cam questions

v8ranger · Post by **v8ranger** » Mon Jan 05, 2009 3:47 am

Ok, I have an E-303 cam in my truck now. I am looking to replace it with something a little more agresive. How does the F-303 compare to the E? Any suggestions on cams? I want more lop in the idle and more mid range power, but also want to stay under $200, would like to stay under $150 but will consider anything in that price range. I am running GT-40 heads and my pistons do have the valve releif in them so I dont know how much lift I can go.

cgrey8 · Post by **cgrey8** » Mon Jan 05, 2009 5:16 am

If you are upgrading from the E303, I don't recommend any of the FMS cams. Only under certain circumstances do I recommend the E303.

But to recommend a specific cam, I'd have to say the Comp Cam 270RFHR (mild upgrade from E303) or the Comp Cam XE276HR12 (all out upgrade). Either of these have faster ramp rates, higher lift, and the XE276HR12 has more duration than the E303. All of that spells more RPM capacity. Although I will warn that both of these cams are really meant to work with aftermarket heads. Getting a cam this large is technically "mismatching" your equipment. It doesn't mean you won't get gains at the top-end. It just means that your cam is efforting to a higher RPM than the heads are really meant to perform in. But either of these cams NEW is priced at $303 from Comp Cams. Although I would expect you could get them for cheaper elsewhere.

Crane Cams is often cheaper and for them, you got the 449601 which is a pretty BIG cam too. Lots of lift and lots of duration as compared to the E303. I'm fairly sure it would lope.

The main reason, I recommend the high lift cams even on stock heads is to get the valve open to its max flow ranges for longer. Even if a head quits flowing after the valve has lifted beyond a certain point, the benefit of continuing to lift beyond that point is you give more time at or above the max lift range. So at high RPMs, that's more time to flow to the max. It also gives you a good stick to work with if you upgrade the heads to some that CAN take advantage of the higher lift. But the same holds true for them. Although I don't generally recommend high duration cams. I'm not a big fan of those. But you were asking for a cam that's "bigger" than the E303 and a quick look resulted in these. Hope that helps...

v8ranger · Post by **v8ranger** » Mon Jan 05, 2009 6:40 am

I was thinking on the lines of something similer to the F or even X cam. I dont know much at all when it comes to cams durations and stuff. Its all greek to me. I do have GT-40 heads that have a mild port job. I am also running a air gap 4brl intake with Edlerbrock 650, large tube shorty headers and t-5 trans. I know if I go to high of a lift cam I have to change springs. I dont think mine are stock but no positive. They are a double springs.
I am looking at this one but I dont know if its for a roller motor
http://cgi.ebay.com/ebaymotors/ws/eBayI ... &viewitem=
Also looking at this one
http://cgi.ebay.com/ebaymotors/ws/eBayI ... &viewitem=
But dont know which one I should go with. Any suggestions?

87ranger · Post by **87ranger** » Mon Jan 05, 2009 6:57 am

comp cam xe274hr, or the summit trickflow stage two cam they are both nearly identical and will give you what you want

v8ranger · Post by **v8ranger** » Mon Jan 05, 2009 7:08 am

Wont that be to much lift?

cgrey8 · Post by **cgrey8** » Mon Jan 05, 2009 9:49 am

I don't know what springs you have to know what your max lift capacity is. But you do run the risk of over-lifting if you get too high of a "big lift" cam. The XE274HR is the one I was referring to above. I mis-typed XE276HR...going from memory again. And yes, that is a good cam for what you are looking for if it won't over-lift you to the point of spring bind.

But I also won't talk you out of that Crane Cam either. It's about as "hot" of a street legal cam as you can get. Nice, high lift, and not too excessive on the duration. In fact, the duration is a little less than the E303...but the high lift and faster ramp rate make up for any losses. But you likely won't get the "lope" you are looking for since it actually will have less duration than the E303. The intake duration and timing of the lobe is what gets the familiar lope sound. Unfortunately, it's also what kills low-RPM torque.

