Effect Of Bigger Compressor Housing. - Engines & Forced Induction

Hey forum I just have a quick question about a turbo's compressor housing. What would be the effect of a bigger compressor housing on a turbo? For example you have a 35r you take the compressor cover off and machine it out a little bigger and put it back on that same turbo what would be the effect on spool, cfm and horsepower? A friend of mine has a 35r he wants to sell me and he says the compressor housing was machined a little bigger. I just wanted to know if it is a worthwhile investment or if that mod just made the turbo worst?

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This is an area which I am curious about too and am not sure what to make of. Garrett don't seem to release different compressor maps for their different housing sizes for the same wheels - but in saying that I have definitely seen cases where people have made more power upgrading to different housings.

I know for sure that people have got power improvements with EVO 16Gs going up to a larger compressor housing size, the same applies to people with XR6 Turbos (going from the stock .50a/r unit to the .70a/r housing seen with aftermarket GTRs) - but at the same time a guy I know did an experiment with a maxed out Garrett "T04Z". The turbo was totally maxed out on race fuel, 770hp @ hubs from memory. He got the compressor housing extrude honed and put it back on the dyno for another go, attempted a full retune to suit and no difference anywhere - it was like nothing at all had changed. They couldn't push the boost any higher, nothing.

At this stage I am playing with the idea that its a case of getting the smallest scroll (will call it tube) possible to flow the amount of air you need, a smaller tube may allow pressure to build a bit quicker but then there will be a point it may become a bottle neck. If your tube is big enough then going up a size may make no difference at all, but if the compressor flow is being restricted by it then I'm guessing an upgrade would be beneficial. Maybe someone like Stao @ Hypergear will have some insight, as he's probably used similar cores in different housing combinations.

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Hmmmmm. So the general consensus is that it will increase lag a bit, but with the potential to raise horsepower and lower discharge temps? Also another quick question can your exhaust pipe/muffler affect your turbo spool? A friend of mine had his exhaust system replaced from the turbo to the catback and now car has picked up lag?

The first exhaust system he had was from the turbo all the way to the middle of the cat, one straight piece of pipe and the turbo spooled by - rpms, he now has a front pipe, dump pipe and decat pipe installed and that is the only change he made to the system and now he turbo spools by -.

That is about rpm's worth of lag picked up and we can not figure why. System has been checked for boost leaks, there are none and also no exhaust leaks pre-turbo.. Like I said only thing changed was the exhaust system from the turbo to the cat back. So could that have caused the lag he now experiences?

This might be obvious to most of you, but it's a very common mistake a lot of people make, and actually leads on to some interesting points others might not realise either, so this is worth a little feature...

While turbine housing size is hugely important, and plays a key part in how a turbo performs, on almost all turbochargers, compressor housing A/R isn't a performance issue at all, so don't worry about it, and stop quoting it, please.

The reason I'm writing this is because it's very common for people who aren't exactly turbo experts, especially when someone asks them what A/R the turbine housing is, or when they're listing one for sale, to quote the compressor housing A/R instead. The reason they do so is pretty understandable, as it's usually pretty noticeable on the front of the compressor housing, so it's much easier to spot than on the turbine, especially when still fitted to an engine.

While it's easy to spot, in 99% of applications it doesn't mean anything, and will be near as damnit optimal from the factory, and with no other options available anyhow.
For this reason, it's only really Garrett and BorgWarner who ever even print it on the housings- It's never mentioned on Holsets etc as they're all damn big and therefore pretty much perfect, and because of this there's no other options regardless.

Having said the above, and maybe it's a coincidence, but the only manufacturers where occasionally a larger compressor housing can benefit some turbos, especially when they're pushed hard, is Garrett and BorgWarner. 
On Garretts the ones that benefit are things like the GT28RS, GTR, and even GT30R, that have optional smaller 'compact' compressor housings available, which make them direct replacement turbos on things like SR20DETs etc. These smaller compressor housings aren't the optimal ones for flow, but they serve a purpose to make larger than normal turbos fit in the stock space, so it's a compromise you may have to make in some applications.
On BorgWarners, some S200s, S300s, and S400s it's an option sometimes too. Thanks to the HUGE amount of OEM engines they're fitted to, Borgs often have a variety of compressor housing sizes fitted to what's essentially the same size turbo, and the bigger ones generally perform better than the smaller ones.
With Borgs it's become such a big thing that these bigger, higher flowing compressor housings have become known as "RACE COVERS", as they're proven to perform significantly better, despite the fact they're mostly the OEM parts from JOHN DEERE turbos; yes, tractors and so on! Of course, "RACE COVER" sounds sexier and sells more parts than "TRACTOR COVER", despite the fact most modern John Deere engines have some of the best turbos in the world fitted to them as standard.

Better compressor housings for Borgs have actually become big enough business that true aftermarket ones exist too, and they're not cheap, but they look cool...