RB exhaust design from headers back

[mopar67440]

Hi all,
Looking for some insight, feedback, and science-based suggestions.
I would like to install a new exhaust on the 67 Dodge Coronet R/T RB this winter. I know the current exhaust has a few flaws resulting in poor exhaust scavenging/intake reversion, compromised low rpm torque, and an interior drone...all caused by a 3" header collector reduced immediately to 2.5" w/ no header extension, no crossover and with 18" thrush mufflers.

I would like to redesign the system to address all 3 issues.
Current engine combo:
- 440 .030 over
- Mild port 906 heads
- KB pistons level to deck (10.5:1) w/ Cometics steel shim gasket
- Mopar 509/292 cam
- 1 7/8 primary tube headers w/ 3" collector
- Was a Torquer II w/ 750 DP holley w/ A/F tuned. Currently running Edlebrock CH28 dual AVS2 650s
- Mallroy Hyfire ignition w/ advance optimized
- 727 auto
- 8 3/4 w/ 4.10 gears

I'm very happy with the way the engine is running, and do not plan to make any changes except maybe going back to the single 4 bbl setup once I access the dual quad setup, and retune carb for any scavenging improvement from the exhaust.

My current plan to improve on all aspects mentioned above:
- 3" mid pipe from header collector to Dynomax Turbo mufflers inlet (3" in/out 21" body). Received mufflers free
- 2.5" crossover at 18" from primary header pipes
- reduced to 2.5" tail pipes to 1969 Mopar chrome tips

Thanks!

 

[493 Mike]

Hi all,
Looking for some insight, feedback, and science-based suggestions.
I would like to install a new exhaust on the 67 Dodge Coronet R/T RB this winter. I know the current exhaust has a few flaws resulting in poor exhaust scavenging/intake reversion, compromised low rpm torque, and an interior drone...all caused by a 3" header collector reduced immediately to 2.5" w/ no header extension, no crossover and with 18" thrush mufflers.

I would like to redesign the system to address all 3 issues.
Current engine combo:
- 440 .030 over
- Mild port 906 heads
- KB pistons level to deck (10.5:1) w/ Cometics steel shim gasket
- Mopar 509/292 cam
- 1 7/8 primary tube headers w/ 3" collector
- Was a Torquer II w/ 750 DP holley w/ A/F tuned. Currently running Edlebrock CH28 dual AVS2 650s
- Mallroy Hyfire ignition w/ advance optimized
- 727 auto
- 8 3/4 w/ 4.10 gears

I'm very happy with the way the engine is running, and do not plan to make any changes except maybe going back to the single 4 bbl setup once I access the dual quad setup, and retune carb for any scavenging improvement from the exhaust.

My current plan to improve on all aspects mentioned above:
- 3" mid pipe from header collector to Dynomax Turbo mufflers inlet (3" in/out 21" body). Received mufflers free
- 2.5" crossover at 18" from primary header pipes
- reduced to 2.5" tail pipes to 1969 Mopar chrome tips

Thanks!

I think your plan is sound but, 2" for the cross over is large enough.
Mike

 

[AR67GTX]

Sounds good to me. I’ve read some places that the X type crossover results in a few added HP over the H style but I have no experience with it. Use band clamps on the inlet to the mufflers to make it easier to drop the head pipes if the transmission ever needs to come out.

 

[mopar67440]

Sounds good to me. I’ve read some places that the X type crossover results in a few added HP over the H style but I have no experience with it. Use band clamps on the inlet to the mufflers to make it easier to drop the head pipes if the transmission ever needs to come out.

H pipe at 2" it is.
I also heard the X pipe results in a slightly different tone. I prefer to keep that traditional RB exhaust cadence.
Band clamps at the muffler inlets. Good idea!

With respect to the science I have been researching behind this...
Would the 3" mid pipe and 3" in/out large case muffler serve as a large enough expansion chamber? I think the total length from header to muffler exit is approx 74".
Also, anyone have insight on the negative exhaust pulse arriving at the intake valve opening point just before the intake stroke? I can look up the cam specs and calculate intake opening point.

 

[Some Car Guy]

If you want to kill the drone, be sure to do some resonators. They won’t hurt flow. The super turbos are good, but like most stuff they will drone.

X pipe, h pipe and no cross all perform the same at the track, so says my exhaust guy who tired all three on his drag car. They can change around the sound some, but it’s not some huge thing. I’ve had all sorts of big blocks with the different combos.

 

[mopar67440]

Thanks Some...
Drone currently isn't too bad with the Thrush. The firewall, floor, doors, rear panels and trunk all have dynamat which made a huge diff.. A 21" muffler should eliminate the remainder.
Car won't see the track, so I'm more interested in optimizing for 2200 thru 5200 rpm on the street while maintaining that all too famous Chrysler RB cadence....which I love!