Dyno Tests - Cherokee SRT8 Forum
WK1 General Performance Discussion If you are planning on modifying your Jeep Grand Cherokee SRT8 with a performance enhancing modification, this is the section to post about it.

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post #1 of 31 (permalink) Old 07-21-2006, 05:11 PM Thread Starter
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Dyno Tests

OK......went to a conservative DYNOJET dyno and we did eight pulls for $100...the guys at Harman Motive are great. All the pulls were within 1-2hp of each other. This DYNOJET dyno is tighter than the older XS Engineering dyno and is a rear / front wheel dyno.......not a four wheel dyno were the drums are literally connected. The Jeep kept whacking out at everyother run due to the sensors the drive system must have.....so we would shut it down and reset for the next run. These were all 3rd gear pulls.

I really dont care that the numbers are (uncorrected) lower, what is important is the baseline and what different changes and up coming upgrades and coolant temperatures does to the POWER BAND as far as the horsepower and torque. We ran (assuming gauge is correct) 195/200......then just past the 215 mark.......then got it up around what appeared 220..........look at the air fuel and the effect from 3500 rpm to 5400...then again at the end of the power band. The final sheet is the stock air box pulled up off the filter box at 215-220........the power band seems smoother and torque went up compared to the other 220 run. Note the difference from the 215 and 220 runs.

Total horsepower (not torque) on the hotter runs is higher BUT that is just in one spot like a blip........over all it makes more power for longer with a slightly lower coolant temp.

Our vehicles have no MAF.....so there is no "learning" the CAI. Any improvements will be immediate and as illustrated by removing the air box on the single run..

Next change to be posted is the X pipe and dual mufflers with the FACTORY REAR STOCK FLOW THRU RESONATORS in place.....I'll think the increase will be minor from swapping exhausts. Dont buy any cat back at least until Thursday night. Its not like these are tuned exhausts.

BTW...these are raw uncorrected #'s that will always be lower than corrected dyno numbers that can be easily manipulated by the operator or read higher for the correction factor.



http://i77.photobucket.com/albums/j6...e-gcsrt8-1.jpg closed air box,stock exhaust, 195 vs 215

http://i77.photobucket.com/albums/j6...e-gcsrt8-2.jpg closed air box,stock exhaust, 215 vs 220

http://i77.photobucket.com/albums/j6...e-gcsrt8-3.jpg pulled air box, 215-220
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post #2 of 31 (permalink) Old 07-21-2006, 05:12 PM Thread Starter
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Re: Dyno Tests

I had the stock muffler replaced with an exhaust consisting of 3" mandrel X pipe with 3" core Bazzani racing mufflers........all components are stainless steel. I do not feel 28hp BUT it does rev more freely. It is louder than stock and has a bark BUT is nowhere as loud as my previous Zoomers system. The stock resonators are still on so ABSOLUTELY NO DRONE at 3000rpm at 80 mph and NO drone at 1600-2000 rpm in 4th or 5th gear. Im very pleased and feel it is a good balance.....considering it is a one of a kind. When you get on it it right at redline shift it makes a distinctive BOOOF sound at the end of 1 & 2nd gear changes. It took 3 hours to fab and install and cost $580.00.

I'll dyno it and post the results.......good or bad. I had the GSM traction control/fan kit installed......that mod definetly has the grin factor.

http://i77.photobucket.com/albums/j6...xhaust0081.jpg

http://i77.photobucket.com/albums/j6...exhaust005.jpg

http://i77.photobucket.com/albums/j6...exhaust006.jpg

http://i77.photobucket.com/albums/j6...exhaust010.jpg

http://i77.photobucket.com/albums/j6...exhaust002.jpg
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post #3 of 31 (permalink) Old 07-21-2006, 05:15 PM Thread Starter
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Re: Dyno Tests

So ........it looks like to air box does have some restrictions and it likes the air box lid open. Does that mean a CAI with an addtional bend downwards works? Or does the one Boosted 717 fabbed out of the stock rubber tube with a large cone filter at the end actually work the best???

Exhaust.......well it was nominal at best........if not a small step back. Maybe having a 3" transition system in the middle of a is too big. You know for 370 cid even 2.25-2.50 pipe is more than enough much less 2.75 and adding 3" may be too much. The dyno tuner made a comment that even on his 800 hp turbo motors 2.75 is the max they run.

Talked to my muffologist (lol) this morning and he is willing to build me a X pipe and two Bazzani cores from 2.5 s/s pipe.......we will see...I think i'm done until the Corsa is out in the next 3-4 weeks.....this is getting "exhausting"..so to speak.

The runs shown are last weeks best compared to this week with exhaust and them air box lid removed.

http://i77.photobucket.com/albums/j66/sgtstanko/1.jpg

http://i77.photobucket.com/albums/j66/sgtstanko/2.jpg

http://i77.photobucket.com/albums/j66/sgtstanko/3.jpg
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post #4 of 31 (permalink) Old 07-21-2006, 05:29 PM Thread Starter
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Re: Dyno Tests

I'll post the dyno sheets but the results for the X pipe removal and Mopar CAI are as follows.........assumptions that the temperature gauge is accurate and 215 deg is in the middle therefore the next (left) lower line is 205 deg and 185......is in fact 185. Same dyno as previous runs, uncorrected. Today in Torrance was overcast with high humidity. So air was thinner than previous dyno runs

Temp 215....stock air box with flat K&N 332 hp.....339tq
Temp 215....Mopar CAI........................337hp....340tq

Temp 185....stock air box w/flat K&N...336hp....346tq

Temp 185...Mopar CAI....341hp....348tq......then we did a back up run to verify this run again..342hp...353tq.

