How to identify airhead cams?

[mattcfish]

Originally Posted by supershaft

Cam specs are all at the crank. A cam that is advanced three degrees is advanced three degrees at the crank (and valves). Six degrees at the crank would adversely effect the exhaust timing too much by most tuners' opinions. Most tuners consider five degrees the absolute max to advance or retard a single cam. Most don't go past three degrees (at the crank). That's 1.5 degrees at the cam. Advanced or retarded at the cam? That all depends if it is gear or chain drive but most all tuners call what ever it is doing at the crank how they are modifying the setup at the cam.

Advancing the sport cam had nothing to do with emissions since the cam was never in a regulated bike. Advancing the 308 had to do with emissions but . . . . I don't know anyone that has changed a cam timing for idle. The timing was changed to lower peak torque around 500 rpm. Most single cams can be retarded and advanced three degrees for a total of six degrees to play with in order to move peak torque around about 1000 rpm.

As I understand it, they advanced the 336's timing before they changed the oil pump drive so this one could be either straight up or advanced.

I have a 336 in my '92 R100 which includes the later, lighter clutch and single row timing chain. 9.5:1 pistons. .025 off the heads. Titanium valve spring retainers. Custom made 4130 pushrods. 44mm intakes with modified valve head/seat ID ratio. Heavily re-shaped ports. Dual plugged with stock bean can and re-mapped ignition curve. Completely re-jetted 38mm Dellorto carbs. Heavily modified airbox. Staintune Sport mufflers. 33/11 FD. The thing makes WAY more power EVERYWHERE in the rev range including at least 1000rpm on top that it never had to start with! I love it!

I ran mine for over ten years without raising my stock 8.5:1 CR or much port work at all with great improvements from around 2500 rpm on up but now I wish I had raised the CR and ported my bike like I did just recently a long time ago. The thing is now running a LOT better than it was and it was already running a lot better than stock!

The strange thing is that valve timing is usually retarded for emissions. BMW advanced the 308 and the 336 the same amount. Some say it's only 3 degrees some say 6. Anybody ever take a degree wheel and figure it out for sure? There's a lot of ambiguity....when and why? Either style of 336 will give you more punch than a stock 308. Since you've already got one, I'd consider using it, especially on a 90S.

[supershaft]

Originally Posted by mattcfish

The strange thing is that valve timing is usually retarded for emissions. BMW advanced the 308 and the 336 the same amount. Some say it's only 3 degrees some say 6. Anybody ever take a degree wheel and figure it out for sure? There's a lot of ambiguity....when and why? Either style of 336 will give you more punch than a stock 308. Since you've already got one, I'd consider using it, especially on a 90S.

A lot of people talk about cams and know even less than I do about them and that is saying something!! People talk about check points like they really don't know what they are. Timing cams in general. This question about specs, for instance. I know you have read what you are reporting numerous times but that doesn't make it right. I do have a friend that is good with cams for working with flat track bikes. Cam specs are at the crank. Period. Look at any photo or read any book and look at the photos. NO ONE has a degree wheel on the cam(S). What the cam is doing AT THE CRANK is all that matters. Some people think that since they know that the cam spins at half crank speed they can take some specs and then do some 'figurin'' Wrong! The figuring has already been done with a degree wheel on the crank. Ambiguity? Not from BMW specs. There is all the BS and there is the specs. That isn't ambiguity.

[EDIT] Oops mattcfish! There is ambiguity in the numbers from BMW too! I knew there was in the wording but . . . .

The same amount? Three degrees is THE classic amount to change a single cam. You can't do it much more without running into troubles and if you do it much less it won't make much difference. The room for adjustment is SMALL. If you think about it, hopefully the need for a mid stroke arrived at TDC and the biggest degree wheel that you have room for will start making sense. The tiniest difference is a BIG difference! IMO rocker arm ratios and swing radii are best left out of the picture if possible! Spec duplication is key and super touchy! I have worked with people trying to time cams with the TDC determined at TDC. WAY too much crank to piston movement at TDC to even get close as far as duplicating cam specs! It's simple geometry!

