valve spring psi for boosted engines

[nosnova]

why does it take so much more valve spring seat psi  with 30 lbs of boost compared to a nitrous engine.

for example my currant nos eng. has the same lift more duration @ .050 a tighter lobe separation
 it has 245 lbs of seat psi and i shift at 8000 rpm and go thru traps at 8200

the turbo eng. has less cam duration a wide lobe separation and the shift rpm is going to be at 7300  to 7500 rpm

[Injected65]

On the intake side the valve spring has to account for the boost pressure in the intake tract.  On the exhaust side you have to counteract the back pressure in the hot side.

Chris

[nosnova]

ok that makes sence so i my valve train has been happy with 245 on the seat and i add 30 lb of boost
how much more seat psi is need another 30 lb of seat psi because of the 30 lbs of boost?
the int valve is 2.125 if that matters or not.

[Injected65]

ok that makes sence so i my valve train has been happy with 245 on the seat and i add 30 lb of boost
how much more seat psi is need another 30 lb of seat psi because of the 30 lbs of boost?
the int valve is 2.125 if that matters or not.

A 2.125 valve will have an area of approx 3.5 square inches.  30# of boost X 3.5" = 105# worth of resistance

Now that is all mathematics, I doubt that you really need a whole 105# more of spring pressure, but the best bet would be to check with the cam manufacturer and see what they suggest for the amount of boost you are planning on running.

Chris

[nosnova]

thanks for the explanation a few people have told me i need anywhere from 320 to 400
but no one gave me that formula so thanks. im going to have a go with the sugestions
of my friend ash and some others on yellow bullet and put the best spring in i can afford then damn
i cant save any money anywhere ion this project lol
thanks for the advice

[ash]

thanks for the explanation a few people have told me i need anywhere from 320 to 400
but no one gave me that formula so thanks. im going to have a go with the sugestions
of my friend ash and some others on yellow bullet and put the best spring in i can afford then damn
i cant save any money anywhere ion this project lol
thanks for the advice

i'm just passing it on from TQ

[nosnova]

ash can u send me that other web site that you and tq goto i lost it
you no the one that  u pay to be a member on
thanks  norm

[Injected65]

Keep in mind that the figures I typed were simply figuring out the area of the valve head and multiplying it times the boost pressure.  The pressure of the spring comes into play during the closing of the valve so it is unlikely that you will see that full amount of pressure exerted on the valve.  On my SBF I ran with about 275-300 on the last cam and with the new cam I am a touch over 400# since we went with a much more aggressive lobe that will get spun higher as well.

Chris

[nosnova]

what was your last cam like and what is concerned  a aggressive lobe
that u had to up your spring psi

here's my specs is this a mild or aggressive cam
 .429 lobe lift  lift @ .200 is .176  dur @ .050 is .257
 .444 lobe lift   lift @ .200 is  185  dur @ .050 is 262

l/s 114

[Injected65]

The old cam was 256/250 duration @ .050 and lift of .704/.704

The new cam is 265/269 duration @ .050 and lift of .753/.753

Keep in mind this is a 365" SBF so the numbers are fairly aggressive.

Chris

[nosnova]

i see you switch to a traditional split on the duration why ? also
you made the cam bigger but i thought the aggressiveness was
how big the lobe lift @ .200 was. somewhere on this site i read that
you could have 2 cams with the same lift but one might have a higher lift @ .200
making it more aggressive i think i worded that right. 

The old cam was 256/250 duration @ .050 and lift of .704/.704

The new cam is 265/269 duration @ .050 and lift of .753/.753

Keep in mind this is a 365" SBF so the numbers are fairly aggressive.

Chris

[Injected65]

i see you switch to a traditional split on the duration why ? also
you made the cam bigger but i thought the aggressiveness was
how big the lobe lift @ .200 was. somewhere on this site i read that
you could have 2 cams with the same lift but one might have a higher lift @ .200
making it more aggressive i think i worded that right. 

Its two different people speccing the cams for me.  The latest one was from Don Bailey and was more in line with the amount of boost and rpm I was planning on turning as well as more suited to some other changes I made to the car.  I'm far from any cam expert so I just rely on what they reccomend.  My basis for calling the new cam aggressive compared to the old cam was the fact that he moved from the .440 lobe up to a .471 lobe.

Chris

[nosnova]

can i asked who speced your 1st cam.
i know don knows his stuff it. will he be tunning it also?

