It appears from many early reports that good headers incrase RWHP by 20+. Why didn't GM equip the C6 with these from the factory?

I can understand why they did not use long tube headers since there is a big emphasis on placing the cats as close the the engine as possible to reduce the time necessary to warm the cats up to operating temperature. However, would this preclude the use of regular headers?

It seems GM really did spend money optimizing things like the rest of the exhaust (many have reported little or no gain from most cat back systems or high flow cats), so why stop there?

Also, some folks have said that X-pipes are more efficient and do show gains over the stock H-pipe. Again, why would GM not use the optimal pipe?

If these things really do continue to prove out as some have said, then I will certainly be getting headers and an X-pipe and whatever other bolt-ons seem to work. But I am curious why GM would not go all the way and supply these from the factory.

All insight appreciated.

Thanks,
Ira

 

I would think #1 would be money and #2 would be they got the HP they needed without them. I'm sure they had a 400HP goal and it was achieved without them.

 

Emmisions and cost.

 

The upcoming C6 Z06 does have factory headers. They look a little different from aftermarket headers (they're hydroformed), but they work the same way. It is one of the tricks they used to pull 500 hp out of the engine.

I think the reasons they didn't use them on the base C6 are 1) they didn't need them to hit their 400 hp target, 2) they're expensive, and 3) if they used all their tricks on the base car, there'd be nothing left to make the Z06 more powerful.

 

Ideal headers will have a narrow rpm range whose lenght will be determined by what rpm is needed for that performance. Using the transmission to keep you in the ideal range. The X pipe or what we used to call a collector cross over is used to reduce the Q of that circuit causing the peak to be reduced but spread out over a broader rpm range to be more usefull at street use. The stock system will give you a flatter curve but not ideal for racing, they're not selling cars only for the track. You might ask a trivia question asking, what physics principle does headers work by. You will get some interesting answers, most wrong.

 

in that order.

 

Your C6 does have headers, shorty's, I was suprised myself when I saw them.

 

Don't all cars have 'headers', just not ideal ones?
I thought headers were just the exhaust collector tubes up to the point where they join on one side.

 

Most cars just have exhaust manifolds, where all four collector tubes immediatly join into one, but the C6 actually has shorty headers stock.

 

Would the headers off of the Z06 LS7 fit on the LS2? If not, which aftermarket headers are the best on the C6? Does anyone have dyno #'s?

 

Id guess burnulli's (sp?), but would probably be wrong as you said.

 

An exhaust manifold is usually made of cast iron and is a big block of metal with passageways molded in for the gas to flow through.

A true header is typically made from a steel plate that bolts onto the side of the engine block with ports cut into it to match the exhaust ports on the engine. Then, individual steel tubes are welded onto the plate, one for each exhaust port and the tubes join together somewhere down their length depending on the design. Thus, headers are typically lighter than a cast iron manifold and allow for more careful planning of the route and lenght of each exhaust passage.

The one problem with headers in the past is that the welds connecting the tubes to the steel plate tended to crack and then leak. I assume this problem has been solved for the high quality and high cost headers available for the C6.

Ira

 

Headers work by connecting the high pressure blow down of the exhaust valve opening with the overlap area when both exhaust and intake valves are open.

When the exhaust valve opens there is still 70+ PSI left in the cylinders, this high pressure blows out of the chamber and down the header pipe. When this high pressure wave encounters the collector the size of the 'piping' changes to a larger size. This change in size, seen by the high pressure wave, causes a negative wave to travel back up the header. [A]

If this negative wave arrives while the exhaust valve is still open, and the intake valve is opening, the negative pressure wave can begin to pull fresh mixture into the cylinder even before the piston starts its downward movement. Since both valves are open, the fresh mixture can be drawn not just into the cylinder, but also into the header.

Meanwhile, back at the collector, the (somewhat reduced) high pressure wave has traveled accross the collector and encountered the end of the collector. This causes a high pressure wave to travel back accross the collector, up the header pipe. [B] If this wave encounters the exhaust valve still open, it can push the fresh mixture drawn across the top of the piston and into the exhaust back into the cylinder just before the exhaust valve closes. Causing a superchargine effect.

By capitalizing on both wave effects, volumetric efficiencies of up to 115% are possible on highly tuned engines.

[A] the length and tubing size of the header determine the amount of time it takes for the wave to leave the cylinder, travel down the header, and then travel back up the header.

[B] The size of the collector determins the strength of the negative pressure wave, while the length of the collector determine the amount of time between the return of the negative pressure wave and the subsequent high pressure wave.

The camshaft needs to be selected with the length of the header, and the size and length of the collector in mind to get the maximun benefit. The Lingenfelter book on modifying SBC engines indicates that making the collector longer improves the low end with little sacrifice at the top end.

 

I never knew there was any science to it other than better flow out the exhaust. Thanks for the education.

 

At least some of this stuff should apply towards college credit!

 

Very good, Wave Propagation would be the fundamental principle. The actual velocity is the sound wave speed, 1100 feet and change per second. A quarter wave is the correct distance with frequency being the Engine RPM’s and everything else you stated.

 

So, all other things being equal (tube length, collector size, etc) which tube diameter is best??

Some say 1 7/8" is too large for most efficient scavenging... Some have said this 1/8" increase over 1 3/4" is better suited to the now stock 400 horse engine in the C6 (essentially the 1 3/4" was suited to the stock C5)

Could the answer be dependant on where in the RPM range you want to gain the increase horsepower?

educate me

 

From what I have read, 1.625 is the idea size for 350 to 425 hp V8 whereas 1.750 may be an attempt at standardization or even more likely just a stock pipe size.

To MitchAlsup:

Again from what I have read, wave effects and volumetric efficiencies also depend on the tubes temperature retension abilities i.e. that thin walled tubes are not as effective as thick walled or wrapped tubes. Moreover, air being "flow" lazy, both from an intake and exhaust sense, that there discussion about the diameters of the whole exhaust system and that diameters should change at different points to encourage scavenging-what are your thoughts?

 

The larger the headers diameter, the less bottom-end you will have, and you will have more top-end(higher rpm's).

 

The speed of sound changes with temperature change. The speed of sound is 1128 fps at 70F. But at an exhaust gas temperature of 500F, the speed of sound is 1519 fps.

The formula is c = sqrt(k * R * T), where c is the speed of sound in meters/second, k is the adiabatic index (1.402 for air), R is the universal gas constant (287.05), and T is absolute temperature in Kelvins.

But things aren't quite that simple for headers because gas has inertia. It takes a while for it to come to speed, stop, and head the other way. These "end effects" mean that figuring pipe lengths based solely on the speed of sound doesn't work out exactly. Flow "impedance" due to tubing diamter also plays a role. And gas temperature varies along the length of the header too, so you have to allow for that. Without access to supercomputer hydrodynamic modeling code, you still have to cut and try on the dyno to see what works best.