View Full Version : Boost pressure drop at high rpm
My boost gauge connected to manifold showed 0.55bar @3500, dropping to 0.35bar @6500rpm. I suspected the turbo was crap, but today I connected boost gauge to the wastegate signal pipe and it shows me 0.65bar @3500 and 0.55@6500rpm. So, at 6500rpm I get 0.2bar pressure drop, about 35% - is it normal for stock intake system (incl. IC)? I could not imagine pressure drop is so huge... What do you reckon? :(
SX Drifting
19-11-2005, 22:15
your sig don't show a stock intake system?
Dont know much about evo's, what type of turbo is it? size etc :confused:
Jumping the gun here but I assume that if your running a stage 2 chip its gonna be around about T28 size and they do not drop boost any where near that amount, mine holds on until the limiter (1.0 bar) but I do have a FMIC and Induction kit so I guess your trouble lies in your stock parts - the standard intercooler is by all accounts very restrictive :nod:
With a T25 and the stock IC it's reckoned you get about 3psi (0.2bar) pressure drop.
Funnily enough I'm going to be in Latvia next week if you want me to take a look :wack:
In my sig these are specs of my old car. This one I am talking about is completely stock, except that turbo is T25G, so a tiny bit larger.
So, do you guys think it is normal to have 35% pressure drop??? Anyway I will clean it today with petrol if it won't be freezing cold
Pete C: cheers, but I don't know what could you see in there :)
But I don't mind having a beer with you if you're going to Latvia - not many people from UK do it :)
What did you forget in our country? :)
You'll always have a bit of a boost lean off at the top end its to do with the speed of the engine and the balance out of gases acting onthe turbines or something like that! anyway its normal!!
The boost will start to drop at higher RPM due to the fact the actuator is plumbed into the turbo housing rather than the plenum. Do a search for 'Mega FBU' and you will find a very simple guide on how to make the change. Using the 'Mega FBU' mod, the car will hold boost right through the rev range :).
is it not down to the fact that the turbo cant supply anuff air to keep the boost up at high rev's??? i get a nice drop over 6000 i've got a t25h running 15psi with volvo fmic
damo
You'll always have a bit of a boost lean off at the top end its to do with the speed of the engine and the balance out of gases acting onthe turbines or something like that! anyway its normal!!Don't know what you call "a boost lean off", but I am mainly concerned about 35% pressure drop between turbo and intake manifold.
intercooler crap ? intercooler pipe cracked and leaking at higher air flows ?
The boost will start to drop at higher RPM due to the fact the actuator is plumbed into the turbo housing rather than the plenum. Do a search for 'Mega FBU' and you will find a very simple guide on how to make the change. Using the 'Mega FBU' mod, the car will hold boost right through the rev range :).Again, I'm mainly concerned about intake system pressure drop. I know I can get constant boost in intake manifold by connecting actuator to manifold or just before throttle plate, but that will overrev the turbo, I'm not sure I want to do that :rolleyes:
intercooler crap ? intercooler pipe cracked and leaking at higher air flows ?One of two variants, but how to check them? I don't want to spend money on welding bungs to every pipe to check pressure...
SX Drifting
20-11-2005, 17:15
Through the standard intercooler this is normal. Mine does it. 11PSI dropping to 7 at the top of the rev range.
The only reliable way to stop it is to change the intercooler as I will be at christmas.
The mega FBU does stop it as well but it works the turbo hard to force the pressure through the intercooler. IMO this reduces the reliability the the turbo.
The Saab and Volvo intercoolers do help but IMO the best intercoolers are bumper mounted like the Apex unit.
its normal.
its to do with the t25 dropping off efficiency and the standard wing mount cooler being crap
its normal.
its to do with the t25 dropping off efficiencyHow?
P.S. My previous stock S13 definetely wasn't loosing boost like this :no:
I tried to take off IC yesterday to clean it but half of bolts broke so couldn't finish it. Will try today
did u have a stock ic on ur other s13 also?
did u have the same exhaust and manifold system?
basically at high rpm its really just heating up gas, and its spinning too fast to be efficient.
And if u have stock exhaust then that will be choking it also
It had same everything except CAT was empty inside. So yes, same IC, and it didn't drop so badly.
I again wasn't able to remove my IC yesterday :( Melted 2 drills to remove sheared bolt and nothing...
At first of all things: for boost drop at high revs is responsible turbo itself. If you using too small compressor to hold target airflow in high engine revs, you wil allways get pressure drop. Just look at the compressor map of your turbo and you will easy find if it is capable to hold targeted pressure at your engine´s airflow and rpm. For example, airflow of ca18det at 6800rpm and 1bar of boost pressure is about 27 lbs/min. For this airflow you will need at least T3 super60 trim compressor to hold this pressure (airflow), and this is ONLY 1bar!
Next thing is restrictions in intake and IC. This will always cause some pressure drop, dependig on quality of IC and air filter.
Then we have restrictions from intake cam, exhaust turbine, exhaust manifold etc....which in result will cause restrictions at trubo compressor rpm and its airflow.
My opinion is that your boost drop is from your turbo....
Guys... I don't know whether you read my first post, or maybe I didn't write it clear enough, but I am worrying about pressure drop ACROSS INTERCOOLER AND INTAKE PIPES. If turbo pumps 1 bar and intake manifold sees 1 psi I wouldn't blame turbo for that! Now of ~8psi@6000 at turbo I get only about 4.5psi in the manifold, and that's crap.
i have , FMIC, walbro, jez stage 1 and the rest is prety much stock and on the 1st,2nd,3rd ger i have 1bar till the red line only on the 4th gear i have a big boost drop 0.5 bar but i know the reson for that big presure drop and i-ll fix taht soon (it is because the way i used to increse the boost), wehn it was stock it holded all the way 0.6 - 0.5 bar, so something must be wrong in ur case, mabe iu ahve an BOV or a very dirty IC (did ur turbo lose oil and it is posible that some oil reached in ur intercooler and made him dirty), or maybe a conection is not tight up properly and u lose boost...that presure drop is not normal from my point of view
Now of ~8psi@6000 at turbo I get only about 4.5psi in the manifold, and that's crap.
The 8psi you get at the turbo is probably measured in the wastegate actuator line, which is connected to the nipple on the T25 turbine housing.
If you want to know the pressure at the turbine outlet, you'd better connect it to the black steel tube that runs along the exhaust can cover (to the IC). You will be surprised that you can see a completely different figure of pressure versus RPM. Pressure measured at that point will drop a lot more at high RPM than the one that you measure at the nipple outlet.
ByReal: yes, prev.turbo was burning oil, that is another reason I suspect my IC :) I will try again to take it off today and clean it with petrol.
And yes, I also think it's not normal :)
JCD: both actuator and 2nd boost gauge are connected to black steel pipe :) So I'm measuring real pressure at turbo outlet :)
both actuator and 2nd boost gauge are connected to black steel pipe :) So I'm measuring real pressure at turbo outlet :)
:thumbs:
Sideways Danny
22-11-2005, 18:06
I'd say it's a crappy IC doing this
It had thorough clean today inside and outside :) And now it goes into bath - don't want to spend extra money on professional checking :)
Sideways Danny
22-11-2005, 23:23
don't bother spending time on the standard IC, it's just bad
Spent already :) Will straighten fins a little bit and see what difference it all made :) Hopefully tonight
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