Why do after markets headers give you more power?

It sounds like headers are a real upgrade option. What is the reason for this? Something about equal length headers?

factory headers are equal length, and any upgraded header needs to be too in order to comply with the factory warranty. i would only use unequal length headers on a car with cat/muffler bypass pipes (i.e. a race/track car). aftermarket headers that adverstise more HP are usually a little straighter and/or larger bore to reduce backpressure. however, a lot of times they produce a weird resonance at certain RPMs.

You also might notice a shift in the powerband because of decreased backpressure resulting in decreased torque down low, but a healthy gain on the top end.

why does a decrease in backpressure result in a decrease in torque lower in the power band? i've never seen an answer to that and i've always been curious. i would assume that a decrease in backpressure would automatically INCREASE power.

Headers work with pressure waves that are created when the exhaust valve is opened and the spent charge is ejected from the cylinder. The individual header tubes need to be long enough that the pressure wave doesn�t arrive at the collector (common connection point for each individual tube of a given cylinder bank) and cause another cylinder to have to overcome positive pressure to empty itself. Ideally, the pressure wave would pass the collector creating a low pressure zone that would aid the next cylinder in both ejecting its spent charge and filling itself with a fresh charge of fuel and air. Headers are tuned to work with a specific frequency and usually it is at the high end of the RPM range where time is not on your side for emptying and refilling the cylinder of spent and fresh charges. So at the lower end of the RPM range, the headers might be working against you. One last note. In order for headers to work, there needs to be significant duration of the exhaust valve opening and there definitely needs to be some valve overlap (when both the intake and exhaust valves are open at the same time) in order for headers to have something to work with. Otherwise, headers don�t have much to work with. I am not sure about 996s, but 993s don�t have enough overlap to make headers very effective. Lastly, intake manifolds work exactly the same way! Hank

Boy,......Hank covered that very well,.... Nicely done. All of the 996's I've seen lose power on the dyno everywhere except at the very top of the RPM range with the headers on the market. IMHO, its a waste of money. As Hank said, equal length headers are designed and made to enhance a specific (usually high) RPM band. Such engines require camshafts that have some duration and overlap to provide the necessary pulse tuning. All emissions engines use short duration, zero-overlap cams that simply negate any worthwhile gains from equal length headers.

wow thanks for the replies! one more thing i know about cars! woo hoo!

I can only say that the GHL on my 964 gave me a good increase in Torque and HP at anything above 2000rpms. (At approx 1950 rpms I had about 2lbft less than stock) I guess the 996 with with variocam etc. is less 'tuneable' in this area?

The factory headers on my C2 were UNEQUAL length. FActory headers are unequal length, mainly to pass noise emmissions. The unequal length headers cancel each tubes sound out to create a more pleasant drive by sound. With unequal length headers the volume of gas emmitted or "wasted" by an parrticular cylinder is not efficient, b/c with different length header tubes the motor can't "waste" all the exhaust since the volume of exhaust wasted by each cylinder are the same. It cant waste all of that exhaust - and I think that is why you get a little more power or response with equal length b/c all cylinders waste the same volume of gas but if there isnt enough of volume in a header tube to waste, I don;t think you get a 100% complete waste in the bottom stroke - you may still have some "old exhaust stuck in the cylinder on the upward stroke there for a poorer/less efficient of air and ful in the cylinder. Am I wrong?

Depends on your model year. Please note that "equal length" is a bit of a misnomer. On my 03 Boxster, while each tube is not equal in lenght, the outer tubes are tapered, thus "speeding up" the airflow, so that the exhaust gas pulses reach the pipe at the same time. Early 996's & Boxsters did not have this type header, but I don't recall the MY for the change. Of course, as previously mentioned here, every header strikes a compromise - do you want better low, mid, or high? Any product that claims to improve all must be viewed as seriously suspect. J