Dr. Dyno
NAXJA Forum User
- Location
- Al Ain, UAE
Not one to rest on my laurels for long, I decided to modify my exhaust yet again to extract a few more ponies. I had done some exhaustive research 
on the subject and came up with a simple formula to calculate optimum exhaust pipe size based on flywheel horsepower output. From the formula, I wrote a chart. According to my exhaust pipe sizing chart, the optimum pipe size for a single pipe system on a 4.6 stroker making 275-300hp is about 2.75". Unfortunately such a diameter doesn't exist in any exhaust shop so I had to get creative.
The rear half of my exhaust was already 2.5" so I figured that making the front half 3.0" would be a good compromise. Here's a schematic diagram of my exhaust system as it is now:
The XJ Borla header comes with a collector that's 2.25" in diameter and that's way too small for a stroker, so off it came and in its place, I had a 3" collector pipe welded in place.
The next piece is a custom 3.0" downpipe that crosses over from the driver's side to the passenger side. It's connected to the header collector by a flanged joint.
This replaced the 2.25" Borla downpipe. Unlike the Borla unit, mine has 45 degree bends instead of 90 degree bends so there's less interruption of exhaust gas flow. My pipe isn't mandrel-bent so it narrows down to 2.75" diameter at the bends, but that's still big enough for up to 300hp.
The downpipe is connected to the center Flowmaster muffler by another flanged joint so with flanged joints at both ends, removal of the downpipe is very easy.
I also had the inlet to the Flowmaster upsized from 2.25" to 3.0" to remove that as a bottleneck. From the center back, the pipe size remains unchanged at 2.5". Since exhaust gases cool as they leave the engine, they contract and occupy a smaller volume so a smaller pipe size can be used at the rear of the system.
How does it sound?
A little louder than stock at idle with a nice, soft burble from the tailpipe so that'll satisfy the vehicle inspectors. At cruising speed it's also quiet enough and doesn't have that horrible drone normally associated with big bore exhausts. Under hard acceleration it emits a powerful, sporty I6 growl just to remind you that it isn't stock. Not too loud.
I love it. Hopefully I'll record some sound bytes and post them somewhere.
Total cost of new header collector, downpipe (pipes are aluminized steel), flanges, and 2 hours labour came to just $160.
_________________________________
1992 XJ Laredo 4-dr - 201k miles - 27k miles on 4.6L HO Stroker
AX15, NP231, D35c, D30
270hp@4950/327lbft@3750, 1/4 mile = [email protected], 0-60 = 5.6secs
Websites - Jeep 4.0 Performance, 4.6L Stroker Build-Up, Dino's Jeep Tricks
on the subject and came up with a simple formula to calculate optimum exhaust pipe size based on flywheel horsepower output. From the formula, I wrote a chart. According to my exhaust pipe sizing chart, the optimum pipe size for a single pipe system on a 4.6 stroker making 275-300hp is about 2.75". Unfortunately such a diameter doesn't exist in any exhaust shop so I had to get creative.The rear half of my exhaust was already 2.5" so I figured that making the front half 3.0" would be a good compromise. Here's a schematic diagram of my exhaust system as it is now:
The XJ Borla header comes with a collector that's 2.25" in diameter and that's way too small for a stroker, so off it came and in its place, I had a 3" collector pipe welded in place.
The next piece is a custom 3.0" downpipe that crosses over from the driver's side to the passenger side. It's connected to the header collector by a flanged joint.
This replaced the 2.25" Borla downpipe. Unlike the Borla unit, mine has 45 degree bends instead of 90 degree bends so there's less interruption of exhaust gas flow. My pipe isn't mandrel-bent so it narrows down to 2.75" diameter at the bends, but that's still big enough for up to 300hp.
The downpipe is connected to the center Flowmaster muffler by another flanged joint so with flanged joints at both ends, removal of the downpipe is very easy.
I also had the inlet to the Flowmaster upsized from 2.25" to 3.0" to remove that as a bottleneck. From the center back, the pipe size remains unchanged at 2.5". Since exhaust gases cool as they leave the engine, they contract and occupy a smaller volume so a smaller pipe size can be used at the rear of the system.
How does it sound?
A little louder than stock at idle with a nice, soft burble from the tailpipe so that'll satisfy the vehicle inspectors. At cruising speed it's also quiet enough and doesn't have that horrible drone normally associated with big bore exhausts. Under hard acceleration it emits a powerful, sporty I6 growl just to remind you that it isn't stock. Not too loud.
I love it. Hopefully I'll record some sound bytes and post them somewhere.
Total cost of new header collector, downpipe (pipes are aluminized steel), flanges, and 2 hours labour came to just $160.
_________________________________
1992 XJ Laredo 4-dr - 201k miles - 27k miles on 4.6L HO Stroker
AX15, NP231, D35c, D30
270hp@4950/327lbft@3750, 1/4 mile = [email protected], 0-60 = 5.6secs
Websites - Jeep 4.0 Performance, 4.6L Stroker Build-Up, Dino's Jeep Tricks
