PeterUK300

iTrader: (1)

Alex
Have you had the up pipe and down pipe replaced or are you still running with cats in place. I only ask as earlier in this thread it was mentioned about overfueling and flaming with the Atmo one and that it can damage the cats.
I ask as I am very interested in one myself and still have the up and down pipes with cats.

PS Alex NO PARCEL YET

All the best
Pete

logiclee

Pete you will have a far greater knowledge of this than myself.

I believe most standard installations of VTA DV's have the unit B4 the MAFM so all intake pressure can be dumped prior to the MAFM and thus not causing fuelling problems. Under normal conditions the DV is closed and the MAFM monitors normally.

I take your point about the Engine management not remembering how much air has passed etc.

But.

A VTA DV confuses the MAFM (point taken about ECU memory) and causes overfuelling and when the exhaust system is hot enough, flames. Is this because a VTA allows air to keep passing the MAFM where a recirculating DV is putting air back into the intake system and no air is passing the MAFM?

Any unburnt fuel in the exhaust system will damage cats (where fitted).

The lambda sensor measures oxygen content to monitor the efficiency of the combustion process. An over rich mixture will cause an inefficient burn and effect the oxygen content would it not?

Just like to add that I am in no way an expert just interested in such things.
TSL and Scoobysport have far greater knowledge than myself, TSL serviced my Scoob, fitted a brake upgrade and I would recommend them 100%

Cheers
Lee



[Edited by logiclee - 1/30/2002 3:20:16 PM]

EvoRSX

Lee,
Does the scooby have the MAFM mounted in the intake pipe after the turbo?
Just interested because I don't know what the scooby setup is but I know what the Evo is like.

Andy

logiclee

Andy

No, just after the air filter housing in the first bit of pipe.

On the Evo is the DV before of after the MAFM?

Lee

EvoRSX

Lee,
The DV is always after the turbo and therefore always after the MAFM. There would be no point in putting a DV before the MAFM because when the turbo is boosting, there wouldn't be any boost pressure before the MAFM.
This is the usual route for the recirc DV that is fitted as standard
=================
11 11
====MAFM=======turbo========DV========intake plenum

When you fit a VTA, all you do is effectively remove the pipe linking the DV back into the intake.

Andy

Edit: Bugger that diagram has removed all of the spaces. Basically there is a pipework link from the DV to the space between the MAFM and the turbo on a recirc valve and a VTA just has a link from the DV to atmosphere.

[Edited by EvoRSX - 1/30/2002 3:54:45 PM]

logiclee

Andy,

I wasn't sure if the Evo DV was VTA or reciculating as standard.

As you stste a DV has to be after the turbo to dump the pressure in the intake system.

Just thinking that on the Scoob when a recirculation DV dumps the pressure back into the intake system and out past the MAFM and air filter the MAFM must see zero flow or even a negative value (if possible?). When air is VTA the flow is out of the intake system via the VTA DV so is it not possible that a positive flow into the intake is seen by the MAFM and this is what causes the over fuelling?

Edited to say I misread your earlier post. Are you saying on the evo the MAFM is after the turbo in the pressurised part of the intake?

Lee


[Edited by logiclee - 1/30/2002 4:22:45 PM]

Cosie Convert

Just a thought, but does the ECU add fuel at high RPM on a closed throttle ??

Regardless of airflow I would have thought the fuel was cut under these conditions ?

Pete Croney

Lee, unburnt fuel won't damage the cats.

EG... put a backbox on a version 3 or 4. It will pop beautifully on over run. The popping is from unburnt fuel igniting in the exhaust. The backbox has done nothing except let you hear it. The cats are still there and they are still working fine.

The cats are quite robust. We once ran my RA up on one of the ramps, with no manifold fitted. The flames hit the floor and the car was 7 feet up in the air. There's quite a bit of force and a bloody lot of degCs in your average exhaust. A bit of fuel vapour (because of the heat its a gas, not liquid petrol) is neither here nor there.

Now an anti lag system is different. The popping and flaming on the STi 4 over run is with virtually no air going through the engine. An antilag system keeps air going through at approx 0.5bar. This creates seriously powerful explosions outside the cylinder. And cats do not normally survive the end of stage 1.

