ADVrider

Go Back   ADVrider > Bikes > Old's Cool > Airheads
User Name
Password
Register Inmates Photos Site Rules Search Today's Posts Mark Forums Read

Reply
 
Thread Tools Search this Thread Display Modes
Old 11-26-2011, 06:47 AM   #76
ontic
 
Joined: Jul 2008
Location: Melbourne
Oddometer: 1,048
Quote:
Originally Posted by Lornce View Post
The 90/6 top ends were the most efficient BMW ever put on an airhead. They actually have a more aggressive squish shelf than the heralded '77 R100S and RS's.

Decoking the heads and pistons makes a dramatic difference in how well they run under steam. If you've never done it, I bet you a donut it's a mess.

Cheers, I never knew that, very interesting to know. (EDIT- crossed posts, now you've added pictures, I'm going to check out my spare parts)
I haven't done it myself yet but there is a bit of top end work in the bikes recent history from previous owner(s). I'll be investigating the top end on the 90/6 soon as I need to do the pushrod seals, so we'll see about that donut then. You might be right.
Before I got the G/S I was going to build my pile of spare parts into a twin shock G/S type thing- spare 90/6 engine, early 100RS frame, choice of gearboxes and final drives and much more.... would have been fun.
I keep trying to bring myself to part out all my parts... but I just can't seem to do it. Maybe I should just hang on to it all and keep a nice project in the bank...
__________________
1974 R90/6
1981 R80G/S
1994 XLR250R
ontic is offline   Reply With Quote
Old 11-26-2011, 07:22 AM   #77
ontic
 
Joined: Jul 2008
Location: Melbourne
Oddometer: 1,048
interesting,
here is some more photos of some 90/6 top end spares, forgive the jaundice, my super bright garage light doesn't photograph too well,



the piston and head squish band/shelf just slot right in to each otherand hold the piston firmly



would this be a donut looser Lornce? (that stuff is like baked on sand)


The two out of four spare pistons I checked both have spots of that beaten up rim/edge.
__________________
1974 R90/6
1981 R80G/S
1994 XLR250R
ontic is offline   Reply With Quote
Old 11-26-2011, 07:31 AM   #78
_cy_
Beastly Adventurer
 
Joined: Sep 2011
Location: Tulsa, Oklahoma
Oddometer: 5,461
interesting that an R90/6 head are that efficient... what about an R90S head?

back on topic: couldn't one achieve same effect by switching to a higher ratio final drive?
then installing a lower first gear?
_cy_ is offline   Reply With Quote
Old 11-26-2011, 09:56 AM   #79
Wirespokes
Beemerholics Anonymous
 
Wirespokes's Avatar
 
Joined: Jul 2002
Location: Jackson's Bottom Oregon
Oddometer: 7,897
First off, the piston never enters the head, at least not the side. Pistons normally stop flush with the top of the cylinder. The squish band is something like .060" or more (don't recall exactly) wide - and that's the distance between the piston and head.

The 90S head is essentially the same - only difference is the intake spigot and the area directly around it.
__________________
Wanted: Dead, smashed, crashed or trashed gauges
BMW GAUGE REPAIRS - TACH*SPEEDO*CLOCK*VOLT METER *PODs & LIGHT BOARD*
Wirespokes is offline   Reply With Quote
Old 11-26-2011, 10:14 AM   #80
disston
ShadeTreeExpert
 
disston's Avatar
 
Joined: Mar 2011
Location: Silver Spring, Md
Oddometer: 8,448
Quote:
Originally Posted by _cy_ View Post
interesting that an R90/6 head are that efficient... what about an R90S head?

back on topic: couldn't one achieve same effect by switching to a higher ratio final drive?
then installing a lower first gear?
Always an option. I think the advantage of the higher gear in the trans is that the standard 4th to 5th gear ratios are considered close, even too close. So the gear modification opens the ratios up a little. Changing the final drive might get you the higher final ratio you want but the 4th gear is raised too and it's still close to 5th.

What I want is a gear set, final drive, whatever, that makes my speedometer accurate.

