| Author | Message | ||
     Gearheaderiko |
I think you may be right! (or I'm wrong-I know that makes some people happy!) Studied the specs of the XL883, XL1200, XB9 & XB12 and when I remembered that the Blast uses a Sportster piston (which is accepted as fact) and it has to be an XL1200 piston, it all made sense. Only ways I can think of to make the XL1200 piston work (and have all cylinders and heads interchangeable)was to lengthen the con rod (as you suggested) or change the stroke (we wish). Rather than make a one off Blast piston, it does make more sense to change the crank assembly as it can only work in the Blast anyway. I know con rod specs are out there, I just dont know where. If you already have the XB12 piston, do you have a Blast piston to compare too? I can measure one if you'd like. I also have spare XB and Blast pistons, if you'd like me to compare the too. I'd like to help shorten your assembly time too. I do wonder why my piston box says XB9 though and I'm fairly certain the compression ratio wasnt 12 or 13+:1. I dont think that would run well on 91 pump gas (unless I had really radical cams-and I dont). The compression tests only showed 210psi, which matched the Blast with a 10.5:1 piston. | ||
| Gearheaderiko |
PS I feel a new thread coming on! | ||
| Berkshire |
LOL - a new thread? and spoil the tradition of long mixed-up rambling threads?! but yeah, a place somewhere for specifications might be useful! Hey, looks like you've righted yourself again - yea! I'd forgotten about the compression height aspect of the P3 piston being a leftover 1203 t-storm part. I guess the piston for a 10.5:1 XB-headed Blast would have an XB12 piston compression height, with the dome of a 10.5:1 XB9 piston. Maybe there's an off-the-shelf 11:1 XB12 piston that would fit the bill. If the one I have comes out to 10.2, that should still work pretty well - it'd be a bit higher than stock XB compression, and we've got thick air down here. Unfortunately, the only Blast piston I have for measuring is the one that's in my bike now. | ||
| Berkshire |
I wonder whether Blast flywheels are nice lightweight XB9 parts, or some kind of de-stroked old knucklehead stuff or something...? It occurs to me that the Blast connecting rod must be quite long in relation to the stroke. I think the the ideal rod length for a high revving 3.125" stroke would be somewhere around 5.75", but that might take some flywheel clearancing, which would mean lightening, which would also be good for revs. The shorter connecting rod would weigh less, as would the shorter pushrods - again, reduced reciprocating mass for better revving. The cut-down cylinder would drop a few ounces too... Combining this with the previously proposed larger cylinder/head bolt pattern for a 4-inch bore would be an obvious choice. I wonder, would that be the big twin bolt pattern? Do twin-cam heads flow good? The possibilities boggle the mind!  | ||
| Fast1075 |
long stroke/short rod = bad Mojo...piston deflection/high skirt sideloads and frictional power losses, difficulty getting good piston/cylinder clearances, possibly problems with skirt lenght... BUT not as much of a problem with such a low revving engine as a P3...but still worth mentioning. And I also think the information is way too hard to find...I cant believe that someone hasn't researched this a billion times (maybe less  ) More likely it's a case of "I figured it out, and if you want to know...pay me".  | ||
| Sarodude |
From what I recall, the Blast rod length is pretty much exactly what the numbers would tell you if you were trying to make up for the difference of piston crown height relative to the top of the cylinder deck on the 3 1/8" stroke vs the 3 13/16" stroke. I believe if you take 1/2 the difference in the stroke and add it to the length of the long stroke rod, you'd get the length of the short stroke (Blast / XB9) rod. -Saro | ||
| Berkshire |
Fast, the Blast doesn't have a long stroke, it has a very short stroke (3-1/8") and a very long rod, and I'm not talking about making it super-short like a 400 Chevy rod (about 1.5x the stroke), I'm talking about making it closer to the "optimum" 1.8 ratio, but still a bit longish, for lower peak piston speed and better revving. As it is, it has a super-long rod, which is overly heavy. The Blast doesn't have to be a low-revving motor - with a short stroke and small displacement, revving is the obvious path to power. Saro, yes, that is exactly where the long rod comes from - they wanted to use the same piston & jug as a 1203, but with the shorter stroke that meant a longer rod was needed to reach from the TDC crank pin to the TDC piston pin. With both a longer rod and a shorter stroke than the 1203, the rod/stroke ratio is somewhere above 2:1, which gives low flow velocity thru the head and means extra reciprocating mass. The piston skirt barely even gets down to the spigot part of the cylinder, so... shorter, lighter rod moves it down to the bottom, cut off the top two cylinder fins so it's zero-deck at TDC - it's all gravy! Instead of using a spacer on the front engine mount, raise the front of the engine - that would make up for the front part of the crankcase hanging down so low! | ||
