| Author | Message | ||
     F1fletch |
Got an email from Erik Buell Racing last night with some questions about an XB12 r project. How to make it more responsive and rev higher ala XB9r. He suggested XB9 pistons. Can someone explain the result, shorter stroke, lighter weight??? | ||
| Greg_e |
The pistons will raise the compression ratio which makes more power (more efficient use of the fuel). I don't think it will change the rev limits as that is based on piston speed and valve train, but it will give you a little boost. | ||
| Chessm |
it's so you can say that you have a hemi | ||
| Reepicheep |
Pistons? Or Pistons and Crank? If you change the crank to a shorter stroke crank, you can get a lot more rev's out of it, and make a lot more power. So an 1150cc short stroke XB9 setup (bigger piston moving through a shorter stroke) will rev up to 7800 RPM or more. Power comes down to torque times RPM, so even if the torque is lower you make it up as the revs climb. Peddling half is hard wins, if I am peddling three times as fast. What limits maximum RPM is the ability of oil to maintain a film as the piston moves across the cylinder. A long stroke motor gives the air and fuel a LOT of time to mix well, so you get good torque and good fuel economy. But for a given engine revolution, it also covers more surface area, meaning it has to cover it faster. So a long stroke motor (like the XB12) will be pushing the limits of the oil film strength at 6800 RPM or something. The short stroke motor (like the XB9) will move a shorter distance with each rev, and so can move slower, meaning you can rev the motor higher. A short stroke 1150cc motor will make more peak power than a long stroke 1200cc motor. This is why those 600 cc inline fours can rev past 15,000 RPM. In addition to being WAY short stroke to start with, they have 4 cylinders, so each has to sweep only 1/2 of the distance a twin would have to sweep. That advantage of revs is why a torqeless 600cc inline four can make 120HP. | ||
| F1fletch |
So I am still a bit confused. The pistons will just net a bit more compression but no increase in rev's. Here is my question and answer from Erik Buell Racing; Without completely rebuilding a fairly fresh motor such as this (12R with 2kmiles), how can I get quicker spin up. higher RPM limit 7500-8000 and a bit more HP. The easy stuff is the airbox, drummer exhaust and your ECU, but after that what next, head work, con rods, lightweight pistons? I understand the beehive springs in the stock XB9R heads work well and changing them is not the first best order if it is not full race (which it will not be). Your expertise is appreciated. Their Answer: Engine mods on XB12's should be limited. Without exception, the people who have tried to push these to high power spend a lot of money and break a lot of parts. Airbox, exhaust and ECM are all good, if you want to go into the motor and put in XB9 pistons that will work. Anything much beyond this and it becomes a money pit. The long stroke just will not allow much for RPM. The XBRR ran the stroke of the XB9, but had massive changes to everything in the motor and still was not reliable. So in the end, an XB9R was my first choice but the deal on the 12 looks too good to pass up. So is he saying without a complete rebuild making it a short stroke just change pistons? Seems like I need a 1050 kit and short stroke crank to do it right.... | ||
| Dynosaur |
Hmm, and there I was thinking that long strokes are limited in RPM because of (i) ring flutter caused by piston acceleration and (ii) you are sucking the air in through a smaller hole as, for the same capacity, you get a smaller bore and hence smaller area where to put your valves. This oil theory is a new one on me... As for using XB9 pistons in a 12; you will end up with a compression ratio that will challenge pump gas (and your big end bearing). Do a search on the site, I am certain other people have done it already... | ||
| Reepicheep |
Nice response! Very clear and frank, the kind of people *I* want to do business with! So yes, sounds like you got it. The 9 pistons give you a little more compression, which will run a little more risk of pinging (predetonation), but will make a little more power. You can mitigate the risk of predetonation a little by running higher octane gas, and by running at cooler temperatures. Next step would probably be taking a motor (either 9 or 12) and machining the cases and making it a short stroke 1350. That would probably be the "money pit" step... though it would be fun when it's running!  The other option that I would have laughed off a while ago, would be a turbo. Eric did it with great results, and the factory had a really nicely sorted one that was too scary for the Harley execs too release to the public. | ||
| Reepicheep |
(and I could be wrong about oil film strength being the limiting factor on our motors... I am not an expert on the topic). | ||
| Greg_e |