Dave · Post by **Dave** » Mon Jan 05, 2009 10:11 am

Chris, the other Comp Cam you listed, Comp Cam 270RFHR should have read RF-HR. The RF means its for retro-fit applications. Had to find a cam under $200 unless it's the F-303 or the Crane one but it says it's for use with the 1.7 rockers but gives the valve lift spec's with the 1.6.
Picking a cam right for the application, sure got to give up something somewhere, whether it going to be top end, low down.
Dave

cgrey8 · Post by **cgrey8** » Mon Jan 05, 2009 10:23 am

Very true. Cam selecting is all about what you want to get out of the engine and what you are willing to sacrifice.

v8ranger · Post by **v8ranger** » Mon Jan 05, 2009 12:32 pm

What I am looking for is some more mid range and some more lop. I dont usualy go over 4500 or 5000 RPM's and I dont race it. Its just a street truck.
Ok, let me ask this, What numbers is the cam specs give the "loppy idle" and is it the higher the number the more lop, or is it the lower the number the more lope. And what kind of lift should I stay around, 512,529 ect.... Low end torque and power is never a problem so to sacrafice a little in that area is ok. I want more pull in the mid rpm range.

Dave · Post by **Dave** » Mon Jan 05, 2009 1:12 pm

The one other thing to throw in the mix is the LSA (lobe Separation Angle).
Shoot me a PM or direct to me with your E-Mail, got some really good cam related info that wxplains what duration, lift and LSA do and some dyno test and things. Chris sure knows and understands it better than I do.
Dave - sitting here learning how to draw in 3-d

cgrey8 · Post by **cgrey8** » Mon Jan 05, 2009 1:30 pm

LOL, that's a really tricky question. But I'll try to answer it in very generic terms, then explain it. Lope is generally created with a cam with a large intake valve duration. Any cam over about 216Â°@.050" lift will generally get a little lopey at low RPMs...some worse than others. But that's not to say that a cam with 220Â° can't run smooth. It can if other specs of the cam are in play. But before that can make any sense at all, it's important to understand why a cam causes an engine to lope.

Lope is created by holding the intake valve open longer than it really should be for the RPM the engine is at. Thus what is happening is the piston will pull in an air charge, then start to head up to compress it but the intake valve is still open...it's close to being closed, but just not there yet. So some of the air charge gets shoved back up into the intake as the piston rises in the bore. When that happens, you kill some of the vacuum the next cylinder pulling air in is trying to create. In carbed engines, this is not ideal because the carb needs vacuum to pull the fuel into the air as the air passes by the pickups in the venturi. So for people with heavy overlaps or wide duration cams (i.e. "big" cams), they make dual plane intakes which literally has 2 plenums...1 pulls from 2 of the 4 barrels of a carb while the other pulls from the other 2 barrels. What dual plane intakes do is separate the cylinders so that sequentially intaking cylinders do not pull their air from the same plenum. That way when you get a big cam that has a tendency to shove a portion of cylinder air back into the intake at idle, it's not affecting the next cylinder's ability to pull air across the carb's pickups. This allows bigger cams to run lower RPMs so you aren't forced to idle at 1000 RPMs just to keep the engine running.

So why do cams hold the valve open like that? That's because at high RPMs, the inrush of air in the intake can make use of that extra open time on the valve. Air, like any other mass, generates inertia and momentum when it gets up some speed. What those cams are doing is harnessing the inertia of the air at high RPMs (when the inertia is at its highest) to let the inertia shove more air into the cylinder despite the fact that the piston is already pushing upward. At low RPMs, the air isn't moving fast enough AND the time the valve stays open is way too long for any shoving to be done. The air has too much time to stop and start moving backwards. But at high RPMs, there's not enough time for the air to "backup" so it's like having a tiny supercharger on each cylinder shoving a little more air into the cylinder that you wouldn't have gotten otherwise. The side-effect is the low RPM lope.

