Diesel guys...Isuzu vs Duramax engine?

Jfet

Adventurer
I have been looking around at a truck upgrade from our Isuzu NRR for capacity reasons and noticed some models of the GMC T-7500 come with a Duramax 7.8L engine and some come with a Isuzu engine of similar displacement. Are these essentially the same engine and about the same reliability or is one preferred over the other?

Also the 2008 model says it has DPF but no DEF needed. What exactly does that mean as far as issues?

Thanks for any help.
 

shadow61

Observer
the duramax in the early years is an isuzu engine, i am assuming they probably still are close to the same since GM owns them now
 

jhrodd

Adventurer
The 2007 EPA engines were the absolute worst. Very frequent cycles on the active regeneration, poor fuel economy, and lot's of intractable problems. The 2010 standards with SCR allows the engine to operate with much lower particulate formation by scrubbing the resultant NOX downstream in the exhaust. Thus less EGR and less load on the DPF. The active regen cycles in big trucks with SCR/DEF are 10 times less frequent. The active regeneration takes a lot of fuel and detunes the engine making it very unpleasant to drive. I was pulling a meager 60,000 lbs on the Coquihalla one time when the truck went into active regen and found myself being passed by loaded B trains like I was standing still.
 

mallthus

Pretty good at some stuff
The 7.8l Duramax (LG4) is the GM branding for the Isuzu 6H engine. There's nothing GM about this engine, other than the branding, and it has nothing to do with the GM 6.6L Duramax L-series V8s.

When GM pulled out of the medium duty segment, the Duramax branding for this engine ended.

Although Isuzu and GM jointly designed the L series engine, that line, and the Duramax trademark, are now solely a GM venture.
 

boostin

Adventurer
The t7500 trucks are neat trucks...run like the wind from the one with the cat engine... heui pump systems get expensive
 

Jfet

Adventurer
The 2007 EPA engines were the absolute worst. Very frequent cycles on the active regeneration, poor fuel economy, and lot's of intractable problems. The 2010 standards with SCR allows the engine to operate with much lower particulate formation by scrubbing the resultant NOX downstream in the exhaust. Thus less EGR and less load on the DPF. The active regen cycles in big trucks with SCR/DEF are 10 times less frequent. The active regeneration takes a lot of fuel and detunes the engine making it very unpleasant to drive. I was pulling a meager 60,000 lbs on the Coquihalla one time when the truck went into active regen and found myself being passed by loaded B trains like I was standing still.

Thanks for all the info guys. So stay pre 2007 but post 2002 so you get a Isuzu 6H engine with no DPF? (before 2002 some seem to use the CAT engine)

Tight band but I will start looking.
 

jhrodd

Adventurer
It was the 2007 EPA mandate but they might not show up until the 2008 model year. I'm not sure that the build date even guarantees what compliance it's under. You will probably take a hit in MPG with a medium duty truck. That's one reason that the guys on Escapees HDT forum like the class 8's they get better fuel economy than the MDT's. JRhetts' FM260 Fuso only gets 6.5 - 7.5 MPG , thats a big jump from the 11-13 that the smaller Fuso's / Isuzu's are getting. If you find a truck you like ask the dealer to download the ECM for you - should get lifetime fuel economy, average speed, percentage of idle, top speed. Also get copies of all dealer repairs. I looked at a 1 year old 2008 Volvo tractor that drove great but had a lifetime fuel economy of 4.5 MPG with only 17% idle. It also had 17 repair orders mostly for the emissions system and was towed twice. The owner had 6 of these trucks and filed for bankruptcy. Volvo was offering a brand new warranty and about 50k off the original price. I passed.
 

riptilyaflip

New member
I just purchased a 2004 Isuzu FRR, which has the 7.8L 6H-series motor.
That motor came in the; Isuzu F-series trucks, GMC WT-series, and a few Chevy Topkicks and 8500-series dump trucks.
Interestingly enough it's the same motor from a 5500 thru 7500 series trucks.
Most are 200 hp @ 2,200 rpm and 520 tq @ 1450 rpm, however, there is a "High Torque" version that is 275hp, 860tq.
From what I could find the, the Hi TQ version was usually in the Chevy 8500 and hard to find.
From an engine spec standpoint the only difference between the two, is a 17.5:1 vs 16:1 compression, every thing else is the same. Also different ECM.
From the research I did, it has a really strong bottom end and is way over built, which is demonstrated by the wide range of vehicle GVW's it was available on.
 

