So, which would make more horsepower/torque in both peak and 'under the curve' in a typical NA smallblock? Right now I'm leaning towards a single 3" in my application, but if need be I could squeeze in a dual 2.5" exhaust. Thanks.

Depends. Are you talking about a 11:1, big cam, NA small block, or a stock 5.2 Magnum? Sounds like you're dealing with a truck since you're even considering a single exhaust. BUT the dual 2.5" will give you more HP potential over a single 3", it definitely won't hurt performance, even in a mild application. So if you can squeeze the duals in I say do it

the dual system will give you the best of both worlds. your pipes will be small enough to keep velocity/scavenging up at lower RPM for better torque, but by having 2 pipes, you can still get high RPM flow out of them.

2 2.5" pipes have a lot more cross sectional area and flow potential than a single 3" pipe.


but, for a truck (assuming that's what this is on since your asking about duals vs single) it can sometimes be tricky routing a true dual system, especially if it's a newer truck where the tank hangs inside the frame rail, in front of the rear axle, forcing your driver side exhaust to cross over to the passenger side.

how "big" is your engine? a single 3" system will still support a TON of Hp. my engine is only lightly modded, and put down 300 hp to the wheels, I do not feel like my exhaust is the limiting factor, or anywhere near choking me down yet. my exhaust will flow more than my heads will

The motor is about 331 cubic inches. The problem is that the spare tire well drops down in the trunk, and the gas tank occupies the space beside it. I could put dual 2.5's that exit in front of the rear fenderwells, with a couple 'bullet' style mufflers side by side. I know for sure a 3" pipe will route the like original exhaust did, and I can use a sideways mounted flowmaster 3" in/out muffler. Shooting for about 400hp @ the flywheel.

What's the vehicle? That might get you some better answers by folks who've been there, done that...

with properly sized headers that go into a single 3" system, you'll easilly make 400 crank hp. My Dakota has the worst headers on the market on them. $120 "shortys" that bolt to the factory down pipes...they are marginally better than the stock manifolds, especially since they are about as compact as the factory manifolds. it then flow through some 2" pipe that dumps into a 3" single run that's about 5 feet long.

I put 295 hp and 330 tq to the wheels, so that's up around the 375-400 ball-park on crank hp, and Like I said, my heads are what's holding me back now, not my exhaust.

I'll give you the numbers to compare. A set of dual 2 1/2 pipes have a combined surface area of 9.8175 inches, while a single 3 inch pipe has a surface area of 7.0686 inches. Thats quite a bit of difference.

Thanks guys, and thanks 70Cuda383 for the real world info.Headers are Sanderson Shorty Headers which will utillize a 3" Y-pipe into a 3" single with Hi-flow cat & muffler to the back. I think thats the route I'll go. Thanks again guys.

BTW, what vehicle is this? Just out of curiosity, always like to hear what people are working on.

Not sure if this would help in your application, but I put dual 2-1/4" from the manifolds into a dual inlet / single 3" out IMCO (Flomaster type) muffler on my '86 Ram W150. It sounds really nice too.

I think the reason is becouse fluids follow a different set of rules then gasses in terms of flow. By that I mean fluids produce more friction then gasses.

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I think you mean cross sectional area not surface area. Those duals have a ton more surface area in the pipes to create friction witch resists flow. Also because the cross section is so much higher in the duals it loses velocity. If you had two exhaust systems of equal cross section then the single would make more power. The single would also be more balanced and scavenge better, sort of like an X pipe but it don't seperate back appart. A single exhaust with an equal cross section will have more velocity, flow and HP. Your examples are not equal though

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This is a very reasonable explanation. I would like to see a dyno test between a dual 2.5" vs a 3" system, but I cant find any examples.

Car in question is a 73 Vega, but I'm sure this theory has universal applications

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