- With a capability of 3000 to 3500 horsepower at an astonishing 200 psi of boost pressure,
the Cummins billet block diesel engine behind me is definitely a work of art.
However managing to keep an engine reliable at that sort of power and boost
level is obviously key.
We're here with Drew from D&J Precision Machining to talk a little bit about what
goes into the tuning of common rail diesel engines in order to make them reliable
and powerful.
So I think, Drew, for a start before we get into the specifics of diesel tuning,
I know that a lot of our followers will come from a gasoline tuning background and
with gasoline tuning we tend to add fuel and run the engine richer than stoich
and that helps coolant control our combustion chamber temperatures.
However diesel is a little bit different so can you tell us how the fuelling affects the
engine performance?
- Yeah we can run a couple of different ways, we can run on the fat side of the fuel curve
like that too, usually 12 to 14 air/fuel ratio is about the best power.
And that is heavy smoke and the fuel is actually cooling just like the gas engine does.
- That's because you're getting a lot of unburned fuel passing through the
combustion chamber?
- Correct yeah unburned fuel passing through the combustion chamber.
You get smoke like the tractor pullers are normally running.
That's 12 to 14 in air/fuel ratio.
And then drag racing, a lot of times we end up running on the lean side of the
fuel curve so typically with a diesel we can get to a certain point, keep adding fuel,
it starts to get super super hot and then you keep adding fuel and it cools back
down again.
So we run on the lean side of the fuel curve, like 19 to 20:1, drag racing a lot of times
we're actually kind of keeping the engine cool with more air so we'll run more and more
nitrous, more and more turbocharger and keep the fuel backed out of it and
then there's no smoke really at that point, it's a lot cleaner, easier to manage,
the ECU can pull fuel at that point and keep the engine from getting hot.
- Alright so there's a bunch of stuff I want to sort of unpack there.
So first of all, if we consider that there is some significant differences with the
diesel engine in that we don't have a throttle body on the engine and instead
the torque that the engine's producing is modulated really by the amount of fuel
we're delivering.
So even if we look at a completely stock diesel engine, it's always going to be running
significantly leaner that stoichiometric.
And what we see there, and this is how a lot of people start making more power,
is as we add fuel, we start to see the power increase but as you mentioned there,
as you add fuel up to a certain point you're going to see the temperatures climb,
the combustion temperatures climb.
So you're monitoring that kind of in an around about way with the exhaust gas
temperature is that correct?
- Yeah we watch exhaust gas temperatures and in this application we actually have the
ECU will pull fuel with exhaust temperatures getting too high.
So if we get to a point where our water injection isn't quite doing what we need to
do or we don't have quite enough nitrous in the engine or quite enough turbo in
the engine, it's climbing exhaust temperatures, the ECU will just start trimming fuel back
and save the engine throughout the rest of the pass.
- So really here the key to diesel performance is we want to combust more fuel and air
without necessarily richening the mixture because again that's where we're starting
to create heat and that can be the enemy of performance.
So that's where you're using boost pressure normally, adding boost pressure,
but you're also adding nitrous so can you talk to us about how the nitrous works?
- So the nitrous comes on in different stages in this drag race application.
We'll use spool jets for nitrous to help spool the turbo, we can get the turbo spooled
within a second, second and a half in this application.
It's a 98 mm turbo on a 410 cubic inch engine at 3500 RPM.
So we add more nitrous to keep the engine cool and keep on the lean side of that
fuel curve.
Now they do make more power on the fat side, so that 12 to 14:1 they will make more
power but in this application we can add enough extra things like nitrous,
boost pressure, whatever we want to make whatever power we really want so we always
try to stay on the lean side of that fuel curve.
It's easier for us to manage.
- In terms of the nitrous setup, you've got stages there as you've mentioned and the
spool valves, when it's all in, what amount of nitrous in terms of a horsepower rating
are you actually adding to the engine?
- In this application we're running over 1000 horsepower worth of nitrous.
We run a smaller turbo, we could always step turbo size up but for us to get out of the
hole and get a good 60 foot time, the turbo's still spooling through that area,
if we run too big a charger it's lazy through that part of the track.
