wastegate confusion

[adi]

right.... im confused.

wastegates...

Lets say that the 7M wastegate is set at 8psi (is it? i dunno) .. and a wastegates job is to bleed off the pressure to stop runaway boost. SO, why then if you fit a freeflow exhaust, or upgraded turbo elkow, does the boost go over that? Or have i got it completely wrong and will the pressure *never* go over the wastegate setting,regardless of what you do to the exhaust etc and that perhaps the 7M wastgate isnt set at 8psi? 12 or something? (by the way i'm talking without the aid of a BC)

the only reason im asking is that my turbos are going to back turbo technics tomorrow morning to have the wastegate reset at 14psi.. and i'm wondering if im likely to go over that because i have freeflowing exhaust etc... I'm already rocketing over that anyway but thats because the wastegates are too small to work with the increased flow and are effectively being bypassed.. or is it.. i dunno.

adi

[Lofty]

This is a good question and one that I may not be qualified to answer but here goes anyway. Turbos are not only truly spooled by the velocity of the gases but really by the rate of expansion. When you have a restiction after the exhaust wheel the gases are not able to expand so easily and form a bottleneck for the gases following on behind. If you remove the restriction the gases can expand and escape more easily which in turn causes more intake air to be compressed, etc. On an efficient turbo/wastegate/exhaust setup the actuator pressure would be constant and you would see no variation. On the 7M-GTE Supra there are obviously some serious restrictions to be removed in the wastegate (turbo elbow) and the exhaust.
Edit: On the 7M-GTE/CT-26 the wategate cannot fully open because it will hit against the wall of the turbo elbow. Is that efficient? No.

[adi]

so, in theory.. if the wastegate is set at a certain level.. and provided its doing its job, then it shouldn't go over that set level so you're okay..


I really and truely dont know if when i get my turbos back that they will stop boosting at 14psi... suck it and see i guess.. i guess all i've lost is time and some skin on my knuckles.. oh and some money.

[Tony Donovan]

This is a good question and one that I may not be qualified to answer but here goes anyway. Turbos are not only truly spooled by the velocity of the gases but really by the rate of expansion. When you have a restiction after the exhaust wheel the gases are not able to expand so easily and form a bottleneck for the gases following on behind. If you remove the restriction the gases can expand and escape more easily which in turn causes more intake air to be compressed, etc. On an efficient turbo/wastegate/exhaust setup the actuator pressure would be constant and you would see no variation. On the 7M-GTE Supra there are obviously some serious restrictions to be removed in the wastegate (turbo elbow) and the exhaust.
Edit: On the 7M-GTE/CT-26 the wategate cannot fully open because it will hit against the wall of the turbo elbow. Is that efficient? No.
Would that make any kind of case for an external wastegate, then? Or is that a 'frying-pan-to-fire' scenario?

[Bondango]

External wastegates are "true" wastegates. they are used with Turbo's that do not use one built into them.
Becuase these turbo's run a lot off boost the waste gate has to be quick and effective so the external one is used.
These redirect a lot of exhaust gasses away from the turbo quickly and re-route these into the down pipe. If you go with a bigger form of turbo, chances are you'l need to run an external wastegate :bigeyes:

[MKS]

im not sure on this one either.. i assumed they would govern it to a certain extent. .but according to the standardf pressure.. if u take away that pressure then i guess it opens mroe or whatever..


cos i got a psi increase from just adding an induction kit...

[Lofty]

External wastegates are used when an internal wastegate would not be able to cope with the volume of air. It helps to combat an overboost situation where the air is forced to exit via the exhaust wheel and causes it to spin even faster pumping more air into the engine, etc. It is possible that this is what Adi is experiencing. A ported wastegate may help in this situation. An investigation of the turbo elbows would also be a good idea to see if things can be improved there too. Another possible solution if this were the case would be "clipping" the exhaust wheel to reduce it's surface area and allow more exhaust gases to escape without driving the exhaust wheel. Performance lower down the rev range would suffer though. This situation generally arises when the input wheel is significantly larger than the output wheel was designed to be used with.

