View Full Version : Exhaust temp and gears - what is correlation?

08-24-2002, 00:52
I've been reading a lot of posts saying that high exhaust temps will present themselves if the (Juice-equipped) Allison stays in 5th too long. The solution is said to be a lockout switch to force the Allison to downshift. Why is this? I would think that if the engine were to push higher RPM, that would raise the temp and vise-versa. Can someone set me straight?

BTW, this board is great. I've picked up quite a bit since I've been visiting.

Joe Duffiney
08-24-2002, 05:41
This forum is great!! Lotsa info. The way it was explained to me ....... more rpm's = more boost = more incoming cool air = cooler temps. Please correct me if I am wrong here guys & gals .

08-24-2002, 06:21
It is commonly held that increasing RPM will increase heat and burn more fuel. However, rpm by itself isn't the driving factor, it's the load placed on the engine that counts most. Lugging and loading a motor is the quickest way to build heat and burn fuel.

believe it or not, increasing RPM--when it reduces the load on a motor--will also improve gas mileage. I had an 87 4x4 Suburban with 31x10s, a 2.5" lift kit, a TBI 350, and it weighed 6000 lbs empty. The 350 wasn't enough motor for this rig. When I changed the gears from 3.42 to 4.10 I improved power and mileage picked up. If I would have checked it, EGTs would have been lower as well.



08-24-2002, 06:53

That's not the case.......EGT's have nothing to do with the OD lockout under normal conditions.....It's for the rigs towing that occasionally get the shift business syndrome.....Therefore GM has decided on the 03's to have a lockout switch.....Most vehicles JUICE equipped will not experience this due to the increase in torque which will assist in holding 5TH allowing for minimal down ,up shifts......If used properly (JUICE) will towing (level 1 2) you shouldn't get any higher EGT'S then you would stock......At least that is what has been reported here......I drive B*LLS to the wall empty level 4 in T/H and never seen excessive EGT'S......The ones that have are using higher level's towing or loaded down....My assumption would be once there up and rolling they take out of T/H Level 1- 2 , into regular drive into higher Juice level approach a grade and will see the EGT'S climb.....Simple solution don't do it.......Other's may be looking for an OD lockout to run a stacked or combination mods to run on a dyno so as not to drive through 5TH gear, into limp.......

MAC ;)

08-24-2002, 07:12
At any given specific load an increase in RPM will decrease EGT's on a diesel engine. This is not the case on a gasoline engine however.

A diesel engine has no throttle plate, and is very similar to a large air compressor. At lower RPM's there is not as much air pumped through the engine as there is at higher RPM's. For a given load, the amount of diesel fuel injected per power stroke is the same at the different RPM's of the different gears.

I know my figures will be wrong, but they are for demonstration purposes only. Let's say that you are running a heavy load at 70 MPH in fifth gear... say 1800 RPM's and 80% throttle to maintain the 70 MPH. The EGT's are at say 1250 degrees. You downshift to fourth gear. Suddenly we're still doing 70 MPH and 80% throttle to maintain our speed, but now we're running 2800 RPM's. The EGT's drop to 900 degrees. We're still burning the same amount of fuel, but because of the higher RPM's we're moving much more air. That results in lower EGT's. A diesel engine does not have a fixed fuel to air ratio so we can move much more air for the same amount of fuel at higher RPM's (lower EGT's).

This does not work on a gasoline engine becuase the fuel to air ratio is fixed. The gasoline engine would not move more air at higher RPM in proportion to the fuel injected. Your only option would be to slow down or reduce the load. This does not take into consideration the torque multiplication of an automatic tranny.

If you see your EGT's on a diesel engine going up too high it's time to downshift. You are much better downshifting than lugging. Lugging a diesel is bad news.

Hope this helps!

[ 08-24-2002: Message edited by: csimo ]</p>

08-24-2002, 07:28

While it's true the gasser keeps a fixed air to fuel ratio, lugging the motor does require more fuel to be burned and thus increases heat. With a gasser, it often takes more throttle at a lower rpm for a given load than it does for a higher RPM to sustain a certain speed. While I know an increase in load does generate more heat, I'm not sure how that translates to increased EGTs on a gasser--I guess it depends on what portion of the engine cycle the increased temps occur.

On a diesel, isn't the major reason EGTs are lower at higher rpms due to less load being placed on the motor? Wouldn't more airflow at a given fuel setting lean the mixture and actually increase temperatures? it strikes me that the coolest condition would be unburned excess fuel being expelled out the exhaust and heat with it.


08-24-2002, 08:04
OK guys, help a diesel rookie out here.

afp/Blaine said:
"believe it or not, increasing RPM--when it reduces the load on a motor--will also improve gas mileage"

csimo said:
"For a given load, the amount of diesel fuel injected per power stroke is the same at the different RPM's of the different gears"

Can you guys give me some more insight to these statements? It sounds to me like you feel I could get similar (or even better) mileage pulling in 4th vs 5th with my Duramax, while putting less stress on the engine? On our last trip, I got my worst mileage in the mountains, where I had to spend some time in 4th gear. On the legs where I could maintain 5th gear almost exclusively, I got 1.5 - 2.5 mpg better. Maybe I've missed your point, but again, I lost my diesel cherry to this truck, so I'd like to learn a "better way" if there is one.

