Discussion and questions related to the course Practical Engine Building
I came across 2 valve spring kits for my boosted engine with maximum boost limit specified for each kit - one is for 30 psi of boost and the other one is for 40+ psi. I realize that it has something to do with boost affecting spring seat pressure but what is exact theory behind it - can anyone tell please?
In short, it's primarily related to seat pressure, or more accurately force, but the former seems to be common use. The manifold pressures are working against the springs to try and push the valves off their seats, and so additional spring pressure is required as the pressure in the manifolds increases.
For example, a 32mm diameter valve has an area of ~1"^2, so for every 10 PSI of manifold pressure there will be 10 lbs force working against the spring and trying to lift the valve off the seat.
What the supplier is suggesting is that there is a 10 lb difference in seat pressures.
However, that is really only a tiny part of the equation. Without actually knowing what the installed valve height is for the spring, and being able to shim to that spec', you are having to trust the supplier - something I would be loath to do without that information, the spring rates, the coil bind length, etc.
It is certainly possible to have a higher spring seat pressure but a lower spring force at full lift, which is also important to control the valve train - heck, a 1mm shim/spacer under the spring may make a 10lb difference on it's own.
You may have seen the problem Andre has had with the cutting of the spring seats causing problems with his valvetrain - cutting the seats also increases the spring length when installed, reducing the spring seat pressure, and has to be corrected with shims/spacers.
I expect you can find plenty of better information, with diagrams and graphs, on-line.
Thanks a lot, Gord!