Vacuum gauge, the wonder tool
There is one tool that can tell so many things about an engine it’s simply amazing. Yes, it’s a vacuum gauge. And it must be the best kept secret to tuning in the world. We’ve walked around performance cars and visited the pits at professional drag races for years, seldom have we seen a vacuum gauge in use. A few cars have them like most BMWs at one time, but they were disguised as a fuel economy gauge. Besides, the vacuum gauge shouldn’t be in the dashboard it should be in the engine room. Every car and boat we’ve raced on an ongoing basis has had a vacuum gauge permanently installed on the engine. Not to be viewed while driving but to be viewed while dealing with the engine.
The engine is in fact an airpump. The better it pumps air, the better it runs. A vacuum gauge measures the difference in pressure between inside the intake manifold and the surrounding air, and therefore measure the effectiveness of this airpump. That means that the vacuum gauge can be used extensively both for diagnosing/troubleshooting and for tuning. A vacuum gauge will be mentioned many times in the tuning section on these pages.
So, if you don’t have a vacuum gauge, get one! You can do like us, have a beautiful AutoMeter vacuum gauge complete with mounting cup permanently fixed to your engine, or you can buy a cheap vacuum gauge as a testing tool and use on all your engines. Absolute accuracy is not critical, it’s more the relative movement that is important. And remember to plug it to manifold vacuum, not the ported vacuum port on the carburetor.
Diagnosing/trouble shooting
Fortunately a lot of different faults can be found with a vacuum gauge, because the behavior of the needle is different to most situations. Here are the 7 most common scenarios (engine idling and warm):
- Steady needle, at 15-22: Engine in good condition. (An engine with long duration camshaft will have a lower vacuum and a less steady needle, maybe 10-15 with 1-2 “jerks”. That’s still an engine in good condition)
- Steady needle, lower than normal: Ignition timing too late or vacuum leakage
- Steady needle like in 1. but occasionally fluctuates at idle: Ignition miss or sticking valve
- Steady needle like in 1. but drops regularly: Valves need adjustment or burnt valve
- Floating needle, maybe in the area 10-20: Carb out of adjustment or vacuum leakage
- Shaking needle, becomes worse as rpm increases: Ignition miss, blown head gasket
- Shaking needle, becomes steady as rpm increases: Worn valve guides
If we’re driving and suddenly the engine behaves differently, then we first check the vacuum gauge, then we know where to start…
Tuning
Most of the tuning actions that we talk about in that section involves a vacuum gauge. While a vacuum gauge seldom can tell you the absolutely best ignition timing, it can get you close and save you a few test trips on the track. In the example in the Section Testing we test all the way from -12 to 3 degrees. Without a doubt a vacuum gauge could have narrowed that down, probably vacuum would drop noticeably below -10 and above 0, would have saved a few test runs. Mostly we adjust carburetor idle mixture and ignition timing simply by turning the screws until maximum vacuum is achieved. That’s a real good starting point.