The first thing street cars have to do is light the cat fast, from cold start. Put them close to the ports and use the thinnest stainless headers you can (so you dont lose calories on your way to lighting the cat).
Second thing you need is the smallest cat you can get away with, because it must stay lit at no load and lots of idling in traffic. Big cats cost big money and wont stay lit.
Forget the exhaust tuning (the old methods) because you need to close the exhaust valve too early to fully scavenge. That will save a nice chunk of money in egr engineering....some engines can run without external egr with this method.
Keep the exhaust port size down, because you are going to run this thing right on 14.7:1 as much as possible. If you get the compression up, the mix perfect, and the burn late enough but still fast...there wont be much exhaust to get out, most of the time. In fact, you'll be building the pressure so perfectly timed that you will need insulators in the lower waterjacket area to keep the last of the expansion from the combustion event...this is getting common for bigger engines. Remember, make the power at lower rpm and the exhaust volume shrinks plenty quick.
Now take the throttle away from the driver. He can offer his opinion with the right foot, the ecu will take it under advisement, and decide how much and when he gets actual throttle opening. You can see this for yourself. Connect a CARB OBD II scantool using the non-oem datalist, graph the throttle angle sensor, rpm, and throttle position sensor. Go for a drive, come back laughing.
Ok...so now that little cat is almost staying alive. Better take gearshifts away from the driver. That way the ecu can decide how much exhaust actually goes through that smaller, cost efficient cat. Slow down the engine, most of the time, and the warranty costs goes down with it.
In a most difficult case, like...say... Big engine pulling hard? Discontinue the manual trans option. This is really important if you think the driver may use downshifting decel often. Everytime he blips the throttle and grabs a downshift, the air fuel ratio passes through 15:1 on the way to fuel cut. 15:1 is the fastest heat build a/f for the cat. Do this very much and the cat is burn-out and warranty cost go up. Just to break him of that bad habit...since you have control of the throttle and he dont....hang the rpm up a ways when he does that part throttle "blip downshift". It feeds some cooling to the cat...decided by the ecu...and gets the driver out of that driving mode because it dont feel right.
The bigger and more poweful the engine, the more gears you need in that automatic transmission, to deal with that carefully sized cat. And now you know why big M-B and Lexus, etc need 8 speed automatics.
Turbos are real touchy, because if you let the turbo get cold, you have a hard time relighting the cat. When I worked on the Turbo Highlander Hybrid project, that is what killed us in the end. By running the turbo off only the forward three cylinder (1-2-3-4-5-6 firing order), we could easily spool the turbo and almost keep the subcat lit on a downgrade. Almost. Remember, hyrbrids STOP the engine on decel and that turbo is cooling off. 3-4 seconds for a relite (on a op temp engine) does not fly with CARB or EPA. That was a bummer, because the combination of rapid spool and instant elec motor torque was pretty fun and real eye opening. We spent some time with Borg-Warner talking after cat turbos, but nobody was going to buy off on this program and it was "done in one".
This is where my experience ends. I dont know the method that make the new turbo cars compliant. We get old, put out to pasture, and shuffle to the barn. I think you can see why removing cats can take down driveability...the ecu looks AFTER the cats, as well as before, and will do its best to stay compliant while protecting the wide band a/f sensors from water droplet cracking from too rich mixtures.
This whole thing is a LOT more than reflash, headers, etc. Entire design is tied to that stock cat.
Sorry I rambled on...its the same story I had to tell my friends at C&D when they dropped by the shop to ask me how they ruined the cat on a pre-release Miata playing on Angeles Crest all day. Its a brave new world, aint it? Tom McCahill is turning over in his grave.
Regards, JimL
