HKS vs 912, a comparitive analysis

Published by: Rizwan Bukhari on 3rd Nov 2011 | View all blogs by Rizwan Bukhari

This might be a very silly question and possibly is comparing apples and oranges. But can anyone educate me about their experiences with the two engines. I am particulary interested in learning about HKS engine. I know I am quite far away from having a four stroke engine but a little knowledge wouldn't hurt.

Thanks.

Rizwan :)

Comments

21 Comments

  • Jim T
    by Jim T 2 years ago
    Here ya go Rizwan,

    Comparing the HKS-700T to the Rotax 912 series, (80, and 100, hp)
    By Gerald J. Olenik, pres. Green Sky Adventures, Inc.
    Please note, this comparison to the Rotax 912 series is not intended to detract from quality of that design. The Rotax 912 series has evolved to be the bench mark for comparison among engines for Light Sport aircraft

    Pricing - The big hurdle. Purpose-built aircraft engines are expensive, that's simply a fact. Actually, engine pricing often seems to be a floating variable depending on completeness of the package and a number of other circumstances. But yes, the 700t is certainly in the dollar ballpark with Rotax 912 80 hp. It really does boil down to what you get for what you are paying.
    Some have asked or commented that the 500 hour recommended TBO is rather small..
    The 500 hr TBO of the HKS-700T is an introductory number. For instance its older brother, the 700e began with 500 hours, then with field experience was increased to 800, and now, it is at 1,000. We expect nothing less of the 700t, but 500 is a cautious prediction for now.
    When it comes to TBO, the 700e at 1,000 hours has a rebuild cost of $2,100 including standard parts and labor. That's typically less than half the overhaul cost of the 2000 hr 912. So, overhaul cost broken down per hour is probably slightly more favorable with the HKS. Of course, those are real numbers based on history. The 700T is still only projection.

    Performance
    From a performance standpoint the benefit of the turbo is enormous at a Density Altitude anything over zero. It actually works out better than the 100 hp 912S in some situations. Output of the HKS 700T would be on par with the Rotax 912S (that's the 100 hp version) at about 7,000 ft density altitude, and HKS surpasses the 912S by 10 percent at 10,000 ft density altitude. This is based on a loss of 3 percent per thousand feet.
    Comparing to the straight 912, (80 hp), the HKS 700T outperforms it from the sea level up. At 7,000 ft Density Altitude, the HKS produces 24 percent more power than the Rotax 912. On up at 10,000 feet Density Altitude the HKS 700T makes 36 percent more power than the Rotax 912.

    Weight
    Next, compare engine weight. If you hang from a scale, each engine, with all the parts one needs to make it work. the 912 will be mighty close to 160 lb. The HKS 700T 135 lb. Let's just call it a 20 lb difference, though it's really going to be closer to 25 lb.
    HKS engines are the personification of fuel economy. The 700-e burns typically between 2.5 and 3.2 gph. These are real street numbers on multiple aircraft with various missions and flight duration. (See my fuel notes from the Quasar Lite) The 700T, with the benefit of electric fuel injection, is reporting fuel burn at least as good as it's 60 hp little brother. That's roughly 1.5 gph less than typical Rotax 912, and perhaps 2.5gph less than the 912 S model.
    Combined weight and fuel economy.
    Let's plan a 3 hour flight, carrying 1 hour reserve fuel. So, we'll need 4 hours fuel. HKS fuel at 2.6 gph. equals 10.4 gallons, or 62.4 lb. Rotax 912 80 hp fuel at 4 gph. equals 16 gallons, or 96 lb. The Rotax 912 starts out at least 20 lb heavier than the HKS 700-E. Then add the extra 33.6 lb of fuel the Rotax will need for the mission. So apples to apples, the aircraft powered by Rotax 912 begins this mission having to carry almost 54 lb, or 40 percent more than the one powered by HKS-700T. And, that is useless weight.
    If you are repowering an existing aircraft, any increase in empty weight must be subtracted from useful load. An aircraft designer may approach this comparison by rationalizing that for every pound of engine weight saved, a pound of airframe weight may be removed from the design. Using our three hour plus reserve mission profile, the weight advantage then becomes 108 lb!

    HKS 700T ROTAX 912
    ENGINE WEIGHT
    135 LB

    155 LB
    WEIGHT OF 4 HR FUEL
    62.4 LB

    96 LB
    TOTAL
    197.4 LB

    251 LB

    4 cylinder smoothness with 2 cylinders?
    The HKS is very smooth. As smooth as the 912? ...Maybe. Low rpm vibes are inherent with both of these light weight short stroke geared engines. When set up correctly, the 912 would have only a slight advantage at idle power compared to the carbureted HKS 700e. Comparing to the HKS 700T, the HKS may have the edge. Electric fuel injection of the HKS 700t. is far more efficient at any rpm or throttle position. The 700T also has lower compression than it's normally aspirated brother. Both of these features contribute substantially to smooth running at all rpms but make a major improvement at idle and reduced power. Comparing vibration up in the normal operating range of cruise power or above, I think the Rotax 912 would have the edge, with it's four cylinders but only slightly..

