kantdrive55

Here is an explination from our Turbo Engineer JWT.


TURBOCHARGER WASTEGATES

Waste gated turbine housing is designed to give faster low end response and stabilize boost pressure when the gate is opened.
To understand waste gating we must remember that a turbocharger is a turbine driven centrifugal supercharger, this means the turbine must develop sufficient power to drive the compressor wheel as pressures and flow increase.
The exhaust gasses leaving the engine are pumped out by the piston on its exhaust stroke; this creates pulses of superheated exhaust gas that drives the turbine.
On a well matched turbocharger turbine pressure will be lower by 5% -10% than the compressor side.
Opening the wastegate diverts some of this exhaust energy around the turbine so that the compressor boost pressure can be stabilized at a constant pressure.
Some turbochargers are waste gated on one volute and some on both;
If the wastegate is opened during acceleration as engine rpm’s climb the compressor speed and flow must also increase to supply the engine demand; (Flow is load) even when maintaining a constant pressure.
If the wastegate is opened too early a portion of the exhaust energy is diverted; and the volume of exhaust gases to compressor ratio is now lower, therefore the turbine exhaust pressure must increase to supply the compressors power demand, and this will cause a negative delta pressure too develop.
(Negative delta pressure (-d/p) turbine higher than compressor)
.
WHEN THE WASTEGATE IS OPENED EXHAUST PRESSURE WILL RISE AND BECOME HIGHER THAN THE COMPRESSOR PRESSURE.
ON OUR TURBOCHARGERS THIS WILL SHOW ON BOTH VALUTES; BECAUSE IT IS WASTGATED ON BOTH, ON THE HOLSET TURBO PRESSURE WILL INCREASE ON THE FRONT THREE CYLINDERS BECAUSE IT IS WASTGATED ON ONE VALUTE.

THE SUPER B WILL NOT PRODUCE A NEGITIVE DELTA PRESSURE UNTILL THE WASTEGATE IS OPENED.



If you are using a turbocharger with a single volute waste gate, the none gated volute will have take over a larger portion of the work load when the waste-gate is opened.
If the wastegate is opened too early extremely high pressures can develop on the none waste gated side. In the case of a inline six cylinder engine the load would be on the front 3 cylinders; on an engine with valve overlap it will cause a large drop in engine power, this is because a negative delta pressure will not allow complete scavenging of the exhaust gases.
On a turbocharger that is gated on both volutes the drive pressure will be divided equally on both; but a negative pressure will still develop if the wastegate is opened early before reaching the desired Max pressure.

Mike Holmen

Ok Pat, sounds good. How was the truck set-up? You running an exhaust manifold? Where was the thermocouples and pressure taps put into the exhaust manifold? I agree with you, but you have to keep the drive pressure at 1:1. At some point you have to open the wastegate to bypass the secondary, its a choke point. Yes, you can put a larger turbine housing on the secondary, but most people want everything fast spooling, tons of air. You open the wastegate to bypass, off load the turbine of the secondary. Thanks buddy, I'm learning, you guys have done tons of turbo(s). You guys do things that are differently, but thats ok. You put in too large of turbine housing, why put in twins. The primary just doesn't light, it actually hurts more than help.

kantdrive55

This isn't specific to a truck that we built that had our turbos on it. This is the way any true compounding twin setup will work. If you phone Borg Warner, Garrett any of the large manufacturers they will give you the same answer.

If the turbos are matched correctly you dont have a choke point with the secondary charger. It will flow enough.

Mike Holmen

Ok, I have to ask this. How much boost do you run on the primary and how much do you load you're secondary? It the law of thermodynamics, you're extracting out energy with you're secondary turbo (doing work with the exhaust energy), once you extract the energy with the secondary, there isn't alot left to drive a large turbo up into its pressure curve and get a decent increase in mass flow. There is only so much energy available in the exhaust, once its gone, its gone. You're talking about two different things. Yes, opening both sides of turbine volute helps exhaust flow to bypass the turbine and balances out the turbine pressure. I don't know why manufacturer's only open one side of the turbine volute. I run a 1/4 open spacer between the turbine and exhaust manifold, which equalizes the pressure some what. I can see the way in which you're setting up you're twins, you're really only loading up the secondary and hardly using the primary, which in my mind, defeats the purpose of twins. Its been sometime since someone has debated the different ways on running twins and there set-up. You can run a efficent wastegate bypass by the secondary, you're not really lossing that much energy. Look at STS turbo system, they put a turbine at the back of the vehicle, not under the hood.

kantdrive55

Mike, This is a quote directly from John our turbo engineer.


When compounding the top or secondary turbo is the main part of the system, it should have good response at the bottom end and it should be a good match to the engine.
The lower turbo will always be larger it has to raise the pressure into the secondary turbo that would normally be 14.7 at sea level; this pressure is multiplied by the centrifugal compressor; depending on the wheel speed it can be increased up to 4-5 times.
If the lower turbo is producing 25lbs of boost the total pressure into the second stage will be 39.7 lbs including atmospheric pressure. This will then be multiplied by the rotating compressor wheel. If it is multiplied by a pressure ratio of 2 the total pressure would be 79.4 lbs. deduct 14.7 lbs from this the boost pressure is 65.4. It is better to Maximize the upper turbo: if it does not produce a negative pressure when used as a single turbo it will not when used when compounding. Providing the wastegate is not opened.
The lower pressure on the bottom turbo will allow it to pass through choke without over speeding protecting the lower turbo.
This can only be done by using the correct size outlet from the lower turbo the correct size piping and inlet on the top.

JT.

Mike Holmen

How are you guys loading up the primary so hard, without opening the wastegate? I might see 25psi+ on a hard wot pull and I would see around 60 to 70 psi on secondary. You guys have to try a set of twins using the garrett ball bearing. They are the ticket to fast spoolin twins and tons of air twins. I have to get out to Van sometime to see how the shop looks. Its been sometime since I been there. I still like playing with the Hx-40, just cause its simple turbo thats pretty tuff unit. If you break, its cheap to fix. There are so many configs on that turbo, its hard to find the perfect set-up for every truck. Some trucks want more turbine housing, some want less.

kantdrive55

When you open the wastegate to drive your primary with a single volute wastegate you end up driving that turbo with only 3 of the 6 cylinders. You still have a negative delta p on those 3. If you leave the w/g closed you have all 6 cylinders driving the charger which is going to be much more efficient.