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I have a 427W over here in the UK and it's hard to get first hand knowledge of what adding cats would do to the power.

Due to its age my car by law does not have to have them fitted, but does have to pass a yearly roadworthiness test which includes checking exhaust emissions at idle.

I'm allowed 3.5% CO and 0.12% - 1200ppm unburnt hydrocarbons.

My neighbours also hate on me for my noisy daily driver :LOL:

Would a pair of high flow 3" sports cats hurt power by much?

If they are not too restrictive I may be able to keep the car quieter, less smelly and not piss my neighbours off any more than they already are!
 

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daily driver............

run the cats

max effort 100% of the time is obviously not what you do.
 

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1983 Ford Fox Body Mustang GLX Convertible
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I have a 427W over here in the UK and it's hard to get first hand knowledge of what adding cats would do to the power.

Due to its age my car by law does not have to have them fitted, but does have to pass a yearly roadworthiness test which includes checking exhaust emissions at idle.

I'm allowed 3.5% CO and 0.12% - 1200ppm unburnt hydrocarbons.

My neighbours also hate on me for my noisy daily driver :LOL:

Would a pair of high flow 3" sports cats hurt power by much?

If they are not too restrictive I may be able to keep the car quieter, less smelly and not piss my neighbours off any more than they already are!
A high flow cat shouldn't hurt your power. By the numbers horsepower will lower. It will be expensive tho when you pick the correct one. Unfortunately, if you want a good cat it is gonna be expensive and you have to get it new. Older performance cats are very restrictive and brands like Magnaflow, Kooks Headers, and Flowmaster make modern cats. Idk what names or brands you have in the UK but if you can get your hands on a couple of Kooks Headers GE-307540 but I've heard they are worth it if you can afford. What I think that will work for you are Flowmaster 2230130, Flowmaster 2230130 Flowmaster Universal Catalytic Converters | Summit Racing

Hope the neighbors are happy.
 

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Discussion Starter #4
A high flow cat shouldn't hurt your power. By the numbers horsepower will lower. It will be expensive tho when you pick the correct one. Unfortunately, if you want a good cat it is gonna be expensive and you have to get it new. Older performance cats are very restrictive and brands like Magnaflow, Kooks Headers, and Flowmaster make modern cats. Idk what names or brands you have in the UK but if you can get your hands on a couple of Kooks Headers GE-307540 but I've heard they are worth it if you can afford. What I think that will work for you are Flowmaster 2230130, Flowmaster 2230130 Flowmaster Universal Catalytic Converters | Summit Racing

Hope the neighbors are happy.
I doubt my neighbours will ever be happy!

Will look into what's available over here, would a decent pair not restrict a 500hp engine too much?
 

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Also interested in this. My car smells horrendous. And the smell sticks to everything. Gotta shower after every drive.
 

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many moons ago back in my 302 days making 332/332 with the factory shortblock, i changed from an off road to high flow bassani x pipe with long tubes and only lost like 4rwhp
 

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I doubt my neighbours will ever be happy!

Will look into what's available over here, would a decent pair not restrict a 500hp engine too much?
Not being a jerk, but let's be honest:
The RPM range at where high flow catalytic converters could kill horsepower is an RPM range you will rarely spend any time at, even if you are hitting a drag strip every weekend, or hitting a road courses.
I hope you can find what you need.
 

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I doubt my neighbours will ever be happy!

Will look into what's available over here, would a decent pair not restrict a 500hp engine too much?
On my s trim 352, put down 636rwhp thru aod using 1 3/4 shortys, and mac cats with a custom cam to pass emissions. I think i still have one of those cats brand new in the box in my basement.
 

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Any of the 300 cell metallic substrate GESI cats are effective, durable and last a long time. Kooks sells them through summit racing and jegs (part # 307540 3"in/out). Stay away from anything cheap ceramic core, even most Magnaflow converters. They don't last because they use small amounts of precious metals and tend to fail quickly if slightly overloaded with HCs. You will want to keep an eye on A/F ratios especially on a carbureted engine to make sure you don't overwork them. As for HP loss, you won't be seeing more than about 5hp with appropriately sized metallic core cats. They won't do much to reduce noise, maybe take the edge of high rpm raspiness.

I run cats on all of my cars, not just because I don't want to smell like a refinery after driving the car, but also not wanting to be a hypocrite and hate sitting behind a car at a light that makes my eyes water. Its completely unnecessary and stupid to not have a catted exhaust these days, even on an older car with a healthy cam. If you're good about keeping the engine running well, there's little to no detectable impact on performance running good quality high flow cats. Having to breath unburned HCs and CO is really bad for you, even in small amounts.
 

