jason5

iTrader: (2)

does higher ratio rockers change the duration of the cam or just the lift and if it does how do you figure out the new duration

Ben R

Rockers change the lift.

If you're looking at LS1 cam specs, they're probably given in gross lift and they're assuming you have a 1.7 rocker. Take the lift given, divide it by 1.7, and then multiply that number by your new rocker ratio.

Example: If my cam has .600" lift and I have a 1.5 rocker but want to use a 1.8 rocker I would do this.

.600 / 1.5 = .400

.400 X 1.8 = .720

Your cam would now have .720" lift.

narcszm

it's not easy to calculate. the above post talks about how to calculate max lift, which is good for clearance calcs and max flow determination, but that's only part of the story. You need the .004" and .050" durations to help you determine the ramp rate of the lobe, which can help you calculate what your increase in duration and lift will be.

Yes, higher ratios increase your duration. For 1.85:1 rockers multiply duration by 1.088 (increase of 8.8%) and for 1.80:1, multiply by 1.059 (increase of 5.9%). If you do the same for your lift (as shown above) you'll be pretty close on what your new valvetrain events and lifts are.

jason5

iTrader: (2)

what about 1.75 ratio rockers

mrr23

iTrader: (3)

no, cam duration isn't changed. period. the valve still opens and closes at the exact same time the valve events are ground into the cam. if the cam opens the valve at 42* and closes at 50*, increasing the rocker ratio doesn't change that.

now what people confuse as duration is the speed and distance at which the valve opens at a given LOBE lift. using some numbers. let's map a cam. we'll use 0* as the opening point and 90* as max opening. lets say this is with 1.7 rockers as well. it gives an 'EFFECT' of changing the duration of a cam.

0* = 0 lift
15* = .100 lift
30* = .200 lift
45* = .300 lift
60* = .400 lift
75* = .500 lift
90* = .600 lift

now we change to 1.8 rockers
0* = 0 lift
15* = .105 lift
30* = .211 lift
45* = .317 lift
60* = .423 lift
75* = .529 lift
90* = .635 lift

what it changes is how far off the seat the valve is at a given LOBE duration.

when manufacturers tell you duration @ .050 lift, they are refering to LOBE lift. not VALVE lift.

Patrick G

iTrader: (14)

Actually mrr23, you're not exactly correct. Higher ratio rockers WILL change duration some. Not from 0 to 0, but they will at .006, .050, .200 and anywhere in between. Think about it. With a 1.85 ratio rocker, your valve gets to .006" lift just a little sooner than it would if it had the stock 1.70 rocker. Getting to the lift a little sooner means the duration at .050" will be around 3 degrees or so bigger on a stock LS1 cam. Not a huge change in duration, but you can't go on the record and say that higher ratio rockers don't change duration without qualifying it further.

Do they change it at 0 ramp? No.
Do they change duration at .050"? Yes, but not enough to keep most people from stepping up to a cam with REAL duration on it.

mrr23

iTrader: (3)

again, you physically cannot change the duration of the CAM by adding rockers. the ramp and duration is ground into the cam. period. what you are forgetting is that duration measured is on the LOBE lift of the cam. not the VALVE lift. when they measure a cam to get duration at given lifts, it's on the cam LOBE. not at the VALVE. you are confusing VALVE lift and LOBE lift.

here's a cam card right off comp cams site. at the bottom they give you the LOBE lift at .050. that will never change by changing rockers. CAM CARD

what you change with rockers is what happens at the VALVE. not at the CAM. think of it as VALVE duration and not CAM duration. it's time to discern between the two.

so cliff notes:
does change rockers change duration?
CAM duration = no
VALVE duration = yes. if you have to call it duration. i call it valve speed.

and his question was does it change duration of the CAM.

Ragtop 99

iTrader: (7)

You need to be clear whether you are talking about actual valve lift (duration is effected by rocker ratio) or tappet lift (the cam lobe itself). The tappet lift can never change; it is ground into the cam as mrr23 states and that is what I beleive that comp uses to measure duration. Here is Comp's diagram that illustrates that when Comp says duration at .05 they mean tappet lift.

mrr23

iTrader: (3)

thank you ragtop even crane uses tappet (lobe) lift as seen here on this cam card

reason why is because of the different ratio rocker arms out there. the only constant when measuring duration is on the cam itself.

Patrick G

iTrader: (14)

Lift and duration at the valve is the only thing that matters in a performance engine. We work off of "valve events", not cam events. High ratio rockers will affect the duration at the valve and that's all that matters to the racer.

Don't try to school people into confusion. Trying to show people how smart you are will take them down the wrong path. Tell the people what they need to know, not how much you know.

mrr23

iTrader: (3)

patrick you are the one leading them down the wrong path. even you said

you didn't specify CAM or VALVE. so, before 'schooling' us, go back and school yourself. the man asked if it changes CAM duration. and we are answering his direct question. we all know the motor only cares about what happens at the valve. but answer the question as asked.

mrr23

iTrader: (3)

well here starts another pissing contest that i'm involved in.

GrannySShifting

iTrader: (10)

I like it when someone else else says it and I didnt have to type a thing. Is there any coincidence that the **** match contains one person in common?

Effective lift and duration chance, except at the very initial open/closing point. Whoever said you multiply duration with 1.7 rockers by 8.8 percent going to 1.8s, that doesnt work a 230 cam woudl become a 250 cam

mrr23

iTrader: (3)

**** off. you know i'm right. i only argue when i know i'm right. patrick g is right the second time around. he just forgot to mention that he was talking about VALVE lift also in his first response.

Importdestroyer

iTrader: (1)

Will duration change with variable rate rocker arms, such as the ones offered by Crane?

I wanted to get a TR 224 similar cam with less lift to go with my 1.89 variable rate rocker's but since Ill be in California and the cam may pass by itself I need to know if the duration is affected by these rocker arms.

mrr23

iTrader: (3)

cam duration = no
valve duration = yes

the TR224 is still a 224 @ .050 LOBE lift where it's calculated.

Importdestroyer

iTrader: (1)

I am in the process of selecting a california legal cam to go along with my 1.89 variable rocker's. I am looking at a TR 224 similar cam albeit with less lift, I am concerned with ramp rates and if duration will be added with these rocker arms.

It is detrimental that It isnt as I will be on the ragged edge of legality with the selected duration and LSA of the TR224 cam, a few more degrees of duration may make it a pain in the *** to get this thing registered a few years down the road.....

Im also concerned with ramp rates, in my opinion and correct me if Im wrong a cam with a set amount of lift and less duration would have a higher ramp rate than another cam with identical lift and more duration because peak lift would be reached quicker?

So maybe a cam with more duration would be easier on the lifters than a stock 02 LS6 cam with very little duration but .551/.545?
__________________

mrr23

iTrader: (3)

if you are concerned with emissions with the tr224, why not go for the VHP 047 cam instead? or the new VHP 056 cam? they work great with the rockers.

Importdestroyer

iTrader: (1)

Thanks for the advice, If I only could email VHP. Access to a phone out here is difficult, no email address on the site as far as I know. Ill look into it when I get back to Hawaii.

mrr23

iTrader: (3)

http://www.vincihighperformance.com/CONTACTPARENT.HTML the contact us page has all the info.