jayg

Does anyone know the formula off hand?

junkyarddog

3.1428 X radius squared
or diameter squared X .785

jayg

thanks dog!
If anyone is interested her are the results.
One 3.5" pipe = 9.62
Two 2.5" pipes = 9.8125
One 3.0" pipe = 7.065
One 4.0" pipe = 12.56
As you can see, one 3.5" is very close to two 2.5" pipes.

Apeiron

Those equations will only tell you the cross sectional area of the pipe, which isn't the only important variable affecting flow. The 2 2.5" pipes will have a greater surface area inside than a single pipe of the same area, which will impede flow. Most people I've heard from say that a single 3" pipe flows about the same as 2 2.5" pipes.

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1984 Silver Z28, 383 cid 4 bolt, ARP Fasteners, 2 1/2" Cowl Induction Hood, 600 cfm Performer Carburetor, Torker II Intake, Performer RPM Heads, XE268H Cam, Magnum Roller Tip Rockers, MSD6AL Ignition, Blaster2 Coil, Recurved HEI Distributor, 700R4, TransGo SK, 2.73:1 Limited Slip Dana 44, Terminator Headers, Custom 3" TIG Welded Stainless Exhaust, Flo-Pro Twister Muffler, 4 Wheel Disc Brakes, Oil Cooler

CamaroMike

Apeiron, your theory is shaky. What exactly does surface area do to impede flow. Cross Sectional Area is the name of the game, not surface area. Unless lamilar flow characteristics affect the exhaust then after the header (as a general rule) bigger is better.

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84 Camaro ZZ4 with HOT cam. 1.88 60' (12.98 @ 105MPH E.T.)
Other Mods: You name it and I have probably changed it.

91 Firebird 5.0L TBI

ASE Certified CHEVY DEALER Auto Tech
LIVE AND DIE BY THE ALMIGHTY BOWTIE!

[This message has been edited by CamaroMike (edited November 10, 2000).]

Grim Reaper

There are other issues too. Like you need 2 mufflers with duals (twice the cost and maintenance), and 3rd Gens were not blessed with enough space to squeeze dual 2 1/2" exhaust with mufflers without rubbing.

I've talked to muffler shops about installing duals and most wouldn't even consider it and the few that would, would only instal a 2 1/4" at a VERY OUTRAGEOUS PRICE and still said that I risked scrapping on the smallest of speed bumps.

For all the cost of dual 2 1/2" and the maintenance (I have owned many dual exhaust cars), I will pass on the little (if any) performance gain it would have over a single 3 1/2" exhaust. BTW, a single 3" will be sufficient well into the 400HP range before the benefits of a 3 1/2" single will be noticed (other than sound...bigger DOES sound meaner).

Apeiron

Fluids flow slower along the walls of a pipe. Given the same total cross sectional area, a dual pipe setup is more restrictive than a single pipe setup. To put it another way, the combined cross sectional area of a dual pipe setup must be larger than the area of that of a single pipe in order to flow equally well.

Bigger is still better when comparing similar systems. A single 3" pipe is better than a single 2.5", or dual 2.5" is better than dual 2" and so on.

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1984 Silver Z28, 383 cid 4 bolt, ARP Fasteners, 2 1/2" Cowl Induction Hood, 600 cfm Performer Carburetor, Torker II Intake, Performer RPM Heads, XE268H Cam, Magnum Roller Tip Rockers, MSD6AL Ignition, Blaster2 Coil, Recurved HEI Distributor, 700R4, TransGo SK, 2.73:1 Limited Slip Dana 44, Terminator Headers, Custom 3" TIG Welded Stainless Exhaust, Flo-Pro Twister Muffler, 4 Wheel Disc Brakes, Oil Cooler

[This message has been edited by Apeiron (edited November 10, 2000).]

Grim Reaper

But a single 3 1/2" won't buy you anything (other than lightening your wallet) over a 3" on an under 300HP engine.

Bigger IS better when the alternative is restrictive. But a single 3" isn't restrictive until higher HP.

82z

What apeiron is trying to say is that the greater surface area of the 2 2.5 pipes decreases the flow as compared to the 3.5 because of the greater friction caused by the greater surface area of the pipe. In other words on a <420 horse engine the 2 2.5 pipes are just for bragging rights because of the increased friction as compared to the single 3.5 pipe will more than likely not increase performance any significant amount. In all honesty, a realalistic street motor like mine or Apeirons will run perfectly fine with a single 3 inch pipe. I do plan on installing a custom 3 inch y-pipe with a step to 3.5 2 or 3 feet back which in THEORY will create a low pressure area and increase scavenging. I have no idea what to do for a muffler though. This is all in theory and I will have to try it out. I

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82 z28 350cid, vortec heads, comp 262h cam, Holley 600cfm carb, 2in twice pipes, MSD ignition, turbo 350 trans, 3.73 posi

Apeiron

That's true, too. We're talking about raw flow numbers, and not considering the effect on engine performance.

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1984 Silver Z28, 383 cid 4 bolt, ARP Fasteners, 2 1/2" Cowl Induction Hood, 600 cfm Performer Carburetor, Torker II Intake, Performer RPM Heads, XE268H Cam, Magnum Roller Tip Rockers, MSD6AL Ignition, Blaster2 Coil, Recurved HEI Distributor, 700R4, TransGo SK, 2.73:1 Limited Slip Dana 44, Terminator Headers, Custom 3" TIG Welded Stainless Exhaust, Flo-Pro Twister Muffler, 4 Wheel Disc Brakes, Oil Cooler

CamaroMike

But dual 2.5" exhaust has more cross sectional area than a sigle 3.5" pipe. (3.5" is what I would rather have). I really think the frictional losses are negligible.

