gerry72

Put a wooden cam in it. They are the best.

Blown69

Comp Cams 268H: I am putting this mild cam in a 468 BBC. Does any one how chopy the idle is?
Specs are as follows:

268/268 advertised duration
218/218 duration @.050"
.485/.485 lift with 1.7 rocker ratio
110 degree lobe separation
RPM operating range 1500-5500

Ganey

In a 468 BBC it's a cam for a truck. Expect a good idle.
In a 350, it can have a nice cammed idle.

jackson

for a 468BBC ... mild is an understatement ... you might be able to leave a dime on the filter lid & read the date while idling. Don't confuse guidelines for a sb350 w/ what a bb468 would like ... inches do matter.

Blown69

Sounds like it will like my 308 gear and 4 speed

jpatrick636

Let me make a slightly different suggestion. Go out and get yourself a program like "Desktop Dyno". They are not that expensive. Then you can plug in your specific combination and play with different cam profiles (as well as other variables) and see the results. These are great learning tools.
One great advatage to these programs is you can see things like average torque and horse power, which (in my opinion) is the best way to pick a street combination. The highest average in a given peak RPM and range will serve you best.

In other words, if one cam 1 gives you 400 hp @ 6000 rpm and 390 ft lbs of torque @ 4800 rpm with an average hp/tq from 1500 to 6000 of 220/210 and cam 2 gives you 390 hp @ 6000 rpm and 390 ft lbs of torque @ 4800 rpm but has an average of 240/235, I would go with cam 2. Why? Because on the street you would have more power available at more rpms.

Choosing a cam with the highest averages also gives you a very balanced motor that is not "peaky" and hard to drive. It has already been stated that torque rules the street. I agree. Adding to that, I would say that you want the most torqe you can get under the most conditions. You don't want a truck motor that runs out of breath at 3500 rpm and you don't want a race motor that does not come alive until 4000 rpm. You want power off idle, mid range and high up. The best indicator of that is the average numbers. Just knowing peak hp and torque in not enough and the only way to get averages is by trial and error on a dyno (big bucks) or with a dyno program (little bucks).

Since you have asked the cam question, it follows that your next question might be heads, intake, exhaust etc. Here again, a dyno program is a great learning tool!

Edzred72

Two good books that explain cam function & selection are
"How To Build Big-Inch Chevy Small Blocks" by Graham Hansen
"How To Build Max Performance Small Block Chevy's On A Budget" by David Vizard. Both are SA Design publications. I found the Hansen book the better of the two when it came to building an engine from the ground up.
Both give in depth cam technology, written in easy to understand text.
Availabe at bookstores everywhere.
Eddie

Blown69

"for a 468BBC ... mild is an understatement ... you might be able to leave a dime on the filter lid & read the date while idling"

But the lobe seperation of 110 will not let it idle that smooth

427Hotrod

I can understand how confusing it all can be. I study up on it all the time and everytime I think I know something, someone builds something better.

Everyone has given good stuff here, so I'm not sure how far you want to go. The websites are good at giving you the basics to work from.

Here's some more real world stuff to put with all the other stuff.

There are a couple of ways to get more air/fuel in a motor. Better heads/induction/exhaust and then you can manipulate the cam to do it. If you do both you make the big power. Heads are the biggest obstacle to power. You can use a mild cam with killer heads and make good power. You can use a big cam and weak heads and have a stone. But you CAN make weaker or stock heads perform much better than they do if you select right cam.

So as mentioned, you have to start with what components are limiting you and what you intend to do with it. Trans type, converter, gearing, weight, emissions etc all come into play here too. You can manipulate the cam events to get what you need.

Let's start with lift. This is how far valves open. Pretty much nothing bad can happen with additional lift IF there are no mechanical issues with doing it. Most stock GM small block heads can handle right around .500-.525 lift max before you have clearance issues with valve guides etc. The newer Vortec handle maybe .480 I think, but they are easily modified to allow more. But this is something you need to definitely check as you get closer to the limits. Big blocks can handle .600+ with no big issues. Now all of this assumes the proper springs etc are in place and they don't bind. Piston to Valve clearance is determined much more by the opening/closing points of the valves as well as where you have it installed at with respect to TDC than max lift. At max lift the piston is on the way down on intake valve and is going away from it. At max lift on exhaust valve, it is at the bottom on the way back up. You just have to make sure you don't start opening it too early or close them too late and you won't have any issues. The closest points of contact will be 10-20* on either side of TDC. But you do have to pay attention to valve reliefs in pistons and valve size. For example, if you install 2.19 valves in a big block head on a motor that had the stock 2.06's, you have to look close. The valve notches on the 10.25 pistons are usually deep enough, but the radial clearance around the edge of the pocket will kiss the 2.19 valves when you approach .600+ lift. You just have to check stuff when you start swapping parts. But anyway, generally lift is always a good thing. Get the most you can get by with. If using stock type heads that peak flow at .500 lift, there's no real need to stick a .575 lift cam in it though.

Duration is the next biggee. Cam mfgs usually give you two sets of numbers..and if you do some digging you can get more to help pick stuff. They always give you ADVERTISED and .050 duration. Adv is generally the total duration in degrees that the valve will be off the seat. .050 is just that. They rotate cam and after it lifts valve exactly .050" they start counting. They rotate it to max lift and then keep rotating it until it is .050" from being closed. That's the total degrees of duration measured at .050. So why is that important? They long ago found that many cam mfgs all measured stuff differently and this is a way to get everyone somewhat on the same page. You will find that even in the ADV duration numbers, some start counting when valve moves .004" of an inch and others start at .006". This can make a big difference in the ADV number and make it hard to compare stuff. You just have to do the best you can. Generally, the closer together the ADV number and the .050 number are together, the more *aggressive* the cam. Meaning more performance. For example a cam with 280* adv and 255*@.050 numbers is much more aggressive than a cam with 280* adv and only 230* .050. For a given total duration the one with 255*@.050 numbers is moving the valve off the seat much quicker. It will have the valve open 25* longer than the other cam yet still close it at the same time. This opens the *window* of the valve much earlier and larger than the other cam does and holds it longer with respect to piston position. This will obviously allow more airflow into the cylinder.

