Cam selection "rules"

[HSSS427]

I've scanned a lot of the posts here, and read several books and articles, but am still trying to understand what the basic "rules" are that guide how to determine various cam specs. Seems that most of the articles describe how a cam works, what the various definitions are, but don't really tell you how to determine correct lift, duraction, lobe separation, advance, etc other than to "call a cam manufacturer." As such, I would like to get some commentary going on what to consider to answer the following types of questions. In lieu of this, could we attach a wire to Keith Craft or George Anderson's brains and have it produce a print out?

1. How do you determine the appropriate amount of intake lift, exhaust lift? I've read something that lift should at least be .25 of the diameter of the valve. Is that correct and what factors would cause you to go with less, or more lift? ie type of use, rpm range where you want most HP, or most Torque?

2. What are the key factors that determine the appropriate duration? When do you want more, and when would you want less?

3. What factors drive determination of correct amount of overlap?

4. How do you determine how much advance?

5. How do you determine appropriate lobe separation?

6. Other key rules or questions I've missed

7. Best books, sites or articles for answering the above

In short, if I know my engine type, CC size, valve sizes, tranny, gears, carb, intake, desired use and rpm range, and velocity of an unladen sparrow, how do I then determine the right cam specs.

My situation, for use as an example is as follows:
- 427 side oiler, cast iron block, .030 overbored
- Shelby American aluminum heads w 82cc chambers, 2.25" intake valves, 1.75" exhaust valves
- Aluminum police interceptor medium rise intake
- Demon 750 cfm double pumper carb
- MSD 6AL electronic ignition
- Tremec 5 spd tranny
- 3.54 rear end
- Primarily street use
- Run 93 octane premium pump gas
- Usual rpm range 1500 - 5000

Trying to do more than just pick a cam here, as I'm also on the pursuit of higher knowledge - which isn't very high given where I'm starting from ;-)

Look forward to being enlightened!
Thanks!
Scott

[BB427]

Way too much information to put in a post.

Buy and read:
camshafts, How to choose and time them for maximum power
by Des Hammil
64 pages
about $30.00

Have fun
Brent

[avanti-176]

We also need to know if the Unladened sparrow is...

AFRICAN or EUROPEAN?


Dan

[HSSS427]

I don't know that..........aaaghhhhhhhhhhh - and I am cast into the pit for stupid cam questions!

[HSSS427]

Found the book on Amazon, but it kinda got panned in the reviews, such as:

"Too mutch filler!, January 18, 2002
Reviewer: A reader
This book is not very useful. The only think worth mentioning is degreeing the cam. It includes no REAL information on selecting a cam. The important information from this book is also available with a whole lot more in the book:
The Step-By-Step Guide to Engine Blueprinting : Practical Methods for Racing and Rebuilding"

If you think it answers the questions beyond just describing how the cam works and the terminology, then I'll get it. But this review sounds just like my previous experience.

Thanks!
Scott

[ERA Chas]

Hsss,
Not to side track your question on cams, but I too have Shelbys stage II and I cc'd them and found 78.2 to 79.7cc across all eight. Compression is 10.33 with standard bore and pistons -.003 down.
Is 82 cc the advertised spec or did you measure?

[jdean]

For a car to spend any significant amount of its life on the street the rule of thumb is to install a cam that is two steps down from the one you'd like to brag about.

For a race only car, seek professional help.

[Thomas Gayer]

That's an easy answer.
Buy an XE 282 HR and build your motor around it.
If you are looking for more than 650 HP I cant help you.
It may sound a little lumpy,
but we are not driving Hugos here.
good luck with your build.

[Excaliber]

I have a 427 S.O., high rise heads and was faced with the same problem. WHAT cam and how to decide?

Interesting comment concering lift being a ratio of valve size, I never heard that before. Cam profiles are EXTREMELY complex, a worthy goal to research deeper into it.

I based my basic cam profile on: Typical cruising rpm because I wanted primarily a "street" application. I wanted a nice torque curve in that rpm band and was willing to give up so high rpm performance. My rear gear at 3:31, top loader, 60-65mph, 2800 rpm or so. Then I looked at specs that fit that profile. Like 1800 to 6200 rpm range kind of thing. Then I checked with George at Gessford and told him the range I was looking for. We talked about what I USED to have, how it worked and what I was looking for now, my heads, compression, etc. HE then picked my cam based on his experience with MANY side oilers in Cobras.

