# Engines: Cost and weight per HP

Discussion in 'Firewall Forward / Props / Fuel system' started by Vigilant1, Jul 29, 2018.

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1. Jul 29, 2018

### Vigilant1

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Looking at the various discussions here about engine weights and cost, I gathered some numbers (below) for rough comparisons.
1) I looked at engines that could be bought new and completely assembled
2) Weights and costs are from "the internet", don't include exhaust systems, and are subject to a lot of variables.
3) To some extent, none of his matters. If you've got a particular type of flying in mind, then you may need a particular engine.
4) Costs are initial purchase price. Maintenance costs and resale value may also be important factors.
Edited to add: See later post for edited chart with corrections and additions.
Numbers:

Engine.................HP.....Cost ($)....Weight (lbs)...$ per HP.....Lb per HP
Lycoming O-360.........180....27,500.(1)...305.............153..........1.69
Rotax 912 ULS/S........100....19,437.......125.............194.........1.25
Aeromomentum AM13......100....10,495.
(2)...201..(3)........105..........2.01
VW 2180cc.(4)...........76.....7,175.......167..............94.........2.20
VW 1835cc (5)...........60.....6,025.......165.............100.........2.75
VW 1835cc "basic",(6)...60.....4,750.......145..............80.........2.42
Harbor Freight670cc.....22.......584.(7)....97..............27..........4.38
"" + hub&bearing (8)....22.......734.......104..............33.........4.70

Notes
(1) Price through Van's direct purchase program. May be higher elsewhere?
(2) Price is for the Aeromomentum AM13 "low profile" version. The upright version is $8495 (3) Weight is for the AM13 engine, radiator, and coolant (4) From Hummel engine site, includes dual ignition system, starter, alternator, carb, Force One bearing and prop hub (5) As above, but simple shrink fit prop hub (no Force One bearing and hub) (6) Hummel engine site: Single ignition, hand propped, no alternator, with carb (7) With ubiquitous "20% off" coupon (8) Estimated price and weight of additional bearing and prop hub:$150, 7 lbs.

Observations:
1) Mass production does a great job in lowering prices
2) Weight: The formula for a cylinder's included volume compared to its surface area is relentless. Bigger engines generally have more HP per lb. The Rotax stands out in this regard--but they sure charge for it!

My own very subjective opinion is that, for fun flying, there's slowly decreasing utility for each HP added after we get safely airborne. More HP is almost always more fun, but the first 25 HP that may get us airborne and enjoying the magic of flight surely gets us more "fun per HP" than the 25 HP difference between a 160 HP engine and a 185 HP engine. So, in that regard, the small industrial engines that provide that first, "highest utility" 20-25 hp at a lower absolute $$/HP are very impressive standouts--if we have a plane that can take advantage of it. Luckily, if our idea of minimalist fun flying requires 60-80 HP, there are still good$$/HP (and lb/HP) choices in that range, too.

Just some not-very-original rambling on this Sunday . . ..

Last edited: Jul 29, 2018
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2. Jul 29, 2018

### pictsidhe

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Current price o the HF is $730. They have money off discounts occasionally, but not often. curently a$670 one. There was a $600 one til Feb. 20% coupons don't work on engines. The industrial V twins have a heavy flywheel that won't be needed in DD. That could be -10lb. 30hp 810cc Briggs tip the scales at 87lb out of the box. -10lb for flywheel =77lb. Price is usually around ~$100 more than HF if you look around.
23hp 627cc Briggs are 77lb out of the box. -10lb = 67lb. You'll pay maybe $1200 for one of those. There's a bit more power that can be had from the industrials, but that's either time,$, or both.

1/2 VW, 87lb, ~32hp,

3. Jul 29, 2018

### Vigilant1

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Thanks, I'll fix the chart, add a Briggs and a 1/2VW. I didn't realize the HF 20% coupons weren't good for Predator engines (and lots of other stuff, too :dis: )

4. Jul 29, 2018

### davidb

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Easy to see why Rotax has a monopoly with SLSA.

