There were some posts a while about about people who had installed dual exhaust and loss torque at low RPM and the MPG went down.

Well I installed a dual exhaust system and the same thing happened to me.

I friend of mine told me to call Borla and order a Borla 2 1/2" venturi. I ordered one and had installed at my local shop. It worked great!! The venturi reduces your pipe from 2 1/2" to 2 1/8" for about a 3" inch section of your exhaust pipe.

Torque is back and MPG is at or above factory levels.

It cost for the Borla venturi is $24 and my shop charged me $20 to install.

Very simple install. Slice your pipe ahead of your muffler, slide in the venturi and weld the pipe.

 

So what you are saying is it was a waste of time and money for the larger tubing and mufflers

 

Yes, that was my interpretation as well. Put duals on, lose torque and horse power. Then, add a SMALLER diameter pipe, similar to what you had BEFORE and all is well. Yes, let me rush out and do this mod.

However, I will admit for those who have already had this exhaust installed and experienced the loss of torque and horse power, they can spend a minimal amount and regain it. That's a good thing and I think that was the original poster's intent.

 

I remember as a young lad, my dad was modifying his old cj5, and told me that as you free up your airflow, it moves the hp and torque curve higher up in the rpm range. Same still holds true with these newer motors. With the poor power curve we have in the jk already, moving it up even futher would definitely not help.

 

Wouldnt be even cheaper to just take a BFH and put about a 1/4 dent in my exhaust? It would accomplish the same thing, we are not talking about a huge amount of restriction here?

 

RedneckJeep -- thanks for watching my back.

That is exactly what I am saying. The dual exhaust with huge tips.... looks and sounds awesome.

I had a 2005 Rubicon and added an intake and a Flowmaster muffler...sounded great and performance improved.

When I got my 2008 Rubicon Unlimited, I took the same approach. First the intake --looked great, sounded great and helped performance and MPG.

Then I installed a dual exhaust system based on a Flowmaster 50 series "Camaro" muffler. To my surprise and disappointment, low end torque was affected and MPG went down.

A friend told me about the Borla venturi......$44 dollars later....all is fine.

The low end torque issues appear to be gone and my MPG is now better than stock.

I get a ton of compliments on the looks and sound!

 

I wont go so far as to say this doesnt make sense... BUT, one way for ANY engine to increase performance is by making the flow of "air" more efficient. Polishing intakes, less restrictive filters, open exhausts/headers/mufflers, duals, no cat... It is this way on EVERY engine from motorcycles to quads to snowmobiles to hotrods, to muscle cars... Why would JUST the JK be different. It goes against all conventional wisdom to think that a high flow intake and exhaust would decrease HP and TQ. Perhaps its in the programming of the computer and the fuel injection? Perhaps that is why Jet makes a stage 2 chip? Something seem strange with this though...

 

I see what you are saying but I think it all has to do with back pressure. There is a sweet spot for back pressure. When you lose too much, you start to lose power. I would assume the same the other way (too much restriction, lose power). With the smaller minivan engines we have, we are getting too much exhaust flow for the engine to benefit. Now on big blocks and what have you, 3" exhaust will do wonders, especially if you have the polished intakes, less restrictive filters, etc. I could be wrong so..

 

RubiJK, That was my thinking when I installed the intake and exhaust. The muffler shop that did the install does a "ton" of custom HP systems. They had only heard of this situation on some diesel pick ups.

Trust me.... the low end torque was affected and I lost at least 2 MPG.

It has now been corrected and the MPG is better than my stock intake and muffler.

 

all show no go is the way to go. jk.