This reminded me of a thread from 10 odd years ago,
viewtopic.php?f=1&t=17780
There were a few good comments ,
This was my comment from back then,,
""QUOTE Re: the great blown valve spring theory debate!ReportQuote
Post Fri Feb 12, 2010 3:02 pm
Many years ago I had a good running bog stock 200ci falcon 6cyl, I decided to put a turbo set up on it, get it going then when proven and driving , I built another engine (250ci) to put the going turbo setup onto. Definetly NOT high lift, high pressure springs etc in this deal, BUT this is what happened,
The good running engine when fitted with a basic draw thru carbed turbo setup, using only 6psi boost for the initial setup ran great up till 3800rpm where it would pick up random detonation, black exhaust smoke, no more increase in power, AND when brought back to idle it ran like a dog, as if it had got to valve float and pumped up some lifters. I did some chasing/looking, this was my first boosted engine so I was raw on the subject. I ended up being told by the guys at Vine Turbo's to take the preload off of the hydro lifters on the intake only and check back with the result. I did so and the same thing happened, i got some valve train noise at the same time- but it would come back to a clean idle. I checked back to them with the result and was told that half way thru the exhaust stroke that the intake valve can get popped off its seat if the springs cant cope and that this was caused by the pressure pulses/spikes in inlet pressure as other inlet valves close and even though with a turbo the cylinder holds more exhaust stroke pressure than when n/a, the pressure difference across the inlet valve can become greater and a spring that just coped in n/a form would not cope when boosted. Their advice to me at the time was to shim or replace the inlet springs. I used .060" spring shims and when set up with the regular lifter prelaod the engine ran fine with 6psi boost up to 4500rpm which was as far as I was game to take a stock 200ci with boost.
I was told by others at the time that it was my stock cam, my ignition, jetting etc etc, When I ran this by the guys at Vine Turbos they told me that on a stocker, just up the spring pressure, you'll be fine, and it was.
I was later shown a solid cam from a boosted chrysler 6cyl that had a lot of damage to the opening ramp on all of the intake lobes that they claim was a result of springs that did not cope with the intake pressure spikes- causing the lash to be taken up in random spots across the opening side of the lobe, the owner had also had problems with bent intake pushrods.
, and the exhaust manifold and turbo exhaust housing from the same engine that was heat damaged, not-they claim- due to overlap causing fuel blow thru but from the intake valves allowing some a/f into the exhaust stroke which would then burn in the ex manifold causing the damage.
Seemed logical, and seemed to work.
I dont do much with s/c engines but if this theory was applied to an s/c engine, the cyl pressure at the time the trouble occurs in a turbo engine would be lower in the s/c engine which 'should' (theres that word! lol) make the problem worse!
Going on this I agree with some more intake spring for a boosted app, but also the specific combo of boost, cam, spring, valve etc is going to have bearing on the outcome.
Didnt solve much with that, but gives something else to chew over!
Craig. END QUOTE""
Now ten years on to be honest im none the wiser, but im still finding a need for more spring on boosted stuff, i dont really care why anymore but it needs it.
Also, Ford Australias development if the "Barra" 4.0L i6 single turbo engine has followed the valve spring force increase. The 1st series of these engines did not upgrade the springs in the turbo model which is fine till you get up around 13+lb then suddenly they have problems. They run hydro lash adjusters which suffer pump up due to loss of valve control and hold valves open, quit making power and run ragged back at idle till the adjusters bleed out again. These engines can make fantastic power in as stock form, with factory turbo, manifold(in & ex) cams etc, but the springs let them down. In the uprated(higher boost from factory)models Ford Australia simply fitted springs with more seat pressure and problem solved. As in the aftermarket, the only mod internally needed to make big reliable hp is a valve spring swap. Of course you do reach a power level where further engine mods are required for strength. Point is , these like any other boosted engine require more spring force to do the same rpm as boost is added.