Headers or iron manifolds?

[Schurkey]

Will these actually fit a gmt400? I have a 1990 C1500, when I looked them up online it said they weren’t compatible

I don't know ALL the reasons they won't fit, but lack of an O2 sensor bung would rate fairly high on that list.

Didn't even have enough sense to cast-in bosses that could be drilled 'n' tapped; one for O2, another for a Vortec-style EGR fitting. Somebody was not thinking clearly.

 

[1998_K1500_Sub]

Going off memory, the L31s in lighter GVWs have 2" OD pipes (my 99 Tahoe was born with that garbage) and the 8600+ are 2.75" OD. The vans have dual 3" OD, atleast my 97 did and still does. Why GM used the odd 2.75" on the trucks has always been mystery to me considering the vans are 3" OD.

^^^ This post dates back to Dec2023, but it caught my eye recently.

So I checked the downpipes on our 1998 L31 K2500 and the nominal OD was 3" in the straight sections of pipe.

The bends weren't "crush" bends, but the pipe was slightly distorted into an oval, measuring >3" at points.

In one picture below, I have my calipers on a straight section, just prior to the cat for the driver's side exhaust. The transmission can be seen adjacent to it.

The second picture shows the crosspipe from the driver's side; the transmission can be seen above it. Also visible is the torsion bar and the downpipe coming from the passenger side. For reference, my calipers can be seen in the background hanging on the piece of pipe of which I took a picture prior.

The section of downpipe that actually connects to the exhaust flange is perhaps 2.5-2.75" OD. After a few inches it transitions to 3" OD thereafter.

My 1998 L31 Suburban K1500 has the 2” OD pipes, as others mentioned in earlier posts.

 

[L31MaxExpress]

^^^ This post dates back to Dec2023, but it caught my eye recently.

So I checked the downpipes on our 1998 L31 K2500 and the nominal OD was 3" in the straight sections of pipe.

The bends weren't "crush" bends, but the pipe was slightly distorted into an oval, measuring >3" at points.

In one picture below, I have my calipers on a straight section, just prior to the cat for the driver's side exhaust.

The second picture shows the crosspipe from the driver's side; the transmission can be seen above it.

The section of downpipe that actually connects to the exhaust flange is perhaps 2.5-2.75" OD. After a few inches it transitions to 3" OD thereafter.

My 1998 L31 Suburban K1500 has the 2 inch OD pipes, as others mentioned in earlier posts.

That is news to me. Never seen a truck with 3" OD pipes. I have seen plenty with 2.75" though. The 3" pipes make it much easier to swap the muffler later on which is awesome. On my 97 van I cut the bolt-on flange off on one pipe and cut the 2nd pipe to the same length. Then used an aftermarket dual 3" inlet/single 4" outlet muffler and tailpipe for a GMT800 8100 truck.

 

[Scooterwrench]

That is news to me. Never seen a truck with 3" OD pipes. I have seen plenty with 2.75" though. The 3" pipes make it much easier to swap the muffler later on which is awesome. On my 97 van I cut the bolt-on flange off on one pipe and cut the 2nd pipe to the same length. Then used an aftermarket dual 3" inlet/single 4" outlet muffler and tailpipe for a GMT800 8100 truck.

Do you have a part# or application for that muffler?

 

[1998_K1500_Sub]

The 3" pipes make it much easier to swap the muffler later on which is awesome.

So can the larger L31 K2500/3500 OE exhaust pipes be used as a bolt-on retrofit for the L31 K1500 trucks?

Has anyone done this?

It appears likely but I have yet to study it.

 

[Supercharged111]

That is news to me. Never seen a truck with 3" OD pipes. I have seen plenty with 2.75" though. The 3" pipes make it much easier to swap the muffler later on which is awesome. On my 97 van I cut the bolt-on flange off on one pipe and cut the 2nd pipe to the same length. Then used an aftermarket dual 3" inlet/single 4" outlet muffler and tailpipe for a GMT800 8100 truck.

My dually and plow truck are both 3" pre cat 2.75" post cat. The cats have a flare on the outlet though that would support 3" post cat piping if one were so inclined. The 2.75" stuff also flares out to 3" at the flange. This is how I was able to weld in 3" v-bands on the dually.

 

[L31MaxExpress]

So can the larger L31 K2500/3500 OE exhaust pipes be used as a bolt-on retrofit for the L31 K1500 trucks?

Has anyone done this?

It appears likely but I have yet to study it.

IF it clears the 4L60E crossmember mounting location and the manifolds have the correct diameter exits.

 

[99xcss4]

this is for the 454 vortec I could be wrong on this but I think do not know for shore but I want to say that this is the same pipe for 8600gvwr and higher 350 vortec I would love to buy one for my project but they are so F'n expensive because they have one of the 2 cats on them https://www.summitracing.com/parts/MPE-95470

 

[99xcss4]

where do these prices come from 454 back section https://www.summitracing.com/parts/mpe-4451471/make/chevrolet/model/k3500/year/1996

 

[99xcss4]

here is another front section for a 454 https://www.summitracing.com/parts/cto-9703/make/chevrolet/model/k3500/year/1996