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Old 03-14-2011, 08:37 PM   #46
supershaft
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Quote:
Originally Posted by Wirespokes View Post
I never pay much attention to the HP figures either. As a matter of fact, the HP figures seem really low for the kind of power delivered.

Another difference you've got to take into account between later RSs and the 90/100S besides exhaust and dellortos/bings is the final drive ratio. The later RSs have a 32/11 whereas the Ss had a step lower at 33/11.

But I totally agree - there's very little difference between a 90S and the 100S. They're both great, love the paint on them all, but lean just ever so slightly toward the smoke red as my favorite.
The hp figures seem high to me.

Gearing doesn't change HP.

I think there is a big difference. Don't get me wrong, I love old bikes and I love seeing them being put through their paces but the /7 is a definite step forward into the present over the /6! Granted, a '76 /6 is damn near a /7 but . . . .
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Old 03-15-2011, 03:07 AM   #47
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Quote:
Originally Posted by Wirespokes View Post
I never pay much attention to the HP figures either. As a matter of fact, the HP figures seem really low for the kind of power delivered.

Another difference you've got to take into account between later RSs and the 90/100S besides exhaust and dellortos/bings is the final drive ratio. The later RSs have a 32/11 whereas the Ss had a step lower at 33/11.

But I totally agree - there's very little difference between a 90S and the 100S. They're both great, love the paint on them all, but lean just ever so slightly toward the smoke red as my favorite.
Agreed.

Most early production '77 RS's also shipped with 33/11's.

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Old 03-15-2011, 03:09 AM   #48
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Originally Posted by supershaft View Post
The hp figures seem high to me.

Gearing doesn't change HP.

I think there is a big difference. Don't get me wrong, I love old bikes and I love seeing them being put through their paces but the /7 is a definite step forward into the present over the /6! Granted, a '76 /6 is damn near a /7 but . . . .
Many subtle advances in frame and chassis but the engines actually started going backwards with the /7's.

Volumetric efficiencies peaked with the R90S and '77 RS and S's.
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Old 03-15-2011, 05:13 AM   #49
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Quote:
Originally Posted by Lornce View Post
Many subtle advances in frame and chassis but the engines actually started going backwards with the /7's.

Volumetric efficiencies peaked with the R90S and '77 RS and S's.
Interesting.. How did the motors change after '77? I ask because I was researching my '78 S and on the bmwbikes.co.uk site I found a page that said that the '78 S produced 70hp vs the other R100S models producing 65. They also stated that the 90S produced 67hp.

I don't know how to create a link, but the page is:
www.bmwbikes.co.uk/specpages/R100S.htm

For the specs on the R90, substitute R90S for R100.

edit; I figured out the link thingie

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Old 03-15-2011, 05:20 AM   #50
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Quote:
Originally Posted by Lokey View Post
Interesting.. How did the motors change after '77? I ask because I was researching my '78 S and on the bmwbikes.co.uk site I found a page that said that the '78 S produced 70hp vs the other R100S models producing 65. They also stated that the 90S produced 67hp.

I don't know how to create a link, but the page is:
www.bmwbikes.co.uk/specpages/R100S.htm

For the specs on the R90, substitute R90S for R100.
Yes this is also what I noted in the above thread.
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Old 03-15-2011, 09:51 AM   #51
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Originally Posted by Lornce View Post
Many subtle advances in frame and chassis but the engines actually started going backwards with the /7's.

Volumetric efficiencies peaked with the R90S and '77 RS and S's.
Claimed peak hp at the crank peaked with the '77RS but engine improvements kept coming in large doses until '81 and then a little more in '88. I think pretty much everybody agrees that the '77RS made the most power in stock form. If you put R90S carbs on them they would make even more. If you put that very same modified top end minus the iron lined cylinders with nikisil instead on a post '81 engine, you would reap even more benefits. There are very sound reasons why most good running vintage airhead racers have a large chunk of the "vintage" engine converted to post '81 parts and it has nothing to do with the engines going backwards starting with the /7's.

