How to identify airhead cams?

SamH · Jan 1, 2013

I've got two cams that came with my R90 project and I've no idea which one to use when I put it back together. The lobes measure up differently between the two but I haven't been able to find much in the way of information on airhead cams.

Here's a pic of the two.

supershaft · Jan 2, 2013

It looks like it might be a 336 on the our left. I don't know what the marking are on those earlier cams. It could be other cams as well. You might have to time it to find out. If it's a 336 it could be a very early one timed straight up. That I would want to know for sure.

Kai Ju · Jan 2, 2013

Except the left cam looks to be in rough shape, especially the oil pump drive area.
I would try and clean it up before I'd even consider using it.

supershaft · Jan 2, 2013

Kai Ju said: ↑

Except the left cam looks to be in rough shape, especially the oil pump drive area.
I would try and clean it up before I'd even consider using it.
Click to expand...

It might just be stains. I hate stains but the FAA says that stained metal doesn't warrant rejection. It looks ruff but it might be usable.

SamH · Jan 2, 2013

Yes, the cam on the left certainly looks a bit rough but it's really not that bad, the picture looks much worse than reality. It's mostly staining.
How do I go about figuring out the numbers on these two, do I have to have the top end together?

SamH · Jan 3, 2013

Ok, I measured up the cams tonight as best I could. Put them both in the lathe and setup the dial gauge. I could only measure lift.

The cam marked 921 measured 6.75mm lift on all four lobes, x1.39 = 9.3825mm or 0.3693"

The cam marked 630 measured 7.64 lift or near enough on all the lobes, x1.39 = 10.6196mm or 0.4180"

Is lift alone enough to identify these two? Is there a list of specs somewhere?

supershaft · Jan 3, 2013

You could probably get a good idea from lift alone but there are a lot of cams out there now and a lot of them have just about the same lift.

There are usable specs out there for a 336, 308, Andrews, Megacycle, Crane? Chances are it's a 336 but . . . . Either way you will need to time it in order to see how it is timed. Chances are if it's a 336 it is timed 3* advanced but it might be timed straight up since it has the old style oil pump drive. I suggest reading a few books on timing cams. Other inmates know how to post the specs. There is two more recent cam threads that have most of the specs I referred to. Some inmates don't like cam discussions. It gives them a headache. It is a bit more complicated than removing grips!

Disston · Jan 3, 2013

Yeah. But removing grips is were the real money is.

supershaft · Jan 3, 2013

It's been cracking me up.

motu · Jan 3, 2013

I removed and replaced my left hand grip yesterday just to see if I could do it - thank God for ADVRider!

SamH · Jan 3, 2013

I know, it's a good thing this bike's in bits. I only have to worry about installing a new set of grips. Phew.

mattcfish · Jan 4, 2013

SamH said: ↑

Ok, I measured up the cams tonight as best I could. Put them both in the lathe and setup the dial gauge. I could only measure lift.

The cam marked 921 measured 6.75mm lift on all four lobes, x1.39 = 9.3825mm or 0.3693"

The cam marked 630 measured 7.64 lift or near enough on all the lobes, x1.39 = 10.6196mm or 0.4180"

Is lift alone enough to identify these two? Is there a list of specs somewhere?
Click to expand...

BMW specs.
308 has a lift of 9.4mm
336 has a lift of 10.68mm
284 has a lift of 8.62mm

Lucky you. #630 has the lift of a 336. An early one with a key drive on the OP is a rare thing......#921 is a 308.

SamH · Jan 4, 2013

Ahh, finally some numbers! Thanks heaps for that. I'll just have to take it at face value and accept that it's ok to use. I'll see if I can compare the two cranks and figure out if they're timed the same.

I've got the lightened flywheel, 9.5:1 pistons and dual plugged heads so hopefully all those bits will combine to make a nice poweful engine.

mattcfish · Jan 5, 2013

SamH said: ↑

Ahh, finally some numbers! Thanks heaps for that. I'll just have to take it at face value and accept that it's ok to use. I'll see if I can compare the two cranks and figure out if they're timed the same.

I've got the lightened flywheel, 9.5:1 pistons and dual plugged heads so hopefully all those bits will combine to make a nice poweful engine.
Click to expand...

Actually the timing difference is where the key slot is positioned for the cam sprocket. It's just a hair difference. The slot is 3 degrees retarded which advances the valve timing by 6 degrees (cam spins half as fast as crank). There are several different opinions as to why this was done. Emissions regulations, to increase mid-range, or to improve idle. Your slotted oil pump, points style cam probably has strait 0 degree valve timing. Not sure, but I think the advanced cams came out with the bean cans.
I'm using the later bean can 336 on my /6. I've got light 9.5:1 1000cc pistons in Nikasil cylinders, titanium valve retainers, dual plugs, crank ignition, 1983 tranny with ultra light clutch/non-existent flywheel , 32/11 rear end , 2 into 1, and 38mm Dellortos (with spot on jetting). Gotta say I love this combo with the 336.

supershaft · Jan 5, 2013

mattcfish said: ↑

Actually the timing difference is where the key slot is positioned for the cam sprocket. It's just a hair difference. The slot is 3 degrees retarded which advances the valve timing by 6 degrees (cam spins half as fast as crank). There are several different opinions as to why this was done. Emissions regulations, to increase mid-range, or to improve idle. Your slotted oil pump, points style cam probably has strait 0 degree valve timing. Not sure, but I think the advanced cams came out with the bean cans.
I'm using the later bean can 336 on my /6. I've got light 9.5:1 1000cc pistons in Nikasil cylinders, titanium valve retainers, dual plugs, crank ignition, 1983 tranny with ultra light clutch/non-existent flywheel , 32/11 rear end , 2 into 1, and 38mm Dellortos (with spot on jetting). Gotta say I love this combo with the 336.
Click to expand...

