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Strato9r
| | Posted on Monday, August 18, 2008 - 03:38 am: | 
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If I were to do a project like this, I'd follow the path of least resistance, and build an 88 cubic inch (3.813" bore) engine, based on the stock 3.813" stroke crank of the 12. I certainly would not get wrapped up in thinking that really long rods are going to make a big difference, they may make a little bit on a full bore race engine, but on the street, I wouldn't want to push things much past the stock redline anyway. Besides, an engine like an 88 in an XB is going to have so freaking much torque, you won't NEED to rev it to the moon...... |
Bombardier
| | Posted on Monday, August 18, 2008 - 05:45 am: |
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Remember reading that to get more peak power high in the rev range you emplo a longer rod with the gudgeon pin higher in the piston with the same stroke length.
The piston will stay at TDC longer and get a better burn.
Perhaps if you used a set of pistons with the higher gudgeon position instead of the XB9 ones you would still have streetable comp ratios and the peak hp? |
Strato9r
| | Posted on Tuesday, August 19, 2008 - 12:27 am: |
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You could definitely use a longer rod and a piston with a shorter compression distance to increase the rod to stroke ratio, but, as delivered, the 9 and the 12 pistons both have the same compression distance, the 9 piston uses a dome to keep the same compression ratio as the flat top 12 slug. Using the 7.113" S+S rods would mean using a piston with a compression distance of around 1.080", which is about the same as the 96 inch Twin Cam piston, (although the Twin Cam wrist pin is quite a bit larger in diameter; .927", just like a small block Chev.) Using the 7.27" 9 rod would make for a REALLY short compression distance, with the wristpin intersecting the ring lands. I guess you could use the old school method of using spacers under the cylinders, the correct thickness allowing you to use a readily available piston to work with whatever rod you wanted to use, but that means correspondingly longer pushrods, along with intake manifold spacers, possible exhaust system mods, and then getting it to fit where the stock engine was mounted. I would actually like to see some test results on a 12 where the stock length rods could be compared to a longer rod engine, just to see what the difference actually winds up being, as I am a disciple of the Smokey Yunick school of thought when it comes to high rod to stroke ratios. That being said, I've built a lot of 5.63" rod/ 3.75" stroke Small Block Chevys and 6.135" rod/ 4.25" stroke Big Blocks that seriously kicked butt with what would appear on paper to be pathetic, sub 1.5-1 rod to stroke ratios. No, they weren't high RPM screamers, but so much work got done right off of idle with these things that what happened after 6500 was just anti- climactic. For a really good analysis of rod to stroke ratios, you may want to check the Isky Cams website. In the tech section there is a really well written article called "Much Ado About Nothing" that addresses this issue. |
Rick_a
| | Posted on Wednesday, August 20, 2008 - 02:51 pm: |
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12.3:1 would probably be ok for a track bike where the bike is always moving but stop and go traffic may be a different story. I was using 10.5:1 here in Florida and even backing off the timing 5 degrees from optimum did not help prevent pinging. It was only in issue in slow traffic when accelerating from a stop. I've since gone with a 1250 kit and 10.2:1.
Keeping things simple and relatively bolt-on without any wild re-engineering or machine work, I believe an XB9 crank with 3-13/16" big bore (around 1170cc, I believe) with heads machined for Thunderstorm pistons and oversized valves, headwork, and a Red_Shift 585 or similar would be about ideal. Big power, good torque, and good to 7500 RPM. |
Strato9r
| | Posted on Friday, August 22, 2008 - 12:05 am: |
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I really like the looks of that combo, too. To have more of what a 9 does really well; that sweet rush from torque peak to redline, that would rock. Only problem is, it runs so incredibly well right now, with no undue mechanical clatter, and literally zero oil consumption, that I cant really justify doing it at the moment. I've started putting aside a bit at a time to buy a complete kit, though, it's the cheapest way to do it. Then the next time the shop is slow in the winter, I'll just work it through like any other job and get er done for spring.
You know, it's easy to forget when most of your riding is done up here on the prairies at over 3000 feet, then higher into the rockies, what it must be like at sea level, and hot like it is in Florida. My bike has never detonated, no matter how hot it gets. I guess the trade off is that you make a little more power at sea level, once you are out of traffic! |
Jmr1283
| | Posted on Sunday, August 24, 2008 - 10:58 am: |
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I have to say a xb 9 with that set up would be sweat. The 585's are great, great power and easy on the valve train. |
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