Of course, modifying a post '81 engine in it's entirety can net the same thing or better.
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Old 03-15-2011, 09:54 AM   #52
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Originally Posted by BiG DoM View Post
Yes this is also what I noted in the above thread.
Me too. What's up with that?
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Old 03-18-2011, 06:25 AM   #53
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Quote:
Originally Posted by Lokey View Post
Interesting.. How did the motors change after '77? I ask because I was researching my '78 S and on the bmwbikes.co.uk site I found a page that said that the '78 S produced 70hp vs the other R100S models producing 65. They also stated that the 90S produced 67hp.

I don't know how to create a link, but the page is:
www.bmwbikes.co.uk/specpages/R100S.htm

For the specs on the R90, substitute R90S for R100.

edit; I figured out the link thingie

Quote:
Originally Posted by Lokey View Post
Interesting.. How did the motors change after '77? I ask because I was researching my '78 S and on the bmwbikes.co.uk site I found a page that said that the '78 S produced 70hp vs the other R100S models producing 65. They also stated that the 90S produced 67hp.

I don't know how to create a link, but the page is:
www.bmwbikes.co.uk/specpages/R100S.htm

For the specs on the R90, substitute R90S for R100.
edit; I figured out the link thingie
Not sure on Euro-spec bikes, but North America only ever saw R100S's with 38mm exhaust ports. From '77 to '80 they were rated at 65hp. Not sure how they did that in '80 because by then compression was dropped through the floor. ie: For '80 BMW pulled the 65hp number out of their hoo hoo?

Qualifying my earlier statement about engines going backwards after '77: Specifically referring to combustion chamber design. R90/6's, R90S's and '77 R100/7's, RS's and S's had a particularly efficient combustion chamber shape that was made redundant after the '77 model year. Seems fuel burned too efficiently and completely (higher temps) can result in the production of noxious gases. I'm no chemist, but I think the offending gases were called "nitric oxides" or something similar. Anyhow, it was determined that the the squish band style heads, though burning efficiently and making decent power, produced too many of the offending gases. Reducing the combustion chamber's efficiency, by eliminating the squish band shelf in the head and on the piston, is what BMW did to lower the fuel burn temps to reduce production of the verboten pollutants. It also reduced torque and power.

afaik, this was a US EPA mandated change, and didn't affect Euro-spec bikes, which *I believe* used the squshband heads right into the mid '80's. iirc, Rob Farmer posted some pics of a set of UK spec '83 heads and they looked for all the world to be what we here in the Colonies refer to as the "squish band" design.

As we all know (mostly) many advances to the rest of the motor appeared in the years to come. '81 was a banner year. Eliminating the heavy flywheel and replacing it with a stamped steel clutch carrier was a great move. Removing eight pounds of mass from the crank assembly (flywheel) allowed the engine to rev more freely.

Question for you learned engineering and physics types: If the same amount of torque (product of the combustion event acting upon the engine's otherwise identical internal architecture) is used to build revs faster (on account of there's no longer 9 pounds of flywheel spinning on the end of the crank). Is that the same thing as making more horsepower?

That's the only way I can figure out how BMW claims 70HP for their '81 US-spec motors while running lawn mower compression rates of 8.25:1. Unless they're just conveniently quoting Euro-spec high compression power numbers to keep the press packages simpler and rosier coloured?

'81 Was also the first year of nikasil cylinders, which were a revelation for air-cooled 1000cc BMW's. Nikasil is a remarkably hard material. Wearing cylinder bores almost became a thing of the past, even with huge miles piled on. Not certain, but I suspect they may also have reduced some of the parasitic losses associated with conventional iron bores. I'd be interested to hear from anyone who might have good data on that.

Didn't know until SS mentioned it a few weeks ago in another thread, but '88 marked a significant improvement in head port design. I'm guessing that's a big part of the useful torque available from those engines, in spite of their woefully low and inefficient operating compression ratios (again, US-spec North American bikes).

To recap: (US-spec North American bikes)

Combustion Chamber Efficiency: 90/6, S and '77 100/7, RS and S take the prize.

