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2 into 1 VS Dual Exhaust: A Motorcycle Exhaust Comparison

Posted on 19 Sep 2018 in Powersports | 13 comments

We wish that we could offer you a simple answer. We know you’re wondering what kind of exhaust is best for your Harley.  You’re probably hoping that we’ll say: “Here, get this! It’s the best on the market, it’ll look and work great and it’s incredibly cheap!” …If only it were that easy. But, life is never quite that simple, and that’s not something we can offer. However, we can offer you what the pros and cons are of the available options. The options that we’d like to cover are slip on exhausts (mufflers), 2 into 1 exhaust systems, and true dual exhaust systems.

Slip On Exhausts

One thing that a lot of people like to hear is that they don’t actually have to swap out the whole exhaust system. Slip on exhausts are a quick and easy option. When you buy a slip on exhaust, you are essentially just replacing the muffler section of your exhaust system. It’s fast, simple, and cheap, but it will offer less results, both aesthetically and performance-wise. It can change the sound of your bike, it can change the look, but changes to performance will be minimal at best.

Full Exhaust Systems

If you are looking to make a more noticeable change, then you should consider buying a full exhaust system. For the V-twin in your Harley, you have two options and one choice before you can go any further. That would be the choice between 2 into 1 exhaust and true dual.

2 Into 1 Exhaust

With 2 into 1 exhaust systems, the two exhaust pipes combine to emit fumes through one muffler.  Sound will obviously vary depending on the pipes and muffler, but these systems are generally characterized as being higher pitched compared to true duals. These pipes are also typically lighter, meaning you can shed a couple pounds off your bike, especially if you buy pipes made with lighter materials like carbon fiber and aluminum. They generally perform the best at low RPMs, which is where most riders will notice an increase in performance.  With that, though, a 2 into 1 system may hinder top end performance.

This design also aids in one other special advantage, a benefit referred to as scavenging. When the first cylinder sends a pulse of exhaust shooting down the pipe, it helps to pull oxygen into the second cylinder. Then, the second cylinder helps the first in the same way, and the cycle goes on.  2 into 1 exhausts can offer a better balanced airflow in your engine which can result in more torque and a smoother idle.

True Dual Exhaust

Where 2 into 1 pipes are more common on smaller, more sporty bikes, true duals are more common on baggers and touring bikes. They give your bike a more balanced look thanks to their two separate mufflers, each emitting exhaust from one of the cylinders. They perform best at high RPMs, unlike the 2 into 1 systems, which make them a great choice for those interested in top-end performance.  They also provide that stereotypical deep Harley rumble. And if you want the pipes to be louder, you can remove the baffles.  But because this removes back pressure, it might hinder performance.

We know what you’re thinking: “What about that wonderful scavenging phenomenon, I want that too!”. Well, you’re in luck.  Dual exhaust systems can actually be bought in a configuration that is best described as 2 into 1 into 2. Basically, the pipes combine for a short time before splitting again, meaning you can have that classic true dual look and the increased performance brought on by the scavenging effect. 

A Couple Things to Remember

It’s important to remember that while aftermarket exhausts can make improvements in horsepower, torque, and acceleration, results will vary depending on your current exhaust and the exhaust that you switch to. It’s important to look closely to see what differences exist between your new and current pipes. Attributes like shape, diameter, header design and scavenging ability are all worth noting as you consider the amount of improvement you might actually see from your new exhaust.

When you make the change in your motorcycle’s exhaust, the work doesn’t end there. If you’ve succeeded in increasing the airflow out of your cylinders, then the fuel/air ratio should be changed as well. Without a change, your engine might run lean, and you’ll need to either re-jet or remap your motorcycle. This probably isn’t necessary if you only changed the muffler, but the bigger the change, the more important it will be.

Conclusion

You might have been hoping for a more straightforward answer, but a straightforward answer simply doesn’t exist. But, on the bright side, while there is no right answer, there is also no wrong answer. So, do your research, weigh the pros and cons, and rest easy knowing that your new exhaust will work as sweet as it sounds. If you have any tips for your fellow rider, don’t be afraid to make them known in the comments below. 

Check Out All Motorcycle Exhausts

13 comments

  1. Sam / September 21st, 2018 7:24

    Hey Tom,
    Nice detailing. I am thinking of changing my exhaust system. Somehow redirected to this blog. I am interested about 2 into 1 exhaust. By the way, do you sell any or recommend any specific product ?

