasket etc. make sure they are NO LARGER than your ports or they make leak because there may not be enough surface sealing area to compensate for the mismatch between your intake ports and cylinder head ports.

on cyl head, silicone around water jackets with black rtv [DO NOT USE BLUE FOR ANYTHING]. then use gasket cinch on head and on head side of gasket only and same on front and rear seals. then glue those suckers down. the reason for the gasket cinch is so it will 100% guarantee that the gaskets will not move out of place when tightening intake. then apply a dab of silicone under and on top of the four corners where the gaskets meet the front and rear seals. then finally apply a thin layer of silicone [so it doesn't squeeze into the ports] to the gasket and front and rear seal surfaces. install manifold quickly. use a friend so you don't herniate a disc. you can put some big fender washers on some bolts and screw them into the manifold so you have something to grip. carefully drop it down as accurately as possible. put a screwdriver in the bolt down holes to line it up. the silicone will not only help seal [obviously] but it also works like a lubricant so the manifold can slide properly into place when torquing it down. DON' USE GASKET CINCH HERE. let sit at lrast 4 hours.

reassemble with new carb base gasket and be happy [hopefully].

good luck.  

pepsi1 pepsi1
Enthusiast | Posts: 348 | Joined: 09/11
Posted: 01/24/13
06:51 PM

It sounds like you have an intake leak. If the intake leak is on the under-side of your intake, you may see a puff of smoke as engine oil may get pulled into the intake port.(Try this) Let the engine idle for a few minutes, when its hot. Now crack the throttle open just like you want to rev it up....You may or may not get a puff of smoke.
  You can find out if you have an intake vacuum leak. It does take a little doing. If you have a dual breather set-up or a PCV and a breather on one side. Here is what you can do.
  1.Block the side that is easiest to do. I have used duct tape over the breather just double it up....This is only for a test. Whatever you need to do to block that side.
  2. On the other side I try to use the PCV side. Disconnect the PCV. Plug your vacuum source for your PCV hose.
  3.Now on the PCV hose install a vacuum gauge.(you will need to use some enginuity here). If you get a vacuum you have an intake leak under the intake side. Bring the RPM up and watch the vacuum gauge.
  4.If the gauge goes to a pressure side thats okay. You should expect that. Thats crankcase pressure.
  5.If you have any vacuum on your gauge you know there is a leak on the intake.

  Fel-Pro does have thicker intake gaskets for some intake port sizes. If you have to trim any gasket be careful not to trim too much from between the intake ports. Use a sharp razor blade or very sharp exacto knife....

If you removed .040 from the heads. You have to mill the intake manifold. or it won't seal properly. X amount from the intake.

I use Fel-Pro intake gaskets. then I also put some sealer around each of the intake ports, and the water jackets. I di this even when the heads and intake are straight and/or angle milled. I have always used a sealer on both sides of the gasket. I'm talking about a small amount not a glob of it. I never had an intake leak.


pepsi1 pepsi1
Enthusiast | Posts: 348 | Joined: 09/11
Posted: 01/24/13
07:11 PM

I just did a little calculating. For your set-up you nremoved .040 from the heads, my calculating came up with this.

.040 removed from your heads
.040 to .046 need to be removed from your intake sides also.
.069 from the bottom of the intake.If you use a a gasket. If you don't watch the fit at the bottom. To bad the engine is in the engine bay.
Your machine shop guy should have mentioned that. Well all that means is taking the intake off. But try that little test if you want first.
  I had an engine  that my shop set for Plasma Moly rings. (thats what we were Given) "HAPPY HOUR CROWD WON AGAIN".  Well the guy used Chrome backed rings. The engine smoked, the rings never sealed. Maybe in 20,000 miles.The Cross Hatch was not correct for the rings...With that said...
  Thats when I said I needed to find out what is really going on inside the crankcase I can't see. Because the Chrome backed rings never sealed I had a pressure problem. Grin  Cool


beertracker beertracker
New User | Posts: 23 | Joined: 03/11
Posted: 01/20/14
04:42 PM

I have been doing other things on the Pontiac and just got back to measuring vacuum.  I measured 12-13 "Hg at 1050rpm.  Later I will try and reduce the idle rpm.  

HerbHouston HerbHouston
New User | Posts: 13 | Joined: 12/12
Posted: 01/20/14
07:57 PM

Hey Beer!

I've had a similar set up and here are some of the things I did to get things lined out. It is a 406 with 7K3 heads and a comp cams 280H HE cam (480 lift, 280 duration with 110 degrees lob sep). I run a Holley 4160 with custom drilled primaries because in order to get sufficient air flow the primaries were coming off the idle circuit and significantly increasing the idle RPM. After all of that was done the idle could come down from 600-800 no problem, not sure if a similar treatment would work with the Q-jet. Even so the vacuum was 10-12" at idle. I just swapped in some Rhoads lifters as Ken Hand (How to build max performance Pontiac engines) recommends. These are variable duration lifters allowing you to "set" leak down (you already have an adjustable valve train so these will work for you). Best vacuum can be achieved with 0.010" Intake and 0.020" Exhaust leak down being set. Essentially this is like taking .020-.030" off the lift and approx 20 degrees off the duration below 3000 rpm. I picked up 4" of vacuum at 800 rpm idle and it idles much smoother. Now I'm idling at 16" of vacuum. Perhaps if you do a lifter swap and kill the cam a bit at idle rpm the carb will react like a more stock set up. Hope it helps, good luck!  

shyrgfuh70 shyrgfuh70
User | Posts: 93 | Joined: 12/13
Posted: 01/20/14
08:57 PM


i would do the following.

remove dist vac advance hose.

rotate dist until highest idle is reached and/or engine just barely starts to stumble.

retard timing 3 degrees.

set idle to lowest rpm to 600 - 750 rpm the lower number is better.

connect vac gauge.

turn fuel mix screws in until they just stop and cpunt the turns.

turn them both out an equal amount to the average of what they both were.

start eng and once warm turn both screws equally 1/4 turn at a time until yhe highest idle is reached.

reset idle speed to 750.

if engine turns over more slowly when hot, retard timing 3 more degrees.

connect dist vac line to ported vac not manifold vac. if line has vac at idle you may need to drill a small hole in your front throttle plates.