5. Jul 29, 2018

### Vigilant1

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Updated chart, includes 1/2VW and B&S 810cc engine info from pictsidhe.

Numbers:

Engine.................HP.....Cost ($)....Weight (lbs)...$ per HP.....Lb per HP
Lycoming O-360.........180....27,500.(1)...305.............153.........1.69
Rotax 912 ULS/S........100....19,437.......125.............194.........1.25
Aeromomentum AM13......100....10,495.
(2)...201..(3)........105.........2.01
VW 2180cc.(4)...........76.....7,175.......167..............94.........2.20
VW 1835cc (5)...........60.....6,025.......165.............100.........2.75
VW 1835cc "basic",(6)...60.....4,750.......145..............80.........2.30
1/2VW 37HP "basic"
(6)...37.....3,650........85..............99.........2.00
Briggs & Stratton 810cc.30.......830........87..............28.........2.90
^^ "" + hub&bearing (7).30.......980........94..............33.........3.13
Harbor Freight670cc.....22.......730........97.............33..........4.41
^^ "" + hub&bearing (7).22.......880........104..............40.........4.73

Notes
(1) Price through Van's direct purchase program. May be higher elsewhere?
(2) Price is for the Aeromomentum AM13 "low profile" version. The upright version is $8495 = 84.95$/HP
(3) Weight is for the AM13 engine, radiator, and coolant
(4) From Hummel engine site, includes dual ignition system, starter, alternator, carb, Force One bearing and prop hub
(5) As above, but simple shrink fit prop hub (no Force One bearing and hub)
(6) Hummel engine site: Single ignition, hand propped, no alternator, with carb

BTW, chinese scooter CV carbs of about the right size can be had for about $30-$40 each from aliexpress. So, maybe another $100 for a CV carb conversion. 10. Jul 29, 2018 ### Hot Wings ### Hot Wings #### Well-Known MemberHBA Supporter Joined: Nov 14, 2009 Messages: 6,640 Likes Received: 2,532 Location: Rocky Mountains That is what they say but it's worth a try. All they can do is say "no". The 20% coupon I used for the one I purchased, via their web site not from a store, worked. Got the free shipping too. They may have fixed that little back door by now? 11. Jul 29, 2018 ### Topaz ### Topaz #### Super ModeratorStaff Member Joined: Jul 30, 2005 Messages: 13,964 Likes Received: 5,589 Location: Orange County, California Pulling the flywheel altogether on any of the industrial motors is a little more involved than it sounds. Most (if not all) key the ignition off magnets on the flywheel, and use the flywheel for charging the ignition battery. Those functions would need to be replaced, somehow. For the guys who are looking at extensive modifications, EFI, etc., that's likely not an issue. For other guys (like myself) who would be interested in the most "bare-bones" conversion possible, I'd probably be tempted to swap out the steel flywheel for a lighter-weight aluminum one, should the weight reduction prove necessary. With either a no-flywheel or aluminum-flywheel conversion, some means would still need to be found to keep the battery charged. A little wind-driven generator is probably the best bet there. Vigilant1 likes this. 12. Jul 29, 2018 ### pictsidhe ### pictsidhe #### Well-Known Member Joined: Jul 15, 2014 Messages: 7,352 Likes Received: 2,119 Location: North Carolina For the price of an alloy flywheel, you can probably find an electronic ignition. The standard flywheel ignition is rather heavy... A small belt driven BLDC motor seems the simple, light, cheap generator option. Add a reg-rec and Bobs your mother's brother. 13. Jul 29, 2018 ### Topaz ### Topaz #### Super ModeratorStaff Member Joined: Jul 30, 2005 Messages: 13,964 Likes Received: 5,589 Location: Orange County, California Actually, for exactly the same price. Looks like it goes on the PTO end, though. 14. Jul 30, 2018 ### Pops ### Pops #### Well-Known Member Joined: Jan 1, 2013 Messages: 7,505 Likes Received: 6,495 Location: USA. Hummel list their Basic 1835 cc engine as 135 lbs. My firewall forward weight for a Basic 1835 cc VW was 141 lbs, including prop and engine mount and exhaust system. Ready to fly. I cut some of the case round bell housing metal off to save weight. The most bang (HP) for the buck ($) in a 4 stroke, 4 cylinder engine is a 1835/1915 cc VW engine.

