Cam timing

[MaineShark]

I've often seen talk of changing the cam timing by some small amount of degrees.

What does this accomplish?

Joe

 

[Vader]

Advancing valve events in relation to the crank will typically increase the cylinder compression at lower RPM by "sealing" the chamber sooner. This will enhance power (torque) at lower RPM. Retarding the valve events will typically cause higher cylinder compression at higher RPM since the exhaust valve is held open longer to scavenge the cylinder better and promote cylinder fuel/air charging. This results in more power (torque) at higher RPM, which generally enhances the peak HP output.

Oddly, Chevrolet originally had a different scheme when the SBC was first designed. The cam lobe center timing was slightly more advanced. In 1968, Chevrolet changed the base valve event timing by -4° in all V-8 engines. That means a 1967 327 with a given cam profile had the cam advanced 4° compared to the 1968 and later 327, 350, or whatever with that same cam profile. There is speculation that this was done to improve emissions and that later cars were generally lighter (fleet averages) and could survive with just a little less "grunt" (torque) at the lower RPM ranges.

 

[MaineShark]

Interesting...

How much change are we generally talking about?

In other words, if you wanted an engine just for low-end, how much would you advance the cam? If you wanted an engine just for top-end, how much would you retard the cam?

Joe

 

[grumpyvette]

read this

advancing the cam 4-5 degrees will move the engines whole torque curve about 200rpm lower in the engines rpm range, but what you gain on the low rpm range your likely to loose in the upper rpm range , especially on a small displacement engine, so keep that in mind, generally it not worth the time and effort for the results and youll take a small chance on having piston to valve clearance problems


retarding the cam causes the whole torque curve to move in the rpm range (4 degrees moves the whole torque curve about 150-200rpm higher in the rpm range) in effect retarding the cam has the effect of allowing the engine too breath better at slightly higher rpms but gives up some low rpm cylinder compression to do it. its an easy adjustment if your fine tuneing the engines torque curve to a desired rpm range but adjustments of more than about 4 degs advanced or about 6 degrees retard would really mean your better off useing a differant cam. BTW advancing the cam has the opposite effect(slightly gains cylinder compression and lowers the torque range) most cams are not truely strait up if installed on the gears timing marks as they are normally made already 4 degs advanced
keep in mind that retarding or advancing the cam timing changes the piston to valve clearances and you can get into trouble fast if those clearances are not carefully checked when you change the cam timing.
in most cases the factory marks on the timing gear set will allow you to install and run the cam with little or no problem BUT KEEP IN MIND everyone makes mistakes INCLUDEING THE GUYS MAKING CAMS AND TIMING GEAR SETS and if your like me , for several years I installed cams at the factory marked setup points and only occasionally had engines not run quite as well as I thought they should.
we let me say this , since I started checking things carefully with a dial indicator I find just how sloppy the manufacturers tollerances are and that engines are fairly tollerant of running cams that are ground a few degrees off what the spec card SAYS the SPECS are. if your one of the millions of guys that can,t really tell if your running 375hp or 395hp but just want to burn a little tire smoke , don,t worry about it too much but if your one of the guys that wants EVERY LAST HP AVAILABLE than take the time to do it correctly, but be prepared for a shock at just how loose some manufacturers are at matching the cam to the posted specs.

if your still interested heres instructions and tools priced reasonably

http://www.jegs.com/cgi-bin/ncommerce3/ProductDisplay?prrfnbr=3292&prmenbr=361

heres just the video if you have the tools already

http://www.jegs.com/cgi-bin/ncommerce3/ProductDisplay?prrfnbr=30669&prmenbr=361

http://www.jegs.com/cgi-bin/ncommerce3/ProductDisplay?prrfnbr=3293&prmenbr=361

or if you want written instructions
BTW I have both of these degree wheel, the larger 18" moroso wheel is great with the engine on an engine stand but the smaller size is mandatory IN an ENGINE COMPARTMENT

http://www.thedirtforum.com/degree.htm

http://www.hotrodder.com/kwkride/degree.html

good info to know

http://www.symuli.com/vw/camp1.html

http://www.symuli.com/vw/camp2.html


http://www.chevyhiperformance.com/techarticles/95298/

(need more info,..look below)
read these lessons (#1-8)
http://www.newcovenant.com/speedcrafter/tech/camshaft/3.htm
The LSA, or lobe separation angle, is ground into the cam and cannot be changed. It is the angle that separates the intake and exhaust lobe for a particular cylinder, and is measured in camshaft degrees. The intake lobe centerline is measured in crankshaft degrees. The #1 intake lobe centerline is usually between 100° to 110° ATDC and is what you use to degree the cam. The cam manufacturer will publish the specs for the cam based on a given intake lobe centerline. Comp Cams, for instance, produces a large number of cams with 110_° LSA ground 4° advanced, so they list the specs for the cam with a 106° intake lobe centerline. You can calculate the ILC by adding the intake opening angle in °BTDC, the intake closing angle in °ABDC, plus 180° for the distance from TDC to BDC. Divide by 2 and subtract the intake opening angle and you will have the ILC. For example a 12-430-8 Comp Cam lists IO at 34°BTDC, IC at 66° ATDC, so 34 + 66 + 180 = 280. 280/2 = 140. 140 - 34 = 106° ILC