With a VTA you get the extra fuel on over run, because the turbo keeps spinning and keeps pumping air past the MAF. The air just vents to atmo. On a re-circ, it comes back to the turbo feed intake pipe and will not maintain a flow over the MAF... BUT, it will initially blow air back at the MAF and I have often wondered if this is what causes some MAF failures. This part of the intake gets all the sump and rocker case vent gasses, which are oil mists. Also the air which has been through the intercooler holds oil mist. If it were to blow this back on the the MAF... And would the lack of airbox and resonator make it much easier for this blow back to occur, hence more failures with a cone filter??? [just kicking ideas around]

Anyway, normal fuelling is resumed the moment the throttle is depressed

EvilBevel

This part of the intake gets all the sump and rocker case vent gasses, which are oil mists. Also the air which has been through the intercooler holds oil mist. If it were to blow this back on the the MAF... And would the lack of airbox and resonator make it much easier for this blow back to occur, hence more failures with a cone filter???

Interesting thoughts Pete... these things are never easy, are they.

There is indeed a lot of crap floating around through the intercooler etc (just unhook it and see just how much oil is in it), and on lift off, air may well move back to atmosphere past the MAF (which is why you hear it so well when you remove your resonator ?).

OTOH, when I had an "uprated" dumpvalve fitted, it became stuck after 2 monhts... that same oil had diluted the grease round the piston, so it either became stuck on full throttle (making a whopping 0.2 bar of boost) or at lift off (great response after that, but can the VF23 really take the abuse ?). The old standard MY99 BOV is back on now.

I'm pretty sure a MAF can not read negative airflow ... anyone has proof to the contrary ?

Theo

logiclee

Another interesting point, you only have to look at the top side of a standard panel filter when it is changed to realise how much crap is pushed back it that direction.

Pete obviosly has a vast experience of how robust cats can be.
Just seems strange that most manufactures warn against bump starting or running a car with one or more cylinders not firing as unburnt fuel will damage the cat, know such a warning is published in at least Ford, Toyota, Rover, and Skoda handbooks. Not a warning in the MY96 Impreza handbook though. Ford even warn of unnecessarily long cranking peroids damaging the cat through unburnt fuel in the exhaust.

Good thread though.

Lee

Cosie Convert

I believe a MAF works out air flow from the difference between the air temperature beside the hot wire and the actual temperature of the hot wire/film (measured as electrical resistance) The cooler the hot wire the more flow is calculated to be passing.
Therefore regardless of flow direction the hot wire will be cooled and an airflow will be calculated.

Pete Croney

Lee

The reason that ALL manufacturers of cars advise against bump starting is that there is a very high likelyhood of the cold exhaust pipe filling with a lot of highly explosive air and fuel mixture. Then the engine fires up...

I have seen exhaust systems which were 2 dimensional, after such an event.

And this has absolutely nothing to do with the effects of using a VTA dump valve.

Like you, I used to be against them, because I also listened to all the stories. The more I thought about it, the more it didn't stack up. As I prefer to try things out for myself I fitted one to investigate the effects. I found no evidence whatsoever of any detrimental effects and this concurs with the results of other users.

Cossie Convert, you are quite right, you can't have a negative reading, only a reading.

Theo, I remember reading this some time ago. What one were you using?

haynesaj

I have been wondering wether a dump-valve would help with turbo lag when changing gears. I'm not concerned about power or even a whoosing noise. What does everyone think of this from www.vididracing.com, regarding one of their dumpvalves..I think it comes from MRT:

******
You have just found the best Blow off valve that is available for the WRX. GFB's Unique design guarantee’s smooth running and reliable performance. The GFB valve features a “dual” exit system. The Dual Exit system routes roughly 50% of the air back to the Factory Recirculation pipe and 50% of the air out a Billet machined horn. In effect the valve is a Bypass and a Blow off valve in one unit.
The stock system on the WRX is a bypass system. This means that when the throttle plate is shut against the charged intake the bypass valve vents that charge air back into the intake pipe. If the valve were not in place that charge air would back flush into the Super sonic spinning turbine wheel. That back flush of charge air slows the turbo down. Hence, increased lag, as the turbo now has to spool back up one the throttle is opened again. In addition to the increased lag the turbo would suffer Shock to its blades which could cause severe damage.

