Dual 2.5" vs Single 3" exhaust

If you can still find them my friend had a vega with headers and sidepipes made for a vega . he bought it as a kit sounded good to i beleive it was 2 and a half inch pipes ,

Here are my thoughts and theories on this.

Don't forget that with all the cylinders tied together, any reversion is going to be worse, in other words if you install a relatively restrictive chambered style muffler, the feedback (reversion) from the single exhaust that tends to restrict the intake will be worse than if you have an isolated, dual-pipe system because now you have all 8 cylinders acting on the one that happens to be exhausting at the time. A higher flowing muffler will likely not hurt as much but the location of the junction may be more important.

Granted crossovers tend to tie both sides together but not in the same way at the manifolds as a larger single large pipe.

I think if you keep the pipes split all the way to the back then combine them, it may act differently than if you tie them together at the front and run one single all the way. Plus with dual mufflers you can lower (quiet) the exhaust sound level even more due to the added volume of the two muffler bodies. Always remember, you want volume in the mufflers to get the most sound control. If you have multiple pipes feeding/traversing through a muffler, that takes away volume you can depend on for noise/sound absorption.

Then why not run 8 single exhaust systems

Headers are takeing advantage of tuneing, collectors are also needed for this tuneing, the rest of the exhaust system is primarily dealing with just flow. If you understand what is really going on with a tuned length header/collector you will understand why they are run together... the piston forces exhaust out in one big "lump" really fast, this "lump" goes flying past the end of the pipe of the next pipe in the firing order, as that mass of exhaust flies past the other pipe the inertia it has pulls a vaccume on the adjacent pipe, if that pipe is just about to dump the exhaust then the first lump will help pull it out of that pipe, it is the venturi effect. In a normal V8 there is an un even firing order from side to side so you get two lumps really close together, this is where the X pipe comes in, or the old fashioned less effective H pipe. It allows that big supper lump to bleed from one side to the other and allows scavenging from one side to the other, now the ultimate exhaust would be to join the two pipes into one at just the right time for the two big lumps to help pull each other out the entire length of the pipe in a single exhaust configuration. After a certain point in the system there is gonna be no significant tuneing effect and simply raw flow numbers are gonna make the most HP and a single pipe with the same cross section as a dual system will flow more, then it becomes a matter of packageing the system and most cars will have a hard time fitting a single 3.5 inch pipe with out dragging the ground or bending it too tight, this is the only reason dual exhaust became popular for high power machines. A dual system has more heat loss due to surface area than an equal cross section single and heat loss costs HP also.


Now there is gonna come a point where simply raw flow rules supreme even over scavenging but that is top fuel territory (not even nascar or F1) where 8 single exhaust pipes rule but even then they still tune with the primary lengths instead of just running the exhaust right out of the head.

Here is how scavenging works

http://www.youtube.com/watch?v=pzhM9e5ZpbA

See how he lets his breath pull more air to fill the bag with one breath instead of the 6 or 8 individual breaths, that is like how one exhaust "breath" from one cylinder pulls on the next one to better empty the cylinder and if you can get the cylinder more empty then you can pull in more A/F on the intake stroke and make more power. The pipes by them self would never make as much power if they were not able to pull their neighbors exhaust and by running duals you are limiting there ability to help there neighbor.

For sure HotRod Dave you are right on about the tuning but in fact, at the muffler, heat loss gains power. Heat loss brings with it a loss of gas volume and that helps the muffler work a bit easier and this is what you want for good silencing.

Obviously, we are both talking about different ends of the system and I agree about scavenging. Headers and good exhaust systems depend upon it to get fresh air in after getting exhausted air out.

If any others want to see what new cars have on them to get good silencing and performance, check out new Honda and Toyotas. These have split pipes after the axle and run two very large volume mufflers.