Figure 3 is a picture of both an intake and an exhaust lobe of a camshaft, seen end-on. It shows the relationship between the lobes, shows the overlap area, and illustrates this next section.
As stated in lesson 2, overlap has a great deal to do with overall engine performance. Small overlap makes low-end torque but less high-end power. Large overlap reduces low-end torque but increases high-end power.
Overlap is determined by two other cam specifications, Duration and Lobe Center Angle.
Duration is the time, measured in crankshaft degrees, that a valve is open. A duration of 204 degrees means that while the valve is open, the crankshaft rotates through 204 degrees.
Duration is measured on two "standards," "advertised duration" and "duration at 0.050"." Advertised duration is measured from when the valve just starts to lift off its seat to when it just touches the seat again. This is measured in different ways by different manufacturers. Some measure when the valve lifter is raised 0.004", some at 0.006", and some at different points yet. So the industry agreed to another standard that was supposed to make it easier to compare cams. In this standard, the duration is measured between the point where the lifter is raised by 0.050", and the point where it is lowered again to 0.050".
The 0.050" standard is great for side-by-side "catalog" comparisons between cams. But if you use engine prediction software on your computer, the software is much more accurate when you can feed it "advertised" duration numbers.
Lobe Center Angle is the distance in degrees between the centers of the lobes on the camshaft.
To increase duration, cam makers grind the lobes wider on the base circle of the cam. This makes the lobes overlap each other more, increasing overlap. More duration = more overlap.
To increase overlap without changing duration, cam makers will grind the lobes closer together, making a smaller lobe center angle. Less lobe center angle = more overlap.
Overlap and duration are the two big factors in cam design. More overlap moves the power band up in the engine's RPM range.
Longer duration keeps the valves open longer, so more air/fuel or exhaust can flow at higher speeds. It works out that increasing the duration of the camshaft by 10 degrees moves the engine's power band up by about 500 rpm.
A smaller lobe separation increases overlap, so a smaller lobe separation angle causes the engine's torque to peak early in the power band. Torque builds rapidly, peaks out, then falls off quickly. More lobe separation causes torque to build more slowly and peak later, but it is spread more evenly over the power band. So a larger lobe separation angle creates a flatter torque curve.
So you can see how a cam maker can tailor the camshaft specs to produce a particular power band in an engine--

Short duration with a wide separation angle might be best for towing, producing a strong, smooth low-end torque curve.
Long duration with a short separation angle might be suited for high-rpm drag racing, with a high-end, sharp torque peak.
Moderate duration with wide separation angle might be best suited for an all-around street performance engine, producing a longer, smoother torque band that can still breathe well at higher RPM.
Remember, there's always a compromise made in this process.

One last item to consider is the lobe centerline. The lobe centerline is the angle of the lobe's center peak, measured in crankshaft degrees when the piston is at Top Dead Center (TDC). In general (but not always), when a cam is installed "straight up," the intake lobe centerline and the lobe separation angle are the same.
The lobe centerline can be altered when the camshaft is installed, by advancing or retarding the camshaft's position in relation to the crankshaft. Advancing the camshaft by 4 degrees will move the power band about 200 RPM lower in the RPM band. Retarding the cam by 4 degrees will likewise move the power band 200 RPM higher in the RPM band. This allows you to fine-tune the engine's performance according to your needs.

personally I try to stay close to 110 degrees on most carb engines and 112 degrees on EFI engines because I value a wider torque curve more than a few hp only close to peak rpm

if cams are a mystery please take the time to read these, it will get you a good start

http://www.newcovenant.com/speedcrafter/tech/camshaft/1.htm
(read LESSONs 1-8)

http://www.mercurycapri.com/technical/engine/cam/lca.html

http://www.wighat.com/fcr3/confusion.htm

http://www.cranecams.com/instructions/valvetrain/camfail.htm

http://www.mercurycapri.com/technical/engine/cam/index.html

http://www.idavette.net/hib/camcon.htm

http://www.cranecams.com/master/adjustvt.htm

http://www.centuryperformance.com/valveadjustment.htm

http://www.totalengineairflow.com/tech/valvelashing.htm

http://www.chevytalk.com/tech/engine/Cam_Selection.html

http://www.chevytalk.com/tech/101/Cam_Theory.html

http://www.babcox.com/editorial/ar/ar119736.htm

http://www.mercurycapri.com/technical/engine/cam/vtg.html

http://www.symuli.com/vw/camp1.html

http://www.symuli.com/vw/camp2.html

http://home.wxs.nl/~meine119/tech/camqa.html

 

[sscam69]

Damn grumpy vette

That is some EXCELLENT info. I am going to have to read your post in parts! :L

 

[Vader]

...or, call Comp Cams and ask for your complimentary copy of Publication 106. It will explain a lot about cam lobe, profile, and timing theory. They're at 1-800-999-0853 or www.compcams.com

 

[Stallion]

That site is really good.

http://www.compcams.com/information/tutorials/