Port Efficiencies: Post '88 heads (too bad those ports couldn't be combined with a '77 combustion chamber)

Clutch Ass'blys, Nik barrels, elec ignition, quieter rocker assbly's etc: Post '81 motors with further improvements to rocker assbly's after '88 (*i think*)

fwiw, ymmv, imho,
Bob Loblaw (read it out loud)



edit: Big exception to the engine advancement trend was '81 through '84 exhaust valve seat material (which was changed to a much harder inconel-type alloy) in an effort to create valve seats to withstand the ravages of unleaded fuel. These too-hard seats beat their poor exhaust valves into a pulp by 50 or 60k miles. The cure? A return to valve seats just like those used prior to 1981. There was nothing wrong with the original material. D'oh!












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Old 03-18-2011, 09:53 AM   #54
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The non-squish band heads have a squish band but it is angles and is not very squishy. Lots of good running engines tend to prefer angled squish bands. Our engines seem to do just fine with them if you mill the head down enough to get more squishing.

I think you are barking up the wrong tree wondering if the hp actually went back up on some later models. I think they pulled those number out of their ass the whole time. The exact same engine can see all different kinds of hp ratings depending on how it is run up on the dyno let alone changes in the engine itself. I think the sales department gets a stack of charts from the R+D department and picks out which one they think will do them the most good. Something like that anyway.

As I understand it, lighter components won't increase torque but they can increase peak HP IF the engine revs enough to get the hp curve above the torque curve. Hp increases after the torque falls off IF the engine is still revving at a greater rate than the torque is falling off. (I hope that makes sense.) Lighter components can help a quicker rev that has more inertia after the torque is dropping thusly getting a higher peak hp.

I don't know if I would call the raised port floors a significant improvement but I bet they do help.

I don't think I would call the small port heads with raised port floors better for peak power than the big port heads. It's just that many dismiss them altogether and I am thinking wait a minute. R90S's had small port heads! Small port heads work very well for overall power gains. They work very well with 44mm or 45mm intakes. IMO, small ports are easier to get running real well without some serious flow bench time. Small port heads can be hogged out to the same size ports as big port heads but you obviously have more options controlling the shape of the port.

BMW raised the compression back up after the early eighties here in the US to 8.5 and then 8.7. IMO, I think 8.7 is as high as you can go with a stock setup single plugged. I remember all to well many a BMW with stock 9.0:1 CR getting their CR lowered for pinging back in the seventies!

I think the slanted "non-squish band" heads perform just fine if you raise the CR to as high or higher than the "squish band" heads. (In dual plugged heads!)
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Old 03-18-2011, 10:15 AM   #55
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the question is how much reducing heads height ?
on 77 1000cc heads
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Old 03-18-2011, 10:35 AM   #56
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I think I huge step forward besides the '81 on clutch is the single row timing chain.

From what I have seen, those early eighties seats go soft as much or more often than the valve. It's usually a combination of both anyway.
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Old 03-18-2011, 12:17 PM   #57
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I would agree that BMW hp figures are suspect, recoded at the crank with an engine bare of alternator, then 10% added for good luck
A euro 81 on RS/Rt/S model with 38 exhausts and 40 mm Bings will get into the low 50s at the rear wheel. Around 49 for a basic 1000. To get 70+ at the rear wheel you need to spend quite a bit of cash, with someone who knows what they are doing, but on the plus side the bike should remain both reliable and very Roseanne on today's busy roads.
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Old 03-18-2011, 06:42 PM   #58
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Quote:
Originally Posted by supershaft View Post
The non-squish band heads have a squish band but it is angles and is not very squishy. Lots of good running engines tend to prefer angled squish bands. Our engines seem to do just fine with them if you mill the head down enough to get more squishing.
I've been looking more closely at post '88 heads since you mentioned the port floors and also wondered about the usefulness of that angled band. Have yet to compare it to the the various pistons and measure the gap, but it looks hopeful. Might be the direction I'll go with my own GSPD as I despair of finding a solution for dimensional instability with 1050cc bores.

