muffler size/engine heat issues

[Stan]

The MC-5A originally came with a 6V71 or 8V71 so if the engine and HP are raised with a 6V92 then you have to have the cooling system for a 6V92. Obviously you have to have the correct rads and blowers but you also have to have the correct airflow. As buswarrior pointed out, you have to be sure that the blowers can only suck air through the rads. The increased air flow from the bigger blowers has to have some place to go. Have you increased the size of the openings in the floor of the blower compartment, increased the size of the openings in the side doors and have free flow under the rear bumper.

There have been hundreds of threads on cooling MCIs with all kinds of exotic ideas, but almost always it goes back to the radiators and air flow. MCI buses with the two top rads were borderline on heating when new, so boosting HP will always create more heating problems.

[makemineatwostroke]

Stan, Dallas gave you the correct max reading for the 92 they require some back pressure but not much if you do not have a mercury gauge check it with a low pressure air gauge by using the formula psi x 2.07 = mercury

[Lee Bradley]

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

Yes carburetored engines used to have heat riser valves but their purpose was to restrict the exhaust flow enough to force them through a passage under the carburetor. This warmed the carb and intake to give better vaporization and prevent icing. Some cars used an auto choke system with a bi-metal spring in a well in the manifold this was also heated by the exhaust gases flowing through the exhaust passage. In use these systems had many problems; the passage became blocked and no heat was supplied, the riser froze open and no heat was supplied, the riser froze closed and too much heat was supplied and often cracking the intake manifold. Some car makers went to hot water systems and electric choke heaters but finally fuel injection solved all these problems.

[JohnEd]

Dallas,

At last, at last!  Real numbers and a way to go.  Those people that thought they had a plugged up muffler can now be certain of the internal condition and servicability of their equipment.  As is the case in autos, new replacement items are sometimes such very poor performers that they worsen the condition.  Glass pack straight thrus comes to mind.

These bus mufflers are anything but cheap and the labor is serious.  Having it done at a shop is @ $100/hr.  You have taken the guess work out of this heating/power/smoke thing once and for all.  At 1,800 rpm you should have 1.8 inches of mercury, or less, with a 8V92 Turbo.  I suspect the turbo and NA are using different mufflers but 1.8 is all that matters.  Get that or less and exhaust back pressure is NOT YOUR PROBLEM AND TAKE THAT TO THE BANK!  So says DD.  Money and attention elsewhere for the fix.  Thank you, thank you.

Now these guys that get 2 MPG less than a similarly weighted and powered coach have something to look at for an answer.  And something to verify to be working properly.

I know that I should measure this at the outlet of the ex manifold to be sure to get the piping into the measuerment.  I used a turbo boost gauge that only went to 25 pounds to do my car work.  I measure the pressure that presented itself at a 1/4 inch hole I drilled in the ex pipe.  I tried other ports and got "crazy" readings caus the holes there were angled into the ex flow.  Any engineers care to speculate?  My readings were valid cause I only wanted to see a delta and the absolute was not as important.

Sincere appreciation,

John

[compedgemarine]

just my 2 cents but the statement that less backpressure is good and more air flow is better is a little misleading. if you run a carburated or mechanical injected motor and remove the back pressure yes you will flow more air but with no other changes you will run a leaner burn. more air, same fuel = lean burn, less power, more heat. with some electronic motors you can also create a problem if it a system that doesnt monitor certain things or is capable of only adjusting a given amout. changes to exhaust to reduce back pressure can help assuming you make changes to the whole system to work with the changes. if not you can hurt more than just the power it makes.
steve

[skihor]

Dallas, that is just the info that I was looking for.
Steve that was my gut feeling on the change I made to the backpressure. It was not a major difference but one I thought I could feel. It seemed to have a litt less power and seemed to get hot faster. I had to run 1 LB of boost less than before to maintain the temp at approx. 185*.
By the way I talked to a tech at AERO Mufflers today and he stated the 5" IN X 5" out muffler for diesel's has a back pressure of .4 @ 2500 RPM.
Not enough according to Dallas and DD.
THANKS Dallas for that info. It saved me from another $275 experiment.

