Higher compression

[fracture]

I have been considering installing the 9.5:1 compression pistons, replacing the factory 8.2:1 pistons. I am not trying to create a sport bike. I just want to restore some of the power and efficiency that was lost due to EPA regs.

I may have to use higher grade fuel to avoid pinging. I do not want to mess with dual plugging as I do not think it is worth the hassle.

What other issues are to be considered? Will ignition timing have to be changed? Is valve timing compatible as is?

If anyone has done this mod I would like to hear about your experience.

[keelerb]

I haven't done this in a boxer, so will have to let others chime in with better specifics. But having done it other bikes, I would think:

- It will increase power throughout the entire RPM range, as well as increase MPG.

- It may require higher octane fuel. 9.5 isn't all that high, though, so you might need to go from 87 to say 89, probably not more than that.

- It should not affect valve timing.

- If necessary to cure pinging that higher octane fuel doesn't fix, or as an alternative (a poor one, I would think) to higher octane fuel, you might have to retard your ignition a few (maybe 1-3) degrees.

By way of example, I put 11:1 pistons into a 900cc Ducati whose stock CR was 9:1. Power and MPG noticeably increased. I went from 87 to 91 octane (the increased mileage made the pricier fuel probably about an economic wash). And I retarded the ignition timing 3 degrees. It was a worthwhile mod.

Good luck. - BrianK

[20774]

Year? Model??

[MotorradMike]

I think you will find this article interesting. Sorry about the formatting, I had to upload it as text.

[rinty]

I think you'll find...MotorradMike

Ah, the long lost Dr. Curve long rod theory is brought back into the daylight. :laugh

I didn't, ah, read all of it. :D

[fracture]

Year? Model??

'94 R100RT.

[Littleleroy38]

I don't you think you can get away without dual plugging if you increase compression. At least that is the consensus of people whose expertise is much greater than mine.

Vanzen, do you want to chime in here?

[felaw]

I did it a few years ago with a 1908 R100T. I also went up one size on the main jets and moved the needle up one clip position. I advanced the timing a bit. I got a little better performance and a little better gas mileage. However, I did replace the rings and honed the cylinders at the time. Is it worth doing. Yes, but the results, thought noticeable, aren't startling, IMOH.

[Polarbear]

A long time ago too, I did the increased compression piston on my R100/7. NO issues and NO dual plug either. In the beginning, I had thicker base shims in place, but a few years ago, took them shims "out" and the bike runs like a scalded ape today, with no pinging.:) The "earlier years"unleaded gas did ping in my engine and todays gas has to be different, because my pinging is gone for years now. A recent compression test reveiled 140+ on both sides still at 335000 miles! 225000 of that on these "new" pistons...I did the work myself. Bore and refit, were possible on the older iron sleeves only. I do have Dyna 3 in place and it helps a lot too, for older BMW's of the points era. Randy:clap

[bikerfish1100]

probably worth doing if the pistons or rings need to be done anyway, but i sure wouldn't bother if things were running right. The performance gains are there, but not mind-bending.
also, unless i'm mistaken, due to having Nikasil-lined cylinders, you can't easily go to an oversize piston. yes/no?

[20774]

unless i'm mistaken, due to having Nikasil-lined cylinders, you can't easily go to an oversize piston. yes/no?

bf1000 -

I think you're right. Now boring, etc., on the nikasil jugs. You have to buy the whole thing - jugs, piston, rings. I suppose you could completely bore out the linings and send them off to a company which does carbide impregnation, but that seems like a lot of work.

[tvrla]

There's no need to change the nikasils. They can last a couple hundred k with normal maintenance.

I agree that it's possible to increase the compression without dual plugging. It should increase power, decrease consumption, and the engine should run cooler. More gas shouldn't be needed, so increasing jet sizes isn't necessary.

In the winter or cooler weather, regular gas should be fine. Even in the summer, if heavy acceleration or working the motor hard in the mid RPM band is avoided, then there shouldn't be any pinging.

We now have 10% alcohol in our gas, something different from 20 years ago. The funny thing is that alcohol requires a compression ratio of 14 to 1 to get power out of it. So increasing CR will help that as well.

