LET'S TALK ABOUT CAMS!!!
First things first! If we understand that it is actually CYLINDER PRESSURE that turns the crankshaft and that the engine ONLY makes horsepower and torque when the valves are closed then we are well on our way to building an engine that will perform up to expectation!
Can a camshaft make or break a combination? ABSOLUTELY! Should you build your engine around a given camshaft? NO. Because you will be forced to reconfigure the WHOLE VEHICLE if you want it to perform properly.
Less is more. In a typical street environment where the engine must transition from idle to part throttle load and wide open throttle, we must recognize these varying throttle inputs differ from a wide open throttle, high rpm scenerio of drag racing where idle quality and vacuum are rarely considered.
I liken street performance moreso to road course racing than drag racing, where a load must be accelerated out of a corner from a low or medium rpm to the top of the range. Being able to accelerate that load as quickly as possible is the goal. When we are dealing with heavy vehicles with a very limited number of forward gears, WE NEED A BROAD TORQUE CURVE. Drag racing, on the other hand, requires a relatively high and narrow powerband so that maximum power is realized for the entire quarter mile. They can simply gear the car accordingly.
It used to be typical for a high performance engine to need a high stall convertor for optimum performance. Not as much these days due to valve spring technology and metallurgical advancements. Camshafts may now be ground with much more aggressive lobes since we now have the valve springs to control them! For low rpm applications, THAT IS HUGE!
That means we don't need very high compression ratios to make good power as the seat duration may now be decreased for a given .050" duration. That is important because now we don't have to push the limits of compression and possibly incur detonation. Now, if we want to make some real good power, we look at .200" duration numbers as well. To optimize those numbers, we generally look to roller camshafts over a flat tappet. That's the major advantage of a roller camshaft, we can achieve much more lift for the amount of duration needed to meet a performance goal. Camshaft designers call that the "area under the curve".
THAT MEANS MORE POWER EVERYWHERE!!!
Camshaft theory tells us:
More camshaft duration moves the peak torque up the rpm scale at the cost of low rpm torque. Picture a teeter totter where peak torque is not increased, just skewed about a certain pivot point.
More lift increases torque and power throughout the rpm range and also WIDENS the useable rpm range
Lobe Separation Angle (LSA) describes the placement of the lobes in relation to each other, eg.; an intake centerline (I/C) of 114 After Top Dead Center (ATDC) and an exhaust centerline (E/C) of 114 Before Top Dead Center (BTDC) average out to a 114 LSA. If the cam is installed where the I/C number is lower so that it is a 110 ATDC then the exhaust is now 118 BTDC and is considered to be 4 degrees advanced yet it still averages out to a 114 LSA. A wider LSA is generally considered to broaden the power curve while reducing PEAK torque. But, LSA is merely the placement of the lobes in relation to EACH OTHER. So what is the real player here in my opinion....?
Overlap is the amount of time in degrees that BOTH valves are open simultaneously. That can be useful for optimizing cylinder filling at a specific rpm while, generally, narrowing the useable rpm range. It will be useful to remember it is OVERLAP, not LSA in and of itself, that has the real influence on scavenging and the volumetric efficiency curve of the engine because LSA AND DURATION actually determine the amount of overlap created. Therefore, a tight LSA cam may not actually have a noticeably narrow powerband IF THERE ISN'T MUCH OVERLAP CREATED. For example, we have used 110 degree LSA cams and achieved a 2,000 RPM SPAN BETWEEN TORQUE AND HORSEPOWER PEAKS! YOU DON'T SEE THAT JUST ANYWHERE! Look at various Pontiac dyno tests, the majority show peaks within 1,500 rpm of each other, some only 1,000 rpm! So how did we make such a wide power curve? Duration was not the only camshaft parameter looked at.
