Originally posted by John Maher on Cal-Look Lounge August 2010
in response to: not another camshaft thread
Cam manufacturers have no control over the style of rocker assembly being used. In order to help you dial in the cam as they intended they must use a reference point that relates directly to the cam and not something that will give a multitude of different results dependent on type of rocker and how accurately (or not!) rocker geometry has been set up.
Yes, switching from a 1.4 to 1.5 ratio alters duration as well as lift at the valve (once it’s off the seat).
Cam design is a very specialist subject – very few people understand it fully, myself included!
The cam designer will generally design the valve’s lift profile first, effectively working backwards to arrive at the cam lobe design, once rocker ratio has been decided. But now the actual lobe design must be studied to ensure it’s suitable for use with the diameter of lifter being used (more velocity requires a larger diameter lifter) and whether the rate of change in velocity (acceleration) is likely to be kept under control by the valve springs. Then you have a third derivative, jerk i.e. the rate of change of acceleration. Now take in to account the cam designer has no idea what type of valvetrain their cam will be used with eg, super lightweight titanium or heavy stainless valves with chromoly retainers etc etc.
Yes, it’s possible to come up with a design that accelerates the valve off the seat more quickly than most of the common cams and therefore has equivalent (or even more) area under the curve than a longer duration cam. Eg, take a look at Engle’s FK4* series….
You can get a feel for how quickly these cams accelerate the cam off the seat by comparing their ‘running duration’ figures against ‘duration @ .050″.
Running duration is normally taken at .020″ lift by most of the VW cam makers. Subtract ‘running dur’ from ‘duration @ .050″….
the lower the figure, the faster the acceleration is between those two values.
On paper the faster accelerating cam looks good. It should deliver a better spread of torque through the entire rpm range. But in the real world, as many people on here know from first hand experience, cam grinds of this type have a tendency to wear cam follower bores and maybe trash other valvetrain parts. Short of putting your cam on a computerised cam measurement device, the cam card alone reveals very little about the camshaft’s design. Instead we rely on recommendations and/or experience.
It stands to reason there is room for improvement in VW cam design. Most of the grinds available have been around for at least 20 years plus, many of them based on old Chrysler V8 grinds for no other reason than Chrysler used a tappet diameter closer to the stock VW tappet than either Ford or Chevy (both smaller than Chrysler). With the advent of advanced camshaft design software and CNC production techniques, there’s scope for someone to come up with more advanced designs. Currently, Johannes Persson (JPM) is the only person I know of who has looked seriously in depth at aircooled VW camshaft design, specifically from the point of view of optimising duration, velocity, acceleration and jerk and not only as a theoretical exercise. He has taken into account the whole dynamic running situation. In the real world his cams allow higher rpm before hitting valve float compared to the camshafts we’ve all been using for years and this is achieved with less spring pressure than conventional stock diameter dual valve springs, let alone K800s!
With regard to whether it’s necessary to run lifts of .600″ or more…
Another complicated subject but I’ll attempt a brief(ish) explanation. The amount of lift required can be roughly based on valve diameter i.e. the larger the valve, the more lift you need. In very basic terms, the valve must be lifted by approx 0.25 x its diameter just to make enough curtain area to equal the inside diameter of the seat (curtain area = valve diam x 3.14159 x lift). Continuing lift above this point sees flow continue to increase . You can often hit a lift figure equivalent to greater than a third of the valve’s diameter before further lift sees little or no improvement in flow (depends on quality of head work). The actual calculations are a little more complicated but this gives the general idea.
Take a head fitted with 48mm intake valve. You need to see an absolute minimum of 0.472″ lift (48 x 0.25) for the head to start flowing reasonably. If the seat, port and chamber have been well modified, flow should be excellent to around 33% lift (0.623″) or more. If this is a max effort engine, running anything less is leaving potential power on the table.
OK, so if that head’s flow maxed out at 0.623″ lift, is there any point using even more (assuming there are no mechanical limitations to doing so)?
YES, because using more lift increases the amount of time available for flow to enter the chamber i.e. you get a better fill (increase VE) and therefore make more power. Maxing lift at 0.623″ limits the time available for flow at that increment to a fraction of a second. Going further can increase time by a huge percentage.
Like everything else in the engine, you cannot look at one component in isolation and expect to make huge gains. It’s all about how everything works with everything else but the cam and heads should come pretty high up the list if you’re looking for good results.
[Insert topic re WebCam 110 vs Engle 120]
I’ve not used a web 110 so don’t have any personal experience but yes, based on the figures you’ve quoted, the web 110 accelerates the valve off the seat more quickly than the Engle 120 between .020″ and .050″ cam lift. It starts later than the E120 but has caught up and passed it by .050″.
In theory that should deliver better throttle response, plus improved low and mid range performance due to less overlap.
It seems any cam with around a 30° difference between lift @ .050″ and running duration (.020″) can be regarded as high velocity. The likes of Engle 110, 120, FK8, FK10 etc have a difference closer to 40°. This observation is based on cam card data only – like I said before, the cam card doesn’t reveal the full picture. There’s a lot of things that can be altered on the opening and closing side below that advertised (.020″) figure to control how the valve is lifted off and then returned to the seat. That info will never appear on a cam card.
[Next, a question re using 1.4 ratio rockers with an Engle 125 camshaft]
Forget using 1.4 rockers on the Engle 125!!
Duration @ .050″ valve lift increases by about 5° when swapping from 1.1 to 1.25 rockers
and around 12° @ .200″
The graph below shows lift and duration differences at the valve based on 1.1 and 1.25 rockers (Engle 125)
Valve lift for Engle 125 with 1.1 and 1.25 rocker ratio
As you can see, there’s a healthy increase in area under the curve. Even if the port’s flow were to max out at say .400″, adding the 1.25 rockerextends the period of time available for flow to enter the combustion chamber.