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Author Topic: muffler size/engine heat issues  (Read 4037 times)
skihor
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« on: August 29, 2007, 08:45:21 PM »

My MCI 5A has had serious heat issues since I bought it. 6V92, (350HP) auto. 12" X 12" blowers, small pulley,new radiators made w/ dimple tubes, and serpentine fins, original tanks were used. I need more rads I know but this issue is worse than it should be. Also just had both heads replaced due to small cracks in both due to Po's inability to keep the heat in check. Also new turbo.
My question is: The muffler on my bus is 5" inlet and 4 1/2" outlet. It is not straight thru, and the size leads me to believe it might be just a resonator. Regardless on whether it's a muffler or resonator would the smaller size outlet contribute to heating of the motor? I drive it by the boost gauge. The engine heating is directly related to the amount of boost I'm running NOT the amount of pedal I'm using. For instance: I'm in the top of third approx. 50 mph @ 2100 rpm and I'm using enough pedal to have 10 LBS boost, then I shift up to 4th and the boost drops with the same pedal pressure. I can give it more pedal pressure to achieve the 10 LB boost mark. This leads me to believe that it's not as much fuel related as it is an air flow problem. The air in is fine so that leaves the exhaust. Also I can't seem to get the boost past 15 LBS.
Can anyone provide me with a definitive answer on how much this exhaust
(5" inlet 4 1/2" outlet could be affecting my engine??

Don & Sheila
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TomC
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« Reply #1 on: August 30, 2007, 12:26:44 AM »

Highly recommend you change to a 5" inlet and outlet-that's what the trucks have on them.  I installed an 10" x 15" oval muffler that is 26" long with the inlet and outlet on the same end.  When I bought it, all that was inside was the pipe perforated with holes.  Otherwise was just straight through.  Granted it is a bit noisier than stock, but then again, I like the tone.  It made a big difference getting rid of the giant muffler that was stuffed up to the point that it didn't make any exhaust noise.  Mine is made for turbo's and is just one of many sizes and styles that Donaldson has available.  Mufflers are around $250-350. Good Luck, TomC
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Tom & Donna Christman. '77 AMGeneral 10240B; 8V-71TATAIC V730.
donnreeves
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« Reply #2 on: August 30, 2007, 04:30:19 AM »

I have a 36"x10" 5 in 5 out muffler on my 350 HP 6V-92. I have tried using a 4" tail pipe and it has no effect on engine temp or power compared to the straight 5". Mine makes 20+ lbs of boost and always runs 180-185. You can't just add big injectors to a 6V-92 to get 350HP. There are a lot of bits and pieces to go with them or you won't get the airflow through the engine. It will just produce a lot of heat.   Donn
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homermandy
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« Reply #3 on: August 30, 2007, 07:07:05 AM »

My 1991 Eagle DDEC2 6v92TA gets about 23lbs of boost. I removed the clogged muffler and am straight piping. I gained about 3-4 lbs of boos by doing this and throttle response greatly improved. The sound is louder but not too bad for me.  My muffler had about 500K on it and looked pretty bad inside.
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JohnEd
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« Reply #4 on: August 31, 2007, 06:11:11 PM »

On my Lexus SC400 I wanted to evaluate the exhaust system.  I mounted an 18mm nut on the side of the pipe immediately after the cat.  I registered 1.5 pounds of back pressure at WOT at redline.  Not bad I guess but I am not positively sure of the relative merit of that backpressure figuire.  I then cut out the center resonator and replaced it with a piece of 2.5 inch pipe.  My back pressure went to zero...or at least I could get no reading on my gauge.  The system still had two resonators and two mufflers and was reasonably quiet.  I stopped moding anything on that system as zero was as far as I could conceive going.  My question is: what is a tolerable back pressure at the inlet of the muffler on a 6V92 Detroit?  If that was measured and found acceptable you could start looking for a heat problem or more power somewhere where you might be able to realize some gain and some bang for the buck.  NOTE:  I noted that trying to use the O2 sensor port gave ten pound reading, by comparison.  I attribute this to the port being canted slightly twoards the gas flow and I think it acted like a scoop.  What shape port would minimize that action and accurize the reading.....engineer???