You will also often find duration numbers @.200 too. This is another way to look at how fast the cam is moving the valve. For example, you could have a cam with 298* ADV, 266*@.050 and 185*@.200". Then compare it to another cam with 300*ADV, 260*@.050 and 180*@.200". The second cam has 2* more ADV duration, but you notice that by .050 it is smaller by 6* and at .200 it is smaller by 5*. The first cam is a more aggressive cam and will likely spank the second one, even though the second one looks bigger if you just look at the ADV number.

Confused yet? Hang in there..it only gets worse!!

Ok, so we have lift...more is better as long as you don't crash into stuff.

A little more on duration. The reason it's so critical is that you are controlling the amount of air that gets into the cylinder as well as what remains. Generally you put a *bigger* cam in a motor to make more power. That means the valves open earlier and close later because duration increases. The biggee here is *they close later* part. After the intake valve opens and piston goes all the way down *pulling* mixture into the cylinder, it then reverses and starts back up. The intake valve is closing at this point. Exactly when we close it is critical, because as the piston rises we are pushing all that nice mixture we just sucked in there back into the intake manifold. This is where the fine balancing act of cam timing/head flow etc all come into play. The mixture is coming in so fast as the piston gets to the bottom of the cylinder, we can still be *ramming* it in even as the piston rises. So we can't just close valve, we have to keep allowing it in as long as we can until the ram effect stops. So you can see that changing heads, intake etc can drastically change the *ramming effect* and require a different cam. That's why you see cam and head packages sometimes,,,,they've figured out what works through trial and error. But be aware, they are often very conservative and don't always make the most power, but they are a safe bet.

Ok, so piston is rising and we are closing intake valve. When that dude gets completely closed, you are now beginning compression. Until it gets closed, you aren't building much compression. This is an important part. First, you notice that when they put bigger cams in at the factory, they usually increased compression from say 8.5 or 9.5 to maybe 11.0 to 1 right? They had to do that because the intake valve is closing later. That means there is less mixture trapped in the cylinder, so whatever we have in there, we need to squeeze tighter. This is called Dynamic Compression Ratio and there are a lot of calculators out there to help with it. This is why cranking compression goes down with a larger cam...the valve is closing later and there is less stuff to compress. This also why a motor will ping like crazy when you use too much compression with too small of a cam. You are closing valve early, trapping a lot in cylinder and then squeezing it tight. The motor can't handle it without better gasoline and higher octane.

But hold on, this all sounds crazy.....if there is less mixture in the cylinder, then that can't be good right? The big cam isn't helping power any. Often that's correct..that's why you hear people saying that they used too big of a cam and it turned it into a pig. The issue is that they aren't operating it in the RPM range it needs to be. When you increase RPM a magical thing happens. Remember all that *ramming effect* I was talking about? Well as RPM climbs, that really takes over and now all of a sudden I have a REALLY full cylinder and I'm now using those 11.0 pistons to squeeze it tight and making BIG power! So this is why you usually saw the factory big cam/high compression/high Hp cars using 4.10 gears and stuff. They wanted to get the RPM up quickly so the engine would be in the "happy spot" and start making real power!!

I'll hold on there for now.....

Let me know if this is any help and I'll keep rambling. We still have the exhaust event and the next real biggee....Lobe separation angle...to talk about as well as Intake centerline. Then we have to try and add all of them together and come up with ranges that will work for varying circumstances and why some stuff works and some doesn't. You can use some pretty strange stuff to come up with exactly what you want.


JIM

MotorHead

Written in pencil, 2 deg ADV Bushing = 105 1/2 deg

I would say someone degreed the cam and had to put in a 2 deg bushing to get near the 106 deg intake centerline. Comp cams and some others come from the factory 2 degrees advanced I believe.

jpatrick636

Nice job! I wish I had you for auto shop. But that was when Abe Lincoln was president.

LeMans Pete

I was just reading up on this post to learn myself, but isn't the advertised .050" of tappet lift, not valve lift? I mean if it was valve lift, couldn't they just jack up the rocker ratio which would in turn give them more duration, technically? Please ease my mind and tell me I'm correct, but if not, please tell me why its valve lift and not tappet lift.

UKPaul

Jim,
I've been reading various books & articles over the years on cam selection & your post above is one of the most informative & easy to read explanations I've seen Please continue

Hi Jotto,
If you are able to measure manifold vacuum when you get it running, I'd really appreciate it if you could let me know what the vacuum is at idle with that cam

LeMans Pete

Help!

shafrs3

What about durability, I think this poster and others would be interested in how high lobe lifts can affect valve train longevity.

Great post though.

LeMans Pete

I still don't understand how the norm is .050" of valve lift and not .050" of lifter rise. Like I said, you could easily increase the rocker ratio to get a greater duration, which then, of course, would make standardizing pointless. Unless there is a standard rocker ratio as well? But I don't ever see anyone saying, ".050" lift with 1.5 rr"

No one has corrected me, so I'll assume this post is wrong and I'm correct.