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!

mattcfish · Jan 6, 2013

supershaft said: ↑

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!
Click to expand...

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 · Jan 6, 2013

mattcfish said: ↑

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.
Click to expand...

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 · Jan 7, 2013

supershaft said: ↑

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

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.

 <table class="MsoNormalTable" style="margin-left: 100.05pt; border-collapse: collapse;" border="0" cellpadding="0" cellspacing="0" height="177" width="394"> <tbody><tr style="mso-yfti-irow:0;mso-yfti-firstrow:yes"> <td style="width:141.15pt;border:solid black 1.0pt; border-right:none;mso-border-top-alt:solid black .25pt;mso-border-left-alt: solid black .25pt;mso-border-bottom-alt:solid black .25pt;padding:2.75pt 2.75pt 2.75pt 2.75pt" valign="top" width="188"> Nockenwelle 0°
</td> <td style="width:130.35pt;border:solid black 1.0pt; mso-border-alt:solid black .25pt;padding:2.75pt 2.75pt 2.75pt 2.75pt" valign="top" width="174"> Nockenwelle 3°
</td> </tr> <tr style="mso-yfti-irow:1"> <td style="width:141.15pt;border-top:none;border-left: solid black 1.0pt;border-bottom:solid black 1.0pt;border-right:none; mso-border-left-alt:solid black .25pt;mso-border-bottom-alt:solid black .25pt; padding:2.75pt 2.75pt 2.75pt 2.75pt" valign="top" width="188"> Einlaß öffnet 32° vOT
</td> <td style="width:130.35pt;border:solid black 1.0pt; border-top:none;mso-border-left-alt:solid black .25pt;mso-border-bottom-alt: solid black .25pt;mso-border-right-alt:solid black .25pt;padding:2.75pt 2.75pt 2.75pt 2.75pt" valign="top" width="174"> Einlaß öffnet 38° vOT
</td> </tr> <tr style="mso-yfti-irow:2"> <td style="width:141.15pt;border-top:none;border-left: solid black 1.0pt;border-bottom:solid black 1.0pt;border-right:none; mso-border-left-alt:solid black .25pt;mso-border-bottom-alt:solid black .25pt; padding:2.75pt 2.75pt 2.75pt 2.75pt" valign="top" width="188"> Einlaß schließt 52° nUT
</td> <td style="width:130.35pt;border:solid black 1.0pt; border-top:none;mso-border-left-alt:solid black .25pt;mso-border-bottom-alt: solid black .25pt;mso-border-right-alt:solid black .25pt;padding:2.75pt 2.75pt 2.75pt 2.75pt" valign="top" width="174"> Einlaß schließt 46° nUT
</td> </tr> <tr style="mso-yfti-irow:3"> <td style="width:141.15pt;border-top:none;border-left: solid black 1.0pt;border-bottom:solid black 1.0pt;border-right:none; mso-border-left-alt:solid black .25pt;mso-border-bottom-alt:solid black .25pt; padding:2.75pt 2.75pt 2.75pt 2.75pt" valign="top" width="188"> Auslaß öffent 52° vUT
</td> <td style="width:130.35pt;border:solid black 1.0pt; border-top:none;mso-border-left-alt:solid black .25pt;mso-border-bottom-alt: solid black .25pt;mso-border-right-alt:solid black .25pt;padding:2.75pt 2.75pt 2.75pt 2.75pt" valign="top" width="174"> Auslaß öffnet 58° vUT
</td> </tr> <tr style="mso-yfti-irow:4;mso-yfti-lastrow:yes"> <td style="width:141.15pt;border-top:none;border-left: solid black 1.0pt;border-bottom:solid black 1.0pt;border-right:none; mso-border-left-alt:solid black .25pt;mso-border-bottom-alt:solid black .25pt; padding:2.75pt 2.75pt 2.75pt 2.75pt" valign="top" width="188"> Auslaß schließt 32° nOT
</td> <td style="width:130.35pt;border:solid black 1.0pt; border-top:none;mso-border-left-alt:solid black .25pt;mso-border-bottom-alt: solid black .25pt;mso-border-right-alt:solid black .25pt;padding:2.75pt 2.75pt 2.75pt 2.75pt" valign="top" width="174"> Auslaß schließt 26° nOT
</td> </tr> </tbody></table> 

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

Anyone ever checked them?

Voltaire · Jan 7, 2013

What does " remapping the bean can curve " mean?

I thought they had bobweights for advance?:huh

RGregor · Jan 7, 2013

Voltaire said: ↑

What does " remapping the bean can curve " mean?

I thought they had bobweights for advance?:huh
Click to expand...

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

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