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hondacr250r
Dodge Dakota
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4/20/2005
14:07:50
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 Subject: back pressure
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i have the 3.9 should i go with the 40 series......single? or dual? exhaust i heard the dual eliminates to much back pressure....
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hondacr250r
Dodge Dakota
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4/20/2005
14:10:39
| RE: back pressure
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actualy they dont even make a single for the 40 series do they
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N56629
Dodge Dakota
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4/20/2005
15:35:12
| RE: back pressure
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You don't have to worry about backpressure with port injection.
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MattNC
Dodge Dakota
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4/20/2005
20:30:57
| RE: back pressure
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To my knowledge they do have a 40 series single, it was recommended to me instead of duals. If you go dual go with a 2.25 inch pipe instead of the 2.5. That will keep back pressure up and give you the nice tone! Good luck.
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01dak
Dodge Dakota
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4/20/2005
22:02:45
| RE: back pressure
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The only motor that needs "backpressure" to run is a 2 stroke. This myth about 4 cycle motors needing resistance in the exhaust is a load of crap. The more resistance you can get rid of,the better. If you have backpressure you arent scavenging the cylinder and the fresh intake charge has to work that much harder to get in.
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OBIO3
Dodge Dakota
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4/20/2005
23:12:51
| RE: back pressure
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01DAK >> Finally someone that knows the real deal on backpressure > Been watching this back pressure hoax stuff for a long time > I can not beleive nobody understands one of the most important aspects of increasing there HP > Here is the bottom line >> IF YOU CAN'T GET IT OUT YOU CAN'T GET IT IN > Simple as that > You do not want backpressuse on a 4 stroke engine > enough said
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N56629
Dodge Dakota
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4/20/2005
23:23:57
| RE: back pressure
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If you have zero backpressure (theoretically) you have no velocity. If you have no velocity you have no scavenging effect.
Would I be better off with 2" primaries on my headers or 1 5/8"? 1 5/8 has more backpressure.
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MattNC
Dodge Dakota
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4/20/2005
23:49:45
| RE: back pressure
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You may believe what you wish on back pressure, but I have had a v6 before that had a problem and kept creating more and more back pressure and it kept blowing my exhaust manifold gaskits.... Our engines are created to run with so much, it you kinker with that too much, you have unwanted results. If you tune your vehicle to run with 93 octane fule and run 89, is it bad, not exactly, but won't run too the full benefit.
Matt
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Kaderdak
GenIII
4/21/2005
02:21:38
| RE: back pressure
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For starters, I by no means claim to be an expert on backpressure. I do know however that if you have none, you are FUBAR. Too much however is also bad. You need just the right amount so that everything that goes in can come out freely, but still just enough that the escaping gas creates a vacuum and will essentially pull the exhaust out from the other cylinders. YOU NEED BACKPRESSURE. Not very much, but just the right amount.
By no means though will you eliminate too much backpressure simply by changing mufflers, converters, or the rest of the catback. The major creators of backpressure in our engines are the manifolds and the y-pipe. If you've done some serious mods to get the air in, you may be best off to go with headers and a bigger y-pipe. For our stock engines though, stock manifolds and y-pipe are fine.
By all means add a muffler though, you'll still have plenty of backpressure from the rest of the system, plus it'll sound a hell of a lot better!
** '97 Dakota 5.2L 4x4 w/3.55(my best ebay purchase yet) **
** Homebrew CIA, Dynomax Catback, March Underdrive Pulleys, Transgo Kit **
** Borg Warner Brass Cap/Rotor to Taylor 8mm wires to NGK plugs **
** Homebrew dual electric fan setup with 180* thermostat **
** Sitting on BFG 31 x 10.50's **
** Tons more once I'm not a poor college student :) **
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01dak
Dodge Dakota
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4/21/2005
10:07:58
| RE: back pressure
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1 5/8 headers do not have more backpressure!! You can believe this if you want,but the smaller tube creates more VELOCITY to scavenge the cylinder. Your motor cannot produce enough air to cause a restriction in a 1 5/8 pipe. EXHAUST GAS VELOCITY is what you are after. Not backpressure.
