Blow off Valve

[zentac]

Looks like I need a blow off valve, could really do with one before I take it up to the intercooler guy this weekend. Anyone got any recomendations?

Doesnt have to fit a Galant as Im having the pipework made up, but will need to handle about 350-400bhp INSERT INTO post VALUES (maybe one day!)

[Simon]

Saving up for a HKS dump valve....reccommeded by others i know....plus they've come down in price to about £245!! worth a look. INSERT INTO post VALUES (One made specifically for VR4...comes with two valves for different noises!!!!!!!!)

[zentac]

wheres the cheapest place to get a HKS one from?

[Spirit]

Zentac - price up the best deal you can get and then PM me - I have a contact, but will need to call them direct myself

Pete

[haydar]

Originally posted by Simon
Saving up for a HKS dump valve....reccommeded by others i know....plus they've come down in price to about £245!! worth a look. INSERT INTO post VALUES (One made specifically for VR4...comes with two valves for different noises!!!!!!!!)

Forgive my ignorance, but what exactly does a dump valve actually do? How does it improve performance?

[calum]

It's a vacuum triggered valve that lets built up boost pressure in the inlet system escape when you change gear or come off the throttle. The theory goes that it stops the turbo stalling when the pressure wave returning from the closed throttle butterfly hits the turbo compressor side.

A lot of people buy ones that dump to atmosphere 'cos they think it's 'cool'. Quite a few high performance cars have recirculating ones, that dump the excess air into the airbox.

Atmospheric dump valves are not my favourite bit of kit, but that's just my opinion.

Calum

[pjjohnson]

The standard car does have a dump valve, its just not very noisy when it operates, and its made of black plastic. The after market ones are nice and shiney and you can clearly hear them work, but I'm not sure that they improve performance.

[Brind]

I know the guy who wrote this, he doesn't know I just stole it though. :D

Turbos and Dump Valves

Engines gain extra power by having greater air/fuel mix in the cylinders. More air fuel = more bang = more power. A normally aspirated engine INSERT INTO post VALUES (n/a) sucks air in but the amount is limited as air pressure is only so great, thus with n/a engines, more capacity = more power. A turbo forces more air/fuel into the engine than is normally possible, hense the name forced induction, and can give small angines comparable power outsput to lager displacement engines.

A turbo unit consists of two parts. One half sit in the exhaust system and uses the escaping gases to create it's power by spinning a turbine INSERT INTO post VALUES (hense the name) . This is then transfered to the other half, in the compressor side, which then uses this power to spin up the compressor wheel which forces the air into the engine on the intake side. It's very efficient as it uses the engines own power to operate and can increase the output of the engine by upto and over twice as much in some cases. As an example, the Rover T16 2 litre engine creates 140bhp normally aspirated, but with the turbo added, running 8psi of boost, that raises to 200bhp. 1psi of boost is approx 8bhp. This would be roughly the same for most 2 litre cars running a small turbo on a basic level.

That's the easy bit. Now, without any control of this you would simply have more boost the higher rpm the engine runs as more exhaust gas spins it faster. This is where the wastegate comes in. As well as the turbo itself, there is a second 'bypass' part of the exhaust which misses out the turbo altogether, this allows the extra exhaust gases to be 'wasted' and is controlled in much the same way your throat controls whether stuff going in goes into your stomach or lungs. The wastegate is essentially a flap and is, in turn, controlled by a spring of a certain strength in a unit called the wastegate actuator. The spring will open above a certain pressure, and it this that allows the turbo to hold a steady level of boost throughout the rev range. It takes it's feed off a vacuum pipe in the inlet manifold which tells the wastegate actuator what the pressure in the system is. It's all done mechanically, and has a slight delay in it's working which causes overboost, or boost spikes - this is where the boost pressure is temporarily higher than it should be and isn't a good thing because it can cause engine problems.

So, we now know what a turbo is and how it's boost level is kept stable. Now, onto dump valves and their purpose in proceedings. When you change gear, or ease off the power, you don't really want to have too much pressure in the system, this can be vented with the dump valve. It works in a similar way to the wastegate in that it is spring loaded and works off the pressure in the manifold. When you change gear you lift off the acellerator which eases off the turbo and the manifold goes into vacuum again, this vacuum releases the dump valve, opening it up to vent off the excess pressure, creating the *tshhh* sound we all know and love. When you put your foot down again, pressure goes positive thus closing the DV and keeping the boost in system. Now, the general opinion is that all turbos need a dump valve to vent off excess pressure and stop the turbo from stalling with the build up of pressure in the system when you change gear. However, that's not the case. It is only engines with big turbos running high pressure INSERT INTO post VALUES (20psi or so and above) that need a DV to prevent them stalling. Smaller turbos on lower boost will not generate enough pressure to stall the turbo, so adding a DV to vent off the pressure will actually INCREASE turbo lag as it then has to spin up again from scratch. Let's not forget that the engine is still running sucking in air so the back pressure is minimal. This is why cars such as the RSTs came without one as standard, and we all know diesel turbos don't have them. The reason most manufacturers put recirculating DVs in the standard car is to stop it from making any noise and to try and increase the life of the turbo by removing any backpressure from the system. So, not having a DV could reduce turbo life, but no more so than increasing the boost a few psi, so it's not a serious problem running without one. It's also the case that without a DV, you will hear turbo flutter upon changing gear as the compressor wheel chops the air coming back, this is the sound of the great Group B rally cars and is way cooler than any DV.

