It seems this time every year I get questions on tuning and, in particular what to do with a vacuum guage. So here we go with another white paper. I will attempt to cover the normal diagnostic indications, some tuning and maybe a trick or two some may not have used. The vacuum guage is very useful for tuning and diagnostics and I consider it a must have for performance tuning. There are undamped and damped guages available, and while the damped guage may be easier to use for someone who is new at it, I recommend the undamped type for reasons to be explained as we go. As always, I assume no liability for any attempts on your part to hurt, maim or kill yourself or damage your car while using one. To use one, you must warm the engine to proper operating temperature and hook it to an unported (full time) vacuum fitting on your engine, Hooking to the intake directly is best, but the carb is fine. If you are hooking to the carb, the unported is the lowest fitting if there are more than one. First, the common indications that come from the manual with most guages ( with added commentary if I see the need).
1)AT IDLE.Steady needle, at 15-22: Engine in good condition. (most manuals list something similar to this) This is for a stock type engine with factory camming - if you have a performance cam it will not be as high and will not be steady- look for a steady oscillation of the needle in time with the exhaust sound-the reading is all relative so don't get tied up too much with the number.It will also be about 1" lower per 1000 feet of elevation over 1000. If it is not repetitive, see below,
2)AT IDLE.Steady reading lower than normal (3-5 in) Late (retarded) ignition timing , weak rings or a vacuum leak.If you suspect weak rings, snap the throttle rapidly to about 2500 and release-the vaccuum should fall to near zero and then temporarily rise above idle high by 2-4 inches. If this does not happen, the rings are suspect.
3)AT IDLE. Steady reading with occassional (erratic) drop. Sticking valve or ignition miss. If you are inclined to you can spray light oil on one valve at a time if a sticking valve is suspected (with the engine running, yes I know it's messy, don't stick your fingers or tongue in it while it's running) Usually, when you spray the right valve the drop will go away briefly.
4)AT IDLE.Steady needle but drops regularily one or more times per rev-valves need adjustment or burnt valve. (keeping in mind if you have a performance cam,it will oscillate all the time-)
5)AT IDLE. A slowly floating needle (Waving slowly 2-5inches back and forth) Carb too rich or a vacuum leak.A lean carb will cause a more rapid drop of about the same range and typically a sharp, staccato idle sound.
6)AT IDLE,Rapidly shaking needle (again a slight shake is normal with high overlap cams), becoming worse with higher rpm-ignition miss,damaged valve spring or blown head gasket.If it smooths out with increasing speed, it is likely worn valve guides.A head gasket will usually cause a prominant (10") or so regular drop.
7)AT IDLE.Slowly falling needle indicates a possible exhaust restriction.A blocked exhaust will generally cause the guage to drop to near zero as the engine speed is increased and will cause the needle to not return to idle vacuum quickly after the aforementioned throttle snap.
In any of the above tests a weak cylinder will cause less drop in the guage than the rest- this can be useful for tracking which cylinder- see tricks section at the end. The manual with most guages gets fairly in depth with the above concepts so I will refrain from an all day commentary.
TUNING with the guage.One of the primary reasons for the guage is to be able to tune YOUR engine properly. The timing and other settings listed in various manuals is in accordance with generalities developed by the manufacturers and not necessarily correct for your engine. All engines are a little different and each will be in optimum tune at slightly to greatly different specs than the factory, Mr Chilton/Haynes or your buddies is.Vacuum readings are pretty much proportionate to cylinder pressures and are one of your best indications as to the state of tune in your application.....on to some specifics.
All of this assumes you have something close enough to start, but if not set all your idle mixture screws to approximately 1 1/2 turns out from all the way in on Holleys, 2 turns on Edelbrock /Carter/ Quadrajet. These are the screws on the sides of the blocks on Holleys and the two screws on the front of the rest.Set the idle SPEED screw (the one on the side touching the throttle arm) 1 1/2-2 turns from zero ....in other words screw it out until the throttle arm does not move, back in until it touches and then 1 1/2 =2 turns further. Loosen the distributor just enough to be able to move it back and forth- you don't want it any looser than necessary. Start the car and adjust the distributor back and forth to get the highest smooth idle speed and retard it (same direction as rotor rotation) 1 to 2 inches on the guage. Stop the engine (turn it off) and tighten the distributor- if you think it moved, restart and compare the reading to what it was when you adjusted it. If you advance it too far, it will be faster,but the needle will jump around, too far retarded will cause the vacuum to fall.If you are tuning a race engine that does not have a locked out distributor or otherwise trying to set max timing, run the engine at the rpm you are tuning to for that engine- if you are trying to set your curve or the engine will not idle properly after setting the high rpm vac, see the tricks section of this paper. You may also want to read the ignition 101 paper.
Start the car and adjust the MIXTURE screws one at a time to get the highest smooth vacuum reading- if you haven't done this a lot, go 1/4 turn or so and move to the next one -start by going in and out to find which direction you need to go-go multiple laps per carb in order to be most accurate. If you just turn the first one to highest smooth idle, you probably will go too far and end up imbalanced- the point is to have all the screws within 1/8th turn of the same setting- you DON'T want one 1/4th turn out and another 1 turn out. Do all mixture screws and then repeat the situation one more time, but only turn each screw 1/8th turn back and forth in order to get it dead on. Remember you are looking for the highest SMOOTH speed here- if you get the screws a little too far out you will get a "hunting" needle ( waves back and forth slowly instead of being steady- this is all relative -if you have a long duration cam , the needle will be unsteady anyway, but more so if you go too far), too far in will produce a rough idle and a rapidly flickering reading. Once they are all the best you can get them, turn each one back in 1/8th turn- this will give the best lean setting. One small point here- if your screws end up being less than 3/4 turn out, you are jetted too rich and need to go down on the primary jet size and repeat the procedure- more than 2 1/2 turns, you need to richen or go up on jet size and repeat tuning. During this whole procedure, you may find your idle increases significantly- if it gets higher than 1000-1200, back The idle SPEED screw out. You can do this as many times as necessary to maintain a reasonable idle speed and you must keep it low or your mixture settings will be inaccurate.For dual quad or multiple carbs see the white paper on setting up Dual carbs.Once you are happy with your settings, set the idle to your preffered speed.
