Okay guys, first post on here(coming from CorvetteForum), so hello everyone. Anyways, please help me understand cams a little better. I read over the sticky at the top & have went through many other threads, but I am still confused. So, how do ramps & cam duration work in relation to power outcome & head flow & velocity? From what I gather, the less duration w/ the larger lift is best, since is creates more ramp. If this is true than how come cams with larger durations(more aggressive), are often sought after for more power(especially at the strip)? This was also the impression I was under(that larger durations make more power), until I read a quote from John Lingerfelter(posted on here, but I didn't want to accidentally jack the thread) saying that you should pick the cam with the least duration & most lift. This has rocked my world, so I am in utter confusion. Also, if this principal is true, then how come you would need to choose a particular head before you choose the cam. Because if I am taking this quote correctly(which I might not be), then the cam with the least duration & most lift would still create the most power, so there would be universally be a "perfect" cam that everyone would have. This is obviously not true since there are many differentiations & options out there. So, someone help me here, these principals as I have clarified them contradict themselves. One more thing, upon attempting to understand these concepts, I am ultimately trying to find the perfect head/cam setup for the street, from a roll. So could someone also help clarify how higher ramps, duration, lift, head velocity & flow work together BEST for the street from a roll? Sorry for the long post & crazy # of questions. Please answer & interpret whatever you can. Thanks so much guys. *Leaving out smilies for the sake of length

 

Search around for the cam discussions. There were two parts to them and were many pages long.

 

Which discussions are you refering to(I'll do some searching in the mean time, but just in case I don't find it)? I have seen MANY threads on here relating to cams, so could you help me out here & be specific. Thanks, I appreciate it. Also, any additional help still appreciated.

 

Sorry, I thought I knew what it was called but couldn't find it searching.

 

Hey, no problem. I'm still searching right now for it. Thanks though. Anyone else?

 

Not saying there is a perfect cam. If you read the treads and look at the questions they ask you will find peolpe are looking for something specific for power, driveablity, etc. The original thread is what people though of the cam he was thinking of choosing and if someone had run it before. Then someone made the remark he should choose this cam over another or even this cam. The end result was that all of the cams they were talking were in the 233 to 240 range with 112 to 113 lobe separation. What I was trying to tell them is if you want to work in this range pick the one that has the most lift out of the bunch with the smallest duration. I will agree that most 233 to 240 will make more power and torque in the 2500 to 6500 range than a 200 to 215 range. People who use the type of cams are looking for stealth sound. It comes with a price as you give up a few extra horses but you have a sleeper as to speak. Even then you should still use the same concept when looking in this range as well, perfect examples are the TRcheater cam and Lingenfelters GT11 that they came out with recently. The reason some people go with this theory is that given an rpm range the heads, intake, valvetrain can not make use of the extra RPM the engine is trying to gain with larger duration (trying to cheat it to get there). It loses massive amounts of low end for 5hp or so gain up high. In the end it is easier to over durate a motor than under durate. It least if you under durate you will still have more low end that can make up for alot of other things.

 

Mac, excellent reply. I gained plenty of knowledge by simply reading your post. Thanks for the help. Anything else guys?

 

Actually, you CAN have a long duration cam with a fast ramp profile... simply one that opens the valve real dang fast and holds it open longer, then shuts it real fast. The whole deal with short duration and high lift is a more tame approach to getting more power, but it costs you in the way of valvetrain longevity. On the flipside, you can get a slower rate, longer duration cam with the same lift, and sacrifice some driveability for your power, yet squeeze a little more life out of your valvesprings and such, too.