On to upping the boost. To do this you need to keep the wastegate open more/for longer. It can be done with a stronger spring in the wastegate actuator itself, or more commonly by a device that sits in the system between the manifold and wastegate actuator. The device will do is bleed off pressure. The most basic form of such a device is a bleed valve, which you add in and open slightly to vent off some of the pressure from the inlet manifold, for example 3psi. This means that when the wastegate actuator thinks it's 8psi and closes, it's actually 11psi, so you've got 3psi more of boost. At the top end of the scale you can get electronic boost controllers running with instant response solenoid valves. These can be set to various levels and have all sorts of functions. The Apexi AVCR is one of the best of these devices, allowing you to run differing levels of boost over the rev range and between gears! The electronic devices also remove the boost spikes mentioned above, keeping things safer.

Now intercoolers. We know more air/fuel into engine = more power. We also know that cold air is denser than warm air and for the same space has more oxygen in it. So, the colder the air, the more power your turbo adds. Hense the adding of the intercooler between the turbo and inlet manifold. The turbo is spinning away frantically creating friction and pressure, which create heat. Without an intercooler the temp of the air entering the engine could get upto and over 100 deg C, which is unhealthy, as it causes the pre-detonation INSERT INTO post VALUES (or pinking). So, the intercooler, much like a radiator, cools the air going from the turbo to the engine, making it even more efficient. You'll hear people talk of bigger and better intercoolers, all with the aim of cooling the air to keep it denser to get more air/fuel into the engine.

At the end of the day, you can only increase power by adding more air/fuel into the cylinders to burn. It's as simple as that.

[Brind]

He will know now, he has just joined.

Welcome Mook. :)

[mook]

Hehe, hi folks, hope that novel is of some use :D

[king]

turbo'd cars that do have dv's fitted as standard generally use a rubber diaphragm, which can leak at higher boost pressures and may split.
If you do decide on the dump valve it might be worth checking out this one:
http://www.dpmotorsport.com/apex/page3.htm?General%20Accessories&accessories.htm

its a good make and the whoosh noise is adjustable depending on how loud you want to be!!

[calum]

Hi mook.

My 2ps worth:

The wastegate OPENS when the required boost pressure is acting upon the diaphragm.

The wastegate is a valve, controlled by the actuator, which allows exhaust gas to bypass the turbine side of the turbo, thereby stopping boost rising any higher.

This pipe can come from the turbo compressor housing or the manifold. Having it from the compressor housing stops the overboosting effect from being as noticeable as described.

The wastegate has a 'base' pressure at which it is fully open. Not sure what this is on the VR4, but using my celica as an example, the pressure required to be acting on the diaphragm to have the wastegate fully open is about 9psi. To get greater boost pressure INSERT INTO post VALUES (once the operating parameters such as oil temp, water temp, etc have been met) there is an ECU controlled bleed valve.

This bleed valve is connected to a second air connection on the wastegate actuator diaphragm housing. If the ECU is telling the bleed valve to stay shut, then the max boost you get is 9psi. If the ECU is telling the bleed valve to be open, then it bleeds off air, 'fooling' the wastegate actuator diaphragm into thinking that the turbo is producing less boost than it really is.

This has the effect of keeping the wastegate closed for longer, allowing boost pressure to build higher until the new maximum is reached INSERT INTO post VALUES (9+4)=13psi.

There is a MAP INSERT INTO post VALUES (manifold absolute pressure) sensor in the manifold INSERT INTO post VALUES (surprisingly enough) which tells the ECU what pressure the turbo is producing. The ECU uses this and other inputs to decide what to tell the electronically controlled INSERT INTO post VALUES (variable solenoid valve) bleed valve to do.

Most people up the boost on their cars by either:

1) Opening up the bleed valve more, either by drilling it INSERT INTO post VALUES (inaccurate) or using some kind of ecu modification INSERT INTO post VALUES (unichip, superchip, re-mapping, programmable ECU).

2) Dispensing with the ECU controlled bleed valve altogether and replacing it with a mechanical bleed valve which you can adjust yourself.

3) Blocking off the pipe to the bleed valve and inserting an adjustable relief valve into the pipe between the wastegate actuator and the manifold or compressor housing. You can set the pressure you want the relief valve to open at, and then the wastegate actuator diaphragm sees no pressure at all until the relief valve lets it see it. This stops 'wastegate creep'. The relief valve also becomes your boost regulator, as it opens and closes itself as the pressure rises/ falls around what you've set it at.

4) Buying and fitting uprated wastegate actuatorINSERT INTO post VALUES (s).

5) Dispensing with the ECU controlled bleed valve and replacing it with a system such as a blitz boost controller. DUal solenoid boost controllers incorporate the functions of both a relief valve and a bleed valve

Calum

[mook]

Calum > All sounds good mate - I take it you have GT4 then :D

I think I wrote that piece about 18 months ago, obviously more research and so forth done in the interim :o

I quite fancy getting one of these Dawes Devices Hybrid boost controller. £50 for a mechanical device that would appear to be superior to computer based boost controllers, let alone bleed valves. It works with a couple of ball bearings within a small housing to be completely shut until the desired boost level is reached, which allows for very quick spool up and no boost spikes. Not much technical info I agree, but the idea of it sounds good to me. Definately a cost effective replacement for a bleed valve :D

[calum]

You're right - I do have a gt4. But we also have a legnum VR4.

Learning is an ongoing process! You don't need to get it from Dawes - I can put you in touch with a guy who'll sell you one for £20 or £25. My problem here is that i don't know much about the VR4 twin turbo set up. How would a relief valve work when you've got two actuators? Are they triggered from the same line?

Calum