[rinty]

I went to 8.7:1 on my '82 RS, by skimming the heads by, I think, .030". I also had the heads flowed, ported and polished, and 40 mm headers, and SuperTrapps installed.

The end result? It was still an airhead.

I spent a lot of money, but I got it out of my system. :laugh

Just my experience. My suggestion would be to skim the heads and leave it at that.

[swall]

Consider that you will need to use premium fuel in most circumstances and that, depending on your travels, you may find premium unavailable in some locales or on some days. Personally, I'd leave it alone. And I'm speaking from the perspective of owning a '78 R100RS with the 9.5:1 and having owned an '85 R80, with the lower compression ratio. The R80 did everything I ever asked of it and was a sweet running machine.

[8ninety8]

'74R90S CR is 9.5 to 1, after the first overbore and new pistons, bolted up without shims, the CR should now be higher, no? Considering the increased volume, same stroke, same heads and gaskets? It wouldn't be much, maybe a tenth?

[108625]

I went to 8.7:1 on my '82 RS, by skimming the heads by, I think, .030". I also had the heads flowed, ported and polished, and 40 mm headers, and SuperTrapps installed.

The end result? It was still an airhead.

I spent a lot of money, but I got it out of my system. :laugh

I came to the same conclusion when I was thinking about such mods and left it alone. A souped up airhead is is only going to be faster than another airhead.

[lkchris]

I have been considering installing the 9.5:1 compression pistons, replacing the factory 8.2:1 pistons. I am not trying to create a sport bike. I just want to restore some of the power and efficiency that was lost due to EPA regs.

Just so the "history" is correct ...

The low compressions arrived because at the time premium gasoline was not available in the USA. This because gas stations chose to have pumps for leaded and unleaded regular instead.

Rather indirectly, then, due to "EPA regs." There was nothing about how the Airhead engine ran that required low-test fuel to comply.

[DOINTHETON]

did the seibenrock conversion on a 83 r80 i owned a couple of years ago. 1000cc, 9.5 to 1. cost almost $1000.00 dollars. was it worth it power wise? no, i did not think so. premieum fuel, upped the jetting, timing adjustments, would still ping on occasion.

[fracture]

Just so the "history" is correct ...

The low compressions arrived because at the time premium gasoline was not available in the USA. This because gas stations chose to have pumps for leaded and unleaded regular instead.

Rather indirectly, then, due to "EPA regs." There was nothing about how the Airhead engine ran that required low-test fuel to comply.

I seem to remember (maybe from a thermodynamics class) that lowering the compression also allowed leaner mixtures.

I would say that the combination of limited availability of premium fuel (due to phasing out leaded fuel), plus the lean mixtures required for less emissions, led to lowering the compression.

[rinty]

Was it worth it? DOINTHETON

Beautiful stuff though, from Seibenrock

[108625]

Beautiful stuff though, from Seibenrock

Oh, I agree... You can get big bore kits, titanium rods, shorter cylinders and hardware for a narrower engine (more ground clearance) and so on. It would be fun to build a "spare no expense" hot rod airhead. But...
Holy cow that stuff is expensive! I can (and did) build a hot rod V8 with nearly the same budget, and the horsepower per dollar ratio pays off a lot better.
An extra point-and-a-quarter compression in an airhead is worth about 3-5 horsepower, and the bike we're talking about is breathing through end-of-the-line small valve heads and carbs. There's only so much potential there; maybe 67-70 horsepower, maximum? A new Suzuki SV 650 has more. A ten year old Bandit 600 has more. Hell, a thirty-five year old Kawasaki Z1 will beat that, and a forty[I] year old Honda CB750 will match it, stock.

To me, if I'm not going to Bonneville or vintage racing it, the fun of building such a thing would have to be [I]in the building it, 'cause the end result alone is not improved enough to convince me it's worth the trouble.

[rinty]

You can get...108625

Yep, I've had fun drooling over their catalogue, and my tech has had plans to build a Seibenrod for some time.

