Two 6.2s much different performance.

I have a 94 M998 with a GM 6.2. My buddy has an 87 M998 with the same 6.2. When you drive the two trucks back to back, on the same day, with the same payload, the newer 6.2 has significantly more pep, especially at low speeds. Throttle response better and it's faster. Both trucks have the upgraded S3 brain box and new glow plugs. The 94 has a rebuilt IP and a new fuel pump. What are your thoughts on the performance difference? Just age? Could the newer fuel pump and rebuilt IP give it an edge in engine performance. I know it's tough to say given the variables here but wondering general thoughts on the matter considering there are so many trucks with the same 6.2 engine.

If you look around you can find some info on how to tweek the IP for more fuel. Really simple procedure. The 87 6.2 may be a good test mule. You can always put it back to where you started.
I have a GEP 6.5 that I will mess with when I have time. You read all the time about how these engines were de-tuned for the military. We need a post started on how to reverse whatever was involved in this de-tuning.

lowell66dart said:

If you look around you can find some info on how to tweek the IP for more fuel. Really simple procedure. The 87 6.2 may be a good test mule. You can always put it back to where you started.
I have a GEP 6.5 that I will mess with when I have time. You read all the time about how these engines were de-tuned for the military. We need a post started on how to reverse whatever was involved in this de-tuning.

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When you start play with the fuel you better install boost and pyro gauges first.

We find most 'as set' 6.2 and 6.5 engines are pretty detuned. Adding a little fuel (not 'rollin' coal' by any means) and slightly advancing timing from military settings goes a LONG way! I am guessing the rebuild pump got tuned to civilian or slightly more fuel and timing.

For the retuning notes: I have a 6.5L GEP that needed the IP rebuilt. When I sent it off to the shop for rebuild, I requested them to tune it to full 6.5L spec instead of the detuned 6.2L spec (I had provided them the military P/N for the correct configuration and they told me no problem. Enjoy the new performance.) That really lit up the engine from going from an under-performing, clogged IP to one opened up to be optimal for the engine displacement.

For the case of these two engines seemingly having different performance: I'd actually look to your differentials. There are three different differential gear ratios out there on these HMMWVs and I honestly have no idea which model/year is supposed to have what gear ratio. What I do know is that this will drastically effect low-speed power and top speed. If I were a betting man, I'd say that the '94 is probably geared lower and thus is very responsive and quick at lower speeds, but the 87 can cruise at a faster top speed. If that's not the case, then perhaps it's just that the IP is in need of some service on the '87?

I have never messed with these engines yet but have had five different Cummins in Dodge trucks that I played with. Used to drag race and had up to a 50 foot enclosed trailer so all were towing mods. Pyro is nice but unless your towing heavy on long grades probably not needed for these vehicles.

lowell66dart said:

I have never messed with these engines yet but have had five different Cummins in Dodge trucks that I played with. Used to drag race and had up to a 50 foot enclosed trailer so all were towing mods. Pyro is nice but unless your towing heavy on long grades probably not needed for these vehicles.

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Your motor do as you want , but playing with the fuel and guessing is a good way to melt pistons.

When I replaced the injection pump on my 6.2 with a new 6.5 N/A pump it became an entirely different truck. Acceleration was greatly improved and it sounded much less stressed at 55 mph. I installed it at the regular timing mark but obviously the calibrations between the injection pumps are different. I have not adjusted the fuel and barely get a hint of black at full throttle when starting off but it clears up. I know that I'm working on a CUCV in these pictures but the new pump is for a HMMWV.

So let me get this right. You replaced the injection pump on your 6.2L HMMWV with that of a 6.5L (NA) injection pump? Is that correct?

Mostly correct, I replaced the injection pump from my 6.2L N/A CUCV with that of a 6.5L N/A HMMWV pump. The primary difference from what I researched is the 6.5L N/A pump has 0.310 pumping plungers versus 0.290 pumping plungers for the 6.2L N/A pump. The 6.5 N/A pump is tuned to produce 170 horsepower whereas the 6.2 N/A pump is tuned to produce 135 horsepower. The 6.5 N/A pump is working fine in my application.

sandcobra164 where did you find those specs? I want to see what, if any differences there are between the HMMWV 6.5 NA and any other 6.5 NA.

To start with, HMMWV Injection Pump NSN's.

