ROTFLMFAO:notworthy

Have you not seen the Matrix? Its all true you know!

John, good point on carbon related det - bit of a viscious circle that one. Probably dismissed by most as "techie ****e" no doubt though :rolleyes:

As Ali said earlier, we could all just give up trying to help up and let you carry on regardless :)

MB

To be serious (if I may):

On a system witha recirc D/V, when the throttle is shut off, I assume that the air is recirculated to a point after the MAF sensor?

If so, and the throttle butterfly is closed, that air cannot get into the combustion chamber (as minimal air would be getting pulled in iyswim)?

Meaning that when the throttle body is open again, there will be air that the MAF has metred for the current point in time + The air recirculated?

If so, would the car not run a bit lean as there is effectively 2 lots of air going in to the combustion Chamber?

On a BOV valve, the air is dumped straight away, but the throttle body is shut so the air could not have gone in the combustion chamber anyway?

Meaning that as soon as the throttle body is open, the mAF will meter air and will be correct for how much air is going into the engine?

Out of interest, how much extra fuel are we talking is injected with the use of a BOV, and how much extra risk of bore wash is there?

this idea only works if the air after the maf can be pressurised
it cant so it goes back out over the maf so in theory the maf will read it twice and if its still fuelling will add more fuel than a vta which only meters the air once
has anyone actually measured injector duration on gear changes/lift off or is all of the argument just hypothesis?

richie

It can clearly be seen while mapping. You can see it on the AFR gauge too.

MB

could it not be mapped out for those who enjoy 'the sound of steam':D
i'd have thought subaru would have used a lift off fuel cut or can they not for some reason(205 1.9 kangarooing for example)
richie

Agree up to here...

"Meaning that when the throttle body is open again, there will be air that the MAF has metred for the current point in time + The air recirculated?

If so, would the car not run a bit lean as there is effectively 2 lots of air going in to the combustion Chamber?"

The air mass has been measured once and there is conservation of mass in a closed system (or nearly if the airflow into the still quickly rotating compressor is in the inward direction with little chuffing out the air filter). During throttle transients the flow rates in and out of the inlet manifold (as well as in and out of the area between the compressor wheel and the throttle plate in a turbo engine) are different, and the ECU has logic to enrich or lean the mixtures to accommodate these without large excursions in AFR - a modern engine would never pass emissions otherwise as the cats rely on a continuous average very close to stoich to give low emissions of CO, NOx and HC. If you change the system you can run into errors in measurement, which is what produces the transient overfuelling on lift off. The air that is left or not left in the system depending on recirc or VTA is not enough to notice a difference in transient enrichment going back onto the throttle as the flows are far greater.

"Out of interest, how much extra fuel are we talking is injected with the use of a BOV, and how much extra risk of bore wash is there?"

Depends on how much boost, how quick the throttle lift, when decel fuel cut off occurs, volume of pipework/intercooler under pressure. The lift off transients are enough to give flames, pops etc, all of which are probably not ideal for exhaust ports/valves/head temperatures, manifold, turbine, cats and plastic bumpers near drainpipe exhausts ;) However, the little appreciated issue with VTAs and IMHO the really big problem is the continuous leaks on part throttle leading to overfuelling that could last for ages. A ball-spring MBC (lots of part throttle boost) and a slack dump valve (chav tuned TM) with a turbo that is relatively small could be troublesome in particular.

Not without great difficulty if you're running a MAF to calc engine load :)

Its not as simple as cutting the fuel.

You could keep a small pressurised air cylinder with a feed pipe into the intake pipe and have it working on a pressure switch from the VTA, to trigger a spike into the MAF on lift off. It would have to be a predetermined volume calculated from all sorts of things (turbo pressure / engine speed etc)

Most over the top though :)

MB

Thanks for your explanations.
John - If I have understood correctly, your main "issue" with VTA's is the leaking sometimes asscoiated with them. AFAIK, the HKS system works differently, in that it is impossible for the valve to leak (I could be wrong) - therefore am I running a "safer" VTA? I can't say I have ever "noticed" the HKS leaking on part throttle (and I use no mbc to control boost)

How much icreased risk is there of getting bore wash, as I would have guessed that the overfuelling can only be for a VERY short time - likewise, something like anti-lag (which I appreciate actually works as opposed to a VTA) must inherently be more "dangerous" to the turbines etc, as it must inject more fuel than the chambers would spit out from running a VTA?

Sorry if my questions are a bit novice like, but that is what I am, and the only way to learn is to ask!

Very different mentality with regard to a rally car engine, as they are not concerned with being able to clock up 100k miles. Blanking off the DV does indeed put a lot of load on the thrust bearings...

Overfuel may be for a short time of throttle release, but how many times do you do it in a day? Leaking on part boost is constant overfuel.

You may not hear it with the HKS being sequential, although ive not seen a diagram for it.

John will answer the rest :)

MB

I didnt mean in a rally car situation, there seem to be plenty of people running with anti-lag on road cars - perhaps they have the money for new turbos every so often.

