WSsick

iTrader: (13)

I have really been looking around hard to gather a lot of info about cams, as they are "the brain" so to speak of an engine. The thing that has really got me stuck is overlap. I've looked over Pat G's recipe to 500rwhp post a few times and many, many others. I've seen someone's sig around here with the overlap equation but I cannot remember it exactly. IIRC, you must use adv duration right? Anyways, I was informed today that overlap has a lot to do with the choppyness (is that a word?) of the cam in addition to the LSA. So really what I am trying to figure out here is where over plays into the power making of the cam. I'm confusing myself with too much information, misconstrued and correct, so if anyone can help me out here with explaining all the elements which overlap is and what part they play, it would be very much appreciated.

JPH

iTrader: (2)

Overlap is the period where the engine helps pull/draw in the a/f mixture into the motor/cylinder thru the elapsed time and period(IVO/EVC) that the cam provides, and the determined amount has alot to do with the engines VE and power output. Alot of things in an engine build will/should determine how much "overlap" the cam should provide/have for the engines needs/wants. Displacement, compression, rpm range, bore/stroke, etc.. will help determine what kind of cam profile should suit that engines particular goals and the amount of overlap will be determined by lobe size/area and when/where the valve events should or must occur. And YES, Overlap=chop at idle.

16floz470ml

iTrader: (2)

Overlap is when both the intake and the exhaust valve is open at the same time. Typically if you want to rev really high overlap is good in a cam. It is not good at lower rpms which is where the chop is.

WeathermanShawn

iTrader: (1)

I can understand your dilemma. Not all cam 'science' is easy to understand.

I use Pat Kelly's DCR Calculator, which I believe is linked via the stickies or Internet search.

Yes, it uses advertised duration to compute overlap.

Both post replies are excellent replies. They would also probably agree that overlap is somewhat of the 'sirens song' of making cam power.

For example, I have ran cams from -6 overlap to +10. There is absolutely no doubt that the lope/chop of the +10 overlap cam is intoxicating. I would argue that under the best combination of heads, intakes, exhaust, gearing, etc., that the additional mid-range punch (I.E. 2800-5000 RPM's) is where you might see an additional 20 HP/TQ.

Downside is tuning, and some loss of low end driveability (<2000 RPM'S).
But, it can be successfully tuned, and it simply depends on your goals power wise and tolerance to a few quirks driving in lower rpms.

I would suggest there is probably a 'sweet spot' where you can have both. Many have suggested a cam overlap of ~6 degrees will give you the best of both worlds.

But gain, as always it will be what your goals are and the combination of parts that will ultimately determine your exact power band.

Hope this helps.

..WeathermanShawn..

WSsick

iTrader: (13)

thanks to all! so far, nothing be great info and clarification ive been looking for. i might be buying a trans am this week and as expected, i already have plans for modding before i get the car! lol. but i want to have a set plan to do every mod to help out all the others and the way i see it, there is no such thing as too much info on cams. i have been playing around on TSP's website with the custom grinds just to get an idea of what works with what to see if id have any luck with making a goo combo of lobes and what not. i'd like talk to a cam guru about my grinds for a cam jsut to see if i am heading in the right/wrong direction a h/c will probably come in a few months after getting the car and getting it setup properly for a h/c, but there is nothing stopping me from planning it all out now so it is not a rushed decision!


KEEP IT COMING GUYS!

12secSS

iTrader: (30)

I wouldn't call myself an expert, I just know how the valve events work.

Having said that, the time the intake and exhaust valves are open at idle cause the combustion to get contaminated with exhaust. This causes the combustion not to be as "strong", which kind of wants to make the engine shut off. At higher rpms, the exhaust scavenging affect help clean out the chambers and pull in a fresh mixture of air and fuel. So as Shawn above said, it can be used to tune the power band based on where the most scavenging affect really promotes cylinder filling.

BTW, this is the formula I use:
Add the durations of the cam together, then divive the sum by 4, subtract the LSA from the result, then multiply by two and you get the overlap.
(((Intake + exhaust) / 4) - LSA) x 2 = Overlap

Sales2@Texas-speed

iTrader: (5)

You got it! I always use (Intake+Exhaust)/2-(2xLSA)=overlap....but they're the same formula

WSsick

iTrader: (13)

So just to make sure I am doing this right, I will use the MS4 (since TSP responded)
(Intake+Exhaust)/2-(2xLSA)=overlap
239/242, .649"/.609" 111LSA (copy&pasted from the site)

(239+242)/2-(2x111)
(481)/2-222
240.5-222
=18.5 degress of overlap
correct? I wanna make sure I am doing this correctly before I go on.

WeathermanShawn

iTrader: (1)

Yes.

