We Install Doug Thorley Headers On An LS-Swapped 1991 4X4 Suburban

Gray is the middle ground directly between black and white. Sometimes, we find ourselves in the gray area. It’s a period of uncharted territory where you have no experience to make an educated decision. It’s a time of experimenting with ideas to see what will work and what will not. Your choices during this course will affect your actions in the future as you learn from them.

Recently we ran into this so-called gray area with an LS-swap. The vehicle is a 1991 Chevrolet Suburban V1500. In 1988 Chevrolet moved to what enthusiasts now call the “old body style” or OBS platform. However, the K5 Blazer, Suburban, and some 3500 series trucks carried the ever-popular Square Body design until 1991. The problem is finding parts that are guaranteed to fit these vehicles for a swap can be challenging. They’re out there, but if you think all Square Body swap items will fit, you’re in for a huge letdown. To make matters worse, our Suburban is four-wheel-drive, adding even more complexity to the project.

When it came time to select headers for our project, we wanted a set of proven units that would increase the power of our project and hold up to the abuse of off-roading. We reached out to Doug Thorley Headers, knowing that their Tri-Y headers would fit the bill for durability and performance, but we weren’t positive if they had the application we needed. Upon inspection of their website under the LS-swap header page, they listed one that we knew should work, but it was not the Tri-Y design. We decided to take a chance and ordered PN THY-322Y-C, which are designed to work on 1961-’66 GMC trucks over the other headers to see if they would alleviate one gray area in regards to this swap. Thorley headers are built using industry-leading 14-gauge steel tubing with 3/8-inch thick flanges. They come ceramic coated, which will significantly reduce under-hood temperatures and protect the headers from all environments — including off-roading.

The headers showed up, and as soon as we pulled them out of the box, we were hoping they would fit the Suburban because they looked, killer. The craftsmanship is fantastic, and these pipes are heavy duty. Thorley starts with a 3/8-inch thick header flange and attaches four 1-7/8-inch 14-gauge primary tubes. These four tubes neck down into two and then connect to the 3-inch collector creating the 4:2:1 Tri-Y design. The headers are finished in a durable silver ceramic coat and include gaskets, bolts, and reducers. To top it off, Thorley headers are 100-percent American made and backed by a lifetime warranty.

Doug Thorley starts with a 3/8-inch thick header flange and attaches four 1-7/8-inch 14-gauge primary tubes. These four tubes neck down into two and then connect to the 3-inch collector creating the 4:2:1 Tri-Y design.

With everything out of the box, it was time to see if the Tri-Y’s w fit would between the frame rails of the ‘Burb. It was evident that the headers needed to come up from the bottom instead of dropping down from the top due to brake lines and steering shaft being in the way. We first slid in the driver’s side header. After we had it in place, it looked like everything cleared. We inserted the gasket and finger tightened the supplied flange bolts. The driver’s side was in and fit and tucked up to the floorboard perfectly.

The passenger’s side had more room, but we opted to bring the header up from the bottom as well. This method was definitely the path of least resistance as it lined right up with the head. A quick glance told us we were in good shape, so we again inserted the gaskets and finger tightened the bolts.

Both the driver's and passenger's side had ample room between the headers and the frame of the '91 V1500 Suburban.

It was now time to get under the vehicle and see if we were good with the fitment of the new headers, and we were, well, kind of good. GM decided to come off the frame with the fuel lines and route them up the back of the engine to the throttle body injection (TBI) unit. This location put the supply and return lines directly in the way of the collector. If you were going to run cast iron manifolds or shorty headers, this would not be a predicament. Although, this wasn’t a problem for us either because we couldn’t use the factory lines for the LS-swap due to the hose ends.

To make the headers work, all we had to do was reroute the fuel lines, ​which had to be done anyway due to the LS-swap.

The solution to our fuel line problem was pretty simple. We cut the lines back, add b-nuts and sleeves, and flare them. We then picked up some -6AN unions from Nitrous Express. With our fittings in place, we then built flexible -6AN lines to run to the firewall mounted Edelbrock fuel pressure regulator. We wrapped our hoses with some reflective heat tape as an added precaution to make sure and keep the heat out. We also decided to use some of DEI’s Heat Sheath aluminized sleeving to protect the wires going to the starter. Once we were happy with everything, we went ahead and torqued down the headers to the heads and installed the oxygen sensor for our Edelbrock Pro-Flo 4.

The headers tuck up tightly to the engine but still have plenty of room for the plug wires and boots.

Even though the Thorley headers were not designed to fit this 1991 platform, you would never know it without looking at the part description. With just a minor modification to the fuel lines — which were already required due to the LS-swap — the Thorley headers fit perfectly. Our next step will be to fabricate a dual-exhaust system and connect it to the headers. The headers tuck up nice and tight and will allow us to the exhaust over the transmission cross member and still have room to get around the NP241C transfer case.

We are looking forward to getting this rig all buttoned up so we can blaze some trials. Be sure and check out Doug Thorley’s website for more information on their entire line.

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About the author

Brian Havins

A gearhead for life, Brian is obsessed with all things fast. Banging gears, turning wrenches, and praying while spraying are just a few of his favorite things.
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