Air mass meter on Rallycross cars?

[1975DCS]

Hi there,

I am watching Rallycross for 20 years now and noticed that even the current Div 1 cars don't have air mass meters. Kind of strang because all current road going cars seem to have one.

There must be a big downfall with using an air mass meter if they don't use them? I can understand it restricts flow, but a bigger one could be used right? Like Audi did with there S3 engines?

So what is the reason Rallycross car's don't use them and (modified) road car's (like an EVO FQ340) do? What are the pro's and con's?

Thanks in advance!

[Notso Swift]

I presume you mean turbo cars.
I bet they use a MAP sensor
No restriction and mass is calculated, self adjusting for pressure and heat, "all"* aftermarket and a lot of modern OEM systems use them. Flaps, wires and vortex meters are only used because they are cheap, also it stops people from playing too much, I think they car alaso good at low flow and hence allow better emmissions.
(Some cars are fitted with multiple sensors, then in competition they only use the MAP)

* when I say "all" I do not mean an absolute, but you get the idea.

[ian_w]

There are basically 3 methods of measuring 'load' for an engine management system, namely - throttle position, manifold absolute pressure ( MAP ) and mass airflow meter ( MAF ). I have listed them in order of increased accuracy, it also also the same order as cost.

The pressure drop across a MAF meter is very small ( much less than 1% ) so there is no great performance gain by removing it. OEMs use them as they measure the airflow of the engine directly and are thus the most accurate.

MAP sensors as their name implies measure the air pressure in the manifold - from this, a reasonable approximation of air flow can be inferred. The relationship between MAP and airflow is however effected by lots of parameters as air temp, humidity, exhaust back pressure etc - all these relationships have to be characterised. For OEM type systems there is a trade off between accuracy and cost and it is not uncommon to find MAP based systems on cheaper cars.

On a turbo car you need a MAP sensor in order to be able to do any turbo boost control so you can actually kill two birds with one stone - although many turbo road cars actually have both a MAF and a MAP sensor.

Most of the aftermarket EMS systems don't support MAF and thus MAP is the next best option particularly if you have a turbo engine.