So we run a little smaller turbo and more nitrous to make up for it.
- OK and I just actually want to come back and clarify something I said at the start,
I mentioned that this engine is built, or the inlet manifold specifically to support
200 psi and that's just for our viewers, that's not something you're going to be
running on a single turbo so if you were to build this for a different application
with a compound turbo system, this is where you'd be looking at maybe running less
nitrous and using more boost?
- In those applications it's usually truck and tractor pulling and they're not allowed
to run nitrous.
So that's why we would run a compound turbo setup, two to three turbos and usually
in two stages, they can see up to 200 pounds of boost.
This application's drag racing so we're usually only seeing about 100 pounds of boost,
120 sometimes.
- Alright so in terms of monitoring the engine health, one of the key inputs that
we've got with a diesel engine is exhaust gas temperature and it's hard to sometimes
pin down a number that we can consider to be safe.
Can you give us some input on what you're seeing as safe numbers with your drag
racing engines?
- We like to try to keep things as cool as possible, 1650 °F EGT numbers,
the engine can last 100s of passes, we can get over 100 passes out of rings, pistons,
valves, everything.
We start pushing higher than that and things start to fatigue.
Usually it starts in the top ring lands and the top rings start to fatigue,
that's the fastest thing that happens.
So we like to try to do our best to stay 1650°F or lower.
- I think it's important to mention here that the damage from heat is cumulative as well
so what you may be able to get away with for a short pass down a drag strip,
if you're out on the street with your truck towing a heavy load behind it,
going up a long hill, those sort of temperatures could potentially be
dangerous, is that reasonable to say?
- Oh yeah, we like to try to tell customers that are pulling a load or something like
that to stay under 1200°F.
Just the piston starts to gain so much heat and your oil can't pull that heat out of
the piston so 1250°F, 1200°F is what we like to try to tell people for
towing trailers or something like that.
- Now in most instances when it comes to actually making tuning changes,
it's most common really to be reflashing the factory fitted ECU.
With the particular billet engine that you've got here on your stand, you're running this
as a complete standalone which is quite unique, can you tell us a little bit about the
ECU that you're running there?
- Yeah we're running a Bosch Motorsports ECU in this application.
And we're also coupled with the PowerBox system so the PowerBox system can run
everything for the whole truck.
So it's running the nitrous, it's running our water injection system, it's running our
transmission, the spool valves in the transmission, everything, it's all controlled
and managed through that ECU.
So we can make any changes we want, if we want to pull a little bit of fuel on
a shift, we can do that, everything's all integrated together.
- Now we've talked so far about the air/fuel ratio and the fuel delivery but really that's
a small part of the overall puzzle when it comes to diesel tuning.
You've also got fuel pressure to consider and also the injection timing.
Can you give us some insight into how those elements play in together and what your
considerations are when making adjustments?
- Yeah fuel pressure, for the most part when you look at max power, you're trying to
go to max fuel pressure.
So we run 30000 pounds to 34000 pounds, sometimes 36000 pounds.
Depending on the injector setup and what you have.
Timing, we have to make decisions whether we want to make full power on timing or not.
In this application, we can pull a few of timing out of it, it's a little bit safer,
and make up for the extra power with more nitrous, more air, more fuel.
- So you're talking here about basically retarding the timing from where you're
actually seeing peak power and torque in terms of reducing the overall cylinder pressure
and making the tune up a little safer?
- Right, we've got cylinder pressure monitoring equipment in this engine so we've been
able to see say like a 20% drop in cylinder pressure by taking a few degrees of timing
out and only lose maybe 30 horsepower.
So we'll make up for that 30 horsepower somewhere else and reduce that cylinder
pressure because it's so much easier on components.
- It doesn't really matter how much power the engine's making if it's not going to last to
the end of the drag strip so of course considerations like that are something that
are often overlooked.
Look Drew it's been great to get some insight into what goes into tuning an
engine that is capable of 3000 plus horsepower with diesel fuel.
I know you're heading to the dyno with this straight after the show to see what it's
actually capable of so we wish you all the best with that when you hit the dyno,
thanks for the chat.
- Thank you very much, have a great day.
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