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15. Jul 30, 2018

### Vigilant1

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Pops, Scott gives the weight of the 1835cc as "135 to 165 lbs" here, , but where he lists the bare-bones engine on his price list (here) he gives the weight as 145 lbs. So, given that, I figured the only way he could get things down to 135 lbs is with the NiCom cylinder$option that he sells ($350 each). Anyway, that was my reasoning.

Another thing on weight: For any of the engines that can't be started by hand propping, an argument can be made for including the battery as part of the basic engine weight, since you can't get the engine started without it. The battery weight will probably be more per HP for the smaller engines than the larger ones.

I modified the table in post 5, re-adding the 10 lbs for the flywheel for B&S and HF industrial engines. Even the "light" flywheel with the ignition parts (sold by Performance 670) still weighs over 8 lbs, and then you'd need to find some other way to charge the battery, and that's going to weigh something.

Last edited: Jul 30, 2018
16. Jul 30, 2018

### Pops

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I used stock steel 92 mm jugs for the 141 lbs firewall forward weight. A long block with steel cylinders 1200, 1300, 1500, 1600, 1835 and 1915 cc VW engines weights 116 lbs. The total weight is what you add to the long block.

17. Jul 30, 2018

### Dan Thomas

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18. Jul 30, 2018

### Pops

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I believe I can save another coupe pounds. Like I said, I cut part of the bell housing flange off the flywheel end, and replaced the heavy steel case bottom round steel plate with the oil drain plug with one made from aluminum with a small slump and an alum 1/8" pipe plug drain. If I could find thinner steel tubing or use aluminum for the upper part of the intakes and thinner wall steel for the exhaust I could save some weight, the Y part of the intake down to the carb is aluminum. The oil pump cover is alum instead of the heavy thick steel cover.

I could use the 2 pounds saved to upper the engine to a 2180 cc for 15 more HP over the 1835 cc engine, but it cost a lot more. I have a case machined for 92 pistons and jugs and inside the case cleared for the 82 mm stroker crank and have the flywheel drive parts made. Since I will be using a bed mount for the engine, most of the flywheel end bell housing can be cut off to same more weight. Believe I can have a basic flywheel drive 2180 cc engine at 75 Hp with weight of about 138-139 lbs firewall forward weight.
Now if I get my son's SSSC project back that is for sale and make it a lot lighter and install the light 75 HP 2180 cc engine in it , it should climb a lot better than the 1200+ ROC and take off shorter than the first SSSC.

Last edited: Jul 30, 2018
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19. Jul 30, 2018

### TFF

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For new, but used useable engines like a $10,000 Lycoming 360 on barnstormers would be$55 per HP. A $10,000 320 would be$67 per HP. A $15,000 180 from Barnstormers would be$83 per HP. Right in line or better than a 1/2 V Dub. Realistically, powering an airframe is powering an airframe. It needs what it needs. Cheap or expensive is really about how good the airframe is. Is airframe worth the money is the real question. Some are not.

20. Jul 30, 2018

### Vigilant1

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Yes, it does add costs. If I've got this right, the Force One bearing/hub is $378, and there would be some costs for machining the case to accept it. The case would need to be clearanced for the 82mm stroke 2180cc, and you'd need to buy a new crankshaft (about$700 from GPAS). The 92mm cylinders and pistons from the 1835cc engine would still be fine. So the parts to go to 2180cc would be about $1078, plus money for machining work. On the other hand, from the chart it looks like we generally pay about$100 per hp for pre-assembled VW engines, so the cost probably isn't out of line if you get 15 more HP. Still, it is only worth it if you need that 15 HP.