Now why this is such a complicated thing to answer is 2 things affect how long an intake valve stays open:

Duration
Lobe Center

You can have a small duration lobe, but have the lobe ground into the cam such that it is still holding the valve open longer and thus creating lope. So you can't go on the duration number alone. But generally speaking, duration dictates some of the lobe centerline in an effort to get the desired lobe separation (distance from intake lobe centerline to the exhaust lobe centerline in degrees).

BTW, you almost always compare cams via the .050" duration, not by the advertised duration. Although, cam mfgs generally report both so you can get a feel for what the ramp rate is. Ramp rate is how fast the cam is opening valves. Stock cams have very mild ramp rates. They lift the valves very gradually, and drop them closed just as slowly leaving very little time at the max open points. This allows weaker springs to be used without floating the valves as easily, which reduce wear. However one of the "features" of aftermarket cams is they dramatically increase the ramp rate. And with roller cams, it can be increased even further than was possible with flat tappets. With aftermarket cams, the cam lifts the valve very fast to get it open as far as possible, as quick as possible so it'll start flowing early on in the cycle. Then it holds it there as long as it can, then drops the valve as fast as it can. To do this, requires stronger springs or the valve won't close as fast as the cam lobe can fall out from under the lifter at high RPMs. This is why aftermarket cams float valves so easily with stock springs.

v8ranger · Post by **v8ranger** » Mon Jan 05, 2009 2:31 pm

cgrey8 you are a man with tons of info, where do you put it all lol. Ok so a cam with these specs below be good for me? will this give me good mid range and lope?

Advertised Duration 286 int./294 exh.
Advertised Exhaust Duration 294
Advertised Intake Duration 286
Basic Operating RPM Range 2,500-6,000
Cam Style Hydraulic roller tappet
Computer Controlled Compatible No
Duration at 050 inch Lift 224 int./232 exh.
Exhaust Duration at 050 inch Lift 232
Exhaust Valve Lash 0.000 in.
Exhaust Valve Lift with Factory Rocker Arm Ratio 0.563 in.
Grind Number HR-236/359-2S1-10
Intake Duration at 050 inch Lift 224
Intake Valve Lash 0.000 in.
Intake Valve Lift with Factory Rocker Arm Ratio 0.542 in.
Lobe Separation (degrees) 112
Valve Lift with Factory Rocker Arm Ratio 0.542 int./0.563 exh.

ALSO is it the larger the lobe separation the greater the lope? This cam is 112 so does that mean it has more lope in the idle than a 110 ect....???

cgrey8 · Post by **cgrey8** » Mon Jan 05, 2009 4:57 pm

When you start getting into this big of a cam is where the limits of my expertise end. I focus mainly on "streetable" cams. And to me, a streetable cam is a cam with less than 220Â° duration. There's always the exception, but in general if you need to pass emissions and you want well mannered characteristics from the engine, it really should be less than 220Â°.

As for the specs you've listed, the main thing I see problem with is the lift. Just make sure your springs can handle a .560"+ lift. Aftermarket heads can usually handle up to .600". But production cast irons, and yes standard GT40 and GT40p heads are considered a production head, can't get much past about .550" with aftermarket springs. If you don't know what your bind point is, then ASSUME it's on the conservative side...say .535" and don't get a cam that lifts higher than that.

As for the lobe separation, that has more to do with optimizing exhaust scavenging. Just as intake air has inertia, exhaust has the same characteristic. So what high RPM cam mfgs will do is hold the exhaust valve open beyond when the piston has completed the exhaust stroke so that the exiting exhaust gas that's built up an inertia in the exhaust actually creates a vacuum at the port and pulls the exhaust out of the cylinders at higher RPMs. This means, there are times when the intake and exhaust valves are open at the same time. Even stock cams technically have overlap, but in practice, a cam is said not to be an "overlap cam" until both the intake and exhaust lobes of the cam are lifting at or above .050" at the same time. The number of crank degrees that both are open at or above .050" is the degrees of overlap. Both lobe separation and lobe duration define how much overlap you have. Stock cams never have overlap@.050". The E303 is just barely not an overlap cam. If either lobe duration was increased by 2Â° or the lobe separation was decreased by 1Â°, then there'd be 1Â° of overlap where both lobes are lifting at or greater than .050".