[Saywhen]

We knew when we put these springs on last winter they were marginal, but used them anyway because we don't have the best valvetrain. Ran good for about 40 passes then we started seeing signs of valve float.
290# seat
730# nose
7800 rpm don't have the specs here but way less cam than used above due to the 23 deg heads

Tried a little different spring just to get us through the season
340# seat
750# nose

[302tt]

A 2.125 valve will have an area of approx 3.5 square inches.  30# of boost X 3.5" = 105# worth of resistance

Now that is all mathematics, I doubt that you really need a whole 105# more of spring pressure, but the best bet would be to check with the cam manufacturer and see what they suggest for the amount of boost you are planning on running.

Chris

If we have 30 psi boost pushing on the intake then there could be 60 psi pushing on the exhaust valve (estimate a 2:1 ratio).  The lowest pressure the intake will ever see would be just before IVO, but the high exhaust pressure=high cylinder pressure which will help hold the intake valve closed. So I doubt the intake valve would need additional spring pressure on a turbo engine (different for a supercharged engine). IMO, it's the exhaust valve that would need the higher spring load. If the intake boost is equal to the pre-turbine pressure 1:1 then I don't think it would be any different than a NA engine.

[nosnova]

that's pretty interesting theory have you tried that? it seams that everbody is running  a lot more
seat psi so im going to be a follower with them and run some pretty good psi also.
if youve had good luck with lower psi let me know.

If we have 30 psi boost pushing on the intake then there could be 60 psi pushing on the exhaust valve (estimate a 2:1 ratio).  The lowest pressure the intake will ever see would be just before IVO, but the high exhaust pressure=high cylinder pressure which will help hold the intake valve closed. So I doubt the intake valve would need additional spring pressure on a turbo engine (different for a supercharged engine). IMO, it's the exhaust valve that would need the higher spring load. If the intake boost is equal to the pre-turbine pressure 1:1 then I don't think it would be any different than a NA engine.

[junk]

I see the spring pressure theory the same way. It all depends on the EBP. Another thing to keep in mind is cam timing. At all times except for overlap the piston is pushing or pulling air to even things out so the valve pressure differential is a moot point. During overlap is where the pressure differential matters and that is dependent on EBP...........and that amount of time is minimal.

[Forcefed86]

I too believe it is all in the cam. If you don't have an excessively tight LS and you have the proper duration for a boosted engine you shouldn't have any issues. I run 80# valve springs and run 30 psi through my buick motor without any problems.

[ash]

what did you end up with norm ?

[turbotr]

Well it seems obvious to say the boost on the intake manifold side is pushing the int valve open. Ok. But why are we ignoring the other side of the valve here? We can't do that. Pressure on the cylinder side wants to hold it closed. Which side wins? If you look at a cylinder pressure trace from a turbo engine under load, you see that the cylinder side of the valve head is never at a lower pressure than the intake side. During overlap, when the int is closing, it's generally at even higher pressure on the cyl side than the intake side due to exh backpressure. During IVC it's a wash compared to a non boosted case. Don't think it's really sufficent to just say the boost is pushing the valve open, and thus you need a heavier spring.

Similar for the exh side- at EVC it's a wash really, since the pressure on either side of the valve is ~ equal. at EVO, there's still hundreds of psi in the cyl trying to keep the valve closed. If anything, exh backpressure helps get the valve open there.

Learned a pretty good lesson about exh pushrod strength though, regarding how much pressure the exh valve is trying to open against. Weak (and very long) exh pushrods in a high perf Buick v6 flex so much they can seriously divot the head gasket that sticks out into the lifter valley, located like half an inch away (or more) from the pushrod. If you consider what pressure still exists in the cyl at EVO, it's not hard to see that there are literally hundreds of lbs of extra closing force being applied that the exh valve is trying to open against. Umm, so the weaker pushrod tends to flex a bit at that point until the valve sproings open The int pushrods on the other hand, very little divoting compared to the exh side.

TurboTR

[Taff]

My understanding of it is this, stronger valve springs are needed (in any race engine) to stop the valve bouncing back off the seat as it closes. In a turbo engine the extra 30psi of boost in the intake port or ,say,45psi of turbine inlet pressure in the exhaust port may be enough to help bounce the valve back off the seat if the spring is marginal. Hence the stronger spring may be necessary. Cam profile no doubt plays a big part,as a cam that drops the valve hard onto the seat is more likely to bounce it off again with a 'weak' spring.