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Cam Basics

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So you’re looking at a set of cams for your DSM. Save that chill down your spine for after you drive your revised car. Because, if you’ve done your homework, you’ll get a huge smile on your face. But, if not? You’ll get on the phone for a refund. However, choosing a cam for your 4G63 is your responsibility and your fault.

Where do you begin?!! Well, lets start with why you want cams. More horsepower. What is more horsepower? More air flow. What does air have to do with horsepower? Remember this approximation:

1 lb/min = 10.5 Hp

A 400 crank horsepower engine is flowing around 38 pounds of air per minute.

What does this have to do with cams? What is a cam? The 4G63 has two cams, an intake cam and an exhaust cam. The camshafts are straight steel shafts with eccentric lobes. They are connected to the crankshaft with a toothed timing belt and rotating at one-half crank speed. Lifters (or cam followers) translate the rotary motion of the cams into an up-and-down motion that opens and closes the intake and exhaust valves. This entire assembly must function with high precision and high reliability.

The camshaft controls the valve opening and closing points by the shape and rotational location of the lobes. The intake cam controls the intake valves. The exhaust cam controls the exhaust valves. Stock cams and most aftermarket cams are ground to a precision well within one crankshaft degree, ensuring that the valves actuate exactly when intended. Valves open and close letting air and gas into the engine (intake valves) and exhaust gassed out (exhaust valves).

But enough of the blah, blah, blah. How do cams affect performance? How do I choose the right cam for my 4G63? We need to understand the characteristics of the typical cam to know how to choose one for our setup.

Lift
First lets look at lift. Lift is divided in two categories: cam lift and valve lift. Cam lift is the maximum distance the cam profile will raise the lifter above the base circle. The base circle is the part of the lobe profile that does not move the lifter up or down (opposite of the lobe profile). Lobe profile is the contour of the cam lobe that determines how far above the base circle a lifter is moved and for how long it is held off the base circle. Valve lift is the total cam lift multiplied by the rocker arm ratio. In the 4G63 engine the rocker arm ratio is 1.7:1. This is a mathematical way of determining how far the valve lifts from the head. The more the valve lifts from the head the more flow you get into and out of the cylinder. It’s all about flow here!!! But remember, the higher the lift the more critical it is to find the correct spring. One that does not bind because of excess lift or can tolerate the high lift over a long period of time and/or certain number of lifts per minute (rpms).

Let’s see what higher than stock lift does for a 1g stock head with no intake and a ¾” radius entry. Polk Performance did a flow test on the #3 intake runner:

1G head flow numbers and intake designs...long

Stock lift is about 0.375 inches. There is a 7% increase in flow going from stock lift to 0.45 lift. That’s around 14 horsepower on a stock DSM platform! This is just one aspect that can be tweaked for more performance. Add some other tweaks and you can see why cam upgrades make such a difference.

Duration
The next aspect is duration. Duration cannot be discussed without understanding volumetric efficiency. Volumetric efficiency (VE) is used to describe the amount of fuel/air in the cylinder in relation to regular atmospheric air. If the cylinder is filled with fuel/air at atmospheric pressure, then the engine is said to have 100% volumetric efficiency. On the other hand, super chargers and turbo chargers increase the pressure entering the cylinder, giving the engine a volumetric efficiency greater than 100%. However, if the cylinder is pulling in a vacuum, then the engine has less than 100% volumetric efficiency. Normally aspirated engines typically run anywhere between 80% and 100% VE. So now, when you read that a certain manifold and cam combination tested out to have a 95% VE, you will know that the higher the number, the more power the engine can produce.