Long cam/old technology camshaft in a traditional pontiac V-8
If you are considering a long duration cam for your mild street/strip type build such as a Ram Air IV, consider this: Too long of a cam opens the intake valve very early before the piston reaches top dead center on the exhaust stroke, decreasing vacuum and velocity in a positive direction through the carburetor venturi. THAT is where you lose low rpm response, because fuel is being delivered REGARDLESS OF THE DIRECTION OF THE AIR PASSING THROUGH THE VENTURI (REVERSION). IT IS AIRFLOW THROUGH THE VENTURI THAT LOWERS THE RELATIVE PRESSURE BETWEEN IT AND THE FLOAT BOWL CAUSING FUEL MOVEMENT. Once engine rpm is raised to the point that positive flow through the venturi overwhelms negative flow pulses (reversion), then the engine "cleans up" and you are running within that cams operating range. This is where a high stall torque convertor in the 3,500 rpm range will help, as will an advanced ignition base timing (at least 16) with an aggressive centrifugal advance curve (all in by about 2,800). The convertor will "unload" the engine at low speeds and the ignition curve will bolster combustion efficiency since we have poor atomization brought on by the 87 degrees of overlap and relatively low engine vacuum. If the fuel octane won't support your ignition advance curve... ooh... sorry.
Running the proper camshaft in its operating range will increase cylinder-fill and TORQUE at the SHIFT RECOVERY POINT will improve. That is where the engine rpm will drop to upon the upshift to the next gear. If you shift at 5,500 rpm, your shift recovery point will probably be around 3,500 rpm. If that is the case, you will need to be making some steam @ 3,500! Either that, or use a torque convertor that stalls above that number so you will get some torque multiplication. Is it a crutch? On a 400 CID 400hp street engine--- yes. On something around 550hp--- no, not necessarily.
If we take another look at a more aggressive aftermarket camshaft, for example, the X/E 274 from Comp Cams. That cam has 230/236 @ .050" lift, .530" valve lift with a 1.65 rocker and a 110 LSA. Comp says that cam's operating range is 1,800-6,000 rpm. That is EXACTLY where we ran into valve float with that cam! If yours is not delivering the rpm or power you think it should, you probably have a mechanical PROBLEM--- LOOK AT YOUR VALVE SPRINGS! Set your seat pressure at 130# and the open pressure to at least 320# and you should be just fine. Anything you can do to lighten the valvetrain will help stability.
We can say, that cam is capable of delivering over 550hp with well ported Edelbrock heads in a long stroke combination.
Where the RA IV cam needs at least 11:1 to perform well, you only need about 9.5:1 for a 274 X/E.
Don't believe the X/E cam bashing on some of the forums, there is an agenda involved. You WILL get more driveability and performance out of a more aggressive cam lobe. FACT.
Here are a few thoughts on camshafts:
Theory dictates you do everything to maintain port velocity on a low rpm engine. A low rpm engine needs all the airspeed it can get as it must be able to fill the cylinders effectively but a well-sized induction tract can be undone by the improper camshaft selection. It has been found that an airspeed of 240-260fpm in the induction tract DIRECTLY RELATES to peak torque. A smaller AREA port for a given cylinder displacement dictates a LOWER rpm in which that port speed is realized and generally, the horsepower rpm as well.... GENERALLY. But that can be manipulated with the camshaft. Too much cam, and the torque and power peaks will to very close together if the headwork, cam timing, or some other variable is not up to par... and that seems to happen quite a bit apparently.
This will be an eye opening bit of info... find an online calculator and input your rear end gear ratio and your estimated mile per hour and see what your rpm will be. A reasonable guess would be 104 miles per hour for a close to stock combination.
Another thing to think about; Why do people say you should advance the cam in a Pontiac engine? The answer: to build low rpm CYLINDER PRESSURE!
Variable duration lifters. Those lifters are used to tame a cam that is too long. Look out for advisors that tell you to purchase a cam that is too long and utilize that type of lifter BEFORE you buy the cam! I have spoken to a number of individuals that find the low rpm power very lacking... and then, around 2,500-3,000 rpm comes on like gang busters. That hole in the powerband makes the car SLOW.
You can always tell a medium to low compression ratio combination using a wide LSA.... They have a dull "thud" sound in the exhaust.
What if you want to build a LOT of torque, something along the lines of 1.3-1.4 lb/ft per cid?
By all means, put a big cam in it with a LOT of lift! Run a tight LSA and.... ADVANCE THE CAM. Be sure and check piston to valve clearance as it WILL BE TIGHT! In this case, we are looking to build a lot of cylinder pressure in the low rpm range by virtue of its early closing intake valve. Low rpm, high load, early opening intake with a poorly atomized charge and an inefficient combustion chamber all conspire to create a situation where detonation/pre-ignition can rear its ugly head. Even when running a low rpm torque combination such as this, a high stall convertor is DEFINITELY of benefit.
