Hemi people with 3 inch exhaust please.. - Jeep Commander Forums: Jeep Commander Forum
Exhaust Questions or advice about exhaust systems? This is the place...

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post #1 of 28 (permalink) Old 03-16-2018, 12:10 PM Thread Starter
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Hemi people with 3 inch exhaust please..

So, getting ready to do exhaust and am torn to do exhaust but am getting conflicting info on 3 inch exhaust. Everyone on the Challenger and Charger forums state that the Hemi actually loses torque with three inch until you crest 450 horespower.

So, those here who have 3 inch on the Hemi, what system dis you do and what gains or losses did you see? I am hoping for a bit more power and slightly better mpg.
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post #2 of 28 (permalink) Old 03-16-2018, 04:00 PM
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My $.02.... it will make a lot more noise, probably only slightly if at all help power at the very top of the rpm band and hurt power at low mid and mid-high rpm.

If you're looking for a little extra power for a few hundred go with a chip or fastman throttle body.

I wouldn't go 3" until either heads/cam or forced induction.

Check what the SRT GC is running....

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post #3 of 28 (permalink) Old 03-16-2018, 06:33 PM Thread Starter
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I am running the K&N cold air intake with the Traildash 2 tuner already. Waiting to hear back from a member on what SRT headers will work, and then do all the exhaust at the same time, whether it is 2.5 or 3 inch remains to be seen...just waiting to see what others have experienced.
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post #4 of 28 (permalink) Old 03-16-2018, 10:54 PM
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Honestly, I seriously doubt that going to a 3" exhaust will hurt anything. The cats are the most restrictive part of the exhaust anyway, so some larger pipe won't affect airflow much at all. The stock system is also around 2.75", so you definitely don't want to step down to 2.5".

The myth that a larger diameter exhaust will negatively affect low end torque is largely unfounded. Some people say that backpressure is important to maintain low end torque, but that's simply not true. Backpressure will always hurt performance.

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post #5 of 28 (permalink) Old 03-17-2018, 12:12 AM Thread Starter
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People try explaining it as a lack of back pressure, when in effect, what they are speaking about is exhaust velocity. Properly sized exhaust actually helps velocity by almost helping to pull the exhaust from the engine..and overly large exhaust can be a detriment because then the engine has to push it all without the help of the pull effect. I have had the issue prior and know many others that also encountered going too big too soon. If I were forced induction like my turbo car, bigger is better as it helps decrease spool up time which is why I run full three inch from front to rear and no cat on my 4 banger. From what I have read, the stock is 2.5 on the Hemi, but could be wrong.
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post #6 of 28 (permalink) Old 03-17-2018, 02:14 AM
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https://www.google.com/amp/www.hotro...tti-pipes/amp/

https://youtu.be/_PVXvHkr-Vs

http://www.trucktrend.com/how-to/eng...haust-headers/

3 inch is better

But do you need it at your power level? Not really. Unless you plan to make more it isnt really necessary.

Last edited by luckyse7ens; 03-17-2018 at 02:21 AM.
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post #7 of 28 (permalink) Old 03-17-2018, 05:45 PM
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It's true that going to too large of an exhaust will hurt the scavenging effect, but the Hemi is definitely at the horsepower range where that's not going to happen with 3".

The stock exhaust is slightly larger than 2.5", though I haven't measured exactly how much. I've heard 2.75", but I'm not positive on that number.

Many aftermarket systems go for a 3" exhaust, including Gibson, aFe, and MBRP. Magnaflow goes for 2.5", and I haven't found any specs for the Borla/Mopar system. So you can go either way, but if you're adding some extra airflow with headers and a CAI anyway, I don't see any reason not to move up to 3".

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post #8 of 28 (permalink) Old 03-27-2018, 10:41 PM
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I plan on installing a set of heddman stainless headers that I purchased last fall. I was trying to find a cat section that is 3" pipe, but haven't found anyone that specifically lists it. The unfortunate problem with the catback systems that are 3" is that they quality is not as good as the Magnaflow.
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post #9 of 28 (permalink) Old 03-27-2018, 10:56 PM
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Quote:
Originally Posted by Flex View Post
I plan on installing a set of heddman stainless headers that I purchased last fall. I was trying to find a cat section that is 3" pipe, but haven't found anyone that specifically lists it. The unfortunate problem with the catback systems that are 3" is that they quality is not as good as the Magnaflow.
I would beg to differ with you there Flex;

Have you checked out aFe's cat-back systems?

aFe: Advanced FLOW Engineering | Engineered Adrenaline

I have aFe's 3 inch stainless steel cat-back system for the 4.7L; I've had it for a couple of years now and it's legit.


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post #10 of 28 (permalink) Old 03-28-2018, 12:14 AM
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I have the MBRP exhaust and it, along with a K&N CAI and Superchips TD2 on the 87 octane tune, has not only improved power, but fuel economy. I am getting between 15-16 city and 20-21 highway over at least 5-6 fill ups and constantly crunching numbers. Very little to no drone when MDS kicks in depending on speed. Also, it sounds AWESOME! I would love to put some headers on mine, especially since I have the dreaded broken manifold bolts, but I'm not ready to try to find something that's going to require me changing everything else. Only reason I haven't fixed the bolts yet is because it seals itself up after about 30 seconds of running. It will definitely be happening once it warms up here!

Last edited by Colt; 03-28-2018 at 12:20 AM. Reason: Clarification
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