Turbo compressor map question for small displacements

I've done a little more research today, and apparently there are things called surge ports or MWEs (Map Width Extenders) that are machined into housings to help with low-flow/high-boost surge. They're the little ports in the housing on this:



What they do is allow boost to bleed from the back end of the compressor into the front again, recirculating the flow and reducing the amount a turbo has to compress the air (as some of it is already a bit compressed). Housings with these are readily available for sodding great turbos like Holsets or GT35s, but I haven't seen any for something suited to a 1.3l engine yet...

I've also been doing some very rough mapping and come up with these:



These maps are for a few T3s which is one of the only turbos that seems to fit most of the requirements relatively comfortably. I've done maps for 85%, 90% and 95% VE. The green line is for the minimum power I want to make (180bhp), the red line is for a hopeful figure (200bhp), and the blue line is where I'd really like to be (230bhp). The calculations are pretty crude, and are based on making peak boost by 4000rpm. Not sure how laggy a typical T3's going to be on a 1.3l Uno Turbo engine, but for low-rpm surge that might actually be a good thing. Not sure...

If we compare the green and red lines, doing a bit of porting and intake/exhaust work would help make it more efficient (requires less intercooling which is a good idea for an X1/9), but none of them will really support the 230bhp goal. The 95% VE map comes pretty close, but the turbo just runs out of puff at 6500rpm (with another 500 left to go to redline).

Another option seems to be a GT2259 52 trim 0.42A/R:



It'll sort the 180bhp figure as-is, but dips into surge at full boost in low-rpm. I don't actually know what this will do, any ideas? Does it just make less boost at that point, or will surging cause damage to the turbo (I'm expecting the latter). If I have a blow-off valve, will that area of surge just cause the blow-off valve to open around 4000rpm at full boost to vent extra flow (if it's set up right)?

Either way, it's pretty much the only turbo I've come across that seems to be able to achieve the 230bhp goal. All other turbos I've looked at either surge at low rpm, or are too small and choke at high rpm. 4000rpm peak torque looks like it just dips into surge, but perhaps that's fixable with one of these MWE ported housings...

I haven't actually seen any before/after compressor maps for a turbo with and without a ported housing though, so I don't know how they shift the map around.

So, I suppose the exam questions are:

1. If a turbo dips just to the left of the surge line, is that controllable with things like an external dump valve?
2. If a turbo dips just to the left of the surge line, does it just lag a bit and not build boost or will it merrily wreck itself by surging?
3. Does anyone know what a surge-ported MWE does to a compressor map? I know it extends the surge line. I also know it affects efficiency. IS there some sort of tradeoff between surge width and choke at the top end?
4. Anyone have any ideas for other turbos that might be able to produce high boost at low flow without surging? I was thinking about small-displacement diesel turbos, but not sure where to start googling for them.

Should have just stayed with NA tuning. I know what I'm doing there

I was going to reply yesterday to suggest you plot your desired torque curve on the compressor maps your looking at, but looks like you're way ahead of me! I'd suggest you plot a few more points, ideally every 1000 or 500rpm, for the various boost levels you're thinking of running. This should roughly show your surge margin on the way up to max boost (so from 2-4000 RPM). In OEM engine design, you tend to aim for 10% surge margin, but you can reduce that if you know the car is only every going to be driven at sea level for example.


In answer to one of your previous questions, the turbine side has no effect on the compressor map, but will affect the transient response (or spool time). A large turbine on a small compressor may mean you can never reach the upper left hand side of the compressor map as you simply won't have enough flow to spin the turbine to the desired speed at that point. Likewise too small a turbine may mean you end up choking the turbine before reaching your desired flow through the compressor. Realistically this will never be an issue as any off the shelf turbo will have its turbine and compressor fairly well matched.


Surge will damage the turbo, specifically it will wear the thrust bearing out. It sounds like classic turbo noises (shtuu tuu tuu!!) but you'll hear it as you're accelerating.

Ported housings extend the surge margin in the mid-lower left hand portion of the compressor map (but not the upper left hand part of the map). This is really only advantageous on large high flowing turbos where you want a large range of flow rates with a moderate pressure ratio. Or you have a big turbo and are running a high pressure ratio but want to start spooling it earlier without the risk of surge.

See this page for an example of ported and non ported housings: www.turnology.com/tech-stories/power-adders/understanding-compressor-maps-sizing-a-turbocharger/


For your application i doubt it will offer much of an advantage - it looks like you want a high pressure ratio but over a fairly small range of flow rates, much like a typical diesel turbo! (as i'm sure you've figured). A diesel turbo will probably be a closer match in terms of compressor flow (they tend to be tall narrow maps - high pressure ratios but fairly low flow rates) but you need to be aware that they cannot withstand the same exhaust gas temperatures. A typical diesel turbine will be designed to cope with around 750 degrees exhaust temperature, a high powered gasoline engine can easily run EGTs of over 1000 degrees (although I doubt an 80's engine would run that hot). If you go down this route I'd suggest you get an EGT probe and you may end up running very rich under high boost to protect the turbine. Or just run it super rich and hope for the best!


As for what actual turbo you should use, i've no idea! But you're certainly going about it the right way. I would say a compressor map is a must given you want to run so close to the surge line (and the max speed line in some cases) and you should re-visit the map once the engine is up and running and you can verify your flow rates. I will also say that the T3 frame is probably a bit big, and that the 'T' series turbos in general are a very old design - a GT22 or GT25 would be a better choice. Or there are of course several other manufacturers, Borg Warner (VAG, Porsche), MHI (Mitsubishi, Subaru), IHI (usually on subarus, some toyotas?) etc

That turned into a bit of a ramble, but i hope some of it was useful

Oh, the Ford Ranger turbo is a GTB22V in case anyone was wondering.

Edit: just read through the Turnology link and perhaps a ported compressor housing is what I want. Here's before and after maps for a ported housing on the same turbo:

Non-ported


Ported


You can see the efficiency islands have shrunk (as has peak efficiency: 72% before and 70% after). However, it has shifted the map to the left at the top which is what I'm looking for. Very interesting. I wonder whether the placement of surge porting has a big effect on where the map shifts around (would preclude amateur modification if it does).

The second map does look pretty similar to the GT2259 I'm looking at so there's a possibility it already has some form of surge porting (although most of the pics I've found suggest not).

So, if anyone's still following it turns out that the GTB2256VK from a Ranger is a variable vane turbo. No wonder it's got such a broad spread of efficiency. I wonder if aftermarket ECUs are able to control variable vane turbos...