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HKS Frequently Asked Questions

  1. Why does the 700E have such a high operating RPM? Is this normal for a  four-stroke engine?
  2. Why does the HKS have a gearbox, isn’t direct drive lighter and simpler?
  3. Why does the HKS have separate oil tank? And what is a “dry sump” engine?
  4. Didn’t the original HKS have overheating problems?  What has been done to fix the problem?
  5. Can the HKS engine be used in pusher installations? 
  6. What type of engine mount does the HKS have? Do you sell mounts?
  7. What type of fuel does the HKS require? Can it use 100LL?
  8. What is the fuel consumption of the HKS?
  9. What type of propellers work best for the HKS? Are they available?
  10. What type of exhaust is required?  Can you run straight pipes?
  11. Are the carburetors used on the HKS altitude compensating?  Why don’t they  use Fuel injection?
  12. Why is the HKS engine so expensive?

1. Why does the 700E have such a high operating RPM? Is this normal for a  four-stroke engine?

The 700E was designed to get two-stroke like power output from a four-stroke engine.  The way to get more power from a given size engine is to make it do more work, meaning more RPM.  The operating RPM range of the 700E is normal for an engine using modern motorcycle technology.  The Rotax 912 four-stroke has a similar operating range.

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2. Why does the HKS have a gearbox, isn’t direct drive lighter and simpler?

The larger the propeller, the more thrust it will produce, particularly at the relatively slow climb speeds of light aircraft.  More low speed thrust means more climb.  As well, there is the great benefit of much lower propeller noise than direct drive. The 700E gearbox has two ratios available 2.58 and 3.47 to 1.

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3. Why does the HKS have separate oil tank? And what is a “dry sump” engine?

The 700E has a separate oil tank to reduce the package size of the engine.  Most conventional aircraft engines are dry sump for the same reason. A “wet sump” engine has it’s oil supply in the oil pan at the bottom of the engine. A “dry sump” engine has a separate scavenge pump to take the oil from the bottom of the mini-sump at the bottom and a supply pump to send oil under pressure through the engine. 

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4. Didn’t the original HKS have overheating problems?  What has been done to fix the problem?

The new version of the 700E has a redesigned lubrication/cooling system. The changes made included, increased the cooling oil flow to the heads, adding a second scavenge pump and altering the material specification for the head and valve seats.  These changes were confirmed through an exhaustive 2 year test program.  HKS is now swapping out the original engines in the field with new models in honor of the warrantee.

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5. Can the HKS engine be used in pusher installations? 

Yes.  As a matter of fact the new version of the 700E has been redesigned to work better in the pusher configuration.  The biggest change is a larger and deeper dry sump for a wider operating angle of attack range, and the increased oil supply to the heads as was discussed above.

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6. What type of engine mount does the HKS have? Do you sell mounts?

The 700E has a base mount similar to but slightly different than the Rotax.  There is also a requirement that the engine mount has a hole for in the center for the sump to fit into. HPower does not currently make custom engine mounts.  We have worked with the many airframe manufacturers to develop mounts.  We feel that this is the best way as they know their planes requirements.

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7. What type of fuel does the HKS require? Can it use 100LL?

Automotive Unleaded-high test is best.  92 octane Unleaded is typical.  100LL  Avgas can be used during cross countries without worry.  Constant use of leaded fuels requires different lubricating oils to be used to avoid lead deposits. 

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8. What is the fuel consumption of the HKS?

The fuel consumption in typical operation is between 2 gallons and 3 gallons an hour.  In our testing, the fuel consumption we have noted is 2.5 gallons an hour  @ 75%.  The Dyno testing according to the JIS (Japanese Industry Standard) requires the engine to be tested in a manner that does not reflect propeller loading.  This makes HKS’s consumption numbers very conservative.

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9. What type of propellers work best for the HKS? Are they available?

The highest performance propeller we have tested for the 700E, is the Powerfin "F"  and  "B" model propellers.

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10. What type of exhaust is required?  Can you run straight pipes?

The 700E was designed as a complete system.  There are requirements for back pressure and overall length of the exhaust as a part of the system.  The intake and exhaust manifolds are optimized for a flat torque curve.  

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11. Are the carburetors used on the HKS altitude compensating?  Why don’t they  use Fuel injection?

The Bing C.V. (Constant Velocity) carburetors are capable of 50% mixture compensation to approximately 10,000 Ft.  due to the CV design which uses a diaphragm-slider to vary the mid range mixture.  The Rotax 912 four-stroke uses the same carburetors.   HKS has great experience with fuel injection,  But the added cost for digital fuel injection would be high, around $1500.00 an engine. 

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12. Why is the HKS engine so expensive?

The 700E is sold as a complete system.  Many of the parts included are not included in other competitive engines. The other problem is the volume of production. The 700E is made up from all custom high performance parts and the limited quantities it is produced in make it expensive.  As well, you are paying for the HKS warrantee and the support provided by the OEMs and HPower, Ltd.    

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