LS376/480 - Major power loss.

Discussion in 'Chevy' started by TXFlyGuy, Feb 9, 2018.

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  1. Feb 9, 2018 #1

    TXFlyGuy

    TXFlyGuy

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    See post #3 for accurate data.

    Thanks to Steven in Houston for working with SAM-Tech and getting the numbers for us.
     
    Last edited: Feb 9, 2018
  2. Feb 9, 2018 #2

    delta

    delta

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    I ran into a exhaust restrictor on a 70's Rolls Royce one time. It was located between the manifold and pipe, and the center hole (from memory) wasn't any bigger around than my thumb. The back pressure it must have had something to do with it's ability to scavenge.https://en.wikipedia.org/wiki/Scavenging_(automotive) I don't know how they got it right on the merlin, but it seems Titan could have started out with long stacks, and incrementally shortened them to find the right mix on the dino.
     
  3. Feb 9, 2018 #3

    TXFlyGuy

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    Another person was looking into this. Some sort of insert to help with back pressure. To my knowledge, Titan has never done any dyno testing on any of the engines they fly.

    We just got updated and corrected info in this morning. The original test was done with long tube headers. Then the Mustang style short stack headers were installed. There is very little drop off in power until above 4000 rpm.

    At our max continuous power setting of 4500 rpm, the torque peaks at 430 lb ft, with the horsepower at 370.
    The max cruise setting of 3800 rpm is 415 lb ft, and 300 hp.

    This is better than we originally thought. And it will give the T-51 a power to weight ratio of 5 to 5.9 lbs/hp.
    A4B770A8-E1B6-4E0D-B863-790033D7B144.jpg 3FFEAEED-C0D5-4BEA-93E1-EA10F72C01AC.jpg
     
  4. Feb 9, 2018 #4

    BJC

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    That will be fun.


    BJC
     
  5. Feb 9, 2018 #5

    TXFlyGuy

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    Yes, I think so. While we did lose some power with the short stack Mustang exhaust, it is better than we first thought.

    Curious as to the exhaust systems on the Stewart and the Thunder Mustangs? It would be possible they also will lose a bit of power. Supercharging could offset that.
     
  6. Feb 9, 2018 #6

    pictsidhe

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    20hp drop at 4500rpm? Not a huge deal. If you have the budget, you could play with headers as you may be able to find more power with a different header bore.

    Best option: Make up a straight set of the required length, maybe start a bit bigger than the titan pipes. Do dyno pulls while progressively sleeving them down internally by inserting bits of pipe. Once you know best diameter, make some proper headers.
    Cheaper: try sleeving the straight bits of the titan headers. Restrictive washers won't work half as well as sleeves.

    edit: Looking at the curves, the titan headers may be a wee bit small. Sleeving what you can get at 1/8" would confirm that with a bigger power loss. Power above 4500 won't matter in your plane, but gives tuners an inkling into what is happening.
     
  7. Feb 10, 2018 #7

    TXFlyGuy

    TXFlyGuy

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    Someone suggested lengthening the tubes a bit. I was reading also that you don't want the diameter of each tube to be too large, as it will actually slow the velocity down.
    Correct, the overall loss is minimal and still gives us plenty of power (torque).
     
  8. Feb 10, 2018 #8

    pictsidhe

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    There is an optimum diameter for each length. There is an optimum combination of length and diameter for each rpm. The trick is to find the optimum that works best with your engine and size constraints.
    Longer will likely be better, but you'll still need to find the right diameter. I bet that if did the dyno work to determine optimum diameter, Titan would trade the info for a new set of curvy headers using those dimensions.
     
  9. Feb 10, 2018 #9

    Toobuilder

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    The short stack loss was predictable as the sunrise. Exhaust scavenging on a NA engine is very important, and these pipes are killing it. The camshaft change probably made it even worse. The Merlin gets away with it for one very simple reason - it is supercharged, so the intake charge is going to get into the chamber unless you weld the stacks closed.
     
  10. Feb 10, 2018 #10

    TXFlyGuy

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    The only real loss is above 4000 rpm. We can live with that as most of the flying will be done below that figure. The dyno shop told us to expect up to a 50 hp loss.
    P1020702.jpg P-51 Precious Metal Exhaust 3.jpg Jurca Mustang 4.jpg Sugar on Kit Planes.jpg P1030218.jpg
     
    Last edited: Feb 10, 2018
  11. Feb 10, 2018 #11

    TXFlyGuy

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    While a number of stacks are pictured above, can any one explain the design and shape of the early Mustang exhaust, as well as the P-40 exhaust. Of course, they were both Allison powered.
    P51A-Alison-exhausts.jpg
     
    Last edited: Feb 10, 2018
  12. Feb 11, 2018 #12

    jac

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    The end shape of each exh stub and its position in relation to the airflow & boundary layer will have a huge effect on scavenging at speed, something you wont be able to re-create on the dyno, guess its a case of test fly and use some onboard data collection a bit like some race car series as a pax on initial flights.
     
  13. Feb 11, 2018 #13

    BJC

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    Kent Paser experimented with exhaust nozzles on a 320 CI Lycoming. With a 1 1/4” nozzle on a 1 3/4” exhaust, he gained about 5 MPH at 7,000 feet. He lost HP below 6,000 feet.


    BJC
     
  14. Feb 11, 2018 #14

    jac

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    While I remember those articles vaguely, were they single pipe per cyl or in pairs or 4 to 1 collector. I do remember doing some exh testing on a 2L Pinto powered Hydroplane with both 4/1 headers and 4 individual stacks (both with anti reversion steps @ port face). Engine ran nicer on the 4/1 setup but was actually fastest @ top end on individuals, albeit being a pig @ low RPM and slow off the mark.
     
  15. Feb 11, 2018 #15

    mcrae0104

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    Paser did the restricted nozzles first on a 4-pipe system, then did it on a crossover (2-into-1). He did not try a 4-into-1, or at least did not write about it.
     
  16. Feb 11, 2018 #16

    Will Aldridge

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    I believe that's called a fish tail exhaust. I can't tell you its technical merits just know that's what it's called.
     
  17. Feb 11, 2018 #17

    pictsidhe

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    Those pipes act as nozzles. Fast exhaust gases give thrust. This effect gets better with speed. The simplest way to optimise this effect is to reduce the exit area until the hp starts to drop. Or you can do speed tests, but that's harder to measure so accurately. I linked a paper in another of your threads.
     
  18. Feb 11, 2018 #18

    TXFlyGuy

    TXFlyGuy

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    Thanks for the feedback. We will look into a new, more efficient design. Something we will have to explore on our own, however. Titan already has too many pots on the stove.

    edit: Just did a bit of reading on the fishtail design. It seams to have originated as a way of quieting the exhaust noise from racing bikes. Cannot find anything on it being more efficient.
     
    Last edited: Feb 11, 2018
  19. Feb 11, 2018 #19

    TFF

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    When WW2 planes were new, there was not such a thing as ashless oils. Starting a fire from stuff coming out of the exhaust was real. Most exhausts are an attempt to keep burning stuff a chance to extinguish. Some to keep the fire signature low for night. All of course with an eye of easy to make, good performance, aerodynamics, and not too out of the box to be squished by management. If someone came up with a bundle of snakes exhaust then ,they would have been laughed out the door before they could have bolted it on.
     
  20. Feb 14, 2018 #20

    TXFlyGuy

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    BTW, I contacted the folks at Stewart. They have the same type of exhaust. So those without a good supercharging system will suffer a power loss as well.
     

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