For many years this item was available on Jim Roche's FSU website at the address: http://mailer.fsu.edu/~jroche/road_rod.html. It is no longer available there (or anywhere) so here's my copy.
BASIC ROAD-ROD TUNING: The first stages to take toward a fast reliable, BMW Twin.
Dear Editor and Fellow Club Member:
I got many calls and several letters in response to the short vs. long rod piece in the November issue. Members seemed starved for nuts and bolts information about the BMW engines especially the 1970 to 1984 twins although we did get some questions about the K's and our collective "new baby" the R1100RS.
I have some observations about this new 1100 twin that I would like to expand on in the March issue. These involve handling characteristics, power curve adaptability in real road conditions, service tips, future performance needs and gearing possibilities down the line. For now lets just correct for our members the misinformation published by Cycle World, Motorcyclist, On The Level, and our own Newsletter. These four sources have suggested that the 1994 model will come with a 3:00 rear end drive - dropped down from a 2:81. Forget it! All R1100RS's yet produced have had a 34/11 rear (3:09) and this is likely to be the same for 1994. All transmission ratio numbers that were published with the new GS report were also incorrect and were specifications from a current K transmission. Don (our editor) and the others were given a press release from BMW that was wrong and therefore it was not his or their fault. I'm sure the other tuners who saw those figures just laughed like I did.
Almost all of the written questions I received and the phone conversations revolved around what could be done or had been done to big bore twins thru 1984. Because thi s was where most interest seemed to be for now I feel it only appropriate to start with basic "road-rod" tuning tips for these engines with emphasis on the 1000cc bikes.
What struck me most as I talked to people about their modifications and high performance desires was the amount of mis-information going around and secondly, the extent to which race engine building techniques are being applied to road-rod street machines - as in, "if it works on the track its bound to work on the open road." Tuners know this is not always true and in some cases can actually frustrate the builder and exacerbate a problem he had set out to solve.
I would like to respond to the letter I received from a member who is a MSF instructor, AMA member, and future road-rod builder. By focusing on his problems and concerns while building his 1977 RS to road-rod specs perhaps many questions being asked from other members will be answered. To Andy Van Herwynen in Appleton, WI. the following is for you:
Dear Andy, you got the flywheel lightened and balanced but there is more to it than that. Make sure that the pressure plate assembly, complete except for the driven disc, is also balanced to your flywheel. These components should be clearly marked with paint in such a way that all future assemblies will assure their always going back in the same exact positions.
Take weight (12 oz.) off of the pressure ring on the old style clutches before the balance job and if possible have both pressure plate and pressure ring "hard face surfaced" to stop any further wear of these crucial surfaces. The clutch must be stiffened up on the old models or they will slip as more power is built into the engine. In the 1970 thru 1980 bikes there are three basic ways to do this: bring the spring closer to the pressure plate; bring the pressure ring and pressure plate closer to the spring; or bring the pressure plate alone closer to the spring by installing a thicker driven disc between the pressure plate and pressure ring. To bring the spring closer to the pressure plate place a .035" to .065" hard steel do-nut ring shim between the flywheel and the spring base. These large transmission and differential shims are available from your local "Ring Power" heavy equipment shop. This adds some weight but is the least expensive route to a stiff clutch and the flywheel need not be removed. To bring the pressure plate and ring closer to the spring remove the flywheel and machine off .040" to .085" of its face surface. This is the face area where the six bolts hold the clutch assembly to the flywheel. To bring the pressure plate closer to the spring install a three or four wing metallic "competition" type driven disc which is a lot thicker than the original disc. Its overthickness requires that washers .040" thick be placed between the pressure ring and the flywheel face and on some bikes the pressure plate bolts will now contact the transmission case just slightly and scare you when the engine is first started. Don't worry - the tick will soon go away as the soft aluminum is "adjusted". New style clutches are tough but can be improved by machining their fulcrum ring contact point .030" - .050" to the outside. This increases spring pressure a whole lot. C&C offers this piece al ready machined and ready to install for about $90.00. They to should be hard surfaced but are not as susceptible to scoring as the old type. Keep in mind Andy, that the best solution is to adapt a new style clutch and flywheel, to your old (pre 81) style crank. This gives you a bullet proof clutch and gets the total weight of everything down yet another five pounds over even the most carefully lightened "old style" flywheel and clutch assembly. Swap transmissions also or replace the input shaft of your old transmission with a new type input shaft. One word of caution however - new style input shafts have 10mm less spline length engagement when mated to their driven disc and therefore tend to wear quicker than the older types.
