Fine Engine Tuning With A Vacuum Gauge

Here's everything you need to know about using a vacuum gauge as a tuning device. A few great ways to dial things in, and one that definitely sucks.

[mikesawyer]

If I close my eyes I can hear my dad talking cars, and car maintenance. I wish I would have listened a lot closer to my dad. Uncle Tony you rock.

[classiccars]

allways manifold vacuum on dis advance is a emmision thing
As many of you are aware, timing and vacuum advance is one of my favorite subjects, as I was involved in the development of some of those systems in my GM days and I understand it. Many people don't, as there has been very little written about it anywhere that makes sense, and as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. I finally sat down the other day and wrote up a primer on the subject, with the objective of helping more folks to understand vacuum advance and how it works together with initial timing and centrifugal advance to optimize all-around operation and performance. I have this as a Word document if anyone wants it sent to them - I've cut-and-pasted it here; it's long, but hopefully it's also informative.

TIMING AND VACUUM ADVANCE 101

The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.

The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.

At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).

When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.

The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.

Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.

If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.

What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.

Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.

For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts.

[charlesvaughan]

Your 30 second explanation of how to tune the springs in an Edelbrock helped me understand it better than I ever did reading the manual that came with it and watching a bunch of other videos. Thanks, Uncle Tony!

[jfeliciano]

You are a national treasure... Every old guy I meet tries their best to hold on to this information and then wonders why nobody want to use a carburetor and swaps everything to FI.

[moparedtn]

Once again, the Man dispenses the gospel to the masses as simply as possible, therefore making
it totally understandable and accessible even to the novices.
This channel may be a lot of things, but the one that's always impressed me about Tony is how
willing he is to share knowledge and help anyone who wants to know.
BTW, this old goat just learned a new trick with that "checking the spring tension on the needle valve"
bit on the Edelbrock. I've always just sort of stumbled my way along, using the infamous "chart"
we all get with the AVS carbs as guidance until I hit on something that works.
Tony's way saves a TON of time!
Thanks as always, keep 'em calm over there in the flatlands neighbor,
-Ed on the Ridge

[MrYAMAHA]

Refreshing to see old some school tuning again, no laptops required, thanks to uncle Tony.

[StuartKReilly]

Mister Tony. I'm only 29 but I've played around with and drove classics since I was old enough to drive and before then some. If I would have had even a quarter of the knowledge you have on these carbureted engines when I was younger I probably woulda kept a lot of my old projects. Now that I'm getting back into it with a new project in the age of Youtube and Google I've made it a mission to arm myself with actual facts and info about keeping these cars running and your videos have been the biggest help out of anything else I've watched. So thank you for sharing everything you know about old school cars with young bucks like me.

[hectorrivera]

Nice video with some good information. Here are some important clarifications:
1) vacuum should be checked with the air cleaner ON. Checking it without the air cleaner results in inaccurate readings, and your adjustments will be wrong.
2) Before adjusting the idle mixture screws, the engine timing at the distributor should be checked, and the idle speed should also be adjusted. Both of these come _before_ tuning the idle mixture screws with a vacuum gauge.
3) The engine should be HOT when checking vacuum. Uncle Tony started up the engine and immediately checked vacuum. I assume the engine had recently been running, but the point was never addressed.
4) The idle mixture screws should be turned in sequence, 1/4 turn on one, then 1/4 turn on the other. That way they're equal. If you adjust one screw for absolute maximum vacuum before adjusting the other screw, it's guaranteed that the screws won't be equally adjusted at the end of the process. And both screws should be equally adjusted.

[spankyham]

These are the kind of tech videos I really like and get a lot of useful knowledge out of. I like knowing the "why and when" to something and not just how.

[mattbauckman]

I’m a strong proponent of full manifold on vac advance. I’ve curved the distributor on my 396 to run this way and it’s a raped ape. Excellent fuel mileage too. Bone stock original 325 h p cam, 89, 000 original miles. When set up properly you can achieve a strong running engine either way. Whatever works for your application, as I’ve seen it both ways over the years. Thanks for sharing your knowledge Tony. The vacuum gauge is one of those forgotten tools, but not for us old iron guys. Now break out your dwell meter and REALLY confuse the younger generation 😂

[Czechmate_e]

I love this channel. So many cool nuggets of information and tips. I'm a big 80s-90s euro guy, but I did own a 74 "Gold" 318 Duster for about a year. Was too young, an inexperienced to properly rebuild it so I sold it. Eight years later after watching much of this channel, I'm getting excited to give it a try once again.