    Reply
    • Dave P / July 20th, 2019 2:30

      Thunder header (in my opinion) makes the best 2 into 1 exhaust on the market. It has a great combination of power and sound For Harley Davidson motorcycles. I have run one on my last 5 bikes ranging from 1989 to 2005 in several models, from superglides, Dynas, FLH standards to Road Glides. I’ve been highly satisfied with the extra performance and great sound it has produced in all of the models I mentioned and I highly recommend the Thunder Header.

      Reply
  2. Chris / March 23rd, 2019 2:19

    Backpressure does not cause scavenging. Scavenging is caused by the free flow of exhaust gases through the pipes – backpressure impedes exhaust gas flow.

    Reply
    • Joe D'Avello / May 9th, 2019 5:13

      Is there anyplace in the article that indicates that back pressure causes scavenging? I think Tom gave a good, basic explanation.

      Reply
  3. Mike / April 16th, 2019 11:09

    Scavenging is an application of Bernoulli’s principle of fluid dynamics. A 2-1 system doesn’t increase scavenging via increased back-pressure, but by further lowering pressure in the exhaust. As a the exhaust “pulse” moves past the opening of the second pipe, it pulls air out of the second pipe. This is because air that is moving at a rate faster than surrounding air has less pressure, further lowering the pressure in the second pipe and increasing airflow.

    Reply
  4. tim north / May 15th, 2019 17:22

    If you haven’t bought new exhaust yet, stay away from D&D. Not because of the price but because the rear pipe runs forward and then back. Puts out a lot of heat. I have even ceramic coated the headers with no change in heat. Heat shields do not work. My girl wont ride because of the heat. LOL I might keep em.

    Reply
    • J grey / July 5th, 2019 22:33

      MGTOW !!!!!!!!

      Reply
  5. Gary / June 27th, 2019 12:47

    I PUT ON D&D FAT KATS 2 INTO 1 AND LOVE THEM. I HAVE A 2014 CVO 110 HERITAGE DELUX. THEY LOOK AND SOUND GREAT. MY WIFE DOESN’T HAVE A PROBLEM WITH HEAT? I HAVE FULL HEAT SHIELDS.

    Reply
  6. Mark Nelson / July 10th, 2019 18:34

    I put a Freedom Accessories 2into1 on my 08 Road King. Added Woods racing cams, Thundermax tuner, and a Arlen Ness Big Sucker 2 intake. It definitely made a huge difference in torque and low end quickness. Now I don’t have to worry about passing someone up a hill.

    Reply
  7. Louie / August 3rd, 2019 4:50

    Just got 2019 street glide special 114 any suggestions on which system would give the most power increase ?

    Reply
    • Tom / September 22nd, 2019 9:51

      I have a 2019 Street Glide Special M8-114 Engine, with Screamin Eagle Stage-1 (Street Cannons slip-on mufflers, SE Ventilator Air Cleaner & Stage-1 OE Tune), and Screamin Eagle Stage-2 Torque Cam (and Stage-2 OE Tune). Needless to say, the bike runs well and has great torque (especially low-to-mid range) while also having good upper RPM HP. I’m looking at several exhaust systems myself, including the Screamin Eagle Hi-Flow full exhaust system that has the “Cats” relocated to back into the mufflers (system is $1400-1500 from Harley). My preference is a 2-into-1-into-2 type system. Looking at several now from S&S, Bassani, V&H, Thunderheader and others. In the past, I’ve had Bassani Systems which were very high quality and performed well. I know lots of people that are really into Thunderheader products. I haven’t decided yet which direction to go yet, but am leaning toward the HD-SE system or the S&S system at this time. I’ll post an update if and when I have something more to share, pass on.

      Reply
  8. Rodger / August 4th, 2019 15:55

    When I modded my Road King 88 to 95in I went with V&H pro pipe, K&N Intake, stage 2 cams and ECM flash, V&H fuelpak and cleaned up the head porting. Being able to tune the exhaust and fuel pak numbers was a big factor in setting this motor up. Wanted the usable power curve in the low and midrange for 2 up and touring. In my case I wanted the torque curve and the HP curve similar. Having torque and hp from 1200 to 4000 rpm covers my needs with the motor. Now for gearing…..that jump between 2nd and 3rd on mountain roads..

    Reply
  9. Ben krom / August 17th, 2019 20:50

    On stock exhaust there’s a bulkhead between front and rear baffles in the mufflers. I hole saw ed the bulkhead out and that woke the bike right up and still had a respectable 92 disciple. Bike is fuel injection so no rejeting needed.

    Reply

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