[Lofty]

cos i got a psi increase from just adding an induction kit...
This is for a different reason. The air filter will provide resistance to air. A more free flowing filter will provide less resistance. The input wheel creates a partial vacuum to pull air through the filter. With less resistance, more air can enter. More air = more combustion gases = more energy for the output wheel = faster input wheel = more air, etc.
The engine and turbo acts as a huge pump. Removing resistance in the the inlet and backpressure in the outlet will mean that the pump requires less energy to overcome these problems, so more energy is available elsewhere.

[Adam W]

The wastegate, once fully open, can only move a set amount of exhaust gas through it's hole. Whatever is left of the exhaust gas HAS to go through the turbine. On a stock car, the wastegate would be able to bleed enough exhust off that the turbine never got above a certain speed (which would spin the compressor wheel fast enough to generate a certain boost pressure). Once you ditch the cats and put a bigger exhaust system on there, the backpressure from the system goes right down and the preasure differential across the turbo goes up - in simple terms, more exhaust gas is coming out of the engine more quickly at this point. Even with the wastegate fully open and bleeding off as much as it can, the remaining exhaust gas is higher than before so the turbine spins faster, generating more boost. More boost = more power = more exhaust gas produced = even more boost. aka runaway boost or boost creep. If you then fit a bigger wastegate it can bleed off more exhaust and keep the boost where you want it.

[formal]

Where is the belt that makes the turbo go? I can find the belt for the steering pump and the belt for the water pump but not for the turbo!

Why do I get more boost when the weather is colder? Is the belt slipping when it is warmer outside? :ewww:

[Adam W]

You're thinking of superchargers dude :) No belt on a turbo :yes:

[adi]

External wastegates are used when an internal wastegate would not be able to cope with the volume of air. It helps to combat an overboost situation where the air is forced to exit via the exhaust wheel and causes it to spin even faster pumping more air into the engine, etc. It is possible that this is what Adi is experiencing. yep.. thats a pretty good explanation of what is happening to mine.. the wastegate was never enlarged with the original builds (something they now do) and was too small to let the gases escape.. it's being done as i type..


On a stock car, the wastegate would be able to bleed enough exhust off that the turbine never got above a certain speed (which would spin the compressor wheel fast enough to generate a certain boost pressure). Once you ditch the cats and put a bigger exhaust system on there, the backpressure from the system goes right down and the preasure differential across the turbo goes up - in simple terms, more exhaust gas is coming out of the engine more quickly at this point. Even with the wastegate fully open and bleeding off as much as it can, the remaining exhaust gas is higher than before so the turbine spins faster, generating more boost. More boost = more power = more exhaust gas produced = even more boost.ah right.. got you.
So when asking them to enlarge the wastegates, do i ask them to be set at a certain pressure? How would they know what this will be? Surely when i ask them to be set at 14psi, i mean after the intercooler drop.. how would they know what to set them at in order for me to get 14psi at the inlet manifold? I just want to be sure before i make an idiot of myself on the phone.. :D

[darket69]

Where is the belt that makes the turbo go? I can find the belt for the steering pump and the belt for the water pump but not for the turbo!

Why do I get more boost when the weather is colder? Is the belt slipping when it is warmer outside? :ewww:


:rolleyes: How a turbo works (http://www.turbocalculator.com/turbo-works.html) :duck: :happy3:

[Lofty]

So when asking them to enlarge the wastegates, do i ask them to be set at a certain pressure? How would they know what this will be? Surely when i ask them to be set at 14psi, i mean after the intercooler drop.. how would they know what to set them at in order for me to get 14psi at the inlet manifold? I just want to be sure before i make an idiot of myself on the phone.. :D
They will probably enlarge or most likely just port the wastegates to as large a size as is practical. The pressure set on the actuators will be 14PSI at the turbo. Depending on the effienciency of your IC and piping you will most likely see less pressure at the manifold. This is why EBCs are good because they use manifold pressure as their source. The variation will probably be only marginal though. Why not get them set close to stock and get an EBC?