All I've ever read is that the faster the piston moves in the cylinder, the lower the final RPM. Longer stroke means it must move faster to cover the displacement. Valve spring float and distortion also come into play which is one reason why Titanium springs and airbag inserts are in use. Lighter valves (titanium again) also help since there is less mass to try and turn around. As far as making it rev quicker, lightening the rotating parts always helps when in neutral and somewhat when in gear. Lightening the entire machine will help it spin up much faster when in gear, the motor can only go as fast as the rear wheel lets it go when in gear, so you need to fix acceleration issues at a very basic level. Reducing the gearing (giving more leverage over the tire) will also let you spin up faster with the decrease in top speed. Inertia, it is your enemy. | ||
| F1fletch |
Erik Buell Racing's response back was; XB9 pistons bolt right in with no other changes, and they increase the compression ratio. You really must run premium fuel, and a good race map, but it is a very inexpensive power upgrade without the risk of higher RPM's which are the kiss of death to 1200's. Looks like the first step before conversion to a short stroke build. Wanna V8. Sounds like my LS2 GTO 400/400...lol | ||
| Firebolteric_ma |
You'll never use all the power. | ||
| Steve_a |
Because a 12 is just a stroked 9, both run 3.500 inch bore pistons. The 12 is dished, the 9 is flat-topped. A 9 piston in a 12 bumps compression about a point. A few engines were built with this configuration that produced about 100 hp at the rear wheel with a stock Xb exhaust. Good gas is a must, and ideally you'd have an 08 or later bottom end with the stouter big end bearing. | ||
| F1fletch |
Should I be concerned about the bottom end in the 04 model? Right now it has 2k miles on it but has already had a pipe airbox mod and ecu put on. | ||
| Sifo |
I would expect that any modifications to the engine would imply an ECM tune to make it work well, or in some cases to not create a grenade. Bumping the compression would be an example of that. I would imagine that it would have to be "de-tuned" just a bit to keep it in one piece using pump gas. It wouldn't be worth the time and expense to me personally. The best thing I can think of to improve acceleration on my XB12 is to loose 30 pounds of fat. I don't really consider the XB to be a motor that I would hot rod. It's an old design that already is hot rodded. If you really must though you could look at a cam change. I don't have a recommendation personally, but it's easy to do and a wide variety are available from a number of sources. That might go well with higher compression, especially if you find one designed to go with that mod. You can lighten up the components in the primary case simply by spending money. Plus when the primary explodes it's cheaper to fix! The reality is that the best bang for the buck would be to sell your XB while it's basically stock and by an 1125 or any other stupid fast bike that will be stone cold reliable while being fast. You can still get some bargains on 1125s in this area that were used as demo bikes. | ||
| Sifo |
Aren't there kits to put in slightly domed pistons with some minor rework of the head? I seem to recall reading about that some years ago. It's supposed to provide a better squish area in the combustion chamber as well as higher compression. | ||
| Aptbldr |
Will raising compression necessarily, XB9 pistons, increase heat? | ||
| Sifo |
This is the sort of piston/head set up I was talking about...  Will raising compression necessarily, XB9 pistons, increase heat? Most things that create more power create more heat too. | ||
| Dynosaur |
Because a 12 is just a stroked 9, both run 3.500 inch bore pistons. The 12 is dished, the 9 is flat-topped. A 9 piston in a 12 bumps compression about a point. Have you ever seen an XB piston, be it 9 or 12? Neither is flat and neither is dished... As for going up a point, I think you will find it is more than that, and we can work it out from Buell data... The service manual tells me both are 10:1 compression ratio, the 9 is 984cc (492 per cyl), the 12 is 1203cc (601.5 per cyl). The important thing to note is that the cylinder head is the same part (so are the cylinders and pushrods - but the stroke is shorter and the con-rod longer). (swept volume + combustion chamber volume) / combustion chamber volume = 10 combustion chamber volume = swept volume / 9 So, for the 9, the combustion chamber volume is 492/9 = 54.7cc For the 12, the combustion chamber volume is 601.5/9 = 66.8cc The difference between these is the difference in the lump of metal on the piston crown between the 9 and the 12. If you took the 9 piston and used it with a 12 crank and con-rod, your compression ratio would be: (601.5 + 54.7) / 54.7 = 12:1 Pull the squish down to 30 thou (but machine the buldging eyebrows down - those who have seen the 9 piston will know what I mean), and you push this up towards 13:1. Bit more than the odd point eh? More importantly, quite high for a large bore single plugged head that makes close to peak torque from around 3000 RPM and up. If it was do-able with pump gas, why didn't Buell re-use the 9 piston rather than invest in a new piston design for the 12? | ||
| Fast1075 |
