Problem with turbos after rebuild/hi flow (R32 GTR)

[GtstGtr]

Hi, could anyone help with this problem please?

I've had the standard turbos reconditioned, when this was being done it was recommended to fit a slightly larger compressor wheel to each turbo & machining the housings to suit. The car is standard apart from air filters & a larger exhaust (HKS dump pipes, HKS front pipe, cat replacement pipe & 3in cat back)

Now when driving at a cruse/light acceleration (0 vac/boost) at anything below about 4,500rpm the recirculation valves are constantly cycling open close open close. When this is happening, if the throttle is opened a little more it gets worse (manifold pressure surges from 0 up to 5psi & back to 0 again & again)

if the throttle is opened almost fully, this surging stops & the engine begins to accelerate. It also seems to be running allot richer when accelerating with these turbos.

Most things have been checked & the only problem seems to be that the turbos are pumping too much air at the wrong time or something like that.

Has anyone had there standard turbos reconditioned and had bigger compressor wheel's fitted with no problems?

Does anyone know if something could be done to correct these problems without going back to original size compressor wheels?

Thanks

[Steve]

Sounds like you are experiencing compressor surge. Not entirely uncommon when people high flow turbos.

It is caused because the turbine wheel/housing is now miss matched to the compressor whee/housing.

Get the exhaust housing machined out.

If you do a search, this problem has come up before. Also, BOOSTD had a few problems in this area when he changed the compressor wheel on his turbo.

[Guest RedLineGTR]

Take them back to the shop.

[JimX]

Originally posted by Steve

Sounds like you are experiencing compressor surge.  Not entirely uncommon when people high flow turbos.

It is caused because the turbine wheel/housing is now miss matched to the compressor whee/housing.

Get the exhaust housing machined out.

I've been told that my power dip/surge problem is also possibly due to compressor surge. I get different symptoms though, it's all fine until about 5000rpm, then the power starts to fluctuate (and even then this problem didn't happen while the car was <190rwkw or so). No weird sounds or drops in boost, just alternating loss and gains in power twice before redline. Does this sound like it could be the same thing?

When you say to get the exhaust housing machined out, do you mean the bit that bolts onto the manifold or the bit that bolts onto the dump pipe? I had a look at mine when it was off and it didn't seem like the latter would accomplish much because I'd still be stuck with the smaller wheel (it would just open out to the dump pipe a few millimetres sooner).

And to do the former I guess the manifold would also need to be machined out to be of any use.

[BOOSTD]

Oh wow my specialty,

GtstGtr Your problem does sound like comp surge as Steve said. There are a stack of ways to get around it. Firstly depending on how bad it is. If it isn't to bad a small amount of efficientcy can be taken from the comp wheel buy machining a small lip into the snout down near the compressor inducer blade. This lets air escape past the blades when the turbo wants to surge. Will reduce flow but maybe only a couple of kw's worth. All standard GTs25T turbo's have this if you want to have a look at what I 'm talking about. Basically what is happening is your small turbine is spinning up the compressor faster than it was designed to which brings it on boost earlier, therefore into the surge zone of the compressor map. An adjustable cam gear can help you control how quickly the turbo comes on boost also. With my own turbo experimentation I found that my turbo needed to spool up a great deal later,1000rpm later in fact and there for the only way was to machine out the turbine snout--->increase the clearence between the wheel and the housing. Power increased by over 50rwkw.

What you want !

1. less inlet resistance, less outlet resistance and/or

2. More Lag

Fixes

1. Bigger A/R comp cover

2. Machine a grove into the comp snout

3. Control boost with adjustable cam gear or two stage boost controller ie 12psi till 4000rpm then auto switched to 15psi

4. Machine out turbine snout and leave original wheel (grealty increases flow and gives more lag, however less aggressive boost response as compared to a larger wheel)

5. Larger turbine wheel.....works the same as no4 but will give better spool up and cost alot more.

6. More restriction in the inake before the turbo would help to.

[TTT]

Steve has the problem spot on.

Redline GTR has the resolution spot on..

[BOOSTD]

Actually , having read the original post again. If you can get the blow off valves to bleed more air at part throttle you will most likely solve the problem. Might be an idea up grading to a larger flowind BOV.

[Sydneykid]

Hi GtstGtr, are you saying that they didn't they replace the ceramic turbines with stainless ones at the same time?

[GtstGtr]

Thanks for your replies everyone.

Compared with standard turbos would there be any advantages using these mismatched turbos with either the exhaust or compressor housings machined out to compensate?

The ceramic turbines have been replaced with some sort of steel turbines, as far as I know they are the same spec. The compressors that were fitted supposedly pump 10% more air each than standard.

[BOOSTD]

They will make more power , but, you will find boost would have to be bought on later to eliminate surge.

[Guest RedLineGTR]

if its a good turbo shop they should be able to tell u everything...what will work and what will not...the question u are asking they should know if they know thier stuff..if not i wouldnt of gone to them..my 2 cents might not be the case.

[blind_elk]

Originally posted by RedLineGTR

if its a good turbo shop they should be able to tell u everything...what will work and what will not...the question u are asking they should know if they know thier stuff..if not i wouldnt of gone to them..my 2 cents might not be the case.

If it was a good turbo shop, they would have got it right in the first place?

Another possibility is the turbos have been re-assembled "out of phase" (a mate had some on a 33 GTR, and they had been assembled so the wastegates weren't operating at the same pressure).

Watch the boost gauge when the problem is occurring - apparently, compressor surge causes the boost to jump all over the place.