| Ezblast |
Gravey.....hmmmmm - yum - lol - This is getting interesting - more details! EZ | ||
| Reuel |
I'm confused. 210 PSI for 10.5:1? I thought that the ratio worked with standard pressure, which is 14.7 PSI at sea level. 2:1 would be 14.7, and 10.5:1 gives you 139 PSI. I never actually saw anything written about compression ratio--I just assumed the above. Can someone enlighten me? | ||
| Gearheaderiko |
One of us doesnt understand the question.  I'm sure that for any given compression ratio there is an optimum PSI that can be achieved if everything is perfect. However with engine wear etc., the actual PSI will be less. So you could find the minimum compression ratio an engine would be if given the PSI reading, but you couldn't actually tell what its supposed to be if everything was in perfect order. If any of the engineers on the board would like to explain it better, feel free. Or maybe I didnt understand the question? The above assumes that the cams have no overlap. | ||
| Gearheaderiko |
"I believe if you take 1/2 the difference in the stroke and add it to the length of the long stroke rod, you'd get the length of the short stroke (Blast / XB9) rod." We dont at this point believe the Blast and XB share the same length connecting rod. At this point its conjecture unless we actually measure the conrods or get some specs from Buell. I'm pretty sure the various engine builders know the answer (as well as others) and we can surmise the answer from parts cross referencing, but it would be nice, at this point, to say without doubt, what fits what and why. I had been going on the assumption that the Blast and XB9 share the same relative piston and connecting rod specs, since they have the same bore and stroke. This apparently is not the case. | ||
| Reuel |
Ok. Here's the question. After taking into account the actual compression, not the BDC to TDC compression ratio, as in after cam overlap and all that stuff is factored in, what's the PSI per point of compression? I thought it was 14.7 PSI, since that's standard pressure at sea level, not accounting for weather variances. (Message edited by reuel on August 03, 2008) | ||
| Berkshire |
There's no conversion factor between mechanical compression ratio (BDC to TDC) and the pressure you'd measure with a compression tester - engines with really high compression ratios can make less pressure on a tester than low-compression engines. You could figure static compression ratio - the difference in volume from where the intake valve closes to TDC, but even that doesn't equal the "test pressure" multiplied by 14.7, because it doesn't take the dynamics of intake tract flow and compression into account - the cylinder won't be at atmospheric pressure when the intake valve closes because the piston is moving and pushing a pressure wave ahead of it. At the same time, the air/fuel moving thru the intake has inertia, which keeps it pushing into the cylinder even after the piston starts back up. At some point the two air masses meet and the presure wants to go back out thru the valve and on out the carburetor - and sometimes it does! So, did the valve close in time to prevent that from hapening? depends on valve timing, RPM, and who knows what else, but one thing is sure - all this stuff affects test pressure and there are more obliquely related variables than you can shake a stick at... Lookit: a "tuned" intake port has a ram-air effect that can deliver more than 100% volumetric efficency in an N/A motor under certain conditions - 5.0 Mustang, anyone?! Basically, a compression tester is good for telling if you have cylinder sealing problems, period. I'm wondering where you got that 210 psi figure - the only similar number I see above is "2:1", which is a rod length-to-stroke length ratio, meaning the rod is twice as long as the stroke - a 3-1/8" stroke and rod that's 6-1/4" long. I think the Blast rod is even longer than that, but I don't know for sure... | ||
| Gearheaderiko |
He got 210PSI from my post-what my Blasts tested out with 10.5:1 pistons. When cranking an engine over most of the "tuned intake port" or "ram air",etc. isnt go to come into effect. If you just study one engine with similar modifications, a compression test will give some idea of what the actual (static) compression is. That usually means a test of various engines has already been done. It can also give you an idea of what grade of gasoline you can get away with. But, no, you cant just take any engine, do a compression test and tell what the ratio is, not at least without further information. I also read (on the web) about the 14.7 formula, but it didnt correspond to actual reality numbers. Perhaps the question is if you take a perfect cylinder and piston with absolute seal and slowly compress the air inside, what would the compression PSI be at say 2:1,3:1,4:1,etc. I've heard the answer before, but I dont know where to find the info. | ||
