6.5 Turbo Diesel Performance

6.5 turbo diesel performance is one of the things that makes owning one so worth it. Not in stock form necessarily but with the ease of what amounts to bolt on mods. 

In stock form and in well maintained shape, the 6.5 can give reasonable performance for a daily driver or occasional hauling. But, try to go fast or hook up a decent load and start some real towing and you likely will experience disappointing power and overheating. 

The original GM 6.2 liter diesel was engineered more for fuel economy than torque with the 454 cubic inch gas engine being the real torque monster at the time. Continuing with that, the 6.5 was meant to appeal to a mass market that needed an engine that would offer reasonable torque for hauling and good fuel mileage in addition to having acceleration qualities close enough to what most were used to in a gas engine and the ability to manufacture them in a reasonable time at a reasonable cost. 

As a result, we have a good all-around motor that could really use some improvements in many areas if we want more power in an engine that will last.

Power! Where Do We Find It?

There are three main components to getting more power out of a diesel engine…

  1. Fuel
  2. Air
  3. Cooling

That's it. Add more fuel to a cylinder, and you have more power. But, you need to be able to pull the air to go along with it and with more fuel burned, we need to manage the heat that goes along with it as well. 

On the mechanically injected engine that we will be primarily dealing with, we need only get into the injection pump and turn up the fuel screw. (The electronic DS4 has the same concept, it’s just managed with a change in programming.) Easy enough, right? Not so fast hot shot, first, we need to be able to tell if the modifications that we do are not going to harm our engine. Remember, any time you add fuel, you need to make sure that you can manage the heat. In order to do that, we need to have a way for the amount of heat to be reported to us in the form of gauges.


Gauges are the first thing that you should do before adding any power. It is frighteningly easy to put your engine in a position to be damaged without knowing things like exhaust temperature, turbo pressure and transmission temp along with the stock gauges of oil pressure and coolant temp. My particular truck didn’t even have a tach, so I added one as well. 


Along with coolant temp, arguably the most important gauge that you can add is the exhaust temperature (EGT) gauge. EGTs can rise fast with regular driving and can positively fry your engine when towing. While these motors can survive an amazing amount of abuse, heat will lead to a very early demise. Along with the coolant temp gauge, an EGT gauge will tell you how hot you’re running your engine before you have a chance to burn it up, giving you a chance to back off of the pedal to keep the heat in check. This is the first gauge that you should consider adding to your truck.

Turbo Pressure

In stock configuration with factory management, there isn’t much to worry about with the turbo. But, along with more fuel, we will want more air and the way we do that is through the turbo. In adding an adjustable waste gate controller or a whole different turbo, we will need to know where our turbo PSI is at. Any more than 14 PSI from a stock GMx turbo and we will get a lot of back pressure that will lead to less power and much more heat. Exactly the opposite from what we want. A boost gauge will tell us where we are at and allow us to adjust from there.

Transmission Temperature

The last of the “power three” in gauges is the transmission temperature gauge. Your transmission will suffer greatly from heat when towing if you aren’t able to monitor it properly.

All of these gauges will tell you when to back off of the throttle if anything is getting out of hand and save your engine. Without them, you are just flying blind and relying on luck.

Now That We Can See, Let’s Find The Power!


The best place to start on the trail to power is the exhaust. Since you can’t put in what you can’t get out, 6.5 liters of air need to escape as easily as possible in order for the engine to produce all the power that it’s capable of. A freer flowing exhaust will do that for you. There's no use going through the time and money to put more fuel through and adding a bigger turbo if you can't even get the exhaust out. 

I can sum up the stock exhaust in one word; terrible. The tiny, mangled cross pipe alone should be reason enough to change to a better breathing system. And a better exhaust is one of the few upgrades that you can do without needing gauges. Just changing to a 2 ½” crossover with a 3” down pipe into a 4” lower will give you a noticeable power increase, better mileage and a better running engine.