I believe, IIRC, that due to the design of the HKS item, it is practically impossible for the item to leak boost (certainly to what it has been tested to - which is way more boost than i would want to run on a piddley VF-28 :) ) You guys may be able to prove me wrong on this one though

Not studied the HKS design, but the issue of leaking boost is unlikely to happen on full boost, it is on part throttle where the pressure across the throttle is highest - the turbo may be working at say 20 PSI boost, which drops to say 10 PSI after the throttle. As long as the largest pressure drop you can make on continous throttle is less than the cracking pressure of the dump valve I think you should be OK. The OEM dump valve would probably be open in the above situation - but it doesn't matter, it is just recirculated having already been metered.

Here are the diagrams for hks:

http://www.hksusa.com/content/pdf1583.pdf

let us know what you think (sound aside ;) )

Looks to me like it can still leak on part throttle, through the smaller port. Although I cant work out how the seat differentiates between light and full boost unless the spring is progressive, or there's 2 springs?

Pretty sure the spring is progressive though, the end of it being less stiff than the base.

Looks as though it would be pretty secure at idle though, as any pressure is on the inboard side of the seat.

MB

Thanks, it is adjustable and will open more gradually, but it could still be closed at idle but open on part throttle and cause overfuelling if it was not tight enough. At idle there is about 9 PSI across the throttle and therefore working against the dump valve spring. With part throttle boost you could have more than this.

So with some turbos you might be in the position where you have to have your dump valve tight to stop overfuelling (from leaking MAF metered air on part throttle) but then run into compressor surge if your part throttle boost is frisky. If your part throttle boost is flat the car can be less exciting to drive.

With the recirc, you can recirc the air to stop the compressor surge but maintain the correct AFR.

There is a bit more to this than most people realise.

BTW the Forge VTA has a different arrangement, but functionally would appear to work very similarly to the HKS - it has a small and a large spring inside.

Yep, the HKS uses an adjustable nut to adjust the spring rate, and it needs to be spot on. As John says, too loose and it will lift early and may cause idle issues. Too tight and it will only lift on higher boost, and also slam shut too quick making the car quite jerky to drive. This also will put a bit of shock load on the turbo bearings.

The Forge is essentially the same, but to adjust the spring rate you have to replace it with softer or harder ones, or pack it with 1 / 2 mm shims.

Standard one doesn't suffer this as it just leaks back to the intake.

MB

So, would the Hks item be stronger/better than the standard item if it had the recirc option fitted (i realise that would make it a v. expensive recirc).
I believe the d/v is set to specific cars needs when bought with that cars fitting kit. Since fitting mine, I cant say I have seen any degradation in performance or jerkiness in changing gears etc. i drive most of the time on part throttle (yeah, i know -pussy), and have never noticed any problems.

HA HA HA HA

BEST DESCRIPTION EVER!!!!

for the record i use a slightly modified TAS Piston recirc valve

noo ppptttttchooooooo from my car thanks and it still hasnt blew up yet??

Did anyone ever find with their DV's that they play a game of tennis with the wastegate? Unless you spent hours tweeking the spring so it soft enough to open without chatter yet strong enough to stay shut at idle?

DV opens, wastegate opens, dv shuts, wastgate shuts, repeat for 100 times each second..sounds just like a rally car :D

Cant be good for the turbo though :o

i think DVs are ok if your under 30!! im 25.......... ;)

Think you're right there. Just doesnt look right with 'old' people driving around with the vta blaring :lol1:

That could be why some like the sound but wouldnt fit it to their cars ;) (ahem...JB...ahem) ;)

:D

I'm too young (30) to be a GP according to most of my patients who choose to comment on this issue instead of answering my simple interrogation (in between "Oh dear, old age doesn't come by itself, aren't the policemen young"). Think pslewis was in the other day ;)

I'm too old for a Scooby with a loud dump valve.

Maybe if I buy a BMW I would look older and wouldn't need a dump valve, but the patients would miss the throbbing old Scooby (I always know when you're coming doc) :eek:

E=MC2 theres a sh1tload of energy available!! the lead suit might just stuff the handling slightly though!!

the only reason a recirc doesn't make a noise is because its released into the intake system witch acts as a silencer, so stil the same energy loss.

although with a vta if you rigged up some sort of pipe system (any plumbers looking for abit of o/t!!) that dumped the charge out down the side of the wing in a sort of jet style, turbo lag cured, i thank you.

stevie

just thinking, for anyone running more than standard boost you might get punted off the side of the road when you change gear, better use a 't' piece an abit more pipe to the wing on the other side, just to equal things out abit!!!

the dump jet is born!

stevie

stutter dv
 

While you are on the topic i am after a stutter dump valve, and i am still young enough to have one, just (25). Can anyone recommend a model and make.

cheers

A what? If you want it to chirp like a rally car, just fit an aftermarket DV with too strong a spring, or blank the DV off altogether.

Its really not advisable to do that though, long term on a standard setup....

MB

I hear the Gareth Gates spikeyhairedpr!ck makes all the noises you require :D

Second what DBM says

I have heard a scoob in preston with one, the only way i can describe it is the noise of the supra in the original fast & furious.