guys what % of max boost that a turbo can produce is the most efficient, im running 60% of max which is 1.3 bar on IHI Vf, just wondered when the charge temp offsets any gains from the higher boost i have a top mount intercooler from a later wrx on a classic 2.2 stroked engine cica 300hp when turning the boost up it seems to go really well for short blasts but then behaves like it's getting heat soak when pushing on for any length of time it was supposed to be mapped at 1.4 bar but it seems to go better for longer at 1.3 bar 60% of max 1.4 bar is approaching 70% of max !

 

I suppose it will depend on quite a few factors, Volumetric effiency of the engine for one, exhaust choice. As an example take 2 turbos both indenticle, one a hybrid of the other. They both boost to 1 bar but one makes more power? One has an rpm of 130000 and the other makes the same boost at 100000 rpm, this creates less back pressure and less heat soak, another plus point is there will be less heat soak in the incoming charge, a denser charge means more bhp. So in answer I dont know.

 

A compressor effciency map loks at boost (OK pressure ratio but lets assume atmospheric in!) and airflow, it could be that your IHI is in a good efficiency zone at mid rpm, but at the top end the extra airflow is getting it into hairdryer mode (no boost, lots of hot air) so you then may want boost mapped against rpm, the other factor is underbonnet heat management, lagging DP and turbo (and Upipe), as well as shielding the turbo to stop heat soak into the TMIC as mush as possible whilst encouraging the airflow from the scoop to keep it cool (if you have the extra vent over the tuurbo still available).

Simon

 

You need to look at a compressor map for your turbo and spend a few hours learning about how to read one and spec a turbo. It will all become clear. You are right in suspecting that turbos are not most efficient at max boost. For example, IIRC a TD05-16G can be run up to something like 1.8 bar but it's well below 50% efficiency at that kind of boost level. In fact I seem to recall that a 16G runs most efficiently at around 1.0 bar i.e. on a 2.0 Subaru lump, at 1.0 bar you'll get the best spread of efficiency across the widest rev range.

Running high boost (i.e. inefficient boost) isn't so much of an issue if you have an FMIC that will handle pretty much any kind of charge temp for however long it needs to. With a TMIC you don't have this luxury. I would say you'd be better off going for lower, more efficient boost and keep your charge temps down.

I had a VF35 which I think has the same compressor side as your VF30. Mine was mapped for 1.4 bar but that was pretty much the most you could safely do with it. The simple fact is that this is a turbo designed to cruise at 1.2 bar even though it may technically be capable of more. VF compressor maps are a pain to find though.