Boost creep and the solution [Archive]

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Damo

23-02-2009, 01:02

I have been looking into boost creep and how to solve it. Also to find out why it happens as I did not under stand it properly. On searching this forum after a bit of time I came accross this post by Mark from TurboDynamics which explains it really well and the solution :thumbs:

http://www.sxoc.com/vbb/showpost.php?p=4300967&postcount=105

Hi all

This is a topic that is very hard to explain without diagrams and pointing at things, however i will try my best.

lets start with the turbine side of the turbo. To stop the turbo from making more boost than desired the wastegate opens at preset level and all the gases IN THEORY should leave through the waste gate port, however this requires airflow to do a 90 deg turn through the wastegate port (please remember this point). however the design of a turbine housing is such that when air enters the housing all it wants to do is to keep going around the turbine wheel and exiting through the exducer of the turbine. Air will continue to do this and cause the turbo to contniue to make boost as it has no desire to make a 90 deg turn from its natural flow path to exit out of the wastegate this is known as boost creep.

now imagine if you put your hand over the exducer of the turbine and blocked the air that wants to follow its natural path out of the turbo, the air will now be forced to go out of the next easiest place which means leaving from the wastegate port. the same effect would be achieved if there was some backpressure in the exhaust, it would slow the exit of air from the turbine exducer and therefore making it just as easy to leave through the wastegate port as the pressures become more balanced in the housing and in the turbo elbow. This back up of pressure also keeps oil in the centre of the turbine housing and makes oil leakage less likely, which as we know oil leage would cause the car to smoke.

The design of an internal wastegate is, and was, always a comprimise. for it to operate correctly it needs back pressure which as you quite rightly stated slows airflow comprimising performance marginally. the ideal soloution if want to run with hardly any backpressure (in the form of a 3 inch straight through exhaust) and to not suffer boost creep, is to fit an un-gated turbo with an external wastegate.

As you said when a free up the flow, the car makes more boost, and when freed up too much they make too much boost, which is boost creep.

I hope this explains backpressure and its importance when applied to inernally gated turbos.

Mark

Scougar

23-02-2009, 12:03

Very good explanation, even I understand it :thumbs:

Makes a good point about back pressure and internally gated turbo's as well, and also raises the question of reliability of a - internally gated - turbo when running hardly any back pressure.

m.d.

24-02-2009, 15:55

very useful and simple to understand quote that, nice find

keggy

24-02-2009, 22:49

good read:thumbs:

overdrive

27-02-2009, 11:06

Good explanation! Explains why i have been recording over 1 bar on my boost controller now my cat is gone...

fixup1982

27-02-2009, 19:25

Very interesting must get my gt2871r welded up but then i do not know if i could chop my tomei manifold. :wack:

well done damo :clap:

fixup1982

28-02-2009, 10:54

Very interesting Damo i was thinking of welding the gt2871 shut but then i dunno if i could chop my tomei manifold. :wack:

rearwheelsteer

04-03-2009, 15:52

:nod:

richy200

16-03-2009, 18:53

m.d.

20-03-2009, 17:33

i thought that i would also add something ive recently discovered.

if you have a very large turbo you will normally experience boost creep when running at lower boost settings.
this is totally cured when the boost is cranked up higher.

the guy who is mapping my car told me that his impreza cannot run less than 1 bar boost without creep but when he upps the boost to the 1.8 bar he ususally runs there is no creep at all.
i also find that i get creep when running standard wategate pressure yet anything more its a perfectly flat boost graph.

so if you are experiencing creep try running the maximum boost that you will be running with your set up and see if you still experience it, you may find that the standard internal waste gate set up is totally fine for your set up this way. so dont worry strait away if you have just fitted a new turbo and take it out the first time on low boost and find you get creep.

the other option if you are generally going to be running lower boost settings is to run a smaller turbo and more boost rather than a too larger turbo and low boost.
running the biggest turbo you can is not always the best idea, a large turbo will need to have a much better flowing waste gate to hold steady at lower boost levels.

i hope this makes sence.

RADS_S13

31-03-2009, 17:02

makes a lot of sense, allso make it quite obvious why you get a boost increase by ust fitting a free flow exhaust and filter and FMIC.

aldo56

13-04-2009, 09:48

Brilliant explanation!

