Skyline Owners Forum banner

1 - 20 of 24 Posts

·
Registered
Joined
·
469 Posts
Discussion Starter #1
I'm new to all this turbo malarky. Can someone explain what blow off valves and turbo timers do, or point me so a website which explains such things.

Cheers
 

·
Registered
Joined
·
2,433 Posts
Hi mate!

Blow off valves (aka dump valves) have a very simple yet useful function....

When the turbo is 'working', ie you have the foot on the gas and its producing 'boost', what the turbo is doing is compressing air (to get more into the cylinders - more air in (with fuel) gives more power). This air is passed thru the intercooler to cool it (cos when you compress it it get warm, but the engine needs air thats as cold as possible to get power) then it goes thru the throttle body and into the engine. Now, if the turbo is compressing this air, then as you can imagine there's a lot of air under pressure whooshing towards the throttle butterfly (go faster device) which is open when you are pressing the go pedal. If you lift off the throttle to change gear or slow down, the throttle butterfly closes. Where does the air go???? There's a lot of it under pressure, remember. Well, normally it get trapped and produces backpressure (ie it pushes back against the turbo) which can damage the turbo over time. It forces the turbo to slow down/stop completely very suddenly. So, when you put the foot down again, it takes a wee while for the turbo to spin back up to operating speed.

A dump valve vents this excess pressure created when you lift off the throttle. It sounds wicked too, as well as keeping the turbo spinning and preolonging its life.

A turbo timer is another cool device. Your turbo spins incredibly fast (up to 80000rpm for some), and its bearing/shaft is usually lubricated and to an extent cooled by engine oil. If you are running hard for a while, things get hot in there. If you were to just stop the engine and leave it, then the hot oil sitting in the turbo shaft and bearings slowly bakes the turbo, which generally isn't good for it. A turbo timer will keep the engine running at idle after you take the key out and lock the car, for a preset amount of time. SOme can even work out themselves how long the car needs. This ticking over allows cooler oil to circulate around the turbo, as well as generally letting the car cool down inside the engine itself. Great bits of kit, again, prolonging the life of your turbo.

However, it is illegal in the UK to walk away from your car and leave it with the engine running. Even if it is locked.
They usually have a hadbrake sensor which, when the handbrake is let off, will kill the engine immediately (in case someone goes in thru the window and tries to steal your car), although it'd stop anyway when the timer ran out.

Hope this all helps, was typing rather quickly so it may not make sense!
 

·
Registered
Joined
·
469 Posts
Discussion Starter #3
Cheers

Thanks for the info, that all makes sense.

No doubt more daft questions will follow once I make up my mind which GTS I want and get my mits on one.
 

·
Registered
Joined
·
2,433 Posts
You're more

than welcome mate! Us GTS guys have to stick together in the face of ludicrous power and Attesa (The variable 4WD GTR thing!)!

Have a look on www.rb20det.com and www.skylinegts.co.uk for some handy info on GTS's etc etc.

Both sites are run by guys on here, mine has no info but loadsa pretty pictures! (I think)
 

·
Registered
Joined
·
469 Posts
Discussion Starter #5
Am I being thick ..

So what's the difference then between a dump valve and the wastegate. Don't they both do the same job of stopping the boost pressure getting too high ?
 

·
M's Factory
Joined
·
5,240 Posts
Dump Valve is Intake side of the engine, Wastegate is Exhaust housing of the Turbo.
 

·
Registered
Joined
·
2,433 Posts
Not thick mate,

just learning!

The Waste Gate is a core part of the turbo system.

The Turbo is essentially a shaft with a compressor on one end and a turbine on the other.

The turbo works by compressing air, which the compressor can only do if its spinning.

I will post some pics on this thread later to make it easier to explain, but for the next hour or 3 this will have to do....

The compressor/turbine unit therefore spins as one. How is it spun....

Well, genius bit of kit really...

The exhaust gases as we all know come out the engine and then down the exhaust.
Well, the Turbo is bolted onto the exhaust manifold. There is a gate called the Waste Gate in there, which opens/closes to direct exhaust gases.