I fantasized a long time about building an uber airhead, and enjoyed riding this (non - Seibenrock) one for many years:

http://rinty.smugmug.com/photos/213513380_hbVSA-M.jpg

http://rinty.smugmug.com/photos/213513470_szQjR-M.jpg

http://rinty.smugmug.com/photos/213513608_fMBEn-M.jpg

Built by Dave Anderson, of Anderwerks, the feature sheet was two pages long.

[DOINTHETON]

have owned three r90's and three r 100's. the last one, a 77 i just recently sold.kinda ratty cosmetically but absolutely the strongest runner of them all. would of run circles around my seibenrock r80/1000cc conversion. only r bike i owned that would loft the front wheel with throttle only in first gear. for sale again too ($2100.00).....do i see a cheape cafe in my future?

http://img.photobucket.com/albums/v224/billpack/DSC02087.jpg

[sumran]

'74R90S CR is 9.5 to 1, after the first overbore and new pistons, bolted up without shims, the CR should now be higher, no? Considering the increased volume, same stroke, same heads and gaskets? It wouldn't be much, maybe a tenth?

Increasing the bore should be neutral as it will have the same ratio. It is a ratio of the highest volume to the lowest volume. Decreasing the clearance between the top of the piston and the cylinder head will increase the ratio. Increasing the clearance will decrease the ratio. Eliminating the shims will increase the CR.

[fracture]

I appreciate the responses. I have thought of doing this when I need to have things taken apart for other reasons, such as head work, replacing pushrod seals, etc. Right now it runs well so there is no need to do anything.

True, the gains are modest at best. As was mentioned, just about anything else outperforms an airhead. But, for me anyway, it is not about how fast it will go, how much power I get out of it, etc. All I can hope for is to make a pretty good bike a little better.

The cost could be prohibitive. But, the way I see it, if sound economics was the only factor any of us used to determine if we should ride a motorcycle, I would bet that very few would be riding.

[barryg]

The '88-'95 R100s were peppy but definently not fire breathers. Those bikes had the small carbs, small valves, wimpy exhaust, and mild cams to match the low compression. All of those features have to be addressed as a package to make the bike come alive performance wise.

[rinty]

Fracture:

Concerning the performance differences of the 1000's, all three series of them run within a fraction of a second of each other in the quarter mile. The best times are about 13.3 for the pre-'80s ones, to about 13.7 in the post '88s.

According to Bill Stermer, in his book R 100 RS, the post '88s pull a lot better in the mid range, but give up some top end performance.

...if sound economics was the only factor...few would be riding

Agreed. It's about trying to make a good bike a bit better. In any event, for solo riding, in most situations, the 1,000s have adequate power. I figure any avocation costs money; only some are more expensive than others.

[RandyB]

IIRC, Matt Parkhouse was working on a book about the 1000s. Boomers, I think he called them.

[8ninety8]

Sumran, then, theoretically if the bore were increased infinitely and the stroke and volume of the combustion chamber remained the same, the CR would still be 9.5 to !?

Wouldn't forcing a larger volume of air into the same space increase pressure? Or, is cylinder head pressure different from CR? Not that it matters, keeps alzheimer's at bay.

[keelerb]

Sumran, then, theoretically if the bore were increased infinitely and the stroke and volume of the combustion chamber remained the same, the CR would still be 9.5 to !?

Wouldn't forcing a larger volume of air into the same space increase pressure? Or, is cylinder head pressure different from CR? Not that it matters, keeps alzheimer's at bay.

If the bore's getting bigger and the stroke isn't changing, by definition, the volume of the combustion chamber is getting bigger (and the CR is increasing).

[sumran]

I will confess I am not an expert, but I never let that get in the way of posting.:blush

If I understand compression (and geometry) correctly, it is a ratio of volume. so if you had 10:1 on a 500 cc volume only 50 cc would be left at TDC. A bigger bore and pistion would increase the volume (displacement) at BDC, but it would increase the volume at TDC proportionately. Therefore the ratio would stay the same. Displacement and power would increase, assuming fuel burning efficiency was constant and properly metered.

