Why is power per psi/cube not the same from gas to diesel?

[thunder550]

I've been thinking about this for a little bit comparing my current D-max truck to the gasser I built a few years ago trying to figure out where the disconnect is.

My old truck was a 408ci (about 6.7L), turbocharged, about 9.4:1 compression, and I ran it about 16-18 psi, with meth injection. It dynoed 575/640 at 14psi, never dynoed it at higher boost but ran low 11's at 120mph at the higher boost level in the 1/4.

D-max is 6.6L, runs about 30 psi, 17.5:1 compression. I haven't dynoed it but it's run a low 13 at 102 mph in the 1/4.

Both trucks weigh about the same. Gasser was a 2001 4x4 ECSB, this one is a 99 RCSB 4x4. The higher weight of the D-max/Allison combo probably just about evens out the difference between the body styles. D-max truck may even be a little lighter.

So...we have two trucks, about the same weight, about the same displacement, but the gasser running half the boost and lower compression makes quite a bit more power and runs a lot faster. That's my question...why? Is it that the gassers run higher RPM? I was shifting around 5500 RPM, so it's not like it was revving to the moon. Cam profiles? Better breathing? Is there more potential energy in a shot of gas/meth/air than there is in a shot of diesel/air?

 

[JoshH]

Less boost doesn't necessarily mean less air. What size turbo was on the gasser?

 

[thunder550]

GT4202R: 102mm w/ 53 trim (74.7mm inducer)

 

[hondarider552]

I asked myself the same question when my buddies junkyard 6.0 LQ4 went 140mph with a S480 on a stock bottom end...

 

[JoshH]

GT4202R: 102mm w/ 53 trim (74.7mm inducer)

That's a big turbo. If you put that size turbo on a Duramax, it would make well over 600 HP if you gave it enough fuel.

 

[Kappa9012]

it's not a difference in fuel as diesel actually has a higher BTU/gal than gasoline. I sort of suspect the amount of fuel running through the engine is a big factor. you're comparing a blow 406cuin gasser running 2 bars of boost, to a basically stock duramax running mostly stock power. Assuming the weights are the same the trucks have to be very different power levels.

To get 575hp out of a duramax you need to be retuned at a minimum and i'm not sure 30psi doesn't seem like it would be enough air to get that kind of hp out of it. I guess it would depend on how free flowing the air system was.

 

[Balindir]

Gasoline for one has more energy density.... Gas 45.8 MJ/kg, Diesel 45.3 MJ/kg. Secondly I think the character of the combustion is very critical. Think the energy stored in a the fuel mixture of a gasoline engine is compressed and released all at once in a powerful "explosion", and since gasoline engine require a much for stoichiometric mixture 14.7 a/r, not as much air is needed to burn the given fuel. Diesel on the other hand is a "slower burning" fuel, and also is injected into the cylinder over an elapsed time... there for some of the energy of the "explosion" is being lost before the cylinder reaches maximum cylinder pressure... now granted this is why diesel have so much torque... the continuous injection keeps pushing the piston down over a period of time and not if one single explosion (like in a gas engine.) Lastly a factor may be the fact that diesel engines dont need to be quite as stoichiometric (hense lots of smoke with a rich mixture) but regardless of that the desired a/r for is a diesel is somewhere in the ballpark of 18, so 22% more air is required. I might be crazy too... take it for what its worth and don't bash me too hard if I'm wrong on something.

 

[jkholder09]

There are 20 branch chain hydroxarbons in diesel and 8 in gasoline. Diesel has more energy as far as I have read.

But to stay 14.7 to 1 with the turbo on the gasser you probably had large injectors.
So you were putting more fuel in.
Try calculating this again by the actual weight of fuel being used per minute.
I think you will realize the difference

from mobile

 

[thunder550]

The injectors I had in the gasser were low impedance, I can't remember the lbs/hr rating but I think it was around 85 (stock for the 5.3/6.0 is 28 I believe). They had their own driver box to go with them. So with the size of the turbo and the boost it was making, it sounds like the airflow through the engine was quite a bit more. I guess that makes sense since it was revving 2000 RPM higher.

Do the Duramax engines require that much boost with a big single or twins to get up to the same airflow? I know they are designed for low end grunt, and 640 ft-lbs in a D-max comes will give much lower HP numbers that 640 ft-lb in a gasser they don't rev as high. Anyone know how the Duramax heads flow comared to LS-style heads? My turbo setup on the gasser was a log manifold style, with a factory driver's side manifold and a crossover pipe, so I can't imagine it was any more efficient than the factory Duramax setup.

 

[JoshH]

The injectors I had in the gasser were low impedance, I can't remember the lbs/hr rating but I think it was around 85 (stock for the 5.3/6.0 is 28 I believe). They had their own driver box to go with them. So with the size of the turbo and the boost it was making, it sounds like the airflow through the engine was quite a bit more. I guess that makes sense since it was revving 2000 RPM higher.

Do the Duramax engines require that much boost with a big single or twins to get up to the same airflow? I know they are designed for low end grunt, and 640 ft-lbs in a D-max comes will give much lower HP numbers that 640 ft-lb in a gasser they don't rev as high. Anyone know how the Duramax heads flow comared to LS-style heads? My turbo setup on the gasser was a log manifold style, with a factory driver's side manifold and a crossover pipe, so I can't imagine it was any more efficient than the factory Duramax setup.

I don't have any numbers, but I wouldn't be surprised if a stock Duramax head flowed half as much as a stock LSx head.

 

[jkholder09]

I don't have any numbers, but I wouldn't be surprised if a stock Duramax head flowed half as much as a stock LSx head.

I can't remember numbers either but the diesel heads do flow a good deal less in most cases josh.

 

[Dave c]

Diesel has more BTU's per gallon, this equals heat energy potential.

Density: 7.076 lb/gal (diesel) 6.15 lb/gal (gasoline)


Heating Values: Diesel: 19300 Btu/lb (136,567 Btu/gal); Gasoline: 20300 Btu/lb (124,845 Btu/gal) - Approximately 45-47 GJ/T


American standard testing methods (ASTM)