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Turbocharger wastegate operation [draft] 
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Joined: Tue Jan 15, 2008 2:31 pm
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Location: Home sweet home!
This is a work in progress, I'll keep editing it and remove this line when I'm done.

How a gate operates and the effect of various parameters


diaphragm area
valve area
spring seat pressure
spring rate
intake restriction from compressor housing to inlet valves
pressure drop and hose placement

Wastegates are NOT controlled solely by the intake pressure, they are controlled by intake and exhaust back pressure at a specific ratio that is the same as the ratio of diaphragm area to valve head area vs spring SEAT pressure and the two pressures acting on the two surfaces. it is this simple.

Basically, if your setup is mint, your boost pressure and back pressure will be roughly equal, but this is not a linear relationship on a given setup.

If your setup sucks, lots of restrictions, sharp corners, chinese coolers with square tanks, etc, the turbo has to work harder, and the back pressure is up, and more exhaust ends up still in the cylinders when the valves close, and you make less power, and your engine runs hotter.

The answer is of course, boost control to correct for these losses. However, with such a weak spring, its never going to be fully shut as the exhaust pulses are strong enough to push it open as they come out rather than go through the turbo (path of least resistance).


How to plumb/hook up your wastegate

There are only three regions that you can hook your gate control line up to.
  1. Between turbo and intercooler
  2. Betwen intercooler and throttle plate
  3. Beween throttle plate and inlet valves

The vacuum/pressure feed to the wastegate should only come from one of two places depending on turbo sizing relative to anticipated power usage.

In the case where the turbo is "larger" than you need, the feed should come from immediately in front of the throttle (2). By in front, I mean between the intercooler and the throttle and as close to the throttle as possible.

In the case where the turbo is marginally sized and will be highly stressed, the feed should come from the compressor housing (1).

The reason for these is that in the case where the turbo is bigger and the engine is most stressed, the boost should be kept under tight control at the engine to avoid engine damage. In the case where the turbo is most stressed, the boost should be kept under tight control at the compressor to avoid excessive compressor pressure, rpm and associated end thrust loads that could damage it. The reason that you get two different results from these arrangements is quite simple, your intercooler and plumbing drop some pressure at various flows. This is very much load dependent. I.E. if at 3krpm you get 15psi at the turbo, and 14 at the throttle, then at 6krpm you are likely to get 15psi at the turbo and 12 at the throttle. Whereas, if you had it plumbed to just in front of the throttle the pressure there would remain constant if all else in your system is right. In practice you will lose virtually no pressure across the throttle when wide open unless it is grossly undersized for your application.

Under NO circumstances should the feed EVER be connected to the manifold after the throttle. (3)


The diaphragm should not be exposed to strong vacuum as it places unnecessary stress on it and could cause a premature failure. (unlikely, but possible)


Consider what the wastegate sees when you lift off the gas at 140mph for that corner with the cliff next to it, the pressure in the mani drops (as it should) however, this removes pressure from the gate (as it shouldn't) causing the gate to try to close (which it shouldn't) and the boost pre-throttle to increase (which it definitely shouldn't), and consequently the pressure drop across your throttle is similar to what it was, but from a higher base, and boost in the manifold (if your setup is efficient) STAYS THE SAME, what this means is that you take your foot OFF the gas, and the car keeps accelerating, VERY bad news. This won't be very noticeable if your setup is fairly restrictive and average, but if you have a good flowing setup, it could easily kill you!

A side effect of this is that if you are running your turbo close to the limit, your boost will spike high as hell and potentially damage the turbo from overspool. In fact, if you are running your turbo ON the limit, and the engine has margin (usually the other way around) you should have the gate hooked pre cooler to protect the turbo from excess pressure requirements as mentioned above.


End Reasons.

In the mean time, if you don't believe me, see Tial's diagram here :


Discussion about off throttle spool etc with it wrong and right
Argument 1 : higher boost outside the throttle during lift off maintains turbine rpm and provides instant shot of air on reapply
Argument 2 : off throttle spool = close to surge line and turbine rpm = low anyway, so not much difference except that pumping losses will be a lot lower with lower outside throttle boost during part throttle operation. there will be associated fuel efficiency benefits from this.


Discussion about "throttle response" vs. turbo response vs. wastegate operation
Steady state torque to TPS relationship
Lifting when in boost
Laying into the gas from vacuum at various RPMs

Turbo,engine setup pair boost to rpm relationship

Wastegate spring rate and boost vs spool vs boost control


Effect of boost control on basic principles
Basically there is none with respect to plumbing, but ... TBC

Calculating the exhaust back pressure

Calculating the exhaust back pressure based on the known physical parameters of a particular gate and the boost pressure seen is fairly easily done. Here is an example based on the setup on my ute.

My gate came with a spring of unknown tension, so i applied 3.9kg load to it freestanding, and it compressed from 104 free length to 99. These springs are fairly stiff and large, so always be careful with them. ... tnG=Search

When installed in the housing it is compressed down to 41.3 + 16.8 - 8.7 mm = 49.4mm

From 104 with our 1.282 kg per mm spring rate gives ( 104 - 49.4 ) * 1.282
70kg installed seat pressure. (no wonder i had issues even with longer bolts getting it apart nicely)

The exposed diameter of the head of the valve is 39.5mm

39.5 / 2 = 19.75 / 25.4 = 0.77756 inches squared and times PI is = 1.9 square inches

For the gate to be fully open another 21.5 mm of spring needs to be compressed with 27.56kg of force.

The effective diaphragm is about 73.6mm in diameter

73.6/2/25.4 = squared times PI = 6.6 square inches

Now, if we combine the two areas it gives 8.5 square inches with 70kg * 2.2 for pounds = 154 / 8.5 = 18.2 psi assuming 1:1 inlet:exhaust pressure.

Because I made 17.5psi with that setup, we can calculate exactly what the exhaust manifold pressure was to achieve it. Remember, lower = better and more power. ... tnG=Search

20.2psi this is pretty good IMO. It's worth noting that with my 44mm wastegate sprung wide open the setup made 3.5psi by 7k RPM. This also shows how free flowing thing are on that ute. As you can see the response is not linear, but it's not too far off. It will certainly be interesting to see what sort of pressure I have in the exhaust at intermediate and higher boost pressures.

I hope you found that enjoyable :-) If you spot any typos or technical mistakes, please let me know in the comments thread above.



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Wed Mar 26, 2008 12:06 pm
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