When the power of an engine has been considerably increased, the amount of air that is being consumed can get so high that the airflow meter becomes a restriction. It's literally strangling power. Another problem is that the airflow meter signal might have reached its ceiling - the output voltage not going any higher, despite more air passing through it.
In this situation it's now becoming more and more common to swap-in a larger airflow meter and then modify the factory ECU software - but what is involved? And how much does it cost? We talked to Mathew Spry and Stef Graf of ChipTorque about this type of upgrade on Nissan engines.
Most airflow meter swaps use larger meters from the same manufacturer's vehicles. For example, the replacing of a Nissan 200SX SR20DET hot wire airflow meter with a Skyline RB25DET hotwire airflow meter is a common swap - in fact ChipTorque have recently finished the upgrade of a Nissan 200SX airflow meter done in this way. The company had initially fitted a Rover meter in place of the Nissan design but Matthew Spry said that with the Rover airflow meter, fault codes had been continually set on the ECU.
"At that point it's too hard," he said of the swap. "So you stop and go back to Nissan ones."
Stef Graf said that the most common swaps they see are on Nissans.
"Z32 on a SR20, or RB20 on an SR20 are common," he said.
"For Subaru WRX we do some work with the cast aluminium replacement Blitz airflow meter body that the factory sensor slots into. It's about 5mm bigger but [without remapping] it tends to throw the idle off."
Rather than change the airflow meter because the airflow meter has reached its maximum voltage output, Matthew said that most airflow meter replacements were undertaken because of the airflow restriction.
"You can usually live with the airflow meter signal being maxxed out," he said. "If you max the airflow meter at 14 pounds of boost - and you actually want to run 18 pounds - you can get your fuel and timing right by putting in the settings on the last rows of the map.
"But you may get a rich spot or you may get a slightly retarded spot before you need to be that retarded and rich."
However, a car will probably go through that part of the map in half a second, Matthew said.
"You won't even feel it, because it's still making more boost than it did before. Engineering-wise it's not a nice thing, but in reality it works alright."
But upgrading the airflow meter to a larger size not only prevents this situation developing but also drops flow restriction. But what about the ECU recalibration? An airflow meter that is larger will likely have a different voltage output for the same mass flow and so the air/fuel ratios will all be incorrect.
When tuning an engine after an airflow meter swap, ChipTorque actually change two parameters in the ECU software. Firstly they alter a major calibration number, revising it until the closed-loop learning indicates that little correction needs to occur to get the appropriate air/fuel ratios at light loads. This software calibration number can be altered to take into account changes in injector size, engine capacity - or airflow meter output. The tuning of the calibration number is carried out on the dyno, with an accurate air/fuel ratio meter sniffing the exhaust.
"I pick a point where you will end up driving the car a lot of the time - usually, 3000 rpm and 8-10kW of load," Matthew said. "I then move the main calibration number until the learned air/fuel number is correct."
In this way the tuner is actually using the closed loop learning to tell him where he is in the accuracy of the main calibration.
Another advantage of changing the main calibration number - as opposed to just moving the output signal of the airflow meter with an interceptor, for example - is that the ignition timing remains unaltered with the new airflow meter in place.
The second parameter that needs to be altered is the calibration data of the airflow meter itself. The output curve of voltage versus mass airflow is just that - a curve, rather than a straight line with an equal slope for different airflow meters. So that the ECU will work correctly with a different airflow meter, not only does the main calibration number need to be altered but so too does the airflow meter calibration table.
Matthew said that when he does a Nissan, he takes the calibration table for that airflow meter from the donor car's software and places it in the software of the car being modified.
"If you put the right airflow meter calibration table in and get your required calibration number right, in a Nissan everything else is pretty nice. With the right calibration, they really do drive like a factory car."
However Matthew said that he had done a 200SX fitted with a Q45 Infiniti airflow meter - larger again than an RB25DET design - and it wasn't as nice to drive.
"It was too big and it lost resolution," he said. "But the RB25 and 300ZX ones are just nice. They work well - and I can't see any reason why it wouldn't [successfully] drive through an emissions test."
And how does this sort of approach compare with ditching the airflow meter entirely and going to an aftermarket engine management system?
"To get any aftermarket system to drive as well as a factory system is not something that anyone can do under a thousand dollars of tuning. There's just too many hours to get it right, to get it nice. I'm not talking just driving, I am talking how many cold starts do you want to do to make sure it's right? How cold can you get the engine? What about when it gets hot?
"I have no doubt the aftermarket systems can drive the car as well as the factory, but it's the hours of tuning [needed to achieve this] that people don't take into account."
So what sort of cost is involved in an airflow meter upgrade? Stef said that the price of the used airflow meters varies a lot.
"It's what price you can get from an importer - I've seen between AUD$250 and $395 for a Z32 airflow meter," he said.
"We're experimenting with an airflow meter body that is being made for us that so far is successful - for that we're looking around the $250 mark. To do the tuning on an S15 Nissan 200SX it's $870 plus the dyno time - so you're looking at around $1100 plus the price of the airflow meter."
If the customer wants to buy the chip configured as much as possible for the new airflow meter - but without any real-time tuning - the cost is $870. Stef said that taking this approach had a reasonable chance of success, although the company could not say that that the results would be spot-on.
So when should you look at swapping-in a larger airflow meter? A rule of thumb is at about the same time as the factory injectors need to be upgraded for fuel flow. Simultaneously doing an airflow meter upgrade and going to bigger injectors also means that the management needs to be retuned only once - and importantly, the tuning and dyno cost remains the same as for doing just the airflow meter upgrade.
So you probably wouldn't do just the airflow meter change by itself, but instead as part of a substantial upgrade including larger injectors - and possibly a different turbo and intercooler. Done in this way, the overall engine management cost compares very favourably with cheaper programmable management. And with likely better driveability and economy...
Keep an eye out for those airflow meters!