Why do I see on here and on websites (summit) that the standard point for measuring duration is at 0.050" valve lift? Why is it not tappet lift?

I mean couldn't a company then just jack up the rocker ratio for a measured valve lift? So if I used a set of 1.7 rockers, my duration would be longer at 0.050" valve lift then a set of 1.5 rockers.

And wouldn't that duration change depending on if its a roller or flat tappet? I mean don't roller cams have a different profile (i.e. more steep) so they could reach 0.050" valve lift sooner, meaning they would have a longer duration?

I'm trying to figure this cam thing out, but this valve lift idea is making me

 

pretty please?

 

It's so that we, the buying public, don't have to get out our calculators and multiply lobe lift x rocker ratio(s).

 

Correct me if I'm wrong but the duration is the same no matter what the rocker ratio is - the ratio only determines the valve lift. As far as I know all cam manufacturers advertize with duration at .050" lift and the max lift numbers are with standard 1.5 ratio rocker arms.
That way you (the customer) can easily compare different brand cams

 

No ... duration does change as RAR changes ... not a whole lot but a healthy lobe at 1.5 will produce about 2-3 more degrees at 1.6.

It's probably a misprint or typo or oversight or brainfart ... duration is usually intended to be measured at a specified lobe or tappet lift usually 0.006" or 0.050". If not ... I've sure built & degreed in a whole buncha motors all wrong.

 

How can duration be the same at 0.050" valve lift with different rocker ratios? I mean valve lift is the rocker ratio x lobe lift. So if you have a larger rocker ratio, that means you'll have less lobe lift. Less lobe lift means you're barely on the lobe when you start measuring duration. Same when you come off.

Thats what I'm saying. If you're measuring in valve lift, then you would have to divide by the rocker ratio to get lobe lift.

My question: Why is it not standardized to lobe/tappet lift???

 

In camshafts the ramp ratio is all different but in a typical cam, the ramp that puts pressure on the oil cup is normally all in at .050. It is a gentlemans agreement so the consumer can compare cams for a certain duration/RPM range.

 

Ok Thanks Jackson. I've actually seen it posted like that many times though as valve lift, and with people specifically saying, "when the valve is 0.050" of its seat..."

Also, wouldn't it affect it if it was a roller cam as well? I figured it would affect it alot more than a few degrees per 0.1 addition to a rocker ratio. What about a roller cam though?

 

I understand the need to standardize, my problem is how people are saying its standardized: at a specific valve lift and not lobe lift. So I'm trying to find out which it is, valve lift or lobe lift.

 

0.0006 is also referred to as advertised (seat) duration. This figure hardly changes w/ different rocker ratios, the @ 0.050 duration will vary a couple of degrees depending on the rocker ratio. The figures are always TAPPET lift, 0.050 corresponds to 0.075 valve lift for a 1,5 ratio.

 

Here: shows both the seat and adv. duration specs and everything else.

 

This is from one of my Camaro sites:

Duration @ .050” – This is the duration when your lifter is .050” off your base circle of the camshaft. This number is best used to compare camshafts between brands. Expect a number like 220*. A “small”, or stock cam might be in the 202 range, where as a full blown race cam would be in the 260* range.

This is from the Crane page:

Duration is the period of time, measured in degrees of crankshaft rotation, that a valve is open. Duration (at .050" lifter rise) is the deciding factor to what the engine's basic RPM range will be. Lower duration cams produce the power in the lower RPM range. Larger duration cams operate at higher RPM, but you will lose bottom end power to gain top end power as the duration is increased. (For each ten degree change in the duration at .050", the power band moves up or down in RPM range by approximately 500 RPM.)

What is the difference in Advertised Duration and Duration at .050" Lifter Rise (Tappet Lift)?

In order for duration to have any merit as a measurement for comparing camshaft size, the method for determining the duration must be the same. There are two key components for measuring duration-- the degrees of crankshaft rotation and at what point of lifter rise the measurements were taken. Advertised durations are not taken at any consistent point of lifter rise, so these numbers can vary greatly. For this reason, advertised duration figures are not good for comparing cams. Duration values expressed at .050" lifter rise state the exact point the measurement was taken. These are the only duration figures that are consistent and can accurately be used to compare camshafts.

And more info here:

http://www.4secondsflat.com/Cam_Design.html

 

Someone needs to correct the file at summit. This is straight from their website:

But I got it now, not valve lift, lobe lift. Thanks.