Cam selection

Cams..

There are SOOOOoooo... much data out there that is causes way TOOOOoooooo much confusion!! Go to Buddy Rawls web site.. check out "Cam Truth", then Call a few cam grinders and ask them.. But reading Buddy's page IS a good place to start... or end!!

Just me.........................

Thumper

Here's Buddy Rawls site http://www.wighat.com/fcr3/index.html

And here's the Cam Truth page http://www.wighat.com/fcr3/camtruth.htm

vristang said:

Thanks for the link guys. It has a lot of good basic info.

I am actually looking for more detail though. Something that says...
You won't pass emissions with XX degrees of overlap.
Intake and exhaust duration should be sized per (whatever) to meet XXX rpm needs.
Things like that.

I am looking for some concrete guidelines, or maybe not even as firm as concrete (even like mud would be better than what I have now). All I have been able to find so far can only be used to define camshaft definitions and terms.

Thanks again

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That kind of information doesn't exist. An engine that's not efficient will fail with a stock cam, but an efficent engine with the same cam will pass. Their are no hard and fast rules for what you're looking for. I think only actual testing will deliver the data you seek. And that'll get expensive very fast.

I have a book by Richard Holender (sp?) with all the different combo and parts he's tested over the years. He does a lot of MM&FF's true hardcore tech stories.

Of course the custom cam guys aren't going to cut and grind different cams for you. And they aren't gonna tell you everything you need to make your own cam. They would be giving away the knowledge they use to make a living.

vristang said:

Thanks for the link guys. It has a lot of good basic info.

I am actually looking for more detail though. Something that says...
You won't pass emissions with XX degrees of overlap.
Intake and exhaust duration should be sized per (whatever) to meet XXX rpm needs.
Things like that.

I am looking for some concrete guidelines, or maybe not even as firm as concrete (even like mud would be better than what I have now). All I have been able to find so far can only be used to define camshaft definitions and terms.

Thanks again

Click to expand...

LOL

just to reiterate, that info does not exist!

too many variables!


and buddies site has a whole lot more than basic info, once you can understand it, and explain it, you will then understand why your question above has no answer.


as for a book, search richard holdener, i do believe he finally got it on the shelves...... and it sounds like what your looking for.


maybe ask a more specific question about cams, you started to......but then trailed off.....


good luck

This could potentially be a long one........

vristang said:

I refuse to believe that there are no guidelines to selecting an appropriate cam. For example, there is a practical limit to how much overlap/duration can be run on a street car, before so much engine vac is lost that power brakes no longer function.

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ok, here is why.
lets say you have a stock head, with a 124cc intake port, you can run x amount of duration and overlap, and still produce significant vacuum to run your power brakes, and still have clean enough emissions to pass inspection. now, swap those heads with a 200cc intake port, and add an intake manifold to match, now that cam would be huge, vacuum would dissapear, the car would not idle below say 1000 rpm, have nasty hydrocarbon emissions, and be a real pita to drive.

vristang said:

All I am saying is that the site referenced above does not even suggest any kind of overlap duration numbers. This is just one example of the kind of info I am looking for. What are the upper and lower limits? I can find definitions of overlap, duration, and valve timing just about anywhere.

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the site listed tells you so much info.......... hell, i have been into this crap for YEARS, and i had to read it ten times to get all i could out of it. duration numbers, believe it or not, do not tell the whole story, there is FAR more involved.the correct way of going about selecting a cam is to determine the optimum valve timing events for a given combo, THEN the duration numbers fall where they may, those numbers are not all that important! (the actual valve open/close events are)
say the cam designer feels the intake valve should open at 5* BTDC for your combo, and should close 26* ABDC, he doesn't sit around and say, "well, i think i should run 211* duration......no, he determines where that valve should open in relation to the crankshaft, and where it should close, and if any overlap is neccisary based on mucho info, like intake capacity, exhuast system, cu", weight, gearing and a ton of other factors.

there just is NO cut and dried answer to your question, take two cars, both 3300lb stang, say they have stock heads, intake, and exhuast, but both want a cam to pass smog, and give them some power.........well, lets say one car has 2.73 gears, and a stock converter, and the other has a 3.5k stall, and 4.10 gears, the cams will be different.....change the heads, and exhuast on one of those cars, and now the cams are even more different..... yet many here and on the corral would say "run the "e" cam!" (for either combo)

many people run the f cam on p headed, or regular gt40 headed cars, say 145cc intake port..... well, i had an effective 190cc intake port, and ran an f cam......... car was a turd below 4k, these other guys with the smaller intake ports have good power starting at about 2800 or so....yep, i had the WRONG cam. Buddy specced me a cam for that engine, and when i got it, i near shat myself......it was tiny by comparison, but the car picked up everywhere, even top end. it was something on the order of 212/218 .510/.520 on a 116 lsa Vs. a 224/224 .512/.512 on a 114 lsa..... with the f cam, i had like 10" of vacuum at 650 idle (way to low) after raising it to 950, i got it to 15", just enough to make it daily freindly. buddies cam was more like 16" at 650.

To use an example brought up by nosmatt, how should the cam dimensions be changed when moving to a larger volume port? Or how should cam dimensions change when moving form a heavy vehicle to one that is 500lbs lighter? These are not trick questions, and if Buddy Rawls can figure it out, I would think most people could at least understand it, even if they can't apply it as effectively.

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AGREED on the last part. absolutly even.


if i could explain fully on how the valve timing events should change when "x" is changed here or there, i would be a cam designer myself

i can give another example......

say you have a 302, you want 350hp, if you use a gt40 head to get there, you may have to use a cam with alot of duration, and alot of overlap to help pull the a/f charge into the combustion chamber.
now, change to a large port head, say an afr 205, the cyl is filled so much more quickly when that valve opens, that the duration those valves stay open is not nearly as long as the small port head.(being that the port flows so much more, and the cross section of that port is so much larger----meaning it kinda has more in "reserve" sitting in that port already) now that overlap is not needed at all, and in fact, if it had as much (as the gt40 head setup), it may allow cylinder pressure to actually bleed off, and thus power could be down....

now, how they come up with the actual timing events, as far as i understand it.......

take into consideration, you have 300 cu", you need to fill that space as effectivly as possible in the given rpm range it is to be used, first you must know what that is..... now, you need to determine how much air is needed, and when you need to open the valve to achieve this. knowing how much the head flows is paramount, as is the cross section of the port etc... now that you know when the valve must be opened, and how long to hold it open, you know your duration, now you gotta move onto the exhuast, and using all the info for the entire exhuast system, figure out where your valve events need to be to fully evacuate the cyl, and at the same time, use the exhuast draw on the cylinder as it is exiting to help draw in the intake charge (where overlap is quite useful) now this affects the events you already had for the intake, so these will need to be modified.....eventually you come up with the events you need.
now you can start looking at different lobes (in cam manufacturers catlogs) for the intake, and exhuast that will help you meet your goals, and have the cam ground with "x" lobe for an intake, and "y" lobe for the exhuast, and wherever you want your centerlines etc...now your lsa is determined, not the other way around.



fwiw, IMHO, a larger port head, with a smaller cam will be the more docile combo, yet offer the top end, and torque the smaller port head would......and, it appears that the oem's are doing this type of thing as well. the 4.6 4v cobra engines have crazy large intake ports for only 281 cu", and tiny ass cams to make em livable. kinda like the ls1 stuff as well.






finally...........this is all how i understand it, after more than a few year studing it, and building different stangs, some of which worked great, and others that did not!