Peter, I define seriously as in creating poor responsiveness in the normal low to mid RPMs that most people drive in. Your examples surprise me as with but a very few exeptions the opposite of your experiences are the reports I get. Check out this post.
http://lextreme.com/forums/showthread.php?t=4388
Hiccup installed headers, 2.50 pipes off the headers, Dr.Gas merge collector 2.50-3.00", 3.00" center cat and the stock rears. Now you can certainly argue that the stock rears may be an issue, fair enough, but in this case his low end was hurt. He went to 2.00" mandrel piping to a 2.50 center cat and reports massive power improvement accross the range.
VSsc400 from Club Lexus installed dual 2.50s and reported excellent results a couple of years ago. I conversed with him and he kept the first 2 feet of stock pipe off the cats before going with a wide open dual 2.50 and X pipe. I was surprised that he did not loose low end but he reported it was great and better than his 2 previous attempts.
That said he PM'd me some time later to inform me that he added a couple of sections of 2.25" and his low end noticeably improved. Here is a quote of his in a thread in response to another member (not me) who suggested 2.25". "i have the 92-94 1UZ-FE with 10.2:1 compression. That will soon change to 11.5:1 compression and it will receive 272intake 288exhaust cams with 162lb springs so the 2.5" would be what I'd need at that point. I didn't want to have to do the exhaust again again when I put the cams and raise the redline to 8525rpm. But I do agree with you that 2.25" is what the 4.0L calls for."
I had another guy recently write me complaining that the dual 2.25" X pipe system on his 290hp 1998 SC400 hurt his low end. He went back to a dual Y pipe design with my nozzle collector and merge collector combo in 2.25" dual to 2.50 single sizing and reports the low-mid power is back again.
Check out his post #16 in the following thread:
http://lextreme.com/forums/showthread.php?t=4058&page=1&pp=20
Go to the Dynomax website and review their flow chart using 2.2cfm per HP to gauge the power supported by their mufflers. This flow rate was developed by David Vizard who also helped develope the Walker Super Turbo muffler which was the best performance muffler for a long time. Their data shows each 2.50" muffler part#17293 on the following link (straight through like straight pipe) supports 1197cfm or 544hp for the single muffler.
http://www.dynomax.com/documents/ultrafloss_specs.pdf
Personally, I would run much more than a single 2.5" muffler on anything over 300hp and in fact dual smaller ones for the most part on a street car with noise concerns... but it is interesting to see this type of info and also interesting that Lexus used a single 2.36" center pipe on their 300hp/310tq GS400 engines.
I also understand and preach that the exhaust is like a wind instrument and changing things will alter the harmonics and such performance. A one size fits all is rarely obtainable and recommendations such as slap a pair of "whatever" size pipes will perform differently depending on design.
Bottom line point is that most 1UZFE engines are 300hp and less. A single 2.50 pipe PROPERLY installed is not going to restrict that power level very much if at all. Dual 2.50"s will support MUCH MUCH more power and in the experience of many including myself does not enhance and usually reduces the responsiveness and power of the engine in mid to low ranges. Larger pipe weighs more, it is louder and is harder to package. If it made more power I would be the first to recommend it. I just do not see it helping over a well designed 2.00-2.25" system.
Peter, this is why I post as I do. Your experiences surprise me. I know you tried a single 2.50 system early on. Do any of your countrymen who you have dyno info on have dual 2.25 systems. I know you have the RUSH headers which have larger primary pipe size than I recommend and in fact you have asked me that question also. One day I would love to try multiple different systems and designs on a dyno mule. My beliefs are ALWAYS open to scientific reform so all data is welcome. As you point out standard chassis dynos are not good at gathering mid and low RPM power as the auto trannies tend to downshift. They are also not good at measuring engine responsiveness which is another factor in how a car feels. I know of a GS400 who replaced the stock Y pipe with a slightly larger design and reported a loss of power accross the RPM range. He then dyno'd and discovered to his surprise a 1hp and 6tq gain! It was the loss of responsiveness that created this feeling and of course the low RPMs were not able to be compared.
This is a complicated subject. I will continue to add to the info base.
