Need help with exhaust questions on 1990 K3500 7.4L

[William Keiter]

What I see online looks like the cat is coward the muffler, single cat. I believe it’s a resonator behind if I understand.

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[Schurkey]

I'd be more likely to believe the muffler goes behind the catalyst. I think the prior illustration is incorrect.

 

[andy396]

I'd be more likely to believe the muffler goes behind the catalyst. I think the prior illustration is incorrect.

I’m with you, but my ‘95 had the original exhaust system on it and it was as shown in the illustration I shared. The second illustration is probably correct if you had the big pancake style cat.

 

[1ofthesedays]

Depending on how GM certified your truck, you might be buying TWO catalysts.
My '97 has two, originally leading into a dual-inlet, dual-outlet muffler. I don't know how the '90s were certified. The GM service manual would have that info.

The shell of the new catalyst(s) will be stainless steel, but any pipes they replace will likely be "aluminized" mild steel. Aluminized steel won't last as long as the OEM pipes. It would cost extra to have new stainless steel pipes installed. Pretty-much the same deal for the muffler. The aftermarket replacements don't last as long as the originals. unless you pay extra for Stainless.

The later, Vortec 7.4L exhaust had a VERY restrictive muffler. I don't know about the earlier trucks. The Vortec muffler had big inlet and outlet tubes, but the internal tubes that you can't see when the muffler is installed, were very small. The "hot tip" seems to be to scrap the OEM muffler, replace it with a pair of aftermarket mufflers installed "on edge". Oval cases installed vertically, side-by-side--sort of like this: (o)(o)

Crap. My illustration looks like a pair of boobs instead of a pair of mufflers viewed from one end.

Depending on how GM certified your truck, you might be buying TWO catalysts.
My '97 has two, originally leading into a dual-inlet, dual-outlet muffler. I don't know how the '90s were certified. The GM service manual would have that info.

The shell of the new catalyst(s) will be stainless steel, but any pipes they replace will likely be "aluminized" mild steel. Aluminized steel won't last as long as the OEM pipes. It would cost extra to have new stainless steel pipes installed. Pretty-much the same deal for the muffler. The aftermarket replacements don't last as long as the originals. unless you pay extra for Stainless.

The later, Vortec 7.4L exhaust had a VERY restrictive muffler. I don't know about the earlier trucks. The Vortec muffler had big inlet and outlet tubes, but the internal tubes that you can't see when the muffler is installed, were very small. The "hot tip" seems to be to scrap the OEM muffler, replace it with a pair of aftermarket mufflers installed "on edge". Oval cases installed vertically, side-by-side--sort of like this: (o)(o)

Crap. My illustration looks like a pair of boobs instead of a pair of mufflers viewed from one end.

 

[William Keiter]

Well I’ll be dipped! The exhaust shop called shortly after dropping off truck and said the cat goes behind the muffler. On top of that they said it needed to be heavy duty or it would just melt down and need to be replaced again . So the quote went from $250 on the phone to almost 4x that due to adding the pipe behind the muffler(previous delete) and gasket on drivers side leaking, plus whatever tip they wanted to put on end. For the price I should’ve been able to run an entire system with headers and dual exhaust...it be like that sometimes.

 

[Schurkey]

I'd be more likely to believe the muffler goes behind the catalyst. I think the prior illustration is incorrect.

Well I’ll be dipped! The exhaust shop called shortly after dropping off truck and said the cat goes behind the muffler.

Thanks for that. I learned something today.

 

[William Keiter]

Installed cat, smog passed but spent way more than expected

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