here is a link to an AF post that is highly informative regarding U-bend delete gains

http://www.automotiveforums.com/vbulletin/showthread.php?t=365241

or, just read on...

By Jim Wierzbicki on August 15, 2002... Thanks Jim!



For quite some time, there has been many claims and conjectures about the performance impact from removing the factory U-Bend in the exhaust system on the GM 3800 Liter V6 (Naturally Aspirated L36 and Supercharged L67). For those who are not familiar with the U-Bend, it is a bend that GM designed into the exhaust pipe exiting the catalytic converter. (See Figure 1) The purpose of the bend it to permit the O2 sensor downstream of the catalytic converter to be mounted in a position that would provide the maximum protection from possible road hazard damage. The downside of this configuration is that the bend poses a moderate restriction to exhaust flow. A popular modification is to replace the U-Bend with a straight pipe that contains a fitting for the O2 sensor. The sensor is typically mounted in a horizontal position to provide accessibility and keep it out of harms way as much as possible.

To help resolve these claims and conjectures, I obtained a test vehicle (Pontiac Grand Prix GT, L36 engine) that had a complete factory exhaust system. This factory exhaust system included cast iron exhaust manifolds, catalytic converter in the down pipe, U-Bend, resonator and dual rear mufflers. The testing would involve replacement of the U-Bend with a straight pipe that would include a port for mounting the post-cat O2 sensor.

Prior to the installation, I collected performance data to baseline the car’s performance. After this data was collected,

I pulled down the down pipe with the U-Bend and replaced the bend with a straight pipe.
Once this modification was competed, I repeated the performance tests. The weather conditions for the day were warm and humid. During the testing with the U-Bend, the temperature was 83ºF, Humidity of 78% and a barometer of 30.02 "of Hg. The engine was thoroughly heat soaked and running 192ºF by the PCM’s Engine Coolant Temperature values. A total of 12 passes were used to collect all the performance data, with 4 replications for any one parameter. The results for replicated runs of each parameter were averaged together in the final analysis. The test conditions with the U-Bend removed were nearly identical, the temperature held at 83ºF, Humidity of 83% and a barometer of 29.95 "of Hg. The GT was running on 93 octane, has 8,000 miles on the engine.
As can be seen in the charts, the gains in peak HP and Torque were ~5.0 HP and 2.9 ft-lbs respectively. Also noteworthy is the torque and resulting HP curves were benefited throughout the entire RPM range. This data does not support many beliefs that removing the U-Bend will improve the upper RPM power & torque and sacrifice low end power & torque. A 1% increase in volumetric efficiency and total volumetric flow rate was also seen, which corroborates the increase in HP and Torque. Also interesting is the slight increase in overall spark advance through reduction in Knock Retard. About 2º of advance was picked up. This increase in spark advance and the slight improvement in volumetric air flow is what produce the small but significant performance gains. My conclusion is this modification has benefits all round.

While these results were obtained on a relatively stock GT, they are representative of the gains to be made on more significantly modified L36 engines as well as L67 engines. The more modified the L36, the more restrictive the U-Bend becomes and the more it constricts power increases. I would estimate proportional increase would be observed on an L67 supercharged engine. If this is the case, the U-Bend replacement on an L67 engine would extrapolate to a gain of 6 Peak HP and 3.5 ft-lbs Peak Torque. While these gains are not huge, they are significant and provide an edge over cars still running with the U-Bend in place.


Good luck with seeing the chart !

cool. 2* timing advance because of lower KR.

I'm telling you, if Larry(Big-L) doesn't know it, you don't need to do it.

Read it before. Good article on for those who are non believers.

Kustomkid,
What's up with the hood on that GTX? It appears to be a stock hood. Did GTX's come with the standard hood, or is that a clone in the making?

Original location of article.
http://www.easyperformance.com/

I say get a whole DP, not just the ubend removed. If the ubend alone gives 5hp and 3ft-lbs, then a good DP must give 4X's that. The stock DP is HORRIBLE, i've got my stock one sitting in my garage. I can't believe how it's designed. Same thing with the stock manifolds, before I ported them I couldn't believe how restrictive they're designed. What's up with GM and making our cars slow on purpose? They could be faster if the parts were half-decent from the factory...

$$$ is why. Also why just stop at the Ubend cause some people just don't want to put alot of money into mods just a few dollars. Ubend delete is a pretty good bang for the buck. Compared to some of the other mods that are available for our rides.

I got my DP for $185, 3" into 2 1/2" w/no cat. It costs on average what, $90 or so to get a ubend delete done, depending on who you know or where you go. Not much more.

Won't help those guys who have to pass the sniff test. Also to be street legal you have to have a cat. Just a word of caution.

$30 in my neck of the woods for a u-bend delete. So, it's a very affordable, if not small, mod to do. Downpipe is in my future, but after the L67 swap

Yes, to be street legal in "most" places you have to have a cat, but when you're in Texas and your car is registered in Pennsylvania w/PA plates, TX cops have no idea what the emission laws are for PA. My stickers say 7/03 on my windshield right now, i've been pulled over for speeding and the cops don't say anything to me about it. They can't.

Just be careful because lady luck has to be absent one of these days...

Thrasher did an exhaust backpressure study.
http://www.thrashercharged.com/tech_htm/exhaust.shtm

Manufacture's have to balance reliability, good MPG, cost, and minimum noise pollution. They must appeal to the masses in order to sell the most cars. A 220 HP car with drone in the cabin won't sell as well as a 200 HP car with no drone.

Exactly. Keep in mind what trim level sells the best. And who owns the majority of the GPs? Most of those people would not buy my GTP with the sound it makes when cold with that CAI I have. Or the louder exhaust note.