The Mopar CAI does make more power than stock box and there is more timing obviously at the lower temps. Also after removing the X pipe motor made more power.
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post #5 of 31 (permalink) Old 07-21-2006, 05:30 PM Thread Starter
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Re: Dyno Tests

The dyno runs showed consistent power was back with removing the X pipe. Also they show the affect of coolant temp on what I believe is the timing curve and I was surprised by the Mopar CAI. I should add these are uncorrected and it was very humid in the Torrance south bay.......otherwise for you # mongers add 8-10 more minimum......but that is not the point I set out to prove.

http://i77.photobucket.com/albums/j66/sgtstanko/4.jpg

http://i77.photobucket.com/albums/j66/sgtstanko/5.jpg


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post #6 of 31 (permalink) Old 07-21-2006, 08:07 PM
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Re: Dyno Tests

Great write up! I wish I had the time to do some experimenting.
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post #7 of 31 (permalink) Old 07-21-2006, 11:18 PM Thread Starter
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Re: Dyno Tests

This is why I believe most after market exhaust dont work, especially poorly engineered if any. You cant just paste some parts together and tell us the consumer they work.......just add other parts and my exhaust will shine.

This is for the performance geeks.......long shot, but SRT8Jeep emailed me and asked if I thought reversion thru the X Pipe could be causing the horsepower drop.....compared to the stock muffler dyno runs. So I started thinking....looking in archives..... doing some research and found that first of all H,Y and X pipes were primarily developed in Nascar, F1, etc and later adopted to street/race cars. All applications require that the exhaust be of equal lengths starting at the headers or exhaust manifold running down just past the transmission and at that point the X pipe is installed then the exhaust continues. Most street applications from current manufacturers using dual exhaust and some type of "balance tube" is GM on the GTO, Corvette and Cadillac.......Ford uses it on the Mustang.....and Chrysler uses it on the SRT8 300 and Charger/Magnum. They all have one thing in common.........equal exhaust tube size and equal length dual exhaust, typically down the center of the vehicle.

Problem.......the Jeep! Designed as a single exhaust vehicle with the gas tank in the way. SRT8 comes along and installs an engine that requires dual exhaust. So they engineer it the best they can.......duals (2.75") with a longer length on one side. The driver side is longer starting at the manifold and coming over to join the shorter (passenger) side. Then they criss cross it into one muffler.....(inside the stock muffler)...the longer side exits at the flow thru chamber but wait, you will have uneven pulse waves at the muffler SO...they install a short metal block to slow down the shorter side but chamber them together in the first 1/4 of the muffler. My humble opinion.........they quieted it down from being raspy, but more importantly they attempted to tune the muffler by equalizing the pressure, but also allowing the (longer) flow thru side to "pull" on the shorter side (vacuum) with no pulse back to the exhaust valve........and us technoids know this disturbs the incoming charge and depending on the cam overlap.......could cause some reversion......loss of power......bottom line they tuned the stock exhaust to compromise an uneven exhaust length........and in my humble opinion that is why the the driver side rear resonator pipe has the loop, to equal the total length of the exhaust system.

Question??? When have you ever seen a performance vehicle with one side of the exhaust longer (total length) than the other???

We come along.......wanting more power.......rip off the stock system or portions of it, not fully understanding. The BIGGEST mistake is from cat back installing an X pipe, where an X pipe has no business......and installing a exhaust system that does not take into account an existing system of unequal length and a tube size of 2.75"!!!

X PIPE....developed to act as a "diffuser".......as the exhaust pulse comes down one of the two Jeep header pipes (which are shorties and uneven lengths as well) it hits the X pipe and "sees" an expansion section. This section acts to slow down the flow and increase pressure. This in turn creates a negative pressure reflection which travel back up the opposite pipe. If the header or header section is tuned correctly then this negative pressure pulse arrives at the exhaust valve of the opposite cylinder in time to help scavenge the last of the exhaust gasses from that cylinder as well as pull on the arriving fresh air charge which is the overlap period of the camshaft.

Again to the "gear heads" that read this it will all make sense and to some non gear heads as well.

Conclusion; What was I thinking????? Why did I have a X Pipe designed and installed in the location I did???? The Jeep has an unequal length factory headers and substantially unequal length cross over center (catalytic) pipe/pipes. Any X Pipe is going to disturb the pressure wave activity. If one now understands the function and where and why it's used and placement are critical to engine performance....then one can now assume that on the Jeep an X pipe designed exhaust will hurt overall performance, and that is why I lost power on the dyno my second time. You just can't put one in because it's suppossed to work IF YOU DONT KNOW HOW AND WHY IT WORKS AND YOU DONT ENGINEER IT AND PROVE IT WORKS!!


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post #8 of 31 (permalink) Old 07-22-2006, 01:45 AM
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Re: Dyno Tests

my head hurts....
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post #9 of 31 (permalink) Old 07-22-2006, 04:33 AM
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Re: Dyno Tests

Someone said Zoomers bolts-on after the stock x-pipe. If so, then everything should be fine, correct?
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post #10 of 31 (permalink) Old 07-22-2006, 07:11 AM Thread Starter
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Re: Dyno Tests

There is no stock X PIPE........if your looking (under vehicle) at the stock pipe where it crosses over that is two seperate pipes just criss crossing. They send the driver side cat exhaust and cross it over to flow out the shorter passenger side and they take the shoter passenger catalytic pipe and flow it out the so called restricted "longer" driver side.

No I dont feel the X pipe on Zoomers is engineered or well balanced. Look at B&B, Corsa or Magnaflow, they still attempt to swing out the driver side exhaust in the rear not straight shot it to make it shorter.
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