2000GT4.6

How camshafts work:

This link pretty much explains everything you need to know about the physical operation of cams in our modular engines:

http://auto.howstuffworks.com/camshaft1.htm

Terms of cam specs explained:

Lobe: This is what physically opens and closes the valves (thru the lash adjusters and rocker ams).
Lift: How far the valve is opened
Duration: How long the valve is held open
Centerline:The position that the intake lobe hits peak lift, which is at the center of the lobe, in crank degrees
LSA:The number of degrees between the centerline of the exhaust and intake lobes
Overlap: The time where both the intake and exhaust valve are open. This creates a scavanging effect, where the exhaust gases leaving at a high rate of speed create a vaccum and pull fresh air into the cylinder.


Aftermarket cams in general:

Generally speaking, when installing an aftermarket camshaft, the idea is to increase the duration and lift as much as possiblewithout sacrificing too much low end torque, without killing drivablity (if thats a concern)and without pushing the cam's powerband past the point the rest of the engine (heads, shortblock) will support.There are other things to consider (like LSA, and cam timing for example) but this prettymuch sums it up.

Almost any aftermarket cam is going to give up some amount of power down low, but increase power up top. The more agressive the cam (the more lift/duration) the less low end, and more top end power a car will have, and the farther the cam will want to rev. In other words, the more agressive the cam the more power you will make (to a point) and the higherthe powerband will be.(pushed to the right on the dyno sheet).

I hate to point to a LT1 website here, but simply put this is an excellent writeup and it gets the point across faster and more clearly than I could. This explains what each camp spec does, and how it effects horsepower and torque, along with timing and a bunch of other stuff:

http://www.lt1howto.com/articles/camshafts.htm


2VCams:

The most important thing to realise is that simply going with the biggest cam possible WILL NOT give you the results you need. Going with that monster cam might sound appealing, but its simply not going to run worth a crap on a stock motor car, and your going to hate the results if this is your DD.

There are several different companys you can buy from. VT engines, Comp cams, Crower, and a few others do most of the acutal millwork. If your getting your cams "custom" ground by a local shop, more than likely they are sending them to a large manufacuter to be ground. Most of the modular cams are listed in "stages" as in a "stage one cam". Some of the other companys label them differently, but if you look closely the specifcations usually line up fairly closely.Most modular cams will run you between 500 and 600 dollars for a set (2).

The stage 1 and stage 2 n/acams are for stock head, and usually stock shortblock cars. The stage 1 will sound a little less agressive at idle, and make a little less power up top, but will pick up horsepower sooner than a stage 2 cam. The stage 1 cams do not require a valvespring upgrade.A stage 2will be more agressive atidle, and make slightly more peak power, but notcome on as fast, and will loose some torque down low.The 2(and anything larger) will require avalvepsring upgrade. I recommend a beehive spring ala comp cams, although more options are becoming available. Anything over astage 2 simply will not work well with a stock shortblock/head car. The rest of the engine simply cannot support the RPM and flow the cam wants andyou would loose alot of power in the lower RPM range.

If your not running a stock shortblock/heads car, I highly recommend skipping any of the generic cams and going right to a custom grind cam. There are several big name shops (MPH to name one) that have become very good at grinding custom cams. Although you will see gains with a "stage" cam at this point only a custom is going to give you the absolute maxmium for your setup. This goes for both N/A and FI setups. Most of the time custom ground cams are the same price (or very close) to a generic cam, so there is almost no reason not to do this.

F/I Cams:

First of all, F/I cams, at least in my experence never seem to sound as agressive as a n/a cam. You can have a pretty wild FI cam and it doesn't sound nearly as lopy as a much less agressive n/a cam. This is because LSA gives you the "lope" that everyone loves so much.

The "blower" cams, or FI cams, have less overlap(and higher LSA)than a n/a cam. This is the primary difference, as you loose boost when you have more overlap.

Here is the complete VT stage 2 "blower" cam spec, alongside the VT stage 2 n/a spec:

.530/.570 lift 220/230 .50 duration 112 LSA +4*
.560/.575 lift 225/235 .50 duration108 LSA

Notice the LSA, is less (lower number)on the n/a cam. The lower number the LSA is, the less seperation there is between the exhaust and intake lobe centerline. Decreasing this number results in more overlap. A 4 cycle engine uses this overlap to allow fresh air to enter, and thus force the exhaust gases out.

Making this longer on a n/a setup allows for a 4 cycle engine to get more of the exhaust gases out and get more fresh air in. On a FI setup however, if the LSA is too large you are literally loosing boost right out of the exhaust(both valves open, no compression taking place)This results in a boost drop, and you loose horsepower. This is also why you are more often than not OK(not optimal)with running a n/a cam at lower boost levels. Once you crank it up however, you really start to loose boost and you needa progressvly smaller and smaller pulley to run the same amount of boost, which drops efficentcy, heats the incoming air, and generally makes it make less horsepower per boost (and pulley size).