2910-01-199-2355 6.2 Prior to 1990
2910-01-326-9221 6.2 after 1990
2910-01-467-9029 6.5 N/A Detuned (what I used)
2910-01-414-1272 6.5 Turbo

Now I have to go research again where I got the rest of the information. I did the injection pump swap in November so what I read is still fresh on my mind. Having said that, no research paper would be complete without providing references so I'll edit this post as I find the references.

My first edit to this post is the thread I started below. I used the stated horsepower ratings cited in the tech manuals for each particular set-up (CUCV and 6.5 N/A HMMWV. More to follow.

https://www.steelsoldiers.com/showthread.php?182614-M1028-Fuel-Issues&highlight=

Second edit. I found an updated list of this posting but my browsing history only goes back to December so I'll have to search for the page I pulled the updated information from.

https://www.steelsoldiers.com/showthread.php?110581-List-of-Stanadyne-DB2-pumps

Third Edit, Stanadyne pump listing from manufacturer's website.

http://www.stanadyne.com/dealerportal/ssi/english/Parts Bulletin/119.pdf

Fourth Edit, Post #9 where a member found the new injection pump part numbers. Mine is a Stanadyne DB2831-6277 now and it works great on a 6.2L N/A in a CUCV.

https://www.steelsoldiers.com/showthread.php?183953-Humvee-Stanadyne-DB2-Injector-Pump-bulletin

As far as what differences there are between the injection pumps, I'll wait with you for someone else to explain that. I can replace them but cannot rebuild them with my level of knowledge. My basic explanation in my situation is the injection pump with the 0.310 plungers that I used on my truck is tuned for a N/A 6.5 to make 170 horsepower and is likely pushing more fuel to the injectors. Since my 6.2L is down 0.3 Liters of displacement I'm probably not making 170 horsepower but probably close to 160 or so which feels more peppy than the 135 or so it was rated at with the old pump. The old injection pump was failing so it probably wasn't delivering fuel correctly so let's say the engine was making say 110 hp (speculation). With a new healthy pump, it felt like a new truck with the increased power.

Thanks!!

So, color me stupid BUT I don’t see how increasing fuel increases EGT’s. I see advancing the timing and running fuel lean increasing EGT’s.

I am weak in Diesel so maybe someone might explain?

Thanks for the research and knowledge!

TOBASH said:

So, color me stupid BUT I don’t see how increasing fuel increases EGT’s. I see advancing the timing and running fuel lean increasing EGT’s.

I am weak in Diesel so maybe someone might explain?

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Diesel engines don't control the air going into the engine, only the fuel. Idle is pretty lean and goes richer the more you apply throttle. The more fuel, the hotter the burn.


Mike

TOBASH said:

So, color me stupid BUT I don’t see how increasing fuel increases EGT’s. I see advancing the timing and running fuel lean increasing EGT’s.

I am weak in Diesel so maybe someone might explain?

Click to expand...

Yep you are thinking gas engines where lean burn raises the heat of combustion and EGTs and rich mixtures cool the burn and drop EGTs. In diesels the more fuel, the more heat (more EGTs).

Make sure you are not losing throttle movement through loose linkages/cable stretch from the foot pedal to the injection pump. Floor insulation can also limit movement. I have seen humvees that when the pedal was to the floor, the injection pump was less than 65% throttle. Also make sure the linkage from the injection pump to the trans is setup correct (cable just tight at idle). These 2 things are the first thing i look at for better performance.

Little more info...

https://www.flashoffroad.com/Diesel/injectionpump/mechanicalpump.htm

Been following this thread for a bit now...

Does anyone have GOOD, solid, data on what range of EGTs (and measured from what point in the exhaust system) are NORMAL for stock/OEM 6.2L DT, NA, TD engines and, likewise for the same of 6.5L series engines AS ISSUED TO MILITARY use.

Seems to me this should all be compiled somewhere in the annals of GM engine testing, dynamic as well as field, and/or from the Stanadyne website, possibly some obscure TM or another.

Cursory perusal of the babbling cesspit that is the global innerwebs leaves me breathless, bereft of any useful data.


Consider: so you want to “tune” your particular engine, and, to be on the the safe(r) side, install an EGT probe and gauge.

1 - are the numbers you get under “normal driving conditions” really “normal,” or just for your particular truck? Location, altitude, ambient temperature, exhaust system glass/lava glass wrapped or no?

2 - are you the first person who ever actually adjusted the settings?

3 - how high an EGT is too high (and again, at what measuring point)?

4 - What EGT will “melt your pistons”?

Details, details.


Inquiring minds want to know.

Thanks.

Norm