Lift vs Duration

[JBirch]

I am looking at getting a set of cams for my Nissan 3ltr DOHC (its a combination of 3ltr block and twin cam head, not a production motor). The motor is non turbo. The main use of this car is club days, motorkhanas etc and not everyday driving.

The question I have is am I better off to sacrifice some lift for additional duration or try to keep as much lift as possible and settle for less duration.

Ultimately I would like to get some custom billet cams with both high lift and long duration but I can not justify that sort of expense at this point in time.

Thanks for your help!

 

[ivymike]

Higher lift doesn't necessarily add anything, depending on where you are w/respect to flow.

Do you know what the right duration is for your system? Longer is not always better.

 

[patprimmer]

Why is it a choice. Most cams with higher lift also have longer duration.

If you get shorter duration, but higher lift, you will need stronger components or you might suffer substantially reduced component life.

With a twin cam, you can play with lobe centres to move the peak power points and width of power band vs peak power level over a narrow range around a bit.

I guess the head is off a 2.5 or 2.6 litre engine.

I guess the extra capacity is mostly due to increased stroke, and probably reduced rod to stroke ratio.

Longer stroke larger capacity engines generally tolerate long duration cams better due to higher piston speeds and higher air speeds.

Short rod engines reduce the efficiency of the ports and require longer duration cams to reach high rpm, however they also put extra strain on pistons and cylinder walls, thereby reducing durability at higher rpm, making long duration cams unwise if durability is required.





Regards

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[thundair]

Patprimmer is correct in that the lobe centers will have a lot to do with your RPM and torque requirements.

I would suggest making a spread sheet and use some of the free software to calculate your needs. For instance if you want to have power between 8,000 & 10,000 RPM you would calculate the tuned length of you exhaust and intake.Also the dwell time that you rod ratio gives you will determine the intake opening knowing you want to chase the piston down the bore. There has been a lot of information recently on exhaust closing early and even the ratio of intake to exhaust valve size.

check out this forum

http://speedtalk.com/forum/viewforum.php?f=1&sid=8890a8dde1ec6ad06c83a7e1bc8d3b05

I don't know anything but the people that do.

 

[whitevette]

Hi, JBirch!
This is my very first post on this site (& they're all different!), so I'm learning to navigate, too!
Your question as to go HP or torque when considering a cam profile... just bear in mind a couple of things: 1) Torque moves a car; HP sells the car! At 5252 RPM, HP=torque! Higher RPMs may generate more HP(the rate of doing this "work"), but you'll pay dearly for these higher wear rates! A Formula I engine winds & winds...for one race! Your car has to last!
Steel billet cams are nice...but they don't do anything extra , just bump the valves up & down! Cast iron works fine!
Regarding the "lift vs. duration" argument, more lift stresses components more, wears lobes/lifter faces faster, and sounds more impressive to fellow bench racers! Duration ( and its companion - overlap ) affects the gas(eous) flow much more than lift! Years ago, one of the cam regrinders stated "25% of the flow occurs during the first 0.003" of lift." Hmmm.... What does this say for higher lift? Stick with duration changes on a cheap regrind, and LET THE REGRINDER PICK the best profile for your application! He know his business much better than anyone else!!! IMHO...
-whitevette

 

[patprimmer]

25% of flow with 0.003" lift! garbage, unless it's a model aircraft engine.

It is a simple matter to look up some cylinder head airflow figures for most popular cylinder heads. Professional porters and aftermarket head suppliers promote this data.

Lift has more impact on idle than duration, garbage again.

I would not take any advice from any grinder who advises that 0.003" lift gives 25% flow, however cam grinders with a proved track record can be a useful source of advice, but keep in mind, they have a profit motive behind the advice they give.

Always ask for opinions to be backed up by data before parting with cash.



Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[GregLocock]

How thick is your boundary layer? I bet it is a damn sight thicker than 3 thousandths of an inch.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.