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Xb12r
Posted on Sunday, October 05, 2003 - 07:56 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only)

Is this right, Are they the same cylinder bore and will it give you more comp 10-1 to 12-1.
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Spiderman
Posted on Sunday, October 05, 2003 - 08:11 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only)

12.5 to 1 according to Aaron. I was thinking about doing that to my M2 But it didn't seen feesable on a street motor.
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Aaron
Posted on Sunday, October 05, 2003 - 10:06 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only)

I just redid the calcs and it comes out 12:1, don't know why I said 12.5 before ... I think maybe I was just repeating what someone else said ... here's the numbers ...

10:1 on an XB9 is 547cc above the piston at bdc and 54.7cc above the piston at tdc:

547 / 54.7 = 10:1 compression ratio
547 - 54.7 = 492cc displacement

Likewise. an XB12 at 10:1 has 667cc above the piston at bdc and 66.7cc above the piston at tdc:

667 / 66.7 = 10:1 compression ratio
667 - 66.7 = 600cc displacement

So anyhoo, if I'm doing this right, an XB12 with an XB9 piston would have basically the same volume above the piston at tdc (54.7cc) and 600cc more than that at bdc:

654.7 / 54.7 = 12:1

That's *still* too much for street use IMO
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Xb12r
Posted on Monday, October 06, 2003 - 09:29 am:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only)

AAron
I was told this is ok in street use because the xb12 does not turn high rpm.
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Aaron
Posted on Monday, October 06, 2003 - 10:50 am:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only)

When you light the fire and pressure starts to build, the piston has to move down and create more room. If it doesn't move out of the way fast enough, pressure builds too high and pockets of unburned fuel that have yet to be reached by the flame front will explode from the pressure increase, before the flame front gets to them.

This is why your motor tends to ping more under load. With a load on the motor, the cylinder pressure has a harder time moving the piston out of the way and making more room. Pressures go too high and the fuel explodes before it burns.

It's also why pinging tends to be a low to mid range rpm problem, and more of a problem in the higher gears.

So anyway, I don't agree with whoever told you that.

People run 12:1 in street motors, I know of several. But I think you'll create a motor that's real picky about the gas you put in it and the conditions it's operated under. And I think you're talking about a very marginal at best performance advantage over a well configured 11:1 motor. There's a diminishing return from more compression, particularly with streetable cams.

Your intake close point is also a critical factor. The later you put it, the more compression you can get away with. But the later you put it, also the more you tend to lose bottom end and shift the powerband upward.

My M2 runs 11:1 with a 58 degree intake close (Red Shift 585's). Made 121.8hp on the dyno a week ago, on 1250cc, with a nice broad powerband. Personally I think that's the way to do a street motor.

My race motors run more compression and also run intake close points in the 64 to 70 degree range. The 1350cc motor in Susan's bike is power peaking at about 7700rpm. But, with both bikes, I was fighting detonation issues with 110 octane gas. I went to 120 octane and it's working better.

Detonation is BAD for motors.
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Kaudette
Posted on Monday, October 06, 2003 - 10:57 am:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only)

Can you guys enlighten us on how this is possible swapping out pistons to maintain the displacement but increase compression???- is the 12 piston "shallower" than the 9?? Wouldn't the stroke decrease annull some of the desired benefits??

12:1 compression - that is pretty high - I'd imagine that the innards & valve train would take a beating...
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Aaron
Posted on Monday, October 06, 2003 - 11:17 am:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only)

The 9's piston has a dome. The 12's piston is a flat top. Same pin height. Same heads. Same bore.

Changing pistons doesn't change stroke, you have to move the crankpin to do that.

The 9 has a 3.125" stroke with a 7.27" rod. The 12 has a 3.8125" stroke with a 6.926" rod. Note that the difference in the rod length is half the difference of the stroke.

edited by Aaron on October 06, 2003
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Glitch
Posted on Monday, October 06, 2003 - 11:21 am:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only)

Aaron: What you described above, is that detonation? Or is that something different all together?
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Kaudette
Posted on Monday, October 06, 2003 - 11:51 am:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only)

Aaron,

Thanks for the mechanics course. I figured that the piston on the 9 would somehow eat up more "volume" than the 12 - having seen the above calculations... dome vs. flat makes that clear.

