Exhaust cutout OK on pentastar? - JeepForum.com
 
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post #1 of 9 Old 05-20-2015, 10:58 PM Thread Starter
XJMcPhee
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Exhaust cutout OK on pentastar?

Hello all,

After recently dropping the muffler off the JK, I've grown to love the rasp and loudness of this new powerplant. It makes running through the gears far more enjoyable. The thing is, its ok most of the time, but I still liked how quiet the JK could be.

I ran a cutout on my '99 XJ with the 4.0, and never had any problems. However, the straight six was a far simpler engine.

So here are a few questions to anyone willing to chime in:

1. Does less back pressure hurt the engine? Does it effect the VVT or other high tech parts?

2. Will opening the cutout while the engine is running effect anything computer-wise?

3. Is there any advice you could give on exhaust setups you have or are currently running?

Thanks, Jake.

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post #2 of 9 Old 05-21-2015, 12:35 AM
smcutter
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I'm looking forward to jwmbishop's response!

For the record, I'm probably judging you for your bad grammar or spelling.
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post #3 of 9 Old 05-21-2015, 09:00 AM
jwmbishop
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1. Does less back pressure hurt the engine? Does it effect the VVT or other high tech parts?
Less reversion (or more) CHANGES the volumetric efficiency of the engine. Hurt or help is relevant only to the tuning target. Not sure where in the line you are adding the cutout - that has an effect as well. The VVT is fairly static - that is changing the exhaust wont change the actual valve timing - BUT that change could be outside of best performance for the timing - in otherwords undoing the VE gain that the valve timing is adding by lowering the VE with less exhaust reversion ... its my opinion that the majority of the low RPM (where we need it crawling) of the 3.6 is FROM the VVT - so believe we should not be counter to that. In other words - long pipe is probably best.

An open, short pipe exhaust tends to decrease torque production (by reducing VE) at the lower range of RPM - 1800-2500 - and increase it at 3000-4000 (or higher if cam and valve timing is aggressive). So to really USE that change to your advantage you would also want to be deep in gears - to get into the meat of the new torque curve with less throttle.

2. Will opening the cutout while the engine is running effect anything computer-wise?
It would effect just about everything computer wise - you are changing the VE at just every point of RPM to throttle combination possible - so thus changing the total air pumped at any given rpm/load/throttle combination and the total amount of fuel to properly burn in that air flow! However it wont break anything. It compensates.
I have a "simple" computer - MSD Atomic EFI with timing control - on my 387" bowtie. When I open the electric cutouts - it immediately stumbles, lopes and loads down - briefly until the map learning recovers (and while its doing that the idle adjustment count drops to zero and the injector cycles increase once settled the idle count runs 25-29). The first couple times I romp down on it it falls off at 1200 RPM briefly then recovers well - and again "relearns". Open pipe will pull the tires loose at 35% throttle from any RPM/speed. I am glad I did not run the cut outs when it was on the 750Holley - it typically takes all of a good day to retune a Holley and plot weather compensation marks. Going the other way it idles 500 RPM higher with a LOT more idle adjustment counts (like 45-50) for about a minute and once settled in (idle count goes to the normal 11-15) and takes about 45% throttle to get em protesting once above 30MPH or 1900RPM.
This engine has a much higher VE to start with than the JK - so the JK the difference should be less drastic - and the recovery far faster as the computer system is much more refined.

3. Is there any advice you could give on exhaust setups you have or are currently running?
Quicktime makes the best electrics cost/performance wise. For the high RPM gain/low rpm sacrifice - you really want to have em as close to the headers/manifold as possible BUT MUST be at LEAST 18" of pipe downstream of the O2 bung (unless you have deep pockets for o2 sensor replacement - they will run hot at less). If you want exhaust note more than an alteration of the torque curve - stick one cutout just in front of your muffler - maintaining the tail pipe length/diameter has more effect on the reversion than removing the muffler.

OR just go with the dynomax VT - its quiet with a nice tone until you drop your foot then it goes to a open straight through path and the note THAT brings. (Haven't heard one on the JK yet - but LOVE em on the bowtie! - that is the setup I am going to - but that is not budgeted until 2017 unless I rip my OEM up on the trail). They're kinda like having cutouts only when you really need the gains an open pipe brings (45% to Wide Open Throttle) and uncut when THATS better (idle, light throttle cruising and deceleration).

And either way - if you go electric - use a pipe post cutout. Dumping straight out the valve runs that valve hotter and it will corrode the seats faster. I NOW have em sandwiched between the y pipe and a four inch turn down... I had to rebuild em after two years as the valve started leaking.

J Wm Bishop EA, ASADE
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post #4 of 9 Old 05-23-2015, 08:13 PM
Kiwi Jeeper
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Dynomax VT seems to be an interesting proposition. Has anyone used it on a JK 3.6 Pentastar yet?

