POWER DOWN AFTER BOV FITTING

[carpcrew]

hi ive fitted a turbo xs bov and it sounds great but i noticed a drop in power as i found yesterday as a vw gti kept up with me, before i fitted bov car would throw you back into your seat ( great feeling ) but now it seems to be a graduale build up, car has a big 5 turbo, hks induction etc, cant work out whats wrong, any help would be great, thanks rob

[ben281083]

ive got the same problem, put a turbosmart one on and at the same time put original airbox in and lost alot of power, and the dump valve doesent even sound as good as the standard one,
if you have any luck with this let me know,, |Ben

[jd]

check all the hoses for leaks.

[ben281083]

all hoses checked and nice and tight,, dont know what i can be now, any help?

[KaraK]

get rid of it? pure VTA is a bad thing you are always going to have problems. Get either a decent recirc or a hybrid.

[Gogsie78]

it wont be holding the boost fully, i had one and got rid of it because of the same reason.

Now car pulls well again. If they were meant to have Blow of valves Subaru would fit them

[mikef04]

remove bov and blank off and run without, much better others will disagree but in 2 years i have had no problems

[jd]

Subaru do fit blow off valves, they are recirculating.

I agree with the remove and blank the pipes, but that epends on what turbo you have fitted.

[Tidgy]

i've never understood this blanking off the vent as it were. where does the extra pressure go when you lift off? other than backwards the wrong way through the turbo, which surley will stall it? (not that im talking from experiance here i, just thinking about physics lol and also that no track cars i know of run like that)

as far as a the vta being a bad thing, thats rubbish, as long as its the correct fitment then it wont be an issue at all. i've had one on mine for 4 years and never had an issue at all.

[KaraK]

Originally Posted by Tidgy

as far as a the vta being a bad thing, thats rubbish, as long as its the correct fitment then it wont be an issue at all. i've had one on mine for 4 years and never had an issue at all.

To be more specific - running a turbo'd car that originally had a re-circ valve and has a MAF with VTA valve without specifically mapping for it is a bad idea, you might not see the problems over a short period but they are there.

I ran a classic with a VTA for 8-9 months and then swapped it back to a stock re-circ, the difference in how the car drove and ran was very noticeable. Said VTA valve was chucked into the parts graveyard in the garage and I never looked back!

I'm shamelessly stealing this from another forum but here is a more technical explanation as posted by a development engineer at prodrive, he's talking specifically about VAG 1.8T units here, and from what I've seen the scooby ecu is definately more tolerant of VTA than the 1.8T is but the general engineering principles should be the same even if the ECU doesn't complain to the driver as much:

A modern engine management system, ie the Bosch Me unit fitted to 1.8T's has "adaptive" learning on the fuel, ignition and airflow side.

Because Me is a Torque based structure it's calculation of engine torque verses driver demand is critical to the driveability of the car and it's performance / durability.

When you fit a "leak" in the intake system (open circuit valve) the original calibration of the MAF sensor to manifold and cylinder filling modeling will not corespond. However due to the 20% allowance in the long term adaptive values the ECU will relearn you engine and "leak"

At idle the inlet model calculated airflow will exceed the MAF meters measured output, and depending on the state of your particular components - ie MAF ageing / contamination, throttle plate leakage, Fuel tank purge vapour concentration this may, or may not push the adaptive to it's 20% limit. If it hits the limit the ME unit will run in FMEM mode (Failure mode and effects management) causing reduced system efficiency. The Me unit will use the switching signal from the lambda sensor to return fuelling to lambda 1, storing the correction as a map agaisnt airflow. and add this correction to the fueling calc when operating at non closed loop conditions, ie WOT, fuel injector reenstatment (after overrun shut off, traction control intervention etc.) Now depending on how you drive and how sensative you are this may or may not be felt by the driver during certain manovevers. The throttle plate position will also learn the new airflow to maintain control of idle speed, but you may notice poor engine load rejection, ie turn on the aircon and the engine speed varries etc. or engine speed flares on starts or when operating PAS when parking.

