Boosting HP in my 8.3 - Cummins response


New Member
Have been thinking of upping my HP from 250 to around 300. Here is Cummins response to my question.

Summary: Toughest Standards Comments


Thanks for your Email message.

Be advised that your RV chassis has been engineered for this horsepower and torque rating and this engine has been certified for its present rating. If you modify the engine by uprating to a higher NON-Cummins rating, you will assume all warranty responsibility for the engine and probably the chassis driveline components.

Since we build the diesels and sell them to the chassis builders, it is very difficult for us to give you any definite answers about uprating the engine. We build many different engine configurations for many different equipment manufacturers and they determine which engine they need for their particular application and chassis setup. The RV chassis builders design the chassis for a certain size engine output and sometimes changing the engine output results in other problems.

We suspect the key issue for uprating horsepower is the age of your RV and what brand chassis you have and what driveline and cooling system it has. Is your current transmission capable of more lb-ft torque and is your current cooling system capable of cooling a larger hp engine? If your transmission is not adequate for added torque, then Allison would not likely be responsible for any transmission failures.

Whether or not your current cooling system (radiator, fan, etc.) would be adequate for more hp and whether the transmission is capable of any considerable increase in torque, are key issues. Also, the torque of these engines is produced at 1600 rpm and maximum horsepower is produced at rated rpm (2600 to 2800), so there may not be that much difference at 2200 rpm (cruise speed)?

Since all Cummins engines are certified by the EPA, any differences between the various ratings would have to be changed. For starters, we know that the fuel pump and injectors would probably need to be changed. Also, if the engine is still under Cummins warranty, there is an uprate charge, to maintain the warranty coverage on the engine. If you choose an aftermarket uprate kit and it causes damage to the engine, then Cummins would not participate in cost of repairs.

According to our records on engine uprates, the major change is pistons due to the higher cylinder pressures and temperatures of the 300 hp versus the 250 hp engines. The Bosch fuel pump would also need to be changed but we show that the turbocharger and injectors are the same in both ratings. Your local Cummins distributor or dealer can supply you with cost information on making all the necessary changes to uprate to 300 hp.

There are also other variables involved that relate to fuel mileage and since the engine is only one part of the equation, it is very difficult to predict mpg on any specific vehicle. There are factors such as rpm, speed, wind, aerodynamics, load, rear axle ratio, tire size, etc., that really affect MPG, more than the engine.

However, we talk to lots of RV'ers like yourself and in most cases they tell us that there mileage runs in the 7 to 12 mpg range, of course, depending on weight, speed, rpm and the other mentioned variables. Usually, the diesel engine will use about half as much fuel as a gas engine and the diesel will last twice as long as the gas engine.

Since the chassis builder has engineered that engine and rating for that chassis, it is likely that they would not accept any warranty responsibility for driveline failures, if the engine is uprated in horsepower and torque, beyond the driveline capabilities.

The RV industry has a 'rule of thumb' on diesel powered RV's for maximum weight to horsepower ratio, which is 100 pounds per horsepower. For example, a 300 hp diesel engine should be capable of powering a total combination weight of 30,000 pounds, which would include the total weight of the RV, tow car, etc..

Another rule-of-thumb we have heard is that a suggested maximum weight-to-torque ratio is 30 pounds per lb-ft. For example, a 660 lb-ft engine should be capable of powering a total combination weight of 19,800 pounds or a 1000 lb-ft engine could power 30,000 pounds.

Cummins does not approve or disapprove, aftermarket engine uprates. We are aware that there are aftermarket upraters around but their practices are not approved by Cummins, the RV builder or the EPA. Changing an engine to a non-Cummins rating would cause warranty implications on the engine and driveline, where failures would not be reimbursed by Cummins nor the chassis builder.

We thank you for your interest in Cummins products. Please let us know if you need assistance in locating the nearest Cummins-authorized Dealer or Distributor Service Provider. For assistance in locating a Service Provider, feel free to use Cummins North America Dealer Locator, which can be found on Cummins website:

Please let us know if you have other questions and if away from your computer or have a time-critical request that needs more urgent attention, feel free to call us toll-free (from North America) at 1-800-DIESELS (343-7357).

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Customer Assistance Center
Cummins, Inc.
Columbus, Indiana, USA


Boosting HP in my 8.3 - Cummins response

Bill, You have not given up yet have you :cool: . Just as a suggestion, since you have an older engine, have you given thought to possibly having your engine checked as far as the injectors, air fuel ratio, manifold pressure. Your injectors amy not be atomizing the fuel correctly and your turbo may not be moving enough air at the required pressure to make the rated 250 hp. Also one thing I found out is the diesel residues build up in the exhaust system and increase back pressure, which is a killer, which in turn hampers the turbo efficiency.

Just as an after thought, most damage is done to turbochargers, not while the engine is running but when they are shutdown. :dead: , due to lack of lubrication. There is an after market kit that is a small reservior which feeds oil to the turbo for a short while after the engine is shutdown.

Just food for thought from an ole man ;) .


New Member
Boosting HP in my 8.3 - Cummins response

Yup - you are correct. I will be having these things checked out as soon as the bitter cold weather breaks up here (it's 14 below zero right now).

I am going to look at the footfeed control system also and the injector controls as this system is electronic controlled to the injector pump.

Not that it is bad - just that I have driven a lot of diesels and for some reason this one is just a little sluggish on the uptake. Of course - that may be because of the control systems too as I did have some trouble getting the RPM down to under 850 so it would go into gear on a coupe occassions.

The report above - is from Cummins and naturally they are going to CYA. The only thing is - my unit is far-far from being under warranty so that is a moot point.

I am still leaning toward the banks system simply because it is such a simple one. Plus - I will bolt this thing on myself and that will force me to become acquanted with the old girl up close.

In the meantime - thanks for thoughts - Hope things are going well down there.

God Bless

Bill & Judy