Specific Fuel Flow - LS Series

Discussion in 'Chevy' started by TXFlyGuy, Apr 1, 2017.

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  1. Aug 15, 2017 #41

    TXFlyGuy

    TXFlyGuy

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    At sea level, 2600 rpm would be 180 hp. I figure close to .43 lbs / hp / hour.

    Tuning and programming is currently in progress.
     
  2. Aug 15, 2017 #42

    pictsidhe

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    It wasn't at sea level, it was at 7,500'. 23/30 x 180 = 138. If you look up the standard atmosphere, that's on the money. I also added in a temp correction, as the weather wasn't the 70F of standard conditions: 561/581 x 138 = 133. Lastly, as that was looking well out of tune, I arbitrarily knocked 3hp off. If it had been at sea level and 30" mp, it would have been running reasonably well. But it wasn't and it wasn't.
     
  3. Aug 15, 2017 #43

    TXFlyGuy

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    Thanks for the clarification. My engine, the LS376/480, is quite different with the Hot Cam. We will also have the new HD gearbox (2.21-1), plus the new design 96" propeller. In addition, we have the high speed wing.
    So the factory numbers will not apply to my aircraft. It will be interesting to see how it all plays out.
     
  4. Aug 15, 2017 #44

    pictsidhe

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    Ummm?

    To get an accurate bsfc, you need to dyno it and get the actual power. My calculations are probably within 10%, maybe 5%, if it was a stock engine as you said it was... Assuming sea level power at 7,500' on a NA engine, in hot weather is just fantasy.
     
  5. Aug 15, 2017 #45

    pictsidhe

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    And real hp, not the inflated numbers they give hot rodders.
     
  6. Aug 16, 2017 #46

    TXFlyGuy

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    The sea level hp at 2600 rpm is 180. You are free to extrapolate the numbers to any altitude you like. It's not my plane, and I was not there.
    The numbers from GM are far less than real world, being on the conservative side by 5 to 10%.
     
  7. Aug 16, 2017 #47

    pictsidhe

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    Claiming a bsfc based on a sea level hp while flying at 7,500' is stretching things an awful lot. The air is thinner up there, with no turbo, that means less power.

    Did you know that the easiest way to boost the hp of a European built engine by 5-10%, is to simply ship it across the Atlantic? Unfortunately, that's not a joke.
    I suspect that GM figures are actually on the money. I can't see them just giving away 10% of an engine's power. Joe hot rodder is more likely to take his car to the dyno shop that gets 650bhp, than the one who only gets 610bhp from the same engine. It's vanity sizing for engines. The big manufacturers can't do that as they will test and call out their competitors. Accurate dynos are everywhere now, it's the 'correction' factors that get creative.

    I'm getting confused as to what plane you have?
     
  8. Aug 16, 2017 #48

    TXFlyGuy

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    I have the T-51, fast wing version, with the LS376/480 GM Hot Cammed Engine, new 94" propeller, and new HD gearbox (2.21-1). It has not flown yet. The aircraft in question is the Titan factory model with the stock LS3.

    The sea level hp number was published as a reference point. You are correct that at 7500 feet, the power would be about 20 to 25% less. I figure a 3% loss in power per thousand feet of altitude.
     
  9. Oct 29, 2018 #49

    TXFlyGuy

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    You can do the math...

    T-51
    Standard wing
    84” prop
    LS3
    2800 rpm
    26” mp
    165 knots IAS
    10.775 gph
    About 195hp, based on the dyno charts.
     
  10. Oct 29, 2018 #50

    rv6ejguy

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    I did and the math shows the numbers can't be correct because the BSFC would be .33- better than any aero diesel now flying.
     
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  11. Oct 29, 2018 #51

    TXFlyGuy

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    Well, all the numbers are what was given to me by another builder. The horsepower figure was a guess of sorts, taken from the charts we have from the dyno shop.

    I’m guessing the altitude was low, perhaps near or slightly higher than pattern altitude.

    Yes, I was skeptical over the .33 number also.

    Figuring 180hp, that would be .36/lbs/hr/hp. Still a respectable number.
     
    Last edited: Oct 29, 2018
  12. Oct 29, 2018 #52

    rv7charlie

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    I think you're getting bit by thinking you can get power at altitude numbers by looking at a typical published dyno chart. Unless you specifically request it, all numbers on a dyno chart are going to be with sea level manifold pressure (meaning wide open throttle *at sea level*), with the brake loading the engine to obtain the desired rpm for torque measurement.

