Catalytic Converters

Ready, set, flow!


Cats, not the furry four legged meow gremlins that that make you sneeze and place foot prints all over your freshly Zaino’d car.  I am talking about the tree hugger’s solution to pollution and the horsepower-robbing enemy underneath your car.  Every new car is shipped with catalytic converters in order to clean the exhaust emissions from your car to keep the air clean. 


Unfortunately for us drag racer’s, the time is coming (and already arrived for some cities) for emission’s test for everyone in the United States.  That means we have to find the highest flowing catalytic converters we can so we can pass emissions test and minimize horsepower loss.


Why use a flow bench


How do you go about finding out which catalytic converter will sacrifice the least amount of horsepower?  You can track/dyno test each cat or flow them on a flow bench.  Seeing how the later choice is the cheapest, and easiest, we ended up at White Performance in Kingsport, TN.  Flowing pieces on a flow bench will allow you chose pieces that minimize flow restrictions and maximize horsepower. 


First you have to find a flow bench to use.  White Performance had a Super Flow 600 flow bench and was the site where our test took place.  They manufactured an adapter to connect the catalytic converters to the flow bench prior to our visit. 


All units were flowed at a 20.4” of H2O pressure drop and converted to 28” of H2O.  Both results are given in Table 1.  Why such an odd number?  David Vizard’s exhaust flow research is the precise reason we flow exhaust pieces at 20.4”.  Pieces can be flowed at 28” or 1” and converted to 20.4” in just a few seconds with simple math.  In this case we wanted the actual reading at 20.4” of H2O for accuracy since the research references a specific pressure drop.


David Vizard’s flow research shows the required air flow a muffler needs to attain, in order to keep horsepower loss due to back pressure to a minimum, is 2.2 cfm for every one horsepower.  This general rule set by David Vizard keeps horsepower losses under one percent.1 


The 2.2cfm per one horsepower represents a free flow condition and may be more or less depending on your car.  We use that standard as a basic guideline with our test results.  If the muffler has to follow this rule, then so should the rest of your exhaust system, including your catalytic converters.


Flow test and results


Five catalytic converters were chosen for the flow test.  The passenger and driver’s side converters off of a 99 LS1 F-Body, TTS Bullet, FLP, and Carsound converters.  The factory catalytic converters were modified from stock.  Each was opened up to have a 2.5 inch entrance and exit to match those of the competitors.  Of these, the passenger side converter has been regarded as the lowest flowing one for reasons expressed later in this article.  Enough rambling, let’s get to the flow test.


Table 1



Inlet Diameter (in.)

Outlet Diameter (in.)

CFM @ 20.4” H2O

CFM @ 28” H2O








Passenger Side Catalytic Converter





Driver's Side Catalytic Converter





TTS Bullet Catalytic Converter





Carsound Catalytic Converter





FLP Catalytic Converter






Results discussion


On a stock LS1 F-Body, the driver’s side catalytic converter has been praised as the higher flow converter of the two that are on the car.  A few reasons for this assumption have been:  The driver’s side converter can be found on past Corvettes.  The fact that the F-Body has a converter off of a Corvette surrounds the converter with an aura of performance history.  Also, the converter is smaller and sleeker appearing than the passenger side converter and that gets the mindset that it has to be better. 


The theory that the driver’s side catalytic converter is the “high flow” converter of the two found on LS1 F-Body, ranks up there with high performance muffler bearings.  The data above shows that the passenger side catalytic converter flows more than 35 cubic feet per minute (cfm) more than the driver’s side cat.  That is a significant difference. 


The real shocker of our flow test involves the TTS Bullet catalytic converters.  The total combined flow of two TTS catalytic converters would be 554.2 cfm.  Compare that to the modified factory catalytic converters of 567.8 cfm.  Something is definitely wrong with this comparison.  The TTS cats are not even close to delivering the flow of either FLP or Carsound.  Installing a set of TTS Bullet cats on your car would be taking steps backwards compared to the stock units found on LS1 F-Body’s.  How TTS can sell these catalytic converters with their excellent long tube headers is beyond anything imaginable.  TTS does have a new oval catalytic converter that should outperform their current catalytic converter.  The new oval unit is currently being tested for OBD II compliance.


The FLP and Carsound converters strutted their stuff on the flow bench.  Both sets flowed over 190 cfm per pair more than TTS converters.  These test results were what we expected to find with every set of aftermarket high flow catalytic converters.  No one should hesitate to place these to converters on their car.


Mathematical Models


Each pair of catalytic converters restrict about eight (8) rear wheel horsepower on stock LS1’s.  David Vizard’s research shows that 2.2 cfm of flow at 20.4” of H2O will support 1 horsepower.  2.2 cfm is the optimum flow through the exhaust system and reflects open pipe flow.  Removing the stock catalytic converters on the LS1 usually results in a gain of 8 rear wheel horsepower.  Assuming that the average LS1 puts 300 horse power to the ground, that is a 2.67% increase in power. 



        Stock catalytic converters:



Final thoughts


Adding a cat back to your exhaust system helps, but by no means makes it more free flowing than replacing the stock exhaust manifolds on back to your muffler tips with headers, larger diameter pipe and high flow mufflers.  Replacing just your catalytic converters with high flow pieces may not necessarily help flow if you have a factory muffler on your car.  The same could be said if you have a high flow muffler or run an exhaust cutout, but have factory cats installed.  So be smart and plan your exhaust system out and realize that replacing just one piece of the exhaust system isn’t the end of the job.





1.  Vizard, David. How to Build Horsepower. Volume 3.  Page



Web Author: Eric Barger
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Revised: June 07, 2007.