Jfet

Adventurer
I just purchased a 2004 Isuzu FRR, which has the 7.8L 6H-series motor.
That motor came in the; Isuzu F-series trucks, GMC WT-series, and a few Chevy Topkicks and 8500-series dump trucks.
Interestingly enough it's the same motor from a 5500 thru 7500 series trucks.
Most are 200 hp @ 2,200 rpm and 520 tq @ 1450 rpm, however, there is a "High Torque" version that is 275hp, 860tq.
From what I could find the, the Hi TQ version was usually in the Chevy 8500 and hard to find.
From an engine spec standpoint the only difference between the two, is a 17.5:1 vs 16:1 compression, every thing else is the same. Also different ECM.
From the research I did, it has a really strong bottom end and is way over built, which is demonstrated by the wide range of vehicle GVW's it was available on.

Riptily, very interesting! I have a few questions for you.

The FRR seems to be the lighter version of the series, correct? The body builder guide rate it at 19,500lb GVWR, the same as the NRR. A 6800 pound front axle vs the 10,000 pound front axle on the FTR,FSR,FVR. The T7500 are similar but with a 12,000 pound front axle.

So the FRR is a NRR with a stronger engine. How are you finding fuel economy? With our 12 foot high camper mounted on our NRR and only 13,000 pounds vehicle weight, I am getting about 9.5mpg keeping it at 60mph. I expect a decrease when fully loaded to 19,000 pounds. With the drop in fuel prices this doesn't bother me very much. I like that the 7.8L has more HP and torque (although I thought the HP was 230).

Since we need a higher weight rating, I have been skipping the FRR and trying to find a FTR,FVR,FSR or GMC T7500. If I didn't need the capacity for our particular build, I would definitely consider the FRR but probably stick with the NRR and weak engine.

http://www.isuzutruckservice.com/pdf_redirect.php?reference=BodyBuilder2004FRR&size=80
 

riptilyaflip

New member
I haven't had a chance to check mpg. accurately. During the test drive it seemed to get about 12mpg. I bought it I Calif. drove it for a few hours, and had it shipped to Colorado, where I put it in storage until winter gone. There's too much snow to get it up to my house without chains.
I bought it to convert into a 4x4 expedition vehicle, so the front axle capacity didn't matter. I plan to put Dana 80 axles front and rear.
I wanted the extended cab, 6 cyl turbo, 6 spd auto trans, and the ability to put a 16' camper body on the back.
The 7500 does have 230 hp, although it's the same identical engine with a different ECM, however the FRR is the same body width as the FG, 19.5 wheels and air over hydraulic brakes. Whereas the 7500 is a foot wider, has 22" wheels and air brakes, just a lot bigger truck all over.
With an aftermarket engine module, different exhaust, and a Fass lift pump system, I'm told I can get 40-45% more hp/tq and 30-40% better mpg, which should bring it up to 280 hp, 728 tq and 15-16 mpg, which would be awesome. ( Time will tell. ) However it all shakes out, I'm really looking foreword to the build. I've been researching the build and looking for a truck to start with for 2 years.
 

DzlToy

Explorer
^^ FTR and FRR cabs are exactly the same. The FRR setup is a few inches wider than an FG (79 vs 82 IIRC).

The FTR only has large fender flare extensions to accommodate the 41" tall factory tires on wider wheels/axle.

If you remove the large gray outter fender and the huge FTR bumper, you have an FRR width cab. The cabs are identical on all F-series trucks in the US w/r/t dimensions.

The 7.8L can be turned up to 300 HP and 860 TQ as noted and the dealer CAN do this for you (not to say they will). I have been told, "only if your build/spec will take that power level" and "it depends on who is doing the programming" It was expensive, but comparable to an aftermarket "chip" such as Steinbauer, which is in the $1500 range IIRC.
 

Britboaters

Observer
How is your 4x4 build going ?

I saw you're planning on a 4x4 conversion of an FVR. How is progress ? I'm planning something very similar myself.
Thanks,
Bob
 

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