Why are compounds good for towing?

[stibuilder]

So, it seems to be a general consensus the compounds are the best setup for a truck that tows and wants more power while towing.
Now my question why/how? When you make more power you need to add more fuel and air right? So you do this by adding more boost. More boost and more fuel creates more cylinder pressure and heat, however its been said that twins are lowering guys EGT’s as much as 300 degrees.
I’m just trying to figure out how this all pencils out. For the sake of the discussion we will just consider a S475 over stock LBZ valley turbo.
Is it simply creating a whole new efficiency map of the valley turbo with the additional flow created by the atmospheric turbo? So the air is just cooler at the same boost pressures?

Thanks up front for the comments.
:thumb:

 

[jlawles2]

From what I understand, compound turbo's help with intake air heating. You can flow more mass (thus creating more boost) while staying in the efficiency band of each turbo. Once a turbo reaches the top of its efficiency map, it starts it adds more heat than airflow. The intercooler then has to work much harder to try and cool the charged air. Higher intake temperatures result in higher exhaust temperatures.

 

[IOWA LLY]

Set your cruise at 65 mph and watch the EGTs. Then start to accelerate. What did the EGTs do? They went up right? But why? Because you added more fuel to the injection event. What happens when you let off the throttle? The opposite happens. Now of course the boost will go up as you accelerate, and down when you let off the throttle. But it’s still a decent example. The more fuel for any given amount of air (up the point of saturation) will heat up the exhaust gas temperature. So with the twins you are basically adding much more air to the mixture which has a cooling effect.


Any fuel only has so many BTUs of potential energy in it. And it takes a certain amount of energy to move your truck, or pull your trailer. So fuel amount required stays roughly the same. You have just increased the amount of air, which drastically changes the air fuel ratio which cools it down. It should also in theory make it more efficient, however I’ve yet to see a fuel economy increase with twins over a single.


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[Dozerboy]

X2
It all depends on the efficiency map of the turbo. On the stock turbo we add power by over fueling it making the turbo work outside its peak efficiency. Adding a second bigger turbo increase the size of the efficiency map. Thus making more efficient use of the fuel and less heat. Twins also make more TQ at a lower RPM helping it tow better, but also at the cost of more stress on the engine. Guys with twins tend to kill engines faster then guys that do big singles.


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[Chevy1925]

in a diesel, you cant go lean like a gasser (well you can if you use nitrous but thats a different story) so when you put a second charger in front of the stock charger, you increase CFM into the engine and potentially boost (i say potentially as it depends on tuning). this now allows the engine to burn up even more fuel that may have been unburnt prior to adding the second turbo (hence higher EGTs). Your EGT's are a good indication of partially burnt fuel as well as the black smoke you may or may not see out the exhaust. More air will cool them because you are much more fuel up within the main firing event. Cylinder pressure will not reduce but will either be the same or increase some depending on the tuning, its cylinder temps that will be cooler. you also put the turbos in a better map between the two so intake charges are cooler. On top of that, drive pressure is much lower so the exhaust gasses are flowing out more easily and you dont have that excess pressure on the exhaust stroke of the piston nor super heated gasses lingering around on the exhaust side.

now due to design, twins allow stock spool up with much more air through out the power band like a larger single would so as it had been said years ago "you get your cake and can eat it too".

 

[stibuilder]

Ok, so coming from "gas" land this is all very interesting and makes sense on many levels. So if i wanted to get the best best running truck after doing compounds ill need to get it on a dyno and have it tuned it sounds like.

 

[Chevy1925]

Ok, so coming from "gas" land this is all very interesting and makes sense on many levels. So if i wanted to get the best best running truck after doing compounds ill need to get it on a dyno and have it tuned it sounds like.

if the engine is stock along with stock fueling (IE: injectors, cp3, etc), a dyno is not needed unless you are trying to push the most out of the engine/turbos without blowing the engine up (this is where Marks dyno setup is VERY handy).

there are plenty of tuners out there who have well developed tunes for s475 over stock setups on stock fueling that will get you a nice tow tune.

