alright this might be entirely too overdone but i have never heard this argument yet so here we go.

i have 2 cams. both have the same duration of 224/230 at .050 degrees. both cams have nearly identical lift.

cam 1 using XE lobes 3314 and 3315

advertised duration is 276/282 on a 112 LSA would have an overlap of 55 degrees.

cam 2 using old-school lobes 3110 and 3112 (cc305 and 306 uses these same lobes)

advertised duration is 284/290 on a 112 LSA would have an overlap of 63 degrees.

now with that said which cam would you guys guess makes the most power? the cam 1 is gonna hold the valve where the heads flow the best the longest, but then you dont have as much overlap as the cam 2. so what is better for power.... overlap or the different lobe design? wouldnt cam 2 have a better lope to it also because of the extra overlap? im just not sure what characteristics i want to sacrifice in getting one cam or the other and was hoping some of you guys with more experience could give me a little input for a solution. thanks

 

Alright, someone is going to take issue with this, but it's what I have found. Screw it, it's not like a agree with everyone anyways. Agreement is for frat boys. (oups did I just say that!)


Since they are the same lift the power is going to be almost the same. I would go with the 33XX series since the overlap is LESS and run a lower LSA. The thing about a lower LSA is not that there is more overlap, it's that it moves the intake valve closing point to a earlier spot and makes the DCR higher, which will produce more power everywhere. Overlap is something I try to get rid of as much a possible. I don't see the need for it because it screws up the tuning events for me and the heads can use less low lift flow then. I'd rather have more low lift flow an be able to use it.

Out of those 2 cam I want the 33XX series because I don't want idle and driveablity issues and the throttle response should be better. If you want a lopier idle put those lobes on a 110LSA or get some of the 319X lobes on a 110LSA and make more power with the same duration.

Also the 3314 has more .200 duration at 145 vs 136, that means you actually have more effective lobe area, which leads me to belive that that cam will make more power.


Fire away,
Bret

 

What is DCR?

 

those 319x lobes have too high lift for me. i have already installed 987 springs and 1.6 pro mags all on stock heads. i just need the cam. that is interesting... the cam 1 on a 110 would have less overlap than the cam 2 on a 112.

 

Overlap smoverlap... I could care less about overlap myself.... well not really but it's secondary.

What you want is a cam with the best area-under-the-curve. Yeah..... that group of words you see in every magazine out there.
The funny thing about all of it is that everyone thinks of max lift as if it coincides with max piston acceleration. If you ever do a little basic math, you'll see that they just don't coincide with each other. Either way to it, we're talking about "time" here and is the flow benefit occuring at the right time..... that's a question that can get you scratching your head in a hurry. I've seen some PV diagrams from simulation software and I think that would be the way to judge what's actually happening there cause then you have an idea of the pressure differentials, intake vs exhaust. In this example you want the cam with the most lift area and thats gonna be the Xtreme lobe. With all the complicated things at work.... I think it's tough to make a real arguement one way or the other here.

-Mindgame

 

Here's an explanation from Rich Krause which is very good:

--------------------------------------------------------------------------------
Originally posted by WS6 TA
rskrause- can you define dynamic compression ratio and how you calculated it?




The easiest way to explain dynamic compression ratio (DCR). Is to consider that compression doesn't start until the intake valve closes. Even with a short duration cam, this does not occur until the piston is already a considerable distance up the bore. My cam is a mild 214/224 duration. Even with this cam the intake doesn't close until 67 degrees ABDC. At this point the piston is already 0.9" up the bore! If you add 10 degrees duration the intake doesn't close until 72 degrees ABDC. Doesn't seem like much, but in this 5 degrees the piston moves another ~0.1" up the bore.

So, the main variables are static CR and intake duratiuon, with lesser contributions from the lobe separation and intake center line as well as the rod length. There are formulas you can use, but don't bother as there is a great downladable Javascript calculator at http://cochise.uia.net/pkelley2/DynamicCR.html
which also has the formulas if you prefer to do it the hard way.

Run a few numbers using the calculator. It's an eye opener and explains (among other things) why it's possible to run sky high static CR NA on pump gas (if a very big cam is used). DCR under boost may also be calculated, but isn't as straightforward.

I run ~6:1 DCR to accomodate both nitrous and the blower. Oh, and BTW, contrary to what many people say this does not make the car a "slug" off boost.

Rich Krause

 

since duration and lift are identical on both cams... those 2 variables we can pretty much cancel out of this discusion. but yes opening and closing events are what i am trying to determine as i dont want the intake to stay open too long.

 

And the thread, which for me, gave a better understanding of the subject.

http://web.camaross.com/forums/showt...threadid=46128

Thank you Chuck for "DCR".

-Mindgame

 

OldSStroker,

THANK YOU! that was very helpful. i just dont understand how to figure the BDC/TDC and open/close point of the valves and how LSA affects them.

 

Try this link to Comp Cams Catalog:

http://www.compcams.com/catalog/

I suggest you use the "PDF Version Here" top center.
Go to page 8. See the diagram.

I fixed the link in the thread referred to above:

http://cochise.uia.net/pkelley2/DynamicCR.html

 

Since you did the 987's stick with the 33XX's then.

The cams are only the same in .050 duration and real close in lift. They are not the same in lobe area which you can see in the .200 duration numbers, so there is more cam with the 33XX lobes.

If you are concerned with the intake closing point........ Since it also has less overlap I would just drop the LCA to get the intake valve closing point to happen ealier so you can gain some DCR, which Rich and OldStroker talked about above. DCR is really a big part of power production and big cams will kill it. It's also almost independant of knocj to almost. (meaning you can add DCR and not add knock unlike adding CR itself) If cam one still has less overlap AREA on a 110 then you can go to a 109-108 to see how that works, but on a street car and with that duration it could be too little.

Does this make sense with the DCR explaination?

Bret

 

yes that explanation was supurb!

would the overlap of the 2 cams be a good indication of the cams manners on the street. for instance, cam 2 has more overlap than cam 1 on the same LSA so cam 2 will idle worse. going to a 110 LSA on that XE will only give me 4 more degrees of overlap over the 112 and if i went all the way down to a 108 it would still have less overlap than that cam 2!.. would this mean the idle is still better? in fact, on a 108 it would have just 1 degree less overlap than the XE 230/236 everyone runs so could i go as for as to say it idles just slightly worst than even a cam that size?

 

Overlap area @ .050 and total overlap is what shows the street manners and idle.

I wouldn't go down to 108, 110 is probably the optimum point for that cam. Going to far will put the Intake Closing point in the wrong spot.

Bret

 

alright, but how do you know where the the closing point is on a 112 vs 110 vs 108... how can this be figured?

EDIT: nevermind i found that DCR calculator.

 

so ILC is 112... correct?

anybody know the head gasket bore? deck to piston clearance? and the dish/valve relief/dome volume?