How Far Can an L5P Duramax be Pushed?

Those words are straight from the mouth of self-described data and forced induction junkie Gale Banks, who’s also the president of Banks Power in Azusa, CA. Gale has been hard at work using his engine and engineering prowess to find the limits of a stock L5P Duramax and sharing his exploits on the Banks Power YouTube channel. 

“There’s a huge information vacuum out there,” Banks says. “What we’re doing is we’re talking about the things we’re creating and the discoveries we’re making. To be interesting, you have to have some teaching. I try to put new things in or explain things in new ways. It’s not just pure entertainment. The YouTube channel is our experiences and our learnings based on the things we’ve done. I just decided, if we’re doing this stuff anyway, why not take folks along on the adventure?”

An adventure is certainly what viewers have received. Along with the L5P Duramax series, Gale has simultaneously been building a diesel-powered monster truck engine using both a supercharger and twin-turbo set up to reach a 1,400-horsepower goal with a 427 cid 7.1L Duramax. You might be asking, why is he doing these engine experiments? Gale is looking to develop a V8 diesel marine engine and he wants to make it as perfect as possible.

“That’s why I’m really curious about how durable one of these things are,” Banks says. “I want to do a Duramax-based marine engine. I want to do one with good durability that’s close to 700 horsepower and I needed to know how to get there.

“The other part of it is, a version of it will be a super turbo. In dyno 1 is the Super Turbo Duramax and in dyno 2 is the L5P durability experiment, so there’s a commercial reason for doing all this. The marine industry is very small by comparison to automotive, but I can see guys buying a hot rod street engine that is 1,000 horsepower turnkey.”

Gale has been utilizing the engine dyno, his Banks Power iDash Data Monster gauges and his vast engine-related knowledge to put this L5P Duramax through its paces. However, he didn’t want to kill this engine like an idiot. He was looking to do it smartly and while increasing the horsepower along the way. 

“I don’t want a peripheral to kill the engine,” Banks says. “If it needs a bigger turbo, it’s getting a bigger turbo. If it needs more intercooling, it’s getting more intercooling. If it needs more oil pump, it’s getting more oil pump. If it needs more oil cooler, it’s getting more oil cooler. If it needs more water pump, it’s getting more water pump. If it needs more radiator, it’s getting a bigger radiator. I want to find the physical limits of this engine. I want to push it until I put the crank on the floor or the heads on the ceiling.”

A stock L5P Duramax delivers 445 horsepower. At the start of the Killing a Duramax durability testing, Gale figured he would be able to double the stock horsepower without going inside the engine.

“The first thing we do is put enough sensors into the program so that we have a window into all the elements of the functioning of the engine,” he says. “You don’t want the coolant to get too hot. You don’t want the oil to get too hot. You don’t want the oil pressure to diminish – all the subsystems of the engine – oil cooling, oil pump, water pump, radiator capacity – all of this can kill the engine when the engine is physically sound. I don’t want to kill a physically sound engine with a thermal problem. I want the engine to fail due to a physical problem. I would see that the engine got what it needed so that wasn’t the reason it was failing.”

As Gale and his team quickly upped the horsepower levels of the stock L5P, one of the first components to go was the stock turbo. To remedy that, Gale opted for a 76mm Precision turbo in its place.

“The whole goal of turbocharging, supercharging, intercooling, etc., is to increase the air density going into the intake manifold and from there into the cylinders of the engine,” he says. “Less air density means the engine turns the same rpm, but it’s making a hell of a lot less power. Manifold air density is such a bitchin’ tool. Guys talk about boost, but boost is only part of the equation and it isn’t even all the air pressure.”

In fact, Gale has also been measuring exhaust system backpressure to ensure this L5P is pumping the maximum horsepower it can.

“Exhaust system backpressure also impacts horsepower,” Banks says. “Every pound of backpressure you can get rid of goes all the way back to the piston rising on the exhaust stroke. If you can get rid of backpressure, you can liberate horsepower. The amount of backpressure matters, no matter where you reduce it at.”

