exhaust diameter question
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Lead Driver
Joined: Mar 2008
Posts: 7,175
Likes: 377
From: Near Los Angeles
exhaust diameter question
Hey guys--
My Hedman header came with the collector attachments/pipes so they can be welded to for the rest of the exhaust system.
I notice the pipe is 2". It is a dual set up. From what I've read, the best size is the 2.25" diameter pipe.
Should I stick with what Hedman sent, or switch to the larger 2.25" pipe?
My Hedman header came with the collector attachments/pipes so they can be welded to for the rest of the exhaust system.
I notice the pipe is 2". It is a dual set up. From what I've read, the best size is the 2.25" diameter pipe.
Should I stick with what Hedman sent, or switch to the larger 2.25" pipe?
Postmaster
Joined: Dec 2006
Posts: 3,603
Likes: 10
From: Spokane, WA
I don't remember which one but one of the muffler manufactures has a chart that recommends pipe size according to engine size. 2" or 2 1/4" works fine on these engines to maintain desired velocity.
Post Fiend
Joined: Jan 2007
Posts: 7,021
Likes: 4
From: Almost Heaven
Keep the 2'' half way and open it up to 2.25 or 2.50. Ideally you want to start narrow and open it up as you get to the rear bumper.
I agree completely. No need to go any larger than 2.25 on a nearly stock 300.
I agree completely. No need to go any larger than 2.25 on a nearly stock 300.
Mountain Pass
Joined: Dec 2008
Posts: 101
Likes: 0
From: Scotts Valley, CA
FWIW, I have (2) 2" headpipes off the EFI manifolds, into a Flowmaster Y, and 2 1/2" with a generic turbo all the way out.
Post Fiend
Joined: Jan 2007
Posts: 7,021
Likes: 4
From: Almost Heaven
Ah, no. If you're going to use different sizes, it's the other way around. As the exhaust cools, going down the length of the truck, it requires less volume, not more.
FWIW, I have (2) 2" headpipes off the EFI manifolds, into a Flowmaster Y, and 2 1/2" with a generic turbo all the way out.
FWIW, I have (2) 2" headpipes off the EFI manifolds, into a Flowmaster Y, and 2 1/2" with a generic turbo all the way out.
I have read through dozens of posts and have done a lot of research on this. I'll stick to my own method. A good way to find out is to take a truck and run it on a dyno with the exhaust identical, except for which end has the larger piping. I'll not swear to anything but from what I have read it's best to run narrow piping at first and then open it up.
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Elder User
Joined: Oct 2008
Posts: 708
Likes: 1
I would use the two inch pipe that is supplied to go back from the collector. And then go from that 2 inch into a 2.25 or 2.5. I don't believe the size after the 2 inch is really too critical unless you make it huge. But my thinking is that the pulse of hot gas from a single cylinder exhausting will travel down it's own header tube and then at the point it reaches the collector it will take some of the positive pressure from inside the other header tubes and drag it out with it to make a slight vacuum. This will then scavenge the exhaust pulses that are next in line. If you use a little bit larger pipe right at the collector the exhaust pulse will immediately loose a little velocity when it exits it's tube and enters the collector and transfer a slightly weaker vacuum signal back up the other header tubes. Reducing scavenging slightly. So I think using the 2 inch piece, however long it is, 12 or 18 inches or so, immediately after the collector, and THEN going slightly larger will improve your scavenging. That is probably why they supply the headers like that.
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Post Fiend
Joined: Jan 2007
Posts: 7,021
Likes: 4
From: Almost Heaven
I would use the two inch pipe that is supplied to go back from the collector. And then go from that 2 inch into a 2.25 or 2.5. I don't believe the size after the 2 inch is really too critical unless you make it huge. But my thinking is that the pulse of hot gas from a single cylinder exhausting will travel down it's own header tube and then at the point it reaches the collector it will take some of the positive pressure from inside the other header tubes and drag it out with it to make a slight vacuum. This will then scavenge the exhaust pulses that are next in line. If you use a little bit larger pipe right at the collector the exhaust pulse will immediately loose a little velocity when it exits it's tube and enters the collector and transfer a slightly weaker vacuum signal back up the other header tubes. Reducing scavenging slightly. So I think using the 2 inch piece, however long it is, 12 or 18 inches or so, immediately after the collector, and THEN going slightly larger will improve your scavenging. That is probably why they supply the headers like that.
