| Author | Message | ||
     Spatten1 |
Recently I've had the pleasure of dissasembling an XB9 engine which left me with a few questions. What limits RPM, thus power on these engines? My understanding is that it is typically one of two things- piston speed or valve float. 1) Piston speed (if that is the limiting factor)- what is the limitation on revs on these engines? Is it the siamessed rod configuation? Is it longer stroke necessetated for a two valve head? 2) Valve float (if that is the limiting factor): The XB hydraulic lifters are VERY HEAVY. An over head cam engine only deals with the reciprication weight of small/light valves and springs. The XB engine has a lifter that weighs more than a typical valve and spring put together. A second valve question is: Are you limited by the large and heavy valves in a two valve head, as opposed to four lighter valves? Next thought, if the piston speed is what limits RPM, then it would not matter if the valve train is heavy, as long as the valve train's limitations were at a greater RPM than maximum safe piston speed. In that case you might as well have self-adjusting lifters. | ||
| Hughlysses |
I've never seen it addressed on the XB9's, but Erik Buell himself was quoted in Cycle World as saying the roller bearing in the big end of the rods is what limits the RPM on XB12's. The engine can be revved higher than the factory set limits without immediately grenading, but the bearing life is drastically shortened (which I guess would eventually lead to a grenaded engine). I'd imagine at some (higher) RPM limit, the bearing becomes a limit on the XB9's as well. The XBRR shows that piston speed is not the limit. These engines were turning 8500-9000 RPM IIRC and I believe have about the same stroke as the XB9's. No doubt valve float would be a problem at some point; the XBRR's addressed this with titanium valves and very light weight pushrods (carbon fiber?). I'd bet other valve train components were changed as well. | ||
| Djkaplan |
The stock lifters, pushrods and rocker arms on a Sportster/Buell engine are heavier that their counterparts on a big block Chevrolet engine. | ||
| Aeroe |
Spatten1, I'm not totally sure, but I'd be willing to bet that if you were looking to increase the rpm's that much you're going to need to take care of both. Once you push the pistons, etc. past their limit, you're going to have to push the valves, then the pistons, then the valves. And so on and so on. | ||
| Alex |
First of all the heads/intake setup limit power. With reworked heads You get more power at the same rpm so pushing the red line shouldn´t be the first step. If You want to increase rpm on a xb12 the major problem is in the lower connecting rod bearings. The forces acting on the rods tend to deform the lower part of the rods making the lower rod races elliptical instead or round resulting in additional stress on the bearing rollers which leads to pitting and bearing failure. The same will happen to a xb9 engine but at higher rpm as the shorter stroke doesn´t produce the same forces on the rods according to less piston accelleration and decelleration. (Message edited by Alex on December 14, 2006) | ||
| Slaughter |
Assuming you're on the XB-9, it's motor over-bored to 3 13/16 makes an EXCELLENT race motor. Lighten/balance the reciprocating bits - flywheel/crank, rods, pistons - and she'll turn quicker. The weak point IS the bottom end. Hitting the rev limit at high RPM's beats the snot out of the bottom end. Stock lifters/rods work OK - though I've gone to semi-solids. Depending on my mood, I tend to keep redline at 8,000 You need to consider heavier springs, probably roller rockers - ESPECIALLY with higher lift cams and higher RPM's. If I had to build a race motor again, I'd still start with the short stroke motor (XB-9) and over-bore it. Throttle response far superior to the -12 and it makes good power. | ||
| Spatten1 |
I have no desire to do anything to hop my engine up. I just want it to hold together for me. If I decide that I still need more power after gearing down I'll probably have to get another bike. The cost of trying to make an XB engine run with a more modern engine combined with potentially less reliability has no appeal to me at all. I was just curious after seeing the internals. It is amazing that these engines perform as well as they do. Some improvements are obvious like the rods which are obviously very thoughtfully designed and the coatings on the pistons. The basic powerplant looks like it would have a governor and a PTO. The rockers were probably never designed to rev more than 5k, I mean they are HUGE cast pieces. The lifters are ridiculously heavy, the rod bearings are not made for speed, etc. I think that Buell has done very well with what they are given to work with, which was obviously designed to make 50 horsepower and push sportsters down the road. I'd bet the performance and reliability of the 9 and 12 is far beyond what anyone at HD every thought possible 10 years ago. | ||
| Trac95ker |
I think the limitation at higher rpms is the oiling system. XR750 flat trackers live at 8 to 9,000 rpm the whole race but have a different oiling system. | ||
| Spatten1 |
Aren't the XR750 engines completely different than the EVO sportster engines? | ||
