Is my turbo too small? (loosing boost high end)

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axle
3
Joined: 22 Jun 2004, 14:45
Location: Norfolk, UK

Re: Is my turbo too small? (loosing boost high end)

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You don't need a Blow Off Valve as this simply release pressureised air from the intake side when the throttle plate slams shut. The BOV is opened by the vacuum created on the other side of the throttle plate. it does this simply to try and stop compressor stall/chatter.

The wastegate intercepts air destined for the turbine and diverts it once the right pressure is reached in the intake (reference line attached to the turbo housing usually). The spring that actuates the valve on the wastegate opens at a predetermined pressure, as this spring moves up and down/and varies the bypassing of the turbine, the turbine is kept spinning at the right speed to move the compressor enough to compress the right amount of air.

So A) you need a wastegate if you want to control boost.
B) The wastegate pressure reference can be over-ridden by an Electronic Boost Controller or a Manual Boost Controller (bleed valve).
C) Some race applications don't have wastegates cause they don't want to limit boost - some drag applications.
D) VNT/Variable Vane Technologies don't need a wastegate as the turbo's vanes etc acuate to do the same roll as the wastegate.
E) Wastegates are either Internal or External to the turbo housing. Internal WG's are found on small frame such as T3 turbos and ext WG's are used on anything T4 sized and upwards.

HTH

If boost pressure is tailing off over a certain RPM then the turbo isn't capable of supplying enough air at that pressure for the wastegate to need to operate. So yes a bigger turbine and compressor is needed.
- Axle

G-Rock
0
Joined: 27 Jul 2006, 20:05
Location: Ridgetown, ON

Re: Is my turbo too small? (loosing boost high end)

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It takes a big man to admit he has a small turbo.
While we're at it I have a confession to make. My wife sometimes tells her friends (about me) that "it may be short..but it sure is skinny!"
There, that feels better. It doesn't lose boost though, not yet anyway.
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NickT
2
Joined: 24 Sep 2003, 12:47
Location: Edinburgh, UK

Re: Is my turbo too small? (loosing boost high end)

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It might not be the turbo that is the problem. Yes there are limits on the amount of air a turbo can move and ultimately the power output of the engine its attached to. But I would be more interested in the fuel injectors. It has been quite a common problem that they simply run out of capacity and cannot deliver enough fuel.
NickT

bizadfar
0
Joined: 03 Jan 2007, 15:51

Re: Is my turbo too small? (loosing boost high end)

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NickT wrote:It might not be the turbo that is the problem. Yes there are limits on the amount of air a turbo can move and ultimately the power output of the engine its attached to. But I would be more interested in the fuel injectors. It has been quite a common problem that they simply run out of capacity and cannot deliver enough fuel.
TT injectors are about 380cc, that is sufficient. (im not so sure on the 180hp version, could be 316cc)

An A4 was using <300cc to run a K03.

Bolt on a K04-23 (for TT), you can continue to use the same injectors(make sure you get it checked, again im not certain on the 180hp version TT). Most people who do the K04-15 on the A4, use S3/TT OEM or OEM spec injectors.

External wastegate is not needed.

Also the downpipe/cat right after the turbo, you should replace that since it is very restrictive.

axle
3
Joined: 22 Jun 2004, 14:45
Location: Norfolk, UK

Re: Is my turbo too small? (loosing boost high end)

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NickT wrote:It might not be the turbo that is the problem. Yes there are limits on the amount of air a turbo can move and ultimately the power output of the engine its attached to. But I would be more interested in the fuel injectors. It has been quite a common problem that they simply run out of capacity and cannot deliver enough fuel.
That'd would present as a different set of symptoms. Lack of fuel would just see the engine expire! (Unless the ECU could detect it going massively lean). If the turbo wasn't undersized it would be able to keep pushing Xpsi of pressure right through the rev range.
- Axle

djones
20
Joined: 17 Mar 2005, 15:01

Re: Is my turbo too small? (loosing boost high end)

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The car does have a different down pipe from standard (not sure if i mentioned that).