Next Andy you said the "heads got dual plugged" - but wait a minute, lets look at that job a bit closer. Many people are getting messed up right here. I've seen so many sets of dual plugged heads where basic conceptual mistakes have been made and the damage can not be un done without great "weld back up" expense. Right off - make sure that the new plug comes in exactly at the same angle as the old one and that the two respective plug tips are equal distance from the piston and center point of the combustion chamber. Ext reme care should be taken to ascertain the "deck height" of the new plug. Note that the original plug hole is chambered on the combustion chamber side so do the same on the new one - the chambered area around the plug tip serves as a built in "flame pock et" and is very important to "flame front" start up. Of course some people do all of the above OK and still make the biggest mistake of all. Don't put a 14mm second plug in a big valve BMW twin. The casting is thinner where the new plug is going and wi ll be more heat stressed when a hole is punched thru it. The amount of head material between the new plug and the valve seats in a big valve head gets crucial when a 14mm plug is used especially if a short thread (low deck) is also used - which is almost always the case when one sends his heads out to a far away shop. Instead insist on a 12mm or 10mm plug with long threads as your 2nd plug even though the cost may be higher. More head material will remain, on the "thin side" and less chance of head cracking and seat distortion will result. Always - 12mm plug max diameter when dual plugging.
By then Andy you said that "since the valve seats needed some attention, they got a three angle cut, and all of the valve guides were replaced." Its too late now but lets hope your seat cutter knew what he was doing. So often people will interpret "three angle cut" to be just that and start grinding away with enthusiasm. By then its too late and what you thought was going to help - has now hurt. Keep in mind that you want a nice wide .075" - .095" contact face on both your intake valve and its seat. The exhaust can be a little thinner .065" - .085" but not much. You want a nice wide face on your valve and seats, not a thin whimpy one. Think of this operation as a single angle cut with two "feather touch" cuts on each side. Don't sink the seat no matter what - keep your valves high. When three angle cuts result in a narrow contact area this lessened surface increases seat pressure while at the same time reducing heat transfer from valve to seat. Instead go the other direction if possible and use a 30/60 cut (like the newer BMW's) instead of a 45/45. This will increase contact area, boost heat transfer, and lengthen the time between valve adjustment s. Don't make this common over-cutting mistake when preparing your BMW heads.
Valves can be even more complex to cut than seats but the basic wide contact face must still apply. Remember one valve lets stuff in while the other lets stuff out. Apply these rules - keep a 90 degree cut on the outer edge of each valve. This should be .040" thick on your intake and .070" on your exhaust. This important area of valve margin must not be thin especially on the exhaust - these 90 degree rim cuts help direct flow and make your valves "work". The valves can receive several angle cuts also - four on the exhaust and three on the intake. A 30 degree undercut on the back or stem side of each valve should be done along with your regular face and margin cuts. But then don't forget a radius type cut on the lead edge of the exhaust - think of this as rounding the front edge only - just before the 90 degree rim cut area starts. This helps exhaust flow but would kill intake. Exhaust valves can also be "ditch c ut" but we will discuss that in a future "race engine" piece.
Guides are not just guides, they are also seals and therefore it is crucial that they be selected with care. The ones being sold recently thru Capital Cycle Corporation for instance should be strictly avoided as they are too short and have a sloppy fit. C&C on the other hand offers an excellent guide for less money. Factory part guides fall somewhere in between these two. The best ones however are very different in design from stock. Because just a few thousand molecules of oil mixed into your combustion chamber can reduce the burn time (power of explosion) by so much, the seal of a valve guide is everything. A road-rod engine should use guides that feature a tight fit bore and a teflon lip seal on the top side. You can purchase these seals (Yamaha SR 500) and machine your guides to accept them or purchase them outright from Manley Valves, Branch Flowmetrics, or C&C. These tight bore guides with teflon scrapers are the best solution to a power-robbing oil mist leak problem.
Andy goes on to mention that he "installed a offset cam key to retard cam timing, changed to a K&N air filter, upped the idle and main jet sizes in his rebuild Bings, and raised their needles one notch." If anything, camwise, spring for the money to purchase a Factory 336RS cam while some are still available and avoid all others that anyone might try to sell you. It is possible to change the cam in a BMW without pulling the transmission but you better know your stuff before trying this. The last run of 336 cams are all square drive oil pump types and if you get one of these you must pull the transmission to replace the original oil pump of slot key design with the square hole type which is supplied with the new cam from BMW. While its true that other cams are touted as being better than the factory 336 RS cam, I disagree. Perhaps for the race track or for the person who wants to brag about how high his engine will rev - go ahead and install some other c hoice. I maintain that the 336 with its .424" theoretical valve lift and its soft ramps is the best one available for road-rod sustained high speed use where a engine will not be turned over 8000 RPM's. It has closer lobe centers than most other cams, after market or stock, which helps it maintain a broad power range at upper RPM's. It loves to work between 5700 and 7700 RPM's. Other "hot" cams have less duration with lobe centers spread apart. They will give higher peak horsepower but require more RPM to do it and must work in a narrower rev range. Their quick pop lobes and tight curve toes also add unnecessary stress to a road-rod valve train which should be made very light and easy to maintain.