Some Other things you can do (tricks that you may not be aware of)
Before you try the next one, make sure your ignition is off/ coil is disconnected to prevent an unwanted fire up. Most engines with show 1-4" vac rolling over on the starter (with the plugs in). On long overlap cams it could be quite low-this is one of the reasons for getting an undamped guage (also the biggest face you can get). There are a few things this will help with. One is unbalanced cylinders or valve adjustment that is inconsistent. If you see one or more spots where the vacuum drop is less than the rest, you have a valve too tight (or an intake too loose)(this assumes a known good engine-if your valve seats are leaking/rings are gone etc, this test could indicate those problems) You can use this somewhat as a quick poor mans leak down- if the engine normally reads 2" for example and all the sudden only pulls 1", you have a problem. The reason this test is handy is that the guage movements will be slower and somewhat easier to see especially if you are new to it.If you see a bad spot, there are two ways to isolate it without using other guages. One is to pull one plug at a time (and replace it before moving to the next), and redo the test. The cylinder with the plug out will register as a large drop- you are looking for the one that leaves the rest consistent. The other way, if you have a timing light that will cooperate, is to hook up the timing light and aim it at the guage. The strobe effect will "freeze the needle on the corresponding cylinder. Again, you are trying to isolate the low cylinder. In order to do this properly, you must hook the light to the wire 180 degrees out from the cylinder you are testing. This is a bit confusing maybe, so I will attempt to explain. Remember the normal engine is a 4 stroke with events of intake,compression, power and exhaust in that order. The vacuum signal is generated on the intake stroke as the air rushes to the cylinder. The timing light will trigger on the power stroke. If you abbreviate the cycle as I C P E yo can see the timing light is 180 (distributor) degrees from where the vacuum is created. If I haven't lost you, the easy way to get around this is to hook the light up to the cylinder 180 out from the cylinder you want to test. (or simply hook it up in order and use the below chart to see the cylinder you are testing) Remember on these tests a LOWER reading indicates the bad or inconsistent cylinder.
(1-6) (8-5) (4-7) (3-2) /(6-1) (5-8) (7-4) (2-3) (this chart is for engines with a 18436572 firing order) The way to use this is as follows: the first number in any pair indicates the wire you are hooked to, the second number is the cylinder you are seeing when the strobe hits the guage. In other words, as an example , if you have the light hooked to wire 1, the guage is indicating what is happening in cylinder 6. If you are hooked to 8, you are seeing the vacuum for cylinder 5 and so on. So if you see less drop when you have the light on cylinder 1, the problem cylinder is 6. You can also do the timing light test with the engine running, which will yield higher redings, but they will be quicker- in this case the timing light is very handy. ( just so you know , shorting each plug wire with the engine running will show the bad cylinder also, but some ignition systems don't like that approach too much)
Another one-this is for those who have problems with other methods of making sure you are on the right cylinder when initially putting a distributor in a new engine for fire up and insuring they are not 180 out. You want to be on TDC (or a little advanced) on the power stroke with the number one plug ready to fire with a new engine. Using the above pairing diagram ,hook your timing light to number 6 and pull all plugs except #1. When rolling the engine on the starter, the cylinders that have no plugs will not create a vac reading, only number one will. So you want the needle to jump when the light triggers,If it does not, to verify you are 180 out, hook the light to #1 and retest- if the needle jumps when the light triggers you are 180 out. (aim the light at the guage when doing these tests as well as the above-do not blind your dog and watch to see if he jumps when the guage does.....) One other note of caution here- On any engine but especially if it is a new flat tappet engine, you don't want to spin it much until the cam is broken in and the oil system pressurized.
Another use is to set your timing curve. Set your timing at idle as explained in the tuning section,check what the timing number is with your light and record it. Now run the engine to the rpm that causes the timing to go as far as it will and repeat the vacuum setting excercise. Check the timing at that rpm and record the number. Before shutting the motor off reset the distributor to the proper idle timing to insure it will start properly when you restart and then shut it down.All of this should be done with the vacuum advance disconnected and plugged if you have it. Now lets look at the numbers. Let's say your best tune at idle was at 18 degrees, and your best reading was at 36 degrees on high rpm. What that means is you need 18 degrees of total advance on your centifigal weights. In many cases this means you will need to modify the curve in your distributor- for instructions on how to do this see the paper called ignition 101.One thing the guage will not tell you is what springs to use to get the maximum advance at the proper rpm-that is what a tachometer is for- for example if you had to rev the engine to 4500 rpm to get the maximum advance, your springs are too strong or your advance weights are sticking. For tips on setting springs and how to modify the curve, see the ignition 101 paper.
One last one and I will give up for now- for those of you concerned with fuel mileage, run a long hose inside the car and hang your guage in easy sight. The best mileage figures will be obtained by maintaining the highest possible vacuum at all times. (you may have seen the little mileage meters- they are actually vacuum guages with red, yellow ,green instead of numbers) Enough for now ,hope this helps.