This question has been posted by me and another guy on this board in various posts and has yet to yeild an answer and hopefully some one can answer this question for us!

What is a more determining factor affecting performance when it comes to cams: Duration or Lift? Why?

 

both. i think that theres a balance of lift and duration that effect ultimate power output. you guys arent even talking about ramp rates, which also have a large effect on HP output

 

Exactly. They are both related to air flow. Lift allowing the valve to open further and intake and export more volume of air. And duration, being the amount of time the valve is open or the amount of time the chamber has to intake air and export air.

Lift and duration is like love and marriage. (Finish the song here)

 

i would say lift has a greater impact than duration... lift always allows more air in assuming the head flow isnt a limmiting factor... duration lets in a different amount of air in depending on the acceleration of the pistown downwards. also valve open and close has to be timed just right for peak power so its not like adding x amount of duration lets in the x amount more air.

i think that rocket said it best when he said lift is what makes power on honda heads, duration is just there to acheive that lift.

 

i hate to reply to my own posts but im sick of editing for laptop induced spelling errors

also 1 big thing is that increasing duration while keeping lsa's ect the same will shift the powerband up.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by JCushing &raquo;</TD></TR><TR><TD CLASS="quote">also valve open and close has to be timed just right for peak power so its not like adding x amount of duration lets in the x amount more air.

i think that rocket said it best when he said lift is what makes power on honda heads, duration is just there to acheive that lift.</TD></TR></TABLE>

I disagree. I think duration makes more power than lift. Look at a JUN cam versus a Toda cam for example. Granted ramp angle has a lot to do with it but a JUN 3 cam which had .5mm less lift than a Toda Spec C cam will outperform the Toda cam everytime in the upper RPM where piston speed is at the maximum.

Adding X amount of duration does allow in X amount of air. As long as the valve is open there will be air flowing in. You can't avoid that. But your right when suggesting that "X" amount when increasing X amount of duration may not be adding value to making power. More clearly, just becasue you increase duration does not mean the new found air will be optimized becasue the intake of the air and the closing of the valve needs to related to the position of the piston and the flow capabilities of the head.

 

you do realize of course that we both said the same exact thing lol

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by asubennett &raquo;</TD></TR><TR><TD CLASS="quote">Exactly. They are both related to air flow. Lift allowing the valve to open further and intake and export more volume of air. And duration, being the amount of time the valve is open or the amount of time the chamber has to intake air and export air.

Lift and duration is like love and marriage. (Finish the song here)</TD></TR></TABLE>

I agree. very well said, nice analogy as well.

 

this question can't be answered. ramp and duration are somewhat related. longer duration usually have longer ABDC valve closing degrees. power output also depends on the ports, CR, and blah blah matched to the cam.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by JCushing &raquo;</TD></TR><TR><TD CLASS="quote">you do realize of course that we both said the same exact thing lol</TD></TR></TABLE>

LOL, I just re-read it and I suppose they are fairly similar. Prehaps more of a clarification along with my belief that duration impacts peak numbers more than lift.

 

Duration has the geater effect on performance.Lift will usually improve the air flow and make more power,but not much.When a valve is at it's peak lift,it's only there for such a short time that adding 1-2 mm of lift isn't going to affect much in terms of airflow.The ports on a Honda Vtec head are good enough that adding lift helps because they will flow at higher then stock lifts,but some heads don't even flow well up to there stock lifts so increasing lift does really nothing except put a little more area under the lift curve.Duration is what keeps the valve open longer and determines the rpm range the motor will perfom best at.

 

well.... let me take a wack at it

comparing two cam profiles where one cam has more lift and one has more duration......

the cam that has the higher lift would probably make more cylinder peak pressure but less often during the compression stroke.

the cam with the higher duration would probably make less peak cylinder pressure but a higher mean effective pressure. Meaning the cylinder pressure would not increase and decrease as quick as the cam with higher lift.

And the higher duration cam will probably benefit more from high compression compared to the cam with only higher lift and less duration. generally speaking

 

Thanx I appreciate the information in this debate, there are plenty of good theories here to give me an understanding!

Another question I have is:

1. What are ramp angles, and what do they affect?

2. What does LSA mean?

 

LSA is your lobe seperation angle and it means the difference in degrees of the centerline of the intake and exhaust cam lobes. the farther apart (larger lsa) the less time that both intake and exhaust cams are open together ( this is called overlapwich produces extra power due to the vacuum effect of the exhaust gasses leaving making a low pressure area in the cyl drawing more air in the cyl through the intake.)

its one of those things that kinda needs to be kept constant when comparing things like duration because it can affect power itself. ie changing it makes an extra variable when comparing cam specs. LSA is altered primarily through cam gears on dohc engines and is buiult in the cam in sohc.

ramp angles affect how quickly the valve opens and kinda hand in hand how long in degreed the valve spends at a specific lift or how quickly peak lift is acheved. not a whole lot to explain there and since its not given by cam manufacturors its not huge point of discussion. one way to determine ramp angles (other than by looking at the lobe itself) is too look at duration at .5" 2 cams can have the same advertised duration nbut the one with more duration at .5" should make more power all things being equal.