| Reuel |
Ok. My assumption was correct, but did not involve buttloads of other variables. | ||
| Fast1075 |
My mud truck (jeep) has a 2.5 four banger in it..the bore is 3.87 with a 3.19 stroke...it has the whole gamit of possible modifications and will reliably turn to 8K+ rpm...toss in a 75hp nitrous shot for grins and giggles. The upper limits for rpm (assuming the rod strenght is up to it) is the piston speed (generally around 4500fps)...no problem there...I have a nice easy to use calculator for that...I think the weak links in the Blast engine would be the main bearings and rocker arm strenght...assuming the needle bearing rod end will handle the pounding and the rod is a very good forging...Eventually, I will find the upper limit....it's my nature. | ||
| Reuel |
I'm thinking the top speed was 8400 RPM. Someone here figured it out a while back. | ||
| Gearheaderiko |
Berki: Flawed logic. A knowledgeable Badweber pointed out to me the one missing puzzle piece. If you increase the stroke and use the same piston, you increase the compression ratio. Therefore the XB12 piston must be different, less of a dome, to compensate for the added compression. So both pistons may still be interchangeable. However, adding an XB12 piston to an XB9 lowers the CR. Using the XB9 piston in an XB12 raises the CR. So if the XB9 & P3 bore and stroke is the same and the con rods are the same length, you need an XB9 piston in this scenario for a Blast. That would also explain why my piston box says XB9. I'm still unclear as what XL parts match with the Blast, 10 deg. domes and 15 deg. domes etc., but it seems a minor quandary at this point. Still would like to have facts, based on actual specs would be nice! Would also be nice to know what pistons would yield what compression ratios. I'm sure Buell/HD pistons are cheaper than CP or Wiseco! Happy to hear contrary (or any) opinions or facts. I'm also sure this info is covered on the twin side, XB forum. Thanks Saro! | ||
| Berkshire |
That puzzle piece wasn't missing - I knew it from hot rodding Ford V-8's: a given set of heads will yield a higher compression ratio on a 400 than a 351, and lower still on a 302 (assuming flat-top pistons or identical domes). Many aftermarket heads are made with different chamber volumes for this reason. That's why, after thinking a bit, I called back to make sure I shouldn't have gotten an XB9 piston. That's also why the compression ratio will be lower with the piston in a Blast than it would've been in an XB12. This was one of the things that was bothering me when I first posted on this subject. I think there must be a difference in the XB9 vs. XB12 & Blast compression height (CH, the distance from the pin centerline to the flat-top part of the piston, also called pin height). CP's web site lists the CH for their 1203 XL pistons as 1.207" but I can't find CH for the XB9 anywhere. Buell could have used the XB12 rods in the XB9 - the piston pins would have to be 11/32" lower, which is a bit much for performance use (1.55" CH), but not unheard of. It would've made it mechanically impossible to use 9 pistons in a Blast or any other Buell or XL motor though. Buell could've used rods from the same motor that the Blast rod came from when designing the XB9 motor, but I was told the Blast rod is *different* - not the same as XB9, or any other Buell or XL. And considering the revving nature of the 9 motor, the logical thing to do would have been to design a new lighter-weight high performance rod, and make it slightly shorter than the big-twin rod in the Blast. Why shorter? The Blast/1203 piston has some material removed from the underside of the oil ring land on the sides, in order to have clearance to get the pin in & out. This is necessary because the pin is so close to the top of the piston (low compression height). It would be logical to increase the compression height just enough to get the pin down to where the material thickness under the oil rings wouldn't need to be compromised - maybe 1/16" or so. I have a hunch that's what they did... Now, they *could have* used rods in the XB9 that were exactly 11/32" [(3.8125"-3.125")/2] longer than the XB12 rods, which would have meant that the pistons would have the same compression height as an XB12, but I don't think they did. First of all, they couldn't have used the SAME pistons in the 9 and the 12 due to the aforementioned compression ratio issue, and if they couldn't be the SAME pistons, a slightly different piston has no advantage bean-counter-wise over a "more different" piston, so why not lower the pin & shorten the rod slightly - it's win-win. Another reason I think XB9 & XB12 pin height is not the same - aftermarket XB9 & XB12 pistons are listed separately from each other and don't show an "alternate" compression ratio if used in the other size XB motor. Secondly, Wiseco has a 10:1 XB9 piston and a 12:1 XB12 piston, both of which have the same dome and valve reliefs, but different part numbers. If they're the same bore and same dome and valve