MattyGee

17-04-2009, 21:30

:o ignore me, didnt mean to post here

diamondsink

04-08-2009, 12:33

But doesn't the back pressure also act on the waste gate exit in the elbow. Surely the back pressure needs to be more at the turbine exit than the waste gate to have any affect?

wantedS13

11-08-2009, 19:04

You could also make an improved version of the divorced elbow.

You have these screamerpipe elbows for internal gated turbo's, and however they are gay, you could weld a pipe to that screamer and let it joint further up the line, at the downpipe. This should solve the creep problem as well.

dan23

01-11-2009, 20:13

would not a easier way to stop the boost creep to just fit a boost controller?

Polo

03-11-2009, 23:07

There are alot more factors to why boost creep occurs. On my 30/76, the WG hole on the internal gate was too small to run lower boost at high RPM, i took the exhaust housing off and used a die grinder to make the hole bigger, sorted the problem a treat:nod::thumbs:

diamondsink

19-11-2009, 19:51

Spent some time looking into too and got the out come that a external wastegate is really the only way to solve it properly.

DavS1RST

23-11-2009, 14:18

I thought (and am aware) that its either one of 2 things, there is a restriction in your exhaust (would need to be something like a 2" pipe in a 3" exhaust or something similar), or your wastegate is simply too small. Basically what mark says, but simplified.

I've found porting your wastegate where possible to be a good enough cure.

rodi200sx

13-01-2010, 10:55

i have found something about boost creep
( i hope that i say for same think???)

http://www.autospeed.com/A_111119/cms/article.html

i improve boost curve ( decrease lag???)
they fit long hose with reservoir before the wastegate actuator
it a good solution??

rodi200sx

13-01-2010, 11:02

excuse me
creep=== overboost
i don t understand

Kinugawa Turbo

14-02-2010, 10:13

good explanation!!

Kinugawa Turbo

14-02-2010, 10:14

Good explanation!!

martinbattye

19-02-2010, 11:18

Creep.. = excessive backpressure in the wastegate.. (no more airflow available)

overboost = a boost control issue caused by poor wastegate control

redsx94!

24-03-2010, 20:32

creep i suppose is a type of overboost i suppose but it's called creep because as the car goes up the revs the boost also keeps going up (creeping up). mainly because when you fit a really free flow exhaust the gas can flow through to easy and give less back pressure making the wrong balance of air at the turbo. the engine can put out more air now and it wants to get out. the problem is your wastgate can only flow so much so when creep occurs this is because its to small and can't cope with how much air is trying to get out so the turbine is the only other place it can get out causeing the turbine to spin faster and faster the higher you rev thus causing more and more boost the higher you rev

DavS1RST

06-05-2010, 00:59

Boost creep is when the boost rises at high revs because the turbo wastegate is too small or there is a restriction (exhaust too small for instance) which causes the turbo to keep producing boost uncontrolably.

Overboost is simple running more boost than you desire, like when you pull the hose off the actuator...it will overboost past the wastegate or boost controler setting.

EMENEAR

01-07-2010, 14:52

redsx94!

03-07-2010, 20:46

As above

shorty687

14-07-2010, 23:17

Think i got my head round this.. basically the less restrictive the exhaust system, the more it can flow, this allows the engine to though-flow a greater volume of air resulting in the wastegate not being able to deal with the volume of air passing thru/by it?? exhaust gas cannot escape and so continues to spool up the turbo creating more boost leading to a viscious circle???

have i got it right?

big_mac

16-07-2010, 19:55

creep would be reduced by a restrictive exhaust. A free flowing exhaust would cause the turbine to spin faster, not slower.

incorrect.

a restrictive exhaust (one which has measurable backpressure) will decrease the flow of gasses through the wastegate as there is not as big a pressure difference as there would be in a larger diameter/free'er flowing exhaust.

the reduction in the efficiency of the wastegate causes more gas to go through the turbo increasing its speed(and therefore boost) rather than being bled of and keeping the boost pressures consistent.

fit a larger exhaust and you will see that the boost will be more stable :nod:

Jezz_S13

19-07-2010, 14:12

incorrect.

a restrictive exhaust (one which has measurable backpressure) will decrease the flow of gasses through the wastegate as there is not as big a pressure difference as there would be in a larger diameter/free'er flowing exhaust.

the reduction in the efficiency of the wastegate causes more gas to go through the turbo increasing its speed(and therefore boost) rather than being bled of and keeping the boost pressures consistent.

fit a larger exhaust and you will see that the boost will be more stable :nod:

He's right.
A restrictive exhaust reduces creep.

martinbattye

25-07-2010, 19:01

plus 1..

think TT supra.. take CATs out.. = huge boost creep. thus many use a reducing ring in the exhaust to increase backpressure.

