How far can you go including boring cases for a good streetable XB9R. I know its 1050cc roughly without case boring.

Im just curious how a short stroke 1200cc or more would compare to the longer stroke XB12 motor. Didnt the XBRR race only bike use a shorter stroke but a massive bore?

I know most people just seem to get the 1200 crank but i would like to hear the pros and cons of other options. The shorter stroke might hurt lowend but rev a little faster, is there any other major draw back?

A short stroke XB 1200-1300cc streetable motor just sounds interesting to me.

The 1050cc kits include 3 9/16" pistons and do not require crankcase boring. You can go to 3 13/16" bore but the cases will require boring. 3 13/16" is the biggest you can go in a stock Buell/Harley crankcase.

The Buell XBRR has a different crankcase that can handle the 4.079" pistons it uses. The cases were made by Buell and Eric got the racing people to allow him to use them in road racing, since they're not stock.

The horsepower that an engine makes is dependent on it's Piston Area, not it's displacement, not it's rpms, not it's torque, and not it's stroke. The difference that the stroke makes is in the rpm that the engine can rev to. This is usually measured in the avereage speed of the piston as it moves up and down and is generally limited to 4,000 ft/minute for street quality parts and up to 5,000 ft/minute for drag racing quality parts (crankcase, crankshaft, connecting rods, pistons, etc.). For a street engine with a 3.125" stroke the rpm limit is 7680 rpms and for a 3.81" stroke it's 6300 rpms. An engine with a lower rpm for it's power output will make a broader power bandwidth (more low end power) than an engine with a higher rpm power. So the XB12 crankshaft is better for the street because it can't rev as high but would give more low end power. However, Eric Buell needed different HP numbers for the XB12 than he got with the XB9, which would both make the same HP because they both use 3.5" pistons but at different rpms because of the different strokes. So he overreved the XB12 and made it's power at 6800 rpm, using a higher piston speed than the XB9 and reduced it's longevity. He knows this and put a rev limiter on the XB12 at 6300 rpms but with a 3 second delay so that drag racers could use the engine up to it's 7,000 rpm redline but road racers could not hold rpms above 6300 rpms for very long.

The most important thing to know about making HP is that it is dependent on Piston Area. The Japaneese 4 cylinder 600 cc engines have about 22 sq in of piston area (about 125 HP), the Ducati 1000 V-Twins have about 24 to 26 sq in (about 130 to 160 HP), even the Harley V-Rod has about 24 sq in of piston area (115 HP). But the 3.5" bore Buells have only about 19 sq in (about 92 HP without over reving the XB9 and 103 with over reving the XB12). Stock Buells can never make as much HP as a Ducati or a V-Rod but the Buell XBRR has 26 sq in of piston area and can make about 120-130 HP in street trim (they make 150 runing on racing gasoline). The Buell does have a disadvantage because it is air cooled and the others are liquid cooled allowing them higher compression ratios and still run on street gasoline.

The thing about street engines is broad power bandwidth. It is achieved with a low rpm engine. Long strokes require low rpms and make the best street engines.

What Eric Buell needs to make for us is a bike with the XBRR crankcases (needs a new frame to hold the engine) with it's 4+" pistons and the XB12 long stroke. He should build 4 valve per cylinder heads to let it breath. This engine could have a 6300 rpm redline with lots of bottom end power and provide about 120 HP at the engine and run with pushrods and not need DOHCs. He should also look into liquid cooling because it will provide even higher compression ratios and more engine efficiency and more power.

The best engine for the street would be a big American V-Twin with lots of piston area and a 4" stroke with a low 6,000 rpm redline with liquid cooling making 130 (or more) HP. The XBRR has started to show the way.

Rod
XB9SX, 2006 Buell
XV920R, 1981 Yamaha

Thank you.

I ride a VTX and my friends XB9R since his work takes him out of town for months. The VTX has a 4"+ bore and stroke but the long stroke redlines at 5650rpms and i really like the way the loaner XB9 revs.

Im just trying to decide if i want to build a XB9 or just buy a XB12 and be happy with just the typical stage one. I know both have a higher redline than my VTX.

Im still waiting to see if the XR1200 is actually released in the US first but for now im leaning real hard towards the XB12X Uly.