Floyd

iTrader: (9)

This is a continuation of one of Stephen's threads....

Stephen was having trouble with an induction kit that lost him power low down. An explaination came back that said it was something to do with the MAF sensor not reading the air correctly.

For starters then, the MAF (Mass Air Flow) meter reads 'mass x velocity', so the theory behind greater mass of air but less velocity isn't true. As one goes up the other goes down and the relationship is maintained. Or am I wrong? Does it work in a different way?

Secondly, Stephen added an upgraded air box to turbo inlet pipe. This was larger and is suppossed to increase response and power. Now I always thought that reducing restrictions in the induction side, reduced pumping losses, so this must be good, right? But this is generally true for normally aspirated cars where the piston is pulling against a vacuum. In a turbo car, you can have positive pressure, which elieviates the pumping loss.

So, when the turbo is off boost, there will be vacuum and the better the induction path, the smaller the pumping loss will be. This means that there will be greater air flow, more power low down and better response.

When the turbo is on boost though, greater air mass onto the turbo will slow it down. Hold your hand over the end of a hoover pipe and the fan spins faster because it is releived of it's load and working in a partial vacuum. Thus the turbo is working harder to push more air that it receives, if it only gets the air it was designed to get then giving it more hinders it? Does this mean that there is little high boost gain from increased efficeincy on the induction side with a turbo?

This is the opposite to Stephens findings though - he had lost power low down and gained up top! Although he did say (I think) that he had better response on the road at lower revs.

I'm confused!!!! Where's the theory going wrong?

F

[Edited by Floyd - 25/10/2001 14:16:34]

Floyd

iTrader: (9)

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Floyd

iTrader: (9)

Oh that's good then, I must be right! I didn't realise how magnificently clever I am.

I suppose this is 'ignore Floyd' week and I shall now go back to the muppet zone, where I'm fully appreciated. Pah!

F

WREXY

Floyd,

I wish I knew. All I can say is that I had fitted an exhaust to my car, midsection (catless mid) and backbox and it felt like I gained a bit up top and had similar, if slightly less power to standard down low. Then I added the APS induction kit and I gained power, well it felt like I gained power down low and right through the rev range. I also found that I had heaps better throttle response and also some pops and bangs at low revs when letting go of the throttle, but this was probably due to the blow off valve. This was before I installed my EBC (electronic boost controller). I don't know if people are getting the loss of power at low revs due to downpipes and the APS kit. Maybe the two don't match. As for me the APS kit made a huge improvement to my car, MY00.

Can't say much more as I don't know the technical side of this.

Cheers,

Wrexy.

[Edited by WREXY - 10/26/2001 5:28:40 PM]

Floyd

iTrader: (9)

Wrexy

Thanks for that.

I want to do something about improving the induction side of things but was wondering which way to go and are there any real downsides.

I've looked at the APS kit and there are some good reports, but from my experiments, there aren't many gains form sucking cold air from the wing. Perhaps the best gains come from the filter and ram pipe design?

The larger airbox to turbo pipe looks like it should work but for the effort and cost, the results that I've seen were mixed.

That's why I'm trying to explore what is really going on on the induction side first.

F

AndyMc

Hi Floyd

The MAF doesn't measure mass x velocity.
It measures mass flow by calculating how much heat is lost to the air flow from a hot wire situated in the airflow(or a hot film depending on the type).As the airflow cools the wire down the MAF increases the heating current to maintain the temp of the wire and its is the size of this current that is directly related to mass flow.The temp of the air is also measured by the MAF so it knows exactly what temp it should maintain the wire at, this also allows it to compensate for warm or cold days.

The problem is the wire doesn't cover the entire cross section of the airway through the MAF so it can only measure the flow near the wire and assumes the flow elsewhere is the same,this is why the air needs to enter the MAF flowing just as the MAF manufacturer intended.Some MAFs have flow straightening vanes to help make sure the air is flowing properly.
For example if you were to put a 90' bend just before the MAF a lot of the air would flow along one side and a proportion could be missed by the wire which would then cause the MAF to read low.Anytime you change the design of the intake there is a chance an effect such as this might occur.A similar effect seems to be happening with the APS kit.

Next you asked if it was a good idea to reduce the flow restriction of the intake system of a turbo car and the answer is yes it is a good idea.The fact the car is turbo charged is not relevent .Even though the turbo is making a positive pressure, overall pumping losses will still be reduced,(in fact the induction stroke is actually a power stroke in a turbo engine).

When you put your hand over the end of a Hoover it is obvious that the flow is stopped as you move your hand away the flow picks up again.Maximum flow only occurs when you take your hand away completely.The fact the fan slows as you take your hand away does not matter the flow is still the greatest when your hand is not there.In the same way the fact that the fan spins the quickest when there is no flow also does not matter.It is flow we are concerned about not the speed of the compressor.

Andy


A couple of other points
If you free up the intake to a turbo it will be able to flow more air at the same compressor speed.

Overall less energy will be required to compress any given amount of air so the car will come on boost earlier.

The turbo has to do a lot of work on the incoming air to compress it.The compressor requires more tham 20 HP to drive it and this energy has to come from the exhaust gas.The process of extracting energy from the exhaust gas causes back pressure and the more back pressure the lower the power.
If you can reduce the inlet restrictions so that it is more efficent you may now only need 18 HP to drive the compressor so less needs to be extracted from the exhaust gas and the back pressure will be lower.

So removing restrictions on the inlet to a turbo helps in two ways
1 it reduces pumping losses on the inlet side.
2 it reduces back pressure on the exhaust side.

Floyd

iTrader: (9)

AndyMc

I hear what you are saying and this is what I thought, but consider this. If you had a water pump, which needed to flow X gallons/min at certain revs/current i/p then feeding the pump more water via a bigger pipe on the i/p side would not increase flow but decrease it and unless more current is used. The weight of water would now slow the impeller blades and the current draw would increase, not to mention the smaller pipe on the o/p side now causing more restriction.

I think what I'm trying to get my head around is that there is an optimum volume of air on the input to match the o/p pipe size for a given rev. Any oversizing of the pipe may be detrimental as is the air flow over the MAF.

F