You definatly do not want to run a n/a cam on a blower if you can help it, and most people have NOT had as good results as I have had running a n/a cam on a blower car.However, you can do it at lower boost levels, and the cam will sound more agressive do to thelower LSA number. I am somehow not loosing boost, but everyone else that has done this has to pulley up. Sometimes I wonder if VT didn't accidentally send me a blower cam intead of a n/a one.

LCA vs duration vs overlap:

Duration effects overlap just like LCA. The longer the duration, the longer each valve is open during each rotation, and thus the more overlap.

This is going to sound crazy, but stick with me here. Imagine you have a cork rod and a small rubber ball. Cut the rubber ball in half, and place a pin in the exact center (thru the top)of each half. Now, pin the two halfscut side downinto the rod.Now imagine lookingdown therod (looking at the end down it)If you were to take a measurement, in degrees, between each pin, this is your LCA. If you were to look down the length of the rod, you could see that part of each half ball overlap the other (if they are close enough.. on a

lizzyfan

Good stuff Scott!!
Sticky up!

97 5.0L

just a question. I put in some comp cams into mine- xe274's. I do not have F/I now (except nitrous) but I plan to get a procharger D-1 will these be a good F/I cam? the LSA on them is right between the two you listed. it is 110

nathansee3

That there is a great write up. I could read it over and over. Thanks for postin that.

NoRdO

Great write up.

2000GT4.6

Since you have a stroker, and your planning on going with a D-1, that tells me your wanting big numbers. the XE278 is not going to be an ideal cam. basically, after you go beyond the horsepower limits of the stock shortblock, and past 10 PSI, your going to want a blower specific cam with a larger LSA number. The boost loss is progressive, so you will have to pulley up farther at say 15 PSI than you will at 10. (you loose more boost at 15 PSI than 10 per pulley size).

Edit: Ran out of time there, here is a better answer. The reason you would not want to run the X278 is not only the LCA, but the size of the duration.

Duration effects overlap just like LCA. The longer the duration, the longer each valve is open during each rotation, and thus the more overlap.

This is going to sound crazy, but stick with me here. Imagine you have a cork rod and a small rubber ball. Cut the rubber ball in half, and place a pin in the exact center (thru the top)of each half. Now, pin the two halfscut side downinto the rod.Now imagine lookingdown therod (looking at the end down it)If you were to take a measurement, in degrees, between each pin, this is your LCA. If you were to look down the length of the rod, you could see that part of each half ball overlap the other (if they are close enough.. on a 4 cycle engine they always are).

Now, imagine what would happen if you increased the size of the halfs by say 10 percent, but you kept the LCA the same. The pins are in the same place, and the centers of the halfs are in the same place... but the halfs are larger. This means there is going to be more overlap.

Thus, the larger the cam , the more you have to increase the LCA number to keep the overlap the same. A cam with a duration of say 225/235 (a stage 2 n/a cam) with a LCA of 108 is going to have less overlap than a cam with a 235/245 duration and the same LCA.

I am going to place some of this up in the OP.

01GT4.6

an easy way to figure out how much chop (or lope) a cam will have it like this:
The more overlap the choppier the sound.
Figuring Overlap Of A Cam

- Add the intake and exhaust durations
- Divide the result by 4
- Subtract the LSA
- Multiply the results by 2

sweet99

I have Sean Hyland #2730 cams which have .495"/.495" lift and 230*/230* duration at .050" at the valve on 114* LSA's. There is not much bottom end with these cams even though I have an automatic and 4.10's with a 3000rpm stall converter though the cams are advertised as having a power band of 1500-6000rpm.
Comp told me to go with their .550"/.550" and 234*/238* on 114* LSA's with +5*. They said that a 2400rpm stall was required. WHAT! If I ost my bottom end ith the SHM's why would larger cams give it back?
How about Hitech's stage 2's with .550"/.500" lift and 234*/230* duration at .050" on 111* LSA's-1*? Hitech says the 4.6's intake is the weak spot and thus they run more on that side? Anyone?
What cam specs would give similar low end to stock PI cams but rev to 6100rpm? THX!

GrayGhost01GT

Nice Write up man!!!!

2000GT4.6

The only cam that is going to give you a nearly identical TQ curve down low is going to be a stage 1 cam. However, I did not see really large losses in TQ when istalling my stage 2s, and that was even with a REALLY ****ty tune down low (like 15:1 a/f ratio). I am refering to VT cams, but anything with similar numbers should be about the same.

A stage 1 should rev to 6100 or really close, even on stock heads. My stage 2s were pulling hard at 6000 where I shut it down for stock shortblock saftey.