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What was going on here?

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Chumpaumpalumpa

Proven Member
1,554
303
Jun 16, 2014
La Habra, California
So in an endeavor to learn what was going on with the car, I have some questions about the turbo system. So my turbo is internally gated and vents to atmosphere off the O2 housing. Its obvious when it opens because it gets much louder. I had a Holset HX-35 actuator on it and bench testing it with an air compressor, it would START to open at 20 PSI. It would not FULLY open until 30 PSI. I am running 22 PSI. The turbine housing was cracked pretty bad (pics here http://www.dsmtuners.com/threads/what-housing-can-i-use.497022/ ) and there were a couple small pre turbo leaks and one very small one on the IC piping where the nipple for the feed to the MBC is.. It would spool very slow from 5-10 PSI so it was easy to hear when it started to crack open. The MBC (a Hallman unit) was backed all the way out. From my understanding, this means the wastegate should open up at wastegate pressure (20 PSI in my case) and is not possible to open sooner. I made sure the MBC was hooked up properly and the wastegate was even 6 turns (1 hole) tight. It clearly sounded like it was opening at 5-7 PSI and sometimes even 10 PSI (it varied and didn't seem consistent with anything i.e. temp, elevation). On top of that the boost would spike to 25 PSI in 4th if I let it. How could the gate be opening that soon? or was it something else entirely that I was hearing? You could smell a bunch of exhaust when you heard it open as well. Does anyone have a clue what was happening?

Edit: Is wastegate spring pressure referring to fully open or starting to open?

I have since gotten a new hotside and new gaskets and have done all I can to take care of any leaks when the car goes back together. Though if an item is typically forgotten about please remind me.

Things I've since gotten:
New hotside
Head to Mani gasket
Mani to turbo gasket
O2 housing gasket
DP gasket
Fixed IC nipple leak
GTP35 actuator (Starts opening at 15 PSI)
 
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Doesn't matter where the gate starts to open versus fully open as long as you have control of boost...although if the spring is too soft, it'll open when the backpressure in the housing overcomes the spring's ability to hold the flapper closed. If the gate is fully-open and the boost continues to climb, that's boost creep.

If you're talking about a GTP38 Garrett turbo...the OEM actuator on those things is something like 6-8psi. It's pretty weak...so that would be the reason you're hearing it open as low as 5psi.
 
Also it really doesn't take much to regulate the boost. I did some calculations on it....and it's about 10% of the total exhaust flow on a setup like mine. The 38mm ect I had once opened less than .0625" to control the boost, and that was measured.
 
Doesn't matter where the gate starts to open versus fully open as long as you have control of boost...although if the spring is too soft, it'll open when the backpressure in the housing overcomes the spring's ability to hold the flapper closed. If the gate is fully-open and the boost continues to climb, that's boost creep.

If you're talking about a GTP38 Garrett turbo...the OEM actuator on those things is something like 6-8psi. It's pretty weak...so that would be the reason you're hearing it open as low as 5psi.
Things I've since gotten:
GTP35 actuator (Starts opening at 15 PSI)
It was also off an eBay GTP38 and It started opening at 15 PSI on the bench :idontknow: The one that was opening as low as 5 (on the car) was the HX-35 actuator (started opening at 20 on the bench). I did not think it was possible for the HX-35 actuator to open at such low boost which is what was confusing to me. My thought on the boost creep was that since it was only starting to open at 20 PSI, that it was not able to open enough to bleed off enough exhaust to keep the boost under control. Does that sound right to you? The GTP38 actuator is not on the car yet (car is all apart) so I can't say how that performs yet.
 
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Also it really doesn't take much to regulate the boost. I did some calculations on it....and it's about 10% of the total exhaust flow on a setup like mine. The 38mm ect I had once opened less than .0625" to control the boost, and that was measured.
Thats interesting. Really, only .0625"? I would've expected at least .25" thats great info thanks. So since I'm running less boost than you (22 PSI target), and have a smaller port to work with (right around 27mm I believe) does that mean that I'd have to have the gate open more to bleed off even more of the exhaust to hit my target boost?
 
without going into lots and lots of math and all the science.

compressor power ~ pressure ratio^1.3 * mass flow * compressor efficiency

So up the boost and you need more power, up the mass flow an you need more power.

the turbine power looks like

turbine power ~ pressure ratio^1.2 * (mass air flow + mass fuel flow)/turbine efficiency

wastegate flow ~ sqrt(pressure ratio) * density * circumference of valve * lift


At steady state turbine power = compressor power. you can then figure out about how much wastegate flow is needed.

**** the above equations are not the real ones, but greatly simplified just to give you the feel for what is happening here. ***
 
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