Quik

iTrader: (7)

would 918s hold the amount of power i want to make? this will be a 408 around 9.1 cr with cleaned up 317s and probably a LS2 intake. im looking to make around 9xx rwhp on pump gas. cam isnt decided yet, nor is rpm range.

would 918s be good for boost or should i go with a dual spring that has better seat pressure?

Steve Bryant

iTrader: (1)

The 918's should be good through about 6,500 RPM's at least. You might want dual springs if you want to spin the engine faster or want more seat pressure or just the assurance of dual springs in case one breaks.

The 918's are good springs for performance applications. The reasons listed above are really the reasons to consider dual springs, rather than the fact that you are running boost or a power adder.

Steve

Quik

iTrader: (7)

see i question cause i thought i seen on tech where they didtn have enough pressure and the springs were causing the valves to float under moderate boost

Grumpy5.3

iTrader: (14)

I think you have good reason to worry, I do not believe the seat pressure is goint to be able to handle the boost you are going to run...

Stoichiometric

iTrader: (10)

You can do some math to figure out the amount of pressure (pounds per inch) on valves under boost. Valve area X Boost pressure. Subtract that from your spring rate during boost. The exhaust valve is worse because you can have tripple the intake pressure in the exhaust before the turbo.

Steve Bryant

iTrader: (1)

Think about the four strokes for a minute:
Intake: Combustible mixture entering cylinder (you want the intake to be open anyway).
Compression: The pressure inside the cylinder is pressing hard to seat both valves.
Power: See explanation for Compression Stroke.
Exhaust: Gases rushing out of cylinder (you want the exhaust to be open anyway).

The only time that I could see this being a problem is right after the piston is at bottom dead center during the transition from intake to compression. I'm open to a more informed opinion. But under moderate boost, maybe 8 or 10 PSI, I can't see the seat pressure being inadequate on the intake valve. I don't think that it would ever come into play on the exhaust valve.

Steve

Quik

iTrader: (7)

im sure at low boost it would hold but not around the levels i was thinking of. im looking in neighborhood of 18-20 max. and average of 12

over on tech same thread was posted and none say 918s are worth anything for boost

Wilde Racing

iTrader: (26)

For what it's worth my 918's held just fine at a max of 15 psi with 317 heads. but of course I was spinning a max of 6200 rpm......

BigKID

iTrader: (5)

I am interested in this since I will be running a set of 918's for a while untill I do my head swap. I will only be running ~8psi which translates to an extra 50# of pressure on the intake valve but I am also not running and overly agressive cam either. According to Comp the 918 specs are:
Seat load = 130@.800
Open load = 318@.200
Coil Bind = 1.100

These specs were taken from TSP's catalog:
Installed Load: 130 lbs @ 1.800"
Open Load: 318 lbs @ 1.200"
Spring Rate: 313 lbs/in
Coil Bind Height: 1.085"
Outside Diameter: 1.290"/1.060”
Inside Diameter: .885"/.656”

silver-mod-o

iTrader: (15)

go with some patriots Quick... much better spring IMO...