camshaft rules of thumb?

[Marx3]

Hi, I am very interested in cam specs, especially understanding them. I guess you have to have alot of experience over severeal years before you get really hard at determining which cam is better.
In my country ( and in all others countries I guess  ) people make the usual mistake when choosing cam: they go for the bigger the better ending up with that lopey 400 hp sound, but in reality they have an engine that is working against itself.

Is it possible to make some quidelines for camchoice? assuming that one always wants the cam that works best for the engine NOT the cam that sounds the best.

CUI vs CR vs VE ? ??

[Cliff Ruggles]

Camshaft choice can certainly be a "bundle of snakes".  Most folks get all caught up in the "bling" when it comes to camshafts, and get drawn in to advertisements from companies making cams claiming all sorts of power gains at every rpm.

I've been full circle with cams in the engines we build here, and use and prefer more "tradition" grinds with more off seat duration and wider LSA's.  Pretty much the opposite of what most companies recommend.

I could literally write a book on the subject, but will say that folks should look more at the actual duration when comparing cams, than the .050" specs.  Every moment the valves are off seat are opportunities to move air thru the ports, things just don't start at .050" tappet lift.  Actual specs determine the overlap and intake valve closing point as well, and have a great impact on cylinder pressure in any given application.

As a general rule I think many modern cams leave a lot of power on the table with short seat timing events combined with tight LSA and advanced intake closing points.  These cams are typically "raspy", aggressive idle, throw a lot of power at you early and narrow up the power curve at the same time.  This typically has the user thinking he's making a butt-load of power with the new cam, at least compared to a grind that delivers smooth idle, strong/broad torque curve, and "locomotive" power characteristics.

Here's a clip of my car at the track with it's "boring" smooth idling "tractor" engine.  I actually have to retard the timing 4 degrees and shift at 5000rpm's to stay roll bar legal:

http://www.youtube.com/watch?v=6zVdoLR-VzM

[HeathD]

What are your views of roller cams/roller lifters/roller rockers?

Flash bling, strip only or a reliable alternative (albeit expensive).

Nice video too.

Heath

[Shark Racer]

I like roller cams a lot. Improved valvetrain life, decreased friction, more lift for the same duration, no need to run any special high zinc oil. A little extra cost up front but it's win win win otherwise!

[Ethan1]

I like roller cams a lot. Improved valvetrain life, decreased friction, more lift for the same duration, no need to run any special high zinc oil. A little extra cost up front but it's win win win otherwise!

 I do as well and just for those reasons as well. Cost more, but are well worth it, not having to worry about wiping a lobe on FT cam because of lack of zinc in todays common oils.

[billy 2]

  I agree as well for all the same reasons. billy 2

[Cliff Ruggles]

We use roller cams only in engines built here since the retrofit parts have became readily available.  It just makes sense to take "scrubbing" a lobe out of the equation, and I sleep a lot better at night!.....Cliff

[Marx3]

I never really understood what to use duration at 050 for... Besides maybe getting a picture of how steep the ramp is.
I think the whole idea behind getting nice civilized idle and response and that strong locomotive feel makes so much more sense than getting some flashy cam that sound great in idle but never really work with the engine since most people do a cam change only. A good example is this cam:
http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=708&sb=2

into a completely stock smog Mopar 400 from the mid-70's with 7.5:1 cr.
A friend of mine was advised that this cam was a fine choice for a 400 engine.
I enlarged some things on the idle system on a nice late 70's Pontiac 800 cam unit.
It idles fine, but sound a little like it's got 400 hp, but it has sub 10 hg vacuum in idle and I doubt it makes over 250 hp at the crank.

Are there some guideline to determine minimum lsa from? Or is it also determined by the adv.
duration? I guess you could as much lift and duration into any engine combination if only you where not plagued by overlap?

[Shark Racer]

That is a VERY aggressive cam for that little compression ratio. You need to get the compression way up to appreciate a cam like that - the cars that were typically matched with a cam like that had compression ratios around 10.5-11 - on iron heads. (at least on Chevys!)

Duration at 050 coupled with the LSA will determine how well it idles, how much suction is available at idle. I have a much milder cam, 216/216-110 on a roughly 8:1 CR and it's also too much cam for that ratio.

Honestly, I'd go with a cam in the 20x duration range at most with that CR. Better yet, up the CR (thinner head gaskets, smaller cc combustion chambers, different pistons, deck the block, some combination thereof) and optionally go down a step or two on cam.

[Marx3]

Yeah dynamic compression ended up around 4.5:1 sad thing is it as a reputable shop that suggested that grind. Jesus Christ... Engine is in a completely restored 68 charger that is going to be sold to free up money for the next build. By that time I am helping him out on building the engine.

Aha! Duration at .050 and LSA determines idle/vacuum quality... VERY interesting.
Can you elaborate?

[Shark Racer]

At a basic level, increasing duration shifts the power band of a cam upwards and makes it run "worse" at lower RPM. This is due to reversion. More duration = more likelihood of overlap = more reversion.

The tighter LSAs also increase overlap, as LSA is a measurement of the angle in degrees between peak lift on exhaust and intake lobes.  Thus a tighter LSA hints at more overlap and a looser LSA less. Overlap is what kills idle as it becomes difficult to get a consistent vacuum signal. This is why people have to run higher idles - to get a better, more consistent vacuum signal.

PHR has a decent article here: http://www.popularhotrodding.com/enginemasters/articles/hardcore/0606em_camshaft_specs/viewall.html