X51 oil pan and 997S X51 headers installed
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I noticed the car feels like it has more go in 2nd or 3rd at 4k rpm which I thought was torque. Could be my imagination though.
I guess I'll really know once I get to the track and get a chance to see if my speed down the back straight has increased or decreased.
chsu74 - I'll ask my shop about that. Thanks for the info.
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Here's a side by side shot,--but recall, those were taken off a 3.8 M97 engine.
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The stock headers are truly abysmal, I don't get why they did it like that--unless it was an easy way to get a bigger power gap out of the higher end models.
Anyway, freer flow will net you higher hp, but it will be at higher rpm. Low down in the rpm range you'll lose some torque, but really not enough to be noticeably detrimental just swapping out the headers. Besides, great excuse to spend more time above 5000rpm.
I wish I knew the specifics of why all that is.
Anyway, freer flow will net you higher hp, but it will be at higher rpm. Low down in the rpm range you'll lose some torque, but really not enough to be noticeably detrimental just swapping out the headers. Besides, great excuse to spend more time above 5000rpm.
I wish I knew the specifics of why all that is.
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It's all about exhaust velocity and scavenging. Mike Kojima, who is very well-known in Honda/Acura and Nissan communities as well as NASA racing, explains this in his book. Google doesn't allow copy'n'paste of text (for obvious reasons), but I found another source where he pretty much states the same thing.
And if you doubt his credibility, Mike Kojima has been around for many many years, particularly in the 300ZX TT, Sentra SE-R, and Honda/Acura communities. He's an engineer at Nissan to boot.
The point is to not blindly increase exhaust pipe diameter, but selecting the correct diameter that lowers backpressure w/o sacrificing exhaust velocity. So depending on how bad the stock exhaust is, it is possible to have your cake and eat it too, that is go w/ a larger diameter exhaust pipe to lower backpressure, but still maintain exhaust velocity.
///Michael
And if you doubt his credibility, Mike Kojima has been around for many many years, particularly in the 300ZX TT, Sentra SE-R, and Honda/Acura communities. He's an engineer at Nissan to boot.
To save costs, your typical stock exhaust uses small diameter, crush bent pipe. Crush bends are easy to make in mass production. However, crush bends can reduce the flow of a pipe by up to 50%. Your typical exhaust system made by the local neighborhood muffler shop is also crush bent. The best exhaust systems, like most Japanese pre-made exhaust systems, come with mandrel bends. Mandrel bending is done by a special machine that uses a non-crushable insert or mandrel that goes into the pipe while bending to prevent it from becoming crushed.
Some self-proclaimed motor gurus state that you should not run too large of an exhaust tube on your car because engines need a certain amount of backpressure to run correctly. Although the statement about not running too large of a tube is correct, the assumption about engines needing backpressure is not. Remember this, it is one of the most common performance misconceptions out there. Read on and you will be able to argue with and break off any self-proclaimed expert on the subject!
You need to have the lowest backpressure possible to produce the maximum power by keeping pumping losses low. Too big of an exhaust pipe causes power loss, especially low-end torque. This is because a big pipe has less exhaust stream velocity than a smaller pipe. Velocity is essential to get the best scavenging (or sucking) effect from tuned headers, which we will discuss in more detail later in a future installment. To simplify things, if the exhaust gas flow is kept high with good velocity, a vacuum can develop behind the closed exhaust valve allowing even better scavenging when the exhaust valve opens on the next exhaust cycle. Good scavenging is even more critical on valve overlap, the part of the 4-stroke cycle where both the intake and exhaust valves are open, especially with longer duration performance camshafts.
If the exhaust pipe is too large, the flow will be sluggish with low velocity and the scavenging will not be good. Remember that a good exhaust has low backpressure and high velocity. The only possible exceptions to this rule are for turbocharged or nitrous motors. It is almost impossible to put too big of an exhaust past a turbocharger as a turbo depends a lot on the pressure differential across its turbine to get power recovery efficiency. A turbo engine can have an exhaust gas volume about 1.5-2 times more than an equivalent displacement naturally aspirated motor. NOS motors also have a pretty high exhaust volume and require a bigger exhaust if they are to be optimized for NOS operation.
