This is interesting! Why is that the case? Would it be possible to actually measure turbo RPM, or to modify the boost gauge sensor to measure the output pressure?

 

I was thinking about this the other day...a turbo RPM gauge would be a very cool piece of bling.

 

egp is NOT essential. it is something to be avoided if at all possible, on all engines.

the thing is, it is impossible to separate egp from velocity and the desirable scavenging effects of the gas's inertia. they go necessarily hand in hand.

remember, it's all about increasing volumetric efficiency. the more intake charge you can get into the cylinder before the valves close, the better. in the NA engine, carefully constructed exhaust systems can use the pulsations of exhaust gas to actually create a negative pressure at the exhaust port while it remains open. this increases the evacuation of spent gasses, and increases the ingress of intake charge. ergo, greater VE and greater torque. because the pulsations are tuned via the dimentions of the exhaust piping, different headers will result in different ranges of rpms where this scavenging effect is most efficient.

the very same thing occurs in a forced induction engine, but it's a bit easier since the intake charge is literally being forced into the cylinder--but the egp must still be overcome while the valves are overlapped, which is why it is still detrimental to the FI engine. it is precisely this comparison of pressure ratios of compressor vs. turbine that determines how "good" a turbocharger can be, since it directly reflects efficiency. older turbos generally produce greater egp for a given boost pressure. trust me.. if a turbocharger could be developed which has zero egp and still produced boost, it would be the best thing in the history of internal combustion engines!

jm2c
ken

 

well, it's the case because essentially you're not really interested in what the turbo is doing by itself... you're interested in how the turbo is interacting with your engine. measuring the output pressure of the compressor is interesting, but it's not as important as knowing the pressure of the air actually going into the cylinders.

it essentially boils down to the product of the level of restriction in the intake tract and the total volume of airflow. make either one high, and you will see very disparate pressures at compressor vs. manifold, and vice versa.

during idle, with throttle closed, the turbo output pressure will be much closer to atmospheric than the pressure in the manifold, where on our cars it's about -20inHg. that's a good deal of vacuum! the restriction is high, the flow is low.

during WOT at lower rpms, when the throttle butterfly is open, the difference in pressures will be minimized--almost zero. here we have a low restriction, low flow condition.

as the rpms increase and the total air volume increases, we have a low restriction and high flow condition, and the pressures will begin to diverge again, with the manifold pressure being lower than the compressor pressure.

naturally at part throttle, mid rpm conditions we have some balance of the two factors of restriction vs. flow rate.

an rpm sensor for the turbocharger would be interesting, but just about impossible to do cheaply.

to measure the output pressure, simply get another boost gauge, and plumb it into the compressor output nipple, as opposed to the manifold. the commonly used BPV signal hose is fed from the manifold.

hth
ken

 

This is what I was saying...

restriction ----> back pressure (egp) ------> velocity of gas

Velocity is essential so some sort of back pressure and restriction are essential... I agree with you.

 

I think it would create turbulence, and slow the velocity.

 

If it gradually decreased from 3" to 2.5" you could maintain laminar flow, but just bolting a 3" pipe to a 2.5" pipe would make the gas basically have to go around to 90 degree corners. This would definitely create turbulence.