C5 FLP Install

E-mail jmX with corrections

Tools Needed:
- Set of metric sockets, deep and normal (from 8mm to 15mm is good)
- Set of Metric wrenches (8mm, 10mm, 13mm, 15mm, 18mm, 19mm)
- Metric "Gear" wrenches (not absolutely necessary, but REALLY handy)
- 7/8" open ended wrench (for 02 sensors)
Parts/supplies you will need:
- FLP header package (I purchased mine from Thunder Racing)

- 2 hpipe to catback gaskets (2x GMPN 10276792)
- 2 AIR tube gaskets (2x GMPN 12553617)
- 2 O2 sensor bung plugs, if not using cats (NOTE: NGK WR4-1 (pn 4652) spark plugs will also plug an O2 hole)
- 2 O2 sensor SIMMS if not using cats

- Oil drain pan
- 1 quart of oil
- Tube of Anti-seize
- Tube of dielectric grease
- Tube of antiseize


One of the most common big power mods done to these cars is headers. That's for a few reasons. One, the price for performance is GOOD. Two, the sound will be changed quite a bit (esepcially without cats), and three, they are fairly painless to install or have installed. The motor stays together and many dealers will still warranty the engine even with headers.

Pictures of the parts:

The FLP header system (cats not shown).

Lets begin:

Part 1: Removal of stock H-pipe

Time to complete: 0.5 - 1 hour

First things first, get your car up in the air. What I do is drive the front end into Rhino ramps (black plastic ramps available at walmart or various other places), make sure the wheels are pointing straight ahead, block off the front wheels so they dont roll, then jack the rear up by the center of the rear crossmember and place 2 jack stands under the crossmember. Throw a light under the car, and we're ready to begin!
Get underneath the car with a deep 15mm socket wrench and some WD-40 or equivalent. You should see where your H-pipe mounts to the exhaust manifold. Spray down the 3 nuts on the studs coming out of the manifold with WD-40, and then try to break the 3 nuts free. Initially they may be tough to break free but should come loose with some effort. KEEP THESE NUTS as you'll need them for the FLP's.

While you are under here, follow the wire off of the O2 sensor until you see an electrical weatherpack connector. Disconnect this connector. Look for the wire coming from the REAR o2 sensor as well, and unplug it too. The connector will be right next to the one for the front O2 sensor.

Repeat for the other side of the car.
Now go to the rear section of your stock Hpipe and find where it connects to the muffler pipes. Unbolt these 4 15mm bolts, and the muffler/exhaust tips should sorta droop down. Keep these 15mm bolts as you'll need to reuse them for the FLP's.
The H-pipe is now supported by 4 bolts. 2 in the front, mounted to the bottom of the motor, and 2 in the rear mounted to some springs hanging from the torque tube/tranny.

First, remove the 2 supporting 15mm bolts. Keep these as you'll need to re-use them for the FLP's. Keep the front of the hpipe either supported by a helper, or put a box or a jack stand under it.
Now, unbolt the rear two supporting bolts that hold the exhaust springs to the exhaust. These 2 are 13mm bolts. Keep these too.
That's it! Now you've got the hpipe/cat assembly totally free. It should look something like this.

Part 2: Removal of stock manifolds

Time to complete: 0.5 - 1 hour

Pop the hood, disconnect the battery, and pull off your fuel rail covers.
On each side of the engine, there is a metal AIR tube bolted onto the exhaust manifold. Each is held onto the exhaust by 2 10mm bolts. Remove them. On the drivers side, the AIR tube immediately connects to a rubber hose. Using a flathead screwdriver, undo the plastic clamp on the rubber hose and remove the AIR tube totally from the engine bay. On the passenger side, this will not be possible.

While on the passenger side of the car, using a 10mm wrench to unbolt the oil dipstick tube from the exhaust manifold and with a firm tug, pull the dipstick tube up and out of the engine bay. Don't lose the dipstick bolt.
Now disconnect all 8 plugwires from the coilpacks. Try not to pull on the wire, but instead pull on the boot that connects to the coil. Do the same for the spark plug end of the wires and set all 8 wires aside. Next, disconnect the wiring harness that goes to the coilpacks. Its a large white weatherpack connector, labeled in the photo.

