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Old 08-06-2005, 02:31 AM   #1
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1998 Cobra + Magnaflow Cat-Back + JLT CAI = 293 HP



I have to say that I am happy with the results. The only other modification to the engine and exhaust is a tune up that included Ford Motorsport plug wires.

I have to thank everyone who has contributed to this forum. Without the info I gathered here I doubt I would have chosen parts that would have worked so effectively.

My next planned modification is pulleys. Then maybe a dyno tune by Alternative Auto Performance.

By the way if anyone in Northwest Indiana is looking for a Dynojet I encourage them to go to Mroz Complete Auto in Merrillville. Dan's a great guy, who also likes his Fords.
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Old 08-06-2005, 02:47 AM   #2
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you still have stock h-pipe and you still dynoed 290+? I wish I could do that I only got 270 268 with O/R h-pipe with flows and Fram air hog. good numbers
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Old 08-06-2005, 09:38 AM   #3
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Unbelieveable with only those two mods......

Either the dyno is uncalibrated or you have the king of the factory 98 Cobra freaks.
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Old 08-06-2005, 11:04 AM   #4
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Kinda of believeable. But I bet he doesn't have a big gear in his car which is the reason of the high #'s. I'm guessing the stock 3.27's. Am I right? -Mark
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Old 08-06-2005, 11:12 AM   #5
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Now, I'm not the sharpest knife in the drawer - but I always understood gears not to make a differance in HP/TQ numbers.

Please feel free to correct me.
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Old 08-06-2005, 11:13 AM   #6
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Yes I still have the factory H-pipe. I thought the numbers seemed a little too good to be true at first, but looking at some other posts on this site, the numbers don’t seem that far out.

The test was done on a Dynojet, and I am not sure how the machine is calibrated. I would say that if anything was off during the test it would be the SAE correction factor. I’ll do some research on the correction factor and see what I can find out.
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Old 08-06-2005, 12:11 PM   #7
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OK. I found this formula for the Dynojet SAE CF calculation:

CF= 1.18 x (29.22/Bdo) x To+460 / 537) - 0.18
Where:
Bdo = Dry ambient absolute barometric pressure
To = Intake air temperature in degrees F

I found it here: http://www.bishopsperformance.com/dynoinfo.htm. I encourage everyone to check the site out. There is some good information there.

When I plug in my numbers into the formula as it is written I come up with 1.0366. However if the formula is calculated in this manner:

CF= 1.18 x [(29.22/Bdo) x (To+460) / 537] - 0.18

I come up with 1.11689.

Other research shows that the Dynojet itself comes with the sensing hardware, so I would assume that the software does the calculation. If that is the case the formula must be evaluated in the manner of the second example, as the chart shows an SAE of 1.12.

Can anyone confirm this?
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Old 08-06-2005, 12:37 PM   #8
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Quote:
Originally Posted by VeEight
Unbelieveable with only those two mods......

Either the dyno is uncalibrated or you have the king of the factory 98 Cobra freaks.
I don't feel like looking for a bigger graph, but you can see my wife's car picked up 20+ HP with the JLT, in some places on the graph. The stock intake tube necks down to about a 1/2" ID on one side (where the strut tower brace dimple is). It made a huge difference in back to back dyno testing, and seat of the pants improvement. This car had 120k+ miles on it at the time too, and other than 4:30 gears and the intake tube it was 100% stock (on the before pull it had a clean K&N drop in filter). So in short, I believe your numbers!

graph
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Old 08-06-2005, 03:59 PM   #9
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Quote:
Originally Posted by JESTER
Now, I'm not the sharpest knife in the drawer - but I always understood gears not to make a differance in HP/TQ numbers.

Please feel free to correct me.
have read several times that gears(especially in the 4's)can have a negative impact on dyno numbers,but can't remember the publications or details right now.anybody have any first hand experience so we can put this baby to bed??
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Old 08-06-2005, 04:22 PM   #10
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Ok, my 98 Cobra with a Magnaflow Catback and 4.30 gears put 260 HP to the rear wheels, SAE corrected. Stone stock, it also made 260 RWHP. From what I can tell, the horsepower loss of the 4.30 gears was almost exactly offset from the gain due to the catback.

