Boost control.. Not via wastegate.

I'm in the process of sorting out a bypass valve for my compound  twincharged build. I'm ditching the mechanical valve i have and going with a servo controlled butterfly valve from a Toyota Previa. 

I'll be using a PWM signal from the ecu converted to voltage to the servo controller.

I intend on using a pwm boost control output in closed loop mode. But instead of the ecu handling an external wastegate it will be in control of a bypass valve in the boost side of the turbo/supercharger. 

I'll run no wastegate on the turbine. 

The servo bypass will vent air for both bypassing and boost control.

Make sense?

Opinions? 

Seems to me you'd be working that turbo awfully hard, it'll be trying to compress the entire world. Should decrease backpressure pretty nicely, though. 

so instead of slowing the turbo down by bleeding exhaust with a wastegate you are wanting to keep the turbo spooled and just bleed off the excess boost pressure instead?

It's inefficient and hard on the turbo but it'll work...very nicely, strictly from a performance perspective. Quick and precise control of the bypass valve will be important.

How will you keep it from over speeding ? 

scottdownsouth said:

How will you keep it from over speeding ? 

I too would be worried about spinning it too fast, especially if the turbo is “sized appropriately for the engine” by normal measures.

edizzle89 said:

so instead of slowing the turbo down by bleeding exhaust with a wastegate you are wanting to keep the turbo spooled and just bleed off the excess boost pressure instead?

 

 

Yes.. Got it

GameboyRMH said:

It's inefficient and hard on the turbo but it'll work...very nicely, strictly from a performance perspective. Quick and precise control of the bypass valve will be important.

My worry is not having enough resolution with the valve... Might be tough to control

therieldeal said:

scottdownsouth said:

How will you keep it from over speeding ? 

I too would be worried about spinning it too fast, especially if the turbo is “sized appropriately for the engine” by normal measures.

Turbo is over sized for the engine.. I think. The exhaust turbine is huge and running turbo only i can barely push 6psi at red line.

The supercharger that it blows into creates enough of a restriction to stop it over spinning. 

The supercharger that it blows into creates enough of a restriction to stop it over spinning. 

 So to be clear the bypass valve will be after the supercharger? If it was between the turbo and the SC it would remove that restriction and allow the turbo to overspeed.  

Vigo said:

The supercharger that it blows into creates enough of a restriction to stop it over spinning. 

 So to be clear the bypass valve will be after the supercharger? If it was between the turbo and the SC it would remove that restriction and allow the turbo to overspeed.  

Yes Correct. After the charger..

The bypass also vents back to turbo inlet, which is basically to atmo. Turbo never sees any pressure spikes at all. Its spinning all the time. If my bypass was plumbed back to between the turbo and the charger the Turbo would be in a more normal environment and blowing against a closed door when the throttle is shut. 

Currently the turbo never sees boost pressures above 12psi. 

Im running an 18cm/1.3AR housing on a 2.3L could i even over spin it?? 

In thinking about it more I doubt it would overspeed since that bypass being open would negate the boost from the supercharger as well, so you're basically looking at a low flowing na 2.3 worth of exhaust flow. At that point it depends on whether the valve would ever be open at WOT and high enough rpms that the 2.3 could overspin the turbo. If you are never WOT and over say 3000rpm with the bypass valve open i dont see how the 2.3 could overspin the turbo. 

I think regardless of what's happening post super charger the Turbo is just blowing into a charger that gulps 62cubic inches of air per revolution.

Geared at 2:1 that's 124 cubic inches per engine revolution.

Seems simple when you look at it that way? 

Turbo thinks it's bolted to a bigger cc engine and never blows on a throttle plate. 

I think I may have actually stumbled upon the answer unintentionally when I first got this thing going in the early days.
I was having trouble keeping the boost in after the supercharger. Due to bypass plumbing and spring issues so compounded boost was really low. Maybe 6-7 psi. 
But my boost gauge between turbo and charger was still building to 10+ psi.
Therefore its (pretty much) irrelevant what is happening post charger.
Its definatly spinning fast but I dont think it's over spinning.

So maybe it's not overspinning but if the turbo is having to make boost that dictates that it also restricts the exhaust in some proportion to that. If that's only occurring at throttles and rpms where you can afford to have a little backpressure that's not getting you anything then no big deal. 

So if there is no chance of it over spinning then why do you need to control it ? Let it make boost all the time ? Depending on the turbo, but I've seen them go from 30k to over speeding or snap the compressor shaft off in a second. It's hard to beat a wast gate controlled by boost. 

Vigo said:

So maybe it's not overspinning but if the turbo is having to make boost that dictates that it also restricts the exhaust in some proportion to that. If that's only occurring at throttles and rpms where you can afford to have a little backpressure that's not getting you anything then no big deal. 

Boost will always have some sort of associated back pressure. Lets see if the charger can compound beyond that back pressure with no wastegate on the turbo.

scottdownsouth said:

So if there is no chance of it over spinning then why do you need to control it ? Let it make boost all the time ? Depending on the turbo, but I've seen them go from 30k to over speeding or snap the compressor shaft off in a second. It's hard to beat a wast gate controlled by boost. 

That's the intention. Hoping the larger turbine housing will do its thing. 

It's always under load at higher rpm so can't freewheel to self destruction. Its like its on a larger cc engine thats never below say 25% throttle.

Air being brought through it by the charger keeps it spinning 24/7 but once the turbo really spools the charger keeps it loaded. 

Atm I get 12psi from turbo to charger and 27psi after charger. 

Im looking for a more friendly part throttle drive. Hence the servo controlled valve.

Boost will build super quick then start to vent at higher rpm. It'll sound mental.