I also think SPD's tuning principles are based on the fact that he is in California, certainly the most strict state in the US when it comes to emissions testing (and well-trained police officers that can tell by sight (and smell!) if your car has been "illegally" altered). Certainly if I had to pass a California sniffer test or an AWD rolling emissions test, I'd want that DP cat firmly in place.

The CHiPs and LA local 5-0 in California are walking encyclopedias of car parts and modifications; a friend of mine drives a pretty tweaked Eclipse in San Diego and he was stopped a while back. The officer proceeded to not only list every part he had changed out (after a 20-second glance under the hood) but how much the parts cost (installed) and how much HP he was running over stock... I told him not to install the full Ti exhaust at that crazy angle!

 

aren't there a few companies that provide hi-flow cats in their downpipes anyways? I'm guessing if you want to upgrade everything with "big" piping, you might as well get one with a hi-flow cat to pass smog, and keep emissions low...as they can be anyways.

 

FWIW, you can pass the CA sniffer with a single high flow cat. It was done recently by a NASIOC bay area member with only an MRT high flow.

I left the stock 3rd cat in...and removing it later made no difference. So my real world conclusion is that the DP cat is MUCH more restrictive than that teensy little 3rd cat.

 

I am looking into this now but I was told that all AWD cars are exempt in maryland because they do not have all wheel drive machines. Maybe someone from the area can confirm this.
Kilby

 

I guess one more possibility is that you can only free up the exhaust so much before it is not much of a restriction problem... In other words once you remove two cats the exhaust is pretty much as free as it's gonna get/needs to be for moderate power levels... That could explain what you experienced and still have what SPD says hold true. Who knows... The only way to prove any of this would be to remove each cat individually and dyno in between.

 

I didnt read all of the posts on this subject so I am sorry if I am repeating anyone, but one thing I kept seeing was people saying that the stock DP is not that restrictive, etc.....

well, here's my 2¢ on that.....

I agree...the actual DP is NOT that restrictive....even with the cat.....HOWEVER.....the problem doesnt lay in the pipe itself or the cat even.....the problem is the flange that the DP uses to connect to the turbo outlet......the stock flange is VERY restrictive and not very efficient....the wastegate gasses are shot out of the turbo at a very high velocity and SMACK! right into a steel wall....then they need to find their way over to the exhaust opening and somehow merge with the exhaust gasses exiting the turbo jetting into the DP tube, and do this SMOOTHLY in order to get good exhaust flow. Most aftermarket DPs take care of this problem with a bellmouth design (Helix) or a separate downtube for the wastegate gasses (Perrin DP)....this eliminates that "wall" and allows the gasses to flow smoothly and quickly when they merge with the exhaust gasses. IMO, THIS is the reason I bought a DP....the laws of aerodynamics/physics still apply here...the smoother, and more evenly that the gasses can flow/merge together will give you better performance. Whereas if there is an extremely turbulent mess in the exhaust flow....well, your engine will not be happy.....Have you ever crushed an exhaust pipe on a car and tried to drive it? It doesnt run very well does it? its like trying to blow air into a bottle...once the pressure gets too high, you cant blow anymore air into it until something gives (the bottle blows up) This is an extreme situation, but it illustrates my point.

Less restriction = better performance....up to a point anyways....there is a line that, when crossed, will make your car run weaker....such as running 4" exhaust on a civic. The engine doesnt produce enough exhaust to benefit from a 4" exhaust system, so matching your exhaust diameter to the engine produces will give you optimal performance.

am I still on subject? hmmmm...LOL sorry about the book I wrote.....I just started typing and before I knew it I had 17,000 words written. LMAO I'm going home now.

-Ted

 

I was under the impression that turbo cars have all the backpressure they'll ever need. Freeing up the exhaust and reducing backpressure would yield huge gains. The only torque loss you would recieve would be under no-boost conditions. I think the downpipe is really restricitve and the optimal setup would be no exhaust to relieve the exponential amount backpressure in the car. So I believe Bigger is Better, or the more free flowing the better.

 

With a turbocharged engine you obviously want to do anything you can to increase the velocity and pressure of exhaust *before* the turbo (uppipe) and one of the ways to do this is obviously lowering the pressure after the turbo (downpipe, midpipe, exhaust.) However, there is a law of diminishing returns at work -- at some point this levels out with the max air you can intake.

After I got my boost gauge installed, one of the things I noticed right away is how rarely I am actually boosting with the turbo. This isn't because I am a timid driver, but because life (traffic, speed limits, fuel consumption) basically limits full boost to acceleration lanes and late-night trips to the store. A no-exhaust car or one with a totally catless 4" setup is fantastic for the race course, but isn't a practical daily driver by any means (which is another goal mentioned by SPD -- to make a car fast and yet streetworthy.)

 

what's so hard about swapping the turboback once every two years?

honestly, it takes a bit more than an hour.

ken

 

remember, boost gauges measure manifold pressure, not compressor output pressure.

ken

 

This is not true. Back pressure is essential to engines. The reason it doesn't matter on a turbo car as far as exhaust is concerned is because the exhaust has to spin the turbo, this creates plenty of back pressure... So the more you can free up the rest of the exhaust the better. As someone mentioned earlier in this thread (I think it was about a preludes, which I can't say anything about because I know nothing of tuning preludes) you can make a NA car slower and less powerful by freeing up the exhaust too much... You did have one thing right about the velocity, this is important. Think about the water flowing out of a hose fairly slow, now constrict it by putting your thumb over it, the water shoots out much faster right? By increasing the back-pressure you increased the velocity. It's quite a complicated subject and can be pretty engine-dependent at times. Some engines indeed run better with no exhaust.

Anyway, seems like we should write this guy at SPD tuning and ask him if he has results that can prove what he's saying. Everyone here seems to conclude that the stock downpipe is constrictive even at moderate power levels... I think I'll write him tomorrow. Thanks for all your input guys.

 

When you put your thumb over the hose the same amount of water still exits the hose.