• Hello Guest, welcome to the initial stages of our new platform!
    You can find some additional information about where we are in the process of migrating the board and setting up our new software here

    Thank you for being a part of our community!

Cosbysweater's Turbo LS 244

Had to leave for a little, updated!

Awesome, thanks for the whole riveting saga in one megapost.

For others reading this - Erik's core issue was wastegate flutter. From the video he sent me it did sound a bit like compressor surge, but with a deeper tone and lower frequency of oscillation.

A good way to think about this type of wastegate issue is by visualizing the forces acting on the gate itself (draw a free-body diagram if you're an enginerd). Below is a handy exploded view from Turbosmart.

4779.jpeg


Forces acting on the valve are:
  • On the bottom face, exhaust backpressure (pre-turbine) times the valve face area - acting to open the valve, pushing left in the image above
  • On the top side of the valve face is the post-turbine pressure (or atmospheric if dumped to atmosphere) times valve area minus stem area - pushing right
  • On the stem, pushing right (acting to close) is the spring preload force when the gate is closed, or spring compression times spring constant k while opening
  • On the stem, pushing left and acting to open the valve, is control pressure times diaphragm area
  • Finally, depending on boost control configuration you can have pressure acting on the top of the diaphragm, times diaphragm area, pushing right and acting to close the valve

My take on what was happening below....

In Erik's test case, with both ports open to atmosphere the diaphragm wasn't doing anything at all to open or close the gate. Only the spring preload and the forces on the valve face were in play. With the 4psi spring the preload force is very low. As pre-turbine exhaust backpressure builds up in the turbine housing, only a very low pressure is needed to overcome the spring force and open the gate, releasing all the exhaust energy, killing turbo speed and boost pressure. As the gate opens and boost & engine power fall downward, the pre-turbine pressure would also go down, to the point that the wastegate starts to close again, under spring force. As the wastegate valve is closing the flow area is getting smaller between the valve and seat, which speeds up the exhaust gas velocity, which actually lowers the pressure on the valve face even more through Bernoulli's principle, so the gate slams shut. Now that the gate is shut the exhaust gas energy starts to build again, up to the point that the spring pressure is overcome again, and the valve starts to open a second time. And so on, etc. etc...this happens in cycles until Erik gets tired of it and lifts off the throttle.

The video was interesting for sure and got me thinking about wastegate control stuff that I haven't dealt with in ages.

By going to a smaller wastegate valve diameter the pressure forces on the valve is reduced, because F = pressure x area. Same gas pressure but reduce the valve area and the opening force goes down.
 
Gotta respect the amount of work you've put into this car. I wish I had 1/4 the tenacity you have. Nice work!
 
The next problem will be it slamming into the highway median when you smash the accel pedal. Good work!
 
here are some random pics from the whole fiasco...




with it non intercooled the hood wouldn't close, so ratchet strap to the rescue.



here I am mechanically checking fuel pressure




her is a 44mm mvr that I had on my s366 vs the 60mm. look at that difference!



I got frustrated so I tested the new drag wheel setup
 
At what point do you ditch the intercooler? Is it just too much air for unnecessary pipes, hoses, clamps or routing issues, etc?

I have a very small intercooler in the car, and I just wanted to make sure it wasn't a restriction. It's a 22x7x2.5. need to upgrade it but for now it will do.
 
I'm confused: Was the wastegate not functioning correctly because there was no boost/vac signal routed to it? (ie both ports open to atmosphere)

or

That the wastegate was being forced open regardless of spring and boost reference applied to keep it shut by exhaust back pressure?

I always assumed the spring meant 4psi of boost (applied to the gate, not exhaust pressure) to open it.
 
Erik can correct me if I'm wrong, but I don't think he was applying any pressure to the top of the gate to attempt to keep it closed. The "bolt mod" effectively does that mechanically - seemed like a good positive way to test performance with a known closed wastegate.

The flutter issue was happening because there was too much force on the 60mm valve trying to open it. With various different springs he was able to move that flutter point around but not eliminate it.

Again, Erik please correct me if I'm wrong!
 
I plumbed boost lines in every way possible, I ran reference to the bottom, top and bottom, just the top, and no reference at all. This is with both a 4 psi spring and about 30psi worth of spring. And it the issue would go from happening at 4psi with the 4psi spring with the vacuum routed in all of the above described ways. And with 30psi worth of spring it would happen around 15-16psi, with the same symptoms. With the bolt installed in the top I limited the wg travel to only a few mms and it was finally able to control the boost. Just too much surface area on that valve
 
Back
Top