
A 55.8 Increase in Horsepower After Big Bore Upgrade! Can This be Right?
So, it wasn’t just the fact that I had bigger jugs I could put on my bike right now and reap the benefits of a larger displacement in hopefully more horsepower and quickness. But, with this idea in mind, I decided to dig little deeper into finding out what kind of horsepower upgrade I would be looking at, considering I am not replacing the cams, using stock valves and heads, and leaving the current K&N intake intact.
I had sent some eBay-bought cylinders for my Road Star to Patrick Racing to be reconditioned and to also be bored out to a 1775 cc configuration. The process began with an inquiry with Pacific Coast Cruisers website and a conversation with Greg.
He gave me the address of where to ship the cylinders to Patrick Racing in Prescott, AZ and about eight weeks later, they came back to me in pristine condition. The kit also included new JE piston heads, wrist pins, rings, and retaining clips.
I was really stoked about getting them back and was wanting to pull the other cylinders off and put the newer – bigger ones on right away. But, rationality got the better of me and I figured I had the new cylinders ready to go if and when the ones on the bike ever needed to be replaced. (I just rebuilt the engine that is on it now less than a year ago and I don’t even have 10k on it yet).

So, it wasn’t just the fact that I had bigger jugs I could put on my bike right now and reap the benefits of a larger displacement in hopefully more horsepower and quickness. With this idea in mind, I decided to dig a little deeper into what kind of horsepower upgrade I would be looking at, considering I am not replacing the cams, using stock valves and heads, and leaving the current K&N intake intact.
I started comparing the Yamaha “Star” line of motorcycles to see exactly how they were upgraded from the factory and what kind of jump in hp and torque I could expect to see with the cylinder/piston upgrade. The comparison is looking only at the bore diameter and stroke length and not taking into consideration different cams that are used in each configuration. So, here we go.
The first choice should be the obvious one and is the stock version of the daily rider I currently own and it is the 1999-2003 Yamaha Roadstar 1600. The specs on this bike are as follows:

1999 Yamaha Road Star 1600:
Engine type: 2 cylinders, 4-stroke, 48° V-twin
Displacement: 1602 cc (97.88 cubic inches)
Bore × stroke: 3.74 inch × 4.45 inch (95x113 mm)
HP: 62.13 HP (45.7 kW) @ 4000 rpm
Torque: 134 Nm (98.83 lb-ft) @ 2250 rpm
Carbureted: 40mm Mikuni Constant-Velocity with TPS (throttle position sensor)
At the time this bike made its debut in 1999, a Harley Davidson Road King rocked 67 hp with a 1450 cc air cooled v-twin with fuel injection but boasted only 81 foot-pounds of torque at 3100 rpm.
The base price for the Road King was $15,725.00 as compared to the Road Star’s $10,099.00 sticker price. However, this post is about the Yamaha line.
So, during 2002 the Star division of Yamaha came out with a bike with a little more displacement at 1670cc but it was still Mikuni carbureted. The horsepower increased slightly as well with the larger displacement.

2004 Yamaha Road Star 1700:
Engine type: 2 cylinders, 4 stroke, 48° V-Twin
Displacement: 1670 cc (101.9 cubic inches)
Bore × stroke: 3.82 inch × 4.45 inch (97x113 mm)
Hp: 84.3 HP (62 kW) @ 4400 rpm
Torque: 135 Nm (99.57 lb-ft) @ 3750 rpm
Carbureted: 40mm Mikuni Constant-Velocity with TPS (throttle position sensor)
So, from the 1602cc, 95x113 engine to the 1670cc, 97x113 displacement, the horsepower gain was 22.17 along with the increase of 68 cc’s. This equals to about a 3.1 horsepower gain to 1 gain in cc. So, approximately a 3.1:1 ratio.
Also, during this time the Star division began producing the Road Star Warrior 1700 which has identical numbers to the Roadstar 1700, only it was fuel injected instead of carbureted.
The Warrior also claims a slightly different configuration than its sister with a wider rear tire among other cosmetic differences.

2004 Road Star Warrior 1700:
Engine type: 2 cylinders, 4 stroke, 48° V-Twin
Displacement: 1670 cc (101.9 cubic inches)
Bore × stroke: 3.82 inch × 4.45 inch (97x113mm)
Hp: 84.3 HP (62 kW) @ 4400 rpm
Torque: 135 Nm (99.57 lb-ft) @ 3750 rpm
Fuel Inj.
In 2006 the Star division began producing the Stratoliner and Roadliner models of a large displacement cruiser that claimed 1854cc’s or 113.9 ci. Here is where the similarities in the Star line stop because the Startoliner incorporates oversized intake and output valves as well as a longer stroke at 118mm. The bore diameter is also increased to 100mm instead of 97mm.

2006 Yamaha Stratoliner XV 1900:
Engine type: 2 cylinders, 4 stroke, 48° V-Twin
Displacement: 1854 cc (113.9 cubic inches)
Bore × stroke: 100 × 118mm
Hp: 98.6 HP (72.0 kW) @ 42500 rpm
Torque: 166.7 Nm (123 lb-ft) @ 2250 rpm
Fuel Inj.
At the time of their release in 2006, the Star Stratoliner and Roadliner motorcycles were state-of-the-art and at the top of the food chain for that year. No other cruiser (including Harley Davidson) could compete with these pinnacles of Japanese engineering.
These powertrains would go on to become standard equipment on further Star line of cruisers – most notably the Star Venture and Star Eluder models. So, the jump in horsepower to the 1854 cc powerplant is just under 100 at 98.6.

However, it still shows a significant increase from the 84.3 in the 1700. But, to get this increase, the valves and other components have been modified beyond what I intend to do with my 1600. Also, the stroke length has been increased from 113 to 118mm.
If I stay within the stock formula for my bike and go with just opening up the cylinder bore to 100 mm (which is what it is after the modification from Patrick Racing) and keeping the stroke length at 113, I still have the ratio that I came up with earlier which is the 3.1:1.
So, if by opening up the cylinder to 100 mm increases the cc’s of my bike to 1775, I can then subtract the larger (1775) from the smaller (1602) and get a difference of 173. Dividing the 173cc difference by 3.1 gives me 55.8.
This number would then be added to the baseline 62.13 horsepower of the stock 1600cc engine which would equal approximately 117.9 horsepower.
I highly doubt that this will be the actual horsepower output of the engine and I won’t know the actual output without putting the bike on a dyno, but it gives me an approximation that I can live with until the dyno can be accomplished.
Thanks for stopping by today and I hope you found the article informative and/or entertaining. If you have something to share about today’s post, please leave me a comment below. Until next time – Happy Living!

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