As Gale continued to push the limits of the stock L5P, his team continued to ensure they weren’t doing it hap hazardly. 

“I won’t be proud of the fact that engine is killed,” he says. “There are a lot of guys out there who will be entertained by it, especially if there’s smoke and fire and a large explosion. But if you do it right, there isn’t. First of all, we don’t want to make smoke. Smoke is unburned fuel. You injected it and you didn’t use it. That’s stupid. If you see smoke, you put in too much fuel or you don’t have enough air. Smoke is the product of inadequate air, but it’s also the product of injecting too long. 

“There’s a certain crank angle under which you start injection and you end injection. I found with most modern diesels, it’s about 40 degrees of crank angle. When you inject so much fuel that you don’t have enough air, you can do that within my 40-degree window. You can over fuel and the end result is engine destruction because in a diesel when there’s inadequate air, things get very hot. The burn continues right into the exhaust system and through the turbocharger, so you over temp the turbine wheel and there’s lots of things that go on. 

“Air/fuel ratio is very critical. If you do it right, you can keep within the thermal limits of the engine and the engine becomes very efficient comparatively. Fuel efficiency is measured on the dyno pretty elementally, but there’s a thing called brake specific fuel consumption (BSFC) that measures the efficiency of the use of the fuel. I try to uphold a lean fuel, highly efficient engine as I increase the power.”

With the help of the new 76mm Precision turbo and larger, 50% over injectors, the L5P was now making 852 horsepower in Banks Power’s dyno cell No. 2. However, the team could tell they were still only scratching the upper limits of this Duramax.

“After making 852 hp, the engine got an improved air intake system by laying down the two filters that used to be standing upright,” Banks says. “We got rid of the 90-degree bend and moved the turbo up and out of the valley. Now, the air is making a straight shot right into the compressor. To get the turbo up, we got rid of the aftermarket up pipes and fabricated our own 2 1/4˝ pipes with very gentle bends up into a 3˝ forged collector. We also got rid of the stock turbo mounting foot as it became a major choke point.

Manifold air density is such a bitchin’ tool. Guys talk about boost, but boost is only part of the equation and it isn’t even all the air pressure.

“We also replaced the worn-out rubber damper with a viscous Fluidampr so we don’t break the crank. We did all of this to find power where most people don’t look. We reduced the parasitic losses.”

Gale had also changed the 50% over injectors to 100% over injectors and combined with the aforementioned modifications, that allowed the L5P to pick up another 60 horsepower.

“We made 912 horsepower with a stock L5P Duramax out of someone’s pickup truck,” he says. “This thing is stock from the bottom of the oil pan to the top of the valve covers. We haven’t unscrewed a bolt on the outside of this engine except to upgrade the oil cooler, the vibration damper, the turbocharger, the injectors, and to straighten out our big ass filter arrangement.

“I was thinking if I could double the horsepower, I’d be happy with it. We’ve now done that. Aaron, who is one of my dyno guys, has been telling me 1,000 horsepower the last couple of go arounds. I’m starting to believe Aaron’s right.”

Now that the Duramax is to this point, Gale has reached the limits of certain components. More specifically, the 76mm turbocharger, 100% over injectors and the camshaft were no longer helping. Gale also had to get creative with his dyno set up in order to accommodate the new horsepower levels he was reaching.

“I’m out of turbocharger at 912 horsepower,” Banks says. “We are totally out of camshaft. We’re at 3,400 rpm with a camshaft that is so mild it’s beyond belief. I can’t believe I’m even at 3,400 rpm with that camshaft. And we’re out of injectors. 

“We’ve had to hot rod the dyno too, which we haven’t talked about on YouTube. I’ve hot rodded it by extending its rpm range. Stock it was 4,000 rpm. I changed a bunch of stuff inside the dyno to get it to 7,200 rpm or so. Now, I’m running out of controlled water valving, so we did a secondary parallel valve system so it can take more torque. I had to hot rod the dyno because we’re at 100% at 912 horsepower. The engine is making almost 1,400 lb.-ft. of torque at 3,400 rpm. God only knows what the torque peak is.”