Up the system from the exhaust is the fuel. We need to be able to feed the engine with more fuel if we expect it to give us more power. To do that, we need the injection pump to feed the injectors, the lift pump to push through the FFM (fuel filter manager) and feed the injection pump and the tank to flow free enough to feed it all.

There are two modifications that we can do to help the lift pump do its job before we consider installing a better pump like the Walbro. 

  1. The oil pressure switch controls all power to the lift pump. With so much stress on the tiny contacts, these switches go bad regularly and are a real bear to get to for a change. Now there is a new harness that still uses the oil pressure switch for the signal to a new heavy-duty relay with heavier gauge wires feeding the lift pump. This allows the lift pump to run at its full potential to begin with. 

  2. In the fuel tank is a sock that screens the fuel before entering the fuel lines to the lift pump. This sock can get plugged from solids and algae and needs to be replaced or can be removed and replaced  with an exterior filter set-up before the lift pump.

  3. Now that we have the lift pump flowing as much as it can to the FFM, we want that fuel to be able to get from the FFM to the injection pump as easily as possible. If you have ever seen the size of the line here, you have probably wondered what GM was thinking. We have a large hose feeding into the FFM but only a very tiny line going into the injection pump. Several years ago Tim Outland came up with a simple mod called “Feed That Beast!” (FTB) that increases the size of this line to help prevent fuel starvation under heavy load. Originally developed for the DS4 electronic IPs, it works with the DB2 pumps as well.

After these mods, we can look at putting in a heavy duty Walbro FRB-5 lift pump (The FRC-10 for electronic.)

Up the line from the FTB mod is the injection pump itself. The mechanical DB2 pumps are considered by some to be pretty limited in the fuel that they can get to the engine compared to the DS4, but they can give us plenty of power to go fast and pull hard so we won’t be getting into that argument. They are also fairly easy to modify to get that power. Just a turn of the fuel screw inside the housing is all it takes to get more fuel to the engine to match what the turbo is pushing in to make more power. If we need more fuel, there are modified pumps like the Bull Moose pump from Conestoga Diesel that will likely be all the pump you need and quite frankly should quiet any arguments about getting power with a DB2.

Lastly are the injectors and are the most basic. Stock. That’s all you need. Marine, HO or high pop injectors can be beneficial in some instances but stock, genuine Bosche/Standyne injectors will perform just fine and are cheaper. The important thing is to have fresh injectors, all with matched pop pressures. So that pretty much guarantees that you will need to change yours out. People have tendency to run them for many more miles than they should (me included). But I can’t stress enough how important it is to have good injectors. If they aren’t producing a well atomized stream, hot spots in the cylinder can crack pistons and other catastrophic damage. If they have 80,000 - 100,000 miles on them, change your injectors people. 


O.k. now we are to the part of getting more power that is the most effective and the most fun. The turbo.

GMx turbos are notoriously inefficient. They spool quickly so they show us power quickly and that’s what the masses wanted to see when GM was marketing these motors. The problem with that is with such a small turbine wheel size, it was only efficient to about 14psi. Anything more than that and it started to resist the flow of air from the exhaust (back pressure) and caused high EGTs. 

However, even with a turbo as inefficient as the GMx, adding a manually adjustable waste gate controller and keeping it at 14 psi will REALLY wake up your motor, especially with the previously mentioned mods. It’s cheap (About $100 in 2015), easy to install and gives you control over your boost numbers.

All turbos wear and will need to be replaced and some of us just want to go to a bigger turbo for more power, a cooler running, more efficient engine and better mileage. We can have all of this with the turbo that I think is one of the best and that I have personal experience with… the A-Team turbo.

Developed specifically for our 6.5 engines by Dennis Koss with much engineering input from others like Tim Outland, the A-Team turbo is a custom, hand-made turbo that taps the full potential of our engines. With a very reasonable price, unmatched quality and incredible customer support along with a large community helping each other get the most out of the turbo; you can’t go wrong with this mod, period.

Next in the trail to more power is partnered with the turbo. If we are trying to pull a massive amount of air with the turbo, we need to be able to have access to all the air we can get and the stock flat panel air filters just can’t cut it. A ’97 and up round filter is a fairly cheap switch that will flow the air you need but a custom air intake like the S&B intake will really provide all the clean air that your higher flowing turbo will need.