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Discussion Starter #12
Any of the 300 cell metallic substrate GESI cats are effective, durable and last a long time. Kooks sells them through summit racing and jegs (part # 307540 3"in/out). Stay away from anything cheap ceramic core, even most Magnaflow converters. They don't last because they use small amounts of precious metals and tend to fail quickly if slightly overloaded with HCs. You will want to keep an eye on A/F ratios especially on a carbureted engine to make sure you don't overwork them. As for HP loss, you won't be seeing more than about 5hp with appropriately sized metallic core cats. They won't do much to reduce noise, maybe take the edge of high rpm raspiness.

I run cats on all of my cars, not just because I don't want to smell like a refinery after driving the car, but also not wanting to be a hypocrite and hate sitting behind a car at a light that makes my eyes water. Its completely unnecessary and stupid to not have a catted exhaust these days, even on an older car with a healthy cam. If you're good about keeping the engine running well, there's little to no detectable impact on performance running good quality high flow cats. Having to breath unburned HCs and CO is really bad for you, even in small amounts.
Sounds like they are worth fitting, I will be running stand alone engine management so getting the fueling right shouldn't be a problem.
 

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It's definitely interesting to hear the different (read:mature) responses on Forums with regards to running cats or not vs some of the BookFace groups.

I run cats. Not worried about the loss of 5HP on a street car.
 

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I've worked in electronics manufacturing my entire career. You won't find many of these chemicals in factories any longer, and access is much more restricted than is was 35 years ago. One thing I like to do is guess which chemicals (besides the obvious, like ethanol) I can detect in my exhaust. I have a 427w with a decent sized cam, so there's lots of choices, especially when the engine is cold:

Gasoline (86290-81-5) 100
Benzene (71-43-2) 0.1 - 4.9
n-Butane (106-97-8) < 10
Ethyl Alcohol (Ethanol) (64-17-5) 0 - 10
Ethyl benzene (100-41-4) < 3 n-Hexane (110-54-3) 0.5 to 4
Methyl-tertiary butyl ether (MTBE) (1634-04-4) 0 to 15.0
Tertiary-amyl methyl ether (TAME) (994-05-8) 0 to 17.2
Toluene (108-88-3) 1 - 25
1,2,4- Trimethylbenzene (95-63-6) < 6
Xylene, mixed isomers (1330-20-7) 1 - 15
Naphthalene (91-20-3 )0.1-0.5

Wish I had cats at times. My initial interest in cars as a kid was party due to the nice smell of car exhaust in the 60's and 70's. Now in the best case it smells like the old bars where I grew up, worst case it makes my eyes water and organs ache.
 
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Yeah, I too had to deal with my fair share of MEK and carbon tetrachloride in my electronics days. The stuff in modern gasoline is really nasty garbage. I still use Xylene and Toluene to boost octane in the so called premium 91 gas we have here. Those are some horrible chemicals to be around.

The reality is when they started taking out the lead in gasoline to accommodate the use of converters, they had to up the VOCs and other aromatic HCs to boost knock resistance (octane rating). Tetraethyl lead gets a bad rap even though very small quantities were required to increase the knock resistance. In contrast what they replaced it with is so much worse in raw vapor form and in exhaust form coming from a cold, freshly started engine. After the second you turn the key and until the converters get to operating temp, the exhaust has some really nasty byproducts in it that are extremely bad for you, even in small quantities. Only when the converters are up to full oxidation temp, they get rid of virtually all of that, but only if they're working right and if the engine management is running in closed loop fueling mode.
 

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I'm putting down 400RWHP through a 2.5" dual exhaust with a pair of Magnaflow spun-metallic hi-flow cats. I have them for the anti-stink effect as well, and will never run an off-road pipe again.
 

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If you can get flow ratings for the converters you're considering, you might be able to estimate the HP loss based on engine displacement and the rpm in question. Most ratings correspond to about 1 psi pressure drop, but the actual exhaust flow will vary (either higher or lower than the flow at the rated pressure drop) so the pressure drop will rise or drop accordingly.

Rule of thumb at least used to be 1 psi of exhaust flow resistance (aka backpressure) cost about 2.5% power (Marks' Mechanical Engineers' Handbook). 3% might not be far off.

Back in the 1970s and their low-flow cats you'd have been looking at 3 psi or more with dual cats and 7.5% - 9% power loss with dual cats. Most exhausts back then were single, and trying to run your engine through one of those would have been at least 10 psi and 25% power. 10% - 25% then, 3-ish % now.


Norm
 

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How are those cats holding up for you?
They've been on for five years, but probably less than 10K miles. I don't have to have emissions testing here, but the stink is still being held at bay. (Not a great answer, I know.)
 
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