Through all these enhancements, Gale stresses that he hasn’t been inside the engine to make any upgrades, and it goes to show that if you build horsepower correctly, many components don’t need to be unnecessarily upgraded.

“We have not been inside the engine,” he says. “We haven’t even checked the valve lash. I’m telling you the thing is bone stock. This is the thing – the knee jerk building of engines by removing stock parts that are still perfectly good… is wasted time and money, and I’m a guy who sells speed equipment. If you’re honest with people, they appreciate that and they come back.”

Now that the L5P sits at 912 horsepower, to get beyond this point, Gale says a camshaft change is necessary. The team will also be going to larger S&S injectors, an S&S Bosch CP3 12mm stroker pump, a new oil filter for less pressure drop, and a larger Precision turbo is being built. 

“I might hit 1,000 horsepower with this thing and only with a camshaft change,” Banks says. “I’m not changing the rockers, the pushrods, the springs, the retainers, the seals, none of that. I’ve developed a cam working with COMP that almost coil binds the top springs, but not quite. I’ve limited the lift. It has more intake duration and more exhaust duration, but only on one side of the lobes. 

I’m trying to keep the engine on a single turbo… I’m out of turbocharger at 912 horsepower… If I’m lucky, we’re going to blow this pup past 1,000 horsepower.

“I’m not putting a racing cam in here. The overlapping is basically stock. I’m just opening the intake earlier and opening the exhaust earlier. I want to hopefully get up to 3,800 rpm and have the horsepower peak be there. That allows me to have lower cylinder pressure. In other words, rpm, if the engine will tolerate it, is a good thing because you can run the same cylinder pressure. I’m also going to change the firing order on the engine.”

What the new camshaft will help with is the L5P’s intake ports, which act as a governor for the engine. To progress beyond 912 horsepower, Gale had to find a solution.

“That governor is the intake ports,” he says. “The L5P has a lot better exhaust ports, but diesels depend on swirl – the rotation of the air in the cylinder – for the combustion process to work properly. The swirl comes from a restrictive, high-velocity intake port, which I call the swirl port. There’s two intake valves and two exhaust valves on most modern diesels. One of the intake valves is the velocity one and the other one is the mass flow one, but it also helps keep the swirl going. 

“To get that to happen, you have to restrict the flow and gain velocity. That, plus the valve size and the amount you open the valve, are all restricting what I can do. I’m trying to keep the engine on a single turbo as well, where I’m not compounding turbos or any of that jive. I’m limiting out on compressor outlet temperature as well. I’m at 460-degrees F compressor outlet right now and I’m holding that as my maximum. How do I get more air in there? Open the valve earlier and hold it open longer. 

“To do that I want to go to higher rpm so I don’t depressurize. It’s called blow down. When you open the exhaust valve, you’re blowing down the pressure in the cylinder. Well that diminishes the thrust on the piston, so you don’t want to do it too early, but if you go up in rpm and you have a gas dynamics thing going on, you can open it earlier and still get enough thrust on that piston to improve the power. The new cam is going to be a magic change. 

“The new pump and injectors will give us the fuel, the cam will process more air/cylinder firing and the turbo will give us improved mass air flow and less backpressure. If I’m lucky, we’re going to blow this pup past 1,000 horsepower.”

For Gale, at the ripe age of 78, it’s still all about discovery.

“The ideas are endless,” Banks says. “Every project we get into there’s cool shit. I come up with these ideas by asking how did that happen? What’s responsible for the power increase? What’s inhibiting the rest of it? There’s so much discovery and so much that’s not in the textbooks. That’s why I’m doing the series. There’s no limit to the opportunities we have. The only limit is our lifetime. If I go out the back door still thinking like this, it’s going to be a good life.”  EB