[ 08-24-2002: Message edited by: bluenote ]</p>

08-24-2002, 08:23

I do not want to go too far down this path, and let me be clear here. I am speculating on diesel performance based on my experience with gassers. I am new to diesels, and I am not 100% sure the all the factors that affect a gasser affect a diesel the same way.

I do know "lugging" a gas motor kills the mileage. The load an engine sees is more of a factor than RPM--with a gasser. However, if the motor isn't currently experiencing a heavily loaded condition--like running on a level freeway even when towing--lower RPM will improve mileage.

I used to determine when I should downshift based on how much throttle I needed to maintain speed. This is easiest to do on a manual. I always figured in general, the more gas pedal I needed, the more fuel I was burning. My towing MPG with gassers seemed to bear this out. Also, my 4x4 Suburban with 31" tires loaded the TBI 350 quite a even when bit even when empty. Changing to 4.10 gears picked up an additional 1 MPG.

I THINK the same principal applies to a diesel, but I do not know to what degree nor am I 100% certain. I do know the engine needs to be loaded for the turbo to make power. The more the engine is loaded, the more fuel it burns. My take is the best mileage will occur with the lowest rpm obtainable without signifcantly increasing the load on the motor. The again, that is based on gasser expereince, not diesel.

It stikes me best mpg may occur with a lower rpm with a lighter load, and a higher rpm with a heavy load. The exact load and rpm would have to be found by trial and error.

At this point let me "bow out" and let the guys with diesel experience tell us how it really is.....


[ 08-24-2002: Message edited by: afp ]</p>

08-24-2002, 09:59
bluenote, actually both are technically wrong (even my own statement). Please ignore my previous statements regarding amount of fuel injected since they just confuse the issue.

If you are pulling a 30 ft. fifth wheel down a flat highway with zero headwind you need a certain amount of power to maintain your speed. The RPM of the engine does not matter in regard to the power required to maintain your speed. If you are in 5th gear at 1800 RPM, or 4th gear at 2800 RPM it does not matter. The amount of power to maintain your speed is exactly the same.

In reference to the above situation your EGT will be lower in 4th gear because you are pumping much more air(which is cooler) to generate the same amount of power.

If you have a pyrometer installed this is very easy to demonstrate for yourself.

08-24-2002, 18:42

When I encounter a long grade I shift into tow/haul mode empty or not. I see EGT drop from 450-500 down to 250-300 probably just from the increased rpms. I don't know if anyone else is seeing this happen?? Then to me it makes sense that higher rpms move more air and thus you get lower EGT's under a load or not.

Am I wrong??? I'm no expert here.

JMHO and my $.02 cents


08-25-2002, 17:49
Thanks for all the help. This clears it up a bit.

I'm starting to search for a reputable diesel shop around town to install my gauges (boost and pyro). I read the article that TDP has installing gauges, but I'm not that handy. I figure I'll go to a shop that specializes in diesels, compared to simply going to my GM dealer here in town (which I'm not too happy with, but that's another story).

08-26-2002, 01:00
I'll help you put them in. I put mine in myself. Got your email. I'll call you this week.

08-26-2002, 04:17
Gents: See if this helps...

Consider a Diesel pulling a hill at say 2500 rpm putting out 200 hp. The fuel consumption will be some value based on the engine efficiency at that rpm and the EGT will be some value.

Now consider upshifting the same engine to a higher gear. Say you need an engine speed of 1500 rpm to maintain the same speed in the higher gear. The engine has to put out the same 200 hp to maintain speed. How does it get this extra HP at a lower RPM? The RPM has gone down so the torque has to go up to generate the HP required to maintain speed.

The engine will require approximately the same fuel flow as before but with the lower RPMs, note that you are injecting more fuel per firing cycle. More fuel burns a higher percentage of the air charge per firing cycle. This generates higher cylinder temperature/pressure (more torque) and higher exhaust gas temperature.

So: For the same HP at a lower RPM, cylinder temps must go up and EGTs go up since more work is being acomplished per combustion cycle, even though the engine is doing the same amount of work overall.

Conversly, if you upshift, maintain the same speed, EGTs go down. (the engine is doing less work per firing cycle)

A side note is that the theory says that due to the higher cylinder temp, the fuel efficiency should be better at a lower RPM.

Thanks, Lloyd

Jeffrey Shaffer
08-28-2002, 23:08
It seems that a times you might need more rpm's lower gear to get your egt down. Where do you have a lock out installed or can you do it yourself I heard one guy had one installed does anyone know about the lockout it sure sounds like a good thing as you cant select 4 gear on an Allison. Thanks Jeff

08-28-2002, 23:58
Whooaaaa!..... To much info for me. I too am planning on installing the lockout. Things just get to warm with the added power on the long grades. I need a better way than mashing the peddle to shift to 4th to cool things down. This is the info you all need.http://forum.62-65-dieselpage.com/ubb/ultimatebb.php?