GT1549 Compressor

RX8 Project – Part 10, Turbo!

So this is about the time this whole project started getting a bit out of hand, when I decided I was going to need more power…significantly more.

I looked into what options I had –

Option 1 – I could stay naturally aspirated and probably skim the head to increase compression a bit and get more out of it but tuning in this way can be very intricate and looked to be more involved than I wanted for the amount of power I could expect.

Option 2 – Supercharger, there are a few options here. Realistically the most common supercharger these days the Eaton M45 found on the modern Mini cooper S is just too small for this so sticking with the positive displacement type we can get an M62 from a mercedes CLK230 and with the right pulley ratio it would probably be ideal for moderate improvements. For real degrees of silliness an M90 might well be needed and these are a little harder to find.

Option 3 – Turbo, this gives a huge amount of options due to the prevalence of turbo engines at the moment and would give potential for significant power gains comparatively cheaply and without needing to align belts.

After debating for a very long time the best way to go for a road car I settled on option 3 primarily for the simplicity aspect – I know very little about the intricacies of high compression engines and I know superchargers require a level of alignment very difficult to achieve with DIY manifolds! The next obvious question is how much power? Well following finding out from Noble that the rods in the engine fold up at something a bit over 300bhp I decide that from a cost and complexity point of view I’d aim for about 280bhp as a limit so I could keep the amount of parts I needed to a minimum – famous last words!

Now there’s a huge online argument about whether two smaller turbos or a single larger one gives the best throttle response and performance. This isn’t an argument I want to get into but in my case I decided twin turbo was the way to go for two reasons. Firstly because I could close mount them under the engine to keep the overall engine package as small as possible and so simplify the pipework on the exhaust side. Secondly because due to the government publicising the benefits of diesel there are now loads of small cheap turbos about for very little money..

Getting into sizing most of the information is that Noble used two T25 turbos. Taking a look at

We can see that for this engine at 6000 rpm and 0.7 bar of boost we need about 27 lbs/min of total airflow. Next we need the T25 Map for a common inducer size:

T25 Compressor map

Looking at the map for the normal T25 turbo we can see that with two turbos to share the load and so only needing about 13.5 lb/hr at 1.7 pressure ratio the turbo is right in its optimal zone. Not a bad choice all in all but these are old design turbos and as a twin turbo configuration the actual  amount of available exhaust will be limited so the turbo may not spool until a bit high up the rev range so I started looking at other options which would give a good improvement across a wider rev range. To achieve this a smaller exhaust housing was needed and this is where the diesel engines come in. Turbos used for diesel engines tend to have smaller exhaust housings for this very reason and they’re abundant. This led me to the GT1549, this is a manufacturer specific version of the GT1548 turbo, people have reported them to be good for 180-200bhp which is right in the area we want.

GT1548 Compressor Map

In many ways a similar map to the T25 but the spindle speeds are noticeably higher. The unit as a whole is much smaller but will have less weight in the rotating components and as a result of the smaller exhaust housing the turbo should generate boost at lower RPM. I used to have a map for the exducer which confirms this but have since misplaced it. Now before anyone tells me “you can’t use a diesel turbo on a petrol” consider this – this same turbo was used on both a huge range of diesel engines but also on the Saab 9-5 V6 petrol. That said there is also a VNT version of this turbo (GT15xxV), VNT turbos don’t last long on petrol engines by all accounts.

So here we are, the turbosGT1549 x2 :

So there you have it, a short post but a complete change in the direction of the project from where it started off and we’re only just getting started!



5 thoughts on “RX8 Project – Part 10, Turbo!”

  1. Nice to see this project coming along nicely with this awesome twist to it.
    Definitely looking forward to future updates.

    Still using the standard ECU or going custom with this?

    1. Hi,

      No I didn’t get the standard ECU for the V6 with it anyway but was always planning to go custom, that turned out to be a lot harder than I thought since I wanted to keep all the features, VVT for two intake cams, coil on plug, sequential injection and boost control is quite a list! I’ve ended up with a Megasquirt 3 with the expansion board which was about the cheapest way I could find of doing it all!

      I’ll try to do a proper post about the ECU and engine harness shortly!


  2. Just stumbled on your posts and found them really interesting. I’ve been planning using the same engine and gearbox in a project but a concern was would the gearbox cope with the increase in torque (especially if tuning the engine)?
    To date I haven’t found a definite answer and would be keen to hear your thoughts on this.

    Keep up the good work and look forward to your next post.

    1. Hi, glad you’re enjoying it!

      So the gearbox as you have evidently found is a matter of some debate on the internet. After an extensive search people seems to suggest it’ll cope with up to about 300 (400Nm in real units) but there is little real evidence for this. If we assume this is reasonably accurate for my application it should be ok as I’m not going for huge power but with more power (say the 3L version with turbos) you could quite easily exceed this if you wanted to. That said you would have a very fast car!

      From what I have found the gearboxes in the RX8 seem to fail most often during ‘spirited’ driving and experience catastrophic synchro failure. Interestingly shortly after coming to this conclusion I found this device . As such it seems to be often reported as very weak but consider this : I have also found that the same gearbox is used in the Ford Ranger pickup behind a turbo diesel – an application where it sees plenty of abuse with high loads but not the same slamming between gears – and while they do fail it’s usually due to wear at high mileage with very little maintenance.

      Based on all of this and the fact I can buy another for very little money I figured I’d give it a try! Your mileage may vary, good luck! If you beat me to it please let me know what happens!

  3. Yeah I’ve sort of come to the same conclusion to try it and see how the gearbox lasts as no one seems to know for sure.
    I do like the idea behind the synchro saver you found, I’ve never seen them before, will definitely be fitting one.

    Also I didn’t know the boxes were used in the Ford Ranger which is interesting as the brother in law had one and it took a fair bit of abuse.

    Looking forward to the next instalment.

    Cheers Euan.

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