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Back pressure?

TheFox88

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I've seen a few times in threads about different exhaust setups that having piping that is too large (like 2.5"?) will cause you to lose backpressure and there fore performance. I was wondeirng exactly what kind of loss of performance this will cause. I'm primarily looking for better gas mileage and a little better performance, how will 2.5" piping affect this? Also will a flowmaster 40 series single-in singleout work well for my purpose? Thanks in advance!
 



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Chew_12

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Wow Flagstaff, Nice. You shouldnt worry about backpressure unless your going true duals. Lots of people on this board have 2.5. It should be what you are looking for.
 






TheFox88

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Sweet dude! I should be doing it this month somtime over at muffler magic. Good to see someone else from the highlands. What year are you?
 






Chew_12

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I have year and a half left of school. I guess im techinally a senior. Are you going to NAU?
 






TheFox88

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i'm still in high school. i'll be a senior this coming year.
 






mitch

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I have exactly what you are wanting to do and so far they are great. But since flowmaster doesnt make a kit for the 40 series (only delta flow) you will have to make custom pipes or get someone else to do them
 






baff2004

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Hey you guys in Flagstaff? I go to NAU.... wheeling there is awesome, I'll be a Sophomore there.
 






jimbo74

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2.25" is normally the best... i am sure everyone has their own opinions but 2.5" is also decent.. and bigger than that on a v6 and backpressure will be too little for its own good
 






Chew_12

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baff2004 said:
Hey you guys in Flagstaff? I go to NAU.... wheeling there is awesome, I'll be a Sophomore there.

Yep Im going to NAU too. Im a Senior now... Have you seen the green lifted explorer around campus? Thats is mine.
 






baff2004

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Yeah I think Ive seen it parked close to the bookstore...but Im not sure if thats yours...is it?
 






Chew_12

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Yea that should be mine. I have been living at McDonald and that is Next to the book store
 






Ronin8002

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Backpressure is never a good thing. What most people experience when they go with too large exhaust for their application is a loss of exhaust velocity at low rpm's. This is basically because the larger piping allows the exhaust to cool a lot more before it reaches the end of the tailpipe, which means the gases become heavier and cause a restriction....which amounts to loss of low end torque. Top end power is not negatively affected in these situations though...usually it is improved actually. But since you don't drive around at WOT all the time it makes your truck feel like a dog.

3 inch single exhaust is good for 250-350 HP.
2.5 single or 2.25 single are good for a mildly modded v-6....only go bigger with heavy forced induction or nitrous use and you have every other bolt on.
 






TheRookie

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MaximumViolence said:
Backpressure is never a good thing. What most people experience when they go with too large exhaust for their application is a loss of exhaust velocity at low rpm's. This is basically because the larger piping allows the exhaust to cool a lot more before it reaches the end of the tailpipe, which means the gases become heavier and cause a restriction....which amounts to loss of low end torque. Top end power is not negatively affected in these situations though...usually it is improved actually. But since you don't drive around at WOT all the time it makes your truck feel like a dog.

3 inch single exhaust is good for 250-350 HP.
2.5 single or 2.25 single are good for a mildly modded v-6....only go bigger with heavy forced induction or nitrous use and you have every other bolt on.

WELL SAID :) I am glad someone else understands this. Whenever I say this on here i get ridiculed and flamed so expect same. I have tried explaining it so many times so many ways. Even got into the science of scavenging a few times. No suck luck, Thanks for in insitefull and accurate post
 






TheFox88

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Well. i now have the 2.5" piping, custom bent, with a delta flow 40. I have not seen any loss in performance, in fact a bit better acceleration and what not. Definitley a great mod! Thanks for all the help guys! :thumbsup:
 






rookieshooter

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Even got into the science of scavenging Quote: rookie

Are you talking about when both in. and ex. valves are off their seats (split overlap)and the momentum of the exhaust gases lets fresh intake air to start coming in thru the intake even before the piston starts down to cause a neg. press. in the clyinder. This is what I call scavenging. One of my best friends is Herb Gebler who designed and made some of the best headers around . I would even help load up all his u bends and pack them to Richard Pettys and other NASCAR teams. then all the stuff would be down there when he arrived to make the headers. This was in the late 60's and 70's. He is the one who also made my headers for my drag cars. It was nice to read your post and Maximumviolence. On my aluminum big block chevys he would actually use two different dia. pipes due to the difference length of the runners in the heads. I would like also to mention about the pressure pulses. If you could actually see each individual pressure pluses as they are going out the header tubes they would look tear shaped with the big end going out first. The small end creates a low pressure area behind it and helps bring the next pressure pulse along with it. Just like drafting in NASCAR Herb and I talked in great lenghts about this. That is another reason to have the tubes the right dia. so these pressure pulses don't get all messed up in the tube. For a lack of a better discription they need to fit just right. As you know, hold a small piece of paper behind the taipipe and you can see the pressure difference I'm talking about. As the pulse comes out the paper blows away and then you can see a slight drawn back in due to the neg. area behind the pulse. Just my 2 cents worth.
 












rookieshooter

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You are the first one to have ever told me that. You no your sh-t
 






CDW6212R

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It is good to read a few nice comments, instead of wrong, wrong, wrong, etc.

Then, ponder this; what good was an H-pipe two decades ago, and now an X-pipe?
In the end, my questions are basically, what downside is there still in an X-pipe, and how can it be bettered? My hint is "think of a header collector." Regards,
 






TheRookie

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If you could actually see each individual pressure pluses as they are going out the header tubes they would look tear shaped with the big end going out first. The small end creates a low pressure area behind it and helps bring the next pressure pulse along with it
This is exactly what we call scavenging in the exhaust world. Definately a well worded post, thanks rookieshooter :thumbsup:
 



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rookieshooter

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CDW6212R said:
It is good to read a few nice comments, instead of wrong, wrong, wrong, etc.

Then, ponder this; what good was an H-pipe two decades ago, and now an X-pipe?
In the end, my questions are basically, what downside is there still in an X-pipe, and how can it be bettered? My hint is "think of a header collector." Regards,[/QUOTE

Just my thinking,
H-pipe, to equalize pressure pulses. Then X-pipe comes along to try to do it better by reducing the angle of the cross-over pipe. Of course all of this is due to the un-even firing order of the engine. One pulse can be shorter due to the fact that its exhaust port is closer to the collector because of its location in the head. Hint-collector, OK let me think, remember I've been out of active racing since the eary 70's. We used to take bolt-on collectors that started of long and then would hack-saw off an inch at a time to tune the headers for low end or high end performance. By cutting off the collector this would change the dynamics of the pulses as they come out the end of the collector and meet the atmospheric pressure. This would change the scavenging effect. So with that said, can we improve the X. How about the length of the runners of the X, the angle of the X in relation to the primary pipes and then experiment with pipe diameters in the X pipe itself. Forgot to mention downside. Is it because no matter what it is not as good as open headers?

Am I any where close? Do I even make sense?
 






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