The Stovebolt Site Forums The Shop Area Engine Shop Cam Spec Question

I've been around cars 'n trucks all my life and never really understood cam specs. Someone would say he's got a 3/4 race cam and I'd look at it and say, "man it sure is lumpy" and it did seem that the lobes were taller or wider at the tops. But to be honest I really was parroting something I'd heard from someone before. And I know that an RV cam will give you a little better performance than a stock one but you can still it'll run fairly smoothly.

When it comes to numbers I haven't got clue one ... for example I'm looking at an ad for POWERHOUSE, they've got a bunch of performance cams listed;

SMALL BLOCK CHEV HYD

923 420 443 270 280 $39.00
922 443 465 280 290 $42.99
1015 465 488 290 300 $49.99

Is there a web site that I can go to or is there someone out there that can explain these numbers to me? And also what the significance is of each of them. (other than the one with the $ sign in front, I know what that one means)

The numbers in the 400s are the valve lift numbers. The valve lift is usually specified as lobe lift times rocker arm ratio. Cam specs assume stock ratio rockers. The numbers in the 200s are the duration numbers. Typically specified as either advertised duration or duration at .050" of lobe lift. The duration at .050" lobe lift numbers are usually much smaller than the adv duration numbers. They are also more accurate at describing the cam's profile.

Theres a lot to cam selection than just lift and duration numbers. Theres the Intake Center Line, which is where the intake valve is fully open in relation to piston location. There is Lobe Separation Angle that determines valve overlap. There is also the intake closing timing, where the intake valve closes as the piston comes up on the compression stroke. This has a lot to do with compression bleed-off at low-midrange RPM.

All of these parameters have an effect on how an engine runs. Tighter LSAs have more overlap and thus more lope at idle. Changing Intake Center Lines shift the power up in the RPM range. Typically higher valve lift numbers are accompanied by longer duration numbers, as the cam lobe's ramp has to be lenghtened to increase lift without overaccelrating the valvetrain.

Actually a larger lift cam may appear to be smaller than a stock cam. This is because you can't make the lobes larger than the cam bearings, it wouldn't fit in the engine! So you make the base circle of the lobe smaller so the total diff between base circle and nose of lobe is larger.

The shape of the ramp is a big factor too. Comp Cam has an Xtreme Energy line that uses fast ramps with a larger LSA to provide more cylinder filling and build dynamic compression with low static-compression engines. This helps an 8.5:1 engine behave like a 10:1 engine without sacraficing as much idle quality or idle vacuum. Theres also 'single-pattern' and 'dual-pattern' cams. Single pattern cams are more common, lift and duration are the same for intake and exhaust valve. A dual pattern cam has lift and duration numbers for the exhaust that are larger than the intake. This is to help some cylinder head designs breath better, cuz some heads have small exhaust valves or restrictive exhaust port configurations. Some engine families like single patterns, some run better with a dual pattern cam. Depends on engine, head selection, manifold selection, valve size and piston shape.

For example, Pontiac engines like about 10 degrees more duration and more lift on the exhaust because the exhaust port is restrictive on some iron D port heads.

Much larger than stock cams recommend stall coverters to revv the motor into the new cam's powerband quicker. Your motor must have the compression, manifolding and RPM range to use a "hot" cam as this. Depending on a lot of factors, a cam with more than about 230 degrees of duration at .050" lift usually requires an increase of compression, valvespring changes, aftermarket manifolds and a stall converter to work really well. There is no free lunch. You've got to build compression and shift the powerband up in the RPM range to make big power.

clear as mud?

chip

Chip wow, lots of facts. I think today a 3/4 cam will have 200-215 degrees of duration at .050 lift. A cam with 210 degress in a 283 is getting a little wild, the same cam in a 377 cube small block might have a barely noticable lope at idle.More cubes can handle more cam ,all else being equal.I've heard some cams have more duration on the exhaust to improve midrange torque.A small block Chevy has good stock exhaust ports comapared to other designs, yet many small block cams have the dual pattern with more on the exhaust.
The cam ,exhaust system and intake must be in harmony to get the get good smooth power.Change one component and generally you may have to redesign the other two systems to get the most from the change.

