Exhaust Upgrade May-22
Goals for this project included:
• Reduce decibel level and improve sound quality
o Maximize muffler length to reduce decibel level
Better protect hearing
o Increase side pipe diameter for a deeper sound
• Eliminate exhaust leaks at header/collector joints and header gaskets
o Eliminate contribution to backfiring and flow impact
o Eliminate exhaust fumes/odor from engine bay
• Improve flow characteristics
o Reduce reversion contributing to blackening of oil and impacting performance
o Ensure adequate exhaust scavenging and velocity
o Achieve positive performance impact by design
• Improve side pipe levelness, body orientation, and clean overall appearance
o Achieve a clean, simple, integrated look
Single open straight pipe appearance
Subtle raw metal look, yet protective coated
Larger external tube diameter for look and sound
• Other
- o Improve distance and consistency of pipe to body exit cutouts and to body
o Add O2 sensor bungs in collectors for future exhaust monitoring or EFI scalability
o Improve rear anti-vibration side pipe mountings
This has been a fun project with several improvements. After researching, I decided to use Classic Chambered CobraPack Hi-Flow Cobra mufflers and am pleased with them.
Prior mufflers were 22’ with the prior packing blown out. New mufflers are 3” flow path 36” mufflers, with packing. The mufflers have 4” OD smooth bodies. For a consistent smooth single, straight pipe look, we used matching 4” collectors and tips. The muffler seams were built up with TIG welding and ground/polished down to a smooth single pipe appearance, then ceramic coated.
Before Photos
After Photo
CobraPack Muffler Photos (example photos from Classic Chambered site, not my actual muffler)
(outside of muffler))
(inside spiraling)
The existing headers were maintained, with previous rough edges ground down. Prior header ceramic coating was kept, with the ground down areas touched up with high temp exhaust paint. The prior slip on, one bolt header/collector connectors were imprecise and leaked. These were converted to four bolt flange connections. Remflex high heat flexible graphite header and collector crush gaskets, and flanged nuts, were used to ensure sealing (worked well). The flanges were carefully oriented for welding at the proper angles to achieve the desired side pipe level, orientation, and distance from body. This included mating the headers to the merge collectors.
Photo of header/collector upgrade
Since we went away from four tubes exiting the body to a single 4” pipe design, we added an anti-reversion torque cone inside the collector to increase velocity and exhaust scavenging. It looks like a straight pipe with no muffler from the outside (like a semi-truck stack pipe). Yet most of the pipe is actually muffler, with functionality to ensure velocity and anti-reversion (the magic happens inside).
Photos of anti-reversion torque cone
Photos of body exit planning
Photos of side pipes during seam smoothing
The side pipes were extended back further utilizing available space. The longer pipes with turnouts further back now makes it easier to put a foot/leg down for ingress/egress. The simple fact the exhaust tip is a few inches back further helps with cockpit noise as well.
Here are some photos and a video from our two shop-organized test and tune track days at the skid pad during this exhaust upgrade (also testing some other suspension/brake upgrades and engine tuning). If you turn it up on a large computer you can hear the full and deep sounds and pulses the Classic Chambered mufflers and large pipes create.
First Test & Tune Day – The Ridge Skid Pad (crank volume on video for exhaust sound)
The pipes were just tacked together at this point for testing, and were later gap filled/smoothed and ceramic coated. In person, the sound is reminiscent of the old thunder boats.
Photos at initial test & tune day:
Video – initial skid pad session exhibiting new exhaust sound
https://youtu.be/uZyngBczbSw
While I’d intended to finish in satin black as before, I was quite enamored with the raw, metallic silver look at the track before finishing. Working with the ceramic coater, we used his Chromex color but held off on the final bead blasting to leave off the final polish level, then we hit it with a scotch bright pad to give it a raw metallic look, still with ceramic coating protection.
Second Test & Tune Day – The Ridge Skid Pad
The pipes were coated now and we firmed up the coilovers, which helped with body roll. The car felt balanced and predictable, particularly on throttle coming out of corners and changing directions. It did pretty well, considering it has an iron headed 385 series big block (the proverbial 700 pound lump) in the front end. We’re still considering adding a tubular rear anti-roll bar and increasing the strength of the front tubular anti-roll bar.
We dialed in the brake bias at the track after changing rear pads.
Photo of brake bias tuning
Our son’s (family) friend is a professional photographer who’s started his own business and he captured photos and videos at the track. The below video sequence gives various sound shots around the autocross course, where you can hear the new exhaust. These test/tune sessions were my first experience with skid pad, figure 8, and autocross sessions. The video also captures my first ever donut session (just had to try it, the low-brow yay-hoo that I am). The Cobra was a star, with its 90’ wheelbase, it just naturally wanted to perform donuts. That was during the session practicing launch control, working to maximize traction at launch, which was also educational with the Jaguar posi rear end.
Video – Test & Tune Day #2
https://youtu.be/dO1TpLDPF2A
Overall, the exhaust sound is deep and purposeful, while less peaky on the decibels, particularly while cruising (easier to talk). It’s a nice blend of livability and punchiness. So far so good, and a fun day had by all!
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