2.5" Manifolds?

[Erik the Awful]

I'm planning the exhaust for my 2WD '89 Standard Cab Stepside. I'll be swapping the seized up motor for a Vortec-headed 9.5:1 350 with a Lunati 20080720 (advertised duration 262/270, lift .507/.515).

I want to avoid headers. I don't like the leaks, the extreme underhood heat, and I don't want it being obvious when I pop the hood. I want to run a 2.5" exhaust, but the only 2.5" manifolds I've seen are the center-dump Corvette manifolds. They won't look right, and it doesn't look like there's room to turn the exhaust back.

Is there a 2.5" exhaust manifold that will fit our chassis? I've seen the turn-back truck manifolds, but
they're only 2". I haven't seen anything else.

If there are no other options, has anybody tried using a torch to heat the Corvette manifolds and cheat the angle rearward? I know I can make a decent flowing 90* that will fit, but it'll look janky.

 

[smdk2500]

Have you checked the 3/4 ton and 1 ton appications? I don't know for sure what the outlet size of the manifold is. If my 95 wasn't currently sitting in a pool of mud i would try to measure it for you.

 

[b454rat]

Don't see how manifolds run any cooler than headers when they both have hot exhaust running thru them. I had a set of Flowtechs on an 89 I built 20+years ago. I got the Y-pipe, and got rid of the cat, running 3" single into dual inlet Flowmaster, dual 2 1/2" out. I never had any leaks, and gave it a **** ton of torque. A trick local machinist/engine builder told, put hi-heat gasket sealer on the gaskets both sides. I did this to any vehicle I put headers on, or had the exhaust off for any reason. Helps alot.

 

[454cid]

Does Sanderson, Fenton, or Hooker have anything that fits?

(Apparently, Hooker has some cast headers now.)

 

[Erik the Awful]

As far as I can tell, the 3/4 and 1 tons with a 350 have the same manifolds, but the downpipes immediately expand to 3". I have set of the downpipes on my '99 Suburban, but right after the manifold they're about 1 7/8" diameter. I want 2.5" from the collector on.

I've cooked stuff underhood with headers before, but not with manifolds. I don't want to deal with wiring repair, heat-shielding transmission cooler lines, and re-routing fuel lines if I don't have to. Yes, copper permatex is a god-send for fixing header leaks, but I don't want to use it as a crutch if I can get a manifold with a good mating surface. Manifolds are also quieter. Finally, I'd like to keep the engine bay looking as innocuous as possible.

The only 2.5" outlet manifolds I've found are the Corvette ones, and Sanderson's manifolds that specifically say they won't fit Vortec heads. Plenty of manifolds for LS motors now, but not SBCs.

 

[L31MaxExpress]

As far as I can tell, the 3/4 and 1 tons with a 350 have the same manifolds, but the downpipes immediately expand to 3". I have set of the downpipes on my '99 Suburban, but right after the manifold they're about 1 7/8" diameter. I want 2.5" from the collector on.

I've cooked stuff underhood with headers before, but not with manifolds. I don't want to deal with wiring repair, heat-shielding transmission cooler lines, and re-routing fuel lines if I don't have to. Yes, copper permatex is a god-send for fixing header leaks, but I don't want to use it as a crutch if I can get a manifold with a good mating surface. Manifolds are also quieter. Finally, I'd like to keep the engine bay looking as innocuous as possible.

The only 2.5" outlet manifolds I've found are the Corvette ones, and Sanderson's manifolds that specifically say they won't fit Vortec heads. Plenty of manifolds for LS motors now, but not SBCs.

LT1 F-car manifolds for a single cat car. Drill the flange for the missing bolt holes. The Vortec exhaust ports are raised slightly compared to the older small blocks.

Some of the old TBI G-vans had 2.25" outlet manifolds on the 350s.

Finally GM also had some really nice stainless steel tubular shorty header style manifolds on some of the mid 80s 350s. You can get them new from Dorman. From what I remember they are 2.75" outlet.

Years ago I rebuilt the 305 in my 83 G20 van. Short block was built back to stock with the flat top pistons 0.040" over, used a Comp HE268, used undercut Precision Engine Parts valves, Z/28 springs, a performer rpm q-jet intake with the EGR deleted and swapped the whole exhaust system off a 1985 G30 van with those manifolds and true duals. The stainless manifolds and tubing were in great shape but I replaced the long rotted out factory mufflers with magnaflows and put a X-pipe in front of them. Between the fresh engine and ditching the factory manifolds, 2" Y-pipe and pellet cat it felt like I had 2 stock 305s under the hood.

These are the factory headers I am talking about.

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[L31MaxExpress]

One other thing. You might be able to use Express van manifolds. My manifolds were either 2.25 or 2.5 ID. I forget which, but the pipes started out at 2.5" and bumped up to 3" within 6" of the engine. That being said Thorley Tri-Ys knocked over 1/2 second off my 0-60 compared to those crappy manifolds and that was with the stock GM 300 hp Marine aka HT383 or Ramjet 350 196/206 cam in it. I had the stock 3" cats and a dual 3" in/single 4" out walker dynomax bus muffler with a 4" tail pipe before swapping the headers on. I swapped the stock cats for thunderbolt 3" high flows and dropped another 0.3s off the 0-60. Headers and high flow cats dropped the 0-60 almost 0.8 seconds. That was a bigger difference than the cam made!

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[Erik the Awful]

Thanks! The more I look at it, the more I think I need to try and fit the vette manifolds, and if that falls through, I'll be stuck with headers.

 

[someotherguy]

Some of the old TBI G-vans had 2.25" outlet manifolds on the 350s.

My '94 C2500 ecsb (7200 lb GVWR) 5.7 had them as well. Discovered this when I was pulling the Y-pipe and broke a few manifold studs, figured weather was cold, I'd just swap some other manifolds on instead of fighting the broken studs. Nope. The others were 2", so I went to work pulling the broken studs and replacing them. Not sure if "all" TBI 2500's and 3500's had the 2.25" as the other examples I had at the time were different combos or the engines were already removed and sold, and I sold them complete with manifolds.

Richard

 

[L31MaxExpress]

Thanks! The more I look at it, the more I think I need to try and fit the vette manifolds, and if that falls through, I'll be stuck with headers.

The LT1 F-car manifolds I mentioned are even better than the Corvette manifolds, FWIW and they are rear dump. The runners are almost completely cast seperate to the exit.

If you are really stuck on manifolds I would look into the modified version sold here.

https://www.castheads.com/manifolds-carb/exhaust-manifolds/lt1-exhaust-manifolds/