The waste gate's position is controlled by various factors which aren't important, the important bit is that the waste gate's position directs exhaust gases either down the exhaust pipe, or onto the turbine wheel at the back of the turbo (which makes it turn kinda like a water wheel at a water mill). So, the more exhaust gases that pass via the waste gate over the turbine, the faster the spool (the turbine/shaft/compressor unit) spins, so the more boost you get.

Essentially, the waste gate is the bit of kit directly responsible for how fast the turbo spins - ergo how much boost you get (the compression ratio of the air entering the compressor to the air exiting it and going into the engine).


Lucky I've got a spare turbo lying about somewhere I can post pics of later.


A dump valve, on the other hand, Releases this compressed air from the turbo when the throttle closes (you lift your foot off the loud pedal) to prevent backpressure against the compressor.

There is a big pipe basically, which goes from the turbo's outlet (after the compressor) into the inlet manifold, which has the throttle in it.

If you shut the throttle, all that compressed air is trapped in the pipe, under high pressure all of a sudden, and that pressure pushes back on the compressor of the turbo, which forces it to stop/decelerate VERY rapidly, which ain't good for either the turbo, or your spin-up time. The dumo valve opens at a set pressure level to release this air (sounds AWESOME too - Gran Turismo stylee) hence reducing wear on the turbo.

Hope this helps, was again typing in a hurry. Shall post some pics of the R32 stock turbo later which will hopefully make things clearer.



Stewart
 

·
Registered
Joined
·
469 Posts
Discussion Starter #11
Aha

Thanks for the excellent explanation and taking the time to post the pics.

So to sum up is it like this ....

Without a dump valve when you lift off the throttle the pressure on the intake side builds up too much and stalls the turbo. At the same time cos the boost pressure has got so high the wastegate opens and directs the exhaust past the turbine so it really does stop.

With a dump valve the extra pressure is released on the intake side, so that turbo doesn't stall. In addition the boost pressure doesn't get so high, so the wastegate remains at least partly closed so the exhaust gasses keep the turbo spinning.

When you don't have a dump valve is there any other form of pressure release on the intake side ? If not, don't you then get massive overboost when you lift off the throttle, potentially causing detonation and damaging the engine - or does that fact that the thtottle butterfly is nearly shut stop too much air getting in ?

Thanks again for your help
 

·
Registered
Joined
·
2,433 Posts
Re: Aha

Without a dump valve when you lift off the throttle the pressure on the intake side builds up too much and stalls the turbo. At the same time cos the boost pressure has got so high the wastegate opens and directs the exhaust past the turbine so it really does stop.
Close...... The pressure on the intake side can stall the turbo (force it to stop), or at least create large 'shock loads' on the turbo shaft by this forced deceleration. The Wastegate moves purely to control the speed of the turbo, so what could happen is the wastegate being closed (directing exhaust gases over the turbine to spin the turbo) for a split second after you lift off... So the combined effect of the exhaust gases trying to spin the turbine and the pressure on the manifold side = lotsa stress on the shaft. The wastegate will open, however, to allow the turbo to decelerate so you are 99% correct, it just may not open that quickly.

With a dump valve the extra pressure is released on the intake side, so that turbo doesn't stall. In addition the boost pressure doesn't get so high, so the wastegate remains at least partly closed so the exhaust gasses keep the turbo spinning.
Boost pressure will still get as high as without a dump valve (unless its set to release at lower pressures on an adjustable one), but it will release when necessary to stop the backpressure on the compressor. Turbo will spool up faster with a dump valve correctly fitted because there is not backpressure on the compressor when you change gear, so even though the wastegate will open as you change gear(releasing the gases spinning the turbine), the turbo will continue to spin thru its own momentum, so that when you put the foot down again it has less accelerating to do!

