A riddle I can't solve

[motorheadsw20]

Hi All,

A funny wee issues crept up during Manfield - and while I've taken all the logical steps I've been unable to figure it out ... almost like a maths test

Boost_issue.jpg (200 KiB) Viewed 2241 times

My expectation was once I swapped the spring (I thought the wastegate was faulty) I'd get 14 PSI boost with the solenoid OFF, but I only got 8.5PSI. Once the solenoid was ON I was expecting the same 15 PSI release - but no working at 18PSI without a problem


Any further troubleshooting suggestions are welcome

[GDII]

So you get 7psi with a 14psi spring?
Have you tried running the wastegate off the turbo only with the solenoid bypassed?

[motorheadsw20]

Yes that's correct 7 PSI on a 14 PSI spring.

When the solenoid is in line though it blocks the port so effectively the bottom of the wastegate is connected to the turbo - so theoretically I should not be able to run 7 PSI (unless there's 7PSI vaccum on top of the wastgate )

I have not tried eliminating the solenoid from the equation completely - I shall try that after work and report

[Benckj]

Check the selenoid port on turbo side is NC.

[motorheadsw20]

So I completly removed the solenoid from the equasion

1. moved the hose coming from the turbo straight to the bottom of wastegate
2. and top hose of wastegate I'm venting to atmosphere

this way with a 14 PSI spring I shoud boost to 14 PSI

weirdly enough no - only 8.5PSI ?? (what I thought was 7ish - when I data logged it was 8.5) but it's nice and steady boost, no spikes so seems controlled

20200108_183358.png (66.01 KiB) Viewed 2219 times

white number on top is the timeline / green 8.53 is the boost

... as mentioned if I leave it up to the ECU/Solenoid I get solid 18 PSI which leads me to believe it's not a boost leak... turbo side port is NC

Just thinking out loud - could the exhaust gases create so much pressure (~5.5 psi) to open the waste gate from the bottom? 14psi spring =exhaust pressure 5.5 psi plus turbo pressure 8.5 psi but then with increase of rpm the boost would logically drop (if the exhaust back pressure was driving the opening of the wastegate)

[GDII]

It's possible the spring just isn't doing the job it's meant to. It may start to open at lower than 14psi but be fully open if it gains 14psi. Is it a large wastegate? Might be super efficient.

However you state that the 10psi spring worked as designed until recently? Which makes my theory a little invalid.

[motorheadsw20]

Yep until the run at Manfield all was good - so no idea what happened since I did not change a thing

It's a 38mm tial wastegate

In hindsight it had a 10 psi spring and judging by my previous data logs it was popping open at 6.7 psi (after Manfield) ... so maybe the springs are not quite doing their jobs

[GDII]

I think this is a normal trait of them opening a bit at lower than the specified number. But it shouldn't stop it making all 14psi. But then how would I know, not owning a turbo car ever...
38mm is a decent size but it's no 44mm.

Plenty to learn from others issues and trying to figure it out. Hard to sort things when you can't see the car or run tests yourself. Internet diagnosis is always hard.

[Benckj]

Generally springs start opening at 1/2 their rated tension. It may only open a small amount but due to the WG size it would limit boost. If you build pressure quicker it should spike up to your rated spring tension.

If you want to test use a compressor and turn regulator down to 15psi. Inject this directly into WG to see how it reacts. It’s also benificial to use same pressure to check intake for leaks. Just need to block off filter and TB side. It’s surprising what this test uncovers.

[GDII]

Generally springs start opening at 1/2 their rated tension. It may only open a small amount but due to the WG size it would limit boost. If you build pressure quicker it should spike up to your rated spring tension.

If you want to test use a compressor and turn regulator down to 15psi. Inject this directly into WG to see how it reacts. It’s also benificial to use same pressure to check intake for leaks. Just need to block off filter and TB side. It’s surprising what this test uncovers.

Definitely some good suggestions there.

[Malcolm]

Just thinking out loud - could the exhaust gases create so much pressure (~5.5 psi) to open the waste gate from the bottom? 14psi spring =exhaust pressure 5.5 psi plus turbo pressure 8.5 psi but then with increase of rpm the boost would logically drop (if the exhaust back pressure was driving the opening of the wastegate)

Sort of...

I'm not an expert on this, but I have done a bit of engine development work with exhaust pressures being monitored. Especially on smallish factory turbos you can often get pressures as high or higher on the exhaust side than what you have on the compressor side, particularly if you're pushing the turbo flow well beyond its best efficiency.

The spring doesn't really care about pressure, it cares about force. The force on the diaphragm is generated by the pressure acting on the area of the diaphragm (or in fact the difference between the pressure on either side of the diaphragm). The force on the valve is generated by the pressure in the exhaust manifold acting on the area of the valve. If you measure it up you'll probably find the diaphragm is a much larger area than the valve, so even if you have the same pressure in the exhaust manifold, the force should be less so although it will make the wastegate open sooner, it shouldn't be quite as large an effect as the pressure on the diaphragm side.

[motorheadsw20]

Thanks heaps guys for the suggestions - definitely some good and useful things to try out

I'll do the intake leak test over the weekend and will see how that goes and will post an update