Welcome to www.JeepStrokers.com!!
Not yet a registered user? You are required to Register before you can post within this topic.

How to pick a Cam FAQ

All FAQ's Go here... Most basic questions can be answered if you look here!
Post Reply
User avatar
Flash
I love JeepStrokers.com!!
I love JeepStrokers.com!!
Posts: 693
Joined: February 17th, 2008, 10:45 pm

How to pick a Cam FAQ

Post by Flash » March 13th, 2008, 9:34 pm

We need some one that know Cam spec way better then i.........but i will pass on my thought and opinion on the subject.
When looking at a cam spec you need to know basically what each means so that you can take 10,000 cams and narrow then down to a hand full and then you have to decide which cam manufacture you want to use...........Now your down to a couple of cams.....

Some have cam spec "@.050".......and some use "Seat to seat". A cam that give you both spec, the @.050 will always be lower. Example of a 206* my be equal to 260* Seat to Seat.
NOTE: a different cam manufacturers same 206* may be rated at 240*.........confused??????? the speed in which the lifter raised off of the cam(or the ramp angle of the cam lobe) will decide how many degrees has past before the cam lobe has raise the lifter .050"

Every thing that i talk about will be Seat to Seat, as i think this will tell the most true about a cams design!

LIFT. .400" would be considered a stock to RV cam................... .450"= Power and with a well ported head .500'+ may work well(As a valve open, velocity or air speed increased up to to point of the ports efficiency............any more valve lift past this point will gain vary little more in volume........Just some food for thought)

DURATION. This is the time in with the valve is open(intake or exhaust)......240* good bottom end torque...........280*
Loss on bottom end (not a rock crowler cam!) but make up for it in the top end HP(Eng will rev higher and still make power)

OVER LAP. This is the time were the exhaust valve is closing, and the intake valve is starting to open. the smaller the number here, the more bottom end torque you will have......on the other side of the coin the higher the number gos the more bottom end Torque you will loose in order to gain top rpm power.........The higher the #### the more the eng will want to lop at a idle and the less over lap you have the smoother the eng will idle............ also the eng Vacuum at idle will go down.......If you choose a cam with enough over lap you vacuum brake boost will not be able to work properly(usually in the 60 degrees and up.....Torque cams, you would be wanting to look in the high 30* to 40*s)Advancing the cam 2* to 4* for a cam that is to big for the eng or intended us, Will get some of the vacuum back, but will lower the top rpm of power.

IVC (Intake Valve Close) Is always go info and this will give you a good idea as to what kind of compression you will need to get a good DCR (Dynamic Compression Ratio)

Make Sense????
Let me know were i lost yeah at and i will try and make it clearer.

Here is a beter, and more accurate discription that i found.
EDIT:....EDIT OK, it has taken a while but if you click on this image it will come in full and CLEAR.
Injoy....
Eng Cam Info 1.jpg

Flash
You do not have the required permissions to view the files attached to this post.

User avatar
Flash
I love JeepStrokers.com!!
I love JeepStrokers.com!!
Posts: 693
Joined: February 17th, 2008, 10:45 pm

How to pick a cam FAQ

Post by Flash » March 27th, 2008, 10:33 pm

In the processes of preparing for this i learned some thing :o :D Will get to that later ;)


First lets start with the stock Renix cam(Once again all the cam spec's that i choose will be seat to seat.)
.405"( lift,) int and exh.
270*(Duration) int and exh
46* (overlap)( the amount of time that both the intake and exh valve are both open)
75* IVC( necessary to figure out DCR)

.405-.440 would be considered stock to mild performance
240*-260* of duration would be considered, mild performance and above 260*, would start to affect idle and bottom end Power.
35*-45* Overlap.... mild w/ good idle and vacuum(You could us power brakes, or vac assist brakes with not problems.
60*+ Degrees of overlap would require manual brakes or a Hydo booster ( Power brakes assist by hyd (power steering pump) brake system)
IVC.......the higher the number the lower DCR or the compression that the Eng See's when it's running.


NOTE: I have run all "three" of these cam on my desk top dyno and will give the numbers .......as it will show the basic number don't tell the hole story.
All cams were run with the same eng specs, 3.875" bore 3.895"stroke, stock 7120 head and 9.44:1 SCR.(I have to input a lot more spec's, but these are the important ones for here.)