| Diablobrian |
The valve train is not the limiting factor. The bottom end can only stand up to so much for so long on these motors. That is where the weakness lies. The upgraded "silver" main bearing is a worth while upgrade if you have your motor apart. Because of the size of the bore the pistons have to have a lot more mass than a comparable displacement In-line 4. This mass has to accelerate and stop at the top and bottom of each stroke. this puts tremendous amounts of pressure on the bottom end. There is a theoretical maximum speed for a piston as well, I forget what it is at the moment, but it is less than 1100 feet per second (roughly the speed of sound at sea level) With a long stroke motor it takes fewer rpm to reach this limiting point. Beyond that speed pistons and rods fail catastrophically very quickly. Then there are the side loads to think about too, the pistons are forced to follow the cylinders and with longer strokes the rod is deflected from this straight path to a far greater extent than in a short stroke motor because of the distance off of center that the crank journals are circling in causes the rod to try to push the piston sideways against the cylinder bore. That description is a little awkward, but a 3 1/2 inch stroke crank would have a journal that moved within 1 3/4 inch of center causing much more side loading than... a 2 inch stroke that only moves 1 inch off of center. | ||
| Reepicheep |
I think piston speed is limited by the ability of lubricants to maintain a film on the cylinder walls as well.. not sure how far that is beyond the other practical limits. An 8000 rpm larger bore XB9 sounds like a LOT of fun. | ||
| Rogue_biker |
I thought the XB9's do rev to 8,000 RPM? | ||
| Slaughter |
I thought the XB9's do rev to 8,000 RPM? I forget the stock redline (7800?) The difference is occasionally hitting above 7500 - versus continually running up around those RPM's - street v. racing. I can "afford" to be bouncing off of an 8300 redline because Bartels Racing does the work on the motor. If I paid for my work, I'd set the redline at 7500 and not worry. | ||
| Sloppy |
Piston speed is an excellent indicator of how long you want your engine to last -- hence, what your max. rpm should be. If you live for 1/4 mile drags, then you could run piston speeds to 6000+ fps. Then expect a daily rebuild. Road Racing, 5000 fps. This would require a weekly rebuild. But, if you only want to rebuild your engine every 200,000 miles, then you should plan on a more reasonable 3000 fps. So what limits rpm -- that would be where the design of the engine comes in. Since it's a long stroke motor, you are "forced" to keep rpm's lower due to piston speeds. Since rpm's are lower (relative to IL4's), you don't need the complexity of OHC -- instead you have CIB (Cam in Block) which typically is limited to 9000 rpm due to the weight of the valve train. But again, since you have a long stroke motor, you are quickly pushing piston speeds compared to IL4's due their shorter stroke. I'm not sure about the limitations of the bottom end of the engine as it is basically a two-stroke design and they can scream... so I'm assuming it's a function of weight. Ferarri's F1 used to use roller bearings (in the 80's?) for their bottom end and those engines really scream! I guess the take away here is the engine is designed as a series of components. If you want to change the application of the engine, then you need to look at all of the components. What's important here is not what the RPM of the engine is, but instead, what the Piston Speed is. If you compare all the competitors, you'll see that while RPM are significantly different, Piston Speeds are very similar. As an example: 2000 Buell 6500 max rpm. 4200 fps XBRR Buell 8000 max rpm 4200 fps (advertised) XBR Buell 7500 max rpm 4460 fps Yamah R1 10000 max rpm 3800 fps From this you can quickly see that while the R1 has greater engine speeds, the Buell's carry greater piston speeds and thus are under greater strain on their bottom end. Think about that the next time you run a Buell to the red zone... Buells are finely tuned machines, and it's impressive the Erik is able to get this much power AND efficiency out of a long stroke twin. | ||
| Spatten1 |
Great piston speed post. One thing I'd like to point out is that there are plenty of high revving V-Twins from many manufacturers. They are shorter stroke too. I think we do a disservice when we compare Buells just to IL4s when there are plenty of high performance Vs out there for comparrison. | ||
| Ducxl |
Ducati 996,yeah! 11,000 redline baby! And with bigger diameter pistons than my 1200cc Buells. Of course i still love my Buells' tho | ||
| Sloppy |
It's okay to compare the performance of IL4's with Twins as long as we look at peak air flow through the engine rather than displacement. As an example, a Buell Twin at 1200cc pumps as much air as a 600cc IL4, hence the power numbers for both types are "fairly" similar. However, a 1200cc IL4 will blow a 1200cc twin out of the water simply because the IL4 can pump more air. The adage of "no replacement for displacement" just doesn't ring true when you're talking about power... (Message edited by sloppy on December 14, 2006) |