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84 Camaro ZZ4 with HOT cam. 1.88 60' (12.98 @ 105MPH E.T.)
Other Mods: You name it and I have probably changed it.

91 Firebird 5.0L TBI

ASE Certified CHEVY DEALER Auto Tech
LIVE AND DIE BY THE ALMIGHTY BOWTIE!

[This message has been edited by CamaroMike (edited November 11, 2000).]

Grim Reaper

Really, the actual difference in cross-sectional area is quite small. A single 3.535"=2.5" duals or 2.475" duals=3.5" single for cross-sectional area. The .035" single or .025" dual difference is so small that I defy anyone to notice a difference between the two in terms of performance (if any). Definitely not worth it in terms of cost.

Terry Kennedy

iTrader: (5)

Why don't you just get a mufflex 4" and be done with it!! Yea yea it's expencive but not much more than what it will cost for duel's. Not to mention if you ever want to upgrade well you already have it.

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88 iroc 350 tpi, slp cam, headers 1 3/4, siamised runners, accel 58mm tb, comp 1.6 RTR, flowmaster 3" catback, th350 with holeshot 3000 stall, 3.27 9 bolt posi disk rear, Vortech S-trim Supercharger, K & N and 24# SVO injectors

91 RS t-top's, tbi 305, 15.7 in 1/4,
x911 kenwood cd player 2 12" solo-barics L7's,RS 5.2 pack in kick panels, impulse 6x9's, a ZX460 and more to come soon.

88IROCs

I am a fan of dual exhausts, but for the reasons stated by Glenn, I have decided to stay with singles on my IROCs.

The numbers stated above for cross-sectional area are incorrect, as they are based on the outside diameter of the tube. All pipe is sold based on it's outside diameter, which is irrelevent to it's flow characteristics. To obtain the correct cross-sectional area of the inside of the pipe, you need to subtract the pipe thickness first. So if you have a 3" pipe made from 16 ga pipe(0.0625"), the inside diameter becomes 2.875"(3 - (0.0625 + 0.0625).

Some comparo's:
OD/ID(")...............Inside Area(sq. in.)
2.5/2.375....................4.430
3.0/2.875....................6.492
3.5/3.375....................8.946

So in fact, the single 3.5" pipe has more inside area than two 2.5" pipes.

Something else to consider is the inside surface area, as measured along the inside walls.

Another comparo:
OD/ID...................Inside Surface
2.5/2.375..................7.461
3.0/2.875..................9.032
3.5/3.375.................10.601

As you can see, two 2.5" pipes would have approximately 40% more inside surface area then a single 3.5" pipe. The frictional losses from this extra area would(I believe) be minimal, because the gases along the wall flow very slowly(relative to the gases nearer the center). More important is the cooling effect caused by gases in contact with the inside walls. As these gases cool, they draw heat away from gases nearer to the center. As gas loses heat it loses velocity, the added inside surface of the dual pipes should cool the exhaust more than the single pipe and cause a corresponding loss in velocity.

<u>82Z,</u>

I am running a 3.5" system from the y-pipe back(Random 3.5" cat). I stole a page from Borla's book when it came to the muffler. After the axle, I y'ed out to two 2.75" pipes. One goes to the muffler(Borla ), and the other side goes to a cut-off plate(with a 1.25" hole). I can't tell you what difference(if any) the 3.5" system made over a 3" one, as I upgraded the whole system at once(from a TES/Flowmaster system to a SLP 1.75"/custom 3.5" system). I do know the entire upgrade made a difference in performance and aestetics(more growl and 4 intercooled tips out back).

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He who hesitates,... is lost!

[This message has been edited by 88IROCs (edited November 19, 2000).]

Grim Reaper

Who did the 3.5" system? Did you personally order it or just find an exhaust shop that either ordered it or fabricate it?

88IROCs

The only place I could find that sold complete 3.5" systems was Mufflex. However despite two phone calls, they would not sell me the system without mufflers. So it was left to me to do things myself(kind of). I bought u-bends and straight pipes from "Headers By Ed". I cut the pieces myself and had a friend weld them together. I cut the y-pipe that came with the SLP's(voided waranty #1), and widened the exit to 3.375" OD. I mated that to a 3.5" Random Tech cat(originally meant for a LS1: lots of cutting and voided warranty #2) which I got from EBay. I designed the y after the axle on KeyCad(a 1/2 decent 2-D cad program), then cut, bent and crushed pipe until it all fit and sealed. More welding. The Borla muffler and tips(driver's side only) were bought from CarParts.com for $229(US) after rebate. Unfortunately, the pipes from the muffler to the tips were too long. More cutting and welding(and voided warranty #3 ). I found the extra pipes and tips(for the passenger side cutout) from an ad in the local Auto Trader($175 and more cutting, welding and voiding).

$1200 and at least 4 voided warranties later: I have the exhaust I wanted. It has the sound and looks I was after(ahhh sweet vanity). It is doubtless overkill for my motor at present(Dyno-ed in June at 261 rwhp with the TES/Flowmaster combo), but should see me through planned upgrades for this winter(cam and heads definitely, a new custom chip probably and, lower manifold and runners possibly).

After that I'll probably sell the damn thing, as my collection of vehicles(three at present and negotiating for a '72 C25) and engine parts(just bought two 400 blocks) is starting to get away on me

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He who hesitates,... is lost!

84TransAm

HEY JAYG, nice to see someone else from the west end. What type of ride you got and where do u go to in Richmond??

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--Steve S-- 84 Trans Am 305 HO 5 speed, completely stock will soup up when i get some $$$$$$$$$$

jayg

Hey Steve, I have a 92 Formula 350, soon to be 383(maybe 406) and six-speed. I usually run autocrosses in Dinwiddie with VMSC and NASA-VA. I live near springfield and hungry rds. Talk to ya later. Jay