[brianhyslop]

Thank you SO much! I was about to take the insurance off my 81 Malibu because I could not rid it of a off idle stumble!! Would stumble at cruising speed and for a moment when leaving a stop sign. Lost my pops to Covid last year and don’t know any old school guys that know their way around a small block. Went out after work with vacuum gauge after watching this video last night before bed. RUNS SO BADASS NOW!!! Driving it to work tomorrow

[zxej]

The lost art that many mechanics never had.

[bbbcid]

Every old-skool car guy has a vacuum gauge and dwell/tach in the milk crate that's in the trunk. I don't actually know where the milk crates came from, I just know I always had a few, lying around. Maybe they grow in old car trunks.

[BeardedWrencher]

Uncle Tony, talk about perfect timing for this video! I'll be taking an extensive road trip soon with an old carb engine and was looking to do a solid tune on my Weber carb before I head out. I had been planning on doing some research on this but haven't done anything yet. I purchased a vacuum gauge from china freight the other day and was hoping to get this done sometime during this week.
Wouldn't you know it, you uploaded a video that answered all the questions I had and explained it thoroughly for a rookie like myself who's hoping to keep the carb alive!

I really appreciate this video and can't wait to dive into this form of fine tuning myself soon!

I'm also gonna be using this method to help my friend fine tune his carb on a 5.0 mustang from an engine swap we recently did on it.

Really appreciate your videos! They're definitely better than enough for government work. Hahah.

I appreciate your videos and thanks for what you do!

[keithcornell]

i learn so much from you .since im old school{62 years old) and can only work on old cars and own just pre 80s cars your the man.watching you explain and back up what your saying with demos brings me back to my days of auto shop in the 70s keep up the great work your doing.OLD SCHOOL RULES

[UncleTonysGarage]

Ok, let me explain something about the ported vs manifold vacuum as it applies to distributor advance.
If you have it hooked to manifold vacuum, it RETARDS the timing as soon as you touch the gas. The exact OPPOSITE of what you want to happen when the engine meets the initial load.
Hooked to ported, it is inactive at idle as well as initial acceleration. Ported, it comes on when it is needed. Manifold, it turns off when the engine wants advance the most.
Vacuum advance is intended for LIGHT throttle cruising, NOT acceleration.
Make sense?
I do not care which manufacturers used manifold vacuum, when they used it, or why. For best performance and to be used as originally intended, it should always be hooked to ported.

[NebukedNezzer]

remember when JC Witney sold variable ignition timing kit so you could adjust your timing as you were driving. My first car was a 48 chevy. I tried the experiment of hooking the vacuum advance to the manifold vacuum. No, that did not work well at all. On my 48 chevy and the other old standard trans cars I had. one good way to adjust timing was this. with a hot engine and the gas you use(usually the cheapest regular). put the car in high gear at less than 20mph. step on the gas and as you accelerate it should ping a bit till you get to 30mph and then not ping there or above that. adjust the timing till you get that. now when you drive normally you get all the advance you can use because you do not normally lug the engine below 30 in high gear. you can cruse in high gear at 30 ok just not give it a lot of throttle. this would get the best gas mileage. Love your videos as you teach how this stuff is supposed to be done. if we just listen to the idle as we adjusted the mixture screw it will idle at max rpm if the idle mixture is rite. lots of stuff was done with no gages at all. do you need to have the vacuum advance working. Yes for mileage and no for power. since the vacuum advance is ported. if its not connected at all the idle is the same. also at full throttle there is very little vacuum that matters not much either. only daily drivers need the vacuum advance to work. when the vacuum advance is broken(or stuck) I would know because my mileage would drop off enough to know something was wrong.

[Eurostang]

Long time UK sub here. Working on a Buick 215 derived V8 (Rover) which I've de-euro'd and converted to 4 barrel Edelbrock. Just wanted to say I love your videos and you've been such a massive help and pretty much the total source of all the knowledge to do the stuff I do on my channel. Cheers mate!

[chrisbieling]

Took me 64 years to find someone who knew what they were talking about, ……and could (between spitting, shaking and thinking faster than any Ferrari mechanic, ) teach me in four point something minutes, the most valuable, take it anywhere, knowledge that you can get! I don’t care what you wear, Tony, you just gained a believer!!!