I really liked that straight pipe idea even though I had one lit off at 6am right next to me and I thought Judgement Day was upon us.  I think it actually vibrated me around in the bed....Sweet Lord!  Still, I like the idea of cheap zero back pressure.

John
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TomC
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« Reply #5 on: August 31, 2007, 10:01:11 PM »

With a turbo engine as close to zero as you can get it.  Good Luck, TomC
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Tom & Donna Christman. '77 AMGeneral 10240B; 8V-71TATAIC V730.
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« Reply #6 on: August 31, 2007, 10:33:00 PM »

Tom,

Thank you.  I want to know how much backpressure I would have to have for that to be a problem that I would want to address.  How much is simply cheating me of a little power?  How much is causing excess heating of the engine?  Maybe this is a 4 cycle or 2 cycle pecular data and I would be happy to know anything about either.  I cringe at the thought of changing any part of my exhaust as a "trouble-shooting" exercise.  Yee gads, Kids!  We are mose talented and resourseful than that.  "Hope your new muffler helps with your on-going overheating problem of 3 years".   The HOPE and HELPS part scare me.  3 years is no confidence builder either.  I say this admitting that I am and have been in the same boat many times and some sage advice would have done wonders for my success stats. 

How do I measure the back pressure?Huh?? Huh Huh Huh Kiss Grin

Luv ya'll..really.

John
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"An uneducated vote is a treasonous act more damaging than any treachery of the battlefield.
The price of apathy towards public affairs is to be ruled by evil men." Plato
“We can easily forgive a child who is afraid of the dark; the real tragedy of life is when men are afraid of the light.”
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buswarrior
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« Reply #7 on: September 01, 2007, 08:05:40 AM »

How's the intake? Filter clean and unobstructed?

You maybe want to know for sure that the parts in the engine (injectors/turbo) are the right spec, before considering the extremities.

Oh, the fan door is properly sealing, isn't it? No air through the rads if the fans are pulling air around or through gaps.

good luck! And keep us informed!

happy coaching!
buswarrior
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TomC
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« Reply #8 on: September 01, 2007, 09:32:57 AM »

Whether it be a too small air cleaner or a stuffed up muffler, usually black smoke will be your guide.  If you think it is the air cleaner, simply disconnect the air cleaner and take it for a short, relatively dust free drive to see if there is a difference (a short drive with minimal dust won't do anything to the engine).  If you think it is the muffler, do the same thing-although it will be a bit on the loud side.  Good Luck, TomC
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Tom & Donna Christman. '77 AMGeneral 10240B; 8V-71TATAIC V730.
skihor
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« Reply #9 on: September 03, 2007, 07:03:15 PM »

Well we're back from the Black Hills. I put a Donaldson muffler ( 5" IN X 5" OUT 9" X 27"
body), on before we left. Turn out it was 5" in AND out. I was reading in a flat area of the tail piece. Anyway I pulled it off and there is a perforated pipe the length and there is a disc blocking the pipe in the middle. The exhaust has to go around thru the perf holes then back out the perf holes on the other side of the disc. I figured that has to have alot of back pressure. I bought a new one along with elbows etc... and rebuilt it all. I punched out the disc in the middle and had all of the elbows and pieces welded. DAMN IT'S LOUD NOW. My initial/gut feeling is the 6V92 mechanical wants some back pressure. I think it lacked power, just a little, from before I punched out the disc. and I think the heating may have been just a little worse. It DID NOT help at all. I may be wrong on this but it's just so little change, except for the sound, it's hard to say if it hurt at all. VERY minor IF AT ALL. I'm going to try the Areo muffler next as this thing isn't much more that a straight pipe, and the Jakes OMG.
Anyway I've done EVERYTHING except more Rads SO... that's this winter's project. Two more rads to bring my BTU capability to 20% more than the motor can produce. Then I can use all 350 HP.
Thanks for all of your input guys.

