B&S 49-series (810cm3/49ci) for aircraft use - TiPi's Q&A thread

Discussion in 'Firewall Forward / Props / Fuel system' started by TiPi, Oct 4, 2019.

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  1. Oct 18, 2019 #41

    Vigilant1

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    That sounds well within reason. It's the custom machining and custom parts that can drive the price through the roof. It appears you intend to use as many of the original parts as practical (no new carbs, etc). On that front, if 35HP (for takeoff) is the goal, do you think you'll need to go to the trouble to modify the stock cam grind? Or, even do much work on the induction ports? If you clean things up just a little (e.g. those screws you identified on the carb butterflies, the edges in the carb, etc,) that seems like it might be all that is needed. I know you've built the flow bench, etc.

    Back to the PTO shaft: If a tapered one is deemed best (considering cost, likely performance, risk, etc), then the two options are to buy that crankshaft and put it in,or buy the engine with the tapered shaft already in it (the gaseous fuel engine--for propane or natural gas). That engine has chrome exhaust valves, but I haven't been able to determine if it comes with the standard Nikki carb. I assume that right now your plan is to use the straight 1 1/8" PTO model that you have already.
     
    Last edited: Oct 18, 2019
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  2. Oct 18, 2019 #42

    TiPi

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    for the prototype, I'll use what I have now (the older 49M977 with the straight 1.125" shaft). the generator unit is quite a bit more expensive and has a LNG/LPG carburetor, not a standard carburetor (kit and some parts are the same as the standard carburetor), camshaft, intake valve, fan are all the same. Main difference are the crankshaft, EX valve and intake system.
    If you want a tapered shaft, it might still be the cheapest option to buy a complete Vanguard engine and have the crank machined for your tapered hub.
     
  3. Oct 18, 2019 #43

    PW_Plack

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    There's a guy on YouTube who modded a Kohler 725cc V-twin a few years ago. I communicated with him at the time, and he said if he had to do it again, he'd stick with the stock cam. Even at 5000+ RPM using a chain redrive, he saw no gain from the aftermarket cam. At 3600 RPM, you might actually go backwards.

    The available racing cams, at least for the Kohlers, are intended for applications like lawn tractor pulling, where engines are tuned for 8000 RPM or more. It might be possible to use CAD to arrive at a grind optimized for 3600 RPM, but it seems well into the realm of diminishing returns.
     
    Last edited: Oct 18, 2019
  4. Oct 18, 2019 #44

    Hephaestus

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    See previous discussions, but there's some argument the stock cam may not last. Reinforced or billet.

    Theres a variety of suppliers who can custom grind a cam to whatever profile you need. And theyre not terribly expensive.
     
  5. Oct 18, 2019 #45

    TiPi

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    The stock cam is fine except that the Briggs 49 has a different EX lobe with too much ramp (open too early/close too late) and the built-in decompression.
    A performance cam only works if all other bits and pieces in the intake stream are capable of meeting the new cam's requirements. The single-throat carby and elbow intake wouldn't help, especially at higher rpms.
     
  6. Oct 18, 2019 #46

    pictsidhe

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    Why not have a hub machined to fit the crankshaft? The hubs would then bolt on to any tapered shaft industrial
     
  7. Oct 18, 2019 #47

    TFF

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    On shaft ends, there are a lot of Continental A-65 and others with tapered shafts that have flown and are still flying.
    Although not drive flywheel when I use to race karts with 5 hp Briggs, we would lap the flywheel to the crank. It was the only way to ditch the key and advance the timing without it slipping. Lapping and a key would be pretty slip resistant.
    The helicopters I work on have the transmission input as a regular shaft with a key. I always look at it going why, but it’s not one of the problem areas. I would probably pick taper but both probably would work. I would be more worried about bending the crank with the prop precession.

    Racing the stock Briggs class, very few modifications were allowed. One that was allowed to run any valve clearance. That allowed us to open up the clearance to miss the compression release lobe. Overall valve lift was down, but cylinder pressure went up greatly. Without a custom cam, it’s a way to get more power. With rocker arms you might need to do some creative adjusting or clearancing; that or polish the release lobes down if you have the know how.

    How much are the cranks? $250 and the exhaust valve $20. If you can machine stuff yourself it would not be worthwhile to buy, but if you can’t , make sure you can find a shop that is not going to be more expensive than just buying the part.

    I also don’t trust Briggs rods from racing. We were stuck in the stock class with stock rods. Fine in my lawnmower, but only good for about three hours of racing. We were turning them 5800-6000 but also the cylinder pressure was higher. High throttle all the time. Kohler rods were better and some cheaters would modify them to work. When friends went to the modified classes, they bought Weiss rods and could turn the engines 8000 for years of racing.
     
