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BMC (Bluebird Motor Coach Unique Issues) If you have a unique issue with your BMC coach and it can't be answered in one of the other forums here, then this is where you can list it.....List your BMC Parts here too.

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Old 11-08-2010
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Lightbulb Muffler Delete on a Cummins 8.3L

I figured a clean thread with no distractions about other modifications might be called for when it comes to going to a straight pipe exhaust. Specifically for comments about noise levels and perceived or factual data on hp increases.

FYI: A lot of this has been covered in the sticky thread about Brazel's but it evolved into a lot of other mods and muffler data which might complicate it more than someone just wondering about loosing the dead weight, freeing up some power, etc....

TxDot:

20.15 Exhaust System. Every motor vehicle shall at all times be equipped with muffler in good working order and in constant operation.

Muffler defined: Muffler is a device consisting of a series of chambers or baffle plates or other mechanical design for the purpose of receiving exhaust gas from an internal combustion engine and/or turbine wheels for the purpose of receiving exhaust gas from a diesel engine, both of which are effective in reducing noise.


So essentially in my State and many others a turbo is considered a muffling device (the OR part of this). Check first if you have a yearly state inspection process that you need to pass as we do. Non-turbo diesels must have a muffler.

The setup:
We performed this mod on our 94 Safari of which it's powertrain is nearly (if not identical) to the BMC of the same year. The turbo has a 4 inch outlet, angled down, then goes up to 5 inch into the top of the OE muffler and then a 5" tip coming off that about a foot to bring it outside behind our jack. The inverter is directly above the muffler and it pretty much fills the entire area on the first compartment back from the axles. It's one big muffler. Sticking your arm in the inlet once we dropped it off one can feel a cheesegrater type baffle, the other end has a slightly angled tip so no access there.

How we did it:
Rather than going straight down we used the existing flexible 4" 90 pipe coming out of the turbo (towards the front of the coach). Used a chop saw to cut the 4-5" off and welded on a 4" 90 (reversing direction). Essentially it makes a large 4 foot gradual U shape. Once straightened out and aiming for the rear corner we (actually our friend Doug) re-welded the 4"-5" pipe to go back up and into a 3 foot length of 5" stainless with a sharp edge tip cut to it. Some hangers and bracing and it was good to go. Took about a hour with Doug sourcing a few pieces from his scrap piles and a few off the existing setup. Most of the time was in getting the hangers set properly so its solid and the angle so no fumes build up under the coach. There's less of a chance of that now than there was before as the tip is several feet further back on the side of the coach and theres less angle pointing down.

Conclusions:
  • Noise levels:
    Very satisfying. Sound is a little increased but not atrociously loud. The engine itself is still the bigger noise producer. It's just that little bit different, you can actually hear a little whistle as the turbo spins at idle where as before their was only a slight hint of it. At highway speeds, assuming you have something like a concrete barrier or tunnel to bounce the sound back, you hear a slight raspyness to it. An unintentional benefit is it's j-u-s-t loud enough to where people don't sit on the back corner tooling down the highway anymore. Give us a chance to mess with fuel mods and put some smoke out and I betcha they won't even pass.

    I would not hesitate for a second in a RV park with this. I believe ANY running diesel in a RV park is going to be waking people. It's the nature of the engine, it knocks and makes noise. But that aside the 6CTA 8.3L is no louder idling along at walking speed than the duallys pulling their 5th wheels and bumper pulls or a newer, Prevost, Marathon etc... backing into the spot next to you. Sure it may be slightly more throaty than before but it's not overbearing. Any Harley or a SportBike is much more of a threat to the peace. One needs to think Big Block Chevy vs Small Block, not Sport Bike at 7K rpm vs a moped. Or think of a larger 12-14 Liter Peterbuilt Rig. To me, it's quieter than any of those extremes.
  • Performance:
    Well we did a 3,000-4,000 Governor Spring Kit at the same time so any review is a mixed bag. The kit doesn't affect the fuel so much but that alone changed the feel and shifting of the coach so I don't have definitive data on power. The turbo definitely spools quicker (which I feel is thanks to the muffler delete) and it's no longer falling off before shifting up a gear, which is why we followed Dougs recommendation during the valve adjustment back in March for the$95 Governor Spring. Same kit as a 5.9L, it's not a hot rod so we stayed at 3K. So I've got the guy a hour west of San Antone if you are considering it and are close.

    All said the coach "feels" thousands of pounds lighter after these two little mods. Most certainly it's behavior driving through town, stoplight to stoplight, is VASTLY improved.
  • Satisfaction:
    Tickled pink. Definitely a must do. It'll make you wonder why it took you so long.
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Ken - Austin, Texas - keeping Mom on the road in her '94 34' Safari Serengeti
- 250hp Cummins 6CTA 8.3 Turbo Diesel (#6 fuel plate, straight pipe, 3K Gov Spring, Timing @14.25°, Boost @21psi, Isspro gauges)

(HDT Coach or BMC & Stacker for me in the future.)

Last edited by Driver; 11-08-2010 at 08:30 PM.
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Old 11-08-2010
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