In the event that you have an overlap cam, you have both intake and exhaust valves open enough to flow at the same time. With the intake and exhaust valve open at the same time, you potentially have the exhaust pulling intake air into the cylinder to ensure as much air/fuel is getting into the cylinder as possible to give the best possible power return even at the risk of sucking unburned air and fuel out the exhaust...not the most efficient design, but that's the trade-off you take for optimizing for max HP. The higher RPM you are aiming for, the more overlap and larger duration you want on the cam to maximize pumping air through the engine.

But just like with the intake stroke shoving air back into the intake manifold at lower RPMs, the engine sucks exhaust back into the cylinder when the piston drops down for the intake stroke because the exhaust valve is still open while the piston is dropping down creating vacuum. Excess exhaust in the intake air mix dampens the burn. In small amounts, exhaust air can actually be a benefit to both emissions and fuel economy. But when the amount of exhaust in the combustion air gets excessive, it becomes a deterrent preventing fuel molecules from "finding" oxygen molecules to burn with and reducing fuel economy and the quality of the burn in general. But the details of EGR are a topic all unto itself.

Point being, lopey idles generally aren't caused by overlap...at least I don't think they are. Although with an excessive overlap, you do get all that exhaust sucked back into the cylinder which probably does degrade the combustion, even at idle loads enough to be audible.

Now here's another tangent to this discussion that might be useful to your decision. Installing a higher ratio rocker is kin to installing a longer duration cam. See, cams are spec'd based on lobe lift (i.e. such-n-such duration at .050" lobe lift). But engines don't care about lobe lift. They care about valve lift. So what you can do is install 1.7 rockers and effectively increase the duration of the cam, increase the lift, and increase the ramp rate. Just to use an example, an E303 cam is spec'd as follows:
Duration@.050" 220Â°/220Â°
Valve Lift .498"/.498" assuming 1.6 ratio rockers

However if you install 1.7 ratio rockers with an E303, what you end up with is valve behavior that acts like a bigger cam with more duration and more lift. Said another way, the valve behavior would be the same as if you installed a cam with these specs:
Duration@.050" 224Â°/224Â°
Valve Lift .529"/.529" with 1.6 ratio rockers

The reason I bring this up is the E303 is a PERFECT cam to install 1.7 rockers on. It has a stock-like lift rate which is greatly improved by the 1.7 rockers and you get a significant increase in both lift and duration. I'm not generally a fan of the E303 as compared to aftermarket offerings, but if I was "forced" to run an E303, I would probably run it with 1.7 rockers.

v8ranger · Post by **v8ranger** » Mon Jan 05, 2009 6:27 pm

cgrey8 once again you are a man of wisdom. Another cam to consider, how does the F-303 sound? would this be a better cam then the E? Or should I just get the 1.7 rockers? Do they make a 1.7 that go onto my heads or do I have to get studs?

Dave · Post by **Dave** » Mon Jan 05, 2009 7:22 pm

Yes they make 1.7 rockers for your heads, think they are pedistal mounted rockers. You'd end up with a lift of .529 that should not give you any problems with valve to pistion clearance - but always best to check. Cost of the rockers is about $300 (check Summit or Jegs). Good news is that you would not have to put the whole front end of the motor off to change cam. That's something else to think about in the total price of a cam, all the new gaskets and time. If the motor is in the truck, that means a lot more work to swap a cam.
Dave - What should we call you?
Thru in a cam/head dyno test to look at also, it does include the GT-40 head

http://airflowresearch.com:80/articles/ ... 2/A-P5.htm