You can even do some match porting or a full port and polish job to really get the air flowing.

Has this info helped at all? Well there's a lot more where that came from, baby.

But it doesn't come easy, if you'd like to see more, bigger, even faster, consider supporting the next project on my Patreon!
Just click that big, giant "P" on the right.

If not, well, keep reading!
↓ ↓ ↓ 
↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ 




Notorious for running hot, the 6.5 can use a lot of help in the cooling department. That, coupled with the fact that cooling maintenance is so frequently neglected means almost certain death to our engines if this part of the power trifecta is ignored. 

Before any mods, just cleaning the radiator will do wonders. Pulling the radiator is simple and it’s only about a 15 minute job. Using a good cleaner and rinsing with a garden hose (Don’t use a pressure washer) to clean all the crap out is all it takes.

In the list of modifications that we can do to increase cooling is a high output water pump. 92’ – 96’ engines have a flow of about 75 gallons per minute (GPM) at 2500 RPM and have a terribly unbalanced flow of only about 25% on the passenger side. Changing to a ’97 and up water pump will increase flow to 94 GPM but do nothing for the unbalanced flow. GM part # 88894035 water pump provides all the flow and nearly 50/50% balance of flow. Many people will change to a dual thermostat crossover but I prefer to stay with the single. A balanced pump,  modified fan clutch and 9 blade, 21” composite fan will really bring your temps down to a healthy level, even with heavy loads. A 180 degree thermostat isn't really needed with a healthy radiator and these upgrades. 

We show a full install of the most important of these, the balanced flow pump, the heavy duty clutch, and the huge Duramax fan here. And you can step up to the maximum cooling for your 6.5 with a Champion all aluminum radiator. See how well it improves your cooling from the stock radiator with our comparison here.

So you can see that there are many things we can do to cool our power houses and can go even further with intercoolers and water/meth injection (WMI)! 

So Here’s The Big Picture

Following the trail all the way back, we now have better flow through our air filter, more flow through our turbo and ported heads to let it all in to the cylinders. We have fresh, matched injectors squirting more fuel from our cranked up injection pump, more flow from our FTB mod on the FFM and better harness/relay allowing our heavy duty lift pump to do its job in pushing more fuel. 

All that with the proper cooling mods and you have a powerful, reliable and cool running engine that will likely give you better mileage that will actually pay for all the work done over time. That is, if you can keep your foot off the go-fast pedal.

I bet you can’t! 

There Is One Secret…

With the power trifecta above, you will see an incredible power gain in your 6.5 turbo diesel. However, once all that has been done, we can look at one more area that is often literally a secret. 


One of the best tuning mods that you can do is a gear drive timing set. The stock chain drive timing set will last many thousands of miles but will go bad in time requiring a replacement, has several degrees of timing slop to begin with and that slop increases over time causing a timing variance. With timing gears most of that variance is eliminated which causes quicker starts, better throttle response and more power with better mileage. The variance also never gets worse and the gears last the life of the motor and beyond.

While there are literally secrets in the tuning programs available for the DS4 pumps, the more simple DB2 pumps are a little easier for the regular Joe like me to adjust. A turn of the injection pump to increase timing will give you more power off the line and a quicker spooling turbo but increase EGTs under load. So turning the advance screw in will retard the timing as RPMs increase to keep EGTs down and create less stress on the engine.


So, those are the basics. I guarantee you that the power trifecta and a little tuning will give you a whole new truck. Air, fuel and cooling with a tuning chaser are the building blocks of a fast truck and a heavy hauler that can actually pay for themselves over time. 

That is, like I said before, IF you can keep the foot off of the go fast pedal! : )>

Follow along as we do a detailed 6.5 rebuild with all of these techniques and more!



Return To Top of Page

Contact us  Site Map  Privacy Policy  Advertising Policy  Disclaimer

Copyright© 2016- All Rights Reserved