Flow bench testing an exhaust is only a rule of thumb and its actually a very poor rule of thumb given all the dynamics of an automotive engines spent exhaust gas flow. Its certainly really means nothing in terms of determining quality sports car sound, power/torque, and durability. I have access to a flow bench but never utilized it for testing. I use the dyno, and my ears for qualitative and quantitative results.

100 cell catalytic converters add perhaps 2% extra flow over an equivalent sized HJS German imported 200 cell catalytic converter if all parameters remain constant. To clarify one has to take the same size catalytic converter lets say 100 cell, 200 cell, 300 cell for example like 4.0" 102mm catalytic converter and proceed with testing. One cat can be 100 cell 4" round and the 200 cell cats 5" and the 300 cell 6" round . SIZE matters .

Many times over 25+ years I have seen countless magazine articles and internet blogs/ heroes that say Ive got 100 cell cats on my 804HP McLaren or Dodge Demon and I'm good to goI WOO WOO- Stoked. You look closely and that catalytic converter was tiny and short and YES its 100 cell . The engines 100% choked at that size. Often you dont hear that he / she had to tune it for NO Check Engine Light- that's illegal and in short order that tiny 100 cell cat or any cat that size in Drastically Undersized vis a vi the Power output of said engine............. it will blow up and fall apart into the exhaust pipes. They often dont mention any of the negative issues they experience.

FYI- I was a Chemistry Physics major and its wild and stupid how many people on forums will brag and assume that a 100 cell catalytic converter will simply outperform a 200 CPSI cat and a in 2018 to 2021 many car enthusiasts often assume that a CAT-BYPASS pipe/ Cat delete automatically increases horsepower torque and that any and all catalytic converters are mutually exclusive for performance. I'm here to tell you I've seen it all and quality HJS German extra large EURO Homologated 200 CPSI catalytic converters provide 90% to 92% flow of a straight pipe and clean no Check engine light performance and NO TUNE required. The HJS Emissions technology team provides catalytic converters that are Euro Homolagted, TUV tested approved and certified in multiple European unions. Its a beautiful wonderful environmentally friendly responsible high technology developed by EMITEC a Tier 1 global supplier and HJS in Menden Germany.
Here is something for you you car guys/ gals to absorb:
On all the late model Porsche, Mclaren and Ferrari cars simply removing the OEM cats and the engine might pickup some power fast as the Twin Turbos spike as the boost comes on on off idle to low RPM. The GT2RS and recent Mclaren and Twin Turbo Ferraris and Mercedes all do the same thing. Intial gain down low and then same power as stock all across the RPM band . Bottomline intial spike of power boost off idle to 1500 to 2500 RPM and then the ECU immediately TAKES IT ALL Back to redline. There is no free lunch and that includes TUNING it as well. These late model high performance and ultra performance cars dont want the turbochargers to spike at 200,000RPM and pull back immediately and dont learn to accept it.

Furthermore going catless is irresponsible and you have a completely illegal car described as a "Gross Polluting car" in all 50 states and the car stinks , pollutes and has a Check Engine light P420/P430 code for catalytic converter efficiency below threshold. EPA carb and all foreign governments want to put an an end to these gross polluting vehicles.

At Fabspeed Motorsport we import 100% genuine HJS Emissions technology catalytic converters direct from HJS . Monthly air cargo arrives from Lufthansa. Our engineers consult with Klaus and Sven and ultimately we utilize the largest possible HJS 200 CPSI catalytic converter that will fit on the cars chassis . For example we use 1200 horsepower capable catalytic converters for the GT2RS . At FABSPEED we actually deploy use 2 of these for the GT2RS, Mclaren 675LT , Ferrari 488 pista and F8 tributo and 812 Superfast so the car makes more power all across the RPM band with superb sports car sound, NO CEL lights, no fault codes, not illegal fraudulent tunes, and 100% reliability that European homologated and Autobahn tested.

HJS Germany was so impressed that 15+ years ago Klaus Osterhaus appointed Fabspeed as HJS Emissions Technology their exclusive North and South American importer distributor service center. If you want genuine HJS products with a valid warranty simple call Fabspeed for your doityourself weld in catalytic converters or HJS catted systems.

Very shortly at Fabspeed we should have a GT2RS and a F8 Tributo spider and Ill post up sounds and performance with massive HJS catalytic converters!

^ Joe, I enjoy hearing an expert properly describe my amateur sense of the topic of the dubious pros/cons ratio of going catless or with poor quality cats.

I was reading along hoping to see you comment more directly about the performance levels that can be supported with the OEM cats on a car like a 991 Turbo. FVD says that their dyno testing shows that there is not enough restriction from our OEM cats to get a significant performance gain from changing to aftermarket sport cats, until somewhere around 680 whp is being deployed. You must have explored this, can you comment as to if you have found this to be roughly about right?

Only comment that I will make to avoid a useless WW3 is .......we have used flow benches for many years on cars running 3.6 seconds at +200mph in the 1/8th mile with great results. As have MANY racers.... Especially on exhaust modifications. Turbo's do not like pressure drop or high EGT's...especially when you push them against the map with smaller turbos like Porsche uses...... But we do not sell parts.....just race fast turbo cars.....

onfireTTS,

Lots of great info on this thread and I appreciate the data gathering / sharing. I don't frequent Rennlist but would love to collaborate with you on some projects as another fellow engineer.

If you can contact me at them3maestro@gmail.com (I dont have DM on this forum) Sam will vouch for me :thumbup:

Thanks

Oh noooooooooo :roflmao:

Engineer party let’s do this!

You know it :D Temperature is slipping so soon I'm have her on my new QuickJack for winter maintenance. Lets see if we can tweak the setup a little over this winter too, always good to throw in some fun parts not just oil and plugs lol. Thinking of improving the compressor inlets (over my aftermarket ones) and maybe play with the exhaust? We will see.

Bring it, don't sing it

OK, i get that the straight pipe flows good, but I live in Cali, and I dont wanna get ref'ed. My friends have been there and believe me not, it's a flying ITCH with a B in front.

How much am I leaving on the table with the factory Cats versus straight piping and how much louder is this with a 3" straight pipe exhaust? Or am I just thinking out loud now?

easy fix, text me.

I know next to nothing about car turbines but I do run gas turbines in large model helicopters and have found a right angle bend like that in exhaust tubing produce enough back pressure to produce an ecu generated shutdown- anything to be gained by more laminar flowporting of tubing?

Absolutely true that a nice mandrel transition would outflow that pinched 90.

I'd like to hear the fix too! PM coming your way