Cam's The Big Picture

[wyrmrider]

that .509 is a Big cam designed for bracket racing
good move- a lot bigger than the other purple shafts
cheers
cam you got was like the factory magnum cam with the lobe centers squeezed and slightly more lift

 

[Zell]

Thanks for extra info. Sounds like you totally get where I'm going with this build. I would definitely want a lower idle thank 2500! I should probably see what is a normal idle for others on the forum with various combinations.

I've read lots of good things about Mopar Purple cams.

If my understanding is correct, if LSA shrinks and lift and duration are the same from lobe profile, then there would be more air/exhaust mixing in the chamber. How would these issues then be addressed? Adjust timing to fire sooner?

 

[rumblefish360]

You don’t address them since that is exactly how it is supposed to be. The added overlap from the reduced LCA is out in to add top end HO. The earlier exhaust opening helps drag in air and fuel from the intake track by doing this.

Your idle becomes choppier/rougher. You tune your carb accordingly as normal.

 

[451Mopar]

If you compare the Mopar Purple Shaft 292 duration, 0.509" lift, 108 LSA, that cam has 76 degrees of overlap.
That cam was designed for bracket racing with headers and open exhaust. Rough idle, and usually higher RPM idle, and requiring higher stall converter and gearing.
The Factory 375 HP Magnum cam is 268/284 duration, 0.450"/0.458" lift with a wide 115 LSA, so it only has 46 degrees of overlap.
Being the factory production cam, smoother idle at lower RPM, more low end power, stock gearing (usually 3.23:1), and stock torque converter.
To a degree, you can tune the idle sound and RPM a bit with different amounts of ignition advance at idle, and the idle air/fuel mixture.
Most of the performance cams for what your looking at are going to have "advertised", that look closer to the stock cam, but with lift almost as much as the larger cam, and usually a 110 LSA and split duration with more exhaust duration for use with headers and full exhaust system.
The part that is hard to compare is the cams rate-of-lift. Although the Comp XE268 or Launiti Voodoo 268 cam may have "advertised" intake duration the same as the stock magnum cam, they also lift the valve faster and higher, so there is much more area under the lift curve. The 110 LSA also adds overlap, so these cams have a wide power band. Idle is pretty good with these too.
The Comp XE268 cam would have 54 degrees overlap using the "advertised" specs, and 52 degrees overlap for the Voodoo 268 cam.

On a first look, The Comp cam looks a bit larger than the Voodoo Cam, but the Voodoo cam is actually slightly larger at the 0.050" duration (and above) and the lift is also a bit higher. The Voodoo cam also has almost as much valve lift as the Mopar 0.509" cam with 0.494" intake valve lift, and the Voodoo Exhaust has more lift at 0.513" of lift.
So would the Voodoo cam make as much power as the Mopar 0.509" cam? That depends at the RPM you compare the two cams.
The Voodoo cam would make more low end torque/power up to some RPM where the larger cam would most likely make more peak power.
The next question is which cam makes the car faster? That depends alot on the gearing and converter stall speed.
With a stock converter and gearing the Voodoo cam will leave and accelerate harder from a dead stop. The Big Mopar cam would be slower off the line and maybe up to around 3,000+ RPM, but once the engine is in its operating range somewhere over 3,000 RPM it should out accelerate the voodoo cam. Now using a high stall converter and more gear, the larger cam should be quicker because it will spin up to its power band faster. On the other hand the Voodoo cam would be easier to live with in street driving, smoother idle and such.
On the valve train, the Mopar 0.509 cam is actually pretty easy on the valve springs, it has much more time/duration to open and close the valves. The Voodoo cam is opening the valves nearly as much as the Mopar cam, but in much less duration/time so the valve train is under a higher rate of acceleration, so it is more critical to have the correctly matched valve springs.

 

[Zell]

Thanks, those are very thorough explanations. If lift increases with a new cam, does the overall lift cause more stress on the springs, or the rate at which the spring compression occurs due to the cam lobes profile?

You recommended that the Voodoo cam would require correctly matched springs, would this typically require new rocker arms?

 

[Zell]

You don’t address them since that is exactly how it is supposed to be. The added overlap from the reduced LCA is out in to add top end HO. The earlier exhaust opening helps drag in air and fuel from the intake track by doing this.