Next we need to remove the coil packs from each valvecover. On 97-98 LS1's, each coil pack is bolted to the valve cover separately. On 99+ year models, all the coil packs are on a bracket and the bracket is then mounted to the valvecover. Depending on your year, either unbolt all the coil packs individually (97-98), or unbolt the 5 10mm double-ended bolts (using a deep socket) and remove the coil bracket with coils still attached. On 99+ year model cars, there may or may not be a bracket mounted to the backmost bolt. If there is, undo the nut and pry the bracket off with a big flathead screwdriver, and then unbolt the final bolt to remove the coilpack rail.
Next we need to loosen the drive belt so we can remove the alternator. Loosen the belt tensioner using a 15mm wrench and slide the belt off the alternator, leaving the rest of the belt in place as much as possible in the engine bay (it will stay in place on all the other pulleys if you are careful).
Disconnect the wiring plug on the backside of the alternator, and using a 13mm socket unbolt the thick cable bolted to the back (WARNING: this cable is HOT (12 volts) unless you have discconected the battery. Make SURE you disconnect the battery before attempting this)

Now, unbolt the alternator. There are 2 15mm long bolts holding it onto the alternator bracket. Remove them, pry the alternator out of the bracket at the top (it's tight),and then rock the alternator back and forth to wiggle it out of the bottom of the bracket.
Now lets remove the exhaust manifolds themselves. On each manifold there are 6 10mm bolts holding them to the head. I recommend undoing the backmost bolt first, and leaving one of the middle bolts until the end. Once all 6 on one side are done, the exhaust manifold should be able to be pulled out from the top. Make SURE you disconnected the O2 wire mentioned in the earlier steps before you go pulling the manifold out, as the o2 sensor is still screwed into the manifold.

Repeat for the other side of the car.

Part 3: Installing the headers

Time to complete: 0.5 - 1 hour

Now, finally all the old stuff is out of the way and we're ready to install the new headers! This is a big step so pay attention.

Installing the drivers side header:
First things first. Using a 7/8" open ended wrench, remove the 02 sensors from your stock manifolds and screw them into the FLP's. Make sure you don't cross thread them, they shouldnt need TOO much force to be screwed in. Snug them down, and you're done.

Get under the car and remove the oil filter. No, your full engine's worth of oil isn't going to spill out, don't worry.

Find the steering shaft on the drivers side of the engine and you'll see a black plastic boot on it at the front end of it, where it meets the power steering rack. Either unclip it, or unscrew the clamp holding it in place and slide the boot up some so you have access to the 13mm bolt that goes into the joint. If your wheels are pointing straight ahead, the bolt will be accessable. Using a 13mm socket wrench, unbolt this bolt. Now, try to force the steering shaft towards the drivers side of the car as much as possible (it should move out of the way another 1/2" to 1" since the bolt is out and the joint can pivot a bit).

Next, remove the drivers side valve cover. On '99+ cars it is only held on by 4 8mm bolts. On LS1's (NOT LS6's) there will also be a PCV hose you'll need to use a flathead screwdriver to pry up and out of the grommet). If you have a 97-98, you'll have permiter style valve covers which have a few bolts around the outer edge of the valve cover. Remove the bolts and set the valve cover aside.

Now, after all this work, we can drop the header in from the top. It will take some time to work the angles out just right, but after a few minutes it should drop right in.
Look at the exhaust gasket you got with the FLP kit and orient it so the bolt holes are in the right place, and try to put the gasket in place and get a bolt started through the header and gasket. Once you get one bolt started, try to get another started. DO NOT TIGHTEN THESE YET. Once two are in, everything should be nicely lined up. Put a tiny bit of anti-sieze on the 4 remaining header bolts and hand start them too (DO NOT TIGHTEN!), then remove the first 2 bolts and put some antisieze on them as well and put them back in. Once all bolts are started, hand tighten them all first, then do a pass with a torque wrench at 18lbft (or, just make them hand tight if you think you can estimate torque :) )

Next, apply loctite on the steering shaft bolt and reinstall it. You may need to tap the shaft connector back onto the rack connector with a hammer if it was a tight fit to start with. Torque this to 25lbft.
Finally, fill your oil filter up with fresh oil and reinstall it.
Installing the passenger side header:
Like the drivers side, you'll need to remove the valve cover on this side too. On LS1's there should be 2 rubber hoses going to this valve cover that'll slide right off. On LS6's there is probably just one.

