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Old 06-22-2006, 11:20 PM   #1
Patrick Olsen
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Member#: 120
Join Date: Jul 1999
Chapter/Region: AKIC
Location: Where the Navy sends me...
Vehicle:
1997 Legacy 2.5GT
QuickSilver Metallic

Default OBX header vs. MRT header dyno results

Bear with me, as this is a bit long. What can I say, I like to be thorough with the details.

Back in 2001 I installed a full MRT exhaust on my car. I had it dyno'd and was very happy with the gains - basically no loss at the low end, +14hp at the peak, and +17hp at the redline. Not too shabby. Here's the same-day dyno results:

Note that those dyno runs were on a Dyno Dynamics Low Boy AWD dyno, which is an eddy current type load bearing dyno. Based on my experience, the numbers seem to fall right about 85% of what an AWD Dynojet will show.

Fast forward a bit and Cobb came out with their equal length headers for the N/A EJ25s. Their advertised horsepower and torque gains were very impressive, particularly at the low end and through the midrange. As you can see, the MRT header/exhaust didn't do much until ~4000rpm, but Cobb's graphs for their header showed significant gains across the board:

I'll be honest, I thought it was BS. How could a header show a nearly constant torque improvement across the entire RPM band? It just didn't make sense to me.

Fast forward a bit more and OBX starts selling a knock-off of the Cobb design for a helluva lot less money. Add to that the fact that my MRT header has been showing its age (5yr, 65k+ miles) with some cracking, and I decided it was time to try out something new. I've always wanted to see a head-to-head dyno comparison, but didn't have access to a dyno for the last 3 years or so while stationed in Hawaii. Now that I'm back on the East Coast I've got dyno access, including an AWD Dynojet at Extreme Motorsports all of about 15min from me.

On June 6th I headed over to Extreme and re-performed my "baseline" runs. The car hadn't been on a dyno in about 4 years, and had never been on a Dynojet, so I was curious to see what it would put down on the rollers. The best run netted 168hp and 153ft-lb, which I thought was pretty good, but it was also clear that the issues I've had with timing being pulled in the mid-range were really wreaking havoc. I did 9 runs total, with peak HP varying from 165 to 168hp, and peak torque pretty constant at 153ft-lb.

Yesterday, June 21st, I headed back to Extreme with the OBX header. Because the MRT header/cat terminates in a different location than stock, I had to have a little bit of modification done to the OBX header to make things mate up properly. Basically, the flange just after the resonator on the OBX setup had to be swapped for a new flange. The system now has the OBX header going into the OBX resonator pipe into a 2-1/4" hi-flow cat, then into the MRT mid-pipe (2-1/4") and Stromung axle back (also 2-1/4").

Now, I've always wanted to see a head-to-head dyno comparison of these headers, but it wasn't quite that simple. Obviously there's the 15-day difference in dyno runs. It was definitely warmer on the OBX dyno day, but the SAE correction was 1.0 on both days, so the difference wasn't much. The other issue is that in the past week or so I've decided to try spoofing the ECU by running a resistor in place of the knock sensor (which I believe is not really detecting knock, but is instead overly sensitive to something in the mid-range). So right now I have the knock sensor installed, but not hooked up, and the sensor harness is routed through a 560kohm resistor to ground so that the ECU thinks the knock sensor is still hooked up and doesn't go into limp-home mode. It would have been nice to have time to dyno the car with the MRT header with the knock sensor disconnected, but I wasn't able to do so. More on that in a bit...

Anyway, we put the car on the dyno and here's what it laid down:

That's the second run we did. The first run showed the fuel curve was quite rich in the low end, so we leaned out from 2500-4000rpm by 5% and took 3% out at 4500rpm. (The S-AFC had been tuned with the MRT system, and even with these adjustments it actually adds quite a bit of fuel from 2500-4000, then takes fuel away from 5000 on up). Note that the peak numbers are essentially the same as the car put down with the MRT header installed. Also note how smooth the curve is at the low end - no more intrusive knock sensor.

Now here's where we get into some conjecture. Here's a comparison graph showing the MRT and OBX runs together. I'll explain each of the curves below...

