I don't get it, the ad for the cam in the link below says it has a 0.050" smaller base circle than others, and to check your pushrod length. It then goes on to say that longer valves are used.

I would have thought that a 0.050" smaller base circle could be compensated for using a longer pushrod alone, and don't touch the valve length.

Why would one need to also have longer valves in addition to a longer pushrod ? What am I missing ? Or am I reading/interpreting it wrong ?

http://www.sdpc2000.com/product/1256...rCamshaft.aspx

 

Chris, that add says you need a .050 longer pushrod with stock valves, or if the heads have the longer valves stock length pushrods will work.

 

Thanks alot, it's clear now after a good-night's sleep. I must have been information-overloaded yesterday, or just getting old.

 

People that write ad copy are generally not engineers.. If the cam is -0.050 on base circle you need 0.050 to get back to correct for stock length valves. If you run longer valves you will probably need longer yet to restore the geometry. Rocker ratio cam lift and valve length can keep moving the correct length all around. The idea of just taking up the slack is not going to work in the end. Sure the parts will connect up but the geomtery will be wrong and valve and valve guide life will suffer from side loading. You generally will not get max life out of the cam either if the geomtery is wrong. If you make a simple drawing of the lifter push rod rocker and valve it makes sense pretty quick Ideally you want the rocker oin the center of the valve a mid lift and a minimum amount of of movement across the valve tip with the majority in the center of the valve. This is done to avoid side loading the stems.

 

Thanks. If you did a valve job and found later that one of the seats was cut so much that the stem tip on one valve was 0.050" higher than the rest, after shimming the spring, what other steps would you propose to restore stock geometry on that one valve ?

 

If you have to grind just one seat that much the seat has a problem and needs to be replaced. You might try a new valve or possibly grinding the tip of the one you have if there is enough material and don't end up with rock to retainer interference. You might want to think about what you are doing to the flow sinking valves as well as the chamber cc's.

 

I'd just go with a 0.050" longer pushrod corresponding to the lifter associated with that seat, moving the rocker arm up 0.050" on the stud and maintaining stock geometry.

 

Maybe that article was incorrect in stating that the base circle on that cam is 050 smaller , below is a link that says its larger. Now that would make more sense with a longer-stemmed valve.

http://www.idavette.net/hib/02ls6/page2.htm

The cam I will run is an LS6 grind on an LT1 stick.

 

Chris, what I get from that article is: The Gen III (LSx) camshafts have a larger base circle than previous generation camshafts. But, the LS6 camshaft has a smaller base circle than the LS1 and truck camshafts.

Your engine heads/cam combination is going to be very interesting. Are you planning on dynoing once everything is dialed in?

 

I agree with your interpretation.

My exhaust stem tips are 050 taller than the intakes but I'm swapping gaskets to the impala ss and will put them 025 too high, while lowering the intakes to 025 "too low". Yet all other valvetrain items were identical on impala ss and I conclude a 25 thou difference is within acceptabilitly, half of mine will be 25 above , the other half 25 below stock height and I expect no trouble.

I'll be going with a rocker ratio of 1.5 since that is what the head was designed for.

Yes I will dyno.

Using the 02 LS6 profile on an LT1 should be an excellent match since rod/stroke ratios about same and static comp is about same in each. A little less stroke on the 350 but with the thinner gasket I'd expect to pick up a little extra tq to compensate. My calculations show a DCR of 9.47 for the 02 cam when using 051 thick gasket. swapping to 026 gasket yields 11.08 SCR and 10.0 DCR, using Patrick Kelley's calculator.

I think hitting 400HP & Tq is probably easily achievable below 5800 rpm.

http://www.gmhightechperformance.com.../photo_05.html

We'll see. I still need to find some exhaust valves from LT4 at a reasonable price. They're lighter and better heat transfer. I plan a thermal barrier - I have the technology, equip., and exp. to do that right here in the barn.