Unequal length runners are not perferred in a well tuned motor. That's what
we end up with in a production vehicle.

I wouldn't go as far as saying the tuning is useless. There are gains to be
made from collector tuning alone.

You could get a custom grind camshaft with individual cylinder
timing. I'm pretty sure top fuel camshafts are set up to account for crankshaft
torsion. Now we're just getting a little overboard for a street engine?

 

wow a real discussion on Cam science on the web no less. There is something to consider that has been left off the plate thus far. Interaction during overlap between the intake and exhuast wave action. In a perfect world you would want zero overlap. You just have to work very very hard to achieve those sorts of efficiencys in cylinder filling. Impossiable but maybe ? I would say the header comments are right on the money but the intake wieghs more heavily. Don't forget to think downstream. The collector if its the same size as the exhuast system will act longer with more pipe length till a cross over point. Just stuff to think about.

 

It has been touched upon in previous posts, but not in depth.

On a naturally aspirated motor, cylinder filling within a tuned RPM spectrum
would suffer dearly without overlap. Even in boosted engines, a small amount
is required to totally rid the chamber of waste charge. I would say very impossible
unless you can re-write the laws of Phyics and win an arm wrestling match
with that nature chick

 

I said in a perfect world. But that would require valve areas on par with bore surface area. would take a damn big port and valve to pull it off but would really kewl to see work. My favorite qoute from somebody out there in the engine building world. " the perfect engine requires no air or fuel to make unlimted power,But since we don;t have perfect engines we just work backwards from there to reality"

I will throw one out there how many dual exhuast X pipe guys are getting good results with wider LAS camshafts ?Just a show of hands.

 

Okay, since nobody else will, I'm going to jump out there and make some bold statements and most likely get flamed for it. . .

There are 2 local engine shops that do ALOT of R&D on camshafts because they both specialize in engines with limited modifications to components. I'm not going to name names, but everyone on the site has heard of at least one of them. Anyway, they both build V8 engines using wedge heads (no hemi's here) and some sort of long-tube header with no exhaust. Both have found that the engines make best power and torque when you add as much overlap as you can without piston/valve interference. I'm talking about cutting valve pockets as deep as you can without thinning the piston top too much, just to make room for the valves during overlap. They've even experimented with "just how thin is too thin" and burnt through some piston tops. This means opening the intake valve as soon as you can, and closing the exhaust valve as late as you can.

However, it has been proven (at least on Hemi-style engines), that you can over-scavenge an engine. A well-tuned intake runner, header tube, and alot of overlap and you actually lose a significant amount of air and fuel out the exhaust pipe. Reducing overlap in this case helped power.

Of course, these dyno results totally disregard idle quality and driveability. Furthermore, I've seen dyno result where adding overlap killed hp if the exhaust system is restrictive. This is because the exhaust pressure over-rides the intake pressure and travels into the intake port during overlap.

Mike

 

[QUOTE=GIGAPUNK]

**Example N/A cam :**
Duration 236/242
Lift .568/.576
Lobe Separation (LS) 112

Add the intake and exhaust durations
Divide the results by 4
Subtract the LSA
Multiply the results by 2
Overlap is 7.5 Degrees of overlap


ok....im new at this whole cam thing but if i do that same equation, i get a negative number for my degrees of overlap???

214/230 = 444
divided by 4=111
subtract 112 lsa= -1
multiply by 2 = -2


im very confused..........

 

That is correct.

 

There's no reason to flame this information, but you have to wonder what
RPM these engine's are turning, the displacement, piston speeds, etc.
How much overlap are we talking here? There is a limit of course...such
a cam would be a tough cookie to live with in a daily driver.

I would also question how well this engine works in a track enviroment
under dynamic load as opposed to a dyno.

No doubt overlap makes good power when used in the correct engine combo.

 

One is turning 7500 rpm and the other only 7000 max rpm. Displacement is 350 cid +/-10.

From 20 deg on up to 60 deg at .050, depending on how much room there is for clearance.

One supplied the cam for the national record in his class and the other builds the winningest engines in the region in a highly competitive class.