Usually retarded for emissions? I don't know but it makes since to me to advance them. Why? Advancing them makes them more efficient at lower rpm. Emission testing is done at idle and cruise ie low rpm. Remember that it is easy to get all this stuff mixed up in your head when you are writing about it. Just one for instance: A BMW service bulletin that says that 336's had there timing retarded. Wrong. It's advanced.

[RGregor]

Originally Posted by supershaft

Ambiguity? Not from BMW specs. There is all the BS and there is the specs. That isn't ambiguity.

I've been told once to refrain myself from cam discussions but at least I'll share the information I have.
The BMW service bulletin that is floating around here in Germany is from July 1980 and it says:

The position of the key slot of the cam is changed by three degrees.
The timing numbers show an advance of six degrees with respect to the crank.

Nockenwelle 0°	Nockenwelle 3°
Einlaß öffnet 32° vOT	Einlaß öffnet 38° vOT
Einlaß schließt 52° nUT	Einlaß schließt 46° nUT
Auslaß öffent 52° vUT	Auslaß öffnet 58° vUT
Auslaß schließt 32° nOT	Auslaß schließt 26° nOT

Are these specs correct?
These are the same numbers shown here:
http://bmwmotorcycletech.info/cams.htm

Anyone ever checked them?

[Voltaire]

What does " remapping the bean can curve " mean?

I thought they had bobweights for advance?

[RGregor]

Originally Posted by Voltaire

What does " remapping the bean can curve " mean?

I thought they had bobweights for advance?

Normally you limit max advance by inserting two bushings, replacing the original plastic stoppers in the can.
Geometry of these bushings can be found here (under Mechanik):
http://www.derdicke.de/mot/dz.html

[supershaft]

Originally Posted by RGregor

I've been told once to refrain myself from cam discussions but at least I'll share the information I have.
The BMW service bulletin that is floating around here in Germany is from July 1980 and it says:

The position of the key slot of the cam is changed by three degrees.
The timing numbers show an advance of six degrees with respect to the crank.



Are these specs correct?
These are the same numbers shown here:
http://bmwmotorcycletech.info/cams.htm

Anyone ever checked them?

I was at least one of them.

That I hadn't noticed. Like I just mentioned earlier, I have noticed other errors in the 336 bulletins.

Like I have pointed out many times before, I have never timed mine. I am just going by what I have heard and read and seen with other cams as far as saying that cam specs are at the crank but then again I am starting to see all kinds of weird ways to list cam specs out of Germany such as specs with 0 lift compared to most all other cam sources I have seen and read of. I had noticed ambiguity in the wording but not the numbers until now. If I am wrong I won't feel too bad because when most tuners say they have advanced a cam three degrees, they mean at the lift check point with the wheel on the crank. They don't mean it's really 1.5 degrees because we know that the cam spins at half the speed of the crank? Or put another way: Most tuners are mostly talking and concerned with when that intake valve is closing in relation to the crank just like all the other spec numbers are in relation to the crank. Timing the valves IS the whole point! Pj has timed his.

[supershaft]

Originally Posted by Voltaire

What does " remapping the bean can curve " mean?

I thought they had bobweights for advance?

They do have bob weights in the advance. You can limit or extend the weights' travel with the weight stops thusly changing the advance range in degrees and then you can alter the weight's weight and extend or reduce the curve in relation to the rpm. The weight stops bend back and forth without breaking very easily. I have done it dozens of times.

[Renner]

Originally Posted by RGregor

I've been told once to refrain myself from cam discussions ...

That is unfortunate, as you offer a lot of good and interesting information on the topic.

[Voltaire]

Originally Posted by supershaft

They do have bob weights in the advance. You can limit or extend the weights' travel with the weight stops thusly changing the advance range in degrees and then you can alter the weight's weight and extend or reduce the curve in relation to the rpm. The weight stops bend back and forth without breaking very easily. I have done it dozens of times.

