Tom,
The Allison software has obviously been programmed to downshift the tranny to second upon the TCM receiving the exhaust brake enable signal from the throttle switch & TPS. That is selectable to either 2nd, 3rd, or 4th. The software also prevents the exhaust brake from engaging at idle AND if engine RPM gets too high, it will send the tranny to the next higher gear to avoid over-speeding the engine. The B & C Cummins are fine to 3100 RPM. BUT ! ! ! Just because the tranny downshifts, does not mean the exhaust brake is actually working. Since you stated that yours did just that, I would check it. How? Lift the bed, support it with some sort of device (I use a broom handle) because the struts will not hold the bed up during a road test. Have someone drive the coach, making sure the brake switch is in the "On" position, accelerate to highway speed, and then release the accelerator. Watch for the shaft on the exhaust brake actuator extending to close the butterfly. FYI...Diesel engines have no "Throttle Valve" that closes when the throttle is released. An exhaust brake adds that feature, turning the diesel engine into a low-pressure air compressor, (around 60 PSI). This nets about 300 braking horsepower, give or take at about 3,000 RPM.
No fuel is being injected during this time and
Using your exhaust brake religiously will add about 300% to your service brake life, including the toad brakes. There are two different pressure -control methods in the world of butterfly-valve exhaust brakes. Pac & Banks use a fixed orfice, a hole drilled into the butterfly valve that limits pressure, BD uses a spring in the actuator on Vacuum actuated brakes and a pressure regulator on air actuated brakes. Electric actuated brakes are not worth having. Too unreliable. Please note that the Cummins engine exhaust brake is available in two flavors 40 PSI & 60 PSI. To use the latter, you must replace the exhaust valve springs with the heavier Cummins springs. In the 20 years I have been installing exhaust brakes, I have only done 2 40 PSI brakes. Also, if you are running a tranny other than the Allison WT, check to see if your model has a lock-up style torque converter. The 542/545 series do not. To take advantage of an exhaust brake on these, the engine RPM must be kept above 1800 RPM, the speed required to maintain fluid coupling in the torque converter. It is extremely important to monitor tranny oil temps in these type of trannys because you are shearing the oil during acceleration & deceleration. Electronic transmissions lock the converter during these times, if programmed to operate in conjunction with an exhaust brake.
Hope I wasn't too long-winded....
Billy T

"accelerate to highway speed, and then release the accelerator."
Be sure to GRAB AHOLD of something before this occurs!

"Hope I wasn't too long-winded...."
IMHO: Not at all - a great, informative note.

The RPMs noted in the above comments range around the 2800 to 3100 area. Our 8.3 seems to be limited to around 2400 rpm and hp is stated to be 250. Were the later versions of the 8.3 allowed to rev to a higher rpm to get to 300 or more hp or were there other modifications made to increase hp?

Regards Bud

What does it take to get more HP out of your engine? This one can be tough to answer without research.

Sometimes it's as simple as turning a screw or bolt on the back of the injector pump.

Sometimes it's as complicated as putting in buttons, replacing injectors, turbo and installing larger fuel lines and bigger filters.

Depending upon your air intake you might need to replace the airfilter housing, air induction hoses, air induction tubing, charge air cooler and even the intake manifold.

Is the head on the engine capable of the 300 HP or the higher speeds? Chances are yes, but unless you research it with Cummins you won't know for sure.

Is the driveline (drive shaft, transmission, axle shafts, rear end) built for the added HP and Torque? Calling Spartan and telling them what you want to do will answer most of these questions.

Sometimes getting more power from your engine will involve fixing a mechanical linkage or replacing a weak fuel injector shutdown solenoid.

When was the last time the engine had its rack run? Something as simple as adjusting valves back to its recommended gap will increase HP and torque.

Is your line of thought more along the line of going faster? Or, are you having problems taking off from a stop? If your rear end is set to high or low in the gearing you might need to change ratios.

Look at the RPMs when you floor the engine in neutral. It's governed so it won't go above the "max no load" anyway. What RPM's do you have at "max no load" and what RPM's do you have at the top speed your coach will go... that is, if what you are really looking for is being able to go down the road faster.

If your engine is rated at 3,000 RPM and your tach only goes to 2,400 then it might be a bad shutdown solenoid, a broken or bound up throttle linkage pivot or even crud, carpeting or a mat under your throttle pedal stop... So many variables...

Hi Bill:

Thanks for the comments and quick response.
My question was based upon my engines limitation to 2400 rpm ( the plate on the door indicates it is rated at 250 hp @ 2400), whereas others seem to have been discussing 400 to 700 rpm higher--- hence more hp (assuming a constant torque curve).
The engine runs to 2400 no problem and going down the road faster is not required--- on the level it goes faster than I have any desire to travel!
Up hills we all know the value of hp but again I can live with what exists but am interested how the engines of that era were set up to put out the variations in hp that I have seen reported.

Regards Bud

The 1993 8.3L in your coach was originally introduced in 1985 as the 6C8.3 this engine was designed in partnership with the Case Corporation.

There are 2 common groups of Highway Specs/Parts for this engine.

1985-1990 & 1991-2000

Have you logged onto the Cummins Site and registered yet?

Go to this page and read my posting. It'll tell you about Cummins Quick Serve and it'll give you the information you seek.