Tight toe curves combined with high lift (above .480") require excessive spring pressure (above 130 pounds) which in turn requires stiffer (and much heavier) push rods. Sometimes a shortened guide on the rocker arm side is also called for which lessens stem support at its most crucial area. Stiff s prings to control heavy components increases rocker arm nose pressure and this tends to exacerbate stem shake. Stay away from all of this - go for ultra-lightness, prudent RPM's, (below 8200) medium-low spring pressure (105 pounds max), soft ramps, broa d toe curves, long guides, and reasonable lift (.410 max) - you will not be disappointed.
Offset keys to retard cam timing are being used to get better breathing at higher engine speeds but bottom end performance suffers. Remember, of the four valve events, intake closing is the most important one - pull this back too far toward BDC and you will lose any advantage of velocity that has built up in the intake track.
K&N wet gauze type air filters do not work very well with CV carburetors. You should use stock air filters with your 40mm bings otherwise you won't get good gas suck and carb tuning will be frustrating. Leave your needles in their lowest position so that they extend the furtherest into the needle bodies. This gives you the longest progression curve to work with when deciding on main jet size. I recommend boring the main jet to 3/64" for altitudes above 3500' and 1/16" for runs at sea level but this applies only to a 1000cc engine. The bings are limited in what can be done with them - the best thing is to switch to 90S Type Del Ortos and go from there - but if Bings suit you be sure and polish the dampers, seal the butterfly ends with silicon glue, and for top flow silver solder the butterfly to its shaft body - then hand file the shaft away to almost, nothing when viewed in cross sections (wide open). K&N's work great with Del Ortos and these carburetors give a great boost in power and response.
Andy continued his letter to describe installing lightened wrist pins, and of switching rings, clips, and pins till a closer balance was achieved between the two pistons. He tells of his plans to reinforce the swing arm, add a billet top clamp, put new shocks on the rear, and concludes his letter by asking what I would do beyond the things already described to increase power and/or handling to make his old 77 RS even more of a enjoyable classic.
Here are a few things Andy that you might consider beyond the suggestions I've already made. Balance and lighten your stock lifters by enlarging the oval hole lengthwise and consider installing hardened buttons on their topside if you plan to use a sport cam with larger valves.
Go to a 45mm intake valve. This can be done without changing the stock seat although you should then open up the inside diameter of the seat with a grinder to take advantage of the wider valve. You can also go up one millimeter in size on the exhaust valve but don't do this if you have already installed a second 14mm plug.
Change to after market light weight aluminum push rods or better yet to carbon fiber one of ultra-light weight which are available from several hot rod suppliers. Polish both end tips to a mirror finish and inspect for rough machined areas on the after market rods. Polish aluminum body to high finish.
Remove the excess threaded portion of your valve adjusters once the engine is assembled and valves set. Leave a little (two threads) but not much.
Radius your rocker arms with a good whetrock and check them carefully for wear ridges that build down over engine time. This will decrease stem shake at higher RRM's.
Install thrust bearings on the tops of the rocker arms but make sure these are the shrouded type to avoid excess oil loss and splash. If you want to do a little more here shorten the rocker arm so that the two needle cage bearings are within about 1/8" of each other and then cut the spacer from a old /5 rocker set to take up the space left over. Place steel shims on either end of this new spacer. Make sure that you have not added or taken any length away from stock length of rocker even though you now have a stacked rocker assembly of six or more components.
Clean up the head ports and match everything up - intake manifold to head transitio n. Don't hesitate to cut the intake valve guide away flush with track curve but be sure to de-burr inside edge.
Polish piston top to mirror finish, radius valve pocket edges, and lightly glass bead piston skirts below the oil ring - but not above.
Polish combustion chamber and valve faces while making sure that valve edge remains a 90 degree cut - not rounded, except on the exhaust rim.
Install the Dyna III system with care and glue or lock-tite its small fasteners in place. This unit has proven itself when used on the 1970-1979 models.
