2 inch exhaust with 300+ hp 5.7

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Supercharged111

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So that leads to another question, what diameter is the 5.7 K1500 exhaust manifold? Are they also bigger on the K3500s and Express vans?

Not sure, but they all belong in the trash. Do yourself a favor and at least get some shorties. The manifolds are not 3", not even on the 454, that's why the first little bit of piping is smaller than 3". I know for the 454, I can get shorties and short sections of front piping to weld onto the stock stuff and bolt to the headers. That's exactly what I intend to do.
 

L31MaxExpress

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The 2.75 is post cat, pre cat is 3" except for the very first part by the manifold flange.
Vans are 3" all the way to the muffler. Then that horrible muffler and a 2.75" tailpipe. Even my G1500 had the dual 3" pipes from GM. The 8 lug vans added a resonator on the RH exhaust pipe pre cat. My 97s first mod after tuning, disabling the EGR, a K&N and EGR diffuser blade removal on the throttle body was a dual 3" in/single 4" out muffler and a mandrel bent 4" tail pipe. Made a signifigant difference even with the stock manifolds. 2.5" collectors also did not seem to bother my 350 much making way more than the stock power. The Thorley headers have 2.5" collectors that bump back up to 3" to match the pipes immediately after the collectors. Seems to work very similarly to a merge collector. The headers added ALOT of torque. To the point even with the 3.42 gears and taller than stock 20s it would climb hills at 70 mph locked up in overdrive that it would previously downshift or unlock the converter on.

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That being said my M56S made a peak of 390 hp at the wheels with dual 2" exhaust into a single 2.5" Y-pipe that opened back up into dual 2" only to be chocked down to dual 1 7/8" exhaust inside the factory resonators. At that power level cutting off the rear non monitored cats, a 2.5" dual exhaust with a X-pipe where the Y-pipe sat, Magnaflow magnapacks where the resonators sat and a retune of the fueling only brought me up to 428 rwhp. However at the 7,000 rpm redline it was up 50 hp. The most signifigant gains were above 5,500 rpm. I only gained 5-10 ft/lbs under 5,500 rpm but the gains were from under 3,000 rpm all the way up. 2.5" duals were more than enough for a 338cid engine spinning to 7,000 rpm making close to 500hp at the crank. Most of the time the mufflers will choke the exhaust far before the actual pipes will. The stock setup sounded like a vacuum cleaner with a full bag above 4,500 rpm. With the duals it actually had a nice V8 exhaust tone.

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L31MaxExpress

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The horrible hissing sound is the result of exhaust restriction.
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Here was after the custom exhaust
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YukonKyle

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Not sure, but they all belong in the trash. Do yourself a favor and at least get some shorties. The manifolds are not 3", not even on the 454, that's why the first little bit of piping is smaller than 3". I know for the 454, I can get shorties and short sections of front piping to weld onto the stock stuff and bolt to the headers. That's exactly what I intend to do.

So you get a 2.75 inch pipe but it is choked at the manifold. Makes no sense!!!

I wonder if I get those pacesetter LTs if I should get that 1 ton cat system anyway or just build my own with a header back kit and two high flows cats

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L31MaxExpress

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So you get a 2.75 inch pipe but it is choked at the manifold. Makes no sense!!!

I wonder if I get those pacesetter LTs if I should get that 1 ton cat system anyway or just build my own with a header back kit and two high flows cats

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Manifolds are 2.25 id or 2.5" od on the Vans that had 3" pipes.
 

b454rat

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The jegs off road pipes are 2.25" catless. Granted mine is running but not on the road, but the check engine light doesn't come on. If I do drive it a little, the light comes on for the tranny, not the motor. But I don't know if it will come on for no cats, might have to drive it, dunno. I thought my other Tahoe it would come on shortly after starting it, but that vehicle had other motor issues going on. Nice thing bout the Jegs pipes, only $140. I used Summit headers, bolted up good, no leaks so far....
 
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