2.4 liter Cavileer and gains were about 4 WHP difference. I'll see if I can find the dyno sheets. I dont know any DSM guys running 11s with a 2.5 inch exhaust. My buddy was running around 400WHP and he was 3 inch piping all the way. DSMs need to get rid of back pressure. Some Supras run 5 inch piping. We do not lose low end on a 2.5 inch piping IIRC. The Magnaflow is 2.25, the VRS I was referring to with the Magnaflow muffler is whatever size you choose.
11s might have been pushing it. I'm pretty sure I read about a new record on 2.5" exhaust, but I can't find the link. mid 12s are comon tho. http://www.dsmtuners.com/forums/member.php?u=4521
Also, DSMs don't need to get rid of back pressure. Larger exhaust are needed for more boost. I tuned a low budget talon once and used the stock exhaust with a test pipe and magnaflow turbo muffler. At 13psi and a junk yard IC upgrade, I was able to break the tires free from a roll and spin all the way through 2nd. The car was way faster then stock and it didn't take a larger exhaust. I took it to the track once before the IC install against a 13s mustang. I lost due to FWD traction and mustang hookup. I couldn't catch up in time, but I started to pull readly hard before the end of the 1/4. The 1/4 time was embarasing so I'll leave that out, but it was at 99MPH and I had a LOUSY 0-60'. The IC prolly gave another 30HP, but also a much wider power band. It was a blast on the street, I could just never hook up with the non lsd FWD crap. Point being, the car was way-way faster then stock, and it didn't take a larger exhaust.
As far as not loosing torque on low end with a larger exhaust. It is comon with every other 4cyl car out there. Not sure why the spec v would be any different. Unless a driver likes hanging out at higher RPMs all day long to stay in the power zone, a header with no cat, high flow main cat, and high flow muffler would prolly be better the a larger cat back with the stock cat in place. The muffler and high flow cat will cost less if ya shop around.
Look, the auto after market was a 5 billion dollar buisness last year. A LOT of products get pushed and too many people think that CAIs and larger exhaust are the be all of must have performance parts. Like I said before, the engine needs to be pushing way more gas then stock to really get a full benifit from a larger exhaust. The only way to do that on the spec v is forced induction or head work and cam.
When you read that a cat back gives 4hp on a caviler or any car for that matter, you gotta ask yourself something. How much of that is having a much better flowing muffler and how much is due to the size of the exhaust and then how does the dyno look compared to a stock run? Just because a part gives 4HP doesn't mean that something was lost to get it and peak HP is not always a gain and that's what race shops advertize. Peak HP gains. So out of that 4HP, is it worth to spend $350 or more on a cat back when you can just go to a exhaust shop and get a nicer muffler installed for around $100. Maybe you only get 2-3HP out of it, but how much is that extra 1-2HP worth? You could prolly spend another 150 for a high flow cat and get more gain then just the cat back because stock cats are very restrictive.
I'm not saying, don't buy cat backs, it's not worth it, but when you tune a car, any car. You gotta have a game plan. You gotta know what your budget is, you gotta know what you want to get out of the car and what you want to do with the car.
All I see on this list is buy a CAI and cat back first thing. It's a must. Well, if your going to autox, it would be better to spend your money on suspention and if your going to drag race, tires and motor mounts and a clutch. It just seems like the first thing people wanna do on any car first thing is to get a CAI and exhaust. On any forum, not just this one.
I was talking with a mustang buddy of mine today. We've both been around the block and we both learned a LOT over the years. Anyway, the topic of CAIs came out some how and he had a LOT of trouble with the one that came on his car. It screwed with his maf, and then he tried to have it calibrated right. It cost him a lot of money and he never got it working right. I guess Fords mafs are touchty to the air not comming in just perfect. He put a drop in K&N using the stock air box and his car ran a lot better. He then told me people are running 11s 1/4 with the stock air box.
I got a lot of crap on this list telling me a K&N in the stock air box didn't do much if anything on the spec v. I noticed a nice gain with just a K&N. More then I expected. The surface area of the stock filter is large, the car has a large non restrictive and short intake path to the TB. The only extra gain from a CAI is going to come from a cone filter having more surface area then the stock air filter. Depending on who you ask (I've asked engineers), a metal intake is going to heat up while plastic keeps the intake charge cooler. Also, if it isn't a real CAI system, it pulls hot air from the engine bay.
This turned in to a huge rant, but just think before you jump on the CAI and catback wagon. Ask yourself is it really woth the money. Will it give more HP then a just a K&N and new muffler? Maybe a few, but is it worth the hundreds of extra dollars when you could buy real mods. The header with no cat is the best mod I've seen for this car yet. I don't doubt for a second that it can give 12HP without giving anything up. You could prolly get another 5-8HP by replacing the main cat with a magnaflow high flow cat and they only cost about $100. Maybe $150 installed. If you can find a shop to do it for ya. Cause there are laws about replacing cats. If your factory one some how gets dammaged tho, you can get a replacement legaly. Other then that your car has to be old enough and have enough miles on it to just be able to replace the cat. It's common enough for a rock or sledge hammer to dammage a exhaust system tho.
Anyway, I don't want to start a huge fight, just tring to educate and maybe help out with why you do or don't want to just run out and buy parts just because you can. I've seen it many times. Too many parts and not enough tunning. If you really want to tune a spec v, talk with a old shool tunner. There has to be at least one in your family. Listen to what they have to say. They might have 30+ years under their belt and they could tell you a thing or two about what will work and what won't work. Things have changed over the years, but engines do the exact same thing the did since they were invented. They are just better now days. I also recomend watching the old guys on speed. Two guys garage. They don't talk all gay and they are the most realistic car show on TV. I still learn new stuff from them. Some days they work on older cars and other days they are working on brand new cars. Either way, you should learn something new every time you watch the show.