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"Cold air" intake for turbocharged engine

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Hello all,

I would like to tap into the collective experience of this group to determine if it is worth the effort to fabricate a "cold air" intake setup for my 1986 Porsche 944 Turbo with upgraded K27 hybrid turbocharger and VEMS engine management. As outlined in my new member intro post, I have adapted the factory airbox (which originally bolted to a vane-type airflow meter) to a larger 3" ID turbo inlet boot and enlarged the airbox outlet to match. The airbox lid takes in ambient air from the inner fender well, which I like, but I am afraid that it might be too restrictive at higher loads/boost levels (plus the intake air takes a 180 degree turn, see picture below).

I will soon experiment with a simple cone filter attached to the turbo intake boot to see if throttle response/VE improves, and this leads me to my question. I am well aware of the fact that a cooler, denser air charge means more power (a major pert peeve of mine is a stupid cone filter sitting in an engine bay and sucking in hot air), and this certainly makes sense in a normally-aspirated engine.

Of course in a turbocharged application, air is drawn in through the turbo inlet and compressed/heated before being cooled somewhat by an intercooler before entering the intake manifold. With all else being equal, would a "cold air" intake setup drawing in ambient air from outside the engine bay that is 30 degrees F cooler than what a cone filter in the engine bay would ingest result in a 30 degree cooler IAT/MAT, even after being compressed and cooled?

I have seen that OEMs like Porsche and respectable race engine builders alike go to great lengths to ensure an adequate supply of ambient intake air, especially in NA applications. However, in some of the higher-hp turbocharged engines in drag racing and drift cars, I have often seen a huge turbocharger or two in the engine bay with open inlets. At some point is the turbo inlet air temp mooted by the efficiency of the turbocharger boost piping and intercooling systems?

I would love to hear the thoughts of those with experience in the matter!

Attached Files

Remeber that a combined turbine/compressor efficiency is easily below 50% for a not insignificant part of its flow range and that 15 degrees C is 5% lower air density. You not only limit peak power but make less power with the same air if pumping to the same target manifold pressure across the operating range.

I've done intake system development before. Let me say this:

You're greatly limiting yourself if you don't install at least a minimum of temperature and pressure instrumentation. Ideally you would have pressure and temperature at the compressor inlet, compressor outlet, after the intercooler, and in the intake manifold. I realize that's cost prohibitive.

However, for pre turbo air induction systems only (since that's what we're talking about here) at the bare minimum you really should have temperature and pressure at the compressor inlet. Either have it drilled and threaded into the inlet casting or into the piping itself.

The temperature tells you if you're actually getting cold air into the turbo or not. Measurements further down the line in the piping system (intake manifold temperature and pressure for example) risk get mixed up with the efficiency of the intercooler. The inlet pressure, which will be from 0 to 1 bar absolute, tells you how restrictive the intake is. The more pressure drop you get as you boost (meaning: lower absolute pressure), the more restrictive your intake is. The lower that inlet pressure gets, the turbo work harder (increases compressor speed and outlet temperature).

I go into a lot of details about this here https://www.thewolfweb.com/message_topic.aspx?topic=617123&page=6 scroll down about half way.

So when you stick those hot air sucking short cone filters in there, the hope is that the benefits of lowered restriction (including more efficient turbo, which results in cooler compressor outlet temperatures) outweighs the drawbacks of the hotter inlet air. That's going to vary on a case by case basis.

For actual measurement one thing you could do is try to use a Bosch TMAP sensor (combined Temperature and 1 bar MAP) in your inlet to actually measure. https://www.bmotorsports.com/shop/product_info.php/xrf/gshp/products_id/2270?gclid=Cj0KCQiA2NXTBRDoARIsAJRIvLyY921ONeKMrrRZPw6rzzh3LvraqykgZ7eOj70bQQe6HgrYZ7fCh0waAmlHEALw_wcB Your ECU would need to supply 5 volt reference. Other options include buying something that can measure pressure transducers and thermocouples.

If you don't actually measure temperature and pressure in the piping system it's just guesswork. Using only a dyno to look at torque works better on an n/a car. For a turbo car it is better than nothing but still you can't figure out the impact of the varying effects of heat soak, intercooler efficiency, restriction, turbo efficiency, etc. In the attached image for example it shows a plot with x axis of mass airflow and y axis as compressor inlet pressure drop. Each set of line/points represent different stock air induction systems on turbo inline 4 cylinders. You can see the sharp curved ones. Those usually have tiny restrictive dirty side air ducts, small air boxes, or lots of bends in the inlet piping.

Attached Files

Thank you for the detailed reply, that is exactly the type of info I was looking for!

Running 5V reference to a TMAP sensor is no big deal, but I will have to check if my VEMS ECU has a pair of spare analog input channels to enable datalogging of the turbo inlet pressure and temperature (I'm already using a couple of them for EGT and oil temp).

I will get this set up and report the results here. Thanks again!

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