Installing Isky springs on 98 HP500 carb. The Isky spec sheet shows 2 different installed heights possible, one gives 135# at seat pressure and other gives 160# at seat pressure. The engine is still stock and sealed. Which is the correct pressure to install at?

You need to figure out what your installed height actually is.

I need to know correct desired at seat pressure. I can install them at 1.875 installed height and have 135#, or install to 1.812 and have 160#. I guess the question is does a big block with roller cam that turns to 5200rpm need the higher pressure or not?

135 is not enough imo,i would go with the 160.make sure to test for coil bind.

I'm about 95% sure that stock springs were Crane 99896
https://www.summitracing.com/parts/c...6/applications

try to get the seat pressure over 150.I've had the isky 8005s springs in for quite some time and they still spec out to 150lbs on the seat.same as when I set them.

Would that be the 8005-sp springs? I have the 8005-a, I may need to switch

Actually it looks like the 8205-sp is the stock 1.900 installed height, the 8005-sp is 1.875 installed height, both at 150#

Is the cam a hydraulic roller if so 135 on the seat is fine and 335 to 350 over the nose (max lift ) is where you want to be. Going towards 150 you could start collapsing lifters and effectively changing the cam profile. Also putting undue stress on the rollers in the lifters and rest of the valvetrain.

135 seat pressure is not enough.valve float will happen and that will beat the valve train to death.many lifters destroyed from low spring pressure.