Quote:
Originally Posted by supershaft View Post
As I understand it, lighter components won't increase torque but they can increase peak HP IF the engine revs enough to get the hp curve above the torque curve. Hp increases after the torque falls off IF the engine is still revving at a greater rate than the torque is falling off. (I hope that makes sense.) Lighter components can help a quicker rev that has more inertia after the torque is dropping thusly getting a higher peak hp.
Isn't HP basically torque times rpm? As I understand it, reducing reciprocating mass doesn't increase torque, it simply permits revs to build more quickly. To some extent I guess that's equivalent to reduced parasitic losses, which could translate to improved net power production. Sort of similar to the way lowering oil pressure on a race engine reduces parasitic losses.

Quote:
Originally Posted by supershaft View Post
I don't know if I would call the raised port floors a significant improvement but I bet they do help.
No flow bench experience to determine anything, but the shapes look like the classic improvement. I'll give BMW some credit there for doing a bit of homework before going to the trouble of reshaping their casting molds. Have often wondered how those anemic engines seemed to pull reasonably well in the midrange.

Quote:
Originally Posted by supershaft View Post
I don't think I would call the small port heads with raised port floors better for peak power than the big port heads.
I don't recall anybody suggesting that.

Quote:
Originally Posted by supershaft View Post
It's just that many dismiss them altogether and I am thinking wait a minute. R90S's had small port heads! Small port heads work very well for overall power gains. They work very well with 44mm or 45mm intakes. IMO, small ports are easier to get running real well without some serious flow bench time. Small port heads can be hogged out to the same size ports as big port heads but you obviously have more options controlling the shape of the port.
Like building performance heads for a 1200 Sportster out of 883 heads.

Quote:
Originally Posted by supershaft View Post
BMW raised the compression back up after the early eighties here in the US to 8.5 and then 8.7. IMO, I think 8.7 is as high as you can go with a stock setup single plugged. I remember all to well many a BMW with stock 9.0:1 CR getting their CR lowered for pinging back in the seventies!
I found being fastidious about maintaining clean combustion chambers eliminated the need for lower compression on '70's spec airheads. Learned the lesson on my first '74 R90/6 after a routine decoking of the top end. Ran like a different motorcycle after wards. At one point (after a discussion with Snowbum about the use of water injection to control detonation on WWII fighter planes) I played around with a water ingestion apparatus on a mildly tuned RS to steam clean the combustion chambers while underway. Used carefully it worked like a charm. fwiw the alcohol ingestion system on Capri's Luftmiester turbo kits had a similar effect on airhead combustion chambers.

Lornce screwed with this post 03-18-2011 at 06:49 PM
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Old 03-18-2011, 06:58 PM   #59
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From what I have seen, those early eighties seats go soft as much or more often than the valve. It's usually a combination of both anyway.
I've never notice that.

Seats in those heads are hard. Seat cutters that cut cast seats with ease barely touch them.

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Old 03-18-2011, 07:53 PM   #60
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For instance, a lot of tuners have come to the same conclusion that HD heads work best with an angled squish band. Lots of other strong running heads work with angled squish bands. The main problem with ours is that there isn't enough squish but that can be fixed with pistons and/or milling the heads. The stock pistons work pretty good in milled heads. I just wish they were not so heavy!

Torque times rpm for sure below 5252 rpm. Above that rpm too but read up on how hp can still increase after peak torque and the torque is falling off. Things get a bit more complicated above 5252 rpm. I don't understand why hp is even rated below 5252 rpm since below that rpm an engine with a lot lower hp rating can have way more power. For instance, a 220hp aircraft engine that doesn't have enough power to fly a plane that originally had a 180hp engine. The lower hp engine has a LOT more power. How? The 220hp engine is making that peak hp at its 2100rpm red line. The 180hp engine is making that hp at its 1100rpm red line. The 220 hp engine has 550ftlbs of torque. The 180hp engine had 860ftlbs of torque! Big Diff! 300ftlbs!

I have no flow bench experience but I have seen what a couple of very famous head guys have done with quite a few different sets of BMW airheads. Plus I read up on it as much as I can. BMW had the right idea but didn't go near far enough to make that much of a difference. I still bet it helps.

Keeping the crank vent out of your combustion process, your rings and valve guides fairly oil tight, and jetting the bike for best power keeps the piston crowns and heads almost perfectly spotless excepting a thin layer of insulating carbon in my experience. Maybe just a very little bit of build up furthest from the one plug. Dual plug setups should stay perfect.
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