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The modern Harley Davidson engine has very poor power output. Even when highly modified a 130HP engine can cost tens of thousands of US Dollars.

Here are specs on one of the modern engines.

Harley-Davidson Twin Cam (1800cc / 110” 45 degree single plug v-twin with F.I. and 9.0:1 CR) – 95hp & 110 ft lbs of torque

This 1.8 Liters and it produces 95HP?

This has always been the trend with Harley. I've heard all manner of reasoning. It's a cruiser, it's not trying to do that. On and on. Well, guess what. Every single Harley rider I know dumps tons of money into their machines to get an extra 10 or 25 HP. So that's hard to buy. The community of Harley riders globally spends hundreds of millions of dollars on aftermarket parts.

What am I looking for?

A little bit of honesty. I figure this is the place where I can get that.

My Question Is

Why are Harley Davidson engines such poor performers on their horsepower ratings?

They consistently lose to the Big 4 Japanese V-Twin cruisers in terms of performance as well almost everything else but the Royal Enfield.

What is it about the design of Harley Davidson engines that limits their performance?

Thank you for letting me bring my motorcycle vitriol here. You all are champs!

enter image description here

  • My suggestion is that you post the engine that you have in mind for purpose of comparison. That should help to minimize opinion and subjectivity – Zaid Jan 14 '16 at 7:58
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    I guess torque is where they excel. High displacement means high torque at low RPM, and low RPM is where they are driven. I always looked at them as being equivalents for a truck. Big, heavy, slow, high low end torque. – I have no idea what I'm doing Jan 14 '16 at 8:23
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    Me too. Totally agree. Two wheeled tractor. – DucatiKiller Jan 14 '16 at 8:26
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    When I was at HD Uni, our professors who were Japanese specialists as well (for decades), stated the opposite. You can sinks thousands into Japanese bikes and gain very little noticeable performance, while Harleys are dialled down from the factory and offer an expansion of possibilities. – Jonathan Musso Jan 14 '16 at 13:26
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    The way I see it - There are 4 pieces of information required to show the characteristics of an engine - HP, torque, and an RPM for each. Comparing only horsepower does not account for the width of the power band or how well it will do at low RPM. – rpmerf Jan 14 '16 at 13:28
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First of all, I don't really know much about motorcycles, so don't beat me...

For a good comparison, one needs torque/hp diagrams. However, they are not easily to find on the web for certain models. In most cases, you will find plots showing curves before/after tuning.

I tried a little and found these two bikes, which hopefully are comparable in this question.

This are the curves for a Harley Davidson Fat Boy Low. (It has "only" 1584cm³):

enter image description here

And I found something similar for the Yamaha Vmax 1700 with 1679cm³ (I hope, this is appropriate):

enter image description here

This bike is made for lots of torque and high speed. Now, let's compare the curves. I've extracted them and brought to the same unit (mainly torque, but there's also a small difference between imperial (hp) and metric (PS) power):

enter image description here

The Harley delivers nearly maximum torque at 2000RPM, and keeps going until 4000RPM, after which the torque decreases. That's also the RPM where it's finally outperformed by the Yamaha in each, torque and power.

(I've also added the curves of the tuned bikes and there, the Yamaha outperforms the Harley from the beginning.)

It's not clear how exact these curves are. For example, I also found a site stating about 125Nm torque for this Harley, while the curve shows 115 only.

But in general, Harleys are usually driven at low RPM, where they have more torque and more power than that 140PS Yamaha bike.

May be, the Yamaha is an extreme example, but keep in mind, the Harley compared here has just 1.5l displacement.

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    That's a next level effort you put in. I really like the comparison. I think what you have illuminated the real and delta. It seems the Yamaha in this case has almost twice as much power per cc as the Harley. Something else I just thought of, the HD is probably twice as much in cost. Yamaha has twice the power per cc and half the cost per cc. Nice comparison. I'll upvote when the clock flips in about 7 hours. I ran out of votes and don't get them back until the UTC date flips. Great answer! – DucatiKiller Jan 14 '16 at 16:01
  • I guess this chart puts paid to the "but the v-twin has SO much more torque" nonsense. – Tim Nevins Apr 5 '18 at 15:44
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Royal Enfields are poor man's Harley they both have a thing in common , a relatively high displacement engine for less amount of horsepower.

