Emissions continues to be a problem for modified cars.
Most newer cars only need a scan of the cars computer system to pass emissions. But that's not always as easy as it sounds.
We recently had a customer that had bought a Hennessey Challenger SRT600 from out of state and was really surprised to find out it would not pass emissions here in Colorado. Here is what happened:
The vehicle did not have any check engine lights, but for some reason, the emissions testing center could not scan the cars's ecu. Whenever the emissions center cannot scan a car, it automatically moves on to the rolling sniffer test. The vehicle will be driven on a treadmill for cars while a vacuum hose samples exhaust gases.
Many modified cars have had significant portions of their emissions control systems removed. They have also had their computers reprogrammed or tuned to block check engine lights related to the removed emissions equipment.
A quick scan of the computer shows that everything is working properly, even though it's not physically present. When the emissions center cannot connect to the car's computer, the sniffer test is mandatory and cannot be tricked. The vehicle must have legitimate emissions equipment to pass the test. 
Fortunately, we can get vehicles with over 2000 hp through emissions, legitimately. Our Hennessey Challenger customer failed the sniffer test and upon closer inspection, the reason was obvious.
There were no catalytic converters in the exhaust system at all! We installed a pair of our EPA approved high flow race cats and retuned the vehicle on our dyno. Not only were we able to actually pick up power after installing high performance catalytic converters, the vehicle passed the sniffer test with flying colors.
There is no reason a modified car should be cat-less. A modern high flow race cat does not restrict power and passes emissions with flying colors.
