Diesel Performance - Economy Options
© Copyright 2004 Chuck Kopelson 12/30/2004
Updated December 11, 2005

Fuel Economy

The "GM Product Service Training Manual for the 6.2L Diesel Engine" (#16015.05-1D) has this to say about GM diesel fuel mileage:
"The diesel, like any engine, is affected by driving habits. Speed is more critical on a diesel than a gas engine. On the highway, in the 50-75 mph range, the fuel economy will go down about 3 mpg for each 10 mph increase in speed. A gasoline engine will lose about 1-1/2 mpg for each 10 mph increase in speed. This condition is perhaps the most significant factor in obtaining good fuel ecomomy. Fuel economy may vary as much as 5 mpg in a given vehicle with different drivers."

The relationship of gearing to fuel mileage has to be the single most misunderstood aspect of the GM diesel engine. A typical 6.5TD with 3.42 gearing will produce in excess of 20 mpg at 65 mph because the engine rpm at that speed is right at the engine's torque peak of about 1800 rpm. A truck with 4.10 gearing is running at about 2250 rpm at the same speed, which typically produces about 15 mpg. A 500 rpm difference might not sound like much, but in a diesel, that's about half of the useable rpm range. 6.5TD engine rpm (4L80E automatic transmission) and typical fuel economy at 65 mph:
3.42 gearing 1880 rpm (21 mpg)
3.73 gearing 2050 rpm (17 mpg)
4.10 gearing 2250 rpm (15 mpg)
If you do a lot of high speed interstate type driving, the GM diesel will deliver the performance you're after as long as you gear your truck for the speed you want to drive. The a 3.42 gear ratio will allow you to drive at any sane speed. The 3.73 and 4.10 gears are more at home towing at a lower speed.

Of course, with my truck (2.73 gears times 1.92 for the hub ratio = 5.24), getting the MPG I've been getting (11 to 13) seems to fit in if you extrapolate the figures shown above. 

If you have a 4L80E transmission equipped truck, you already have an overdrive gear.

The specs that I have on the 4L80E are as follows:
1st - 2.482:1
2nd - 1.482:1
3rd - 1.00:1
4th - 0.75:1
Reverse - 2.077:1

In a comparison (for those rare Alpha owners that are on the list), the
Allison 1000 specs that I have are:

1st - 3.10:1
2nd - 1.81:1
3rd - 1.41:1
4th - 1.00:1
5th - 0.71:1
6th - 0.61:1 (This is only applicable for the new MW7 Series Allison 1000 that was recently announced.)

Bigger tires affect the ratio.  I get a real world speed increase of about 10% at the same RPM (GPS indicated) versus my spedometer reading.

There is a really cool Java applet on 4x4wire.com that calculates gear/tire combinations for you.

http://www.4x4wire.com/tech/applications/gearandtire/

Enter in all the specs:

Tire Size (for most of us, it will be either 37 or 38)
Transmission (Enter the gearing of the 4L80E, 2.48 1st, 1.48 2nd, 1.00 3rd, .75 4th, 0 for 5th, 2.077 Rev)
Transfer Case Low (enter in 2.72, which is the low gear for the NP/NV 242AMG)
Ring/Pinion (Enter 5.24 for pre-98 trucks, 4.92 for 98 and up)

This will then calculate out and graph speed vs. rpm vs. tire size vs. gearing.  It's a very nice tool.

Performance

Several Hummer owners have asked for information on how to get more power from
the 6.5. The 6.5 NA can easily put out over 200 HP and the 6.5 TD over 275 HP without undue stress on the block or crankshaft. The following is a list of modifications made on my 6.5 NA


1. Port matched intake manifold
2. Heads port matched and ported for better air flow (After market
   heads/larger valves).
3. Exhaust ports and combustion chamber on heads ceramic coated
4. Exhaust manifolds heat resistant coating.  (Jet Hot coating)
5. Exhaust manifolds port matched and cleaned up on the inside. (the fit and
   welding is)
6. Ceramic coated piston tops
7. Intake and exhaust valves ceramic coated.
8. After market valve springs, and lifters.
9. ARP head studs and stud and girdle kit for bottom end
10. HV Oil Pump
11. Remanufactured 6.5 TD injector pump with Artic Kit
12. 6.5 TD injectors
13. Upgraded cooling to rear of heads.  No. 6 hose directly to radiator (2 #6
    ports in top of radiator)
14. Gear drive in place of timing chain
15. Engine balanced to less than .33 grams
16. AMG export exhaust (resonator pipe)
17. K and N air filter (a K&N is ok on a N/A diesel, Not OK on a Turbo Diesel)
    