Once the valve cover is off, the passenger side header SHOULD drop right in with some cussing and some yelling. Keep trying different angles and eventually it'll drop in. It helps to have somebody below guiding it. If it seems impossible, just take a deep breath, see what its caught on, and realize that it *is* possible once the header is aligned just so and it'll drop in with ease.

Finally, just like the steps for the drivers side, install the gasket and bolts (be sure to use the antisieze).
Now, stab your oil dipstick tube back into place (the bolt hole in the header will give you a clue as to which pipes to go between). This part may be easier if somebody gets under the car and feels around for the hole it goes in and guides it by hand. The process can take 5-10 minutes of searching until the tube goes in just right. Once lined up with the hole give it a good push and it'll snap in. Then reattach the holddown bolt to the exhaust manifold/header. This doesn't need to be very tight, just make it so the dipstick doesn't wiggle.
Reinstall your valve covers. 99+ years use 4 8mm center bolts while 97-98 models use perimiter bolts. The valve covers use a rubber gasket so torque isn't important here, the GM book calls for about 8lbft on the bolts, which means just make them tight enough until they compress the gaskets to ensure a tight fit. Once the valve covers are on, on the drivers side you need to push the PCV hose tube into the grommet on the rear of the valve cover, and on the passenger side you need to connect the rubber PCV boot to the rear of the valve cover.

Now reinstall your coil packs. NOTE: I leave off the back hard to get to coil pack rail bolts ('99 and up only) to make rail removal easier in the future.

Reinstall your AIR tubes. You'll need to use NEW air tube gaskets and again you need to hand thread in the 10mm bolts to the exhaust manifolds/headers to ensure you don't crossthread them. The GM spec on those bolts is 15lbft, but I just make 'em hand tight with a small socket wrench (you'll get good at estimating torque values so you don't have to use the torque wrench on all the non-critical bolts). On the drivers side, make sure you reconnect the rubber hose to the metal AIR tube.

Apply a tiny dab of Dielectric grease on the contact tips of all 8 spark plugs by putting some grease on your index finger and touching the metal tip, then reconnect all 8 of your spark plug wires. They should snap once on the spark plug end, and twice on the coil pack end.

Now, reinstall the alternator. Slide it back into place, and get both 15mm long bolts started and tightened hand tight. Plug the wiring harness connector back into the alt, and bolt the 13mm thick power cable to the back of it and make sure the rubber boot conceals the connection afterwards. Push the belt tensioner back down and try to get the belt onto all of the pullies again (hopefully it stayed in place during the install so you'll just need to put it back onto the alternator pulley).

Thats it! The headers are on.

Part 4: Installing the X-Pipe

Time to complete: 1 - 2 hours

IMPORTANT: read this whole section first before starting on part 4. Putting the V-clamps and band clamps on oriented in the proper direction is ESSENTIAL, or you will be pulling them off later to switch them around due to ground clearance issues.

Now for the least fun part of the install, the X-pipe install.

Grab the 2 15mm bellhousing bolts you removed from the stock h-pipe and attempt to install them through the tabs on the ends of the FLP headers. If the bolt holes don't line up, unbolt the header support mounting plate by loosening the 2 bolts that hold it to the bell housing, THEN try to put the bolts in. If that still isn't enough, dremmel out the tabs just a hair on your FLP's and you should be set to go. Once everything is lined up, tighten those bolts down so they are snug. Torque specs aren't very important here (but I'll add them later for anal retentive people).

