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What temp delta would indicate that the IC is good enough for the application ?
Temp delta=(Before throttle body air temp) - (external air temp)
That's a hard question as there is many other things which will come into play with the air temps like, what size turbo and where in the compressor map are you running (if you're pushing the turbo to hard it will generate a lot of heat which the IC won't be able to dissipate) what size intercooler piping, under bonnet temp, placement of IAT, ambient temps, IC unblocked surface area, wind chill factor etc
Typically while im on the dyno, during a run I like to see air temps stay below 35c peak, our dyno bay is typically around 15-20c although like I mentioned there is many contributing factors.
This is a screen capture from logging off of a WRX rally car with a 34mm restrictor (hence the higher post turbo temps).
As you can see, the temp delta between ambient (yes, it was a 40+ degree day when we where tuning the engine, this was done specifically for the environment that the car would run in), is very small, just under three degrees when the Post Turbo temps are close to 180 degrees, so this intercooler is the correct one for the application. I have also run the car in days that are below 10 degrees, and have had to block off some of the intercooler from airflow as it was running too cool.
I have also seen cars running the wrong intercooler for the application that had minimal or no variation between the pre and post intercooler temps, and I have even seen a water to air intercooler that had higher exit temps than entry, as they had plumbed the coolant pipes into the hot side of the engines coolant system, rather than having a separate radiator for the the intercooler.
The sensor locations are;
Ambient - in the air duct post air filter
Post turbo - approx 20cm downstream of the turbo compressor exit in silicon piping
Intercooler - Exit piping of the intercooler, approx 25 downstream
Inlet Manifold - In the central plenum of the inlet manifold, post throttle plate.
The reason for the Inlet Manifold and Intercooler temps being lower than Ambient is that the Inlet Manifold and Intercooler have a constant flow of air over it from the cooling fan, where the air filter doesn't.
If I see more then 25C delta between ambient and post throttle body can I conclude that my turbo is operating at a very low efficiency ?
either that, or your intercooler is not efficient
That's some inmpressive data blackrex! In my own testing, short of the drag applications I ran an ice water/air IC, I've never managed to get a temp delta anywhere near 2-3 deg C - That's truly impressive!
The outlet temp from the intercooler that you can hope to see will be affected by several aspects, but the key ones would be boost pressure, compressor efficiency, intercooler efficiency, and ambient temperature. Airflow into and out of the intercooler are also relevant. Essentially as you increase the boost pressure you're naturally going to put more heat into the intercooler inlet, likewise if the compressor efficiency starts to drop off.
To put some numbers on it though, in the majority of road car situations that I saw across our old dyno running perhaps 1.5 bar boost, I'd normally expect to see a temp delta between ambient and IAT of around 10-15 deg C. Much over that and it would be worth looking at the intercooler design or air flow through it.
I do recall we had a run of chinese intercoolers through our shop that were built with no real internal fins. I'm sure they were cheaper/easier to manufacture, however there was no internal surface area for heat transfer. One was fitted to a Subaru Sti circuit car and it was making very poor power and after a run the IAT was measuring around 65 deg C on a 20 deg C day. We swapped it out for an identical size intercooler which had internal fins and picked up around 50 kW atw (from memory). The IAT also dropped to a more acceptable 35 deg C.
Food for thought.