# Is there a rule of thumb for estimating volumetric efficiency?

Here's a situation I encounter quite often

During diagnosis, I want to determine if the mass air flow sensor(s) [MAF(s)] is/are healthy.

I hook up an OBDII reader to a vehicle that I'm trying to diagnose and get access to real-time mass air flow rate.

By knowing the engine displacement and RPM, I can estimate the volume of air that the engine should be pushing through. However, because of the concept of volumetric efficiency, the calculated and measured values will be quite different, related by the equation below.

``````Measure air flow rate = VE x Engine Speed x Displacement ( x unit conversions )
``````

So bottom line, knowing volumetric efficiency at a known point of engine operation would enable one to look at the measured value and tell if the MAF is reading what it should.

Is there a rule of thumb that would allow one to roughly estimate what the volumetric efficiency of an arbitrary fuel-injected gasoline engine is at a given point of operation? If it helps, I don't mind limiting the scope of the question to engines with single throttle bodies.

As an example, "VE is 15% at idle RPMs", but it should be accurate enough to tell whether the MAF sensor readout indicates significant degradation of performance of the sensor itself.

## Announcement: The Great MAF Experiment is underway

The aim is to attempt to objectively answer this question and your data can help the community potentially discover this rule of thumb. Thanks in advance!

• Just to be clear, this question is about roughly estimating VE, unlike this one – Zaid Jul 28 '16 at 22:54
• I think there may be too many factors involved, namely because every engine is different in how the intake will flow, cam profile, and how the exhaust flows. All of these will have a factor in the volumetric efficiency ... at least that's my gut check on this. – Pᴀᴜʟsᴛᴇʀ2 Jul 28 '16 at 23:24
• You can use online calculators like otctools.com/ve to roughly estimate VE. It requires the car have a maf and doesn't account for elevation, humidity etc... I'm sure there are paid tools that do though. I tried writing a program to do this at one point but I couldn't get the math to work right or the formula was wrong. – Ben Jul 29 '16 at 1:53
• @Zaid I hit enter for a space, and ended the comment early. What I wanted to say I've never encountered a bad MAF that needed that level of diagnostic inquiry. My oldschool shadetree is to look a g/s at idle, spray the magic cleaner, and check again. In the meantime, as Ben said, there's no crime in assumng VE is 100%, especially at peak torque RPM if known. Keep in mind that a scan tool OBD II translation of "220 Khz" or "3.22 volts" may be different than actual ECU scaling of predicted CFM from ECU maps. – SteveRacer Jul 30 '16 at 2:56

You want to get a reliable equation without knowing model specific details! This is not going to happen. Even if you know all the specifics of a particular engine, you are still only going to get so close to the actual VE. Steve Racer hit it on the head. Every EVERYTHING is different. One 6.2L engine built right behind another is not going to behave the same as the one before or the one after. HP numbers, torque curves, etc.. will be all over the place straight from the factory. Good luck!

• I realize that getting an exact value for VE is hard to come by. As I said in the question, all I'm interested in is a rough rule of thumb – Zaid Aug 11 '16 at 14:38
• I was a Honda factory mechanic for years and we had a dyno at the shop. Yes, there is variance but not as much as you claim regarding, "all over the place." If you could provide a citation to back your claim I think you would up the credibility of your reply. – DucatiKiller Aug 11 '16 at 16:55

In modern cars, wide open throttle at peak torque should be roughly around 100% VE.

If you know how much fuel your are burning, and it is burning at the standard 14.1:1 ratio, you can calculate the mass of air. This is how many car computers fine tune the MAF sensors automatically. Basically burning 1 lb of gasoline a minute, means you are flowing 14.1 lbs of air a minute.

The only reliable yet cheap way of calculating intake air flow at a set RPM, is using a orifice plate flow meter. Where you calculate the pressure drop from the intake air flowing through a restriction, the restriction being a metal sheet with a hole in it.

If the hole in the plate is of known diameter and the pressure drop is known, you can plug in the numbers into one of the many online orifice flowrate calculators and see how much CFM you are pulling. When the CFM, air density, RPM, and engine displacement is known, then you can roughly calculate the mass flow and VE of the engine.

• I don't know of any engine that runs stoich at WOT. They usually run rich to help with cooling, and that value varies across engines and manufacturers – Zaid Oct 14 '16 at 19:07
• The engine does not need to be WOT to calculate air mass at stoich. If you are burning 1 lb of gas a minute at half throttle, you are still using approx 14.1 lb of air a minute. – Netduke Oct 14 '16 at 19:16