Don & Sheila

[JohnEd]

Ski,

Talk to your Aero rep again and ask if you NEED some back pressure in a diesel.  I think the ans is the less back pressure the better.  The figure qouted by DD is the expected MAX.  If you have more than that you have a plugged or too small a muffler.  The stock muf was designed for space and noise and weight considerations and it was old technology.  Aero is better these days.  As I recall, back in the day,now, FIRETRUCKS ran without mufflers.  They WANTED to make noise and they got more power from the practice.  My uncle was a fireman in Pa. and I got to ring the bell when I was still in short pants.           

Please confirm:  My engineer neighbor says that you DIVIDE the psi number by TWO(2) to get inches of mercury.  I looked at his formula process and it looked solid.  Anybody?

Thanks,

John

[luvrbus]

In the DD manual its says 
inches of water = psi x 27.7
inches of mercury = psi x 2.04
thats their formula in the manual

[Stan]

John: To convert PSI to Inches of Mercury Multiply PSI by 2.036.

[RJ]

Don -

A few random thoughts on this topic:

Remember the Corvair turbocharged Spyders and Corsas from the '60's?  Chevy designed the muffler on those engines to produce sufficient back pressure at 5500 rpm to cause the turbo to cavitate (or stall, not sure of the correct term), thereby reducing boost and increasing the longevity of the motor.  Granted, the little air-cooled flat six leaked oil and had a tendency to flip off the cooling fan belt, but it was awfully hard to break one.  Point is that the factory sometimes does things to protect us from ourselves. . . LOL!!  But Dallas came up with DDA's specs, so now you've got something to work with.

Do you have a gravel shield/mudflap hanging directly below the back bumper to keep stuff off your toad?  If you do, chances are it's trapping hot air under the engine compartment.  GMC put a full-width mudflap across the rear of the coach directly behind the drive axle.  This created a low pressure area under the engine, thereby helping to draw the hot air out.  Hanging one off the back bumper defeats what the factory was trying to accomplish.

Will have to agree with Buswarrior and others regarding the sealing of both the radiator compartments and the blower compartments.  Bad seals = overheating.

Do you still have operating air-powered dampers for the blowers?  If so, might want to check their operation, to make sure they're opening fully.

IIRC, MC-5As have slotted openings in the side engine compartment doors, both street and curb sides.  Might consider installing an electric fan off a front-wheel drive GM or similar on the inside of these doors to boost the air flow thru the engine compartment, especially when climbing grades.

Speaking of climbing grades, Detroits do best between 1700 - 1900 on a partial throttle when pulling a hill.  You should still be able to accelerate from this rpm, but for controlling cooling, it's best to keep the engine in this operating rpm range on a partial throttle.  If you cannot hold this rpm, it's time to downshift to a lower gear.  (Trivia note:  On a 200-mile run from Fresno to Los Angeles, the time difference between climbing the Grapevine on I-5 at 45 mph in 3rd vs 35 mph in 2nd is a grand total of less than FIVE minutes.  Something else to consider.)

FWIW & HTH. . .

[TomC]

Once again- with any turbo engine, the closer to zero back pressure the better.  Any back pressure will keep the turbo from fully spooling up possibly causing black smoke or slower spool up response time.  On trucks, 5" exhaust in and out of the muffler is always used.  Good Luck, TomC

[JohnEd]

TomC,

I talked with the DD 2 cycle rep here in Eugene.  He said that the DD 6V92 TA needed "a little bit" of back pressure to function properly.  I did not think to question whether he was talking about an electronic engine or mechanical.  AND THEREBY HANGS THE TAIL.  I talked to a local builder with a good rep and he said that as a general rule the lower the back pressure the better the potential engine  performce in all regards....period.  Real wishey-washey like.  He added an interesting exception, however.  He said the newer engines sampled the exhaust back pressure as a computer input and if you changed that it confused the computer...and we all know what that means.  He said he was sure that "some" of the systems COULD NOT be reprogramed and that those would be considered to NEED back pressure.  I think this must be a DDEC III but I don't know.