In simplified terms the power output of an engine is determined by how much cylinder pressure you can build. How much Horsepower you get is determined by how well that cylinder pressure is converted to crankshaft torque and how high the engine can be revved. The stock cams in an XB engine would be "hotrod" cams in a Sportster...they have more duration and overlap...the increased compression ratio helps the engine develop more cylinder pressure in the lower rpm ranges where the long duration cams cause more blowdown than "milder" cams do...more cylinder pressure=more torque.... The warning about using good fuel is due to the comparatively poor quench and low charge turbulence....the bathtub chambers are not the ideal solution... E B R advice to not even try to twist a '12 too hard is some sound advice (if you are looking to have a motor with some SERIOUS power. I truly love my Lightning...it is one freakin awesome machine for street riding. The torque and drive is amazing...but the engine is already as massaged as is reliable...Erik and crew did a good job of taking what is essentially a turn of the century lawnmower engine into something fairly amazing... | ||
| Gentleman_jon |
how can I get quicker spin up. higher RPM limit 7500-8000 and a bit more HP. very simple really. Two easy steps. 1. Sell Buell. 2. Buy Ducati. This of course assumes that you are in the 1% of the ridding public that can actually use more than 85 hp at the rear wheel. If you feel that you are in the other 99% , there are two other easy steps to going faster. 1. Get a good set of leathers. 2. Do as many track days as you can. Get back to us when you make Expert. OK?  | ||
| F1fletch |
I have already had two 1098's and just cannot afford that expensive of a bike any longer. I agree with the comment most cannot use all the power they have and track days and classes certainly help. My original comments and questions were about making a 12r have more of the character of the short stroke mill and for all practical purposes it cannot be done. Each bike has it's charm and positives. The bike I plan to get Is a project bike so if funds permit down the road I would like to build a short stroke 1050 Mill either seperately or out of the 1203 mill (Message edited by F1fletch on June 19, 2010) | ||
| Greg_e |
Then maybe you should just start with the 9R and build it up from there. | ||
| Strato9r |
Piston speed, and piston acceleration/deceleration on a 12 is one of the primary limiting factors. 3.813" is a lot of stroke, more than a 427 Chev or Ford, or a 426 Chrysler hemi. Coupled with the fact that the 12 has a pretty short connecting rod, and you are working with an engine that will be using so much power to overcome rotational resistance, that any power that may have been gained by a higher compression ratio wont be able to show a real gain at the rear wheel, if increased RPM is the goal. I think the factory did a great job on the 12, though; it has awesome grunt at nearly any rpm, and I really dig it in a Lightning or Uly. | ||
| Eldredma |
I was recently doing quite a bit of studying on this exact topic. Increasing the compression of a 12 by using 9 pistons yields a CR of about 12 (as mentioned above and by many folks who have done this throughout the history of this site). However, engine builders will know that increasing from a CR of 10 to a CR of 12 only yields 6-8% BHP or about 7 hp at the crank on a 12. To really take advantage of this though, you would need the best head porting, combustion chamber machining (think squish bands) and exhaust you can afford (see NRHS Performance's articles on this as they are very informative and extensive) and you might as well get "hotter" cams, better springs and larger valves while your in there. As mentioned, all of this will require the best gas you can afford. RPMs are limited by the various effects of piston speed, so red line is red line unless you change the stroke; especially in the weaker 04 model. I'm not as educated about what can be done with a 9 motor, but if you want higher revs and plan on using them enough to make it worth it, the 9 sounds like a better starting point. Either way though, revs in a pushrod engine only lead to heartache. I recently asked Erik what one mod he would do to an XB. He told me that he had a 9 that he did a bunch of engine work to and felt that it "ruined the bike". It was loud, difficult to ride and just not as much fun. His answer was to upgrade the brakes to ZTL2 as he felt that was the only part of the bike he ever over-rode. Now, if you think you can out-ride Erik Buell on his own motorcycle, do whatever you'd like, but hearing that from him made me feel a lot more satisfied with my motor and a lot less satisfied with my riding skills, so I've chosen to work on the second one instead. Of course, if all you want to do is build an engine with impressive specs on paper and don't care if your hard work, money and polish bends a valve stem or pushrod and you have to start all over again... | ||
| F1fletch |
Lots of great comments guys thanks for the feedback. I would love to start with an 07 9R but as facts would have it the bike I should be riding home Friday is a 12R. I have learned a lot and will probably avoid any real engine work unless I can build up a short stroker down the road. Thanks guys | ||
| Strato9r |
Cool whitewalls, Matt! That is one of the funkiest Lightnings I've ever seen! | ||
| Tootal |
What are you trying to achieve? Quicker acceleration or higher top speed? If your after acceleration then just install the XB9 primary gearing and wheelie away! | ||
| Pkforbes87 |
Nevermind. Old thread.  (Message edited by pkforbes87 on April 04, 2011) |