When you don't have a dump valve is there any other form of pressure release on the intake side ?
Not that I know of, but that certainly doesn't mean such a thing doesn't exist. However, it would only do the same job as a dump valve anyway.

don't you then get massive overboost when you lift off the throttle, potentially causing detonation and damaging the engine - or does that fact that the thtottle butterfly is nearly shut stop too much air getting in ?
Thats it spot on mate.... The throttle butterfly (basically a round metal disc in the inlet plenum which is tilted on a shaft by the throttle cable) stops the air getting in, thats why the backpressure occurs on the compressor. All that air is trapped between the now closed throttle butterfly and the compressor.
Ergo - Backpressure and the need for a dump valve.

Man, you are learning seriously fast!

Impressed.
 

·
Registered
Joined
·
469 Posts
Discussion Starter #13
Man, you are learning seriously fast!
Thanks. I've got a reasonable understanding of engines already, but fuel injection and turbo's are pretty new to me.

I test drove a GTS yesterday with a Blitz BOV on it. It seemed to be letting air out quite a lot, not just when lifting the throttle.

Is this normal ? Or are they adjustable - is it possible to increase the pressure at which they open. Is this one maybe badly adjusted ?

Thanks again for your help. Maybe some of your explanations could be put on some kind of FAQ somewhere ?

All I need now is to buy a GTS to play with and see if I'm really getting the hang of it all.
 

·
Registered
Joined
·
2,433 Posts
As far as FAQ's

and explanations go.... Am working on some stuff for the 'How To' section of the forum at the mo, but been held up by my bu**ered brakes....


I've got a Blitz BOV on mine, its got an adjusting nut at the end that you can tighten/loosen but I never touch it to be honest.

If the car has a pod/cone induction it will make a sucking sound when you've got the foot down, which can sound a bit odd til you get used to it. Dump valve makes a sudden 'Whoosh!', whereas induction kits are a constant whoosh/sucking sound.

If it was stuck open the car would be SERIOUSLY under powered as a result, and wouldn't run well at all. Could maybe just be the induction kit sound you are hearing?

Could make a recording of mine sometime over the next few days if it will help mate.... (A'Pexi induction).

Edited to say.... The Dump valve will release when you back off the throttle, not just when you lift off completely. You can make 'em whistle with a bit of practice!
 

·
Banned
Joined
·
8,850 Posts
Dear Dr Jock, mine tends to whistle, " Put some more petrol in" To the tune "nickleodeon",is there any cure? :D
 

·
Registered
Joined
·
2,433 Posts
ROTFL.........

I fear the cure may be somewhat pricey Mr Buster sir..... What you need to do is remove the Geflugenfilter and replace the 'nickelodeon' cartridge with something slightly more up to date. That should cure the problem.


Oh, and 8 track players are not available anymore! :D :D
 

·
Banned
Joined
·
8,850 Posts
Oh, and 8 track players are not available anymore!

That will explain the funny attitude from the staff at HMV then.:(

We need a new Header.... Ask Dr Jock
and for all your personal probs....Ask Uncle Buster
 

·
Registered
Joined
·
469 Posts
Discussion Starter #18
ta

Thanks again for your help.

I just put a deposit on the car and get it in 2 weeks, so expect a zillion more questions once I really start poking about under the bonnet.

You have been warned :D
 

·
Registered
Joined
·
469 Posts
Discussion Starter #19
What about the ecu

When the dump valve is doing its thing won't the ecu get confused as lots of air will have passed through the air-flow meter but won't have made it to the engine as the dump valve has let it all go - won't the ecu think the engines running really lean and try to compensate for this by dumping loads of fuel into the engine.
 

·
Registered
Joined
·
2,433 Posts
Yup!

Thats the problem with the 'hot-wire' air flow meters our cars have.

The car will try to stall after its dumped a lot of air as it drops to idle. Only happens if it drops to idle though, and if its dumped a whole load of air.
You can kinda prevent it by staying in gear down the revs if its doing it approaching roundabouts or stopping, I use my S-AFC to correct for it as it has that facility. Itsnot a major problem, you get used to it.

Congrats on the deposit down! Have to post us all some pics when you get it!

I'll be jealous as hell, cos I think I just blew my car up.
 
1 - 20 of 24 Posts
Top