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-   -   Rocker ratio airheads (http://www.advrider.com/forums/showthread.php?t=854255)

chasbmw 01-09-2013 02:09 AM

Rocker ratio airheads
 
To degree the cam on my R90/6 I need a dial gauge.

I can buy them with a 10mm or a 30mm range, which to buy?

What is the rocker arm ratio on airheads assuming that I take the cam readings off the valve spring collets?

Any advice great fully received.

pommie john 01-09-2013 03:34 AM

The rocker ratio is 1.39 to one.

My dial gauge is only 5mm throw, but it works OK. A 30mm one would definitely make life easier though. The lift at the cam would be covered by a 10mm gauge. ( A 336 has about 10.5mm lift at the valve so only 7.5 at the cam.)

chasbmw 01-09-2013 04:05 AM

Many thanks PJ

Charles

supershaft 01-09-2013 01:35 PM

I have never checked but I have heard and read many tuners saying that rocker arm ratios vary a good deal. I could understand how by the way they work. Personally, I wouldn't use that ratioed side of the arm at all. Ratioed lift and lift check points don't mix.

tofgasp 01-09-2013 02:26 PM

You do not really care if the ratio vary between one rocker and another when timing the cam, the gauge pointer is set 90 on the valve retainer so whatever the ratio variation is, the timing can always be set properly.

It 'll be surely easier and less prone to misreading with a 30mm gauge, but you mostly need to check precisely when the valves begin to open/close (priority on intake closing for the setting), so the max lift is quite useless.

The 30mm dial will be usefull for setting your disc on the TDC first, most on them have a screwed ball pointer that can be repraced by a threaded extension. Check for the same measurement after and before TDC, the real TDC is the average of the two. I've found this really the most important point of the settings.

For info I checked 10,7mm lift with a 336.

supershaft 01-09-2013 02:51 PM

The specs are at 1mm lift. That 1mm does not include rocker arm ratios whether they vary from one arm to the other or not. 1mm lift at the lash adjuster minus the rocker arm's radius is suppose to be 1.39mm minus the radius fudge on the valve. Why throw yet another variable in the mix? Getting an absolute rock solid check point lift is crucial you don't want your readings all over the map by numerous degrees. I am just warning that if you are not super anal about just checking a cam's timing, let alone changing a cam's timing, you are really lost anyway.

pommie john 01-09-2013 03:37 PM

The problems i've encountered when trying different methods of measuring cam lift is that:

1) If you try to measure on the cam follower, it has no return spring, so it travels up the tunnel to full lift, and stays there. You need one hand to turn the crank and one hand with a screwdriver pushing the follower back, acting as a human return spring.

2)If you measure at the cam lobe it's a long way inside the engine and it's hard to get the dial gauge set on it properly.

3)If you measure at the top of the pushrod, the pushrod is loose in the tunnel and flaps around and it's hard to get an accurate reading. You also have the return spring ( lack of) issue.

You simply need to find a method that works for you and ensure it gives repeatable results.
Securing the dial gauge is often the hardest part. Mine has a magnetic foot that is pretty well useless on an alloy engine :)

supershaft 01-09-2013 04:10 PM

I am going to start with method #1. If I can't manage I will get some help. I am not even going to try #2. Why not measure from on top of the lash adjuster when the engine is together? I would fab some sort of luck nut with a pad for the gauge pointer. With the engine together you can not get around the arm's radius effecting the reading a little bit but at least you don't have lift ratio entering into the mix like you do on the other side of the arm.

Securing the dial gauge and a reference point for the degree wheel! It gets real tricky!

tofgasp 01-09-2013 05:13 PM

I fix the dial gauge with an articulated arm using the center stud of the head. It does not move and allow to be in the axis of the valve.
If you've got the valve lift specs without free play, why trying to measure on the lifters ?

supershaft 01-09-2013 05:49 PM

Quote:

Originally Posted by tofgasp (Post 20440540)
I fix the dial gauge with an articulated arm using the center stud of the head. It does not move and allow to be in the axis of the valve.
If you've got the valve lift specs without free play, why trying to measure on the lifters ?

Who times cams with pushrod/rockerarm valve trains with valve lift specs? Way too many variables.

Kai Ju 01-09-2013 06:06 PM

How about modifying a pushrod by pressing a washer onto it near the top that is wide enough for contact with the dial indicator ?
Or bend a piece of aluminum into an L and attach it to the pushrod near the top. ( ziptie/small hose clamp etc )

RGregor 01-09-2013 11:06 PM

Quote:

Originally Posted by supershaft (Post 20440820)
Who times cams with pushrod/rockerarm valve trains with valve lift specs? Way too many variables.

