Big turbo low psi VS little turbo high psi

[pl0x]

I just want to clarify

big turbo low psi = X cfm ,little turbo high psi = same cfm mean engine is under same pressure??

The turbo is working harder but spooling quicker?

[Guest]

Yes and no

Yes smaller turbo will spool sooner but will gneerate more heat into the intake charge in doing so = less power

Higher intake charge temps = engine more likely to detonate = sad times for engine

All turbos have an optimum flow vs pressure ratio at a given rpm

An example is a VF 22 at 20psi is ok untill 5800rpm, this is when the intake temp skyrockets

however same turbo at 17 psi is ok untill 6900rpm

The above figures are from a test done early last year and are approximate values from memory

(Just for those of you here that will now calculate exact flow and P:ratios then check the IHI flow charts !! )

Hence boost should decline in the upper RPM if turbo pefectly matched to mid range

My project car made more power at 18psi than 20psi with the same turbo !! in it's early years of development for this reason ONLY

[Guest]

 pl0x said:

I just want to clarify

big turbo low psi = X cfm ,little turbo high psi = same cfm mean engine is under same pressure??

The turbo is working harder but spooling quicker?

As long as the little turbo is not taken outside its efficiency zone - then the engine is "happy"

If you have pushed the smaller turbo to its limit or beyond to make a certain power level then under stressful conditions ie the track, you could push your engine into detonation.

A smaller turbo will have to spin faster to flow the same as a larger turbo. So in that sense its working harder. But should spool faster all else being equal.

[EJ20T]

having a smaller turbo on high boost will give a much more snappy and city friendly engine, if its overly small it will have poor top end. big turbo and low boost on a small engine is boring. altho will be smoother in torque cuvre, only large engines (RB30 etc) work well like this.