The stock exhaust system on most modern rice rockets is often so well designed nowadays that just switching to a high performance cat back exhaust often does not allow for huge gains of power. With the exception of turbo cars, you can usually expect only about a 2-10 hp at the wheels difference on a stock car. The power gain will usually be the greatest from just below the torque peak to the tach�s redline. On cars that are modified with headers, intakes, cams, headwork, etc., you can expect bigger gains with the addition of a cat back exhaust due to the higher flow volume that these mods produce.
As turbo cars are very sensitive to back pressure, you can expect much larger gains with them, especially if the boost is turned up. A free flowing exhaust usually allows the turbo to spool faster also. A turbo car usually gains from 8-30 hp at the wheels depending on how bad the factory exhaust was and how high the boost is turned up over stock.
Some self-proclaimed motor gurus state that you should not run too large of an exhaust tube on your car because engines need a certain amount of backpressure to run correctly. Although the statement about not running too large of a tube is correct, the assumption about engines needing backpressure is not. Remember this, it is one of the most common performance misconceptions out there. Read on and you will be able to argue with and break off any self-proclaimed expert on the subject!
You need to have the lowest backpressure possible to produce the maximum power by keeping pumping losses low. Too big of an exhaust pipe causes power loss, especially low-end torque. This is because a big pipe has less exhaust stream velocity than a smaller pipe. Velocity is essential to get the best scavenging (or sucking) effect from tuned headers, which we will discuss in more detail later in a future installment. To simplify things, if the exhaust gas flow is kept high with good velocity, a vacuum can develop behind the closed exhaust valve allowing even better scavenging when the exhaust valve opens on the next exhaust cycle. Good scavenging is even more critical on valve overlap, the part of the 4-stroke cycle where both the intake and exhaust valves are open, especially with longer duration performance camshafts.
If the exhaust pipe is too large, the flow will be sluggish with low velocity and the scavenging will not be good. Remember that a good exhaust has low backpressure and high velocity. The only possible exceptions to this rule are for turbocharged or nitrous motors. It is almost impossible to put too big of an exhaust past a turbocharger as a turbo depends a lot on the pressure differential across its turbine to get power recovery efficiency. A turbo engine can have an exhaust gas volume about 1.5-2 times more than an equivalent displacement naturally aspirated motor. NOS motors also have a pretty high exhaust volume and require a bigger exhaust if they are to be optimized for NOS operation.
The stock exhaust system on most modern rice rockets is often so well designed nowadays that just switching to a high performance cat back exhaust often does not allow for huge gains of power. With the exception of turbo cars, you can usually expect only about a 2-10 hp at the wheels difference on a stock car. The power gain will usually be the greatest from just below the torque peak to the tach�s redline. On cars that are modified with headers, intakes, cams, headwork, etc., you can expect bigger gains with the addition of a cat back exhaust due to the higher flow volume that these mods produce.
As turbo cars are very sensitive to back pressure, you can expect much larger gains with them, especially if the boost is turned up. A free flowing exhaust usually allows the turbo to spool faster also. A turbo car usually gains from 8-30 hp at the wheels depending on how bad the factory exhaust was and how high the boost is turned up over stock.
///Michael
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I believe the M97 3.6 headers are quite nice (essentially M96 engine part). I have no idea what they were thinking when they welded together the M97 3.8 headers!
The X51 headers must be working as my service tech always had a smile on his face when he returns from a test drive.
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Here the photos of the stock 3.6l 996 hearers
![](http://i148.photobucket.com/albums/s34/jumper5836/Porsche%20996%20C4S/mods/996headers1.jpg)
![](http://i148.photobucket.com/albums/s34/jumper5836/Porsche%20996%20C4S/mods/996headers2.jpg)
vs the 997 x51 (but hard to compare the difference but x51 headers have are larger)
![](http://i148.photobucket.com/albums/s34/jumper5836/Porsche%20996%20C4S/mods/996headers1.jpg)
![](http://i148.photobucket.com/albums/s34/jumper5836/Porsche%20996%20C4S/mods/996headers2.jpg)
vs the 997 x51 (but hard to compare the difference but x51 headers have are larger)
![](http://i148.photobucket.com/albums/s34/jumper5836/Porsche%20996%20C4S/mods/997x51headers1.jpg)
Last edited by jumper5836; 03-28-2009 at 03:20 PM.
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At least the stock 996 headers are a design that is similar to the X-51 headers. The stock 997 headers look like something you would find on a 6 cylinder Honda!
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