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N56629
Dodge Dakota
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4/21/2005
10:46:27
| RE: back pressure
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1 1/2 creates even more velocity and also happen to believe that it creates even more backpressure. You can not increase velocity without increasing backpressure or restriction. If there is no restriction you can not increase velocity, period.
A garden hose is a classical example. Yes, I know it's not a perfect example. Without a nozzle the hose is flowing nearly as much as possible. However, you can not control the velocity or increase it without installing a nozzle or reducing the diameter. This restriction increases pressure which often referred to as backpressure.
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01dak
Dodge Dakota
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4/21/2005
13:32:28
| RE: back pressure
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Okay,I give up. I guess you guys know way too much for me. Im gonna go out and weld a cap on the end of my tailpipe and try drilling slightly larger holes until I find the power and mileage. Like they say,"Ignorance is bliss".
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N56629
Dodge Dakota
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4/21/2005
13:57:25
| RE: back pressure
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"Im gonna go out and weld a cap on the end of my tailpipe and try drilling slightly larger holes until I find the power and mileage. Like they say,"Ignorance is bliss"."
If that's the understanding you are going away with, the ignorance is closer than you think. Maybe if you studied Bernoulli's Principle and possibly Poiseuille's law you would know what I'm referring to. After that, come back and explain your theory of increasing velocity without changing pressure.
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OBIO3
Dodge Dakota
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4/21/2005
19:30:03
| RE: back pressure
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What you guys are saying about just the right amount of backpressure and velosity is correct IF you was running a 2 stroke > your Dakota has valves and once that intake valve closes the firing process comes next driving the piston down with teriffic power > once that has taken place the next cycle is up to the exaust valve > it opens soon as the piston is traveling up > the pressure is released and the remainder is removed by the upward travel of the piston > now if you was to create the back pressure you people think you need , where do you think it would go with the valves closed > think now > It well be forced into the crankcase past the rings and blow seals and or gaskets out of your engine > the amount that does get by when the engine fires in minimal because of ring design > the rings are pressed against the cylinder walls untill the exaust vale opens > how bout this velocity your saying you need > with valves closed at top dead center , if velosity was there you would be sucking air from the crank case through the rings and with it comes a lot of oil > of all the parts in an engine the cylinder walls, pistons and rings by far get the bulk of the oil > otherwise your engine would fry in very short order > I really don't care a bunch what you people think is correct > unless you was to seriously restrict your exaust systen the worse your back pressre will do is shorten valve life and steal horse power > take a look at the exaust on dragsters > you can almost walk into those pipes and look around
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MattNC
Dodge Dakota
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4/21/2005
21:29:31
| RE: back pressure
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First off... Not to diss you, but learn some grammar and punctuation.
If back pressure wasn't ment to be there, then why is there any at all? If I read your post right, that if there is some back pressure that it will go past the piston rings and into the crankcase. I know some backpressure is ok, but wouldn't it go into the crankcase??? Point is, Engines are engineered to have a specific amount of pressure. If they weren't then having a 2.5 inch exhaust and a 4 inch wouldn't matter. Like I said I have had a problem with back pressure in the past and a Mechanic told me that mine (the computer) thought there wasn't enough pressure so it tried to get it up, constantly blowing exhaust gaskets.
I think we have gotten away from the original post though. Depending on the size of exhaust you go with you should be ok. To small and you get too much, too large and not enough. Take all the previous posts with a grain of salt.
Matt
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N56629
Dodge Dakota
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4/21/2005
22:56:48
| RE: back pressure
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I find the whole notion of "needing back pressure" rediculous. It is a by-product of velocity and nothing else. If there were a way to maintain the velocity necessary for scavenging without any backpressure, I would be all for. Unfortuanately I don't know how to change the laws of physics.
