Silver Rose

I thought I covered this??
The boost is always present on the top of the valve, the increased boost in the combustion chamber is only helping the valve in the top 2/3 of the compreshion stroke and the first 2/3 of the power stroke.

I belive the real point is one made by TurboTommy, the valve is now slow to close due to increased boost countering the valve from closing on time, or think of it as slowing the closing process. This could cause the vavle train to have a slam shut effect on the cam to lifter area and incrase ware quickly.

When is it needed, I don't know. Very seldom on this forum do we here from mega PSI dyno queens having trouble with quickly worn valve trains. So when do you do it?? When you rebuild?? At what PSI??

gt37vgt

at the very moment the valve closes lets say 4 deg after bdc what do you propose the pressure below the valve is ?

back to basics ....it is the VE of the engine times boost pressure in absolute terms ..

so with 90% VE and 20psi the most differential there could possbly be across the valve is about 4 psi
so a likely increase of the
"PRESSURE DIFFERENTIAL" across the valve equates to about 1.5 psi wich is about 8 pounds of seat pressure .

now some one please do the numbers on the increased inertial loads when we increase the redline by 500rpmk

2bridges

rpm, valve train weight, and cam profile are the valid factors.

compression (or boost as a function of) does not directly apply to valve springs pressure as i see it.

APKhaos

FWIW, I pulled the engine in my '89 Turbo S after the first season I had it. It needed a head gasket, but was running strong. Just wanted to zero hour everything and have a good look around, given that I'd just bought the car and wanted it a solid as possible for the next season. At the time, the car had 24K miles, exclusively track use since the mod nineties.

There were no less than six broken valve springs! Fortunately the broken bits had washed straight down the oil galleys and were washing about in the bottom of the pan. Lucky doesn't quite cover it!

The lesson here is that stock valve springs can fail even in a low hour engine when it is run hard [ie raced hard]. Mike Lindsey sells some very nice aftermarket springs.

TurboTommy

???????????
Not sure what you mean by all this, and "pressure inside the valve" in your other post. But, I'm pretty sure that VE wouldn't be a consideration.
The bottom line is that added boost fights the spring to want to return the valve to it's seat. It's not much, but everything needs to be considered when engineering a system.
It's probably right, though, that cam profile and, more importantly the RPM you run the engine at, is of greater significance when choosing valve springs.

To those that are wondering: "how do you know when to upgrade valve springs?"
Well, I ruined a new cam and lifters because I didn't pay attention to this. (Well, there were also other factors involved). Valve springs that are too weak is a real threat, especially on engines that often get run at high RPMs.

spoolin51

BECAUSE ITS COOL!!!

HIGHBOOST

Wow, did some track testing yesterday at work and was unable to be at the computer all day.

Thank you for all the insightful posts, I'm going to read them all when I get a chance later today!