mmthomas

Back in my day HJS made the best catalytic converters for street and race.

Does anyone know if any of the current C8 exhaust components suppliers use HJS as a core?

Does anyone know if HJS is still the highest flowing kettle converter?

ArizonaZ06

I've had my HJS Cats now over 3 years, they match up perfect with my headers and the Z51 performance NPP exhaust. Fabspeed and Soul believe are the largest sellers of these Cats today. These high quality 200 cell Kevlar wrapped Cats never have thrown me a CEL, they are pricey but in my opinion worth it and second to none.

Performance nut

I had read that aftermarket cats lose power on the C8. Not sure if HJS was tested or if it was another cat brand

Kracka

Not true, assuming you're talking about quality cores such as HJS (German) or GESi (North American). The generic Chinese junk is a crapshoot.

HJS = Fabspeed & Soul
GESi = Corsa, Cicio, Kooks, aFe, Paragon, etc.

You may be thinking about Kooks headers, which have been proven to lose power when paired with the factory cats.

Kracka

Same experience with my Corsas, but yes, the HJS 200-cells will ultimately outflow my 400-cell GESi G-Sport S-Wound cores. I took a slightly different approach and had my cats ceramic coated to match my aFe headers; engine bay temps remain consistently cooler than they were when stock. I'm still happy with my setup 2+ years later

Performance nut

https://americanracingheaders.com/bl...aders-from-arh

The larger 6" stock cats out performed the higher flowing but smaller 4" performance cats. Hats off to GM for building a great cat.

Granted they didn't name the cat manufacturer so it's left to interpretation. I haven't seen anyone do a comparison that wasn't trying to sell you their product so I honestly don't know. I can state what they have done with my other cars though. Question is if anyone dynoed before and after on a C8.

Kracka

4.0" is a small-ish core at our power levels, and I think Paragon was (past tense) the only legitimate shop actually using a core that size on the C8. My guess is ARH was attempting to use the same generic 200-cell (see my earlier comment above) they used on their C7 header systems. C7 cats are much more restrictive compared to the C8's (partly because of the inlet flange design), so you could still see gains using the cheap generic 4.0" cores (C7 also had four cats from the factory vs. the C8's two).

itsonlyairandfuel

What did your car gain with your exhaust modifications? dyno or track times? What did you use to monitor engine bay temperatures? I have only installed a soler t body and socal ported intake so far. Trying gather useful data. Thanks.

Kracka

Same oven thermometer I used to measure trunk temps before/after adding the additional thermal barrier. It's also obvious based on how the after-run cooling fan operates after engine shutdown (especially during TX summers).

I'm > +30whp (DynoJet) vs. stock at this point, which includes all my mods, not just hot side. I haven't dyno'd since adding the Borla, but I'm due to get back on.

Driving my car vs. stock is night & day, both in my eyes, and those who own the stock cars. Headers were the single biggest power adder. Overall response and increase in top-end pull is notable. The LT1/LT2 is a total dog above 5k RPM in stock form.

itsonlyairandfuel

Thanks for the info, just ordered the DEI heat wrap. Next come exhaust upgrades. But first LS Fest next week. Hope to see a strong corvette presence.

Kracka

So you know exactly what I'm using for thermal mangement:

• aFe headers: Cerakote Copper ceramic coated (internal) & Cerakote Titanium V-164 ceramic coated (external)
• Corsa cats: Cerakote Glacier Titanium C-7900 ceramic coated (external)
• Borla exhaust: their standard heatshields + DEI Titanium header wrap at the inlets, below the airbox
• Trunk: DEI Reflect-A-Cool (silver) adhesive sheets on the backside of the trunk carpeting

mmthomas

Great info!

Does anyone know the specs on the stock cats?

Sounds like stock maybe 6”/150mm dia, and maybe 3-400 cpsi, whereas aftermarket is 4-5” and 200 cpsi?

What is reason for the O2 spacer? Seems like he would add his hysteresis into a closed system?

Kracka

Stock = 700-cell

The spacer is to prevent a CEL from a cat that isn't quite efficient enough at scrubbing the exhaust to prevent a light.

mmthomas

Wow…scrubbers indeed…700 cpsi stock vs 200 from hjs.

Has anyone measured free flow area or bench tested flow rate between stick and aftermarket?

itsonlyairandfuel

I have asked that question in the past. I suspect it it very restrictive.

mmthomas

so this begs the question what cat Configuration wont throwing a CEO light and will equal the same flow rate as a straight ~3 inch pipe?

For example, if the stock is 700 cell per square inch, and we want to go to a 350 cells per square inch what’s the increase in cat core diameter required ?

Kracka

Fabspeed claims the HJS 200-cell cat core flows 93% vs. a 3.0" straight-pipe. I have not seen this rating for the GESi 400-cell.

BPHORSEGUY

Absolutely UNTRUE. And I tested my buddies 2Ms with a calibrated IR gun and they were 30-50 degrees cooler under all conditions!

mmthomas

What is the diameter and length of the fab speed core that flows 93% of the 3 inch pipe ?

Kracka

Stingray:
​​​​https://www.fabspeed.com/fabspeed-ch...ort-cats-2020/

Z06:
https://www.fabspeed.com/fabspeed-ch...ort-cats-2023/