New high score! 115 wheel kw!
![PB_115.JPG](./download/file.php?id=30309&sid=3646d339888290ad81cc3332fb707933)
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I couldn't crack 105kw, even with the new pump. AFR's were good but more boost just = less VE and the same 105kw.
The same thing happened with the GT17 but at around 85kw so I was pretty sure it was back pressure related, and therefor valve train related.
I ordered a bunch of valve spring shims from precisionintl.com for a couple of dollars for a set. Went ahead and first measured valve seat pressure using a load cell to measure the force to crack the valve from the seat. Comparing that to the workshop manual showed valve seat pressure was well under spec, 240N vs 300N on the exhaust valves. Intake were within spec. I assume it was from previous valve seat work, or maybe a stretched valve. It wasn't a weak spring as changing exhaust and intake springs maintained 300N on the intake.
I went ahead and measured the valve spring bind height using the mill, compared that against the open valve height and worked out how much I could shim it while maintaining 60thou clearance for spring bind.
I ended up fitting 1.5mm shims under the springs and when the head was rebuilt I now measured around 350-360N on both intake and exhaust.
A little bit of math on seat pressure and valve face areas says there was only 34psi of pressure holding the valve shut at 240N. I'd say at 12psi boost pressure, there'd be at least 25-30psi turbine back pressure so I think that was the issue here. Now with the 350N on the seat there's over 50psi holding the valve shut which assuming the same backpressure ratio should easily see me running a bar of boost before valve float issues.
The goal was 120wheel kw so I'll add in another psi and I think we'll get there.
![openheight.JPG](./download/file.php?id=30310&sid=3646d339888290ad81cc3332fb707933)
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![shims.JPG](./download/file.php?id=30307&sid=3646d339888290ad81cc3332fb707933)
- shims.JPG (65.25 KiB) Viewed 3790 times