Don & Sheila
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pvcces
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« Reply #10 on: September 03, 2007, 07:24:34 PM »

Don, I've kind of figured that the back pressure caused by the exhaust passing through the muffler and pipes was taken into consideration by DD and GM when they engineered these busses.

They seem to work all right as long as they are operated as designed, more or less. Forty years ago, people didn't object to smoky exhausts like they do now.

If a person increases the amount of air pumped into the engine by using a blower, does that raise the amount of fresh air that gets compressed? I think that with the ports and exhaust valves open at the same time, the increase is much smaller in a two stroke than a four stroke.

For what it's worth.

Tom Caffrey
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Tom Caffrey PD4106-2576
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JohnEd
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« Reply #11 on: September 03, 2007, 08:46:32 PM »

Ski,

If you have back pressure you will generate a little more heat in the engine.  Way back in the day all cars had what was called a heat riser valve in the end of the exhaust manifold.  You can't make this up....honest.  The job of the heat riser was to PLUG up the exhaust and expedite the engine coming up to operating temp.  At temp a bi-metalic spring would position the buterfly to "open".  They also had thermostats in those days.  So plugging the ex will increase temp....ok.  The rub is "HOW MUCH BACK-PRESSURE"  and "HOW MUCH HEAT"?  I know what the principle is and the results.....HOW MUCH?  I admitt my ignorance and even to the point that I don't know where to go to get the info except here.

For your situation:  You relieved the back pressure and that must have reduced the temp by some amt.  Again HOW MUCH?  Less back pressure will allow you to get more fresh air into the cylinder on each charge cycle and that will result in more power and MORE HEAT from that sourse.  Did you generate more heat than you lost?  Don't know.

I completely disagree with the statement that DD designed the engine to operate with some normal amt of back pressure.  Every tech article I have ever read said "OPEN THE INTAKE" and  "OPEN THE EXHAUST".  If this were a tuned ex system there would be some merit to the back pressure theory but that back pressure would be in waves of reflected energy that stuffs the charge back into the cylinder...deep stuff. 

What concerned me with your post was that you seemed to be poised to plug up your ex to get power back.   If I am wrong on any of this I sure hope you guys don't let me stay stupid.

Thanks

John
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"An uneducated vote is a treasonous act more damaging than any treachery of the battlefield.
The price of apathy towards public affairs is to be ruled by evil men." Plato
“We can easily forgive a child who is afraid of the dark; the real tragedy of life is when men are afraid of the light.”
—Pla
pvcces
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« Reply #12 on: September 03, 2007, 09:05:40 PM »

I'll be eagerly awaiting results.

Tom Caffrey
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Tom Caffrey PD4106-2576
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skihor
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« Reply #13 on: September 03, 2007, 09:14:41 PM »

John,
I think I'm creating just a little more heat than I eliminated by your reasoning of more air to compress. I'm not going back to the Donaldson muffler because I think the (plug/disc) in the tube is a little too much back pressure. The Aero muffler is quite unique in it's construction and claims increased MPG, free flowing, (BLAH, BLAH, BLAH) rhetoric most likely. But it will provide just a little back pressure, which I think my 6V92 wants. Not much, just a little. Plus I can't live with what is basically a 40" straight pipe. LOUD doesn't begin to describe it.
May be a wrong comparison but my 1972 Shovelhead (Harley) has a 12%+ increase in horsepower with a 2 into 1 exhaust with a muffler, over straight, dual drag pipes.

Don & Sheila
 p. s. I remember the spring butterfly in the exhaust manifolds. always rusty always stuck to some degree. We just took the butterfly part out and left the pivot shaft in.
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Dallas
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« Reply #14 on: September 03, 2007, 09:20:53 PM »

According to an old copy of the DD Field Service Data Manual:
Series 92 (non turbo):
  Exhaust back Pressure--Inches of Mercury - Full Load: 1200rpm = 1.5 1800rpm = 3.3 2100rpm = 4.0
Series 92T 8V and 16V
  Exhaust back Pressure--Inches of Mercury - Full Load: 1800rpm = 1.8 2000rpm = 2.2 2100rpm = 2.5
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