Now, grab either your cats or your offroad pipes, and get under the car with a deep 13mm wrench and your v-clamps that came with the FLP kit (be careful, there are 2 different sizes of vclamps. We need the bigger ones for this connection). Try to line the collector and pipe or cat up by hand, and then spread the v-clamp and try to make it engulf the ridge on the 2 pipes. Once you can get the nut started on the long bolt, tighten it a few turns so that the pipes or cats will at least stay in place. Look at the image to see the clamp bolt angles before you install the clamps! You need to make sure the vclamp bolts are aligned like I show for maximum ground clearance.
Next, Loosly assemble the xpipe and extender pipes as shown in the picture. The o2 bungs need to point in roughly the same direction the stock pipe had them. Install the o2 sensors, OR bung plugs if not using O2 sensors in the back. Don't tighten down the band clamps yet. Lay the assembly under the car along with your deep 13mm socket wrench, a 15mm wrench, a 15mm socket, 2 new 3 bolt GM exhaust gaskets, and the bolts/nuts left over from the stock stuff (4 big exhaust bolts, 6 nuts from the stock manifolds (you'll only need 4), and the 2 spring hangar bolts).
Lift the xpipe assembly up into place (helps to have a buddy helping here), bolt on the 2 spring hangars LOOSELY, and while supporting the front of the pipe attempt to connect the front of the xpipe to the offroad or cat pipes you installed above. Again, the vclamps need to be aligned just right so the bolts dont hang down once you are done. See a picture below for the angles I chose to put the v-clamp bolts.

Getting all this stuff lined up and the vclamps started will probably be your biggest challenge during the whole install. The key is to keep EVERYTHING loose until you have all 4 clamps started, then tighten the 4 v-clamps up so all 4 nuts are very snug.
Once everything is loosely connected, use the old muffler pipe bolts and 4 nuts from the stock manifold collectors (there were 6 total when you removed them at the beginning of the install) to attach the muffler pipes to the xpipe, making sure to slide your new GM graphite 3 bolt gaskets in between the flanges before inserting the bolts.

While you are back here, go ahead and tighten up the spring hangar bolts so they are snug.

Finally, tighten the band clamps on the rear of the x-pipe as much as possible (you may need to use a box or open ended wrench AND a socket wrench to keep the nut from spinning as you tighten the bolt).
For reference, the picture on the left shows the rear v-clamp bolt amd band clamp angles I chose for max ground clearance.

End of install

Congrats, you've finished the install! Lower the rear of the car (leave the front on ramps for now), and proceed to the final notes.

Final notes:
First off, its time to fire the car up. Check the oil level and fill if needed. Reconnect the battery, put the key in the ignition and turn it to ON but don't start it. Now, hold the lock and unlock buttons on your keyfob until the car honks. This step is necessary any time you disconnect the battery, otherwise your keyfob won't work! Now, go ahead and fire the car up. Make sure the oil pressure comes up (you DID reinstall the oil filter right!?). You'll probably notice a little smoke and some odd odor from the engine bay, thats normal and will go away with time.
Things to check for right now would be to make sure all 8 plug wires look good and aren't touching the header pipes. Also make sure none are arcing (sparking). I did not use any "heat socks" or extra sheilding for my plug wires.

Next, lay down next to the car while its running and make sure there's no huge exhaust leaks anywhere. It *will* be louder than stock under the car and it will sorta sound like its leaking due to the thinner walls of the headers, but anything that sounds really bad is still probably a leak.

If everything looks and sounds good, back it off the ramps and go for a test drive. You can beat on the headers right away, no breakin of course :). You'll notice a burning smell for the first 100-200 miles, but it will totally go away after that. After you put some time on the headers, maybe a few heat cycles, get the car back up on ramps and RETIGHTEN all the band clamps and exhaust bolts. The heat cycles will let you snug down the vclamps a lot more and give you a tighter seal. I actually did this heat cycle/retighten thing 3-4 times on my car until I was totally satisfied.
So, now that the headers are installed, its time to see how much power gain they gave. I dyno'd before, and after, and then tuned ontop of that. All three lines are shown in the graph here. The torque gains from the FLP headers were fairly impresive, while the peak horsepower gains were about average. The fuel curve was running fairly rich after the headers, so the tuning seemed to fix the topend horsepower issues. Not bad gains for a few hours work. The total time it took me and a friend to complete this using no instructions was right around 4 hours.
Again I'd like to thank Thunder Racing for getting the headers to me ASAP, and for a good price.
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