'12 JK Rubicon V6 3.6L
'15 KL Trailhawk V6 3.2L
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post #5 of 9 Old 05-23-2015, 09:43 PM
rich6700
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I run the dynamax vt exhaust system. What I like most about the system is no exhaust drone.
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post #6 of 9 Old 05-23-2015, 11:55 PM
Kiwi Jeeper
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How much louder is it on take off and normal driving? WoT, I understand it will be louder.

'12 JK Rubicon

'12 JK Rubicon V6 3.6L
'15 KL Trailhawk V6 3.2L
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post #7 of 9 Old 05-24-2015, 01:35 PM
rich6700
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It not really loud at all. It has a nice tone to it. Go on dynamax web site for a sound clip.
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post #8 of 9 Old 05-24-2015, 07:57 PM Thread Starter
XJMcPhee
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Quote:
Originally Posted by jwmbishop View Post
1. Does less back pressure hurt the engine? Does it effect the VVT or other high tech parts?
Less reversion (or more) CHANGES the volumetric efficiency of the engine. Hurt or help is relevant only to the tuning target. Not sure where in the line you are adding the cutout - that has an effect as well. The VVT is fairly static - that is changing the exhaust wont change the actual valve timing - BUT that change could be outside of best performance for the timing - in otherwords undoing the VE gain that the valve timing is adding by lowering the VE with less exhaust reversion ... its my opinion that the majority of the low RPM (where we need it crawling) of the 3.6 is FROM the VVT - so believe we should not be counter to that. In other words - long pipe is probably best.

An open, short pipe exhaust tends to decrease torque production (by reducing VE) at the lower range of RPM - 1800-2500 - and increase it at 3000-4000 (or higher if cam and valve timing is aggressive). So to really USE that change to your advantage you would also want to be deep in gears - to get into the meat of the new torque curve with less throttle.

2. Will opening the cutout while the engine is running effect anything computer-wise?
It would effect just about everything computer wise - you are changing the VE at just every point of RPM to throttle combination possible - so thus changing the total air pumped at any given rpm/load/throttle combination and the total amount of fuel to properly burn in that air flow! However it wont break anything. It compensates.
I have a "simple" computer - MSD Atomic EFI with timing control - on my 387" bowtie. When I open the electric cutouts - it immediately stumbles, lopes and loads down - briefly until the map learning recovers (and while its doing that the idle adjustment count drops to zero and the injector cycles increase once settled the idle count runs 25-29). The first couple times I romp down on it it falls off at 1200 RPM briefly then recovers well - and again "relearns". Open pipe will pull the tires loose at 35% throttle from any RPM/speed. I am glad I did not run the cut outs when it was on the 750Holley - it typically takes all of a good day to retune a Holley and plot weather compensation marks. Going the other way it idles 500 RPM higher with a LOT more idle adjustment counts (like 45-50) for about a minute and once settled in (idle count goes to the normal 11-15) and takes about 45% throttle to get em protesting once above 30MPH or 1900RPM.
This engine has a much higher VE to start with than the JK - so the JK the difference should be less drastic - and the recovery far faster as the computer system is much more refined.

3. Is there any advice you could give on exhaust setups you have or are currently running?
Quicktime makes the best electrics cost/performance wise. For the high RPM gain/low rpm sacrifice - you really want to have em as close to the headers/manifold as possible BUT MUST be at LEAST 18" of pipe downstream of the O2 bung (unless you have deep pockets for o2 sensor replacement - they will run hot at less). If you want exhaust note more than an alteration of the torque curve - stick one cutout just in front of your muffler - maintaining the tail pipe length/diameter has more effect on the reversion than removing the muffler.

OR just go with the dynomax VT - its quiet with a nice tone until you drop your foot then it goes to a open straight through path and the note THAT brings. (Haven't heard one on the JK yet - but LOVE em on the bowtie! - that is the setup I am going to - but that is not budgeted until 2017 unless I rip my OEM up on the trail). They're kinda like having cutouts only when you really need the gains an open pipe brings (45% to Wide Open Throttle) and uncut when THATS better (idle, light throttle cruising and deceleration).

And either way - if you go electric - use a pipe post cutout. Dumping straight out the valve runs that valve hotter and it will corrode the seats faster. I NOW have em sandwiched between the y pipe and a four inch turn down... I had to rebuild em after two years as the valve started leaking.
Thank you for such a detailed response

You really know your stuff. After taking a look under the JK, I couldn't find a segment straight enough to mount a cutout, so it looks like there will need to be some modification near the resonator. The only oxygen sensors I was able to find were at the exit of the head, mounted in the manifold before the cats. I've heard good things about quicktime cutouts. I hear a lot of popping and what sounds like backfire. Is the Jeep burning rich?
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post #9 of 9 Old 05-25-2015, 08:51 AM
jwmbishop
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Quote:
Originally Posted by XJMcPhee View Post
I hear a lot of popping and what sounds like backfire. Is the Jeep burning rich?
A negative exhaust leak (Air siphoning into the exhaust stream) will do that. Lets anything unburned light back off...

J Wm Bishop EA, ASADE
The wagon should, of course, be as light as possible, but strength should not be sacrificed to lightness, for on any but the regularly traveled roads, the wagon will get many a
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