Horsepower Lost with a CAT (Page 1/1)
Dennis LaGrua OCT 22, 10:28 PM
I came across a You Tube video where a car owner was interested in how much horsepower was made by his engine with a catalytic converter in the exhaust as compared to using a straight pipe for an unrestricted the exhaust, The guy did a test before and after and the results show that the catalytic converter makes the engine lose 25 horsepower
Dyno test 252HP @ 210.1 fts lbs torque with the CAT
257.7 HP @217.2 ft lbs of Torque with a straight pipe

Dyno Tests w and W/O a Catalytic Converter

Probably a great idea for the track or off road-legal in the USA

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" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

[This message has been edited by Dennis LaGrua (edited 10-23-2025).]

Patrick OCT 23, 12:14 AM

quote
Originally posted by Dennis LaGrua:

Dyno test 252HP @ 210.1 fts lbs torque with the CAT
277.7 HP @217.2 ft lbs of Torque with a straight pipe



Dennis, where are you getting 277.7 HP from?



So, less than a 5 HP increase on a boosted engine that has 252.8 HP with a cat.

Did they actually state the condition of the cat? Maybe it was in less than ideal condition. A fairer test would be with a new cat.

Nevertheless, IMO the gains on a naturally aspirated Fiero 90 HP 2.5 or even a 140 HP 2.8 would be insignificant (with a cat delete compared to retaining a free-flowing newer cat).

[This message has been edited by Patrick (edited 10-23-2025).]

82-T/A [At Work] OCT 23, 08:10 AM
I think the biggest thing, especially for older cars like ours, is to swap out those old pellet-style cats with newer honeycomb style catalytic converters. I think you'd agree (and I think I've seen you say this before a few times too), replacing those older cats with newer cats really make a huge difference in these older cars. It's too late now, but I'd love to know what a stock Fiero with the old cat, how it performs compared to a stock Fiero with the newer cat. I don't hesitate for a second to say that a 92hp Iron Duke will see a 5 horsepower increase JUST from replacing the cat with a newer one.

For instances like this guy's video... I looked at the cat, you could see some veins in the ceramic and metal honeycomb were plugged. It was an older cat, and I suspect that even though it was functional, it was probably more restrictive than it could have been... but I also wonder why he wouldn't just go with a larger cat. If larger CFM is the goal, they have larger catalytic converters that do meet those requirements.

I'm not a big proponent of all the emissions stuff, because history shows the vast majority of stuff on our cars are knee-jerk decisions that are wildly and poorly implemented. Smog pumps, charcoal pellet cats, EGR systems, AIR injection, etc. All of these things, when implemented didn't NEED to exist when the EPA could have just mandated the newer style of cats we have now, and eliminated all of those things (it was the usual lobbying from the company that produced the old style that won-out, even though it was less efficient).

I'm on my soap box, but yeah... I think in most cases... unless you're on a track and spending all your time at higher rpms... a higher-flowing cat will get you where you want to be.
Vintage-Nut OCT 23, 09:18 AM
Unfortunately, 'old-school' flat-tappets and catalytic converters don't mix well...

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Original Owner of a Silver '88 GT
Under 'Production Refurbishment' @ 136k Miles

Patrick OCT 23, 02:21 PM

quote
Originally posted by 82-T/A [At Work]:

For instances like this guy's video... I looked at the cat, you could see some veins in the ceramic and metal honeycomb were plugged. It was an older cat, and I suspect that even though it was functional, it was probably more restrictive than it could have been... but I also wonder why he wouldn't just go with a larger cat. If larger CFM is the goal, they have larger catalytic converters that do meet those requirements.



I was wondering if the cat used was the one that originally came on this car before the engine was boosted, in which case it was indeed probably undersized for an engine that would now benefit from added exhaust flow. (EDIT- After doing some research on this YouTube channel, it turns out this was the original cat, and the engine was originally naturally aspirated.)

