I have a 2002 LS1, and wanted to ask two "advanced" questions:

1. What are the trade-offs of using, for instance, a high-lift, fairly small cam (216 or 220, for instance) with stock 1.7 ratio rocker arms, vs. using a stock or nearly stock cam like the Z06 cam with 1.85 ratio rockers. Either of these options would be used with stage II LS6 heads, long tube headers, and the usual bolt-ons.

Obviously, I would get more lift, and slightly higher duration (not sure how I would calculate that using the 1.85 rockers) in both cases. It seems the 1.85 would reduce lifter bounce. Both methods place basically the same stresses on the springs, correct?

2. What are the trade-offs between an undercut stainless valve and a stock LS6 hollow-stem valve, in the same Stage II head mentioned above? Is the difference in flow more or less significant than the difference in valvetrain stability? Why don't more head porters use the LS6 valves--I assume due to cost primarily?

 

There's no free lunch.

1.85 vs. 1.7 rockers sounds like a win-win, but there are areas to look at. Yes, you get 8.8% more lift, but you also get 8.8% more acceleration of the valve which increases the loads a like amount. (F=ma). Going deeper, jerk (rate of change of acceleration), and the next 3 derivatives (Snap, Crackle and Pop) are also effected. They have a big say in valvetrain dynamics. This rules out stock springs, and some aftermarket ones.

There is a fairly good article in the Sept 03 issue of Camaro Performers magazine testing the SLP 1.85 kit. In a sidebar, Wayne Williams of Comp Cams is quoted concerning Comp's 1.85 ProMagnum Chromemoly rocker package.

"If there's one thing we would recommend that you do not do, it would be running the 1.85 rocker on the stock spring with the stock cam. The stock cam has a very aggressive ramp and is designed to work with 1.7 ratio rockers. The testing on the 1.85 rocker with the stock cam will break valvesprings. The use of the PN 26918 spring helps this problem, but it is still not a recommended application. If you are going to use the 1.85 ratio rocker, change the cam to a COMP Cams lobe designed for a 1.85 rocker."

Some of that may be marketing, but the bit about designing the cam for the rocker ratio is correct. You may ask why 1.6's in place of 1.5's don't have the same problem. 1.85/1.7 is about a third more increase than 1.6/1.5, so acceleration, jerk, S,C & P get pushed more.

As Rich has pointed out in another thread, rocker ratio doesn't change the cam duration because that's checked at .050 of lifter motion, which is .085 (1.7) at the valve. True, with 1.85s, .085 at the valve is about .046 at the lifter, so some folks talk about lifter ratio changing duration. The engine sees total duration and "area under the curve", so the 1.85's act like a cam with more velocity (lift per degree), or steeper flanks.

IMO, If you've invested $ in ported heads, headers, etc. have a cam spec'd for your engine. An aggressive HR will do what you want. A cam costs a fraction of what a good porting job does, so that's not the place to skimp.

Without back-to-back (A-B) comparisons of undercut valves on flow, anything is just a WAG (or "rectal-extraction" as a friend is fond of saying). The LS6 valve was done for less mass, wasn' t it? F=ma still applies, so as m goes up, so does F.

All of this extra spring load goes right back to the lifter. That's not a nice thing to do to lifters. Also, you probably want to rev past 6k, so here we go again.

My $.02

 

Very awesome reply!