- The OBX HP / OBX Tq curves (navy blue and dark pinkish) are the same as the ones shown on the WinPEP graph above, I just exported the data to Excel (all the data on the graph is data exported from WinPEP to Excel, with the exception of the dashed lines).
- The OBX HP2 / OBX Tq2 curves (yellow and light blue) are the next run we did, with the knock sensor plugged back in and the dummy resistor disconnected. BAM, just like that, down 10ft-lb from 3000-4000rpm. AFR averages about 12.2 through that range, and I'm running 89 octane, so I just can't believe it's really detonation. But, maybe I'll blow my engine up with the knock sensor disconnected...
- The MRT HP / MRT Tq curves (purple and brownish) is typical of the 9 runs I did with the MRT header on June 6th. Needless to say, that's with the knock sensor connected.
- The MRT HP fixed / MRT Tq fixed curves (royal blue and I'll call it aqua dashed lines) are somewhat educated conjecture on my part. If you look back at the stock vs. MRT graph I posted above you can see that the MRT torque curve should be pretty smooth. So I just faired a straight line in from 2400 to 4200rpm to show what the curve probably would have looked like if I had done a run with the MRT header installed and the knock sensor spoofed.

So there you have it. From 2200 to 4000rpm the OBX (aka Cobb aka Rallitek) header averages +8.4hp and +14.5ft-lb over the MRT header (using the "fixed" MRT numbers). From 4000-4800rpm the OBX continues to hold a smaller edge, and then from 4800rpm on up they're basically the same. Although the dyno is just showing WOT results, there's definitely a difference in the ole seat of the pants at part throttle, too.

Pat Olsen
'97 Legacy 2.5GT sedan

PS - My next dyno trick will be to show whether or not the "torque box" does anything. Give me a month or so...
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Last edited by Patrick Olsen; 11-14-2007 at 10:10 PM.
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Old 06-22-2006, 11:47 PM   #2
calmnothing
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Default

Wow that is an impressively thorough writeup. Thanks for doing the comparison. Looks like I will be looking to get rid of my borla for an OBX soon. I am so going to miss the sound =(
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Old 06-22-2006, 11:54 PM   #3
Rally_wgn
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2.5 N/A w/ full exhaust

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Thanks Pat this is great stuff. If its alright by you I'll linky to my N/A white paper thread for easy reference.
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Old 06-22-2006, 11:56 PM   #4
Patrick Olsen
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1997 Legacy 2.5GT
QuickSilver Metallic

Default

Fine with me. Then other people will know, and knowing is half the battle. Yo, Joe!!
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Old 06-23-2006, 09:41 AM   #5
Siper2
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2007 BMW 328xi wagon
Silver

Thumbs up

Wow, Pat, cool.

Hey I meant to tell you this a while ago, but welcome back by the way. If I were still in NH I'd drop in to say hi.
(EDIT: Waitasecond, you're in MD.... Groovy, maybe we'll meet up soon after all...)


Anyhoo.... Yeah, right now I have my trusty setup of stock headers, Stromung HFC, Stromung midpipe with Meineke resonator, WRX axleback pipe welded to a Bosal muffler. Don't ask! Heh...

I thought last night of selling the HFC, and getting the OBX headers. I'll have a HFC welded into the cat-delete section anyway, since I don't want to run catless. I think the OBXs fit properly now.... so all I'll need a shop to do is put the cat and an O2 bung in place, I can bolt the things on myself. Later I'm planning on an i-Speed reflash.


Tempting indeed...
=Chris
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Old 06-23-2006, 12:48 PM   #6
jut60
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1999 2.5RS silver
ECS swap [x]

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nice write up. good to see a true comparison between eq headers (i guess i can't complain as much about the wait i had for the cobb header seeing that the design and performance is fairing well compared to others)
Could you give more details on what your "torque box" comparison will include? thanks
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Old 06-24-2006, 09:37 AM   #7
Patrick Olsen
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1997 Legacy 2.5GT
QuickSilver Metallic

Default

Quote:
Originally Posted by jut60
Could you give more details on what your "torque box" comparison will include? thanks
Current intake (Injen CAI) vs. torque box + Ebay intake tube. I have to mock things up so I can cut the Ebay tube down to what I need - it'll just mate up to the MAF where the Injen intake does. Shouldn't be too hard to do, just a matter of getting off my butt and doing it, and then finding time to get to the dyno and compare the two.

Pat
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Old 06-26-2006, 08:31 AM   #8
jut60
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ECS swap [x]

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Quote:
Originally Posted by Patrick Olsen
Current intake (Injen CAI) vs. torque box + Ebay intake tube. I have to mock things up so I can cut the Ebay tube down to what I need - it'll just mate up to the MAF where the Injen intake does. Shouldn't be too hard to do, just a matter of getting off my butt and doing it, and then finding time to get to the dyno and compare the two.

Pat
thanks, and will be looking foward to seeing the results
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