 

[LWATSON]

I know this discussion is about compounds, but in my opinion compounding off the stock turbo isn't an ideal setup. I personally would just replace the stock turbo with a bigger better flowing drop in charger if you towed. The stock turbo is a choke point if you put an S400 over it and try to push it much at all.

 

[stibuilder]

if the engine is stock along with stock fueling (IE: injectors, cp3, etc), a dyno is not needed unless you are trying to push the most out of the engine/turbos without blowing the engine up (this is where Marks dyno setup is VERY handy).

there are plenty of tuners out there who have well developed tunes for s475 over stock setups on stock fueling that will get you a nice tow tune.

ATP would be the choice in my case they are 8hrs away, I have always had all my cars dyno tuned in the past. In this case it sounds like it might be a smart thing to do since there is not a lot of compounded Kodiaks running around.

 

[Chevy1925]

I know this discussion is about compounds, but in my opinion compounding off the stock turbo isn't an ideal setup. I personally would just replace the stock turbo with a bigger better flowing drop in charger if you towed. The stock turbo is a choke point if you put an S400 over it and try to push it much at all.

I disagree, they have been shown to push to almost 1k rwhp when compounded accordingly

ATP would be the choice in my case they are 8hrs away, I have always had all my cars dyno tuned in the past. In this case it sounds like it might be a smart thing to do since there is not a lot of compounded Kodiaks running around.

By all means, it won’t hurt. Rob will have a good handle on tuning that as well

 

[SickLL7Crenshaw]

I disagree, they have been shown to push to almost 1k rwhp when compounded accordingly

X2

 

[IOWA LLY]

I know this discussion is about compounds, but in my opinion compounding off the stock turbo isn't an ideal setup. I personally would just replace the stock turbo with a bigger better flowing drop in charger if you towed. The stock turbo is a choke point if you put an S400 over it and try to push it much at all.

Have you ever ran compounds? Or a larger single while towing heavy? Are you actually speaking from any personal first hand experience at all?




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[LWATSON]

Have you ever ran compounds? Or a larger single while towing heavy? Are you actually speaking from any personal first hand experience at all?




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I ran an S475 single for a few years and towed with it on occasion, Now I'm running an S475/87/.90 in the valley compounded with a 588 hybrid, been running this setup for a year but I don't tow with this truck anymore. Sorry for stepping on toes, I'm just hear to learn.

 

[TheBac]

Landon, a 475 and 588?!? Holy shit. On the crew cab in the pic?

 

[LWATSON]

Landon, a 475 and 588?!? Holy shit. On the crew cab in the pic?

Yes, that pic is about 10 years old, things have changed a bit since then.

 

[IOWA LLY]

Very cool! I have much more respect for your answer knowing that you have actually ran the setups your talking about.

However, I think towing with a set of compounds is awesome. I’m not a fan of towing with a larger single but to each their own I guess.

What kind of numbers is your setup making? Track times or dyno numbers.


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[LWATSON]

I'll probably lose some of the respect I gained after you hear the numbers lol. It's a stock rod LBZ with LB7 pistons so it's slightly detuned. It did 815/1250 on Marks dyno, I've ran 7.25 in the 1/8th but have made changes since then, seat of the pants tells me it has a high 6 in it now. Original plan was to build a solid 1000 horse setup and work the bugs out and start dialing in the setup with the stock engine. I figured I'd build the bottom end after everything else was ready but to be honest this truck is a blast to drive at the level it's at now.

 

[IOWA LLY]

Ah. Cool project. As long as you understand the risks more power to ya. Those are excellent numbers for a stock engine truck in my opinion.


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[LWATSON]

Oh yea, the risks are obvious, and it may blow tomorrow and I won't shed a tear lol. Even though the HP number seems high for a stock bottom end my cylinder pressure is still below the 3000 psi mark, so who knows? This truck rarely will see the 1/4 mile track so I'm hoping the fact I play on the 1/8 will keep her alive for a while.

 

[zakkb787]

You sir give those of us that like playing with fire some hope.