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Unread 07-26-2010, 05:29 PM   #1
xj_army
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Help choosing a cam for a 4.0 I6

I have a '99 with the 4.0. I have no idea what im looking at when looking at cams and pistons. Its not as easy as quads lol. Can anyone tell me what gains I would get, if they would give a nice lopey sound, and if they even work for my engine. I hate to ask so much but I have no idea what I would want.

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Unread 07-27-2010, 02:51 AM   #2
ClimbStuff
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There are 3 numbers that are important:

1. Lift
2. Duration
3. Lobe Separation Angle

By buying a cam with certain measurements, you can tailor your engine to specific rpm ranges, torque curves, and even throttle response. With each, comes a price. Lift is simple... its how far the valves are opened verses their seated position. More lift equals more air intake, and with the proper fuel trim, more power. more lift also creates more wear and tear on the valve train. Your asking the valve to react very violently when adding a ton of lift and then giving it the beans. Unless you have a serious head on that engine, it's not recommended to venture too far over stock as far as lift goes.

Duration is also very simple... its the length of time (in degrees of rotation) that it take the valve to open from the seated position, reach its max lift height, and then close back to the seated position. The longer the duration, the longer the valve is open and the more you're promoting high rpm usage. Duration has a HUGE effect on where in the rpm range will the power be useful. In an engine like the 4.0, I cant see anything over about 230-240deg @ .050 lift, being useful because we stay around 5500rpm MAX, have no power adders, and are looking for off the line torque verses top end horsepower.

Lobe Separation Angles make things quite a bit more complicated... and if you don't plan on running a hot motor, stick with the factory cam spec on that. It is basically fiddling with the timing of the intake and exhaust valves verses each other... its adding or removing overlap... has a lot to do with extending burn time, cycling unburnt fuel out, lowering static compression to a more usable dynamic compression, blah blah blah. It's really something people only mess with if they know what kind of fire they are dealing with... so just leave that one alone.

Out of the 5, the cam that will least change the stock characteristics yet yield a gain in daily driving would be the Mopar Performance Purple Cam. It has a short overlap, just like stock, .430 lift which is more than enough, and a 240deg duration that will keep the power band low to spin those big tires a bit easier off the line.
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Unread 07-27-2010, 11:04 AM   #3
ClimbStuff
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Now that I'm looking at this again and its not the tail end of a 19hr day, I'm seeing that I may have read the data incorrectly on these cams. I also completely neglected to find out what the factory cam specs are, to determine the change in operation based on the new cam specs. I also neglected to so that any time you change your cam, you should change out lifters as well... so if you can buy a full kit that includes all that stuff, your engine will thank you in the long run.
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Unread 07-27-2010, 12:05 PM   #4
mdm
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Another consideration is that most of your aftermarket cams don't work well with the stock springs. After market springs to match the cam will require head work.
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Unread 07-27-2010, 02:42 PM   #5
1998zj
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Call Competition cams tech line and tell them what you have and what you are trying to do and they can recommend a cam for you. You don't want to get a cam that runs great at 6000 rpm when your intake and heads die off at 4000.
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Unread 07-28-2010, 12:08 AM   #6
ClimbStuff
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Quote:
Originally Posted by mdm View Post
Another consideration is that most of your aftermarket cams don't work well with the stock springs. After market springs to match the cam will require head work.
This is true, but more so for engines that are moving to a hotter cam in terms of raised rpm... with stock springs there may not be enough spring pressure to keep the valves mirroring the cam lobe, instead, at the peak lift, the cam falls before the spring pressure can react, and the valve "floats"... hence the term valve float. The only real concern other than that is that the springs have enough compression distance to accept the higher lift, but I'd like to think that you could easily call and find out if it requires aftermarket springs/guides/retainers etc.
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Unread 10-25-2010, 07:08 PM   #7
Ruffnek
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Mopar makes performance springs, locks and keepers for the 4.0 that should work with any high lift cam.
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