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107 | Analyzing turbo performance

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Very old to the party, but I just saw webinar 107. to recap, I grasp that when pre-turbine back pressure is lower that intake manifold/compressor outlet pressure, more power can be generated (and this stays through throughout the entire rpm range?)

am I right to say that this fact also allows us to pair more aggressive cams on a turbo that has lower exhaust manifold/pre-turbine pressure THAN intake manifold/compressor outlet pressure?

@Andre:

Secondly, in the webinar, slightly after you perform the ramp run on the dyno, you mention @ 58:22 that an exhaust pressure of 161kpa is less restrictive and there is relatively low exhaust back pressure. I do not understand why it is less restrictive and why it is considered to be low when it was higher than the intake manifold pressure of 150kpa. Could you please explain this?

You're correct, a IMAP/EMAP ratio below 1:1 allows for the use of more aggressive cams. In that respect for the want of a better analogy, the engine starts performing a little more like an N/A engine from a cam selection standpoint.

What I said in the webinar is that we have 'relatively low exhaust back pressure' which is correct. The balance is always going to be between power and response and this is a road/circuit car that requires good response. The fact that we're only just above IMAP at high rpm is an excellent result and most factory turbo cars will be in the vicinity of double the IMAp in the exhaust.

Thanks for the time taken to answer this Andre.

Do you have any material I can read to better understand why it starts operating more like an N/A engine at this point?

Wouldnt a ratio below 1:1 mean a lower IMAP value with respect to EMAP value?

Is the only way to measure EMAP using an exhaust pressure transducer?

What Andre probably meant is that NA engines do not have issue with back pressure and if you want to get more power you can just install cams with progressively longer duration for best performance at very high rpm ( when power is the ultimate goal). That might not be the case with forced induction applications when back pressure becomes a restriction from getting the most power stand point of view. However, if you'll solve the problem with excessive back pressure then you can apply the same cam selection strategy as for NA applications - the longer duration the better for getting most power.

And there is not much you can do with measuring back pressure but using a pressure sensor in your exhaust header.