The stock system works well but is not without fault. The Stock bypass valve is a bit hard in the spring area, which tends to cause delayed Release. The delayed release increases lag in part throttle situations. Let me explain: The stock valve tends to vent only at high vacuum. This means a complete and total closure of the throttle plate. Letting off the gas during a shift would cause the stock valve to vent. Backing out of the gas slightly will not. Here is the scenario: Under slight acceleration your turbo might produce just a few lbs. of boost. The car ahead of you lets off the throttle and slows a bit. You back out of the throttle to slow down but you don’t let out all the way, You just want to adjust your speed down a bit…You go back to slight throttle and notice a long time constant before power is produced. This is especially noticeable in 5th gear at 55-70mph. Many times I find myself using 4th gear on the highway as to gain response. Some of this lag is caused by the charge air (although it may be only a few PSI) being shut off in the intake tract and being back fed into the turbo slowing the turbine down. Now the turbo has to spin back up to get back to where you where before you let out of the gas.

The GFB valve does not inhibit this behavior. The new valve vents earlier just under Atmospheric pressure. The result is less lag at part throttle and off and on throttle applications. Due to the higher flow capability Lag at WOT is also reduced.

Build quality of this unit is nothing but first class. The valve is made of primarily aluminum. The Body, top cap, air horn, and pipe fitting is all machined aluminum that has then been anodized. The piston inside the valve is made of Brass for a smooth action and long service life. The Valve has a machined bracket to bolt directly to the stock intercooler. Since the shape of the bracket is identical to the stock unit the stock gasket can be used for an airtight seal. The factory 90-degree pipe fitting for the Recirculation hose fits into the GFB valve with precision and accepts the stock screws.

The GFB valve can be installed in two configurations. The first configuration utilizes the stock Recirculation hose and vents 50% of the air into the stock intake pipe and 50% out the Air horn. The second configuration closes the Recirculation hose off and runs 100% of the air out the horn. A machined aluminum plug and a machined aluminum block off plate are supplied with the valve in case you want to run in Configuration two. Running in configuration two provides more sound on blow off but may cause a slight stumble in some situations.

How is that sound anyways? In my opinion the sound is perfect. It’s not to loud as to draw too much attention to yourself and it’s not loud enough to annoy other passengers but, the sound is there. Light part throttle shifts produce a barely audible PSHSHSH sound. Full throttle shifts produce much more sound and a longer PSHSHSHSHSHS. Personally I would not want the valve any louder or any softer. If you drive your car everyday but still would like the blow off sound this is the valve for you.

DaveW

Pete,

Do you know what spring is fitted to your BOV? If its the same as the MY99 then it will either be a blue (soft) or red (harder). I had the red fitted and noticed the same benefits as you have. Fitting a blue spring and the following happened...It was a lot louder, pops during gear changes, very jerky when coming off throttle. Not to mention the increase in fuel consumption. I've put the standard one back on for now until I decide what to do. Its only a 2 minute job to swap them over anyway, just couldn't be bothered changing the spring.

DaveW

MY99 Decat SS.

Bob Rawle

Bov's with weak springs do leak, even on vacuum there will be some 0.3/0.5 bar of pressure build up in the intercooler, stronger spring keeps it there, weaker spring you lose it. Thats what makes the difference on gear changes. They do work by pressure difference, build boost and lift off, you have boost in intercooler and vac in the inlet manifold ... result large pressure difference and bov opens, while on boost the same pressure is on both sides of the diaphram, its just a matter of keeping the cross sectional areas balanced on both sides.

Pete Croney

Dave

I'm using a red spring.

DrEvil

So in the FORGE VTA you would use a RED spring ideally?

Do FORGE supply these or are you sourcing them from eslewhere?

Cheers, Alex

DT

Any way you can get the best of both worlds? A atmo DV with flames but venting quietly? Not big on having more noise.

EvilBevel

OOps, just reread the thread, don't mind me

Theo

[Edited by EvilBevel - 2/6/2002 11:51:43 AM]