    I did have to experiment with the timing. At first I was getting black smoke under heavy load. After getting the timing correct the EGT came down and the black smoke cleared up.
    
    With these modifications, there is more than a 25% increase in horse power and
    torque when run on a dyno. It is very time consuming and I thought it would be worth it to port match and flow test the intake and exhaust.
    
    With all of the heat resistive coatings the engine runs much cooler when under load. When on mountain grades the fan only comes on less than 10% of the time. The down side to this is that below 30 deg it is very slow to warm up.

Intercooler Article

I totally agree with Clarke on this. You can't take each item off a menu and simply add up the hp gains. There are many interacting factors. The only way to know for sure is to put the truck on a calibrated dyno and measure the hp and torque. While I'm sure you can gain some power (40hp) with these add on's 400hp is a real reach. Unless you don't care how long your engine will
last I wouldn't mess with most of this stuff.  The K&N filter on a turbo diesel alone will let more junk into your engine shortening it's life. See the K&N article on my website.

Added to the equation is the drivetrain. Look at all the work AMG did on the new Alpha to allow it to take 300hp and the increased torque of the Duramax diesel i.e. Allison transmission, heavy halfshafts, new beefed up drive shafts and cardon U joints. If you really had 400 hp I'd expect you to be reporting that your transmission and transfercase (among other things) were torn up.

I went through this same thing when I was younger. I had a 68 Camaro with a 327. I bolted on an Edelbrock high rise manifold, installed a big Holly 4bbl carb and put a new cam in. The advertising for each one of those products said I should gain all kinds of power. When I was done the engine ran like crap. That's when I realized that it would take lots of money I didn't have to modify among other things the compression ratio/ pistons, exhaust, suspension and driveline. I learned the hard way. I spent all the money I made that summer loading trucks on that stupid car. I ended up putting it
all back the way it was. I learned the hard way that there is no free lunch.

I think what you need to do it get one of the new Duramax Alphas or better yet, get a fast sportscar when you feel the need for speed. <g>

I would guess that your a victim of marketing - your probably adding up the claimed HP of each modification. At best all of their (or anyone's) HP modifications  would get you into the low 300's with good reliability - you can't add things up serially. Anything other than the output from an engine
dyno will be guesses or half-baked calculations at best.

320HP and up would require going into the motor to install cams, new injectors, timing chain replaced with a gear drive, low-compression 18:1 pistons, a marine mechanical injection pump to handle the fuel flow. Your red-line would need to be bumped to about 3600,

On the outside you would need a mechanical waste-gate controller running 18+ PSI, a larger turbo (which wouldn't perform as well on the street - more lag), and Inter-cooler setup. You better have a TIT or EGT installed, because these kinds of fuel rates will melt the aluminum pistons quiet
easily if not constantly monitored under load.

And at that, wouldn't be more than 20-40 hours of use before you would be tearing the motor down for a rebuild.

Now if you have an engine dyno print out showing 400 crank HP after repeated runs - and are still putting out the same HP months later, I'll take all of the above back and copy your modifications for my truck.

Clarke Ferber

I'm also running the Heath Diesel PCM, the mechanical wastegate controller and the exhaust system with test pipe. It like night and day, especially for towing my m101 trailer. I'm very satisfied with it. As with Dave mentioned, EGT's are kept under critical. The only thing I am worried about is that I may be overboosting. When I really stomp on it, the boost will peg the needle at 15lbs. This may just take an adjusment from that mechanical wastegate controller, not sure though. I also forgot to mention that the shift points for the transmission have been changed because of the new PCM software and are much harder/firmer now.

