I have some understanding of the effects of different LSAs on engine performance but clueless on the effect of different cam centerlines for a particular LSA, ie 110 LSA on a 108 centerline versus a 106 centerline.
Thanks
Pete

 

Go to the Internals section of the Home page and at the very top you will see JRP's Cam guide. This guide discusses different effects ICL and LSA have on the performance, idle, and drivability of different cam designs. This SHOULD help you out!

 

I read that very informative post before but the only thing I got out of it in regards to centerline is that the difference between the LSA and centerline is the amount of "ground in" advance.
Is that it?
If so what is the purpose of grinding in advance? Why not have all LSAs equal their centerlines and adjust advance through the ignition exclusively? There must be some reason for doing it this way. Right?
Thanks!

 

Centerliens dictate where the peak lift of a cam occurs. If you advance the cam, i.e. move the centerlines to an earlier position, all of the events happen earlier (all of the events being your intake closing/opening and exhaust closing/opening, and your overlap will also occur at and earlier point as well). If you retard the cam then just the oposite occurs. This is a quick and dirty, do some reseach on this are of tech for much much more detail regarding this subject.

 

OK I did more research.
So the centerline determines how long the valve overlap takes place. If you have a smaller centerline then the exhaust valve will be open for a longer period and tend to suck out more of the intake charge. Correct?
This is advantageous at lower rpm because it helps the slower intake charge velocity. At higher rpms this would decrease power as too much of the intake charge (because of the higher velocity of the intake charge coupled with the vacum created by the exhaust opening) would be sucked out.
Also as you lower the centerline the max lift is closer to TDC so you PV clearance is less.

Is that about right or am I still in the dark?
Thanks.

 

Yes. Lower centerline == more advance which will increase "performace" in the lower RPMs.

 

Ok last question and i promise to leave you eggheads alone.
With .567 lift and a stock ls1 block and stock LS6 heads I shouldn't have to worry about PV clearances regardless of the CL, correct? In fact I should probably go with a thinner (.04) gasket as well.
Is there a way to calculate PV clearance on paper? ie amount piston is above the block plus lift less some constant for a stock block/head combo?
Thanks

 

No, that is incorrect. Peak lift makes not one bit of difference on piston to valve clearence. It is valve events that make the difference. The best thing I could tell you is to look at the TREX which for all practical purposes is generally the biggest cam you can stuff into a stock headed car with no piston to valve issues and go from there (yes you can put bigger cams in but probably aren't going to benefit anyone with a stock shortblock unless you like stock shortblock soup). Your two problem areas are intake opening and exhaust closing. So if you look at these events at .050" anything which opens the exhaust valve more then 13 degrees before top dead center and closes the exhaust valve later then 12 degrees after top dead center is going to be almost non acceptable as far as piston to valve interference. This assumes similar ramp rate cams are used, or in other words events at .006" and .200" are not that drastically different from the trex. Good luck and do some more searching. It takes a long time to grasp all of this stuff, there is much to it.

 

Also, I do not agree with what is being said about overlap. It is really dependent on the heads combustion chamber and valve arrangement do determine how much overlap is bad and good. Overlap will do nothing but hurt things at lower rpms as the exhaust gassess will revert back into the combustion chamber cause of the lower pressure in the combustion chamber compared to that of the exhaust system at lower speeds. Now as engine speed increases and the exhaust begins to scavenge then this later closing of the exhaust valve helps to bring some the the intake charge in rather quickly before the piston begins to move down on the intake stroke. In ls1 applications it seems the problem with too much overlap isn't the exhaust valve removing the mixture by over scavenging, but the exhaust system being insufficient and causing the exhaust gases to revert back into the combustion chamber (just like I discussed at lower enginer speeds). Also remember, when tightening the lsa and keeping the same intake centerline you are opening the exhaust valve later which allows less time for the exhaust gases to escape which contributes to the reversion discussed previously. Now with a hemi head and other heads with the valve pointing at each other overscavenging can be come a problem.

 

I think he is only referencing the .567" of lift with respect to different ICL's/cam timing thinking the peak opening of the valve will now occur at a different time. Without knowing the duration of the cam it wouldn't be as easy to say "it's safe."

 

Yeah, maybe so. I guess one way to look at it is that anything that is going to cause the problem events that i've listed (IO and EC) is going to be on a much bigger lobe with much more lift the .567", unless ofcouse you do something crazy like advance/retard a cam 20 degrees or something, h aha.

 

Thanks Technical, I was getting scared there for a minute. I do understand the exhaust opens later but it also closes later as well so it helps pull the intake gases in. ie duration is the same it just "moves around the clock"
I'm cool now, thanks guys.
No way to take pen and paper to do the PV calcs, huh?

 

There is but to be honest I don't know the formulas right off the top of my head. I just use a valve events caulculator that i've taken from this site, it is a very powerful tool.

 

I'm in the same boat as redmist right now. We are running the same cam:

224/224 .568"/.568" 110LSA 106ICL

I just got a call from my head porter and he mentioned that my combustion chambers were at 68.8cc which with stock gaskets equates to about 9.7:1 compression ratio....I'm curious to how much I can mill the heads/shrink the head gasket without worrying about PV clearance (if I have to even worry at all). I'd like to get the CR back upto the 10.2-10.7 range.

 

With that cam (my old cam) i'd say you could more then likely run whatever head gasket you wanted to and not have any issues with piston to valve clearence. Good luck!

 

Various threads at ls1tech speak to the value of piston to head clearances around 0.030" ("squish") to increase turbulence and reduce detonation. Also, your actual IVC point will be around 66 degrees ABDC, so for the frequently-recommended maximum 9:1 dynamic compression ratio for pump fuel, you would want close to 11.4:1 static CR.

 

Your ICL doesn't change your duration or overlap, it does change the timing events of the valves in relation to the crankshaft. Retarding the cam or increasing the ICL will normally help at higher RPM's,while advancing the cam or decreasing the ICL usually helps at lower RPM's.If a cam with 114 LDA is installed at 114 ICL it is called straight-up,if it is installed at 110 ICL it is 4 degrees advanced, or plus 4 degrees.

 

I wish the Comp Cams online catalog was clearer. They have a great visual of cam events on Page 6 of their catalog that can explain a lot.

Bret

 

I'm running close to the same cam, 220/220 581/581 110/106, heads milled 55cc chambers, mls gaskets, no ptv issues.