Now I may not understand compression, geometry or engine theory correctly, so please feel free to help me out on any of the above.:ear

[keelerb]

Sumran, I'm no expert either, but here's how I understand it.

If you increase the bore, you will increase the volume of the area the piston sweeps* - i.e., the displacement. The combustion chamber size, however, will remain exactly the same (unless the piston has a negative deck height). If the piston does have a negative deck height (is "down in the hole" at TDC), you will increase the combustion chamber but only marginally - the piston will only be slightly in the hole, as compared to the distance swept from BDC to TDC. So the changes won't be proportionate - the swept volume change will be greater than the combustion chamber change and this will increase the (static) compression ratio. This is because of step two below - multiplying by HEIGHT. The height of the stroke is far greater than the depth, if any, of the below deck height.

That's probably not 100% technically right (particularly my nomenclature) but hopefully conveys the picture?

This is why the prototypical Harley big-bore from 88 to 95 cubic inches (been on my list for a while but my Enduralast and new shocks for the Harley done ate my big-bore budget for this year), or these days 96 to 103 cubic inches, without any other change, slightly increases the (static) compression ratio.

There are loads of online compression ratio calculators, so you can verify this by inputting a set of data and then changing only the bore. (And speaking of bores, I'll shut up now....)







*How to Measure a Cylinder
Radius=r

pi = 3.14259 (approx.)

Area=A

Height=h

Volume=v

First calculate the area of the circle: A=pi*r*r

Then multiply by the height to attain volume: v = A*h = pi*r*r*h

[sumran]

Sumran, I'm no expert either, but here's how I understand it.

If you increase the bore, you will increase the volume of the area the piston sweeps* - i.e., the displacement. The combustion chamber size, however, will remain exactly the same (unless the piston has a negative deck height). If the piston does have a negative deck height (is "down in the hole" at TDC), you will increase the combustion chamber but only marginally - the piston will only be slightly in the hole, as compared to the distance swept from BDC to TDC. So the changes won't be proportionate - the swept volume change will be greater than the combustion chamber change and this will increase the (static) compression ratio. This is because of step two below - multiplying by HEIGHT. The height of the stroke is far greater than the depth, if any, of the below deck height.

That's probably not 100% technically right (particularly my nomenclature) but hopefully conveys the picture?

This is why the prototypical Harley big-bore from 88 to 95 cubic inches (been on my list for a while but my Enduralast and new shocks for the Harley done ate my big-bore budget for this year), or these days 96 to 103 cubic inches, without any other change, slightly increases the (static) compression ratio.

There are loads of online compression ratio calculators, so you can verify this by inputting a set of data and then changing only the bore. (And speaking of bores, I'll shut up now....)







*How to Measure a Cylinder
Radius=r

pi = 3.14259 (approx.)

Area=A

Height=h

Volume=v

First calculate the area of the circle: A=pi*r*r

Then multiply by the height to attain volume: v = A*h = pi*r*r*h

Thanks for the info. Now I just have to figure out whether it is geometry or engines I don't understand.:laugh But it is too early to think that hard.

[8ninety8]

I guess the first overbore doesn't lower the CR, and that's a good thing. Thanx again for the formula.

[vanzen]

The concept of "upping CR" to regain lost power –
whomever was responsible for that loss ...
is not a new one.

Merely raising CR from 8.2 to 9.5 will have minimal impact on performance #s,
even if you would be reluctant to admit that after spending all that $$ for the swap,
and the continued payments for premium at the pump.

Bear in mind that OTHER changes were also made to the engine by BMW,
not the least of which was a change in cam timing ...
...just as a way of saying that "upping the CR" is not the whole enchilada.

Later engine tuning (w/ lowered compression, small valves and carbs, etal.)
emphasized or "redistributed" improved torque delivery throughout the mid-range rpms
vs high end HP output of early S & RS models,
and effectively retained "felt power" and performance at rpms most commonly used.
Watch your tach – you'll find that you spend most of your riding time in that place !
Plenty of mid-range torque on tap may be where it's at,
and nothing is gained where nothing is lost.
Peak HP & TORQUE delivery compliment each other in a kind of "balancing act"
where Peter is usually robbed to pay Paul.

