Driveability corner

Driveability corner

Warren, Mark

Perhaps you rely on the seat of your pants to gauge engine performance. A volumetric efficiency test can back up your hunch with hard numbers.

The engine really has a simple function-to pump air. If an engine were a perfect pump with no inlet or outlet restriction, no compression or power stroke and no leakage, it would displace the same amount of air as its volume every revolution. The actual quantity of air pumped divided by the perfect quantity equals the volumetric efficiency (VE). VE is measured in percent.

Factors that affect an engine’s volumetric efficiency are intake and exhaust restrictions, valve timing, cylinder sealing, gas inertia, runner design and rpm, among others. Among the external factors affecting VE are, barometric pressure, air temperature and humidity.

In my February 1999 column I discussed using MAF values to determine engine pumping efficiency (you can look it up at I knew what a good MAF reading was for the vehicle because it was brand new, and I created the restrictions. How do you know what a normal MAF reading is? Use VE calculations.

Before we start, though, a few notes and assumptions:

*The vehicle in this test was a 1996 Chevy S-10 truck with the 4.3L engine and the MAF was working correctly.

*The scan tool readings of the MAF were assumed to be correct grams/second readings.

*No correction for air temperature was made; I assumed 77[degrees]F.

*No correction for humidity was made; I assumed 0%.

*There was a .2-second offset on the scan tool between rpm and MAF readings. No compensation was made; rpm was read first, then MAF.

*At 77[degrees]F, 29.92 in./hg of atmospheric pressure and 0% humidity, a liter of air weighs 1.184 grams.

Okay, now let’s do some simple math. We have a 4.3L engine turning 600 rpm at idle. This means 300 intake strokes, or half the total revs. Divide 300 by 60 seconds and you get five intake strokes/second. Multiply 4.3 liters by five intake strokes/second and you get 21.5 liters/second, and 21.5 liters/second times 1.184 grams/liter equals 25.5 grams/second. Hmmm, the actual grams/second measured by the MAF and displayed on the scan tool was 4.5 grams/second. That’s because the throttle was closed and the engine was sucking against a pretty small restriction, so the manifold absolute pressure was reduced to 10 in./hg.

Out of a possible 29.92 in./hg of air pressure, the engine got only 10 in./hg, or about one-third. Take the 25.5 grams/second and divide by 3 and you get 8.5 grains/second. Now, 4.5 grams/second divided by 8.5 grams/second yields a volumetric efficiency (after restriction compensation) of 53%. That’s not too bad, considering that most passenger car engines are tuned to run most efficiently in the 2500- to 3500-rpm range.

The graph on page 14 is the scan tool data used to generate the table above. It shows two snap-throttle events occurring one after the other. I tested this S-10 truck on the road at wide-open throttle (WOT) and in the bay with a snap-throttle test. While the on-the-road test slowed down the rpm rate of increase, the snap-throttle data was very comparable. I’ve concluded that you can effectively determine an engines VE with a snap-throttle test.

Looking at the table, you can see that for idle I did not compensate for the intake restriction on die actual VE calculation, as I did in my explanation. All of the other values are at wide-open throttle, so little or no intake restriction was present. True VE calculations must be done at WOT for accuracy.

The last column in the table shows the actual VE calculations with compensation for the altitude here in Arizona (2500 feet). As you can see, the best VE (85%) was achieved at 3500 rpm-not a surprising result.

So, what’s good and what’s bad? I’m going to have to run more tests before weighing in with a final result. Also, I’ll refine the formulas to include temperature, atmospheric pressure and humidity compensation. I’m told that VE values of 95% at peak performance are not unusual. Can a normally aspirated engine ever do better than 100% VE? Actually, yes. Given the right rpm and tuning, the gas inertia in a tuned port manifold with a well-tuned header can do slightly better than 100% VE.

What does it mean if the VE is too low? Well, it could be an engine pumping problem, or the MAF could be defective or dirty. What VE can do for you is identify a problem and point you down a specific diagnostic path. Happy hunting!

Copyright Hearst Business Publishing Jun 2003

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