Charlie
disston is offline   Reply With Quote
Old 11-26-2011, 02:21 PM   #81
Arkwright
Grumpy ole Git
 
Arkwright's Avatar
 
Joined: Sep 2009
Location: Lincolnshire, Engerland
Oddometer: 101
Quote:
Originally Posted by disston View Post
Slightly higher gear ratio. It goes on the out put shaft. Drops RPM at top speed by a couple hundred. In the world of gear design the gears in the trans are a certain diameter and number of teeth. They make this gear the same diameter but a different number of teeth, I think they add one tooth, not sure though. It seems to work and there are many fans. There are plenty of purest though that say you can't be doing that. By changing the number of teeth you have to change the angle of those. It can't be good, must be Black Magic.

There have been changes in the transmissions through out the years of Airheads. One change was the introduction of the X gears. The X gears have a different angle to their cut. The old is 15 degrees and the X is 17.5 degrees, I think I got that right. There are 3 angle cut gears in our trannies. One on each shaft. Since the final gear of 5th gear is one of these you have to get the higher gear to match your box. They used to be available in either X or non-X but not sure they still are. Have seen that the X gears are still available. There were also several different manufacturers and some are favored for some reason but I don't know which one is which really. I forget what year starts the X gears somebody will tell you though if you need to know. You can use the X gear in the older box if you change a few other gears with it.

600 posts. I have to slow down. I'm going to make a mistake sooner or later. I may have screwed up this entire post for that mater. Just saying...
Don't slow down Charlie, your posts are good common sense. If my memory is correct the change from 15 degrees ocurred midway through '82, the 17.5 degree gears give very little trouble whereas the 15 degree ones tend to wear through their case hardening (DAMHIK......the Mono!) both are currently still available in various forms some of which are not particularly cheap. And yes you are correct, the higher 5th has one more tooth on the same size gear.
__________________
Duncan
R1200 GS (07)
R80 being Cafe Racered (82)
R80 G/S (81)
R100 GSPD (92)
Arkwright is offline   Reply With Quote
Old 11-26-2011, 02:40 PM   #82
disston
ShadeTreeExpert
 
disston's Avatar
 
Joined: Mar 2011
Location: Silver Spring, Md
Oddometer: 8,448
Quote:
Originally Posted by Arkwright2007 View Post
Don't slow down Charlie, your posts are good common sense. If my memory is correct the change from 15 degrees ocurred midway through '82, the 17.5 degree gears give very little trouble whereas the 15 degree ones tend to wear through their case hardening (DAMHIK......the Mono!) both are currently still available in various forms some of which are not particularly cheap. And yes you are correct, the higher 5th has one more tooth on the same size gear.
Thanks Duncan, it's nice of you to say all that.

An interesting thing about the difference between the 15 and 17.5 gears. And a good guess on the change date but I have discovered a real authority for you. Look back at post #58 of this thread, Davorallyfan, not sure I got that name right, and he posts a picture of the part in a BMW bag with the BMW part sticker on it. It says from 4/82. But we'll accept mid year also.

I have to make a new cheat sheet for all the change dates. I had one but lost it recently. I'm gonna start a thread on that, whadayathink?

Charlie
disston is offline   Reply With Quote
Old 11-26-2011, 05:02 PM   #83
ontic
 
Joined: Jul 2008
Location: Melbourne
Oddometer: 1,048
Quote:
Originally Posted by Wirespokes View Post
First off, the piston never enters the head, at least not the side.
I didn't mean to imply that, I was just showing how it matched right up. With all that carbon and the damage on these old pistons I was wondering how close they came though.

Quote:
Originally Posted by disston View Post
Always an option.

What I want is a gear set, final drive, whatever, that makes my speedometer accurate.

Charlie
Final drive change not quite an option on the G/S or ST (well at least not an affordable or easily available option).

And although it is probably tongue in cheek- about your speedo. Does it seem to be out by a certain percentage of your speed or more so an amount of speed... my /6 speedo is out (optimistic) by about 10% and my friends /6 speedo, he swears, seems to be out my about 10kmh across the board. The second problem I think could be largely remedied by taking off the needle and repositioning it relative to the spring... had to do this to my G/S one a fair bit till I got it right.
Seeing as though I drive an old car and old bikes with terribly inaccurate speedos and live somewhere where speed fines are a serious revenue stream, I got a GPS primarily for this purpose.