    Remember that to even get somewhere in the neighborhood of a power estimate, you need a lot of different data points, and none of the posts in this thread include all of them in one measurement point. The latest doesn't include altitude. I'll bet that if you look at your dyno chart, it's 195 hp at 2800 rpm, WOT *at sea level*. I'm also suspicious of the MAP number; if that flight was up at 7k-8k feet, it's 3-4" higher than standard day atmospheric at those altitudes.

    165 kts on <11 gph is nothing for a clean 2 seat airframe; well built 2 seat RVs do it all the time on 75% of 160 hp (closer to 9 gph, with a carb/mag 160 hp Lyc). And BSFC would be around .45-.48 in that case.
     
  13. Oct 29, 2018 #53

    TXFlyGuy

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    The altitude was low, as the owner/pilot was doing videos of passes at the local airport.

    No way to know the actual hp, thus all of the guesswork from charts.

    Just for discussion, my friend gets 185 knots TAS at 8.6 gph fuel burn with his IO-540.
    14” MAP, 2100 rpm.
    FL190. Glasair.
    His fuel burn is so low, that it is not even on his owners manuals charts!
     
    Last edited: Oct 29, 2018
  14. Oct 29, 2018 #54

    rv7charlie

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    At that altitude, it's probably do-able. And his BSFC is still likely between .42-.48.

    If your numbers for the -51 are from 'doing passes', it's a waste of time looking for *anything* on a chart (or anywhere else, for that matter). :)

    As I mentioned, details are important.
     
  15. Oct 29, 2018 #55

    TXFlyGuy

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    We have good dyno data, at sea level for the entire operating range of the engine. Is it not a simple exercise to extrapolate the horsepower numbers at altitude? With a 3 percent/1000’ power loss?

    Running at WOT, whatever that manifold pressure would equate to?

    In the Glasair example, he figured 38% power at FL190.
     
  16. Oct 29, 2018 #56

    rv7charlie

    rv7charlie

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    Yes, more or less, IF you are running the engine at wide open throttle *and* turning it at the same rpm as the dyno run chart *and* retuning mixture for best power as altitude increases *and* you're not doing something like passes down the runway *and* probably 4 more things I'm forgetting to mention. I'm betting that you don't have dyno data for part throttle operation (multiple MAP points) across multiple rpms. You could spend a month in the dyno room collecting all that data.

    Having said all that, what you *can't* do is assume BSFC will remain the same across all those conditions. What you need from the dyno is a pull with your throttle reduced to your expected cruise altitude MAP (~22" ?), with the brake set to achieve your expected cruise rpm (???). Lean for best power, then best economy. Look at those HP & BSFC numbers, and you can assume that both *might* be *slightly* better when you're actually at altitude and WOT at that rpm. You do have a controllable prop, right? If not, do not pass go, and forfeit your $50. :)
     
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  17. Oct 29, 2018 #57

    TXFlyGuy

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    That makes sense. Yes, constant speed prop.
    I’m just trying to get a good estimate on fuel burn to anticipate range and endurance.

    The V8 will burn 8.5 gph loafing along at 155 knots. It burns less at that speed than the V6 Honda.
     
  18. Oct 30, 2018 #58

    pictsidhe

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    A decent dyno shop should have been able to measure BSFC directly, no need for all the aero fudging that those of us without dynos do. You'll know consumption when you know it. But, a WAG of 10gph shouldn't be over 20% out.
     
  19. Oct 31, 2018 #59

    rv7charlie

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    See my previous post. Dyno shops aren't going to test at cruise power unless you specifically ask them to. Their focus is on getting you massive HP numbers, with BSFC a minor footnote on the chart. Even if you ask, it'll still change in the air, because they can't duplicate the lower atmospheric pressure that the back side of the pistons and the exhaust will see at altitude.

    All this may be fun to talk about, but here's the thing: If you know the fuel burn (properly leaned) of another (properly built) example of the same plane, That's also running a water cooled engine, with a similar cooling system, at the same density altitude you intend to fly, then you know, within percentage points that disappear in the measurement noise, what your fuel burn will be at the same speed/altitude. You can then calculate (again within percentage points that disappear in the measurement noise), your fuel burn at some higher speed because power required goes up with the cube of the ratio of the airspeed difference, and so will fuel burn.
     
  20. Oct 31, 2018 #60

    TXFlyGuy

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    This is fun to talk about, for sure. Yes...I have run numbers out the ying-yang to figure fuel burn. But the problem is, my engine has never flown, in any T-51. It’s the LS376/480, with the factory GM Hot Cam option. Plus we have the high speed wing (no longer available).
    Not only has this engine never flown, it has never flown with the high speed wing.
    I have estimates for low speed cruise (160 knots) burning between 8 and 9 gph. And high speed cruise of 220 knots burning 14 to 18 gph.
    We will know soon enough. Maybe mid to late December, this year.
     

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