DavS1RST

28-07-2010, 15:12

plus 1..

think TT supra.. take CATs out.. = huge boost creep. thus many use a reducing ring in the exhaust to increase backpressure.

Taking the cats out increases boost, putting the ring in lowers boost. That's totally seperate from creep.

Creep is when there is X amount of boost, then at higher revs the boost creeps past set actuator pressure.

Again, cause = wastegate/exhaust too small, port wastegate and or fit larger exhaust, problem solved.

Rubix_Cube

30-07-2010, 15:33

+2 Andy...

I seen it for years on Starlets...Boost would be fine, take the cat out (which is on the Downpipe) and bang the boost would be mental. Best way to deal with it is to give the exhaust a little bit of restriction. Some folk think boring out the wastegate is the best way, I think this is an arse for tit way round about it myself.

EMENEAR

02-08-2010, 14:54

plus 1..

think TT supra.. take CATs out.. = huge boost creep. thus many use a reducing ring in the exhaust to increase backpressure.

Exactly. Even removing the CAT on a starlet gtt causes creep. A decent induction kit can even add to creep

Mec

08-08-2010, 17:23

redsx94!

31-12-2010, 11:47

Mmm, strange. I had alot of issues with the gizzmo ms-ibs causing boost spike and also creep.
Fitted an extra straight trough silencer and removed the db-killer, and guess what.
No more creep or spikes at all, and the killer wasn't even that restrictive with a size of 60 mm inside diameter.
So for me the less restrictive the less creep.
Just my 2p.

Cheers, Patrick.

I think the problem is you gizmo lol

I don't know how hard it is to understand that backpressure stablizes boost lol.

It's like commen sense.

Yes it does happen because your wastgate is to small but your wastgates aren't designed to take 3" free flowing exhausts.

superspat

04-01-2011, 19:01

i was running a gizzmo on my old starlet gt turbo and it never gave me an problems,i loved it to bits and it handled what ever i chucked at it, but did have massive boost creep in the gt, i got the waistgate ported and polished and solved the problem. do s13 suffer from boost creep a lot when adding bolt on mods, free flowing exhaust, de-cats, modded induction pipes and filters? sorry im a noob to the wolrd of 200sx

martinbattye

08-01-2011, 22:27

Taking the cats out increases boost, putting the ring in lowers boost. That's totally seperate from creep.

Creep is when there is X amount of boost, then at higher revs the boost creeps past set actuator pressure.

Again, cause = wastegate/exhaust too small, port wastegate and or fit larger exhaust, problem solved.

the ring is there to restrict backpressure.. which slows turbine.. and lowers boost..

the rings are to control boost creep not boost pressure..

GmasterT

25-02-2011, 22:46

Does higher boost encourage the exiting of the gasses discouraging creep, or is that something Ive blatently missed?

Joey200sx

26-02-2011, 12:03

Ok. Now I have my awesome japspeed shotgun exhaust I have a full 3 inch system and am experiencing boost creep, with the boost controller off the actuator is set to 16 psi. This holds for 2 or 3 seconds then starts to climb.

Where would I get these restrictors from to increase back pressure?

I don't want to take off my exhaust but I want to get my car mapped and don't want to have any issues with boost?

redsx94!

03-03-2011, 11:12

Does higher boost encourage the exiting of the gasses discouraging creep, or is that something Ive blatently missed?

I think this is true as creep can only creep so much. For instance my car set at actuator pressure when getting mapped went from 14psi at 3200rpm up to 19psi by 6500rpm. When we upped the boost past the point where creep was at in the first run it eventually flat lined. So now I have it at 22-24psi from 3200rpm and it just sits at that all the way to redline. If your car does not have the right means to run over the creep boost ie bigger turbo, injectors etc then you would have to put a restrictor in or open your wastgate port.