Valve spring pressure for turbo engines

[TheSilverBuick]

Is there a rule of thumb or mathmatical formula to determine how much more valve spring pressure an engine needs for a turbo over naturally aspirated? As it relates to psi of boost?

Example being my OHC L6 engine. Valves are 1.94" and 1.60" with 11/32 stems. I'm looking at upgrading the valve springs while naturally aspirated and it made me wonder if I could simply get an appropriate spring pressure for naturally aspirated and then when I put a turbo on the engine, simply install shims under the spring to up the required amount. So I just have to buy once. But I don't even know what that required amount would be? With my big cam the max valve lift is 0.460" and most aftermarket valve springs are rated to at least 0.550" lift, so in my mind appears to have lots of room to work with in the shimming to increase spring pressure department.

Thoughts?

[Barry Donovan]

inline needs more.
my boxer has the same springs for every setup including insane boost.. never a single worry.

OHC is a huge helper for any engine, especially inline, you my be able to use a v8 rule of thumb...
where is that ford mustang guru with the big turbo on the 2300. I bet he nails it every time.

[TheSilverBuick]

Bump.

[Dynoroom]

Don't worry about it. I can give you the formula to calculate valve area minus the stem but when the valve just opens the pressure almost equalizes. There's not enough valve area to unseat the valve when closed, unless you're running massive boost and wore out springs. So when you do the math you can see it's really no problem. Go on to other things that matter like a good seal between the head & header etc.

[Dynoroom]

The Pontiac is a SOHC with a rocker arm. Run 110 to 120lbs. on the seat and < 300 over the nose.

[TheSilverBuick]

Thanks Mike! I'm shooting for about 110-120 on the seat N/A, so it sounds like I'll do decent when I go boosted. I don't think I'll get to the point, but say 20psi as a likely overall max, with 14-15psi being more realistic. I can use the 110-120 to over 300 guide when I go to set up my good head. Thanks again!

I may have just answered my own question as far as purchasing springs once though. I just pulled an intake spring off the engine and all my assumptions were bad, lol. Due to the spring diameter requirements it looks like I'm going to get a set of springs right now and see about having my good "ported and worked over" turbo head machined for bigger springs. Currently looking at summit it's a Comp 984 spring, which Amazon sells 12 for $40.

[BigAL]

If you look at the math a 2.02 valve would have about 3 square inches of surface area, if your applying 15 pound per square inch(PSI) of boost it would equate to a force of 45lbs being pushed against the valve face at 15psi, which isn't enough to open a valve that has 110lb seat pressure. So like Mike said don't even worry about it. Though I have read and heard about your concerns about boost and spring pressure and how boost can cause the valve to come unseated, I just wonder if that is more orientated towards the import crowd where you can push valves open with your figure tips.

[TheSilverBuick]

If you look at the math a 2.02 valve would have about 3 square inches of surface area, if your applying 15 pound per square inch(PSI) of boost it would equate to a force of 45lbs being pushed against the valve face at 15psi, which isn't enough to open a valve that has 110lb seat pressure. So like Mike said don't even worry about it. Though I have read and heard about your concerns about boost and spring pressure and how boost can cause the valve to come unseated, I just wonder if that is more orientated towards the import crowd where you can push valves open with your figure tips.

The math is good for a static calc, but the valve closing at higher rpm with 15psi on the backside and wanting to bounce on the seat, suddenly 65psi might not be enough to control the valve, aka pre-mature valve float. Or maybe it is? It's not just an import thing, the junkyard LS folks replace the springs in their engines because of valve float around 12-15psi. Granted those are 100k-300k mile engines so the springs have some use on them. And many of those guys wouldn't spend a penny they don't have too, but if $100 e-bay springs get them another 100+HP, it's worth it.

[NewEnglandRaceFan]

I'd think your OHC will be different than a lifter/pushrod/rocker set up.. as far as a formula goes..

[anotheridiot]

But isnt there the line of thought that blower and turbo engines increase cylinder pressure so much that big heavy loaded valve springs just rob power?

[TheSilverBuick]

I've never heard that line of thought.

[BigAL]

But isnt there the line of thought that blower and turbo engines increase cylinder pressure so much that big heavy loaded valve springs just rob power?

That would only apply during the compression and power stroke when both valves are closed. I think the big worry comes in when your pumping 30+ lbs of boost, where that 3 sq-in(2.02 valve) of valve area that I stated above ends up with 90+ lbs of boost pushing on it, then if your only running 110 lbs on the seat you might end up with problems where the boost causes the valve to unseat. Though if your running 40lbs of boost, the pressure on the valve would equate to 120lbs and if you only had 110lbs of spring pressure the boost most certainly would open the intake valve, but that would most likely only happen on the exhaust stroke where the intake valve is closed and there is no cylinder pressure pressing against it.

[NewEnglandRaceFan]

I think the intake gasket will leave the building, way before that.. the way the intake and exhaust manifold are bolted to the head isn't really made for that type seal..

[anotheridiot]

That would only apply during the compression and power stroke when both valves are closed. I think the big worry comes in when your pumping 30+ lbs of boost, where that 3 sq-in(2.02 valve) of valve area that I stated above ends up with 90+ lbs of boost pushing on it, then if your only running 110 lbs on the seat you might end up with problems where the boost causes the valve to unseat. Though if your running 40lbs of boost, the pressure on the valve would equate to 120lbs and if you only had 110lbs of spring pressure the boost most certainly would open the intake valve, but that would most likely only happen on the exhaust stroke where the intake valve is closed and there is no cylinder pressure pressing against it.

Well, half of the exhaust stroke since intake with split overlap is already opening when the exhaust stroke is halfway up. I shouldnt have typed turbo engines, but I have heard it with blower engines. I dont think anyone would even consider running less than 160 pounds of spring pressure anyway, but getting in the 200 plus range is using alot of power to open those valves.