Turbo Choke and Wastegates Q

[talonDSMerr]

How does a turbo with a small turbine housing choke up an engine in the high RPM’s when matched with a sufficiently large wastegate? I understand that choke is the event where exhaust backpressure becomes too high and volumetric efficiency is lost, but with a wastegate that is large enough, wouldn’t it be able to bleed off enough exhaust gasses such that exhaust backpressure remains at a level where volumetric efficiency doesn’t decrease? Or am I missing something here?

 

[LiquidX]

Waste gates have nothing to do with the efficiency of the turbine matched up to the housing. It only bypasses x amount of exhaust away from the turbine. You will still have the same issue regardless of the wastegate size.

I'm not well versed in this field (yet) so I'll just leave my words at that.

 

[razrman8]

Just wait for the master to stop by.

 

[rlacasse1]

I'm going to throw this out there.

You're wastegate works based off boost pressure, not backpressure. So it's possible that the turbo is already hindering performance before the wastegate is fully open or open at all, depending on how much boost you are trying to run.

 

[cookmeup272]

A wastegate is a boost control device....All it does is divert exhaust gas away from the turbine so as to control the amount of boost the turbo produces. Choking is when you have too large of a compressor mated to a small turbine housing. The compressor sends so much air into the motor, that when it comes out the turbine can only use so much of it....since the turbine housing is so restrictive. I tried my best to put it into words.

 

[JusMX141]

A stock 14B's 6cm turbine housing has a 31mm flapper covering a wastegate hole that is roughly 26-27mm. This means that after the wastegate opens, when full boost is achieved, the engine is basically breathing through a 26mm hole.

If there were some way that you could use a T25 with a 66mm wastegate, you could have an insane amount of displacement / power while having the turbo spool at a low RPM. This is how a compound system works- you need good flow from the wastegate on the primary turbo because the vast majority of the airflow the larger turbo feeds will pass through the wastegate on the primary.

 

[talonDSMerr]

I always understood that wastegates work by giving exhaust gasses another passageway past the turbine to keep the turbine spinning such that a constant level of boost is achieved. Pressure difference between the exhaust manifold side of the turbine and the downpipe side of the turbine is what drives the turbine right? Wastegates work to control boost by relegating how much the turbine spins and the way it does that is by keeping backpressure constant, isn't it? So how could there be too much backpressure when choke occurs? If there was too much backpressure, wouldn't it in turn overspin the turbine causing loss of control of boost pressure?

I eagerly await the masters, I don't think it is as complicated as I'm making it...

 

[razrman8]

He has spoken.

 

[talonDSMerr]

So theoretically, a turbo with a small turbine housing to aid spool would not experience choke if it were matched with sufficiently sized wastegate(s)?

cookmeup: I see what you're saying, the turbine housing is too restrictive for the amount of air the compressor pushes BUT, with a large enough wastegate, the extra air pushed by the compressor that would usually bottleneck up in the turbine housing is now instead bypassing the turbine housing and leaving out the wastegate dump. So choke can be avoided with a large wastegate? is my question.

 

[rlacasse1]

A stock 14B's 6cm turbine housing has a 31mm flapper covering a wastegate hole that is roughly 26-27mm. This means that after the wastegate opens, when full boost is achieved, the engine is basically breathing through a 26mm hole.

If there were some way that you could use a T25 with a 66mm wastegate, you could have an insane amount of displacement / power while having the turbo spool at a low RPM. This is how a compound system works- you need good flow from the wastegate on the primary turbo because the vast majority of the airflow the larger turbo feeds will pass through the wastegate on the primary.

So are you saying that a 14B with an external wastegate would be good/benficial?

Or am I completely misreading that?

 

[JusMX141]

So are you saying that a 14B with an external wastegate would be good/benficial?

Only if you're using it as the smaller turbo in a compound, or if you're using it on a large displacement engine which would reach the choke point of the 6cm turbine housing very quickly.

However, the standard upgrade for a 14B is to use the 7cm turbine housing....giving the turbo better overall flow throughout the RPM range on our cars. I don't think anyone with a fairly-stock car has ever reached the choking point of a 14B. Now if you have a 2.4l stroker and you're setting out to run 30psi, a 14B would not be a wise choice for obvious reasons.

 

[razrman8]

Justin, please rub some of your turbo knowledge off on me.

 

[rlacasse1]

Oh, ok. Yeah, on a compound, got you.

 

[talonDSMerr]

Only if you're using it as the smaller turbo in a compound, or if you're using it on a large displacement engine which would reach the choke point of the 6cm turbine housing very quickly.

Hmm, but why does it matter whether or not the turbo is single or in series with another? Why would the exhaust gas go through the turbine housing when it can go through the wastegate instead, keeping backpressure in check and the turbo from reaching the point of choke?

I'm not implying it is wrong, I'm just trying to understand.

 

[donniekak]

Because a small turbo will not make 25psi without 40+ psi of back pressure. Lowering the backpressure lowers the boost. A large turbo can make 25psi of boost with around 25psi of back pressure, this is why larger turbos make more power with less boost.

 

[JusMX141]

^ Right; but lowering backpressure between the turbine housing and the engine via a larger turbine wheel or larger a/r turbine housing will also add lag, and this is generally something that is frowned upon for street-driven 4-cylinders.

It's a delicate balance, and Mitsubishi really doesn't offer a variety of turbine housings for our cars like there are available in the Garrett T3 platform. You can tune the amount of turbine flow a particular turbo has just by playing with the housing's a/r....going from a .48 to a .82 can add 10-12 lb/min of airflow depending on how close to flowing at the compressor's peak you are.

 

[talonDSMerr]

Because a small turbo will not make 25psi without 40+ psi of back pressure. Lowering the backpressure lowers the boost. A large turbo can make 25psi of boost with around 25psi of back pressure, this is why larger turbos make more power with less boost.

Thanks, this helped me gain a better understanding. I had to sleep on it last night but it makes sense to me now. By small/big turbo, I'm assuming you mean turbine housing.

A small A/R turbine housing routes the exhaust gases such that they hit the turbine blades at a more tangential angle, resulting in a shorter lever arm/less torque on the turbine. Thus, the small turbine housing requires higher backpressure to push a certain level of boost. As engine RPM’s increase, the engine is pumping more air meaning the compressor must push more air to keep the pressure ratio (boost) constant. This requires the turbine shaft to spin faster and faster as engine RPM’s increase. The driving force behind spinning the turbine faster is, of course, increasing backpressure (the pressure difference between before and after the turbine wheel). Plotting this on a compressor map, it would be a horizontal line of constant pressure ratio moving right as airflow increases from increasing RPM’s. Once this line reaches the choke line of the compressor is where and why choke occurs… A wastegate does in fact control backpressure (speed of turbine shaft), but the shaft speed a turbine in a small A/R housing must approach to feed an engine at high RPM’s is what induces choke.

I overlooked the fact that a turbine must spin faster and faster to maintain a constant boost pressure in the intake as RPM’s increase. I knew it was something simple! Thanks for helping me understand with all the replies