Example : Royal Enfield Continental GT 535CC engine produces 29hp from 87mm x 90mm

It has a Big bore long stroke design , it gives out excellent low end Torque , I have driven this motorcycle and you get almost all of the awesome torque at around 1500 rpm.

The reason it has this design is because these engines were built for the army way back in the 50s to be driven off road especially in desert and muds where this low rpm max torque is desired so not to get the wheel stuck in mud or sand.

This design stuck with the bike and it has now become classic vintage design.

Choppers are of similar nature for example the Twin Cam engine spoken by the OP has the bore x stroke of 95.3mm x 101.6mm

In contrast the bore x stroke of yamaha R1 is 77 mm × 53.6 mm .

In conclusion, Choppers have the particular big bore long stroke design to have max torque at low rpm which is ideal for a cruiser whereas a sports bike has to rev to higher rpm to reach its peak torque figures which is not very desirable in cruising.

Apart from all the technicality, I prefer the chopper engine to a high revving engine for cruising, the feel is very unique

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    Nice answer. TY. I'm sure you have a lot more exposure to Royal Enfields than I do. I see them from time to time here in the US but they are relatively rare. Agree on the ride and feel of a Harley. It's nice to get on a freeway from where I live and just ride for hours to LA. A real visceral experience. – DucatiKiller Jan 14 '16 at 16:03
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    My pleasure, Royal Enfields are much more suited for tight cities like europe and india , it can barely do 70mph so you can forget taking it to the expressways in the US. But what they offer is pure British Pre war heritage which surprisingly is still captured by the factory near where I live. They are history on 2 wheels something no superbike can capture. – Shobin P Jan 14 '16 at 16:19
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"Why do Harley Davidson engines suffer from such poor performance?"

"The community of Harley riders globally spends hundreds of millions of dollars on aftermarket parts."

You have answered your own question. It is simple economics. Create the demand by building low-performance into the whole image. Now you've just added hundreds of millions of dollars to your bottom line by selling / endorsing aftermarket parts.

It's brilliant.

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The Harley V-twin engine design dates from 1914. It was crude and otdated even then.

Pretty much every part of the engine design is a total cluster****. It is completely unbalanced due to the 110 degree cylinder angle (rather than the ideal 90 degree). It uses overhead valves which limit the rev range to 4500 rpm (power is a direct function of engine rpm) and promote inefficient combustion. It has a long stroke and small bore which reduces throttle response.

A modern V-twin liquid cooled sports bike engine is perfectly balanced and revs to over 9000 rpm. It uses DOHC valves and a high compression ratio. The stroke is short the bore is large to allow free revving. Power output is as much as 150 HP/litre.

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    FTW! It's an old design that hasn't changed/improved much over time. They now have electric starters so they can actually be ridden by "normal" people. Back in the day, you didn't have many accountants riding HDs to brunch, unlike now. All this is the result of design decisions made 100 years ago. BUT, if you want to ride an HD or been seen riding one, they are perfect. There is NOTHING wrong with riding and enjoying a HD, IMO. They have their limitations just like most things in life. – Tim Nevins Apr 5 '18 at 15:53
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Engine compression ratios (pdf) are the single biggest difference between modern car and motorbike engines. This is the major source of the greater power outputs from the much smaller engines in most motorbikes.

European cars tend to top out with a ratio around 10:1, though some of the new 1l ecoboost types are getting up to the 12:1 mark. A 2019 Honda CB650r has a ratio of 11.6:1 giving an equivalent 95hp output to the Harley. In your question you note that the CR for the Harley is 9:1 which is frankly pitiful for a motorbike, basically it's a 2 cylinder 1.8l car engine, not a bike engine, and hence you see car engine RPM ranges and power outputs for an engine of that size.

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