Welcome to Cafe Beemer's technoblog
A collection of technical ramblings and articles
blogline
17/08/11 Measuring compression ratio - method 1 (With a confounding result.)
17/08/11 Measuring compression ratio - method 2 (I don't think anyone else has done it this way!)
20/08/09 Some notes on beancan mods for dual plugging - Part 2
08/08/09 Some notes on beancan mods for dual plugging - Part 1
09/07/09 Have I found some warts?
07/07/09 Dellorto's and the air/fuel mixture curve
29/06/09 Comparison of spark timing curves for dual plugged
airhead engines
29/06/09. Comparison of Spark timing curves for dual plugged post 1981 airhead engines.
If you've researched this topic on the web, you will most likely have found two main camps for the spark timing of a dual plugged airhead engine. Both seem in rough agreement that timing should be retarded by approx 4-6 degrees at maximum advance compared to the stock BMW timing specification. This retardation of the spark (compared to the stock setting) is necessary because - to put it simply- the two flame fronts from two spark plugs get everything happening more smartly.
The two camps may be roughly summarized as follows:
Camp 1) For those who don't want to fiddle with the innards of the mysterious timing canister or 'bean can*' there is little choice but to retard the timing at idle from the BMW spec by said 4-6 degrees. This does not change the shape of the timing curve that the stock bean can delivers, but shifts it in its entirety by the same fixed amount across the entire rev range.
The 'S' mark on the fly wheel (or clutch carrier as the later, lightened items are called) is the stock timing mark at idle and is 6 degrees BTDC. The 'OT' mark represents TDC (zero degrees). So, to retard the stock timing by 4-6 degrees, people are timing their machines at idle so that the OT mark is at, or around the center of the timing viewing port.
The stock maximum advance of approx. 32-33 degrees (depending on the model year) is designated by the 'Z' mark on the flywheel and occurs at around 3000 RPM with the stock bean can. The retardation that is introduced at idle also shifts the point of maximum advance by 4-6 degrees to appox 27-29 degrees. In which case, you and your strobe may just see the 'Z' mark hanging around the very bottom of the viewing port at engine speeds of 3000 RPM and above.
A handy number to keep in mind is that one degree of crank angle corresponds to about 2mm along the periphery of the flywheel i.e. where the various timing marks are stamped.
* 'bean can' refers to the timing canister found in >1981 airhead motors. i.e. when electronic ignition was introduced. Not to be confused with the bean can found in models immediately proceeding 1981 which contained points style ignition.
Canp 2) For those who are brave enough to mess about with the fiddly bits in the bean can, springs can be played with, bobwieght masses changed, and travels altered in order to change the shape of the original timing curve. An infinite array of curves can be achieved in this way.
There is rough consensus within this camp that in addition to retarding the maximum advance by 4-6 degrees, dual plugged engines enjoy curves that are stretched compared to the stock curve so that maximum advance occures at higher engine speeds than stock. Some say 3500 RPM is ok, some say 6000 RPM!
The timing at idle is generally set at around stock (the 'S' mark) and the range of advance that the bean can delivers is compressed by our 4-6 degrees in order that the timing at maximum advance is retarded by that same amount.
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Its just all too fuzzy. Is it worth going to the effort of modifying the bean can for a dual plugged airhead (Camp 2)? In the absence of hard evidence which is relevant to the particulars of my engine, I decided to indulge in a little shed science and find out for myself.
I went about the business of modifying a bean can according to vagaries of the 2nd camp as described above (Go here for details). Installed it in my bike and set the timing at idle to around stock. In this case, just a tad retarded on the stock figure at 5 degrees BTDC. With degree wheel and strobe, the curve of the modified can was mapped and graphed.
The two camps are represented below. The black curve is the stock can with stock timing.
Note that with the chosen timings at idle, the curve for the molested can has maximum advance of 29 degrees occuring at approx 3750 RPM, while the stock can (with timing at idle set to OT) has 28 degrees @ 3000 RPM.
Talking of making things more meaningful, here's some background information:
Bike: Customised 1981 R100RS.
Engine: Stock except for the following modifications:
- Dual plugged using: Stock electronic ignition; 2 x PVL twin post coils in series. (0.7 ohm each); Copper core leads and 5000 ohm NGK caps.
- 336 degree cam, with required mod's to piston pockets and valve heads.
- Heads are stock 42/40mm except for extra plug hole and a little 'cleaning up' of ports.
- 38mm Dellorto PHM carburetors roughly tuned to:
Pilot: 65
Main: 165
Needle jet: 260
Needle: K4 at top most notch.