Matching heads to your cam basically involves a combo that offers the best air velocity given the parameters of each. In a nutshell, a head that has ports designed for high velocity in and out of the chamber will be more user-friendly than a head with abyssmal volume designed for all-out racing. Why? Air has momentum when it moves, and with a high-velocity head, the air will have more momentum to it, in essence "ramming" more air into the chamber with a longer duration cam. Keep inmind, this is all to a certain point, now. You can get too stupid on heads, and you can get too stupid on your cam selection. As a rule, if you want driveability and torque, get about a 210-220ish* duration cam and use it with stock or mildly ported heads... something that'll give you that velocity I was talking about. If you wanna make power at 10,000 rpm, then get a titanium rotating assembly first, THEN get some wildly tricked out heads with ports the size of stove pipes, and about a 260ish* duration cam.

Forgive my being brash if I come across as such... it's late and I'm tired.

 

Basicaly the motor is a pump.
Cam choice is based on how to maximize this pumping effect within a certain rpm range.
there is no best of both worlds. There are cams that have the ability to get a bit of both (under curve trq and high rpm Hps) like Futral (cam motion lobes/grind), but there is always a compromise.
The rest comes in the devellopment of a COMBO. the hard part is matching that combo perfectly. Since it is much easier to match a cam spec to a set of heads, that is usualy how it is done.
Combo does not just stand at the motor. It is really the whole car. from its weight, tranny gear, rear gear, suspension, and stall if auto.

 

Cam Discussion I
Cam Discussion II
Cam Discussion III
Cam Guide

 

The pricipal behind longer duration is this - air will only travel at a certain maximum velocity unless you pump it (super or turbo charge) - so the faster the engine is turning, the less time the cylinder has to fill with air. Keeping the valve open longer gets more air in at higher RPM, good right? Higher RPMs with more air (and fuel) make more power. Add some overlap so at higher RPMs the incoming air helps push out the exhaust and your really maximizing the cylinder fill. So far so good, but at lower RPM, the cylinder is already full because the air has had plenty of time to get there and the valve remains open as the compression stroke begins so you start to lose some of your air, and now your overlap is just pushing your nice compressing air back into the intake - thus lope. So big duration (and high lift) work well at higher RPMS but work against you at lower RPMs. Keep in mind though that higher lift (more air) is good at lower RPM too, but with a smaller lobe you need a serious ramp to get both high lift and short duration, so there is a compromise there. When Lingenfelter said if you're undecided between two cams chose the shorter duration higher lift, I think he was saying that an engine is more forgiving of a shorter duration than one that is too long. If the peak RPM is at 6000 RPM and you only see that once a month at the track - you're better off with one that peaks lower (shorter duration).

That's my .02 anyway.

 

There are some good points in these replies, but you're going to have a hell
of a time understanding all of these points without understanding how
pressure
waves travel; pulse tuning effects power, etc.

Even with many books and threads that I've read, they are just basics of
countless functions happening within the engine.

Not necessarily true. Every engine, and each application requires a specific
design to be most efficient. Engineers are constantly researching combinations
of valve timing, port designs, intake and exhaust tuning to improve pumping
efficiency.


Think of lift as a secondary parameter. If you had a "valveless engine" which
didn't need to "lift" a valve, you would soley rely on duration to make power.

In other words, if your cylinder heads peak flow 300 CFM @ 0.500" valve lift,
there would be no need to lift a valve any higher....if you could hold the
valve at 0.500 lift for 240 degrees duration.

The reality is, it's impossible to design a lifter and valve train that can ramp
up from 0.000" to 0.500" lift instantly. The ramps are needed to control the
lifter and valve assembly smoothly into a transition from zero to peak lift and
back to zero.

Since the lobe is egg shaped, the duration along the lobe profile is always
changing. At 0.500" lift, the duration may only be 20 degrees on a 0.510"
effective valve lift.

In order to allow the lifter to safely climb the ramp and get more duration at
0.500", the cam lobe lift is increased. With rocker ratios in mind, the duration
at 0.500" may now be 50 degrees, but the maximum lift has incresead (maybe
to 0.550" valve lift).

If you could design a lifter to follow a square lobe, you could have 240 degrees
of duration at 0.500" lift which would match the peak head flow.

