I'm going to test some higher valve spring seat pressure on a pump gas blown 440 motor for a customer, I bought some Manley roller springs that have between 315 lbs. to 325 lbs. on the seats and between 640 to 675 lbs. opened at max lift.
I haven't verified the actual lift at the retainers yet with these springs as I am waiting on the new Manton Series 5 pushrods to get here, I'm relying on the cam card and advertised ratio on the Comp Cams stainless steel shaft rockers

300 to 350 is plenty of spring pressure. Thats what I use for B-1 which have a longer valve. Birdtracker

Cab, i read today about this. If you have three inchs of intake valve face exposed to boost, a 10 psi boost reduces spring pressure by 30 lbs! So calling a couple of blower experts would seem like a great idea.

My main reason for using the higher seat pressures is to help it rev easier under vacuum and still have enough pressure at max lift to not allow the boost to hold the valve open and let the lifters float over the nose on this camshaft
This is a mild pump gas blower motor that owner wants more power than it had before, the cam that was in it had under .500 lift I have help him on other cars he has and was surprised that the N/A pump gas street Hemi motor I did for him has more power than this blower moor had, not now
Stroking it, increasing the static compression ratio, putting a set of Eddy E street heads on it and making the cam a lot better should make him grin a lot more now than before

Depending on how crazy you get the exhaust side is where you might want to pay some attention to. Valve material and spring pressures. Doubt you are doing anything very crazy boost wise but just in case

Al, the owner doesn't know what the boost pressure was before, I'm going to advice for 3.0 Lbs. or less on Oregon pump swill at this altitude, 3500 Ft above sea level
The local 1/8 mile track, Madras, is around 2500 Ft above sea level, I don't know if he will ever run it there on a Friday street car night or not
He has another Roots blown street car also so maybe between the two we can swap pulleys to make more or less boost in this car
if not I have Littlefields phone number

How much pressure differential are you expecting across the intake valve when the valve is at max lift?
What about when it is closing?

I have no idea of what your asking on the intake valve pressure differences
I've dyno tested around 8 Roots blown gas motors, 3 hemi and several BB and SB Chevy.
The last pump gas Hemi (4.250 borex4.375 stroke and 9.3 to 1 mechanical compression ratio) made 924 HP at 7300 RPM with 130 F manifold intake temps. with 7.5 lbs. of boost with 12% under driven and 33 degrees BTDC ignition timing, we swap the pulleys and got 13% overdrive and put race gas in it and retarded the timing to 25 degree BTDC. with those changes it made 1027 at 6500 RPM with 12.0 lbs. of boost with 190 F intake temps and then went into detonation 30 minutes later after the best pull at 7.5 lbs. of boost
The dyno operator thought we took out to much timing so I bumped it up to 27 BTDC and it went into detonation at 6300 RPM 25 minutes later than the first pull with 12.0 lbs. of boost with more overdrive Stupid me
Looking back later I should have set the timing at 20 BTDC to start with on those changes with the increase boost
I didn't know how big the intakes temp would make compressing the air that much more Hot air expands, correct
The last N/A Hemi I built and dyno tested was a 572 C.I. with Holley dual dry 1150 CFM throttle bodies on a single plane Stage V inline intake and FHO CNC ported Stage V street heads, it made 799 HP at 6500 RPM on Oregon non ethanol pump swill.
My message is Roots super charger consume a lot of HP when compressing the air

True, and they add a lot of heat to the air while doing it. I've been running a Jimmy on a street/strip car since '78. They look impressive, but they are a terribly inefficient way to generate boost.

In order to calculate the effect of boost on spring load, the pressure in the cylinder (the other variable in the pressure differential) has to be known . Obviously, the pressure in the cylinder varies dramatically during the engine's 720 degree cycle.
A simple boost x valve area calculation is only going to be correct when the cylinder is at atmospheric pressure, which is what the manifold pressure measurement is referenced to.

For a professional answer on the springs, Mike Jones at Jones Cams would be a good person to ask.

What impact does running on pump gas have on required spring pressure?
Are the aggressiveness of the camshaft lobe profile, valvetrain weight, valve face area, and boost not all independent variables?

I've seen many mention this but I don't really follow.

So 10psi through the intake cycle is also filling the cylinder, so there will be a pressure differential across the valve but not by the entire 10psi.

As the piston is coming up and intake valve is closing, there is pressure in the cylinder and as soon as the intake valve touches the seat you begin compression cycle and cylinder pressure instantly far exceeds manifold pressure.

Just not understanding the theory behind that I suppose. (boost subtracting seat pressure). I'm no expert on diesels but from what I've read it doesn't seem like they use much valve spring but some with compound turbos are at 150+psi boost.

More of a problem on the exhaust side of a turbo motor IMO (valve trying to close against turbine drive pressure as intake valve is opening)

But worse yet the exhaust valve is OPENING against a HUGE amount of cylinder pressure on a turbo motor. So that would indicate you'd want less valve spring, not more.


In the end, it's nothing to worry about. Put the spring in it that the cam calls for and be done with it.

I wouldn't say that indicates you'd want less spring pressure as much as the stiffest pushrods you can find (and maybe delay EVO)

I'm totally with you on that!!!! No doubt the exhaust pushrod and cam timing are critical to boosted engines. That is a REAL thing to worry about.

Just making the point that a stiffer spring isn't as much to worry about though. On a boosted motor, the intake MIGHT like a stiffer spring during closing because of the marginal difference between cylinder pressure and intake pressure at peak RPM. The exhaust valve MIGHT want a LESS stiff spring due to opening the valve under high pressure. The exhaust valve MIGHT want a stiffer spring to fight against higher back pressure during closing.

Just pointing out there are a lot of "MIGHTs" in there that probably don't really mean much of anything in the end.

^ I agree