Gottcha,

I played around with the mapping of my ignition but only enough to replace the Boyer and its creeping advance.

The Ignitech is nice bit of kit, comes with rev limiter and you can adjust the curve with the motor running.

Would work a treat on a road bike.

[supershaft]

Originally Posted by Voltaire

Gottcha,

I played around with the mapping of my ignition but only enough to replace the Boyer and its creeping advance.

The Ignitech is nice bit of kit, comes with rev limiter and you can adjust the curve with the motor running.

Would work a treat on a road bike.

I now NEED a rev limiter. Is that curve completely adjustable. So many electronic advances do not have the curve I want. Does the rev limiter cut in all at once or partially?

[RGregor]

Now we all miss things from time to time.
For example I completely missed this

Originally Posted by supershaft

Like I have pointed out many times before, I have never timed mine

Having seen this, some other thread would have been very short.


Back to timing: of course specs are with respect to the crank. Anything else would be pure nonsense IMO.

Back to the question, how the late 336 are timed. PJ has the numbers? PJ?????

[supershaft]

I have timed other cams but not a 336 but, still, I am going to be real surprised if BMW advanced the 336 six degrees and has been calling it three degrees all these years. One thing that is starting to sink in is something that many a tuner has warned me about cams. There are no industry standards! Trends but not standards.

I am just glad we have for the most part moved on past discovering specs with lift check points.

[RGregor]

Originally Posted by supershaft

I now NEED a rev limiter. Is that curve completely adjustable. So many electronic advances do not have the curve I want. Does the rev limiter cut in all at once or partially?

Some useful information may be found here:
http://www.advrider.com/forums/showthread.php?t=830041

Some info Voltaire may not have: you can run it with an inductive trigger like Voltaire or a hall sensor. Means AFAIK that you could use the BMW unit (if you can freeze the mechanical advance).
Some guys over here use a complete can unit from a Volkswagen which has no advance.

In case you are interested I would check these information. As I prefer another solution I don't pay too much attention to the threads about that topic so I'm not a 100% sure.

Alternatively the german reseller here offers the complete package with a trigger mounted on the alternator.
Ignitech does not offer special BMW parts.

[RGregor]

Originally Posted by supershaft

I have timed other cams but not a 336 ...

I do feel the urge to a rather harsh reply to that but, ....

[Hookalatch]

[QUOTE=The same amount? Three degrees is THE classic amount to change a single cam. You can't do it much more without running into troubles and if you do it much less it won't make much difference. The room for adjustment is SMALL. If you think about it, hopefully the need for a mid stroke arrived at TDC and the biggest degree wheel that you have room for will start making sense. The tiniest difference is a BIG difference! IMO rocker arm ratios and swing radii are best left out of the picture if possible! Spec duplication is key and super touchy! I have worked with people trying to time cams with the TDC determined at TDC. WAY too much crank to piston movement at TDC to even get close as far as duplicating cam specs! It's simple geometry!

Usually retarded for emissions? I don't know but it makes since to me to advance them. Why? Advancing them makes them more efficient at lower rpm. Emission testing is done at idle and cruise ie low rpm. Remember that it is easy to get all this stuff mixed up in your head when you are writing about it. Just one for instance: A BMW service bulletin that says that 336's had there timing retarded. Wrong. It's advanced.[/QUOTE]

I have been running my R1200GS cams 9 crank degrees advanced for 5 years and 25,000 miles. Doing so gave me more midrange power (which is what I wanted) with the possible expense of some top end. I rarely go 7,000+ rpm's anyway so I wouldn't notice.

Modern auto engines with variable valve timing, while not Airheads, advance cam timing as much as 7 degrees and retards it up to 45 degrees for a total of 52 degrees variability. Having been involved in drag racing for several decades I am very familiar with the effects of advancing, and in my case more often retarding cam timing to change power characteristics. In my experience cam timing changes of more than 3 degrees are commonly done but usually for reasons other than producing maximum peak horsepower.

Chuck