The goal is to leave the valve open longer at the head's highest flow point.

You're also seeing people talk about reversion and scavenging. This concept
is tough to envision. Just imagine the air moving in and out of the cylinder
is pulsing according to pressure differentials in the intake runner, cylinder and
exhaust system.

You can relate the air pressure pulses to waves of water crashing back and forth.
In one instance of time, if a wave of air is positive moving toward the intake
valve, and the cam begins to open the intake valve as the piston is drawing
downward, you can create an additional charge filling. This is normally referred
to as the "ram effect".

Conversely, if an exhaust stroke has just finished and we're waiting for both
valves to open for the overlap period, it is possible at certain RPM to have a
a high pressure exhaust pulse return back into the chamber and create a higher
pressure than the charge coming down the intake head runner.

Since the exhaust pressure is higher, the charge moves back into the cylinder
and does not allow as much fresh charge to enter for the next combustion
cycle.

In a perfect world, the intake and exhaust pulses would be synchronized at
every RPM and the scavenging effect would be positive all of the time.

Unfortunately, we're dealing with pistons moving at different rates, and changing RPM.

This changes the frequency and wave lengths. An even bigger problem is
the valve timing doesn't change; the exhaust length doesn't change, nor does
intake length change.

Now you can begin to appreciate why chosing a cam for a specific RPM range
is critical. Valve timing is everything. THe engine is a system. If you change
one aspect of the system, you have changed a range of dynamics - ultimately
trading off low end power for high RPM power and vice-versa.

 

Now that's what I'm talking about.

Can you believe I did a title search for "cam discussion part II" and "cam discussion" and "cam disucission part" and got none of those????

Bookmarked now.

 

Wow, thanks for the replys.

777 - thanks for the links... I bookmarked them as well
Everyone - I have seen some excellent posts here, I appreciate it.
Adrenaline - That post was GODLY

Wow, I think I actually understand now. Thanks again guys.

 

I'm gonna argue with you on this point. I don't think that lift is secondary at all, if you can't get enough valve diameter into a cylinder head or bore then you have to make up for the lack of curtain area with lift.

Just because a port stops improving flow past a certain amount doesn't mean opening the valve more is a bad thing, lots of things play into that. Even worse is when a port stalls at high lift even if you don't open the valve that far.

Bret

 

just the info I've been looking for. thanks guys.



fuerzaws6

 

In a conventional engine, I don't think of lift as secondary either. It was just
an analogy to debate the initial quote.

Of course you need to remove the obstruction from the port in order to flow
the air - no argument there at all.

You did however catch my attention with the following:

In my hypothetical scenario where the cam lobe is square (giving us max
duration) and we're getting peak flow at 0.500" lift, what are the additional
benefits of lifting the valve higher?

It would be great to read more about the benefits as I`m not aware of any.

Thank you.

 

Adrenaline,

It's been found by a lot of guys that if your port stalls at all there is a serious problem in the head port that is going to effect power because of the various levels of depression that the port actually sees during operation. Usually if you find a stall point at say .650" @ 28" if you turn up the bench to say 36" then that point could be at a lower lift were the valve is actually open. In the port you can see depressions upwards of 100" during operation so as you can see this can be a big issue that looks ok on a static 28" bench but is worthless on a motor.

As for lifting the valve more than the max flow? Well mostly that works out in the valve control and area of the lobe questions. Your squared off lobe is going to be a SOB to control and it's going to put tons of loads and deflections into the system, if you can get the same or more area as that squared off lobe then you will have better valve control and most likely a higher RPM range due to controling the valve better.

Bret

 

Keep it coming guys , I'm eating this stuff up.

 

That makes good sense now.

The flow bench doesn't do a very good job of replicating the reciprocating
piston that's for sure, nor flow characteristics across all RPM points.

As for the square lobe, let's not waste energy on that. I'd like to see the
poor spring that has to guide the valve down...and keep it down at 6000 RPM!

Thanks for the lesson.