Exhaust is such cool science.
Tom

HotRodDave, thanks for the detailed explanation.. awesome


bboogieart, yup.. the plan has changed. I'm going with a 5.3L from a 2010 Chevy Silverado


So theres no way to run a pipe down the drivers side, the spare tire well is in the way, and to even run 2" pipe both down the pass side, up over the axle to the rear bumper w/ 2 mufflers under the floor pan there is just no way without it looking like a total hack job. I think a single 3" is the way to go for my application.

Check into the Stealth Bomber articles in Mopar Muscle mid 2004. I planned to do the same on my Duster using an x-pipe instead of the Y so I could use an electric cutout on the extra exit but I gor a smokin' deal on a TTI exhaust. Dyno numbers were included. If you are interested I probably still have the mags. IIRC there was only .5" backpressure.
Ron

I can't find that article... would you have a link?

another thing to think about. per unit length and gage of tubing, dual 2.5" will have 67% more material than single 3" while giving only 39% more cross sectional area. due to boundary layer effects with the greater surface area of the dual pipes, you won't see all 38% flow improvement, probably closer to 30%. so you'll be adding a fair amount of additional weight for not as much flow improvement.

Could you fit a 3.5" pipe in there? Your surface area of a 3.5" pipe is 9.621", while, as previously mentioned, the dual 2.5" is 9.817", so basically a wash as as far as that goes, but you have the advantages of a single pipe (less boundry layer, weight, etc). 3.5" pipe is a little tougher to find but not impossible.

The single exhaust needs to be looked at harder, but its not something that would sell well, so the magazines wont talk about it

Our 99 Dakota has a single exhaust, and it sounds like a dual x-pipe in the upper RPM range. Our local exhaust guy built it and told us 5 or 6 years ago the x-pipe thing was just a glorified single exhaust.

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runinonmt, many many thanks my friend!

I'm going to a semi supply store to buy a 3.5" elbow.. IF that fits over the axle, I'm good to go.


Also I agree that the single exhaust for performance apps has to be looked into more.

single exhaust can sound good!

http://youtu.be/u4YXAdjfO6o

Yeah a single exhaust can sound every bit as good as a dual imo. I was talking with a guy last night who owns a 1986 Iroc (350cid). He was telling me that originally he had dual 2" exhaust w/ cherry bomb mufflers on it for a few years. He then went a built a single 3.5" for it ( 3.5 y-pipe, 3.5 main pipe, flowmaster 40 series muffler 3.5 in/out) and told me that the car went 6/10ths faster in the quarter mile (from 13.60 to 13.00)! I had no idea a dual 2" with cherry bombs were THAT restrictive!

I patterned the system on my 5th avenue similar to the stealth bomber. I used "cheapie" summit 1 5/8" headers, to 3" 409 stainless collectors, then mandrel bent 409 stainless to a stainless magnaflow dual 3" in/3" out Y, a 3" stainless flex coupler to a dynomax 16669 3"in/out/internal "large body hemi" turbo muffler (10x5x22" case), 3" 409 mandrel bent over the axle, a 3" glass pack in the tailpipe (really helped with interior resonance/drone) to a gibson 3" turn down tip. sounded very nice...here's a clip I put on youtube before I sold it

http://www.youtube.com/watch?v=QvZ2lqPzogQ

Awesome soound there buddy

My 3.5" elbow is on backorder, time to figure out the rest

Pssst....you know, if you want to run AN fittings on your heater hoses, you can pull out the factory stainless bungs (they are only pressed in) and tap the holes out to NPT. (they are 2 different sized holes)

Thanks for the tip, what do you do on the core side?

I ran AN hose from the water pump to the firewall, then an AN bulkhead fitting, and then used AN pushlock style fittings under the dash side to heater hose to run to the aftermarket a/c-heater box.

WOW! Thats a clean install! I was just gonna flip the core 180 degrees and use the rubber hoses. BUT I'm digging your setup.

Thanks

I took my time and tried to make it pretty tidy at least. I ran the heater hoses under the intake...not really something you can do with the factory F body intake of course.

I ran them up under the A/C compressor then through the bracket. You can kinda see in this pic