Now, obviously BMW recommends that.
And for the specs they provide you have to measure valve lift.
How will you time your cam without proper specs? How will you check the grind?
Or are there other specs around?

supershaft 01-10-2013 11:26 AM

Quote:

Originally Posted by RGregor (Post 20442779)
Now, obviously BMW recommends that.
And for the specs they provide you have to measure valve lift.
How will you time your cam without proper specs? How will you check the grind?
Or are there other specs around?

Obviously? Where? The 336 bulletin I just read has specs at a 2mm (I was remembering 1mm earlier) lift check point "on the rocker arm". Is 2mm another typo? Why would that be on the valve side of the rocker arm when rocker arm lift ratio will completely change any reading? Our rocker arm design is notorious for having inconsistent lift ratios. Most all manufacturers list cam specs at the crank and not valve lift specs at the crank for that reason. Plus, you can always run different ratio arms on purpose! Valve lift specs vary. The cam lobes don't.

My bulletin has the advanced cam advanced at six degrees too. It is starting to look like they call advancing a cam six degrees three (except in the specs). Between that and listing SOME specs at zero lift I don't think I will be surprised at what I find anymore. Plus the bulletin I just read has the specs listed at (+2) degrees. That's the first time I have ever seen that on a cam card not that I have looked at that many of them. Hopefully we have CAM specs and not wishy washy valve specs. At least the specs for BMW cams have a lift check point!

RGregor 01-10-2013 01:05 PM

Quote:

Originally Posted by supershaft (Post 20447122)
Obviously? Where? The 336 bulletin I just read has specs at a 2mm (I was remembering 1mm earlier) lift check point "on the rocker arm". Is 2mm another typo? Why would that be on the valve side of the rocker arm when rocker arm lift ratio will completely change any reading? Our rocker arm design is notorious for having inconsistent lift ratios. Most all manufacturers list cam specs at the crank and not valve lift specs at the crank for that reason. Plus, you can always run different ratio arms on purpose! Valve lift specs vary. The cam lobes don't.

The bulletin I have (the german one) also only says "gauge on the rocker", but then "after 2mm valve lift".
So they must mean to measure the valve side.
Thus the bulletin does not contain cam specs. All data are with respect to valve lift.
The rocker ratio ideally is 1.39, but what I'm not sure of is, if there's a constant ration between lifter travel and valve lift throughout the complete movement. So the question is if you simply can calc backwards to get the corresponding numbers for the lifter side if you want to measure there.

Thus the far from perfect method to measure on the valve side may be the better one.

Max Headroom 01-10-2013 01:46 PM

Quote:

Originally Posted by supershaft (Post 20447122)
Obviously? Where? The 336 bulletin I just read has specs at a 2mm (I was remembering 1mm earlier) lift check point "on the rocker arm". Is 2mm another typo? Why would that be on the valve side of the rocker arm when rocker arm lift ratio will completely change any reading? Our rocker arm design is notorious for having inconsistent lift ratios. Most all manufacturers list cam specs at the crank and not valve lift specs at the crank for that reason. Plus, you can always run different ratio arms on purpose! Valve lift specs vary. The cam lobes don't.

Billy, you've missed the clues given by every engine manufacturer for generations including BMW, when they give cam specs. The usual information given is valve timing specs, along with total valve lift. The term "cam timing" refers to the drive from the crankshaft to the cam, and the relationship between the two.

The cam is primarily there to open and close valves at the pre-determined times, when it's not operating an oil pump or ignition system. This data is based on standard rocker arms, for one thing. A change to the rocker ratio is outside the parameters of factory timing specs.

All cams have specs arrived at by using a degree wheel on the crank to determine the crankshaft position when the inlet valve opens and closes, and when the exhaust valve opens and closes. NOT when the rocker arm, pushrod or cam follower moves.

The 2mm checkpoint is partly to get the cam lobes away from the opening and closing ramps, and partly to avoid valves tangling with pistons.

Valve timing has everything to do with the opening and closing of the valves relative to the crankshaft, therefore it is critical to know the correct check clearance (2mm in the case of an airhead) and the exact crank angle when the valves open and close.

Correct me if I'm wrong but it's my understanding that an increase in the rocker arm ratio would affect valve lift rather than valve timing, using the same check clearance. Any change in valve timing would be minimal..

YMMV etc


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