Specific Fuel Flow - LS Series

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TXFlyGuy

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Here are the real world numbers from the dyno with the LS3 Hot Cam V8.

4000 rpm - 29 inches manifold pressure (WOT)
336 horsepower
440 pounds torque
28 gallons per hour fuel flow

That's .5 lbs / hour / horsepower.

Typically we see on a normally aspirated engine a BSFC value of 0.44- 0.45. This would calculate out to be around 0.85 – 0.87 lambda. On a turbo engine we may increase this upwards to 0.47- 0.52 BSFC, and that would be approx 0.82 – 0.79 lambda.
 
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rv7charlie

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I'm pretty sure that you'll never find any normally aspirated gas burner that will come anywhere near .45 BSFC at wide open throttle at sea level pressures. 0.5 BSFC under the conditions you describe actually sounds pretty good to me. Don't forget that optimum BSFC numbers on n/a engines are never at full power, or at full throttle, if at sea level.

Throw a Lyc on the same dyno, run it at the same wide open throttle, same MAP, and at the rpm specified in the Lyc performance charts for roughly 70% power at sea level WOT (the dyno'd HP on the LS motor is likely about 70% of its full power). I'll bet you'll see BSFC in the same neighborhood, or slightly worse.

Charlie
 

BJC

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Assuming that my parallel valve Lycoming IO-360 produces 180 HP at 2700 RPM, WOT at 1000 MSL would be 15 GPH if it had the same BSFC measured in the LS3, above. My fuel flow indication supports Charlie’s contention; it is slightly more than 15 GPH.


BJC
 

rv7charlie

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BJC, do you have a controllable prop? If so, it would be instructive to look up the 70% HP rpm-point at WOT & sea level, and do a run at WOT with the prop dialed back to that rpm. that would 'duplicate' the test TX saw on the dyno. I'll bet that the fuel flow will calculate to just slightly better than what you're seeing at full power, but nowhere near 0.45.

Charlie
 

BJC

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I have a controllable Hartzell blended airfoil, but I’m not flying right now. I don’t expect to see 0.45 in any normal mode of operation.


BJC
 

TXFlyGuy

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Here are some slightly better numbers.
24 inches mp
4000 rpm
18 gph fuel flow
That is .4 lbs / hr / hp.

This figures to be 275 hp.
 

TXFlyGuy

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Here are the latest numbers...

4000 rpm:
24 inches = 345tq 260hp 19.5 gph
22 inches = 324tq 246hp 18.7 gph
21 inches = 303tq 235hp 16.2 gph

3500 rpm:
24 inches = 335tq 220hp 16.5 gph

From 4000 to 3500 the torque values are about the same but the hp drops about 40.

This comes in right at .45 lbs / hp / hr. Right on schedule for a high performance V8.
 
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TXFlyGuy

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Our engine tuner (Hutter Performance) said the fuel/air mixture is still on the rich side, and they will do further tuning adjustments after we get the engine running on the airplane using O2 sensors. He feels the mixture can be leaned out for better economy.
 

rv7charlie

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He's probably right; I'd bet on another tenth or two of improvement.

It's worth mentioning that the claims you saw over on VAF are likely nothing more than claims. I'm fairly confident that those engines have never seen a dyno; the claims are like the Cessna 172 owners saying that their Lyc powered 172 burns 7 GPH at 75% power.

Charlie
 

lr27

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I'm wondering if there's some clever way to use load cells, strain gauges, etc. to measure torque in flight. Then a lot of this speculation wouod be unnecessary. I suppose a strain gauge with a blue tooth device attached to the shaft would be a problem, though.
 

lr27

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At sea level, one could approximate 75 percent power by backing off the throttle until you got 91 percent full power rpm. But that's assuming the prop allows full power in the first place.

I suspect that power is proportional to the cube of the rpm times the air density., which allows a relatively easy calculation. However, this would only work for one airplane in one configuration. Other airplanes in other configurations, with other props, would have different amounts of drag and wouldn't be directly comparable. Also, any significant compressibility effects at the prop tips would throw it off. Another caveat is that significant induced drag will mess with the results.

One could probably figure out a way to use the indicated air speed, rpm, and air density to figure out power, but it would still only work for that one prop on a plane with a similar cowling.

Or, I could just be full of baloney. If the latter, don't tell my cardiologist.
 

rv7charlie

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If you have a properly calibrated fuel flow instrument in the a/c, the simplest thing is to just properly lean the engine, and find that flow in Lycoming's fuel flow vs HP charts. That 0.45 figure is quite...persistent, for power levels under around 70%. Balanced fuel flow with fuel injection will buy a point or two, and advanced ignition (at reduced MAPs) a point or two more. Numbers under 0.40 for any gasoline piston engine likely to be found in an a/c begin to sound suspiciously like marketing.
 

AdrianS

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Which means that the rest's '75% power' is actually somewhere between 76 & 84 HP (from a 150-160 HP engine). :)
Which is approximately 75% throttle, at 10,000 ft altitude.

p Real (75% throttle) = 75 hp
p at 100% = 100 hp
p at Sealevel = 150 hp
p loss to Altitude = 50 hp
p loss to Altitude = pSea x 3% x Altitude

Altitude = 50/150/0.03 = 11111 ft
 

TXFlyGuy

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If you have a properly calibrated fuel flow instrument in the a/c, the simplest thing is to just properly lean the engine, and find that flow in Lycoming's fuel flow vs HP charts. That 0.45 figure is quite...persistent, for power levels under around 70%. Balanced fuel flow with fuel injection will buy a point or two, and advanced ignition (at reduced MAPs) a point or two more. Numbers under 0.40 for any gasoline piston engine likely to be found in an a/c begin to sound suspiciously like marketing.
My altitude pressure chart shows an average loss of 1 inch of manifold pressure for every 1000 feet in altitude.

As our latest runs on the dyno came in at .45 lbs/hr/hp, with further tweaking we might get that down just a bit. Perhaps .43? We will soon find out as my propeller will be installed this week. So the engine runs on the airplane will commence shortly.

The numbers I heard from Titan for the LS3 are around 13 gph at 22" & 2800-3000 rpm. But that engine has the standard factory cam.

Robinson Aircraft claims around 18 gph @ 3800-4000 rpm.

I called Supermarine Spitfire, who installs LS3's in their replica warbirds. They claim a FF of 14 gph at 3600 rpm. They also claim a cruise of 193 knots / 223 mph at 3600 rpm. Not bad, assuming this is not an inflated value for marketing purposes.
 
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