So you are telling me that a JLT package will bump my car over 30 RWHP? Ain't gonna happen. On an 03/04 Cobra, yes, first gen modular snake, no way.

Also, the graph N2O posted shows a peak RWHP number of only 263. This is a far cry from the 293 RWHP Starman's graph illustrates.

Yes, gears affect RWHP, but probably on the order of several percentage points, not the 10% or more that would be needed to cover a 30 HP spread (based on a 300 HP engine). I feel that I lost 5-10 RWHP by going to 4.30's. The factory Ford catback on the 96-98 Cobras is not very restrictive, so I probably gained back the 5-10 RWHP with the MagnaFlows. I don't have dyno results in between mods, so no proof of my "feelings".

All I can say is Starman has the 98 Cobra King of the Factory Freaks.

Hell, I've read posts of full bolt on 96-98 Snakes barely breaking 300 RWHP with long tubes, off road pipes, custom tune, underdrives, bigger intake tubes, etc.

Here is my factory 98 Cobra dyno run with only the intake silencer tube removed and 22,000 miles on the clock:

One Stock, Run of the Mill, 1998 Mustang Cobra Dyno Sheet
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Last edited by VeEight; 08-06-2005 at 05:05 PM.
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Old 08-06-2005, 05:03 PM   #11
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Here is some more info on the SAE J1349 JUN90 Correction Factor calculation:

http://wahiduddin.net/calc/cf.htm



where:
cf = the dyno correction factor
Pd = the pressure of the dry air, mb
Tc = ambient temperature, deg C


The formula is metric based, but I get a CF of 1.105914 when I plug your numbers in. That rounds to 1.11, pretty close to 1.12 (round off error)

I got lazy and put the calculation in Excel.....

Strangely enough, your uncorrected max RWHP would be: 293/1.12 = 262. That coincides with a basically stock 98 snake.......
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Old 08-06-2005, 05:11 PM   #12
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Heres my 2 stock baseline runs from about 3 years ago on the 2nd run we changed the plugs and filter.. I have not yet had it redynoed since my X pipe, gears an JLT soon thou...


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Old 08-06-2005, 05:52 PM   #13
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I have an AFM powerpipe and I can't see how a JLT would make anymore power than the AFM, wouldn't everyone agree?
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Old 08-06-2005, 06:27 PM   #14
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So THAT must be where Mike Smiths old 98 Cobra went
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Old 08-06-2005, 06:37 PM   #15
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Why does it start falling off around 5800rpm?? Thought Cobras peak around 6200 or so then fall around 6500.
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Old 08-06-2005, 06:42 PM   #16
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Quote:
Originally Posted by Blue98GT
Why does it start falling off around 5800rpm?? Thought Cobras peak around 6200 or so then fall around 6500.
Look at the other three dyno sheets. They all peak between 5500 and 6000 rpm.

Ford officially rates the 96-98 DOHC Cobras as having peak hp @ 5800 rpm. Torque officially peaks @ 4800 rpm.
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Old 08-06-2005, 06:46 PM   #17
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All the SAE factors I usually see are more around 1.03-1.05, in fact one of my dyno's shows a factor of 0.99 ...1.12 is real high and is most likely what's giving you the high numbers...what was the air temperature and humidity that day?
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Old 08-06-2005, 06:58 PM   #18
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Quote:
Originally Posted by VIPERBLUELX
I have an AFM powerpipe and I can't see how a JLT would make anymore power than the AFM, wouldn't everyone agree?
As long as it maintains a 4"ID I don't think it would really make a power difference. I think the JLT would be prettier/cheaper/lighter/cooler to the touch though. I didn't think it would make more power than my C&L 76mm aluminum pipe on my 5.0 but I was wrong
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Old 08-06-2005, 08:30 PM   #19
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I make no claims other than the dyno graph that I posted. I wanted to get a semi-baseline before I made any more modifications to the car.