 

Finally Mike I AGREE

Not so much.....

Yep this is true.

Bret

 

That's not too bad considering the RPM and application. That's about ball park
for high end N/A motors that I've come across.

 

if its my understanding, a 112 would provide 2 degrees more of overlap than a 114. what about those 2 degrees of overlap cause the 112 to be slightly rougher down low and thus more often selected for manuals and the 114 usually selected for automatics? or do other things affect the aspects that make those differences?

 

That's a common misconception that people have based on LSA alone.
You always have to look at the valve events to determine overlap.

Here's a picture of two lobe profiles on the same LSA:



Read up on this link to learn more about cams:

http://www.hotrod.com/techarticles/c...ew_comparison/

There are plenty of cam write-ups out there, but I would suggest reading
articles from known sites as opposed to a no-name home made site.

 

Adreneline,

A good demo would be one of the picture you just showed.

Take several cams of the exact same @.05", total lift and LSA and show different power numbers based on more agressive lobe technology.

IE: 236@.05" an 140@.200", vs 236@.05" and 146@.05", vs 236@.05 and 152@.200"

 

Actually, going from 114 to 112 increases overlap by 4 degrees because you are altering both the intake and exhaust lobe, 2 degrees each.

 

well lets ask this question are they changing the duration of the camshaft ? some of the superstock classes require stock lobes etc and the only way to make power is to try and scavenge the motor. I don't buy the more overlap the more power argument. Its narrow minded. Prostock cams run higher then 120lsa's.

 

I totally agree, but so far you're the only person to take a stab at determining how an EVO point is done. So which of the 8 different lengths did GM choose to tune the LS6 cam for?

Well other than the: as big as possible, until you're blowing too much fuel out of the exhaust valve, race car idea. Which I have no trouble believing, but let's presume I need to design A cam for my car and don't have the resources to go through 20 cams determining where that point is. Theoretically/mathmatically (not experimentally) how is it found?

and in regards to my mis-calculations...
I also forgot to mention that the formula presumes you're running symmetrical lobes. And the formula also uses .5" lift duration, not .06".

And it's all kinda silly anyway because if you're really concerned with LSA, you should probably be measuring the AREA of overlap, not the number of degrees anyway. But good luck getting the equation of the line of ramps that would be needed to do the calculus from the cam designers. I guess that's why we all actually end up dealing with the weak *** LSA number in degrees and then tweak it just a little to off set for aggressive lobe ramp rates according to experience/experimentation. If I'm right about this, this really is getting more to a black art than science.

 

Does that necessarily mean there is less overlap? These Pro Stockers run durations from 280-300 degrees at a min. of 0.050" lift.

 

I've come across some pretty tricky calculators and texts with formulae to
get you in the ball park. I'm sure you'll agree there's no substitute for
dyno and track testing.

Pipemax is a program that is well known and works quite well. I also have a
reference from "Scientific Design of Exhaust and Intake Systems" which I
used to tune my collector length.

I'll dig that up at some point this weekend and post up the formula.

It's a tough subject to grasp. I try not to obsess over the particulars as I'll never
have the funds, or resource to fully understand how each valve event alters
the power curve for any given setup. I simply rely on tech articles and
tested combos, then look at the cam grinds to get my engine to peform well.

I'm sure there's more power hiding in my setup from valve timing, but money
doesn't grow on trees and I hate the thought of ripping the front end apart
to install a new camshaft just to test my math.

 

We dont have prostock motors...

Theres a slight difference between prostock engines and the 350-450 inch stuff we deal with in practical applications. The more overlap the more power, until you hit a certain efficiency where you can back the overlap down a bit, like in the case of a motor with serious compression. You dont think a prostock cam has a TON of overlap in it even with a 120 LSA? 120 LSA doesnt by any means means it doesnt have a bunch of overlap to make power. With a very effecient engine with a good bit of compression in it, the wide LSA and earlier exhaust valve opening doesnt hurt power at all because high compression motors generally generate cylinder pressure much sooner so it doesnt bite into the power stroke.... and theres still plenty of room for a ton of overlap. Somehow I dont think we hit Prostock engine VE numbers though