I also computed..18.5 degrees overlap MS4.. (using fancy DCR calculator)..

..WeathermanShawn..

69LT1Bird

iTrader: (1)

yep, you got it.

AznMuscle

iTrader: (7)

I got the same. And overlap also helps in determining the idel? So with that large of an overlap, that is why that cam lopes like a mother ******?

WSsick

iTrader: (13)

Just in case anyone wants to learn along with me, here is a few more cams:
MS4=18.5 (111 LSA)
TRex= 23 (111 LSA)
Tqr2= 6 (113 LSA)<--my friends cam
TSP 224/224= -4 (114 LSA)
TSP 228R= 0 (114 LSA)
TR Reverse Split= -2.5(114 LSA)
cheaTR= -12 (117 LSA)
(hope my math is right someone correct me if I am wrong, and I'll change ASAP)

Once you start to do the math, it makes a lot more sense of where people's ideas of "streetable" come from. Sure, it is objective but the overlap now spells out pretty clearly how a cams street manors will be. So, using this, I just played around on TSP's custom grinds and came up with this cam grind, just to see how overlap would be and how it would/could compare to a bigger cam(like the MS4, TRex, Vindicator).

242/244 .621/.612 111 LSA (XFI/XFI) =21 degrees of overlap

---I'm not saying this WOULD beat those cams, but I took a little bit from those and tried to avoid the LSK lobes for now, since they are said to be the hardest on the valvetrain. How do you think would work at the track? Obviously, on the streets, 95% of people will say its not streetable.

magic

iTrader: (4)

cam tech noob question but....what are the pros and cons ( IE. streetability, longevity(valvetrain), fuel economy, emissions) of having a negative overlap (less than 0) vs. a positive overlap (greater than 0)....??


Also a comparo between:

TR224 on a 114 has a -4 overlap
LPE GT11 (215/231 and .631/.644, 118 LSA) has a -13 overlap....

TSP 228R = 0 overlap

VS the "Donkey Dick" cams with more than +15 overlap...


And the obligatory "which cam would work better in my combo" question...6.0 LQ4 (stock heads and compression, with LS6 intake with a aftermarket Throttle Body and full exhaust 1 3/4 headers etc etc)....

5_02ls1

iTrader: (20)

myself i hate the big overlap cams period....my first cam 220/224 114 -6 netted me 4mph at the track made right around 380rwhp...on a mail order....great drivability as well...the one i have now was speced out around -1 ...after alot of tuning it idles close to stock but pulls hard to 6800..

5_02ls1

iTrader: (20)

the tr224 on a 114 would peak a couple hundred rpms later..not make as high of a peak # but have a broader powerband than a tr224 on a 112...
the lpe cam would be a high peaking sum bitch...the longer duration cams usually have the lsa lower so the powerband will be somewhat useable...if any of that makes sense...

AznMuscle

iTrader: (7)

As far as I know...emissions wise, the lower the overlap, the better chance of meeting emissions. I don't fully understand overlap, because lets use tr224 for example. Great all around cam, get low end power, and can still pull up top. -4 over lap. We could take a cam with similar duration, or slightly larger, like low 230s, and have say 1 or 2 over lap, yet the 224 would still perform identical. Well that is what I have seen atleast from other peoples dynos. Why is that?

5_02ls1

iTrader: (20)

lsa is like a buy product...the same cam lets say tr224 on a 112 will peak around 6k add the 2 to the lsa 114 will move the powerband up a bit but sacrafice a little midrange power and tq.....a tr 224 on a 114 will peak close to a 228 on a 112 but not make as much power...lowering lsa lowers the peak of the powerband....

more overlap means more unburned raw gases +bad on emissions...

usualy having more overlap increases dcr its like raising compression sorta...if that makes sense...im not the worlds best at putting things into lehmans terms..lol

WeathermanShawn

iTrader: (1)

Overlap.

You'll get many differing opinions.

Emission compliance usually requires -4 to -6 degrees overlap.

My best advice is to pick a cam that has the proper valve events for your application. If it's racing, I am sure many sponsors will be glad to hook you up. The overlap will just be a nice side-effect if the track is your goal.

Have fun.


..WeathermanShawn..

WSsick

iTrader: (13)

hmm, my thread got hijacked. j/k. but i'd rather you not come in and ask "whats best for my setup." trying to be polite but you can always PM or call a sponsor or start your own thread for that. id like to keep this just about overlap and its effects. thanks

magic

iTrader: (4)

Well since the topic was cam overlap etc, I thought it would be a good venue to ask the questions and hopefully figure what all this "greek" (cam overlap and its effects) means and how it can help with cam selection, which Im sure many others who have read this thread are wondering the same?

If Ive , then I appologize if Ive stepped on toes....