There is many more spec's, but these or the most important spec's to understand....to narrow the Forest of cams, down to a couple of bump sticks. ;)

we know this is the stock cam but lets go thru it.

.405" lift, would be considered a low lift cam, and 270* of duration(time the valve is of its seat in Degrees).....is more of a performance spec. and 46* of overlap would indicate a low rpm/High vacuum cam.

"T"(torque) at 1500 rpm 233 and HP at 67
"MT"(Max Torque) came in at 4000 RPM and 278
M,HP (Max Horse Power) came in at 243@5000
DCR is 6.97:1

COMP 68-231-4
.465"/.485" int/exh
250*/256*int/exh
30* overlap

OK! High lift = High performance cam
Duration = mild cam
overlap = stock cam???????
IVC= 56*
So, Hi lift cam and mild every thing else
1500 rpm T=272 and HP 78( Both or more then the stock cam)
MT was 303 at 3500 rpm( more torque at a lower rpm)
M,HP was was 223( less max hp and is at 500 less or lower rpm)
IVC is 56* and created a DCR of 8.06:1 (much higher running compression(DCR)

Crane 753901
.456"/.484"
260*/ 272*
42* overlap
IVC 61*

Lift = High performance
Duration= mild to High performance
Overlap= fit in at mid mild
IVC 61* bring the DCR in at 7.80:1( Hmmmm lowers DCR then the comp cam(you can get away with more SCR) but........the stock cam has the lowest DCR
This shocked me :o and is what i meant above, when i said i learned something.
So, HP lift,
mid to HP Duration,
and overlap that is more close to mild.......so even thow this cam is the biggest cam showed so far, vac brakes shouldn't be affected............

1500 "T" 256 and "HP" 73(More T and HP then stock renix cam)
"MT" 4000 RPM and 301
"M,HP" 5000 and 246

DON'T GET HUNG UP ON THE NUMBERS, COMPARISOME BETWEEN Them, IS WHAT YOU WANT TO LOOK AT.

Of theses cam the crane would be the hot rod cam.

the comp would be the street/rock crawler(most versatile cam in my opinion) cam

And the Renix(stock) cam would be for those that have build there stroker already, and can't get rid of the ping, do to to much SCR ............Up, until to night, I would have recommended the Crane cam to lower DCR

I really want to do a DCR comparison between the three stock cams........But I only have @.050 spec's for the Early and late HO :(
If this completely confused you(first time trying to decyfer cam spec's)
use this chart,
.405-.440 would be considered stock to mild performance
240*-260* of duration would be considered, mild performance and above 260*, would start to affect idle and bottom end Power.
35*-45* Overlap.... mild w/ good idle and vacuum(You could us power brakes, or vac assist brakes with not problems.
60*+ Degrees of overlap would require manual brakes or a Hydo booster ( Power brakes assist by hyd (power steering pump) brake system)
IVC.......the higher the number the lower DCR or the compression that the Eng See's when it's running.


and when you have your cam selection down to a small hand full, come back and read this to make you final decision. ;)


Flash
89 XJ with 300,000 on the original eng

"I've also never completed a motor, yet. My mouth (fingers) is also writing checks my ass can't cash."

User avatar
John
I love JeepStrokers.com!!
I love JeepStrokers.com!!
Posts: 709
Joined: February 13th, 2008, 8:35 pm
Stroker Displacement: 4.6
Location: West Virginia

Re: How to pick a cam

Post by John » March 28th, 2008, 6:38 am

The COMP 68-231-4 is very much as you describe, Have one in the 4.6 in the YJ. I like higher compression motors and accept the build requirement and more expensive fuels for a non daily driver, this cam gives around 18 inches of vac at idle, so the brake booster isn't a concern. Basically I find it acts like a factory cam with improved breathing. The built in lift and 4 degrees advance lets it come on strong off idle. A optimized factory cam so to speak. Really a nice flexible cam it is sweet at factory compression levels and responds well to higher compression builds. Always surprises me that comp doesn't recommend it for fuel injection. I have had no problems with these and OBDI haven't tried one with OBDII.
John

Post Reply

Who is online

Users browsing this forum: No registered users and 1 guest