Slides: 60/5 - Unifilter foam pods (un oiled)
- Exhaust: Stainless 38mm headers (with balance pipe) to custom 40mm 'straight through' perforated pipes surrounded by sound insulating material. Approx 550mm long.
State of tune: Pretty basic if not crummy. The Dells received a cursory rejetting (as detailed above), were synchronised and idle mixture set. With stock bean can set to the above specs (camp 1), the bike felt very constipated for 1st half of throttle range. With the modified bean can, the arsometer registered a noticeable improvement across the rev range but still constipated.
There was nothing for it but a trip to the Dyno man to get a clearer picture of what was going on.
Well, not a very encouraging looking graph, you can see the constipation in the form of huge hole in the torque curve resulting in a flattening of the power curve centered around 1/4 throttle. Also the peak numbers look poor and the power curve really shouldn't fall away like that. Something is seriously amiss. More on this later. (Skip ahead to comments on some warts that may have affected this dyno session- here)
Compared to the red curve of the stock can (camp 1), the blue modified bean can curve (camp 2) shows increased torque and power throughout the rev range and a 3hp difference at peak. This is pretty compelling considering that I just had a stab at the modified curve. You could no doubt make further improvements with a tweek to the can then dyno and repeat 10 times but that would be a bit silly before addressing the other issues which the chart raises.
A key indicator when performing tuning at this level is the presence or otherwise of detonation or knocking. Without a knock detector, we relied on our ears. In a small sealed dyno room with 1000cc's screaming through unrestricted pipes, the 'ear' may seem a somewhat less than accurate instrument if it weren't for a fortunate feature of your average earmuff. The earmuffs we used seemed to attenuate lower frequencies which had the effect of isolating the higher frequencies caused by knocking. I'm pretty sure we would have picked up moderate to bad knocking but non was evident.
So, Is it worth going to the effort of modifying the bean can for a dual plugged airhead (Camp 2)? It seems so. At this stage of investigations there seems to be clear and noticeable benefits. However in addressing other tuning deficiencies, the degree and shape of this benefit will no doubt change.
Stay tuned for the next chapter where the next chart will show the all important air/fuel ratio curve with all it's secrets.
Acknowledgements:
- Oak Okleshen. Oak has a wealth of knowledge about airheads and helped me out with advice on the dual plugging. AskOak@aol.com
- Jim Roche or AKA 'Dr. Curve'. His article on Basic Road Rodding
-Tom Cutter's article on dual plugging is well known. You can find it here.
07/07/09. Dellorto's and the air/fuel mixture curve
Here's the power V's air/fuel ratio chart from the same dyno session as above.
There's a large lean spike in the air/fuel ratio at around 1/4 throttle coinciding with the hole in the torque curve. With the Dellorto's, mixture at 1/4 throtle is mainly controlled by the wedge shaped cut-out at the bottom of the slides. Smaller the cutout, higher the velocities and venturi effect therefore more fuel sucked in. So I swapped the slides (60/5) with ones that have a smaller cutout (40/1).
I also raised the needle by one notch (now at middle notch) because the chart also shows things are a bit lean in that part of the throttle range controlled by the needle and it's jet (centered around 3/4 throttle).
The difference was marked: The bike crossed an invisible line from being a frustrating ride to an enjoyable one! The arsometer registered more grunt at 3000 RPM and the bike pulled harder from then on..
These changes seemed to add a little top end power but is this all I can expect from this modified mill? The elephant in the room is compression ratio. The bike is stock 9.5: 1 (theoretically). And the theory says that aside from the sports cam liking the CR increased from stock, it actually reduces the 'dynamic' CR of a stock bike due to the increased overlap. So increasing CR is important if you want to properly capitalise on the 336 sports cam.
So before the next dyno run I'm going to increase the CR to 10 - 10.2 and see what happens. I would really like to physically measure the CR so I can confirm what I'm dealing with... we'll see.
Stay tuned!
09/07/09. Have I found some warts?
Something has been bugging me for a while: A couple days after the dyno session I took the bike for a ride and noticed that the LH carburator spigot was loose in the head i.e. The carby and spigot could freely rotate. Was this the case during the dyno session?
I've been in denial about the possibilty that I might have missed such a simple thing and that the session was therefore compromised. If this is the case then it certainly would go a long way in explaining the not so impressive peak numbers and why the power curve drops off the way it does from about 3/4 throttle on (Thanks for the heads-up Oak!). The oxygen sensor was shoved up the RH exhaust so would not have fully seen the problem (if it existed).
