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Discussion Starter · #1 ·
Have you ever noticed that GTs' exhaust pipes hang low? I've always thought it unsightly but could only think of one solution.

Well, have you seen the McLaren Mercedes SLR?

They exit the exhaust between the rear of the front wheels and the leading edge of the doors.

Would this arrangement be possible on a GT?

I'm talking with all the legal bits for street use such as a muffler. Is a resonator really necessary? What does it actually do anyways? From the name, it seems like it affects the final sound of the exhaust.

Is there enough room to route the exhaust out the passenger side just in front of the door?

Will there be too much heat?

Imagine the possibilities when the exhaust pipe is eliminated! A totally flat, aerodynamic floor could be made with a diffuser for generating down force!
 

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the most common reason for exhaust pipes to hang low is because people use too larg of a muffler. anything over 11" is too large, it will hang low begause there isnt anywhere to put it and still roll over the rear axle. DynoMax and Thrush both offer 11" mufflers that fit quite well. also if you do the install yourself without a bender it will hang low. I had a shop put my system in front to back and htey put many bends in it so it would follow the contures of the under body.
 

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Discussion Starter · #3 ·
Thanks Chris,

I'm still intrigued whether it would be doable to exit the exhaust ahead of the passenger door. I'd love to build some under-body flat panels and diffuser.

Manny
 

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I think its pretty doable. You dont need the resonator, and you can go with a smaller 'race' style muffler (such as flowmaster's round racing series). Several potential pitfalls, your going to be cutting out a lot of backpressure, that wont help the low end torque (but it will love the revs). The exhuast will be much louder, also dont place the tips too near the car, leave an airgap otherwise you can damage your paint from the exhaust heat. I'd defintily wrap the exhuast with heat wrap or a coating like rallybob used in his turbo cih thread. Good luck to you.
 

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Exhausting Business!

The last thing wanted in any exhaust system is 'back pressure' - it is usually only introduced to damp out other flow or noise problems due to inappropriately designed systems.

The standard cast iron exhaust manifold is a pretty well designed piece of kit with the "sprint" manifold being even better. Later Euro models have larger passages in both standard and fuel injection types and are the 'next step' up in efficiency. Tri-y and four into one collector headers produce more peak torque and power if correctly designed but it is often peakier and not over as wide a rpm spread as the best stock manifolds set ups.

My thoughts are a stock or sprint manifold with 1 5/8" to 2" dual secondary pipes 38" to 52" long before joining into a 2 1/4" to 2 1/2" pipe to a hi-efficiency, oval "Turbo" type muffler then on back over the axle at the same size to an enlarged version of the resonator with 1 5/8" to 2" dual outlets in the stock position with provision for perforated tubular inserts if further noise damping is required to meet legal requirements. A bit of dyno time would be needed to fine tune actual sizes and lengths for each individual application.

It is important to keep heat away from the fuel tank area and aluminised insulation under the tank inside is probably not silly! There is so little area under the car that a muffler plus resonator ( or dual "muffler"!) system helps tame the noise. Tip inserts like those on VW exhaust tips can save an other wise too noisy system.

I am in the middle of designing a system of this type in self-built stainless steel so watch this space and feel free to put forward constructive criticism.
 

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GTJIM said:
The last thing wanted in any exhaust system is 'back pressure' - it is usually only introduced to damp out other flow or noise problems due to inappropriately designed systems.
Actually, the way I understand it, a measured amount of exhaust back pressure is desirable for lower torque characteristics, due to some amount of "scavenging" that occurs. Since most day to day driving is done at lower rpm, "torque" is more appreciated for the feeling it provides than high-rpm horsepower by most taste tests. That said, there's no substitute for the high pitched scream and rush of an engine "on-cam". But too big a tail pipe will actually result in LESS performance for most circumstances. IMHO. And I am also quoting RallyBob, so that makes two of us.
 

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Back Pressure

When discussing exhaust system performance, one term is almost always used INCORRECTLY, and that is "back pressure". It is as incorrectly used as the term "weight" when actually discussing "Mass" and "foot-pounds" of engine torque when it is actually "Pound-feet". Instead of thinking back pressure, one should be think exhaust velocity. The larger the exhuast pipe the lower the velocity of the exhaust gasses. If the velocity drops too low there is a significant reduction in the effects of scavening. The target velocity is 300 feet/sec. for best scavenging and lowest pumping losses.

Taken at face value, increased exhaust back pressure actually results in more HP consumed by pumping losses trying to push out the exhaust gasses, not a good thing. All this info is in Smokey Yunick's Power Secrets... Pages 102 - 105.

