Drag Racing Online

Recently, to commemorate the millennium, I began a series of articles posted on our web site, www.iskycams.com,
entitled “Tech Tips 2000.”

The first ten or so are under the heading “Modern Myths of High Performance.” I decided to write them because I didn't necessarily enjoy repeating myself
over and over in order to undo the misinformation rampant in our industry.

So, like “Tech Tips 2000,” Cam Session is intended to set the record straight with the truth, the whole truth and nothing but, so help me.

Ron Iskenderian: will be setting the record straight in each issue of Drag Racing Online. If you have any questions concerning cams or valvetrain components or tech, just email camsession@racingnetsource.com

Ron:

I am going from a flat tappet solid lift cam to a much more radical solid roller cam in my small block Chevy. The flat tappet cam had a gross lift of .581-inch. The roller cam I am changing to has a gross lift of .640-inch. Is it required to go to .100-inch longer valves or will my standard length valves work the new cam? The specs are as follows: 250/260 @ .050, 106 lobe separation, .640-inch/.640 gross lift. I have the right springs for that lift and I understand that I will have to check piston to valve clearance. Some say the longer valves must be put in and some say they don't. I just want to know if I have to turn in my new standard length valves for the .100-inch longer ones.

Thanks,
Mike

Dear Mike:

Your question is a good one and worthy of a thorough explanation. The determining factor for valve stem length is installed valve spring height. Naturally, higher lift (and lift rate) roller cam profiles require heavier valve spring loads (pressures) to maintain proper valve control at higher RPM. This usually involves larger diameter spring wire and, coupled with the longer travel (allowing for higher lift), begs for higher valve spring free lengths and installed heights to accommodate.

In your particular case, a roughly .060-inch increase in valve lift from .581-inch to .640-inch requires at minimum a .050-inch longer valve. Practically speaking however, you'll find a much better selection of valve springs available if you switch to a .100- or .150-inch longer valve. This is because manufacturers like Isky address the needs of the majority of their customers' applications with their valve spring designs. A .100- or .150-inch long valve should place you in the 1.900- to 1.950-inch installed height range and you will find valve springs not only to suit your application, but those which should afford you excellent service life. Don't forget of course that your longer valves will require longer pushrods to maintain proper Rocker Arm geometry. Also, taller valve covers are usually required in order to maintain sufficient rocker area to cover clearance.

Finally, depending upon the maximum intended RPM range, there are different quality levels of valve springs (relative to service life) for you to consider. For example, in the Isky line at Level 1 of endurance are the #9315 and #9365 valve springs at 1.900-inch and 1.950-inch installed heights respectively. These springs will accommodate the .640-inch lift you will employ up to about 8000 RPM. For Level 2 performance (RPM levels up to approx. 8400 RPM,) the #9365 Endurance Plus springs at 1.950-inch installed height should be considered. Finally, at Level 3, yielding Absolute Maximum Endurance, the #9965 Tool Room Valve Springs for service life at up to 8800 RPM would be appropriate. Your RPM service life requirements and your budget will ultimately determine your final choice.

Good luck in your racing endeavors!