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Old 08-22-2017, 07:33 PM   #1
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Stock Exhaust and Cats

First post in a while! The truck is coming out of storage. I do not need to "smog" my truck, but would like to clean it up as if I had to smog it. Eventually this means getting the charcoal canister and other stuff refreshed, but the first (and easiest) target is the exhaust.

Right now, it has LMC manifolds into true duals (no cats) that dump beneath the bed. The truck is a 78 C10 305 V8.

I would like to get a stock setup with a cat(s), manifolds and proper exits like the truck originally came with. The LMC manifolds should be fine, but I'm at a loss for the rest. Does anybody make repro parts for this? Or would I have to have one fabricated based off the brochures and documents with the truck?

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1978 C10 Silverado
1991 Camaro B4C
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Old 08-22-2017, 11:54 PM   #2
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Re: Stock Exhaust and Cats

It looks like Walker is a source, but their lineup isn't complete that I can see. I found the Y pipe for a single cat, the muffler for a single cat/tailpipe and the single tailpipe for a cat (they list separately for ones without a cat), but they only have what look to be generic cats. You'd need the mid-pipe to join from the cat to the tailpipe, and maybe a piece off of the Y to join to the cat; that's what I didn't see. I didn't see where any of their dual exhaust systems were set up to accommodate cats.
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Old 08-23-2017, 05:38 AM   #3
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Re: Stock Exhaust and Cats

I think 'as shipped' dual cats are a rarity, even in the performance car area. Finding one for a truck shipped as an 'economy' setup is unlikely.

Doesn't mean you can't do one, just it is unlikely to find a factory type setup.

If you are trying for close to factory correct setup, avoid the old style pellet type of cat. Those do rob power. Go for the newer honeycomb style.

Check your truck GVWR of your truck. Might never needed cats. HD trucks of that era avoided many of the emissions requirements. My 75 only had PCV and THERMAC. No cats, pumps, canisters, egr, etc by being classed as HD emission. Restrictions tightened in later years.
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Old 08-23-2017, 09:33 AM   #4
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Re: Stock Exhaust and Cats

a 78 c10 would have had a cat under it. If you cant find the parts on the shelf to put it back most muffler shops can bend and install what you need including a y-pipe. I have done a few of these in the past. Used cats are not legal to be installed unless they have been tested, you wouldn't want to use the factory style cat any ways they will slow the truck down.
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Old 08-23-2017, 10:07 AM   #5
Peter S
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Re: Stock Exhaust and Cats

Not all 1978 half ton trucks came with cats. Mine didn't whereas my 1979 did have one. What determined if it had a cat?

1978 K10 stepside
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Old 08-23-2017, 04:37 PM   #6
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Re: Stock Exhaust and Cats

Same here, my 78 did not have any cats from the factory, 79 did. Both 1/2ton 2wd regular cabs.
-78 c10 short/step: 388cid, M20, 5/5 drop, lots more. Playtoy and first vehicle.
-98 c1500 x-cab: 5.7L, 17" rims, 5/6 drop, flowmaster, helper bags,NBS rear disk brakes.
-06 CBR600F4i: 110mph in 2nd at 14,000rpm= happiness.
-02 Suburban 4x4: stock
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Old 08-23-2017, 11:10 PM   #7
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Re: Stock Exhaust and Cats

Originally Posted by Peter S View Post
What determined if it had a cat?
It was based on GVW (Gross Vehicle Weight).

From 1975 up through 1977, anything over 6,000 lbs. GVW was considered heavy duty (for emissions purposes) and didn't require a catalytic converter.

1978 still used the 6,000 lbs. limit except California where it was 8,500 lbs. For 1979 the limit was raised to 8,500 lbs. for all states.
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Old 08-24-2017, 12:57 AM   #8
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Re: Stock Exhaust and Cats

On an unmolested truck of that year, the transmission crossmember will tell if it came with a catalytic converter or not. The catalytic crossmember has a huge open area on the drivers side to allow for the catalyst. If the crossmember is symmetrical on both sides, it's a heavier GVWR truck that didn't have it.

Check out the link below, that should be the correct exhaust system for your truck and it looks like Walker makes everything you need.

And if your manifolds aren't correct, these should be the right ones:
1977 GMC Sierra Grande
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Old 09-10-2017, 07:24 PM   #9
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Re: Stock Exhaust and Cats

I doing all most same set up I will be using duel's ex pipes set up. I got new 2014 cat's off a camero. 1977 w/ 82 305, no smog pump. all the rest is 77-84.
Walker makes single ex. If you want dual's your going to have to look in to the 72-74 car's that had duals use parts of it. If you want a set pre-made or get muffler shop to make one.
If your duel set is still good all you have to do is get muffler shop to cut pipes put in cat's.
if your tail pipes come out like a " L" behind the tires they should be good
Cats try
1977 REG cab Long Bed with a 1984 Front Clip, 82 305 .60 over 9.0.1 flat top pistons, 4bbl , w/84 SS-intake & cam , w/ 1977 big block rad, 1975 gear box, 1984 master w/ metric fittings top & SAE fitting @ portion block, 1975 3.42 gear open diff. Duel 77-87 Factory air cleaner, duel inlet from rad support . Down sized 9' to new 8' Steel flat bed
Was a 77 454 w/308 gear. Taken out 550 lbs. up front with motor clip change.
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Old 09-11-2017, 09:09 AM   #10
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Re: Stock Exhaust and Cats

I'm a little late to the party but my 1978 had catalytic converter information stickers on the core support and on the driver side sun visor, I also had a big shield that was supposed to protect it I suppose but I took that out a long time ago. That walker stuff seems pretty cool, never heard of it or thought of putting "stock exhaust" on.
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