So, I've been playing with my boost controller, (blitz dsbc), trying to set it up for about 15 psi. I noticed that when the engine is freshly up to temp im getting more boost then when its been running for a bit (about 1 psi difference). Another thing and, the more annoying part, is that boost holds steady @ 14 psi until I reach about 6000 rpms then the boost begins to drop off all the way down to 10 psi once I reach 7000 rpms. I was wondering if anyone has any input as to what might cause this. Thanks oh its a 91 JDM TII and here is a list of my mods.


-hybrid turbo (not sure what the compressor is since I bought it like this)
-full turbo back exhaust (not sure of the diameter but It's atleast 3 in.)
-blitz intake
-chipped ecu, and probably injectors and such cause my afr's are looking good
-blitz dsbc

Thats as far as I know for engine mods. Since all this stuff was there when I bought it

thanks for your input

sean

 

the boost loss at temp is from heat soak. as far as bleeding pressure off, sounds like a boost leak somewhere. does it have an FMIC or the stock top mount?

 

i'm pretty sure that's just a characteristic of your turbo. i believe it's because the hot side is too small. but i'm no expert (i don't know why it does this)
sorry i couldn't be of more help.

 

Thanks for the responses guys. I don't think its a boost leak, cause it has plenty of power and holds boost steady until high rpm's. Someone else mentioned to me that if the hot side is too small it could cause these symptoms, which I guess makes sense. Since if it can't flow enough exhaust out of the engine then it won't be able to flow enough into the engine at that boost pressure, at that rpm. can anyone confirm this for me?

As far as the lower boost when the engine is heat soaked, what would cause this? I could definitely understand a power loss, but what would cause the pressure itself to be less? I would think that it would be higher actually since the hot air would have expanded more. could someone explain this?

I don't think there's anything wrong with my car per say, I was just wondering what could be responsible for the behavior I've noticed. thanks again!

sean

 

losing pressure because of the heat (in my head atleast) is because air is more dense when it is cold, there for when the intake temps rise, and the turbo is already working really hard, it just cant get out that extra psi. if the car has a boost leak it will still feel strong etc. the boost will just bleed off at high rpm's. you could test it by turning the boost down and seeing what happens. also go around and put soapy water on your couplers and have someone rev the engine. if there is a leak you will get bubbles. then again it could just be that the turbo is running out of breath.

put a big single and a haltech on the bitch and be done with it!!!

 

My boost also starts to drop at about 5700 rpm on my car. However I loose like 2 psi. It's on a T04R with .96 a/r turbine. I think it's probably a boost leak.

 

When I turn the boost pressure down to 10 psi, it doesn't drop off at high rpms as it had prior, therefore I don't believe I have a boost leak, though I guess it is possible and it doesn't hurt to check, and the small hot side explanation seems sufficient, lest someone can disprove it. A confirmation on this would be appreciated.

In regards to the heat soak and loss of about 1-2 psi across the entire powerband as the engine runs hotter.

whether the air is hot or cold I believe is irrelevant to the work done by the turbo. The turbo is controlled via pressure so, although there will be less oxygen content due to the lower density of hot air and therefore less power, it shouldn't open the wastegate until it has reached the proper pressure regardless of temperature. In fact I would believe that when the intercooler (yes I have the stock top mount) is heat soaked the pressure should be higher due to the lack of density change as it passes through the intercooler.

The drop in boost pressure when the car is hotter is obviously related to heat, though I believe your explanation as to why is inaccurate. If perhaps someone could add some perspective or perhaps an alternate explanation as to why the boost pressure drops as the engine presumably gets hotter, I would appreciate it. Thanks for the input!

 

if all you are going to do is say we are wrong, then why are you asking questions?

 

I'm not saying your wrong in fact I agree with the proposed small hot side explanation in regards to pressure drop at high rpms, I was just analyzing the information you provided. Which I appreciate. I just don't feel that lower density is an adequate explanation for a drop in pressure when the engine is heat soaked. It could be it just didn't make sense to me the way you explained why. So I was seeing if anyone else could provide perspective on your explanation or perhaps an alternate explanation. My intentions are not to offend you.

 

Engine cfm is proportional to (engine rpm x psi)

An engine consumes more cfm as rpm increases, a compressor will have a max cfm flow rate regardless of boost set.

At a fixed psi a turbo will need to increase the cfm to match the flow demand of an engine with increasing rpm. There will be a point were the compressor reaches max flow limits while the engine is still increasing in rpm. At this point the measured manifold pressure start to drop since the compressor can not match the increasing flow rate consumed by the engine with increasing rpm.

I think at 14psi you have reached max compressor flow at 6000rpm, there after the psi will drop with every increase in rpm. Your turbo will need larger compressor to run 14psi higher in the revs.

I would suggest you look at the option of running higher flowing compressor trim in you hybrid,

T04b compressor are most common with hybrid set ups, I have a couple dyno graphs with large trim t04b’s hybrids doing the same thing as your does at 14psi and 6500rpm.

A compressor with 20% more flow will allow you to hold 14psi for at least another 1000rpm
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since its a hybrid id say the turbine side is running out of flow too...

 

agreed, i ran into the same problem

 

Yea I agree also but that will only be apparent on the dyno as a lack of torque vs psi.... not a 'drop in psi'

 

Since I have no idea what the specifications of the compressor are, it's quite possible its running out of flow. Can anyone explain why there is less boost pressure when the engine is heat soaked?

 

good question, usually pressure drops when the air is colder... the compressor isn't really going to be an issue, it's the turbine housing that can't spool it. also it is an internal wastegate which aren't the greatest things ever. I'm pulling the back off my bnr tomorrow because i dont think the wastegate flapper thing is actually closed all the way, i can hear the damn thing flapping around a little bit during idle and when revving down. boost response... HERE I COME!!!!!!!!

 

I’m sure I have read some where that there is more pressure drop across an intercooler when the turbo/engine and set up are heat soaked.

I assume your wastegate vacuum signal operates of the compressor cover at its set 15psi. But the boost map sensor picks up its signal in the manifold after it experiences the pressure drop across the intercooler.