Thanks Chip helped a lot (I think)

I think I'm getting there ...

so ... would mean the 420 is .042" of lift or .42"? for exhaust or intake and 443 is .043 or .43 for intake or exhaust and the 270 is for 70 degress duration for solid lifters and 279 or 80 degress for hydraulic lifters ?? am I at least in the ball park? What would the 923 mean? Possible a stock number or identifier of sorts ... ?

One more thing ... to measure the lift I would assume you measure across the lob and the at 90 degree to that measurement. That sure equal lift ... and then off to a parts book to see what cam you actaully have ...

So while I'm at it how much wear would suggest that the cam is done for?

Shifting the power band up and down is that what those large timing wheels with all the degree marks are all about?

Sorry for raggin on ya but when it comes to knowledge of engines I'm 2/3's sponge.

Ears and eye wide open and mouth shut as my Pop used to call me ... Kid doesn't say much but he'll repeat back to ya anything you've said in the last 4 or 5 hours ... lol

OK tanks Tony ... yers got posted while I was asking these other questions ... so does that mean that the valve is actually open for 270/280 degress of cam shaft rotation? seems like a hellofa lot when I look at a camshaft ... so it would begin opening at say zero degrees and be max open at 130 (actaully a few degrees prior to 130 degrees) and then start shutting again ... and the amount of time the valve is max open would that be the dwell?

I don't mean to put myself out there as a cam expert. I'm soaking up cam info at a pontiac site, cuz I'm in the planning stage of a Poncho 406 build for my LeMans street/strip project.

Based on the info in the first post, the first lift and first duration #s are for the intake, and the 2nd lift and 2nd duration #s are likely for the exhaust. So it may be a dual pattern cam.
Various cam manufactures play with all the variables to get differing performance out of similar lift and duration numbers.

A 500 lift would be .500" inches or 1/2 inch. To get lobe lift, divide valve lift by rocker arm ratio. You can use high ratio rockers to increase valve lift without a large increase in duration. That helps power in your desired powerband. But then you've got to pay extra special attention to coil bind and P to V clearance.

The cam events are referenced to 360 crankshaft degrees. Dont forget that the crank goes around twice for every once of cam rotation. So if a cam has 250 degrees of duration at .050" lift, that means that from the time the lifter moves .050" upward from seated (base circle) thru max lift to the time its .050" from seated again, the crankshaft has rotated 250 degrees. Usually when you degree in a cam, its referenced to the Intake Center Line. (easiest way to do it).

When I degreed the M4F cam in my 244 (235 bored .060") I installed it "straight up", with the Intake Centerline at 106 degrees of crankshaft rotation past TDC. (howard's spec for Intake Centerline.) When you advance or retard a cam, you move the IC relative to crankshaft rotation, which moves the powerband up or down in the RPM range. Advancing a cam moves powerband downward in RPM. Retarding a cam moves powerband upward in RPM. I'm looking into a trick of using a slightly hotter cam than needed, then advancing it 2 to 4 degrees to move powerband downward in my Poncho, while still streaching the RPM range past 5500 RPM. I'm trying to get 450 HP at the flywheel (high 12s in 1/4 mile) and still be torquey on the street. But with high ratio rockers I want to use, I've got to make sure a piston and valve don't have a one-night stand.

From what I've heard, you only want to advance the cam in a streeter (if needed) so you don't kill all low-end torque. Some racers retard the cam 'timing' to reduce low-end torque to prevent wheelspin and give them more power at the far end of the track.

You can change cam timing by using a combination of offset crank keys and a timing set with 3 keyways: Advanced, straight up, and retarded.

Still kinda fuzzy on if the resultant IC number goes up or down if you advance a cam.

Quick Quiz: Advancing the cam 4 degrees on a stock Intake Centerline of 106 would give you an IC of 102 or 110????

anybody know??

chip

darn trick questions!!! Advancing the cam should increase the centerline and generally lower the power band a few hundred RPM. I installed the Howard's M4F in my 261 at 4 degrees advanced as recomended by Howard's on my timing card.I guess it runs strong, sure will whip the [censored] out of any other 235/261's Advancing or retarding a cam will add a little this way or that on the powerband but won't make a wild cam streetable or turn a dog into a screamer from what I hear. It's common in Nascar to advance or retard cams a few degress as a tuning aid.Like Chip, I read this stuff, but back when I drag raced cycles, I messed around with cam timing.In my case I got a better power band from exhaust tuning than fiddling with cam position.