reliefs, then they should be the same part number (assuming they use the same thickness rings), unless the compression height is different. (or unless it's a balancing issue - i'm not sure how that works on v-twins or singles, but it seems more flexible than on V-8's) Erik, if the "slightly different pin height theory" is true, it's possible that what you have IS an XB9 piston - the difference in compression height could be less than 1/16", and OEM's are notorious for building engines that have very loose quench clearance (afraid of piston/head contact!). If that's the case, then an XB9 piston in an XB-headed Blast might fit mechanically, but have very tight quench, even approaching piston/head collision - did you check your quench clearance? Compression ratio would be high, but detonation would be supressed pretty effectively with the super tight quench. Is your piston CP, Wiseco, JE, or...? Got the part no.? Hmmm... regarding the Wiseco piston mentioned above, it would seem that a given piston would give about 20% higher compression ratio in a 12 than in a 9, which makes sense since it has about 20% more displacement. This points to a 10.5 XB12 piston only giving about 8.6 in a Blast, not anything like 10.2:1, so I'm hoping what I have is a 12:1 or 12.5:1 version. REALLY hoping! | ||
| Sarodude |
I did some really deep meditation. I climbed some really tall mountains. I smok... er, took in some deep inspiration. Then, as if by magic, I came up with center to center rod lengths. Blast: 7.438" XB9: 7.270" XB12: 6.926" That was really draining. I need some rest - or another iced coffee. -Saro | ||
| Ezblast |
Saro - the true Buddha from on high has spoken! Thanks Saro - hows Loli doing these days? - still on the campaign trail for President? EZ | ||
| Sarodude |
EZ- We moved from the foothills of the San Fernando Valley (Tujunga) to a teeny little town on the fringes of the Angeles National Forrest. The air has been better - and so has Loli's health. She rode to work all week. She hasn't commuted on a bike in like 4 years, so yes, her health is better!  And PLEASE don't encourage her about that President thing.... She'll have a Giraffe on the white house lawn and she'll be walking out of Air Force One with a freakin' Lemur on her shoulder. -Saro | ||
| Ezblast |
Sounds good to me! The White House could use some normal folks in it for once! Loli for President! I'm glad she's riding - I'm sure its good for her - she's still one of the smoothest riders on the street I've seen, besides Reg, Ferris, and a few others. EZ | ||
| Sarodude |
BTW, if you do the calcs, you'll see that the XB9 and XB12 rods work out formulaically (sp?) like was discussed earlier. My buddy with the detailed internal info was mentioning some other considerations for the longer Blast rod in addition to the XB9 rod - possibly (case) deck height. I gotta get my S2 back on the road so I can start working on our Blasts. I really miss riding the little beasties... -Saro | ||
| Ezblast |
So - a thought - a Blast piston could maybe work as a high comp piston for an XB9? - hmmm | ||
| Fast1075 |
This much is for sure...with the 3.125 stroke and a rod lenght to stroke ratio of better than 2:1...piston speed is safe to nearly 9,000rpm...no angularity problems either...I need a bigger budget and more play time  | ||
| Blake |
With all else being equal a longer piston rod puts the piston closer to the cylinder head at TDC, thus increasing compression. Stroke and bore are identical; I think the cylinder height is too, so other than rod length, the cylinder head chamber/geometry/volume is the other big variable in this scenario, but if all else were equal (we know it's not), and if the rod is longer (we know that it is), then the Blast piston would have the effect of lowering the compression in an XB9. Yes? | ||
| Blake |
"This much is for sure...with the 3.125 stroke and a rod lenght to stroke ratio of better than 2:1...piston speed is safe to nearly 9,000rpm...no angularity problems either." Interesting. | ||
| Ezblast |
Sorry - my bad - Blake you are correct and Fast you do raise an interesting point. EZ | ||
| Sarodude |
I believe there is no "Blast" piston. From what I recall, it's exactly a Thunderstorm piston. The head has the chamber, ports, and valves of a Lightning head with some external differences. This is why the squish areas are so poorly matched on the stock blast's head (10 degree squish band?) & piston (15 degree squish band?). It was a parts bin bike to be sold on the cheap. Again, the Blast's longer rod (longer than the XB9) is likely due to deck height differences - this coming from a trusted source. What's the distance from the crank centerline to the deck of the case on an XB9? What's that same distance on a Blast? If the info I was provided is correct (the source's memory being the only thing in question as far as I'm concerned), you may find a .168" difference (assuming that there is exactly as much rod added as there is extra deck height on the Blast). Anyway, I gotta go pretend to work for a bit. -Saro |