Your idle becomes choppier/rougher. You tune your carb accordingly as normal.

So since choppier/rough idle = more overlap = more air/exhaust in the chamber, then this would create a lean condition remedied by tuning for more fuel in the mixture?

Does timing typically need to be adjusted with the distributor or is this usually done at the cam-level? I've been reading how many high-performance comp cams are 'degreed' during install.

 

[451Mopar]

Thanks, those are very thorough explanations. If lift increases with a new cam, does the overall lift cause more stress on the springs, or the rate at which the spring compression occurs due to the cam lobes profile?

You recommended that the Voodoo cam would require correctly matched springs, would this typically require new rocker arms?

With the springs, both increases in lift and rate or acceleration cause stress on the valve springs, but given the spring does not coil bind at max lift, the rate of change (acceleration) and weight of the valve train (mass) determines the spring pressure (force) needed to properly control the valve motion. Just the physics of Force = Mass * Acceleration. The acceleration part will depend on the cams RPM (1/2 engine speed) and the cams lobe profile (rate of lift.)
The reality is these cams are still pretty easy on the valve train compared to a solid roller with a drag race lobe profile.

Most likely the stock rockers in good condition can be used, but sometimes the pushrod will break through the cupped area in the rocker arm. Mopar Performance used to sell a heavy duty replacement rocker arm set that I think was stamped from thicker steel? I have never compared the material / thickness of the replacement stamped rocker arms from companies like TRW, and such, to see if they may be better than the original stampings? Usually once getting past about 0.500" lift, upgrading to a stronger rocker arm is recommended.

 

[wyrmrider]

451 Mopar says it well go back and reread the thread and makes some notes did you post up your TRUE compression? and I don't remember your heads or exhaust
I'll take a look back later
zell
Elgin actually makes rockers IDK if TRW still does- it's same as Sealed Power now all bought and sold
I don't think you need to with these cams but you can run Magnum/AMC lifters and oil through the pushrod pushrods I'd do it if buying new pushrods and lifters
stock rockers are good to 550 and slightly more with hyd cams - and you can use lash caps to give a larger wear surface- which we did in classless where stock rockers were required
second post
stock exhausts do not like narrow lca or lots of overlap- you get no benefit
advancing a cam will give you more low end (but you should have bought the shorter cam in the first place
the Voodo 268 is a good place to start check your retainer to stem seal for clearance with that lift range check on springs, also consider beehive- they are lighter and that helps with heavy MOPAR valves
the next smaller 265 VOODO with 1.6 rockers on the intakes gives .523 lift in the intake- about the same as the suggested cam with a .527 exhaust IDK what the lca on the voodos is 224/230 @.050 but it is a bigger fatter cam than MAGNUM or 268 purple shaft and would rival 272 purple shaft with shorter duration
if your compression is up the the 274 XE-HL comp could be considered
LOTS OF DIFFERENCE BETWEEN 265-268 and 274 with these cams ps forget the Comp 268H or Crane HMV Chevy lifter cams- you have to go to much more duration to get the area under the curve- you can get a similar top end but you give up torque and driveability with the 268H you would have driveability but much less area under the curve = power

 

[451Mopar]

I have not built anything with the voodoo 268 cam (yet), but have used the smaller 262/268 cam in a 400 engine that might have had 9:1 compression (or a bit less), and it ran good and used the stock rocker arms.
I do like running 1.6:1 rocker arms, even my mild 9.5:1 compression 360 RV engine has 1.6:1 rocker arms with the Hughes Engines SEH1620AL cam, that is 216/220 duration at 0.050" and 0.528"/0.536" with the 1.6:1 ratio rocker arms (0.495"/0.503" with 1.5:1 rocker arms.)
I think the Hughes Engines cams have even faster cam ramp rates than the Vodoo cams, but are more expensive too.

As Wyrmrider said, the Voodoo 268 is just a starting point. I used it as an example because you mentioned something about hearing a car that was running a Vodoo cam. Knowing the engines true compression ratio will be a big factor in choosing the right cam.