What does this have to do with duration. Higher duration gives you more VE up top and takes VE away down low. So many say that changing duration changes your power band. But it also gives you more peak power if you raise your duration or your VE at high rpms where the engine displaces more volume per minute. For the sake of simplicity, we’ll disregard boost. If the 4G63 engine spins at 5,000 rpms then it has the potential of ingesting 10,000 liters per minute:

2.0L X 5,000rotations/minute = 10,000 liters per minute

This is at 100% VE, where 100% of the cylinder is filled with air or the pressure in the cylinder is at atmospheric pressure. At 7,000 rpms then this number goes up to 14,000 liters per minute. Guys who want power down low will get a cam combination that will yield good VE at 3,000 rpms. Where the engine has only the potential of 6,000 liters per minute. So lets look at a couple of scenerios

Cam A; considered a good street cam, lower duration, closer to HKS 264:

VE = 95% at 3,000 rpms; 95% X 6,000 liters per minute = 5,700 liters per minute airflow
VE = 70% at 7,000rpms; 70% X 14,000 liters per minute = 9,800 liters per minute airflow


Cam B; considered an aggressive cam, for the strip, higher duration, closer to HKS 272 or bigger:

VE = 70% at 3,000 rpms; 70% X 6,000 liters per minute = 4,200 liters per minute airflow
VE = 95% at 7,000 rpms; 95% X 14,000 liters per minute = 13,300 liters per minute airflow

Remember that the more airflow the more power we make. The higher duration Cam B above flows over 26% more air than the lower duration Cam A at peak. It’s not all about peak horsepower but you can get the picture here. We have small displacement engines. So we have to make up for that with boost and higher rpms… Show me a stock 5.0 mustang engine that will rev to +7,000 rpms. The above shows that if no other aspects of the cam are changed except the duration, then significant gains are obtained. So don’t be so afraid to get those higher duration cams.

That’s how duration helps. But, what is duration? Duration is the total angle in CRANKSHAFT degrees that a valve is open. To be useful, duration specifications must reference a lifter or valve lift. “Advertised duration” and “.050" duration” are the most popular durations used by camshaft manufacturers.

“.050" duration” is measured in CRANKSHAFT degrees from the point where the lifter rises .050" from the base circle on the opening side of the cam lobe to the point on the closing side of the lobe where the lifter drops to .050" from the base circle. The .050" duration is today’s most common standard for measuring duration.

The point of measurement for “advertised duration” can occur at any lift above the base circle. The lower the point of measurement for lift, the higher the duration angle. This specification is provided for people who think more is better. Advertised duration is a duration stated by a cam manufacturer as a reference to their camshaft only. Advertised duration has no lift specification point agreed to among cam manufacturers. The same manufacturer may use different lift points on different camshaft profiles to list advertised duration. There is no rhyme or reason to advertised duration among cam manufacturers.

An intake valve can be open when the cylinder has traveled the complete stroke and is actually moving up in the compression stroke. Wouldn’t this force the precious aircharge back out from where it came, losing airflow and horsepower? Yes . . . at certain rpms. Air is “elastic”. It can be compressed (boost) and stretched (vacuum). Air has mass and consequently momentum. At high rpms there is a lot of momentum. So, even though the piston is pushing up in the compression stroke, the air does not fully reverse. But, it compresses a little because its momentum is keeping each particle going towards the piston top as it slows. This causes the air particles to pack itself into the cylinder. At high rpms, the valve NEEDS to be open longer than the intake stroke because the piston travels so fast that it would get to the compression stroke before it could suck enough air into the chamber to be remotely efficient. You have to use the momentum of the air particles more at higher revs to get flow up top.

Ramp Rate
Ideally, you will want the valve to open to its full lift at the instant the duration calls for the valve to open, and close fully at the very end of the duration. But there is no valve spring created that can handle that kind of shock. You have to open the valve more gradual this gradual increase in lift until full lift is called ramp rate. The duration (opening and closing) controls the cylinder pressure capability. Ramp rate is a tuning tool to maintain the intake and exhaust relationships and maximize the cylinder pressure. Basically, the sooner you get the intake to a higher lift once you want it to open, the more air the cylinder can take in over whole open duration. More air = more power. However, depending on the flow capability of the port, with respect to different valve lifts, RPM, and crankshaft angle, a quick ramp may not be advantageous. It all comes down to filling and emptying the cylinder and maximizing cylinder pressure. The 1G head and intake manifold combo seams to respond well to a higher ramp rate. Forced Performance has done extensive testing with ramp rates. Go to their site to see the cam profile that they’ve concluded work best.