1 more point to make regarding duration is that the main reason it will make more power on the top end has to do with the fact that the faster the engine is spinngin (rpm) the less time there is in each intake stroke to take in air. adding more duration will allow more time to draw air in so you wont starve the engine in high rpms... conversly at low rpm the added duration will hurt performance because the added overlap will actually cause the cyl pressure to decline and contaminate the intake charge . this is why increasing duration raises your powerband wile loosing power down low and decreasing it does the opposite.


Modified by JCushing at 5:40 PM 9/9/2004


Modified by JCushing at 6:14 PM 9/9/2004

 

Very thorough description! Thanx!

 

this is an interesting thread ..there seems to be some smart doods in here...how about compression ratio..i noticed peeps are saying you want higher cr for these bigger cams that are out there,whats the story on that?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by spaceman &raquo;</TD></TR><TR><TD CLASS="quote">.i noticed peeps are saying you want higher cr for these bigger cams that are out there,whats the story on that?</TD></TR></TABLE>

To touch on what Jcushing said above about LSA and duration effecting cylinder pressure. Cylinder pressure is the reason why big cams needs big compresion to work correctly. You need to compensate for the missing cylinder pressure that big cams will bleed off.

 

thanks for the info bennett..are you on http://www.azhtsquad.com?

 

compression has a lot to do with overlap and cyl pressure. the higher the cyl pressure the more the air/fuel mixture is heated and compressed the more powerfull the explosion (the easier it is to burn). running higher compression ratios compensates for the bleed off of cyl pressure at higher levels of overlap where cam timinig is bleedinig off cyl power. this is why you match the cr of the engine to the cams your running.

undercamming the engine or running too high of a cr results in excess cyl pressure that is higher making tuning difficult and no gain in power compared to a lower cr because your already buring the mixture to its full potential.

overcamming the engine results in loss of low end power due to loosing cyl pressure. running too low of a compression also slows down the burn speed of the air/fuel wich means you have to advance the timing to give the fuel more time to burn so your not missing the most effective point of crank rotation of the combustion cycle

edit
this also reminds me of why compressiono affects top end power as well. like i said compression increases the burn speed of the a/f mixture. just like lack of duration starving the engine of air the higher the RPM the less time there is per power stroke for the a/f mixture to ignite. if your revving at 10k chances are your nont gonna have enough time get a full burn of your a/f before the engine is already drawn the piston down on its own or your going past the most effecient point (leverage wise) to push down on the piston for the most power.


Modified by JCushing at 4:22 PM 9/10/2004

 

Well said JCush.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by spaceman &raquo;</TD></TR><TR><TD CLASS="quote">thanks for the info bennett..are you on http://www.azhtsquad.com?</TD></TR></TABLE>

I actually have never seen or heard of that site. Shows how much I get out. lol

 

yeah good post jc..now i just gotta figure out what the optimum cr for my motor would be if i were to get some wild cams...asubennett what kind of car do you have? if you have an integra then there is this site for locals as well http://www.aztegs.com

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by hybrid_vtec &raquo;</TD></TR><TR><TD CLASS="quote">This question has been posted by me and another guy on this board in various posts and has yet to yeild an answer and hopefully some one can answer this question for us!

What is a more determining factor affecting performance when it comes to cams: Duration or Lift? Why?</TD></TR></TABLE>

most of those dudes left out the factor of fuel......wat the point of life and duration to increase power bang without fuel ratio adjustment...

im sure lift more power if there is the right volumetric fuel adjustment...and of course lift require duration to make power, they are interrealated and interdependable....so you can't really say lift or duration which one is mkaing more or less...

too much timing in lift and duration, of course will result of loss and this know as overlapping each other...but highpower bang racing engine seem to luv overlapping at high rpm but in our case overlapping is bad...so

lifting and duration all come to timing and adjustment of fueling ratio....(don't forget fuel)

more air for lift doesn't mean more power, velocity of air traveling in intake to lifting is also a factor of increase power bang and important of all..you must have enough fuel to burn with that air...

2hp = +1 pound of fuel pumping per hour (figure it out yourself) 2hp won't count if there not enough air...
lifting and duration are useless without adjusting fueling...

 

THat last post had little if anything to do with his orinal ? nor did it make much sense.

Of course you need more fuel for a motor that moves more air throught it. I don't think the person who posted the original ? needed to be educated on that.


No offense but this last post reminded me of the "Puppy that lost it'd way Story" from Billy Madison.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by asubennett &raquo;</TD></TR><TR><TD CLASS="quote">THat last post had little if anything to do with his orinal ? nor did it make much sense.

Of course you need more fuel for a motor that moves more air throught it. I don't think the person who posted the original ? needed to be educated on that.


No offense but this last post reminded me of the "Puppy that lost it'd way Story" from Billy Madison. </TD></TR></TABLE>

Thanx, your right Im not that stupid but there are some things that I dont know, so I try to educate myself on those matters! Tis thread has been very interesting!

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by hybrid_vtec &raquo;</TD></TR><TR><TD CLASS="quote">Thanx, your right Im not that stupid but there are some things that I dont know, so I try to educate myself on those matters! Tis thread has been very interesting!</TD></TR></TABLE>

ok...fine....take it as a reminder or not is up to you...thread making need to be specific...dont assume anyone know everything....just like writing an essay on automotive stuffs..you know...peace

calm down i did't try ti educate anyone....just that much and making you mad already....bad temper...