For years 1981 thru 1984 things get a little difficult but heres what to do. Get the entire canister with advance and points from a 1980 BMW and retro fit this to your engine. Install a spark enhancer that lowers the voltage over the points and you are ready for dual plugs and hot coils. Using the stock electronic unit in the 81 - 84 bikes has met with mixed results when advance curves were adjusted and double lead coils installed. The Dyna system works well for some on these later bikes but changing to the points type can help you avoid some expensive mistakes later. Never work with the electronic system unless all spark leads are grounded. A ungrounded spark can fry the system and you will be out hundreds of dollars.
Always use extreme care when timing the engine especially when timing the engine after installing dual plugs. Change your advance curve so that full advance is delayed until at least 4400 RPM's for single plugs and 6000 R PM's for double plugs. This curve change is much easier to accomplish with the point type canister units than with the 81 on type electronic ones.
Much discussion has occurred about how to time a double plugged BMW Twin. I believe that total combustion is more efficient and completed quicker in a double plug combustion chamber. In other works "its all over a little sooner". This "little sooner" begins to add up as revs increase so that by the time top RPM is reached the engine is running with too much advance. Using a degree wheel set your full advance timing at 33 degrees BTDC on a single plug engine and 28 degrees on a double plug engine if your revs are to be 8200 or less. Its still not that simple however, please remember that when adjusting your advance curve you still want to be at near stock timing in the lower RPM range. So many people make this basic mistake - they just retard the timing and are baffled at the power loss down low. Think instead of reducing the timing range by five degrees - not retarding the whole system by 5 degrees. And of equal importance when double plugging - move the RPM point where full advance occurs way up as mentioned above . Forget little springs - just lop off the metal tip of your centrifical advance arm s by 1/2" to 5/8" - then fine tune with springs if needed. Keep in mind that 2.04 mm measured at flywheel rim face equals 1 degree of crank movement.
Replace rod bearings, replace and precision hone the small end pin bushings in each rod to match your new lighten wrist pin, balance each rod one to the other, and polish them to mirror finish.
Lighten the big sprocket on the cam by drilling holes in its outer rim area and expanding the large holes already there. Get as much weight off as possi ble and then polish entire piece and radius big hole edges.
Change both swing arm bearings to the wide race type for closer bearing to frame support.
Instead of reinforcing your old swing arm (which almost always warps it) switch to the new large tube type - retain your old drive shaft however as the cush-spring type is too heavy.
Machine your front fork insides to receive at least 6 wiper rings instead of the normal three. Machine your front axle so that it goes 12mm further toward the nut side.
On 1970 to 1979 models change air filter housing to the twin hose type made for about 7 months of the 1978 year. These divide the breather hose into two hoses so that each carburetor gets equal case vent gas and oil mist instead of just the right carb receiving all of it.
Change the two position flapper valve in the top of engine case to the more recent reed valve type. This is an absolute must for sustained high RPM use.
Drill additional holes in your air box shells, back side and on top for better cold air flow.
Of course light weight aluminum or titanium spring retainers and keepers should be used and these are commonly available. Another valve train item that is seldom seen but should be considered are "serrated" or "demple" type spring buckets which you can get from a speed shop. These assure a slight air barrier between the valve spring and head surface - springs run cooler and retain specified pressure - especially at high RPM's.
If you are doing a ring job and don't plan to replace your pistons install the next size over rings instead of the standard set size. Hone your cylinder carefully and then file your own end gaps to .009" when checked at the smallest bore area. So often a rebuilder will replace the rings on his 30,000 mile engine and not take into account this important source of blow by and compression loss.
Bore heads for 40mm exhaust if working on a 1000cc twin. If you purchase a after market exhaust system avoid two into one systems or cross over type headers; use instead a two into two system with traditional 6 degree taper megaphones packed with glass and ending in a 45 degree reverse taper cone whose hole is half again as large as the entrance. The best systems were make by C.J. Axtell in the mid seventies.
As one final note: people always ask, "Jim, whats the most I can get from dead stock parts?" Here it is most simply. 1977 40 mm exhaust heads, 1977 piston and pins, R90S carbs, reed style flapper valve, 1978 one into two air box, and Nykisil cylinders. This should yield over 128 mph when combined with a 2:91 rear end, even with single plugs.
I could go on and on - there is so much that can be done to these old bikes but all of the above can be done fairly cheap ly and anything after this starts costing alot. Still, this just scratches the surface of performance for the old twins. If you have a specific question you would like to get another opinion on please write me.
With respect to all,
Jim "Dr. Curve" Roche P.O. Box 881 Tallahassee, FL 32302 #15227
Jim(Dr.Curve)Roche firstname.lastname@example.org high performance old twins