Stock Twins BPU Boost Query, not making enough boost!

[TheTurtleshead]

Hi Guys,

 

I have recently gone full BPU on my supra, and I have some abnormalities going on with the boost im producing. First, a quick rundown of my BPU Parts;

 

3" Mishimoto copy FMIC

Blitz Nur Spec R (N/AModel)

3" Whifbitz 1st Decat

2 1/4" Second Decat

HKS FCD

Custom Induction kit, ITG filter 83mm neck, 76mm aluminium piping, 2 stock cold air feeds.

 

Turbos are rebuilt J-Spec turbos but with steel fans and shafts.

 

Basically, until I wired in my FCD I was running the above setup with a whifbitz RR, at 0.75 bar boost. This I thought was low at this stage. Once the FCD was fitted, I removed the RR and took it out for a quick test run. I was expecting it to overboost to about 1.6bar(Just to test it! I wasnt planning on running this much boost every day!) , due to the small J-Spec wastegate, but it only reaches 1bar maximum?!? This is flat out in 3rd and 4th.

 

So basically I have an Idea why i'm not getting enough boost, the small second decat, the inertia of the large steel turbo wheels, and the increased backpressure of the N/A Nur Spec, but I though I might as well post a thread, just incase theres something I haven't thought of/ anyone else has another Idea!

Edited October 5, 2011 by TheTurtleshead (see edit history)

[hodge]

Think your 2nd decat may be the problem. If it's only 2 1/4 inch, a 1.2 bar restrictor ring has a 52mm hole which is only slightly bigger than that. Also with a 3" 1st decat you will be getting a bottleneck from 1st decat to 2nd decat.

 

I could be wrong here but worth looking at for sure mate.

[johnny g]

I'm also thinking the N/A exhaust might be your problem, as well as what Hodge said?

[Homer]

Basically, until I wired in my FCD I was running the above setup with a whifbitz RR, at 1.75 bar boost. This I thought was low at this stage.

 

Is the 1.75 bar boost a typo? Do you mean 0.75?

 

If the restrictor ring is removed and you still only see 1 bar there is going to be a mechanical issue. It's either an exhaust restriction or buggered turbo (and with rebuilt ones that's always a possibilty).

 

A 2.25 inch 2nd decat shouldn't restrict enough to keep it below 1 bar without a restrictor.

Exactly what cat back exhaust do you have? Can you post a picture of the back box looking down the rear silencer (there are some strange NUR exhausts out there... e.g. TRD-Rob has one which has a

[Tyson]

Basically, until I wired in my FCD I was running the above setup with a whifbitz RR, at 1.75 bar boost.

 

Bye bye turbos, bye bye engine You should be running 1.2 bar max on jspecs !! Get that boost down asap.

[BABZ988]

1.75 b

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+1 with hodge

steel blades doesn't built boost as quick as j'specs blades but you might have a restrition somewhere!!

Edited October 5, 2011 by BABZ988 (see edit history)

[hodge]

As homer said, 1.75 bar MUST be a typo error.

[TheTurtleshead]

1.75=Typo Its the standard Nur Spec for an N/A I was told it was a turbo system, did not realise until much later. http://www.blitz-uk.co.uk/ProductItem-2419.aspx My turbo's may be J-Specs, but I should be able to run 1.4 bar with steel fans, and even though they will spool slower, I should see the same final boost output. Im thinking it must just be exhaust restriction thats holding it back off to post a 2nd decat wanted thread now me thinks

Edited October 5, 2011 by TheTurtleshead (see edit history)

[Homer]

Okay, that's one item cleared up It's not your 2nd decat causing the issue, there's not enough restriction to limit boost *without* a restrictor ring in place.

 

As above, can you get a pic of the rear silencer, looking down the pipe?

[shaky]

Nice hks hi power cat back on here for £200 notes,rip the cat off,rr back on and your ready to go.

[BABZ988]

you said " until i wired my FCD .... ,at 0.75b "

that mean you were full BPU runing 0.75b ?

Maybe you have a leak somewhere .

I'm currently with a wifbhitz full BPU kit and with the same RR i'm usually overboosting to 1.3b! have done 1.5b when trying parrallele, just once!!

try parrallel mod just to see.

[Blackie]

Prob wont help anything but im only getting .9 on full bpu (3" exhaust all the way through) so im getting a boost controler to sort it out, I noticed you dont have one?

[Scott]

I reckon it will be the small decat causing the issues. A restrictor ring isn't far off that diameter, but the fact that the restrictor ring only has the small diameter for a few mm's thickness means it won't restrict as much as a very slightly larger diameter over the length of a decat.

 

Think of the flow restriction in this way. Imagine running down a tunnel as fast a you can. Halfway down the tunnel there's a hole thatyou have to crawl through. Crawling through the hole will slow you down, but only for as long as the hole is. Shorter the hole the less time you are restricted.

 

This is the reason I went against everyone's recommendation of a 2.5" decat being fine and made sure I got 3". In saying that, even 2.5" will be. Lot better than 2.25".

[Paul Whiffin]

I bet it's one of rubber hoses that join the 2 turbos together collapsed inside. Seen that a few times. Have you checked that?