A dyno can detect the difference of a one degree inlet temperature vs a run before also.

I went from gutted cats to high flow cats with everything Tig welded and road tested within 2 hours and the difference was only just detectable. This with a 4.6litre with 5 speed on the same road as before from low rpm's to 6,200 rpm's.
Granted a plugged up cat or one with broken and melted guts will be highly restrictive. OEM want the best flow and HP numbers for their vehicles staying within the smog laws and limits for their stock production motors.
Reminds me of people who hate Holley carbs thinking they can make it better by modifying it better than the engineers.

 

A dyno can, but I think the noise from variations between runs, such as temperature, humidity, and atmospheric pressure variations will be greater than the difference of one degree inlet temperature, so you will have no way to show that the difference in the dyno run was due to the inlet temp or these other factors.

 

I wanna see those dyno graphs.

 

Close as I care to get to a dyno is the in vehicle G-Tech, in my case 25-90 mph in 4th came up damn near the same in distance and speed numbers this from lugging to 62,000 rpm zone.

 

Sorry, I meant that toward -

"what bear river claims, cats DO restrict airflow and affect the powercurve of engines. and dont try to tell me otherwise ive seen the dyno graphs to prove it."

The point is, probably 99% of the time when good factory or aftermarket cats are used, the difference is not enough to notice.

Beemer Nut, just how does a Ford sound at 62,000 RPM?? LOL Just kidding.

 

Oops me bad unless it's a gas turbine.
This testing was done on the 4.6 litre
LandRover Discovery motor that had minor port
work then intake and exhaust gasket matched.
Added a small cam with chip and Borla's cat back with
CarSound high flow cats.
Now Rover's OEM cats are a sorry restrictive POS,
that seem to work ok with a stock 3.9 motor with auto
trans. With the 5 speed and larger 4.6 that is breathing better
they would choke the motor and make it run hot besides it is
normal for their cats to rattle and fall apart. And ya thought British
had the market cornered on oil leaks and bad electrics.

 

Yeah, I can second that the OEM cats on the Discovery were restrictive on one side (the small round one). The other side however probably flowed more freely than the aftermarket cats you replaced it with, until they plug themselves. Range Rover is a terrible vehicle unless you go through all the electrical and fix them up. A guy I know completely rewired his, and replaced the injectors with those out of the F-150. He also swapped in the computer from the F-150. It has plenty of power now, and also picked up about 2 mpg. I don't know the owner personally, but I know the guy who did most of the work. It has a custom made sticker on the back that says "Hummer Recovery Vehicle"

 

It's funny; the Cat-gut advocates are probably hurting their power output from leaning the mixture more than anything else. In the performance world, exhaust modifications must be accompanied by carburetor/injector fuel enrichment to utilize that extra airflow. A properly tuned engine with a high flow cat/cat-back system will make more power than any hack job gutted cat/glass pack rig.

 

I had a set of round and flat cats off a 95 Discovery that had rolled with only 5,200 miles (dealer demo vehicle, I bet it failed the roof slide) that I used to compare the flow differences between gutted cats, OEM and CarSounds high flow cats. I borrowed a CarSound cat from my friends muffler shop before purchasing a pair of them.
I used 3 commercial grade 6.5 HP (yea right 6.5 HP on 110 volts what a joke) vacuum cleaners hooked up to a section of 4" PVC with PVC caps on the ends, one end drilled out to 2 1/4" to seal against the cats with a vacuum gauge on the other end cap.
I found that both style Disco cats round or flat flowed about the same, gutted a lot better and the matched set of flat CarSound high flows a little less than the gutted cats but much better than either OEM Disco cats.

As far as "they plug themselves" I haven't noticed a difference in performance
or speed from new over the last 57K miles going up the mountains same roads and grades and weight 3 times a year the last 5 years. Same speeds and gears pulling grades from sea level to 9,600 ft. Yes I have notes on the grades with speed results and gear I pulled floored, got to love 5 speeds.

Just passed smog here in Cali before the new fuel tank pressure leak test begins in November 1.

New cat test HC idle 6, HC 2,500 7 (PPM) max allowed 100 and 180 (PPM)
57,000 mile test HC idle 9, HC 2,500 11 (PPM)
%CO2 idle 14.4%, 2,500 rpm 14.6%
O2 idle and 2,500 rpm 0.3%
CO idle and 2,500 rpm 0.00, max allowed 1.00% and 1.10%
These numbers are a sniff test only as the Disco is a full time 4 x 4
and the state doesn't have 4 wheel drive dynos.

So far so good plus almost the same results with a MagnaFlow (CarSound)
high flow cat on a 87 325is BMW.

 

Well, many OEM catalyst substrates in an effort to help them last longer without breaking apart are made using thicker walls than most aftermarket converters. And to a large extent it does help, but the real problem with converters that break apart and plug has little to do with the substrate itself.

Gutting the cats is not likely to lean out the mixture on a FI vehicle, since the computer is just going to adapt. However, it does create an expansion chamber that slows down the exhaust, and also cools it. This so if anything, the gutted cats might perform better than a factory cat, but probably worse than a good high flow cat or straight pipe.

Part of the restriction from a factory cat has to do with the length of the substrate. Most "high flow" aftermarket converters are just 4" long, while OEM cats are often 6" - 12" long depending on year make and model. So between that and the thicker walls you get some performance losses, but its really hard to tell the difference without a dyno.