I looked in the Lunati catalog to see what the recommended valve spring is, and they recommend either the #73100 dual spring, or the #74820 Beehive spring. When changing springs you will likely need spring retainers that correctly match the spring sizes (steps with the dual springs.) The #73100 spring shows a seat pressure of 125 lbs @ 1.850" install height and a spring rate of 333 lbs/in. <- this seems a bit on the light side to me?
The #74820 spring shows 155 lbs @ 1.880 seat pressure and a spring rate of 370 lbs/in. Really not a bad price on these at $153.99 / set.
Last time I priced the Comp beehive springs they were over $200 / set.

 

[wyrmrider]

yes on Hughes vs Voodoo the Voodoo are not as extreme as the CompHL or Hughes- should be easier on the valvetrain and more room for error
were they Harold Berkshires profiles? I can't remember
as you say there are better places to get springs than Hughes or Comp I get the long deal at ISKY so... Chet Herbert is reasonable (went to the 62 AHRA nationals with Lefty)
what I've been saying low compression 383-400 262/*** should work fine is that a voodoo?
look at the 253 Voodoo same lift as the magnum cam but a whole lot shorter
the 258 voodoo has about the same as the magnum cam at .050 but a lot less seat duration so does not need the 115 lca of the magnum to hold down overlap
but seat timing sucks with the magnum late intake closing
I'm not pushing Voodo- never used one but some for customers but comps XL HE series does not go that short I'v used mostly west coast grinders

 

[rumblefish360]

So since choppier/rough idle = more overlap = more air/exhaust in the chamber, then this would create a lean condition remedied by tuning for more fuel in the mixture?

Not Necessarily.

So far everything I’m reading by you is Theoretical and your not ready to anything until you have all the answers to questions that can not be fully and totally answered since you have not committed to anything as well as every engine and combo are different.

IMO, your over thinking this and you should just get to work on the engine instead of asking questions that are splitting hairs.

Does timing typically need to be adjusted with the distributor or is this usually done at the cam-level? I've been reading how many high-performance comp cams are 'degreed' during install.

All cams should be degreed in for maximum impact as intended by the manufacturer of the cam so it will perform the way it was intended.
This becomes more and more important as the cam gets larger and more performance oriented in it’s size.

Once the cam is set in and degreed (Also could be considered timed in, but not normally said that way.) Next step is to time the distributor and that is a rough timing until the engine is started up for the first time. Where the flat tappet camshaft is broken in. Roller cams do not need a break in time.

Once the cam is broken in and the oil changed etc.....


Proceed to set the distributor and then tune the carburetor.

Your questions should be answered by doing and not reading into the answers and then dividing the answers against other possible real or thought up outcomes nevermind the fine tuning aspect or the overthought scientific thought process of what is going on in an engine at the Molecular level and ultra fast motions of the operating equipment.

In other words, just do it!
Now get to work and have some fun!

 

[Zell]

rumblefish360 I would love to get my hands dirty and just jump in, but I am stuck in California for work until I can pop home next time to where the car is. So until then I was trying to get all the info about cams that I could!

Really interesting read, and appreciate the posts from everyone. I have to admit, I've had to reread many of the posts more than once. Can't wait to get home and mess around with this engine!!

 

[rumblefish360]

Stuck in Cali?!?!

OMG Prison sentence! LMAO

WELL, seriously, I’m looking forward to hearing about your progress!

 

[Zell]

Stuck in Cali?!?!

OMG Prison sentence! LMAO

WELL, seriously, I’m looking forward to hearing about your progress!

Thanks! And yes, tell me about it- there isn't much garage space here in Oakland However, last time I was back home I managed to expand the garage by knocking down a few walls! The things we do for cars...

 

[66Satellite47]

A key to cam selection is being honest about what your use will be. If only for real street driving, too big is a bad plan. My experience is drag racing. In the day the 509 Mopar cam was not a top performer, if you had the other parts to take a bigger cam and make more power. If you torque converter, gears or the manual transmission a bigger cam will work fine. Modern cams appear to be much more adaptable with wide lobe centers. There will not be a perfect selection. Also if you have the time basic pocket porting on a stock iron head gives amazing improvement. Then you can really make use of a bigger than 500 lift cam. For basic street driving (if haven't improved the heads) you should stay with something close to the stock dimensions.