I just didn't want anyone, especially someone new to Fieros, to watch this video and get the false impression that ripping out their cat was going to accomplish much of anything in regards to making more power.
Dennis LaGrua OCT 23, 03:21 PM

quote
Originally posted by Patrick:

Dennis, where are you getting 277.7 HP from?



So, less than a 5 HP increase on a boosted engine that has 252.8 HP with a cat.


Typo Sorry about that. Agree that a much larger CAT should not be much of a restriction

------------------
" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

82-T/A [At Work] OCT 23, 03:36 PM

quote
Originally posted by Vintage-Nut:

Unfortunately, 'old-school' flat-tappets and catalytic converters don't mix well...





How do you mean? Do you mean in terms of efficiency?



quote
Originally posted by Patrick:

I was wondering if the cat used was the one that originally came on this car before the engine was boosted, in which case it was indeed probably undersized for an engine that would now benefit from added exhaust flow. (EDIT- After doing some research on this YouTube channel, it turns out this was the original cat, and the engine was originally naturally aspirated.)

I just didn't want anyone, especially someone new to Fieros, to watch this video and get the false impression that ripping out their cat was going to accomplish much of anything in regards to making more power.



Yeah, totally. Back in the day, I owned a Pontiac Solstice (when they were new). I'll never forget the performance increase I saw... it was like night and day. It had the 2.4L VVT Ecotec, and I swapped out the factory log manifold with the clear image shorty header. Made a huge difference.

But, it was when I swapped out the factory catalytic converter with a higher-flowing catalytic converter, and OMG... I went from running mid 15s to mid 14s. Just the catalytic converter, which was intentionally sized larger and sold as a stock replacement performance upgrade for the Solstice, made that enormous difference.

I wish cameras were better back in 2006 and 2007. That really feels like yesterday, but cameras sucked back then, and I just didn't think to take any video. The car was totally different. I was blowing away Lexus LX460s, BMW 330s, 350Zs... I just couldn't believe it.

EDIT: To add to that, I am sure the higher-flowing catalytic converter "enabled" the aftermarket header too fully shine, but damn... what a difference it made. And I never, not once... had an issue with Oxygen sensors, everything operated perfectly.

[This message has been edited by 82-T/A [At Work] (edited 10-23-2025).]

Vintage-Nut OCT 23, 04:20 PM

quote
Unfortunately, 'old-school' flat-tappets and catalytic converters don't mix well...




quote
82-T/A:
How do you mean?



I believe you know that flat-tappets needs "Zinc" or ZDDP {Zinc Dialky Dithio Phosphate} to protect the cam/lifters from metal-to-metal contact.

Unfortunately, Zinc / ZDDP creates a film inside catalytic converts and "poisons" the CAT.

So, many 'modern' motor oils have less Zinc / ZDDP to extend the catalytic converter life, but they also shorten the life of flat-tappet camshafts!

I'm on my third CAT as they're 'cheaper' than rebuilding the engine.......

------------------
Original Owner of a Silver '88 GT
Under 'Production Refurbishment' @ 136k Miles

82-T/A [At Work] OCT 23, 08:40 PM

quote
Originally posted by Vintage-Nut:


I believe you know that flat-tappets needs "Zinc" or ZDDP {Zinc Dialky Dithio Phosphate} to protect the cam/lifters from metal-to-metal contact.

Unfortunately, Zinc / ZDDP creates a film inside catalytic converts and "poisons" the CAT.

So, many 'modern' motor oils have less Zinc / ZDDP to extend the catalytic converter life, but they also shorten the life of flat-tappet camshafts!

I'm on my third CAT as they're 'cheaper' than rebuilding the engine.......





I kind of knew that, but I didn't know it was so pronounced. I guess I just assumed that newer oil was better, and that the synthetic made up for the lack of zinc. But I didn't know it affected cats. Ugh. Yeah, I'd be replacing cats too before I'd want to rebuild an engine.