OK, I'm not a truly-experienced Turbo Boy, but I do have a Mk1 Turbo and I have increased the boost by fitting a bleed valve. I have also investigated fitting a dump valve (a.k.a. blow-off valve). Here's some main points:
- "IHI VL3 turbocharger can't be adjusted" - not sure how you mean
It has a wastegate operated by a small pressure pipe so that the wastegate opens at about 0.7bar as standard. You could either modify the wastegate (bend the operating arm, fit some sort of spacers, or even take the wastegate off (I have heard of this being done without damage due to the quality of the Uno Turbo's engineering!) but I would personally not do these.
- If you are not getting more than 0.4bar as standard, sounds like you may have a pressure leak somewhere. Do a visual inspection of the boost hoses (connecting to the intercooler. I hope it's not the turbo itself.
- Bleed valve fits in the small pressure pipe to the wastegate and works by 'tricking' the wastegate to open at a higher boost pressure. It's easily adjustable up to the limit of the standard turbo, which is about where the mapping in the ECU runs out (1 bar).
- If increasing boost, don't forget to disconnect large round pressure switch on end of plenum chamber - this cuts out ignition at 0.86 bar.
- Dump valve is recirculating type as standard. This works on pressure differential set up when throttle is closed, and sends 'excess' boost back to input of turbocharger so that the turbo is kept spinning. This is technically the best solution. Unfortunately my dump valve seems a bit 'sticky' and with the increased boost I get a 'hammering' effect where the engine seems to momentarily just want to keep boosting as I lift the throttle sharply. This delayed opening of the dump valve also makes life hard for the airflow meter flap.
- Many people fit an external dump valve, perhaps to overcome the jerks and shunts but also to make a lovely 'look at Meeeee...' sound. If you want to try this, make sure you enlarge the hole in the main vertical metal boost pipe where the existing dump valve hose connects. Attach the dump valve directly to the metal pipe if you can (I haven't tried this yet).
- The risk with an external dump valve is that you'll lose the turbo-spinning benefit of the recirculating dump valve; you may get more turbo lag during gearchanges.
- I've decided to clean/repair standard valve, I think external dump valve is probably best for those people with larger turbos. Though I have to admit that a small part of me desperately seeks that cool blow-off sound.
- Larger turbos give more lag and potentially ruin the fuel economy as well. Turbo selection is actually a highly technical process, with the compressor wheel separately specified from the exhaust turbine. Get it wrong, and the result may be worse than standard overall. I've decided not to change. If you do change, you also need to think about increasing the fuelling - which means injector swaps and some sort of replacement ECU - not cheap!
- External dump valves come in single piston and twin-piston. Recirculating standard dump valve has a diaphragm instead. Piston types have a risk of leaking. I presume the twin-piston is less prone, but I don't really know the difference of single vs. twin. I suggest finding a small dump valve recommended for a small turbo application. Otherwise there is the question of how responsive the dump valve would be, and how much noise it would make off what is probably a small volume of air.
- Everyone seems to talk about Baileys dump valves being the best?
Executive summary:
A dump valve will not increase performance, since the Uno Turbo has a recirculating type fitted as standard. An external type intended for small turbochargers will make a nice noise if fitted correctly to the vertical metal boost pipe in front of the engine. Standard boost levels should reach 0.6 bar. A bleed valve allows you to safely increase the boost to 1 bar with the standard turbo. The bleed valve fits easily to the small pressure pipe on the standard wastegate. Upgrading the turbocharger itself is expensive because there are further upgrades necessary to the fuel ECU and injectors. It is a compromise between driveability, economy, and reliability which FIAT/Abarth worked hard to achieve in the original design.
-Alex