I THOUGHT my old solid roller would be a high lift cam, NOT SO! Rollers tend to have a LONG duration over a high lift. This stuff is complex man!

ADVANCE the cam and move the torque curve down, more torque at lower rpm. RETARD vice versa. I degreed my cam "just to make sure" it was what it was supposed to be. I set it "straight up", but here's the kicker. My Comp Cam paperwork says the cam is ground with a 4 degree advance built into it. If it's GROUND that way all ready HOW can it be considered 4 degrees advanced? Advanced from WHAT?

I'm thinking they use the same basic lift and duration for several cam "part numbers" and what they "alter" is the amount of advance or retard ground into the SAME cam when they make it. Thus the "torquey model" is really the "high rpm" model with a different "degreed centerline"? Of course lift/duration would probably change (or would it?) with a new centerline, making it impossible to know if it's really the same cam profile or not!!! There so sneaky those cam guys!

[Timmy]

Selecting a cam was the hardest thing about my build. I can’t sleep sometimes because I’m always second guessing myself. Should I have gone with more lift less lift, single pattern, dual pattern……I like to think we have all been there but I still lose sleep over it.

I’m dropping my 390 in on April 2 so time will tell. I was told “you can always change it if you don’t like it”.

Tim

[CJ428CJ]

If you really want to get a good idea of how one cam will perform compared to another, get yourself a copy of Desktop Dyno software. While the program has it's critics, there are many that will attest to how accurate the software results are compared to actual dyno testing of their motor.

http://www.powerandperformancenews.c...gory_Code=SFTW

[HSSS427]

I did not CC the heads and am using the numbers from the Shelby web site. Seems that could have a big impact. Is there a way to CC the heads or measure them while they're on the car?

Found some interesting theory on cams, heads, etc in David Vizard's book How to Build More Horsepower Vol 1. He's listed several "rules" in his book in the section on heads and porting and polishing. Just now moving to the cam section. He has some interesting discussion on compression ratio as well that makes me feel better about being at only 9.5:1.

However, still thinking I need to get to 10:1 compression by milling the heads and get a cam that increases the intake lift to something more like .560 instead of my current .525. However, he says that the flow on the ports should match the flow into the head, so no use lifting the valve higher to increase the flow if the port doesn't support that volume.

One other thing in the book that seemed to make sense, but I haven't seen much about is coating the chambers, valve bottoms and piston tops with material that keeps heat loss down. Basically keep the heat in the chamber instead of going into the heads and cylinder. Any experience out there with this?

BTW - Excalibur, love your blog. I'm keeping up with your commentary so keep it going!

[Clois Harlan]

Scott,

Call Comp Cams tech dept and give them the above info and they will make several suggestions to you. What ever profile you choose you will probably love. It is all about the sound at the exhaust unless you are racing. By the way a tighter cam seperation will rev faster ig: 106 or 108 degrees.

Clois


Clois

[ERA Chas]

Hsss,
Unfortunately, you can't cc heads on the motor. You can be sure yours are very close to mine as I also cc'd my iron med. risers which had 2.25's and 1.75's [same as our Shelbys], weren't cut, and those were nearly identical to the Shels.
DON'T CUT THOSE HEADS, IMHO! If you want to gain compression work with pistons, gaskets and deck height.
Those heads will support A LOT more induction [and compression] than your small carb and small cam -and still be very streetable.
A cam with lift in the low .600's and durations of 240 - 260@ .050 with a 110 deg LDA will work fine.
You need to really decide how you want to use the motor. Right now, you have pretty big heads [in volume and flow] and miniscule induction and cam. Keith K or some of the other pros can get much more out of those heads.
Best to contact some pro's for advice when you know how you want to use it.
I agree, Vizard's a sharp guy.

[Excaliber]

Gessford is a STRONG believer in 9.5 / 9.8 C.R. for us basically "street" guys. 9.8 was my target with the new pistons.