cliff notes:
test pipe gain over cat ~ 2 mph (~20whp), much better spool up/launch, but not good enough to overcome whp loss from hot humid weather.

Long Version
I made a post earlier about measuring the true, real-world MPH gain from a test pipe at a dragstrip. Well, here are my results.

My best time before installing the testpipe was 12.698 @108.39 with a 1.764 60' time. This was with the stock exhaust, MBC and dynoflash. It was also on the stock clutch, which was pretty much destroyed on that day. This was at Cecil County Dragway, on a nice, 70 degree, low humidity day. CCD is notorious for its mineshaft-quality air.

However, CC was not open, so I put on a UR test pipe at my house, and drove 60 miles to Mason Dixon Dragway (MDD). Now I know from my other car that I always run a few tenths slower at MDD, compared to CCD. So to measure the true effect of the test pipe, I need to do back-to-back runs (test pipe on vs test pipe off). Additionally, the temperature at MDD was near 90, and humidity was at or near 100% -pretty miserable conditions for racing, so that should be kept in mind.

Besides the testpipe, my only change compared to my baseline (12.68@108) is an Exedy twin disk clutch. I made two passes. Here was my best pass:

60'.....1.744
330....5.242
1/8.....8.154
MPH....85.07
1000..10.666
1/4.....12.788
MPH....106.79

No gain compared to the baseline, but you have to consider the weather effect. Launches with Exedy twin felt very good (in my other pass I got a 1.720 60' time). I was confident that this run was as good as it will get, so I removed the testpipe, and put the factory cat back on.

I made two more passes with the factory cat. Unfortunately, the engine bogged hard on both passes. I think the reason for this is the reduced spool-up of the factory cat, compared to the test pipe (I launched the same way). So I guess that I needed to launch at a higher rpm with the factory cat. Unfortunately, time ran out due to the stupid constantly bumping test/tuners in favor of motorcycles, Jr dragsters, and Pro Street cars. I spent all day in line for 4 passes.

So the ET was screwed up on those factory cat passes, but MPH is what we're most concerned, right? Here was my best MPH pass with the factory cat, FWIW:

60'.....2.026
330....5.710
1/8.....8.739
MPH....82.57
1000.. 0.000 (sensor problem)
1/4.....13.506
MPH....104.26

I suspect that the bog did cost me some MPH, and I feel that I could have squeezed out 105-ish mph with the factory cat, if I would have been able to get a few more runs in.

Even if I was able to get a 105 trap speed, I'm nearly 2 mph slower than my test pipe run. This implies that the test pipe is worth about 20 whp.

I think the bigger effect here is how the test pipe helped the launch. With the instant grab of the exedy twin, the prepped track, and the awd, the instant load on the engine really makes it want to bog down. But I felt no bog with the test pipe. The fast spool-up provided the torque to power through the launch, and avoid the bog.

Finally, the biggest effect seems to be the weather. The whp loss from the hot/humid weather cancelled out the gain of the test pipe, plus costed me another tenth. That's a total of about 30 whp loss due to air quality.

Those are the results, sorry for the novel

 

Good post however the 20whp is just your estimation, the dyno is the ultimate judge. I am dynoing my Evo 9 this saturday and it will go without a cat. I will post the results. I am dynoing along with about 10 other Evos.


Carlos

 

Remember to apply correction factor to the number,so the number will fair.

 

I only really care about gains on the track, and it appears that testpipe was worth ~2 mph over stock cat

 

You didn't get a good pass with the cat, so it's not comparable. The engine doesn't bog because of the cat, it bogs because of the driving or the track stickiness. I've been to MD several times. In fact, I went there when stock (13.1@104) and with cat-back/mbc/safc (12.22@110.7). I ran about 10 straight 12.2-12.3s on the stock cat but all on a 93/104oct mix en route to winning the Import class (bracket), then subsequently my stock clutch gave up the ghost, haha.

It was in the 80s, not 90s, so I had better conditions, and I do know that a test pipe helps 10-15whp, but I don't think the 2mph differences are comparable since you didn't get comparable runs. Trap speeds are less affected than ET when you get a bad run, but even a .5-1mph loss makes a huge difference when making this comparison.

 

Yeah, its too bad that I made all that effort to do a controlled experiment, and couldn't get enough passes in to get a strong conclusion. Damn officials were only running test/tune lines half way through before interupting and letting Jr Dragsters, motorcycles and Pro Street pass through...Then of course, someone breaks down, and they have to resurface the whole track...well, you know how it goes.

I wouldn't say that the cat caused the engine to bog. Rather the test pipe seemed to prevent it from happening. I launched the same way in both cases (cat pipe, test pipe): around 5 k/fast slip, tires chirped off the line. With the catpipe, engine immediately bogged. With the test pipe, it seemed that the turbo was already spooled up when the tires hooked and the engine had enough torque to pull through without bogging.

I probably could have gotten a good launch with the cat by launching at a higher rpm, and/or slipping the clutch a little more. I guess the main point I'm making is that the test pipe seemed to help me avoid the dreaded bog after launching.