Backpressure is here to stay, but it belongs in the headers and not in the mufflers. By the time your exhaust pulses reach most mufflers it is to late to do any good.
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OBIO3
Dodge Dakota
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4/21/2005
23:28:04
| RE: back pressure
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Sorry the typing upset you > I'm left handed and it's broke > typing with right hand only > I got one eye working and the other has distorted vision so i don't see a lot of the mistakes > I live with it > Hope it don't get you down to bad
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krellco
Dodge Dakota
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4/22/2005
17:34:22
|  RE: back pressure
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1-you cant have ex sys w/o any BP unless you lose EX man & pipes-no go-burned valves etc.
2-MIN BP is best we can hope for, with HIGHEST EX
gas velocity, to best scavange cyl.
3-With CATS in place & y PIPE (AS PREV POSTD) MOST ABOVE SHOT TO H*LL.
4-Best I could do: left Y (to even EX PULSES) ran thru 3"IN & OUT 30" glasspak(MIN BP) to a splitter(dual pipes reduced to 2.25) & out 2.5 tailpipe G'Paks. MIN BP w/max VELOCITY= decent sound & avg 1mpg increase (added stock K&N & lost fan) on my 01 DAK QC 4.7 AWD. Can FEEL DIFF
& now get BETTER MPG. TOTAL cost:$300
MORAL of Story: with EX SYS,BIGGER aint Better
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Boomer
Dodge Dakota
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4/23/2005
09:25:08
|  RE: back pressure
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In the past month or three, I cannot remember which issue, Car Craft or Chevy High Perf. did an article about back pressure. They hooked up a gauge to the exhaust pipe (tapped it like you would for an o2 sensor), and played with various pressures. They stated that if you have 0-1 psi in the system you are ok. I believe anything above 9psi was bad. I'll look through my old mags and find it.
Personal research...
The only motor that needs back pressure is a 2 stroke, even rotaries don't use back pressure. However, rotaries and 4 strokes used 'tuned' exhaust for the schavenging effect. It is based on the exhaust pulse hitting a slight restriction (at the collector) and returning to the head just prior to the exhaust valve opening. When the pulse bounces back off of the head, it aids in 'pulling' the new exhaust pulse away from the head (hence schavenging). This is done entirely in the headers/exhaust manifold. After that, there is no need for 'back pressure'. Think of a top fuel dragster, NASCAR, etc. Do they have a restrictive exhaust? No!
Another way to look at it, picture a typical car on a dyno. Run the exhaust gases through the normal exhaust and do a pull or two. Then, disconnect the exhaust at the collector or install a cutout. Run the car again, and it will 'free up' power. Not gain, but allow the engine to run without forcing the gases out.
Now, with that in mind, don't go out a install a 4" or 5" exhaust on a gas powered street car. By the time the gases hit the exhaust tips, they(gases)have cooled so much that they have to be forced out even more so.
I hope this helps a little in the reasoning and logic.
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01dak
Dodge Dakota
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4/23/2005
09:59:57
| RE: back pressure
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Thank you Boomer. You are right on and explained it better than I could.
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hybrid
Dodge Dakota
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4/23/2005
20:51:40
| RE: back pressure
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This is funny...
A few people who know what they are talking about have pretty much hit this one on the head (you know who you are)....
Backpressure isn't there to make your engine run right... it's a by-product of your exhaust system.
I'm not going to get into this one again because I don't have time to site here and type all day......
Simply put. A properly designed exhaust system will have good flow, very little back-pressure and high velocity. It's all about compromise. You cant have it all, you must sacrifice some flow for velocity....the back-pressure comes as an end result......it's a by-product....it's going to be there regardless.
On this planet we have whats called atmospheric pressure, unless you find a way to convert that to a vacuum then your going to have back-pressure.(that has nothing to do with the cost of sliced bread, but what the hell...
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