Blow through setups

[jambo_gtir]

Think of like this:

If you have a certain flow rate at a pressure of 5psi, then you have another system that has an identical flow rate (same pipework) but a pressure of 10psi.... then the latter would actually have a higher mass of air going through it.

 

[jambo_gtir]

Sorry Bob, that wasn't in reply to yours lol.

Thanks for cleairing that up,,, I wasn't sure how the Z32 worked but was refering to the standard one.

 

[PobodY]

Perhaps MAF is actually a misnoma; it's not actually weighing the air. It's using a calculation based on the change in resistance of a 'hot wire' to calculate a volume of air. - In theory that should correct for a certain change in pressure because it will behave like a higher flow.

I'm not an expert on this obviously, but anecdotal evidence says that it works.

 

[Trip]

Yes I think it would mate..... thats why they say that any modification you make from standard it is advised you get a re-map. I don't tune cars so can't say for sure but I bet even changing the exhaust and air filter will have effects on the ECU (even if they are so slight it may not matter).?

I agree too on this. for instance, changing the IC will give you a richer (in oxygen) air flow since the air entering is cooler. So a small tweak on the map is always suggested

Yes it uses air flow to calculate the mass of air entering the engine.... in combustion, your trying to achieve the right mix or air/fuel.... so you need to know the mass or air entering the engine. Part of the calculation requires the pressure of the air (which is know at atmospheric). If you put the sensor after the turbo, then it will detect the same air 'flow' yes, but that flow is under more pressure than at atmospheric pressure and so would give the wrong reading for the actual mass.....
Make sense?

read the section: Hot wire sensor
http://en.wikipedia.org/wiki/Mass_flow_sensor

 

[skiddusmarkus]

People who have done this, did you move the BOV to before the MAF or just left it where it is as standard?Mine is an SQV but its recirced back into the inlet.

 

[Spikey]

Mine is before the Maf.

 

[skiddusmarkus]

Been thinking about moving the bov to before the maf on a blow through (as some say it can cause problems with metered air having it after) but surely if its moved before the MAF then when the throttle shuts the air that's between the bov and throttle will still vent so pointless moving it?Its still going to release air thats been metered isn't it?

Also found this bit interesting while searching:

Re: Blow through MAF setup on sr20det.

I found it! Mustang 5.0 magazine, October 2003. Titled Mass Conclusion. Using a flow bench they tested various bends and straights before and after the maf to show the distortion. It is recommended that there be at least 8" in front of the maf to minimize distortion as well as 8" of tubing (curved or straight) behind the maf and before the throttle body. The bends dont matter behind it because the air has already been registered. If there is a bend in front of it you may use a pro-m Univer Plus maf housing which samples the air around the perimeter of the meter's inlet and then sends it down a spiral path calming and averaging it before being sampled.
They were also able to spin the maf in relation to the bend in front of it to reduce the distortion. Cooling fan wash in the engine bay also distorts the signal causing a rich condition so it is recommended that you shield the Maf and filter from it.

Another good reason to shiled your filter then.

 

[stumo]

Think of like this:

If you have a certain flow rate at a pressure of 5psi, then you have another system that has an identical flow rate (same pipework) but a pressure of 10psi.... then the latter would actually have a higher mass of air going through it.

and the higher mass of air will cool the hot wire more....that's why it's called a Mass Airflow sensor, cos it senses the mass of air going past it (ok technically it doesn't, it just gives out a voltage but that voltage relates to the mass of air going past the sensor)

 

[Jon Olds]

Air fuel ratio is based on the mass of air, not the volume. Defo.
I have been considering moving AFM mine closer to the plenum, so its pressurised. Mostly so when my BOV vents down, the mixture doesn't periodically go rich, due to the 'metered' air being dumped.
I am hoping that the installation plumbing may be simplified also.

 

[skiddusmarkus]

Yeah but if you move the maf close to the plenum, when the bov vents, isn't the air from before it and after it going to equally try and vent, so metered air will still get vented all the same?Or is it that the metered air is far less than in a draw through setup as its only venting say 10" worth of it as opposed to what would be about 5' worth that's passed through my maf, the i/c and pipework?That makes sense to me I guess.
Mine is an hks sqv which I turned into a recirc anyway, so I'd probably keep it that way as noise is less.

 

[williams]

would the best solution not be to go to a map sensor instead of map, would do away with this problem altogether.

 

[Jon Olds]

Not sure a map (manifold absolute pressure) sensor does the right thing as mass will vary with temp, for a given pressure. If the BOV vents air before the AFM sees it then only metered air goes into the engine. ie I was going to blank the original BOV tapping and add a new one before the newly relocated AFM.
I have bought a recirc BOV also, but havent tried it, as I'm still trying to get my head around the right/wrong ways to plumb it. And anyway adding a 1" tapping into the existing IC pipework has its own difficulties.

 

[campbellju]

<450hp just get rid of your BOV. Simples

Nothing wrong with MAFs. Nothing wrong with MAPs either but MAF's are less influenced by air temperature. MAP's that need an air temp reading to calculate the amount of fuel required. MAP's have 2 points of failure and two things that need to be calibrated so I personally prefer a MAF to map.

 

[skiddusmarkus]

Well that would be the easiest solution Campbellju, plus I'd be able to sell all my hks recirc into 3" alloy inlet setup to fund a couple of bits and bobs I'd need for a blow through setup.A blanking plug is only a fiver off ebay.
It brings up a whole new debate though about bov vs no bov which I still don't know the definitive answer as people have very strong views about both.

 

[stumo]

just run your BOV without the hose on and see which is better.

 

[williams]

I don't see how a recirc would make any difference?

 

[stumo]

it just stops the gaylord wooshy noise beloved by Maccydee wannabees and ballbag groping traccy wearing scumbags.

 

[williams]

You'll still get that with an open filter on though.

 

[skiddusmarkus]

Its not as loud though, I've had same bov both ways.

Would an easy way to test this be to remove the vacuum pipe on the bov and blank that off?Or is that pipe providing +pressure pushing diaphragm down rather than -pressure pulling the valve open when you lift?

Reading the arguments for and against them made me wonder, if you don't have one and the throttle shuts, air backs up against the compressor slowing it down.But when the throttle opens again, then some of this air should still be in the pipework and under higher pressure than post butterfly so would it try and equalise that pressure by quickly moving to the TB and creating a kind of scavenging effect behind it helping to spool the turbo back up again?

 

[williams]

i doubt you'll physicaly notice any difference, would need to log boost, rpm and throttle position to see the difference.