[A/S] Diesel & stick shift issues

[Stan Nicholson <stan@xxxxxxxxxxx>]

I think Jerry's info is correct - diesels have much higher 
compression (they ignite the air/fuel charge with the heat of 
compression, so they're very hard to crank over), but they don't have 
any restriction on the amount of air intake, so compression braking 
is much less than with an equivalent-displacement gas engine. 
Unless, of course, an exhaust brake of some sort is added.

Having said that, I confess to not really understanding how the GM 
Allison 5-speed auto and the new Ford 5-speed auto really provide 
effective engine braking in "tow-haul" mode, although I know they do 
since I've experienced it by driving both.  The Ford has a 
variable-pitch turbo, so I've been imagining that it may play a role 
in this phenomenon, but that doesn't explain the Duramax/Allison's 
behavior.

I also think Barbara's info is correct about an automatic trans 
typically having an easier time getting a load rolling than a manual 
trans.  The reason is the automatic's torque converter.  Its "stall 
ratio" is typically 2.5:1 or thereabouts (I'm not claiming this is 
the ration for the diesel 5-speed autos), which means that it's 
capable of multiplying the torque by (the stall ratio) times (the low 
gear ratio).  That figure usually is considerably larger than the 
low-gear ratio of a manual transmission.  When that's true, the 
automatic will have an easier time getting a load rolling than a 
manual because it multiplies low-speed engine torque more.

The issue of which type of trans to buy, though, is a very personal 
one.  Both types have been used successfully for years, and both have 
their adherents.  My personal vote is for an automatic - at their 
best, they're effective, convenient and reliable.  Unfortunately, not 
all are that way (Dodge in particular has had problems with 
automatics standing up to diesel torque).

Any corrections are gratefully solicited.  And while I'm at it, 
perhaps someone has an answer for this one:  diesels with turbos are 
notoriously good at towing at altitude because the turbo helps 
compensate for the thinner atmospheric pressure.  I'm wondering about 
the nature and limits of this effect.  In particular, are the 
turbodiesels from the Big 3 capable of providing cylinder-filling 
pressures comparable to those at sea level when the rig is at, say, 
10,000 feet, or is the turbo capable of replacing some (but not all) 
of what otherwise would be a considerable loss of cylinder-filling 
pressure?  That is, do today's turbos eliminate, or just reduce, 
power loss due to altitude?

Stan