At first I questioned the correction factor, but according to the formulas both online and posted here, that appears to be about right. Not to mention I truly believe the Dynojet software makes that calculation anyway.

In doing more research on the Dynojet and how it works I found this site:

http://www.idavette.net/hib/dynojet/

A quote from the site under the “Physics Lesson” section:

“The computer factors out the axle ratio by using engine speed data in the torque derivation.”

According to this info, axle ratio should not affect the Dynojet’s readings.

Last edited by Starman; 08-06-2005 at 08:32 PM.
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Old 08-06-2005, 09:26 PM   #20
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Quote:
Originally Posted by Starman
...http://www.idavette.net/hib/dynojet/

A quote from the site under the “Physics Lesson” section:

“The computer factors out the axle ratio by using engine speed data in the torque derivation.”

According to this info, axle ratio should not affect the Dynojet’s readings.
True, the axle ratio is not needed for the calculations; but the increased friction from higher crankshaft acceleration does occur and does somewhat reduce RWHP. This is why most dyno runs are done with the transmission in whatever gear gives no torque multiplication. This is usually 4th gear (1:1) for a typical 5 speed Mustang.

Just have someone do a couple dyno runs in 4th gear and a couple runs in 3rd gear. The 3rd gear results should be lower. Likewise, dyno runs in 5th gear should be higher as compared to results from 4th gear.

In short, the axle ratio does affect RWHP, but is not needed for the DynoJet's calculations. It basically shows up as increased drivetrain friction.

Maybe there is an ME in the audience that can describe the details a bit better......
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Old 08-06-2005, 09:43 PM   #21
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Hmm, sounds fishy to me too. As another reference, mine did 262/262 on a Dynojet with a Densacharger CAI and Magnaflow cat-back, factory H-pipe, 4.30 gears, synthetics, all else stock. Important thing is to use the same dyno for before and after results and tuning.

Axle ratio will not affect real wheel horsepower much as the power transmission efficiency is about the same, but it will change how an inertia dyno measures it as the rotating inertia of the engine and drivetrain has to be accelerated faster with numerically higher gears, lowering the numbers, but probably not much.

Agree that the computer takes out the ratio for calculating or expressing "equivalent" rear wheel torque as "flywheel" number. Your curves cross at 5252 rpm as they should. The actual rear wheel torque (which is what the dyno calculates from how quickly you accelerate the rollers) is something like 700-1000 ft-lbs (say, 300 at flywheel x efficiency (0.85 or so) x rear axle ratio, assuming 4th gear 1:1 in the tranny). If they gave you a dyno readout that said "270 rwhp and 900 ft-lbs of torque" you'd say "huh?", so they give you an equivalent torque curve. The actual flywheel torque (and horsepower) is about 1/0.85 or 17% higher.

Nice to have a baseline. Happy tuning.
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Old 08-06-2005, 09:49 PM   #22
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More linkage:

http://www.superstang.com/horsepower.htm

Good reading if you have the time....

One example on that page shows a 5.0 dyno comparison between 1st and 4th gear runs. The 4th gear results were 85 hp and 140 lb-ft of torque higher than the 1st gear runs.

Are you sure your car was in 4th gear when it was on the dyno? If it were in 5th, the results would have been higher.... (on second thought, that is not likely as the dyno probably would have been damaged as the rollers would be spinning well over 200mph at redline in 5th gear.... )
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Old 08-07-2005, 12:30 AM   #23
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I don’t know that an ME would help, unless they work for Dynojet. Reference this paragraph from the article I posted a link to earlier

“In the real world, the measurements and computations are not quite that simple, but the complex methods Dynojet Research uses to apply these laws of physics and their mathematics to accurate measurement of rear wheel power is a proprietary secret.”

Grammar errors notwithstanding, it’s evident that no matter how much we debate the topic, only someone that works for or with Dynojet will be able to shed the correct light on the subject.
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