Quote:
Originally Posted by disston View Post
I'm gonna start a thread on that, whadayathink?
Sounds great, and don't stop posting, your posts are much appreciated.
I just have to get used to your new avatar.


P.S. sorry for all the thread drift.
__________________
1974 R90/6
1981 R80G/S
1994 XLR250R
ontic is offline   Reply With Quote
Old 11-26-2011, 06:38 PM   #84
Stagehand
+/- V TDSPP
 
Stagehand's Avatar
 
Joined: Oct 2004
Location: Shawangunks
Oddometer: 25,209
Sorry to continue the thread drift, Ontic, but I've been running 87 and 89 octane in my single plugged PD with the 9.5 pistons, and no pinging at all. Runs great. I dont think 9.5 is too great a CR to run regular or even sub-regular gas in.

My understanding was the speedo's are purposefully optimistic by about 5%. The odo's on the other hand, are usually pretty accurate, depending on your tire size.
__________________
Unintentional psychokinesis.
Stagehand is offline   Reply With Quote
Old 11-26-2011, 06:50 PM   #85
danedg
Horizontally Opposed
 
Joined: May 2008
Location: U-puku-ipi-sing
Oddometer: 6,263
So what ratio does an 95 R100R run?
danedg is offline   Reply With Quote
Old 11-26-2011, 06:52 PM   #86
bmwrench
Beastly Adventurer
 
Joined: Nov 2006
Oddometer: 1,996
Quote:
Originally Posted by Arkwright2007 View Post
."... And yes you are correct, the higher 5th has one more tooth on the same size gear.
Shouldn't that be one tooth less?
bmwrench is online now   Reply With Quote
Old 11-26-2011, 06:55 PM   #87
bmwrench
Beastly Adventurer
 
Joined: Nov 2006
Oddometer: 1,996
This damage, "The two out of four spare pistons I checked both have spots of that beaten up rim/edge " is from the head gasket. It's very common on R90s with some mileage on them. You can fell the piston hitting the gasket on these bikes if you turn the crank via the alternator rotor bolt.
bmwrench is online now   Reply With Quote
Old 11-26-2011, 07:05 PM   #88
Stagehand
+/- V TDSPP
 
Stagehand's Avatar
 
Joined: Oct 2004
Location: Shawangunks
Oddometer: 25,209
Quote:
Originally Posted by danedg View Post
So what ratio does an 95 R100R run?
Think the R100R's are 9.5 as well, since I think that was the cheater upgrade for the GS. Well it used to be cheap, but you could just swap out the 8.2 R100GS pistons directly for the 9.5 R100R pistons without changing anything else. You got all the good stuff, Dan
__________________
Unintentional psychokinesis.
Stagehand is offline   Reply With Quote
Old 11-26-2011, 07:11 PM   #89
Lornce
Lost In Place
 
Lornce's Avatar
 
Joined: Aug 2003
Location: Way Out There.
Oddometer: 17,705
Quote:
Originally Posted by _cy_ View Post
interesting that an R90/6 head are that efficient... what about an R90S head?
The R90S heads are the same internally but the top end works better due to a slightly higher compression ratio achieved with a slightly larger piston dome.

R90S 9.5:1 vs. the the R90/6's 9.0:1.

Lornce is offline   Reply With Quote
Old 11-26-2011, 07:20 PM   #90
Lornce
Lost In Place
 
Lornce's Avatar
 
Joined: Aug 2003
Location: Way Out There.
Oddometer: 17,705
Quote:
Originally Posted by Stagehand View Post
Think the R100R's are 9.5 as well, since I think that was the cheater upgrade for the GS. Well it used to be cheap, but you could just swap out the 8.2 R100GS pistons directly for the 9.5 R100R pistons without changing anything else. You got all the good stuff, Dan
I never knew that?

I thought all the post '81 liter boxers sold in the US had 8.2:1 pistons?

Lornce is offline   Reply With Quote
Reply

Share

Thread Tools Search this Thread
Search this Thread:

.
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On

Forum Jump


Times are GMT -7.   It's 05:42 AM.


Powered by vBulletin® Version 3.8.5
Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.
Copyright ADVrider 2011-2014