Elder User
Joined: Oct 2008
Posts: 708
Likes: 1
Thanks Bill. Although I think you would know more about this subject than me. By the way though, since I'm no longer a crime fighter by night I think it's time to unmask my true identity.................
Hereafter I shall be known as Steve (91Bronc300 just has no personality behind it)
I'm sure you were dying to know
Hereafter I shall be known as Steve (91Bronc300 just has no personality behind it)
I'm sure you were dying to know
Post Fiend
Joined: Jan 2007
Posts: 7,021
Likes: 4
From: Almost Heaven
Thanks Bill. Although I think you would know more about this subject than me. By the way though, since I'm no longer a crime fighter by night I think it's time to unmask my true identity.................
Hereafter I shall be known as Steve (91Bronc300 just has no personality behind it)
I'm sure you were dying to know
Hereafter I shall be known as Steve (91Bronc300 just has no personality behind it)
I'm sure you were dying to know
Alright Steve I'll try not to call you James.
Like I said, I've done a lot of research on exhaust, and although I couldn't begin ti provide concrete facts (if I had anything, it was all on my other computer that is in storage). That's why I won't debate; I can fully understand why people think it should be opposite.
Elder User
Joined: Oct 2008
Posts: 708
Likes: 1
Yeah, me too. I agree with the idea of the exhaust cooling as it gets further down the exhaust pipe and requiring progressively smaller tube diameters to maintain a fixed velocity. But to setup an exhaust system to properly take advantage, after the collectors and then the initial 18 inches or so of 2 inch pipe would be to complicated. You would then have to inlet into a 2.5 inch pipe or so and then measure the temperature and velocity at that point, figure out what your temperature and velocity loss are going to be at the tailpipe, do a bunch of math that's over my head for sure, figure out what the ideal taper is, then have a custom tapered pipe made that's about 8 feet long or so. Any attempt to neck down the exhaust pipe in steps is going to create a reversion where the steps meet that would surely negate any gain in velocity the smaller pipe would give you. So I think the idea of 'neck down the exhaust as the gases cool to maintain velocity' is sound theory but impractical to implement.
Cargo Master
Joined: Jul 2003
Posts: 2,328
Likes: 14
From: Colorado
Yeah, me too. I agree with the idea of the exhaust cooling as it gets further down the exhaust pipe and requiring progressively smaller tube diameters to maintain a fixed velocity. But to setup an exhaust system to properly take advantage, after the collectors and then the initial 18 inches or so of 2 inch pipe would be to complicated. You would then have to inlet into a 2.5 inch pipe or so and then measure the temperature and velocity at that point, figure out what your temperature and velocity loss are going to be at the tailpipe, do a bunch of math that's over my head for sure, figure out what the ideal taper is, then have a custom tapered pipe made that's about 8 feet long or so. Any attempt to neck down the exhaust pipe in steps is going to create a reversion where the steps meet that would surely negate any gain in velocity the smaller pipe would give you. So I think the idea of 'neck down the exhaust as the gases cool to maintain velocity' is sound theory but impractical to implement.
For sure. And on a 150HP 300-6 I can't see how it makes a difference. There is something to be said about "tuning" an exhaust on a indy car, or nascar, or something that will really respond as they have specific 700+ HP engines.
Dude, think about what we have... a 40 year old design one hundred and fifty horsepower 6 cyl engine, LOL!!
Post Fiend
Joined: Jan 2007
Posts: 7,021
Likes: 4
From: Almost Heaven
For sure. And on a 150HP 300-6 I can't see how it makes a difference. There is something to be said about "tuning" an exhaust on a indy car, or nascar, or something that will really respond as they have specific 700+ HP engines.
Dude, think about what we have... a 40 year old design one hundred and fifty horsepower 6 cyl engine, LOL!!
Dude, think about what we have... a 40 year old design one hundred and fifty horsepower 6 cyl engine, LOL!!
As it turns out, you can lose a lot of low end torque if you do things wrong. I did things wrong, but on purpose. I love to hear my truck roar. If I need to pull something my gears do the talking.
