<<My dad is looking to purchase a 2004/2005 Trailblazer Ext and is
trying to decide what engine to put in it.  He has a 21', 3500# travel
trailer that he pulls from end of December to mid April (yes he is
retired).  Currently he has a Chevy Astro van with the 4.3 litter engine
in it and he is happy with the preference of it, but he is trying to
decide what engine he should get in the trailblazer.  The dealer is
telling him that all he needs is the 6 cylinders but I am worried about
the engine longevity.  So I am just looking for a little advise ROB>>

If power is a consideration, it costs $1500 to upgrade to the 5.3 liter
V-8, but only $50 to upgrade the rear axle from the standard 3.42 ratio.
However, boosting the rear axle ratio will considerably lower your
highway mileage with either motor.

Rear axle, 3.73 ratio (GT4)

Rear axle, 4.10 ratio (GT5)

Apparently, the 4.2L motor needs to be run at very high RPM in order to
deliver any real power:

Engine, Vortec 4200 SFI
(275 HP [205.1 kW] @ 6000 rpm, 275 lb.-ft. [371.2 N-m]
@ 3600 rpm) (LL8)

Engine, Vortec 5300 V8 SFI
(290 HP [216 kW] @ 5200 rpm, 325 lb.-ft. [441 N-m]
@ 4000 rpm) (LM4)

The overall gas mileage seems to be about the same for the two motors:

Trailblazer EXT 2WD  A-4 ..... 4.2/6 .. 15/19
Trailblazer EXT 2WD  A-4 ..... 5.3/8 .. 16/19

The main difference is that the 4.2L motor will be running at greater
RPM as it tries to develop the necessary power to pull a trailer. Even
under normal circumstances, the 4.2L motor will be running at higher RPM
as it will downshift a lot more than the 5.3L motor, thus putting more
strain on both the transmission and the engine. Your concerns about
engine longevity are well justified with the 4.2L motor.

On the other hand, you say that your father is happy with the 4.3L motor
in his Astro van and I am assuming that he is towing the trailer with
the van. You can look on the glove compartment label and see what the
rear axle ratio is on the Astro van:

GU4  Rear axle, 3.08 ratio

GU6 Rear axle, 3.42 ratio

GT4 Rear axle, 3.73 ratio

GT5 Rear axle, 4.10 ratio

G80 Differential, locking, heavy duty

G86 Differential, limited slip, heavy-duty, rear

(The 3.08 rear axle ratio was deleted in full-size trucks in 1999 when
GM switched to from standard 15-inch rims to 16-inch rims. The 3.42 rear
axle ratio combined with 16-inch rims is about 6% greater than the 3.08
rear axle with 15-inch rims.)

I don't know what transmission is in the Astro van, but the acceptable
4.3L motor and rear axle ratio of the Astro van may be equally
acceptable in the Trailblazer.

Personally, I would go with the 5.3L motor in the Trailblazer because
much of its initial cost will be recovered when the vehicle is sold.
With the 5.3L motor, the standard 3.42 rear axle ratio would probably be
sufficient, but the optional 3.73 rear axle ratio would assure ample
power for all applications.

http://www.fueleconomy.gov/feg/feg2000.htm

http://www.chevrolet.com/byo/build.cv?make=Chevrolet&makeId1&vFrom=&modelId8&v
ModelName=&subModelId=&year 05

Rob the engine used in the Trailblazer is an inline 6 that develops 90% of
it's torque at 1600 rpm.  If the dealer offers the 24 hr. test drive have
your dad bring one home, hook up the trailer and decide if the 6 has enough
"umph".   All have trailer hitches however doubt they'd have the brake
controlled installed so he would have to take that into account.  I would
suggest something other than the stock 3.42 axle.  Perhaps the 3.73 as an
overall compromise.

I have a 25' Wanderer Glide-Lite trailer.  I used to tow with a GMC Safari
(same as Astro) and found it to be lacking when facing stiff head winds on
the freeway.  I now have a Yukon with the 5.7 and towing is much better.
Both vehicles had/have the 3.73 axle.  I'd recommend he het the 5.3.  No one
was ever sorry they had too much power when towing! Besides, trailers have a
way of "becoming" too small and if he trades up (like I did) the six isn't
going to cut it.

WW