So what the discussion should yield is the point that the exhaust system has to have a pipe size that results in exhaust velocity around 300 FPS within the desired RPM range. If the desired effect is for low rpm torque, that means a smaller diameter exhaust pipe, because as the area of the pipe gets smaller, the velocity goes up.

Sorry to drag out the soap box.
Paul
 

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Here's a trick I saw being used, and was discussed many times, many long years ago on drag racing engines. This is also the basic system used on equal length headers. This was shown to me my Bob Bubenick (sp) who was a friend of the family and the engine builder for Mickey Thompson's Indy race cars. Bob was building an engine for a record attempt in a limited racing hydroplane boat of the 266 cu.in size. He had a 265 Chevy V-8 that he had totally redesigned and rebuilt. I happened to be in the shop where he had the engine on his dyno and was final tuning for max power and RPM, requirements for boat racing. He had the headers mounted on the engine pointing straight out from the head, each header pipe was six feet long and had a white stripe painted on each one the length of the pipe. He ran the engine at top RPM and stabilized it there for about 5 minutes, then shut it down. We looked at each pipe and could see where the exhaust pulses had met, going out and coming back, because of valve overlap. At that point the paint was burnt, so Bob took a hacksaw and cut each pipe at the beginning of the burnt area closest to the head. I asked him why he did that, I was just a young teen-ager beginning to learn about engine mechanics, his reply was, the exhaust was being pushed out of the pipe up to the point of the burned area and then the previous pulse was being pulled back toward the engine. The idea was to have the engine push the exhaust gasses out of the pipe into the air and not have the engine work to pull it back in. The result was a tuned exhaust for that engine at that RPM. Header MFGs use the same technique for their headers based on the average RPM the engine will see most during it's life cycle, and that's where the collectors are attached, so the low pressure area of the collector will extract the exhaust from the header pipes. Today, computers take the information that was generated during all those experimental years and programs have been written to use that basic info to generate the headers of today.

Ron
 

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gtzero said:
Thanks Chris,

I'm still intrigued whether it would be doable to exit the exhaust ahead of the passenger door. I'd love to build some under-body flat panels and diffuser.

Manny

Manny, im sure it could be done if theres actually room for your muffler and you dont need to pass inspection. Because of the width (and front length) of a Mclearen the exhaust header probably ends a few feet from its actual exhaust pipe end, leaving room for tuning. In other words, that pipe sticks out there because thats the exact lenghth the pipe should be and thats where it was able to stick out. if they needed three more inches it would have been further back or sticking out more. as you know the gt is very slim. not much room.
Im certainly no expert but, do some research before you go for it, exhaust is a huge area and can be the birth of new power or the death of power you had.
good luck though, short pipes are definitley fun! I run 18 in 2 1/4 to a generic glasspack and get great response. ( I dont reccomend this if you have a life...or neighbors.)
 

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Discussion Starter · #10 ·
Thanks for all the responses

Looks like I should research exhaust systems a bit before contemplating further.

Thanks again,

Manny
 

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You don't want backpressure, this is true. You want an exhaust big enough to ensure no backpressure, but not so large as to reduce velocity (as Paul said). If you lose velocity, low rpm torque/power and response will suffer.

FWIW

Bob
 

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Header Design Theory

If anyone wants a dose of the theory behind header design, check out www.headerdesign.com. Its some of the better reading out there on the subject. Most of the good information is very difficult to find in writing, since this subject is still a mixture of "black art" and science. Many of the secrets go un-published.

Years ago, most of the mathematics behind header tuning was based upon Inertial Scavenging. That being the physical low pressure area created behind the slug of traveling exhaust gasses. These gases if traveling at an assumed 300 feet per second, one could calculate the length of primary pipe needed to time the arrival of the next slug of exhaust to follow in the low pressure area of the preceeding one.

Now-a-days, the science has moved on to Acoustic Wave Tuning. This says that when the exhaust valve opens, a sound wave emanates. This wave travels not at 300 FPS, but at the speed of sound. Note, that the speed of sound varies with the temperature of the gas. Also, this wave is not sensitive to primary pipe diameter. When this sound wave reaches the collector, it is reflected back up the primary (actually all primaries) pipe but as a negative wave, meaning a low pressure energy wave.

The science and mathematical models attempt to predict the length of primary pipe needed to time this wave from the momemt it leaves the exhaust valve as a positive wave and returns to back to the SAME exhaust valve/cylinder just before and during valve overlap as a negative wave. This problem is complicated by the fact that the exhaust gas is constantly cooling as it is traveling down the primary, thus affecting the speed and strength of the scavenging wave. This is the reason the header coatings and wraps are in such great use.

Its really interesting reading, ...if you are a geek like me :D

Someone hide my soap box...
Paul
 
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