This combination, while popular with guys that are passionate about their Pontiacs, is a NARROW power band combination and, in general, won't compete with the many late model musclecars out there now.
SO HOW DO YOU COMPETE WITH THE LATE MODEL MUSCLE?
It depends. How many forward gears are we going to run? Three? If it is a typical TH-350 or 400, then we need to build a lot of torque over a VERY wide range... NOT PEAK torque like above, but a solid 500 lb/ft over the entire rpm range. Not a powerband terminating at 5,000 or 5,500, but WELL OVER 6,000 rpm.
To make that kind of torque, the simplest way is with cubic inches... a 4", 4.21" or 4.25" stroke combination will suffice and 2 bolt studded mains will hold everything together just fine. The displacement will fall in the 434- 468" range. If you are looking for MORE torque over a very wide range.... increase displacement via a 4.5" stroke (4 bolt mains are advised)--- but the heads MUST be able to keep up with the cylinder demand!!!
Cylinder head selection may be controversial if you are a die-hard Poncho fanatic. I like to keep it simple and get the best heads for the job. When you do that, it makes EVERYTHING EASIER. Fully ported Edelbrock cylinder heads flowing in the 320 cfm range. While we know that flow isn't everything, we also know that on combinations close to a stock TYPE build, flow increases DIRECTLY relate to increases in performance. Is the port AREA too big? No. Not since WE will be camming it for this combination.
Intake manifold selection can be either an Edelbrock Performer RPM or Torker 2, I prefer the RPM because it allows for larger runners. But there are clearance concerns where the T-2 will fit under a Shaker hood while an RPM won't.
We have some HYDRAULIC ROLLER options.
If we are keeping things mild, a Stage 2 "Warrior" cam will deliver ALL KINDS OF TORQUE... right off idle! It sports a 224/236 deg. @ .050" duration, .580" lift on a 112 LSA. You don't want to advance a small cam if you want it to rpm.
If you still want that "IDLE", then go with a Stage 3 "Aggressor", named after its aggressive idle quality... VERY NICE! 230/242 @ .050" with .580" lift on a 108 LSA for more peak torque. Mild duration numbers assure a wide powerband. Again, install it straight up.
If you are running around 11:1 compression with an aluminum head and are looking for a VERY wide powerband... go Stage 4, "Pro-Touring" camshaft with 245/248 @ .050", .540" lift with 1.5 rockers... .594" with 1.65's on a 112 LSA.
What if you have a lighter car like a 1st GEN Firebird or are running a 5- or 6-speed overdrive transmission? Then we can run a smaller engine and/or tighten up the powerband.
Assuming a smaller engine like a 400 or 428:
If you want a mild combination, run our Stage 1 "Daily Driver" hydraulic roller with 224/230 @ .050", .567" lift on a 108 LSA... Again, no cam advance. The above camshaft suggestions apply for slightly wilder combinations.
The increased torque multiplication as provided by the overdrive transmission will allow for less torque at the crankshaft.
We must speak of compression ratios if we are going to talk about camshafts; the two are interrelated because together, along with volumetric efficiency, they dictate dynamic compression. Generally speaking, our mildest cams will work well with 8:1 compression 455's and strokers and 9:1 with 400's. Our Stage 3 cams will work safely up to about 9-9.5:1, our Stages 4 and 5 cams will like 9.5-10:1 compression. These estimations are for cast iron headed combinations. Aluminum headed combinations will want to be between 10 and 11:1. Ultimately, unless we are building your engine, we cannot advocate the use of compression ratios that run the risk of detonation as that would be irresponsible. Cranking compression tests CAN give an idea of the engine's ability to run pump gas safely; we like to stay beneath 180 psi for 93 octane... 160 psi for 91 octane. Again, these are estimations, out west, we hear the gas is inadequate to say the least... so back those numbers down to 160psi for 93 octane and 140 psi for 91.
Ignition timing for higher compression ratios can utilize slower curves and lower base timing.
If you want truly BAD___ cam, check out our line of camshafts! They exhibit high lift and come with varying LSA's for a milder or WILDER idle yet keep the duration in check so tuning is not a problem with a stock type carburetor. Once one begins to go up in duration, then some carburetor mods MAY become necessary. Talk to the guys at SMI for a well-built carburetor http://www.smicarburetor.com/ ... they won't steer you wrong.
More on cams later...