Differences in displacement and FD gearing not withstanding,
I will suggest that a stock mid '80s R80 and a stock mid '70s R90S are quite competitive in terms of acceleration until @ 6,000 rpm.
If you really want to "impress the audience" with tire-burning acceleration –
keep an FD w/ lower gearing in the garage to use on those occasions.:whistle

Retrofitting an engine to "pre EPA" specs will increase HP output ...
but the question to ask yourself is this:
Will the expense and work required to implement these changes result in advantages
that can be appreciated within the context of my use of the bike and usual riding behavior ?

my 2¢

[keelerb]

Vanzen, you probably know more about this than I do (that wouldn't be hard, BTW). FWIW, my understanding is that the first point of compression increase is worth about 5% in terms of increased power, and that the increase percentage halves after that (so a 2% increase in CR equates to a 7.5% power increase). Dunno if that's right.

As I said, a 2 point increase in my Ducati's CR was very noticeable. Half that, well, probably somewhat less, but more than half, if that formula is right (2/3, actually, right?) :whistle

I do find airheads a bit torque-lite (there, I've said it -again!) so I have to think (hypothetically!) that a CR increase, ceteris parabis, as they say, would be particularly helpful. :ear

But others may have different views. (Calling all Beemer weinies, among others!) :doh

FWIW, as always - BrianK

[lostboy]

Raising compression in an otherwise stock BMW will give it more torque, particularly in the mid-range. I never build motors with more than 9:1 CR anymore, unless the owner understands that he will be commited to buying premium fuel and dealing with a motorcycle that is not happy in high ambient temperatures.

This thread was started by someone who wanted to up compression in an'88 on
R100. As far as I know, there is no 9.5:1 BMW piston available for this cylinder head configuration. The early ('81-'84) pistons won't work with these heads. Skimming the heads is a good choice here; bringing the pistons closer to the heads will increase the compression and efficiency by reducing the squish clearance.

[keelerb]

Motoren-Israel has some higher comp pistons for R100s (as well as big bore kits). They ain't cheap! Ones I've seen are a bit higher comp, 10 or 10.5:1, but I imagine you could reduce that, if desired, by shaving the piston crowns or maybe installing a base gasket or the like.

I'm not recommending any of this, just pointing it out! - Brian

[8ninety8]

If someone said this before, I agree that changing to a sport cam makes the engine come alive. I realize that compared to other contemporary bikes, the performance of a really souped up classic Boxer ain't much. Maybe 70 HP.

I haven't done this lately but after I put my 90S together, as I always did with any vehicle, took it out on a straight section and held it wide open, and allowing for whatever variables, the speedo bounced right around 125 mph. I didn't think it was any big deal at the time because there were bikes that could do 140.

But now I'm sitting on something that is 35 years old!

One time on I90, at cruising speed, I began hearing a strange droaning noise. I thought the motor was getting sick, and then I saw a 63 Chevy pickup creeping up on my taillight. So, up to 100 for awhile, until the Chevy gave up. That's my story, and I'm sticking to it.

I salute us all who keep these beautiful bikes in the road.

[barryg]

There is something about doing little things to hop up an old pushrod motor, that just makes one feel good. I got my '74 R90 back in '88. I saw the 90S had a drilled airbox, borrowed a stepdrill and replicated it on my /6. Didn't give me any more hp, but it sure made me feel good. And it didn't cost me anything, which is very important in the hop up feel good mindset.

[RandyB]

. Didn't give me any more hp, but it sure made me feel good. And it didn't cost me anything, which is very important in the hop up feel good mindset.

:laugh

Kinda like putting an STP sticker on your car makes it go faster. Of course that costs money.

[barryg]

Hole drilling the airbox didn't give me anymore hp, but it did lighten the bike a couple of oz.. And we know less weight increases acceleration. ;)

[RandyB]

Prevents it from running lean as you cross Crowley's Ridge as well. All that elevation change. :stick

[barryg]

Yes, I have to rejet my carbs for max performance, but on the positive side I can run a lower grade of unleaded fuel due to less oxygen. :laugh