I want to get your guys' opinion on Desktop Dyno...particularly, the Cam Iterator program. I had it choose the best cam for HP for my current application (stock 350 with TPI and L98 heads w/ full exhaust)

Well, here are the specs of my stock roller cam:

Duration @ .050 = 202int/207ex
Lift @ .050 = 413int/428ex
(-24.5 degree overlap; peak hp @ ~4,300rpm)

After running the quick iterator for best HP, it gave me a cam with these specs for a whopping 72HP gain:

Duration @.050 = 222int/227ex
Lift @ .050 = .413int/428ex
(1.5 degree overlap; peak hp @ ~ 5,000rpm)


Now, if you noticed, all it did was increase my duration, it gave me a cam with the same lift numbers. Is this NEARLY at all possible? I mean, that sounds all too good to be true. I can pick up a worn out roller cam at a swap meet and ship it out to get that custom grind all for under 300$, but would I be right to actually expect those performance gains, or anything close? Have any of you guys used this program to help you decide on which cam to get, and if so, was it accurate?


Thanks guys
_Clark

 

First let's get a defintion of duration. Duration is the amount of time, expressed in crankshaft degrees, that the valve is open. I'm assumuning that the cam D.D. picked left the lift numbers the same but changed the lobe seperation angle and intake centerline. The new lobe seperation angle will change the amount of valve overlap, which will also adjust the power curve within the power range dictated by the cam's duration. A widened lobe seperation angle will improve the idle quality, but hurt the low and mid range power. Tightening it will cause a drop in vacuum, which will negatively effect the idle quality, producing the slight lope we all love so much. In a cam with longer duration, it's possible that a wider lobe seperation angle will create a wider power band with more top end power.

Now that we have established some basic fundamentals, we can focus on your question about the power gains with the increased duration. Short duration cams make good torque since they actuate the valves over a short period of time when the engine speed is slow and there's plenty of time to fill the cylinder. Unfortunately, the shorter duration cam won't have enough time to fill the cylinder at higher engine speeds where there is less time to do the job. In other words, the higher the rpm's, the less time there is to fill the cylinder due to the lack of enough overlap. The longer duration cam has more overlap, and therefore has more time to fill the cylinders at higher rpm's. The longer duration also closes the intake valve later, allowing the piston to push a portion of the intake charge past the intake valve at low engine speeds. This, along with exhaust gas diluition of the incoming fresh charge is what gives the cam its choppy engine idle quality.

In short, for the reasons above, and a few more, yes, changing the duration of the cam could produce the extra 72 hp if the heads and intake could move the air efficiently enough.