Engine compartment aerodynamics getting the hot air out (Silver Eagle)

[tr206]

I have heard about running a low hanging mud flap across the whole width of the bus behind the rear axle to create a low pressure area under the engine is this proven? louvered engine compartment doors which I have. Any other ideas?

[RJ]

GMC started doing that in 1953 with the introduction of the PD4104 "Highway Traveler" coach. Carried the same idea for twenty-seven years, right thru until the end of production of the intercity coaches in 1980. You could also find these on the GMC RTS coaches, too.

Was easy to do on a GM, as the forward engine bulkhead was immediately behind the rear axle, seeing as the powertrain sat cross-ways in the engine compartment.

Do you think GM would have continued installing these full-width mudflaps if they didn't do what they were designed to do: create a low pressure area under the engine compartment to help draw the hot air out?

It's a little more difficult to do with a "T-drive" powertrain like Eagles, MCIs and Prevosts, but it can be done, with similar results.

The worst is hanging a full-width mudflap off the back bumper - it may keep the toad cleaner, but at the risk of the engine running hotter.

FWIW & HTH. . . 

RJ

[tr206]

I didn't know GM Did this. I guess it's worth a shot. Probably best to put it across at the bell housing so the air doesn't start rolling back up before exiting behind the bumper?

[richard5933]

GM didn't just use the mudflap at the rear side of the rear axle, they also used louvered belly pans under the engine bay. The whole system was designed to suck air out through the bottom of the engine compartment and both the mudflap and the louvered belly pans helped create the air flow. It came in through the radiator, across the engine, then down and out the bottom of the engine compartment through the louvers.

[chessie4905]

dont know so much on the effectiveness on the belly pans. I think they were effective in keeping the oil leaks off the rear of coach better. Anyway, few still have the pans.

[dtcerrato]

We've always had three full width to the ground mud flaps. One behind each front & rear axles & one under the rear bumper. Never a problem with running hot although we run stock oem 200+ hp. Never felt the need to go any faster in the bus & very dependable over the long haul.

[buswarrior]

The GM belly pans were not well liked by the mechanics.

Simply discarded, damaged or bent as required, to keep the shop foreman off their back.

No sophisticated reasoning on this one.

Happy coaching!
Buswarrior

[richard5933]

The GM belly pans were not well liked by the mechanics.

Simply discarded, damaged or bent as required, to keep the shop foreman off their back.

No sophisticated reasoning on this one.

Happy coaching!
Buswarrior

They may not have been well liked, but the way it was explained to me was that they serve to prevent any of the turbulence under the bus pushing up into the engine compartment and working against the flow from the radiator fan. Not sure how many buses survive with them intact, but mine are still there and don't seem all that difficult to remove/reinstall. Of course, I'm not rushing to get a bus back on a scheduled run when I pull them.

[buswarrior]

If working on, or with, these old whores was one's job, the religeous pursuits were of no concern.

Tear that crap off there, and run them over with the bus, as it leaves the pit.

The length and breadth of the sinning is evident in how few sets are still in existance.

It is only us busnuts, that place value on the more archane sublties of the designers intent... and will put forth the extra effort...

Are we missionaries, wading into the seedy streets, rescuing these abused perveyors, whose only purpose was the enriching of their masters, and discarded when they no longer could?

Some stuff ain't right...

Happy coaching!
Buswarrior

[RJ]

I didn't know GM did this. I guess it's worth a shot. Probably best to put it across at the bell housing so the air doesn't start rolling back up before exiting behind the bumper?

My Prevost has almost a full-width mudflap - except for a one-foot gap under the driveshaft - hanging between the drive and tag axles.

But for you, with an Eagle having the bogie axle ahead of the drive axle, if you decide to pursue such a project, should figure out how to hang it across the coach directly behind the duals.

Pretty amazing what those old GM engineers were able to do without a computer!

FWIW & HTH. . .

RJ

[6805eagleguy]

Pretty amazing what those old GM engineers were able to do without a computer!


RJ

For sure. They were not dumb!

[dtcerrato]

For sure. They were not dumb!

Those old school engineers - especially the post war civilian engineers migrated from the military at the close of the war into the civilian sector of public mass transportation just like the unused stock piles of military aluminum raw material that was building war machines. It initially went into Trains, planes and busses... and as far as "They were not dumb!" Heck they were the best the planet had to offer. Their designs were so over engineered - to the point to win the world war that our vintage antique busses that were designed for a 20 year service life of constant operation equates to post war & pre war busses in the age group of 60 to 75 years old that are still running like the day they came off the assembly line. Opps! Rant over - carry on...