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NA Gas Vs Turbo Diesel

Discussion in 'The Garage' started by Pookster, Feb 26, 2005.

  1. Pookster

    Pookster 1/2 ton status

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    Not a flame war here, Just explain the physics and mechanics of this-.

    A certain person claimed a BB chevy, NA, will outpull a "Turboed" diesel of the same timeline (pre 2000).

    So lets do a somewhat fair comparison. 6.2 OR 6.5 TD vs a BBC NA (454).

    Figures for a mid 90s
    180-hp, 6.5-liter V-8 (diesel) 400ft lbs (99 claims 215hp/430tq)
    255-hp, 5.7-liter V-8 (Vortec, 210 for the TBI)
    290-hp, 7.4-liter V-8 (Vortech, Dont know what it is for TBI version).

    Correct these figures if you can.

    Anyways, to the point of the question.

    Assuming a 6% Grade for 5 miles, I think the claim that any NA motor, be it gas or diesel, will make it with any grace. In my opinion it is a simple calculation- Less air= less Power. As altitude increases, the amount of oxygen decreases. Less power.

    Take two equally power rated engines. lets say they both have 300hp/300tq at sea level. But one is a NA and one is Turboed. The turboed one will make more power at altitude than a NA would. Exactly what altitude this becomes important, i can't tell you but I know it does make a difference.

    Does the 6.2 NA make as much power as a BBC of the same year? no. Does the 6.5TD of the same year? Comes very close at SEA LEVEL. On a major hill climb, the turbo should deliver a consistantly denser charge of air, which will help maintain the highest possible power output. The BBC, being NA, as the hill climb increases, will begin to starve for air. The diesel will starve too, but the forced induction will help it keep its breath longer.

    The other consideration is, in hill climbs, being able to properly select a power band is more important in a diesel than in a gasser. So being geared properly and tranny gear selection become important as well.

    If the theory I've stated is wrong, please explain.
     
  2. big4x4ride

    big4x4ride 1/2 ton status

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    Just look at the crap i started, and all I wanted was some input on what motor to run LOL
     
  3. rjfguitar

    rjfguitar 3/4 ton status GMOTM Winner

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    I think a 6.5TD would be fairly close with a Vortec 454. We have a Vortec 454 in our '98 C30 dually and it is fairly strong. I think an earlier TBI 454 will get prety much spanked in the hills by a 6.5TD.

    There is something about diesels that puts a lot of torque to the ground, I'm no engine wizard by any means but have driven and operated a lot of equipment wiht both gas and diesel engines. One thing I have noticed with turbo charged diesels is that when coming up to a hill the turbo will speed up trying to make a few extra ponies. I have heard of this refered to as a power buldge with agriculture equipment(mainly tractors) but would think it's the same effect with almost any TD diesel regardless of application. As far as I know an N/A engine has no way of producing any kind of measureable power buldge.

    My money would be on a 6.5TD against any 454 through the years.
     
  4. jarheadk5

    jarheadk5 1/2 ton status

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    I think it's safe to say that any forced induction motor will outperform a normally-aspirated motor of comparable size and construction (gas vs. gas, diesel vs. diesel)on a long grade. Especially when you start getting into the higher altitudes. What altitude is a good question; Rene (tRustyK5) said his NA 6.2 ran well at altitude when he drove to & from Moab for BlazerBash a couple years ago.
    I've read that the reason a diesel makes more TQ per cube than a gasser is the high compression ratio. More force pushing the piston down means more TQ at the crank. I forget where I read that...

    For a non-scientific POV: My dad has been a diesel guy since his high school days; first as a mech, then as a truck driver, heavy equipment operator, and trucking & equipment service owner. In conversations with him, he says turbodiesels in general seem to pull harder with more load on them (up to safe EGT of course) than gassers (NA or forced-ind.). Also said if you ignore EGT, a TD will keep pulling hard until meltdown (valves, pistons, etc).
    From my own experience: I had a Renault Clio when I was in Italy last year, with a 1.8L common-rail TD. At one of the ski areas we went to, the access road was 7km long, and went from around 400ft to 5500ft altitude. The car's power felt the same the whole climb, whereas the guy behind me in a gasser Fiat was struggling to keep up as we went further up the mountain. I also had a much easier time going down; just put it in 2nd gear and controlled the speed with the throttle, only using the brakes just before turns.
     
  5. ntsqd

    ntsqd 1/2 ton status

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    Any engine with forced induction is operating at the same altitude (give or take charge air temp differences), regardless of the vehicle's actual altitude. I drove my turbo'd Rabbit pick-up over Red Mtn Pass (over 11,000 feet). The only reason I had to slow down at all was b/c of the Texan driving a NA 6.2 pulling a stock trailer. The engine didn't know or care what altitude it was at. It only made 7 psi of boost, but it did so everywhere.
     
  6. jarheadk5

    jarheadk5 1/2 ton status

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    I'm gonna disagree with you there. Your indicated 7psi of boost is 7 psi above the atmospheric pressure at the turbo's inlet; it's not an absolute pressure. Atmospheric pressure at "standard" conditions at sea level is about 14.7psi; there's charts to find atmospheric psi at different altitudes but I don't have one handy right now. So at sea level, 7psi of boost from your turbo means the absolute pressure in the intake, after the turbo, is about 21psi. With that same 7psi of boost, but at 11Kft, the absolute pressure in the intake will be less.
     
  7. 1977k5

    1977k5 3/4 ton status Vendor

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    But would the gauge/boost controller change its reading of atmospheric pressure at altitude or not (i.e. is it set at 14.7 psi or does it calibrate itself)? Also, it would seem that the boost controller would boost until it reached the specified psi (even though the turbo would have to work harder to do so). It seems to me that forced induction motors do have an advantage at altitude, but I am just theorizing and don't really know...
     
  8. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    Quick Google search dug this up:

    With a naturally aspirated engine, horsepower drops off 3 percent per 1000 ft (300m) because of the 3 percent decrease in air density per 1000 ft (300 m). If fuel delivery is not reduced, smoke level and fuel dilution will increase with altitude.
    With a turbocharged engine, an increase in altitude also increases the pressure drop across the turbine. Inlet turbine pressure remains the same, but the outlet pressure decreases as the altitude increases. Turbine speed also increases as the pressure differential increases. The compressor wheel turns faster, providing approximately the same inlet manifold pressure as at sea level, even though the incoming air is less dense.
    However, there are limitations to the actual amount of altitude compensation a turbocharged engine has. This is primarily determined by the amount of turbocharger boost and the turbocharger-to-engine match.

    Found here:

    http://www.turbochargersnz.com/about.html

    Rene
     
  9. jarheadk5

    jarheadk5 1/2 ton status

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    Precisely. For example, a turbocharged aircraft piston engine still experiences a little power loss at altitude, but it's nowhere near as bad as a NA engine. But the turbo system on the aircraft engine is specifically designed to deliver as much boost as possible at altitude; so much so that at takeoff power near sea level, some engine/turbo combos dump over 50% of the generated boost back into the atmosphere to avoid over-stressing the engine.
     
  10. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    I noticed a section right under what I 'pasted', and it was talking about altitude compensators. Basically a turbo designed only to retain power in a wide variety of altitudes, not for making extra power. I didn't read much of it though...:blush:

    I love Google. ;)

    Rene
     

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