-Pres

There's alot more throttle response down at the bottom end and when the turbo spools up, this 2.5-ton beast is moving pretty good. BTW, my sensor is on the left turgo inlet, so its TIT. In flat freeway speeds (60-65mph) and around town, the T.I.T's range from 300-500 deg and freeway overpasses (slight grades) no more than 900 deg. Even the engine temps are nominal. Its still in the green but does vary depending how heavy your foot is. IMMHO, I think its running alot cooler because of the test pipe and maybe the thermal wrap i put on the turbo (housing, inlet, outlet and downpipe). As in overboosting, as I said it might just be an adjustment I need to do. The mechanical wastegate controller that supplements the PCM has an adjustment nut for spring tension on the wastegate and I may have it too loose maybe. For instance, as soon as I come off a green light at a stop, the boost will peg the 15 psi mark and thats not stomping on it. Its the same gauge you have, even mid-throttl!
e will peg the needle at the max boost on this particular gauge. The boost is really sensitive with this controller. Its the same controller that "Raptor Torq" has on Predators site if you need a picture of it. If you have anymore questions, let me know.

Aluminum Intercooler

Complete High Performance Aluminum Intercooler and Radiator kit. Includes Intercooler, Aluminum Radiator, Engine oil cooler, Transmission fluid cooler, polished cross over tubes and several choices in silicone tube colors. Adds over 40hp and 50 ft lbs of torque and reduces engine temperature by 15 degrees. (Fans sold separately).

call mark at this number ( 877) 733-2867 with your fax number for dyno figures

Changing the FSD calibration resistor

SS Diesel Supply.

Switching out the stock ristor to a # 9 in the PDM/FSD increase power by 6%
the predator pdm system,  includes the upgraded #9 ristor in the kit already

6% of 189hp is 11 1/3 hp increase so it is safew to say that this kit gives you a solid 5hp.

Performance injectors

These Injectors mated with The performance fuel lifter pump will give you a solid 40 HP increase
http://www.bullydog.com/sale.pl?item=213065

usdieselparts.com

When you combine this Lifter pump with your performance injectors you are giving them the fuel that is needed for them to perform  to their fullest at 40 hp

SMA Exhaust

http://www.smahummer.com/detail.jsp?cid=6&pid=346

http://www.smahummer.com/detail.jsp?cid=6&pid=347

These figures are when you buy SMA 35hp computer upgrade mated with their 3.5in exhaust system and their test pipe.

When u jump to predators 102hp computer upgrade mated with their 4in exhaust system and test pipe, you get a solid 25 HP increase.

Turbo Boost Controller
The controller removes the factory canister and replaces it with a mechanical one. This  removes the stock factory setting of 7 lb. of boost and replaces it with 12 lb. of boost.
http://www.predatormotorsports.com/index.asp?PageAction=VIEWPROD&ProdID=248

The marine version injection pump

for a N/A diesel is aparently for boat applications of the 6.5.  It is externally identical and requires no change of anything else.  It has more internal ports.  Call Edgar at Diesel Specialists 714-528-8442 if your local fuel injector rebuilder can't source the parts.  Edgar gets the parts from a local supplier in So Cal.  My truck has never run so well, I am amazed.

Injection Pump Timing

If you have the right timing equipment, IMHO; the TD does very well at 7Deg BTDC @ 2000 rpm. If you go to far past 8.5DEG BTDC, EGT's and NOX go up, and fast. The closer you get to TDC, there is more knock and the turbo lag is enhanced, but EGT's stay low. Some of the IP's I've seen, have no reference to TDC, or it is too corroded to tell, so we have to set the timing anyhow.

Anybody correct me here, I believe that AMG states 2MM past TDC which is approx 5Deg BTDC for the TD and NA. 

With the marks lined up, the timing is (supposed to be) set at 5 degrees BTDC. Each .8 MM (about 0.03 inches) difference between the marks is equal to about 1 degree of timing. (that's not much of a difference, is it?) The mechanical pencil I use is .5 MM, so I can see even sitting here that the adjustment can be relatively fine. Also realize on the mechanical pump, there is an advance system such as found on a gas engine's distrbutor using weights that swing out to change timing as engine speed changes.