    Two cylinder reasoning and quantifying smoothness.
    Why a 2 cylinder vs 4 cylinder engine design? The HKS incorporates a massive (about 7.5 lb) flywheel to minimize the violence normally associated with 2 cylinder opposed engines. Quite simply, it works. At normal power setting, (in my opinion) the HKS 2 cylinder engine runs as smoothly as any Continental or Lycoming 4 cylinder I've ever flown, and perhaps as smooth as some 6 cylinders. That flywheel may be low tech, but it works. That leaves only two cylinders to which HKS has applied their high tech capabilities where they have created a 4 valve head with oil cooling, a simple pushrod valve train, and predictable integral scavenge pumps for oil transportation. A smart CDI ignition with advance circuitry tops off their high tech stuff. One common concern? What if you loose one cylinder? Loss of one cylinder, (for instance, lets' say one of the carburetors falls off of an HKS 700e) does not have the same effect as losing two cylinders on the same side of a 4 cylinder opposed engine. That's because the massive flywheel. is still there. As far as vibration dampening is concerned, it acts just like two other cylinders would on a 4 cylinder opposed engine. The HKS, in this case would loose slightly less than half power. I have heard reports that the 700e will produce 36 hp on one cylinder. If a carburetor fell off a Rotax 912, violent vibration would render the power of the opposite side cylinders useless. Ditto, if you lost 2 cylinders on the same side of a Continental or Lycoming. Of course, if you lost 2 cylinders of a 2 cylinder engine, it would be infinitely smooth.
    HKS has been changing the way people think about high performance since 1973. Since 1998, HKS has established itself as a viable manufacturer of light weight high performance aircraft engines. The HKS 700-T really is the shape of things to come. ...My words. In the words of one of my contemporaries, "its a game changer"

    Green Sky Home
  • Rizwan Bukhari
    by Rizwan Bukhari 2 years ago
    Thank you very much, that really helps
  • Brian Reynolds
    by Brian Reynolds 2 years ago
    Riswan, One of my good trike buddies picked up a "used" HKS with about 60 hrs on it. I think it was non-turbo 80 HP. It turned out to be a huge waste of time and money for him. He spent about 2 months from the time he got it in rebuiding it and trying to get it to run well on his trike. Not to say the HKS isn't a good engine, but for him it was nothing but a hassel. I have a Rotax 912 UL and love it. 400 hrs no issues not engine outs. Yeah it was a bunch of money up front. I fly betwen 5K and 15K. and wish I had more HP but it runs super nice. My buddy went with a GEO Metro 3 cylinder turbo that he picked up already rebuilt took him about 2 weeks total to have the engine mount and get it dialed in. He spent a 1/4 of what I did and it puts out more HP at altitude then mine. Here is something to consider. What are your buddies using for engines? If you are thinking of getting something else you will loose the trouble shooting knowledge all around you unless you are and amazing mechanic.
  • Jim T
    by Jim T 2 years ago
    When it first came out HKS put out 100 of the 700e engines out for testing. They realized there was a problem. They fixed the problem and sent 100 engines out to the same customers free of charge and asked them to return the old engine when it was replaced.

    30 of the old engines were not returned so they are still out there somewhere. If you buy a used HKS "ALWAYS, ALWAYS, ALWAYS" call Jerry at Green Sky Adventures with the serial number. He will quickly be able to tell you if the engine is one the rejects.

    I work on 4 different trikes each having a different engine. Mine is the HKS 700e. My buddy flies a Rotax 912. I also work or the Rotax 582 and 503. The only problem I have come across is the oil injection system on the 582. A very good mechanic friend told me that he has had the same problem with his jet skis with the rotax 582. I remove the oil injection system on my buddies trike after his engine seized up.

    I have about 300 hours on my Aeros HKS. I bought it with 49 hours on it and it has never missed a beat. It starts immediately every time. I have flown it in 28 degree and it still starts.

    Besides reliability the main advantage is the fuel consumption. Flying by myself I use 1.5 gallons an hour. With my 12 gallon tank I don't have to worry about running out of fuel...EVER! My buddies 912 uses about 3.5 gallons an hour. The 582 and 503 use even more.

    For me the HKS is more suited for my type of flying. I weigh around 200 and I have no problem with passengers up to 230 lbs. Fuel consumption goes down to 2 gallons an hour 2 up.
  • Rizwan Bukhari
    by Rizwan Bukhari 2 years ago
    JIm, I thank you very much for this information, I am still light years away from owning a 4 stroke new trike, but at times I have romanced with the idea of just upgrading my engine on my Airborne Edge X (if possible) to a 4 stroke HKS (if that was a possibility).