Remember, an accelerated ramp rate will probably require a spring upgrade if you plan on revving past the stock rev-limit. As of the completion of this article, I have had good success w/ the FP2Xs (high ramp rate and lift), Manley springs, and STOCK retainers with an 8500 rpm rev-limit. I have reason to believe that it would be safe to take these cams to 9,000 rpms comfortably w/ titanium (lightened) retainers. But I may go dual springs and rev to 10K. I’ll update as I try different combinations.

Cam Timing and Tuning
Some gains can be seen by adjusting cam timing or “degreeing”. Cam timing is usually measured at .050” lift. Remember, .050" timing is a reference point indicating where the intake and exhaust valves open and close. This is measured in CRANKSHAFT degrees at the point where the lifter rises .050" from the base circle on the opening side of the cam lobe, (valve opening reference), and the point on the closing side of the cam lobe where the lifter drops to .050" from the base circle, (valve closing reference). Note that these reference points are the same points that the .050" duration figures come from, only these are in reference to position of the piston in the cylinder near TDC or BDC.

Stock 1Gb 4G63T (manual trans) cam timing events:

INTAKE OPEN 21* BTC = before top dead center
INTAKE CLOSE 51* ATC = after top dead center
EXHAUST OPEN 57* BBC = before bottom dead center
EXHAUST CLOSE 15* ATC = after top dead center

You can “degree” to optimize your cam timing for maximum performance by advancing (turning clockwise) or retarding (turning counter clockwise) each cam. This manipulates your cam centerlines. A centerline is an imaginary line that goes from the center of the base circle through the point of maximum lift on a lobe. It is measured in crank degrees. When running stock 1g cams, I noted significant spool increases and seat-of-the-pants power increases from retarding the stock exhaust cam around 3.5 CAMSHAFT degrees (this translates into 7 crankshaft degrees). The difference between the centerlines of the intake cam and the exhaust cam is called lobe spread or separation (mistakenly called lobe center or lobe centerline). This is the number of CAMSHAFT degrees between the intake and exhaust lobe centerline of a given cylinder. This is the only specification given in CAMSHAFT degrees.

Retarding your exhaust cam or advancing your intake cam increases overlap. Overlap is the angle in CRANKSHAFT degrees when both intake and exhaust valves are open at the same time. This is at the end of the exhaust stroke and the beginning of the intake stroke. INCREASING the duration will usually increase the overlap, as well. Overlap is specified at a lifter or valve lift value. It is said that turbo setups don’t take well to increased overlap. Retarding my exhaust cam and getting significant gains proves to the contrary. Perhaps there is a point where a particularly high overlap would be still beneficial for an n/a application but not a turbo application. But, obviously, a little over lap is good for both.

Below is a general run down on how your engine should respond to advancing and retarding cam timing.

Advancing Intake and Exhaust: This shifts the powerband lower in the rev range. This advances overlap earlier but will not change overlap angles.

Retarding Intake and Exhaust: This will shift the powerband and VE higher in the rev range. This retards overlap later and but will not change overlap angles.

Advance Exhaust Only: This Increases the topend of the rev range, and reduces overlap angles.

Retard Exhaust only: By increasing overlap, this decreases lag significantly and can recover some "streetability" of setups with large turbos and aggresive cams. Doing this will greatly enhance spoolup.

Advance Intake only: By increasing overlap, this can decrease lag significantly, as well, recovering some "streetability" of setups with large turbos and aggresive cams. Doing this will greatly enhance spoolup.
 
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