Another possible gremlin which has become apparent since the session is that the clutch slips at high loads (the clutch is brand new). Again, if this was the case during the dyno session, it would certainly have stuffed things up.
All this is against the backdrop of what my sphinctometer is telling me: That this bike has considerably more grunt than any other stock R100 variant that I had ridden.
The take home message here is that I did not spend enough time making sure the bike was properly sorted before rushing off to the dyno man. The issues outlined above are basic, basic stuff which would have been picked up had I been more patient - let this be a lesson to you all!
I have therefore decided to tend to the above issues, recheck everything, and have another dyno session before increasing the CR. This will give me a picture for the latest carby tweeks mentioned above and will hopefully provide a more reliable baseline against which the effects of increasing the CR can be measured.
Stay tuned!
11/08/09. Some notes on beancan mods for dual plugging - playing with the bobweights
Here are some details of what I did to modifiy the timing curve of the stock beancan as shown in the graph above.
The engine speed at which maximum advance is reached is dictated by the weight of the conterweights and the stiffness of the small retaining springs. Being a 1981 model, my bike was fitted with the earliest of hall effect beancans. These had heavier counterweights. As a consquence, the springs are slightly stiffer than the later versions.
Modifying older counterweights
I began by attempting to lighten the old weights by drilling them. This was disastrous! The older weights are case-hardened and nigh on impossible to scratch, let alone drill (even with tungsten carbide bits). So, if you have one of these older beancans, don't attempt to drill them lest you destroy the weights and lots of good tooling (note cracks in the image above - and that was the good one!). Instead, grind in the safe area indicated.
As I didn't go with this method, it is unclear how far up the rev range the point of maximum advance will be pushed if the indicated area is removed. However, it would be fairly safe to say that removing indicated area would push max advance up 'a few hundred RPM'. You would have to make your own measurements to confirm.
Important:
When removing material from counterweights, both older and newer, make sure this is done as a pair with both weights securely clamped together in perfect register. This ensures that the same amount of material is removed from the same area of each counterweight.
Modifying newer counterweights.
After demolishing the counterweights of my 'older' beancan, it was off to the wreckers where I found a damaged can that cantained a pair of newer (undamaged) counterweights as pictured above. These weights are lighter than the older ones so it followed that using them with the stiffer springs of the older can should push proceedings up the rev range by an amount.
In addition to this, on an intuitive whim, I ground the little knob off the top of each weight as shown above.
The measured result of these mods were to push the point of maximum advance up to around 3750 RPM as shown in the graph above
If you don't have the stiffer springs of the older can to use with newer weights then you are limited to the removal of material from the weights. The good news is that the newer weights are not case hardened and can be drilled which is what many folk have indeed done. So the common trick of drilling a 1/4" is OK in this case.
20/08/09. Some notes on beancan mods for dual plugging - restriction bob weight travel
The last blog entry described how the point of maximum advance can be delayed to occur at a higher RPM. Remembering that with the second camp as described above the timing at idle is set to close to stock. Therefore, in order that maximum advance is retarded by 4-6 degrees, the range of the beancan needs to be restricted. Some people bend the metal 'ears' or stops inwards a little but I think this would be a little difficult to bend both by exactly the same amount. I opted to cover the 'ears' with heatshrink tubing as shown in the photo below.
I used three layers of heat shrink tubing: 2 layers of 5mm diam. tubing following by a layer of 7mm diam. tubing. Both the 5 and 7mm tubing had a wall thickness of approx 0.25mm as measured with a vernier caliper. The results are illustrated in the graph above.
As the graph shows, for the modified beancan, advance at idle was set to 5 degrees and max advance ended up at 29 degrees therefore the modified beancan advance range is 29 - 5 = 24 degrees. This is compared to a range of 27 for a stock beancan (depending on year). So for a stock advance at idle of 6 degrees: 6 + 27 = 33 degrees - the stock figure for maximum advance (again depending on year).
Correct ignition timing is dependant on a multitude of variables. Variables which are often variable through the REV range to boot! The relationship between spark timing and Air/fuel ratio (AFR) is hugely important as AFR influences the speed at which the mixture burns and the crank angle at which peak pressure occurs. Changing one will change the other as amply illustrated in the AFR graph above.
Strictly speaking, I should have started with a much better AFR profile before drawing conclusions about the effects of modified spark timing curves. I have just purchased an Innovate LM-2 broad band digital AFR meter which logs AFR against RPM. A bung has been welded into the exhaust headers to accept the oxygen sensor and I'm looking forward to getting the Dellorto's properly sorted.
Stay tuned!