    Brian, thanks to you as well, your friend's Geo Metro...how many hours he has on that engine? I read somewhere that converting car engines to aircraft engies and have consistent good performance is hard, since the car engines are not designed to run at such high rpms consistently, what is your view on that?
  • Brian Reynolds
    by Brian Reynolds 2 years ago
    There are 2 with the Geo turbo's. One has recently been sold. The guy I currently hanger with has got about 350 hrs on it now. He is a mechanic so when issues arise he has been able to figure them out. Apparently the guy that makes the conversion kit, I think it is called a "Raven" is way sharp on his support. Rick (my trike friend) did have an engine out when we were flying to Leadville, Colorado. A distributor wire broke, he made a forced landing in a field and the racher came out, and as he had some crimping tools Rick repaired his trike and completed the trip. It is not as smooth as my Rotax but he does put out more HP at these altitudes.
  • Captain X
    by Captain X 2 years ago
    The HKS looks great on paper. I write down my fuel usage every flight to get a good personal average for planning, I calculate based on burning a little less than 3.5 GPH with my 912 80HP- that agrees with what Jim says his friends are burning. The article however states 4 GPH for the 912 and some of the other wording makes me suspicious that it wasn't written purposefully biased against the Rotax. I'm really rooting for a competitor to Rotax but not ready to take the plunge yet to HKS. Still the article makes the HKS sound great.
  • Ken Nussear
    by Ken Nussear 2 years ago
    How does the new Thunder aviation motor (??) weigh in here? Does anyone have experience with this one? I think Damien has an HKS - what is your experience Damien?
  • Ken Nussear
    by Ken Nussear 2 years ago
    Specs on the thunder site say.....

    Fuel burn @ cruise 2.8-3.4

    Weight 121lbs

    80 hp and 64 ft lbs of torque

    Twin Fuel Injection

    4 valves per cylinder

    Dual Ignition

    1000 Hour TBO

    Rebuild Cost $900.00
  • Flying  Frog
    by Flying Frog 2 years ago
    I like the HKS, but would not buy one at that price. If my 912S ever gave up and needed changing, then I would fit a 90/100hp BMW motorcycle engine. The Tanarg has a bolt on engine frame so the conversion would be very easy to do. There is a Tanarg with a BMW 1150 flying in Italy.
  • Captain X
    by Captain X 2 years ago
    Did anyone else think that this sentence didn't quite seem to make sense?:
    "An aircraft designer may approach this comparison by rationalizing that for every pound of engine weight saved, a pound of airframe weight may be removed from the design."
    Sure, you can lighten the frame by some amount if the engine is lighter, but I don't think that's a very linear equation. Even the fantasy weightless 80hp motor would need a substantial frame, regardless of it's weight, it is pushing 80HP!!
    That's why I'm just not sure I buy all the statments in the article even though they sound great.

    I think the idea of fuel injection on the HKS and Thunder are great. I often wonder why my "modern" Rotax still has carburetors. Turbo has advantages too, I just remember in the 80's when everything went turbo and there were tons of failures and turbo is not so popular on most cars anymore.

    How much is the Thunder engine (ready to go)?
  • Flying  Frog
    by Flying Frog 2 years ago
    Plenty of fuel injection kits available for the 912 David.
  • Captain X
    by Captain X 2 years ago
    Tell us more, or give link, to fuel injection kits for 912. Do they save fuel, less maintenance, no synching, more power? OK for SLSA (mine is ELSA)?
  • Captain X
    by Captain X 2 years ago
    Links for fuel injection for 912:
    http://silent-hektik.com/UL_912_1.htm
    http://www.aero-hesbaye.be/dossiers/Inj912/inj912.htm
  • Flying  Frog
    by Flying Frog 2 years ago
    http://silent-hektik.com/UL_912_1.htm how about that one?
  • Captain X
    by Captain X 2 years ago
    Thanks Frog
  • Flying  Frog
    by Flying Frog 2 years ago
    See here too http://www.eaa.org/news/2011/2011-09-08_blois.asp
  • Brian Reynolds
    by Brian Reynolds 2 years ago
    My 912 UL AC clipper with the Ixess wing at this altitude of nothing under 5k msl burns 2.8 gph. Thats when it is running at about 4300-4500 RPM's. It has been like that for the 400+ hrs on the engine. Yeah it'll burn quite a bit more when climbing at 5000-5200 RPM's trying to get up the mountains.
  • Rizwan Bukhari
    by Rizwan Bukhari 2 years ago
    I thank you all for all the great information. Next step, start saving up for a four stroke.
  • Captain X
    by Captain X 2 years ago
    for me, 3.5gph is gross overall average, not cruise which is maybe around 3.0 (but I like to fly fast)
  • Abid Farooqui
    by Abid Farooqui 2 years ago
    And save for a Rotax 912S instead. I would. Right now that's the engine to get. You can get a re-built 912S for the same as the HKS 700e.
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