There is a "lab for diesels" in town and they do rebuilds and consulting.  I spoke to the rebuilder and he confessed to not being steeped in the theoretical or abstact specs.  He did say that "all the engines I see from quarries and the such that are used for scrapers and movers have a pipe right out the bottom and they were 6 and 8V92's and 71's.  I mentioned the firetrucks running straight pipes and he said "yeah, and they ran the 2 cycles".  He must be old!

So less is MORE unless you are running an electronic engine that samples the ex back pressure and even then you can probably have the computer reprogramed if you go to a high flow muffler.  Considering a stock muf might cost $1K and after market hi-flow is $250....maybe???  Find out for sure first.

I love it when we all are right, sorta.

Thanks TomC.  I would have dropped this if it weren't for your last.  This was fun!

John

[TomC]

Back pressure is not an issue until just recently.  It would be for Caterpillar since 2002 since their engines have catalytic converters in them.  And now for the 2007 engines with particulate traps, it is also important for back pressure.  But for any of the two stokes, if you have a turbo, whether it be mechanical or electronic, if you have a pre catalytic converter and pre particulate trap 4 stroke turbo, the lower the back pressure the better.  Good Luck, TomC

[HB of CJ]

I have NOT read the other posts, sosss if I am repeating stuff, sorry.  Sounds kinda like you may have some incompatablilty inside the engine, like pistons, timing, blower bypass, turbo type, injectors, rack settings, etc..   Don't want to sound too negative here, but your boost gage should not be responding in the matter you report.  It should be more throttle related than rpm related within the normal operating rpm range---say from 1500 to the governed rpm of around 2150 or soosss.

Do you also have a pyrometer and air restriction gages?  Sometimes a marginal air intake filture/intake system can cause other problems difficult to determine like overheating.  Would seem to me the less exhaust resistance the better with a turboed Detroit.  Not necessarily soosss with no turbo.  If you keep your foot out of it, (and it seems you know how) then your Detroit should NOT be overheating at partial throttle--but only when your stand on it.  Don't know what else.  Good luck.

[Ncbob]

Since most of you don't own MC5A's...might I step in here and add some thoughts based on the fact that I too own an MC5A, albeit with an 8V71 engine.

In this model bus the exhaust feeds off the left side manifold (there's a crossover pipe from the other manifold like most MCI's...but the exhaust feeds into the 5" inlet...goes forward...makes a 180 degree turn and exits the muffler (yes, it's a muffler) through the 4 1/2" outlet to a 'fishtail" to dump it outboard of the body of the bus.  I have no idea how the rest of the MCI's route their exhaust.

The 1/2" reduction in the size of the inlet and outlet would make sense...for inherant back pressure which Detroits need to keep from 'wet stacking' at low idle speeds.

Here we have a completely different situation.  a 6V92 in a bus designed for an 8V71. I take no issue with the repower (wouldn't do it myself because of all the horror stories I've read about cooling those monster 92's)....in addition this one has a turbo.

First, Don, since you can't enlarge the fans (no room) I'd contact Fred Hobe for one of his larger pulleys to drive the fans faster.  You're limited as to the size of the radiators...except thicker. If the Shutterstats are gone...good riddance.  You need to contact a cooling specialist about more core area.

Another thing.  I've found, contrary to what the GM boys say, that a full mud flap across the back AIDS in cooling. It forces the waste hot air out through the side louvered doors (if you still have them) that's the way the cooling system was designed.

If you could somehow use a (for the life of me I can't recall the brand name...but they're at all the big bus shows) Turbo-flow, straight through muffler and find a way to get it outboard of the body without scalding the rears...you might have the answer. The 180 turn has stymied me since I've owned my bus!

It might be that all this is for naught...but then if it opens doors for thought or ideas...that is my intent.

I sure would like to be around for the archiving of this thread and wish all who offered opinions, well.

NCbob