Originally Posted by JRCivic1
Shodan, I wasn't aware that a waste gate could be "too big". Can you offer a practical explanation that would illustrate this phenomenon ???
Well, in the drag racing world, the idea of something being "too large" really doesn't come into the common thought process. That's like a drag racer saying .. "there's no such thing as too much power..
". There really is
such a thing, but the threshold is rather high in that type of motorsport before that logic comes into play. Because most drag racers are using larger turbochargers in which a wastegate larger than most street applications are used, (which thereby equates to the wastegate venting more than enough excess exhaust gases than the driver requires), I can see where the idea of a wastegate being too large doesn't really come into mind.
I'll give this a try. I may not be successful in explaining it without making
a video, (which I don't have time for), so ,please ask subsequent questions if needed.. ok?
Also, you'll see some internal edits for clarity of explanation, and the ability to post additional content as I get this together. so, please be patient for me.
Sizing a wastegate really has more to do with which size turbocharger is associated with its wastegate valve size, in addition to the boost pressure used in relation to the size of the turbocharger for the driver. The purpose (as usual) is most important, we have to really remember what a wastage is being used for; to control excess exhaust gas energy in order to control the amount of converted airflow that is entering into the combustion chamber.. Not just "boost".
But, before I attempt to create a simpler correlation, I'll first establish a few definitions that I'll be using to establish the general
relationship associated between a turbocharger and its wastegate
"Low boost" -
Generally a boost pressure range of about 1Bar (14.7psi or 2.05PR) or lower
- Generally a boost pressure range of about 1Bar (14.7psi or 2.05PR) or above
- A wastegate with an opening valve size of about 44mm -68mm or above, designed to vent larger amounts of exhaust gasses. (This includes the TiAL 44mm, TiAL MV-R, PTE 46mm, Turbonetics 45mm, and Turbosmart Hypergate 45mm and larger)
- A wastegate with an opening valve size of about 28mm - 41mm or below, designed to vent lower amounts of exhaust gasses relative to the turbocharger's ability to process. (This includes the TiAL 38mm, TiAL MV-S (which is actually 40mm) , PTE 38mm, Turbonetics 35mm (actually 40mm), the Turbosmart UItragate 38mm, Turbosmart Compgate 40mm, and all internal gates based upon the size of the turbine housing's opening)
- Turbochargers that are considered to be mid-framed to large-framed turbochargers that have airflow rates over 60lbs/min or larger. (This includes GTX3576R, GT3582R and larger)
- Turbochargers that are considered to be OEM-sized or smaller-framed to mid-framed turbochargers that have airflow rates of 60lbs/min or lower (This includes anything GTX3071R and smaller to the OEM-sized applications.)
Now, When it comes to the ability of a wastegate to control excess turbine exhaust energy, (as a practical matter in normal use and main forms of motorsport), you want to size the wastegate so that it is large enough to vent excess pressure in-between shifts properly to where you don't experience boost "creep" or boost "spikes" (I'm describing this without
getting into the realm of solenoids or other boost control methodology... That's for another discussion), and at the same time retain high-boost recovery in-between shifts with the valve closed.
If you go too large of a valve with a relatively smaller turbocharger, you now have actually created a condition in which possibly excess boost pressure as been vented, slowing boost-recovery time in-between shifts and the turbo's behaviour becomes undesirable. Go too small, and you experience boost creep and spikes relative to ignition timing, and you can easily effect the solenoids ability to regulate valve motion when it needs it most.
Full drag-racing cars utilizing full-throttle shifting, 2-step controls (one type
of anti-lag ignition timing control) and other devices need not worry about going too large, mainly because of the purpose of the car; high acceleration, larger mid-framed or large-framed turbocharger, with little need to vent excess exhaust turbine energy. Cars in other motorsports that utilize more partial-throttle control and have more requirements to assist with maintaining traction in possible more than 2 wheels, with higher torque expectations, have more to worry about than the drag racer.
For the drag racer, in some cases, they are running so much boost pressure with such a large turbo, the car will have an additional wastegate
to act as a blow off valve, because no Blow-off valve spring can handle the high boost pressure numbers that the car may be experiencing.. (i.e. 3.7BAR or more)
So, in the end, here's the correlation:
Large Turbo + Low Boost Pressure ---> Uses large Wastegate Valve or two smaller valves
Large Turbo + High Boost Pressure ---> Uses large Wastegate
Small Turbo + Low Boost pressure ---> Smaller wastegate valve
Small Turbo + High Boost Pressure ---> Slightly larger wastegate valve (Ventilating to the atmosphere vs. recirculation of the dumptube helps with boost-recovery , also
So, basically, wastegates are sized for the power output of the engine and turbocharger used. For the OP's example in which they're using a 52lbs/min turbo such as a T3/T04E 60 "trim" using a 58mm inducer/76mm exducer & 64.73mm Inducer exhaust wheel, 46mm shouldn't be to bad at all, as long as you don't keep boost pressure too low (i.e. less than about 10-14psi) otherwise, you may experience a bit more lag between shifts. This phenomenon can be offset with a good manual or electronic boost controller.
Now, here's where going cheap hurts. If the coil spring in the wastegate is compromised, or the exhaust gas pressure contaminates the metal in the valve itself (usually from an ethanol or ethyl-alcohol based fuel.. If the valve's metal isn't treated at the molecular level with an anti-corrosive). If that valve is compromised, it could seize shut, causing an immediate spike and cause destruction to the engine the turbo or both in a split second. No boost control safety or rev-limiter will prohibit the spike. Higher end valves (that yes, usually cost more) have higher tolerances and better valve treatments than the cheapos. That means they last longer and have less of a chance of destroying your engine. Is a PTE 46mm considered "cheap?".... Well, some say yes, while others say no. I don't know if I can answer that with any impartiality. So, I don't, and stick with TiAL, Turbosmart, or Garrett for my wastegates.