It was quite interesting to see what difference that made actually...

Image

The turbo seemed to spool up faster, hit a new high PSI but then hit some sort of peak and the ECU (or something) cuts it back.

axle
3
Joined: 22 Jun 2004, 14:45
Location: Norfolk, UK

Re: Is my turbo too small? (loosing boost high end)

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The Toyota Supra (my experience of turbo'd cars, inc a 1998 T61 equipped version), comes with a fuel cut feature in the ECU if it sees a boost value of over 1bar. An FCD (fuel cut defeater) is fitted to clamp the signal below the voltage that is fuel cut. Don't know if this is what you experienced but there are ways round it ;)

The larger DP will have removed back pressure and made it easier for the turbine to spin...this will also make it harder for the WG to control the boost as things (spool) happen faster. This better breathing would also mean it would struggle to keep the boost up as the engine's VE would go up.

Stick a bigger turbo on and let us know how it goes ;) :D

Oh and do check for boost leaks!
- Axle

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NickT
2
Joined: 24 Sep 2003, 12:47
Location: Edinburgh, UK

Re: Is my turbo too small? (loosing boost high end)

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axle wrote:That'd would present as a different set of symptoms. Lack of fuel would just see the engine expire! (Unless the ECU could detect it going massively lean). If the turbo wasn't undersized it would be able to keep pushing Xpsi of pressure right through the rev range.
Under normal circumstances yes but a lot of modern ECUs work closed loop with detonation sensors to and it may be automatically backing of the boost or retarding the ignition to protect the engine.

However looking at the graph againI'd say something else is going on. Thinking back my boss had a similar problem with his TT but didn't know about, he just thought the performance was a little flatter than he'd hoped when he brought it, ignorence it bliss. However on the final service before his warranty expired a mechanic took it for a test drive and discovered the problem. From memory there is a diaphragm in one of the sensors or actuators that works with the waste gate and it was perforated. When he got the car back he frightened himself coming off a roundabout, nearly stuffed it into a hedge :shock:

He's in Japan at the moment but I will ask him when he gets back or is next on Skype and let you know what it was.
NickT

bizadfar
0
Joined: 03 Jan 2007, 15:51

Re: Is my turbo too small? (loosing boost high end)

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I think you are talking about the diaphragm based bypass valves which are notorious for failing. Usually the TT one is pretty solid/reliable.

I actually think I have that problem now with mine :|

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Shaddock
0
Joined: 07 Nov 2006, 14:39
Location: UK

Re: Is my turbo too small? (loosing boost high end)

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Your results look typical for a mapped 180 engine.

However,

The 1.8T comes with a diaphragm diverter valve. These can wear and leak with the extra boost from a remap. They should not be replaced with a blow off valve (atmospheric) as the original ones recirculate air back into the airbox. This would throw a fault code and put you in limp mode. Forge Motorsport produce some piston replacements DV’s. and are well worth the investment.

Your N75 valve can wear and can be replaced with a ‘race’ variety, this is rumoured to hold boost better at the top end. You can fit a ‘boost machine’ which can reduce/eliminate wastegate creep, so you hold boost for longer. You can also use this to increase overall boost, but without changes to fuelling/timing you could run lean and melt a piston.

If you have an older model TT then the turbo intake pipe can collapse on itself with remapped levels of boost. Audi fixed this post 2003, although they can still be changed for a more free flowing one.

The back exhaust on the TT is very restrictive a ‘big kink’ as it goes over the rear drive train. You can expect up to +10bhp from swapping this out and a faster spool up time.

The MAF can be a problem, and should be cleaned regularly, but your airflow reading suggests that yours is fine.

Remember:

If you swap out the K03 for a K04 or a hybrid you will put extra stress on the internals. The 225 model doesn’t just have a bigger turbo and an extra intercooler but also has a stronger forged crank, conrods, pistons and pins and is safe up to 330bhp.