View Single Post
jluetjen jluetjen is offline
Registered User
 
jluetjen's Avatar
 
Join Date: Oct 2001
Location: Westford, MA USA
Posts: 8,594
Garage
A couple of observations;
1) Chris I believe is talking about a 3.2 in which we've documented that the valves are larger and heavier then in the earlier 2.2-2.7 family of engines. These larger valves have more inertia.

2) It's not really the "wildness" of the cam that requires the stiffer springs, but rather the accelerations combined with the mass (aka: inertia).

3) Your cam supplier should be able to share with you the valve accelerations for the different profiles. While a 906 cam is more radical then a GE60, the GE60 may have faster valve accelerations. If you were to rev the engine with the GE60 cams to 8500 RPM with stock springs you quite likely will have valve issues that would not have existed if you had used the 906 cams or stiffer springs. The more radical CIS friendly cams tend to have higher accelerations in order to fit the maximum airflow without having overlap or closing the intake valve too late. This is generally not a big issue since most CIS engine's don't rev beyond 6500 RPM. If you were to rev a 3.2 engine with one of these cams to 7800 RPM, you very well may have valve contact issues.

As a rule, if you're trying to do something "off the beaten path" of norm 911 modifications, you'll need to get beyond "hot" or "radical" and review the specific charactoristics such as accelerations and valve train mass.

4) A couple of halfway steps between stiffer springs and not are...

- As Grady described, reduce the mass of the valve train, specifically replace the retainers with Ti.
- Shim the stock springs so that they are under more pressure. Keep in mind that you'll need to confirm that you don't have spring bind if you go down this path.
__________________
John
'69 911E

"It's a poor craftsman who blames their tools" -- Unknown
"Any suspension -- no matter how poorly designed -- can be made to work reasonably well if you just stop it from moving." -- Colin Chapman

Last edited by jluetjen; 01-24-2005 at 03:08 AM..
Old 01-24-2005, 03:05 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #7 (permalink)