  8. Oct 18, 2019 #48

    pictsidhe

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    Opening up the valve clearances to 'death rattle' was one of my tricks on the gsxr stock class drag bike that I mechaniced. No compression release on that.
     
  9. Oct 20, 2019 #49

    Vigilant1

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    In the US, the tapered-shaft propane B&S 810s price out at about $1200, which is $200 more than the other B&S models but less than the Vanguard 810s. If you wanted the tapered shaft and chromed exhaust valves anyway, it would be close to break-even I suppose (after you got the carb straightened out).

    If we taper a straight shaft, we start the taper with a 1 1/8" shaft. The factory tapered shafts start at the same diameter as the bearing, and no carveout for the keyway.

    Maybe it won't make a difference.

    FWIW, I made an inquiry to "Arrowprop" (mentioned here) to get tech info (esp max torque values) for their prop hubs designed to fit straight shafts on small industrial engines. And, I asked about their propellers (wood and composite). I'll chime in if/when I hear from them.
     
    Last edited: Oct 20, 2019
  10. Oct 20, 2019 #50

    TiPi

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    I haven't found a drawing or dimensions for the taper shaft on the 49. These pic from the 54-series (left) and 38-series (right) have different tapers. The main bearing on 54 and 61 engine is 45mm, on the 38 it is 35mm. The Honda drawing for the taper shows 35mm as the large diameter so that sounds about right compared to the pictures. Better than 28.5mm but not 41.5mm.


    upload_2019-10-20_17-20-37.png upload_2019-10-20_17-24-56.png upload_2019-10-20_17-29-20.png
     
  11. Oct 20, 2019 #51

    pictsidhe

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    This is not the taper that we are looking for:

    [​IMG]

    But, it is actually specified. It is from Robin-Subaru. It could be worth trawling through other datasheets from Robin.
     
  12. Oct 20, 2019 #52

    TiPi

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    but it seems to be the taper that is available from B&S.
     
  13. Oct 20, 2019 #53

    pictsidhe

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    Generator tapers are standardised. That was a small one, from a 270cc engine. Smaller diameter than our v-twins The best I've managed on larger engines is a Robin 720 parts diagram mentining 2 1/4 SAE taper. It's a fair chance that all the common SAE generator tapers use a 2 1/4:12 taper. There is also a 1:10 metric taper for the 270, that comes with an M10 thread. The SAE ones have imperial thread. While this isn't a definitive answer to the taper riddle, it's a good clue.

    The Inteks appear to taper all the way from the journal. This is probably for ease of machining. We may need to measure one to see where the useable taper starts.
     
  14. Oct 20, 2019 #54

    BBerson

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    I don't know if the taper is available at discount online sellers. If you special order, they might ask a bunch of application questions.:eek:
     
  15. Oct 20, 2019 #55

    pictsidhe

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    EH90 dimensional diagram Looks a lot like our taper. The angle comes out at 2.27:12 Maybe somebody needs to get their paws on the SAE spec...
     
  16. Oct 20, 2019 #56

    pictsidhe

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    "Airboat"
     
  17. Oct 20, 2019 #57

    BBerson

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    Yeah, but I think they might not be interested in talking about a single unit to an individual.
     
  18. Oct 20, 2019 #58

    karmarepair

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    Tapering sounds counter to "simple, cheap, off the shelf" goals, so I'm grazing other pastures.

    E-mailed the Ace Redrive guy. They use the Fenner B-Loc on their lower pulleys, specifically the B-112 series.http://www.fennerdrives.com/b-loc/_/B-LOC-B112-1-1/8/?s=Znw2OTMsMjMxMQ== over 600 ft-lbs capacity, a little over $100. Straight shaft, straight bore. They have two cheaper series that also have plenty of capacity.

    I think that since the Ace people chose the stoutest speaks well of their choices.
     
    Last edited: Oct 21, 2019
  19. Oct 20, 2019 #59

    Vigilant1

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    TiPi,
    Thanks for the recent glimpse into the B&S 44 and 49 series use in the mower racing world. The "scene" seems similar to what I've read about mower racing in the US: They are doing what they can to keep it a safe, fun, affordable hobby.
    How long does it take them to go 6 laps?
    Are they all still using the stock aluminum pushrods? Seems odd.
    It would be interesting to see how their CHTs behave under load at the races--or on a long straight run at WOT, maybe a bit uphill to keep it under load for a two minute "climb." Perhaps some of those guys even measure the CHTs during races.
     
  20. Oct 22, 2019 #60

    karmarepair

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    I've been a little skeptical of using this series engine, and have been exploring Kohler Command 747cc engines, and have found a spec I like. So I started checking prices...the Briggs 49 Series is a SCREAMING deal. A roughly equal CH750 is TWICE as expensive, and that actually gets you less displacement and a cast iron crank. And it looks like performance parts for the Briggs are more common.
     

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