My mistake on the stroke comment - got mixed up with the calculations around the displacement.

and BTW - your M2 sounds like quite a ride! What are you getting for torque out of it?
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Aaron
Posted on Monday, October 06, 2003 - 12:02 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only)

Glitch ... yes ... read this for an interesting primer on the subject. It's an easy read.

Kaudette ... no worries mate ... here's a little older dyno sheet from that bike:


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Glitch
Posted on Monday, October 06, 2003 - 12:09 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only)

Thanks Aaron.
Who knew that buying a new bike would be such a learning experience!
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Xb12r
Posted on Monday, October 06, 2003 - 12:50 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only)

Aaron
Are you doing motor work on the 12s, heads cams exe
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Kaudette
Posted on Monday, October 06, 2003 - 02:07 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only)

That is one SWEET powerband for your M2! I'll forward it to my brother who rides an M2 and thinks his is king of the hill - that should take him down a step or two on the motorcycle foodchain! I'm not sure my 12S would have the same effect ;)
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Boulderbiker
Posted on Monday, October 06, 2003 - 02:36 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only)

Hey Aaron, I stopped by Nallin a couple weeks ago a couple days after I picked up my 12R, and I talked to you and took a walk through the shop. I like Gary above am curious about work on the 12's. When I talked to Mr. Nallin (I'm horrible with names) he was saying that these bikes already had pretty aggressive cam as well as a very smooth action on the valve train. Since thats the thing that would need the most servicing if I went for more power, what ways could I go about getting more oomph with a curve like that and keep the valve train as durable as possible? I forgot to mention that if possible I'd like to avoid the force exhaust, partially cause its just too loud for as much regular round town riding I do, but I'm still hoping as a goal to go for around the same range of power that you hit with your m2

edited by BoulderBiker on October 06, 2003
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Glitch
Posted on Monday, October 06, 2003 - 03:48 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only)

Boulder:
Not to step on any toes, as I'm not an expert here at all. But, if I remember correctly, the 1200cc kit for the XB9 will fit on the XB12, only making it 1450ccs. And also get the stage 3 head prep. I bet that done would make a monster of an XB!
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Aaron
Posted on Monday, October 06, 2003 - 05:44 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only)

XB12R, yes! We've got engine kits and proven head porting. That M2 above has a set of our Stage 3 XB heads on it. We're digitizing that set of heads and we can make exact duplicates on our CNC machine.

Boulder, you're right, the XB cams are a *nice* grind for all-around street use. Very easy on the valvetrain and don't need a whole lot of spring. You can certainly build a nice motor around them, I'm sure you can get some decent power before they start constraining you.

I'd consider the 585's (what's in that M2) as the next step up. They need more spring than the XB cams for sure, but they're not a brutal race grind by any means. They won't beat the crap out of things like some grinds do.

On your 12R, I'd suggest either a 1250 kit or what Glitch mentions, the 88" kit. With head porting in either case.
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Gearloose
Posted on Monday, October 06, 2003 - 10:55 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only)

Guess I am confused.How many inches is 1450 cc's?
What a person gain power wise going to a 1250 kit on my 12S?
I take it the 1250 kit is not a major job,like a 88 is.Run the stock cams with the 1250 kit?
Thanks
Gearloose
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Aaron
Posted on Tuesday, October 07, 2003 - 08:39 am:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only)

Gearloose ... 1450cc is 88" in Twin Cam land. The XL/XB 88incher isn't quite as big ... it's actually 87.04ci and 1426cc. There's a lot of "rounding" of displacements that goes on in this business.

The 1250 kit is a bolt-on, easy to install. The power you get out of it depends on the total package. The basic range we've seen out of this kit (on tubers) has been low 100's to over 120, just depends on what the customer wants. We don't have enough data on the XB version yet but I'd be surprised if it's much different. It's all the same stuff. You could certainly run the stock cams.

The 88" kit of course requires a teardown and boring of the cases. But man, they are monsters.
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