However in all cases this will result in "incorrect" fueling. Now by "incorrect" i mean, not as the manufacturer intended. A post MAF leak will cause rich operation initially, but the adaptives will pull fuel out and become negative. This tends to cause a rich to lean spike on tip outs and other throttle transient. Now it is extremely diffucult for an untrained observer to spot these effects as they occur mainly on throttle transients, when the average drive may not notice. Therefore you could say "why do i care?". Well, any AFR excursion from the intended fuelling set by the manufacturer will result in non-standard engine operation. because of the adaptives this is unlikely to cause immediate engine problems, but over the course of time will change things like catalyst ageing, exhaust and turbo charger valve durability etc. Manufactures spend millions accruing miles on development fleets so hopefully the customers don't get landed with big bills as time goes on, and with most modern cars life'd at 150k miles (min design life) this is a big task.
It is unlikely that this will result in any performance loss, as at WOT the system is open loop, but you may see the result of an open circuit valve oas over fueling on gear changes etc. (a tell tail puff of black smoke is what you can see, a 1200 degC Catalyst is what you can't see, as excess fuel when injection reenstates and excess air from overrun shut off period combine in cat)

Now as you can see this is a seriously complicated subject and i haven't even mentioned the dreaded EOBD or OBDII words yet. Typically Bosch Me units have approximately 9000 calibratable parameters (constants, maps etc) and an engine calibration program will take a team of 8 calibration engineers 18 months to do the basic mapping and OBD validation. These days it's no problem to do the basic fuel and spark mapping, maybe 4 weeks on a midlimit engine on a dyno, but the diagnostics and emmisions devs takes years.

Moral or the story, before you start playing with something you don't understand, find someone who does!(And not just thinks they do!)

(for anyone thinking, "hey what makes me such an "expert" on this subject?" then i'd better mention the last 10 years i've spent as a senior calibration engineer at Cosworth and Prodrive!)

[Tidgy]

crikey thast some heavy reading lol. i do know some folks who have had issues, but they tend to be when the springs are failing or they have got the fitment wrong.

my car is mapped for one and always has been, but i know lots of folks who fit them and have no difference in performance at all other than the noise of course lol

There is no performance gains to be had either way at normal road level of power but on the same note they dont have a detramental effect, or if so it would be more widley document, somethign to bear in mind is that all cars react differently to different mods, so what doesn;t work on vag may work on a scoob and vise versa.

although redaing that they seem to be talking as if its open on boost and changing the afr, when your on boost its shut, so that bits a bit irelevant. but then it goes on about coming off power when it causes additional fuel to go in due to the maf saying there is more air in the system than there is.

now this is getting a bit over my head lol, but my understanding is, the injector duty is dropped when you come off the throttle, but even so you end up with additional fuel going through the system. the level of aditional fueul isn't high and gets kicked through to the exhaust, mines set like that deliberatly to get additional pops and bangs and flames hehe, which results in the same effect.

so the general gist is it causes additional fuel on the over run, can burn out valves, turbo and cat.

the level of additional fuel there on about is negligable to say the least, again i've been runing deliberate additional fueling on the over run (which will be way higher than the levels talked about in this) and its had no bad effect at all on my turbo. it ran for 2 years before being taking off after 85k miles and in very good condition with no play etc. i notice hes talking about the 150k miles fo the engines life and how many cars will still have the same exhaust on at 150k? even if they stay with cats. they dont they rust and get new ones.

i'd be ionterested in seen exactly what the fueling affect is and how much the real affect is. it should also be noted that scoobs tend to like being run slightly on the richer side of the line.

[slider]

Had my car 9 months now and had a baileys dump valve,drove me nuts so ebayed it,and put standard recirc back on.The car ran just as sweet with no noticeable loss in performance I recently bought a forge dump valve, been on the car 2 weeks now and better than the bailey as it only makes noise when you hit 3000 rpm.
I have spoken to people from scoobyclinic,scoobyworld,& alastair at grahame goode,and they have all said pretty much the same thing ,which is if your car is a later classic (1999 - 2000),and your not looking for mega power,then the standard recirc is the best. a dump valve will do the same job as the recirc ,just make a noise. As for the earlier classics,some dump valves do work well on these,but just blanking it off is a bad idea imho,mainly if it did not have a use then subaru would have blanked it off at the factory.

[jd]

subaru do blank them off at the factory........................well Prodrive is a factory.

[slider]

ok mate,prodrive do indeed,but what about the other 98% of cars that are standard factory fiment.

[Tidgy]

all i'll say is how often to prodrive strip the engines and turbo's down? also they run full blown antilag, not pub antilag aka sounds good but n reality doesn;t do much