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GT
Dodge Dakota
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2/16/2002
20:43:59
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 Subject: another dual exhaust question -sort of
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In reading the endless posts on dual exhaust, it seems to be a concensus that true duals(one from each bank) is not the way to go because of low end torque loss. It also sounds like the majority seem to like leaving the cat and going with a cat back system. Soo... can one or more of the techies here explain what is really going on with the exhaust to make a loss of torque with the above scenarios? Technical but understandable explanation please. Thanks in advance. GT
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Bruce P
Dodge Dakota
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2/16/2002
22:24:58
|  RE: another dual exhaust question -sort of
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(GT) This is a oversimplification... but should be satisfactory to answer your question.
The flow of a gas or liquid in a pipe is influenced by many factors. For this discussion lets assue a STRAIGHT pipe of a given length. We will push air INTO it at a given speed.
At no time will I be discussing what happens when the speed of the flowing gas approaches sonic the speed of sound) All of the physics change at that point.
If our pipe has a large diamator... the speed that the gas WITHIN the pipe is very slow.
If our pipe has a small diamator... the speed of the gas WITHIN the pipe is very fast.
Are you still with me here?
Now... gas that is flowing at a high speed has some measure of MOMENTUM. That is to say that the flowing gas will not want to stop flowing immediatly and the flowing gas will tend to PULL more gas into the pipe if we suddenly tried to plug up the intake of the pipe.
Now... understand that the exhaust flow from an engine is actually many small pulses instead of a constant flow. (the exhaust valves open and close)
Given the above... If we can select a pipe diamator that is small enough to keep the speed of the flow high (without 'pinching off' the overall flowrate) then we would have a 'tuned' pipe that actually helps scavange (PULL) the exhaust out of the engine. Each time an exhaust valve opens, there will be a small vacuum waiting to start pulling the exhaust out of the engine.
BOTTOM LINE: A SINGLE pipe that allows high flowrate of exhaust gas is desirable.
ADDITIONALLY: Some of these principles can be applied to the INTAKE side of an Internal combustion engine too.
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Bruce P
Dodge Dakota
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2/16/2002
22:30:23
| RE: another dual exhaust question -sort of
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One last point...
Some people will say that an engine needs "backpressure" in the exhaust. They are refering to adding dual exhaust or large pipes to an engine and getting LESS overall power/torque
What is REALLY going on is that the speed of the exhaust is so slow that the engine is actually WORKING HARDER to push the exhaust out of the exhaust system. (There is no longer any "flow" that is helping to pull it out.)
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2KV6AUTO
Dodge Dakota
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2/16/2002
23:26:35
| RE: another dual exhaust question -sort of
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Ok I just gotta add something to this. Im not a tech guy but awhile back my friend got an R/T. He did a lot of researching about exhaust and contacted Kenne Bell the company that makes superchargers for the 5.9 . They had an article in their brocure about putting true duals on the R/T. They did several test with the duals, with the single stock exhaust and even with no exhaust on the truck. They said that when the duals were put on it actually robbed the truck of HP. And with the exhaust completely off, the truck just running with headers that it wasnt that much of a HP gain from the factory single side exhaust. Hope this helps out. The optimum exhaust is gonna have a scavaging effect which means it also sucks out exhaust from the engine creating a better flow. If duals are put on the truck it may take away the scavaging effect and make the exhaust "work" harder to get through the system and resticting flow or it may even create too much of a flow and not create the scavaging effect. I hope this helps If Im wrong then Im sorry but this is what Ive heard from alot of drag racers out there.
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Bruce P.
Dodge Dakota
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2/17/2002
17:42:40
| RE: another dual exhaust question -sort of
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(2KV6AUTO) You have it exactly correct!
When the VELOCITY of the gasses in the exhaust is too slow (due to too-large diametor pipes, or no pipes at all), the desired scavenging effects are not suffecient enough to provide HP/torque gains.
The ONLY time you want REALLY LARGE diametor exhaust pipes is when an engine is turbocharged.
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2KV6AUTO
Dodge Dakota
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2/17/2002
20:37:24
| RE: another dual exhaust question -sort of
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Hey Bruce P.
Thanks. Your post was the best tech explanation Ive read on this site. Simple physics. Glad to see someone who actually answers a question on these boards rather than show off about their truck or post another question.
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GT
Dodge Dakota
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2/18/2002
20:55:00
|  RE: another dual exhaust question -sort of
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Ditto on the Thanks for the explanation there BP. It was great to get that type of response on the board. We are fortunate to have several folks on here willing to share their tech knowledge with the masses.
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Mills
GenIII
2/18/2002
21:38:16
| RE: another dual exhaust question -sort of
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we need people to donate to the site and/or join in order to keep good things like this happening!
Go #8
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rod manarin
Dodge Dakota
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9/02/2003
14:18:42
| RE: another dual exhaust question -sort of
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Hi,
Installing a 99 360 into a custom rod project, the engine now has sanderson shorties, previously had single exhaust, with an up stream and down stream o2 sensors, what is required to keep dual exhaust. is it 2 cat converts, will a cross over pipe work, what size pipe and what about o2 sensors, do i install 4 rather than 2? thanks
Rod
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Ray
Dodge Dakota
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9/02/2003
16:12:57
| RE: another dual exhaust question -sort of
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haha rod u are funny (sacastic)....
...great read...very good explanation (in simple terms) on how "backpressue" works.
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Demon Dakota
DakotaEnthusiast
9/02/2003
18:09:09
| RE: another dual exhaust question -sort of
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So explain to me why when I went to true duals (equal length, 2.5" all the way back and removed third cat) I picked up HP and TQ, as well as .3-.4 seconds in the 1/4 mile. Mind you, this was before I went nuts and did all my other mods like the HO Cams/Intake/TB/IAT Adjuster/CAI...
Now I've gone a few steps further in my mods, but I didn't lose anything when I went to true duals with a basiclly stock engine. I do have a 4.7L (vs. posting above about the 5.9L), but it shouldn't make a difference.
K&N Drop in AF (CAI during summer months)
True Dual exhaust and removed third cat
HO Cams & HO Intake
FASTMAN 70mm TB
Autolite 3923 Plugs
TPS @ .76 VDC
IAT Adjuster Mod
14.52@94.2 MPH at Silver Dollar Raceway
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Walter
Dodge Dakota
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9/03/2003
13:14:14
| RE: another dual exhaust question -sort of
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Demon Dakota, it's either in the heads or the exhaust manifolds, it depends on where the scavenging effect is produced. The RPM range sets the length of the collector on after market headers. The length of the tubing from the exhaust port to the collector sets some of the scavenging for a set of headers. The 4.7 heads and exhaust manifolds are different (I believe) from the 5.9 the affect could be better in the 4.7 over the 5.9. I have a book that tries to help you design exhaust systems by teaching you how they work. It talks about at least three if not more systems. Each size engine/head combo works differently so for your "combo" the duals might work. The book goes into things like scavenging, back pressure, tube size, and length of system. I will try to find it and get you all a source. The book explains it much better then I can without taking up allot of room. I'm not trying to step on toes but there has been extensive research in this area.
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