Better better better

[Martin in Bellevue]

I am interested in doing another compression test on yer car. After reading A Graham Bell, I think we may have experienced a change in the effective overlap, by advancing the cam as much as we did.

I'll order one of the available cam gears soon. Would there be a noticeable difference with just 1 degree of advance?

[Bill R.]

they made the cap so big on the 6's the further out the rotor arm is the greater the arc of travel between 60 degrees and that means they can make the contact area on the rotor that much wider to allow for variations in timing...Always wondered about that since on a lot of older cars and trucks a 6 cylinder cap was half the size of these...

I am interested in doing another compression test on yer car. After reading A Graham Bell, I think we may have experienced a change in the effective overlap, by advancing the cam as much as we did.

I'll order one of the available cam gears soon. Would there be a noticeable difference with just 1 degree of advance?

[Jeff N.]

...on computer controlled cars where it was just a body that routed spark... I noticed when looking at them that the outer contact area on the rotor was really wide much wider than an older dist....This is what allows the computer to advance the timing or retard the timing without the rotor position changing... This wide strip is so wide that its always in contact with the cylinder thats currently firing regardless of how much advance or retard the computer cranks into it.. my guess is that the factory sets it up for the spark to occur somewhere near the center of that rotor outer contact area at around 15 degrees advance, this was when the computer retards the spark theres plenty of contact still touching the contact on the dist cap and when it advances the timing theres still plenty of contact with that brass strip..

Yup. This makes sense. Timing is triggered by the CPS. The CPS is tied to the crank which has not changed. The distributor has no timing advance impact at all. The timing is controlled by the CPS and DME.

Now when you advance it by moving the cam gear one or more teeth, I don't know if the cam was just ground different from the dist indexing point or not...

If it helps, the blank appears to be a BMW cam. I suspect that the indexing point is the same buuuut...I'll ask Jim next time I call him.

but if you moved it a tooth or more how far do you have to go until you reach the point that the postion of the rotor will be too far away from the contact on the dist cap when the computer is calling for full advance based on engine speed ,load ,temp, tps setting etc...There will be a point at which that rotor is going to be closer to the next cylinder on the cap in the firing order and it will just jump to it instead... So my answer is that no I don't think it will advance the timing, until you go too far and then it really will advance the timing at high advance conditions...

Exactly. Winfred concurs I believe.

I don't know if you'll hit that point or not... If you do you may have to come up with a way to reindex the dist to back it up some so the rotor contact area is widest over the cylinder you want it to be on... I'm just speculating here since i haven't played with the bmw cam timing and don't know what the limitations are in the movement...but it seems to me that anytime you start moving the cam and the dist moves along with it, you're going to have to eventually come up with a way to move the dist back to its original relationship to the crankshaft position sensor... Since you have 6 contacts in the dist cap and they are 60 degrees apart from each other, you'd have to set the crank on tdc number one and look at the rotor relationship to the dist contact to determine how far you could move it until the next contact in line is closer for the spark to jump to...

Yes. It would sure help if we could figure out what's the max advance the DME adds. A dial-back-to-zero timing light would be good. Grrr. Don't have one.

But Martin is correct in that your static timing at idle should still be unchanged.. until you move it to the point that the next contact is closer...something interesting to think about ... measure the width of the contact area on the rotor and the distance between contacts on the dist cap and then look at your starting point at tdc....It'll be obvious at high rpm ,high load , when you've moved the dist too far...

OK...my grey matter fades here. Maybe its the Martin reference... Lost ya.

[Jeff N.]

I'm guessing 2 will be real nice for you and the Korman would let us try mine in the 5 and 7 degree areas.

[Jeff N.]

Actually George, see the other thread. I think the developing consensus is that changing the mechanical cam timing has *zero* effect on the base ignition timing. Or, did I misunderstand what you said?

This is good discussion...something that hadn't occurred to me either...changing the cam timing will affect the ignition timing because of the rotor being directly tied to the front of th cam. I am not sure it is a bad thing however if the ignition timing goes along with cam timing...but certainly will change when the explosions occur with respect to crank and piston positions...get too advanced with respect to changing teeth...then you dramatically advance ignition timing. I presume it would be best however to use the custom slide in the adjustable cam sprocket versus jumping teeth to advance the cam thereby preserving original ignition timing. Mark D or perhaps Bill may know about the relationship between the ignition timing MAP, the CPS and what control the DME has over changing advance versus the fixed ignition timing of the distributor/rotor connection to the cam. I would be interested as well. It makes sense that the DME has to override the fixed hardware connection of the distributor to supply a graduated advance as the engine climbs to higher RPM. My interpretation is the CPS is primary to injector firing but perhaps it supplies crank position and speed to the DME to coordinate ignition timing advance as well.
George

[Martin in Bellevue]

I'm guessing 2 will be real nice for you and the Korman would let us try mine in the 5 and 7 degree areas.

[Jeff N.]

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[George M]

The above is all speculation Jeff until you measure the ignition timing, though what Bill writes has a lot of merit. Bill has to be correct because there is clearly an advance going on at higher RPM's for the engine to run properly and it is not a mechanical advance as there is no mechanical advance feature of the BMW M-30 distributor...it is fixed and not like old vacuum/mechanical weighted point type distributors with rotating base plate and dwell window I used to play with for hours by changing springs and weights to get the curve I liked on old muscle cars.
As to whether low RPM ignition timing stays the same if you jump the cam a tooth or not...you won't know till you put a light on it before and after like I suggested and also you won't know what the total advance will be either...why I made the recommendation after Winfred mentioned using a light which is the only way to know for sure. Total advance may be changed...expanding on Bill's comments if you move the cam a tooth. What would be interesting is doing the simple math to determine what the net effect would be. Measure the width of the rotor contact and duration in terms of degrees the rotor stays in contact (with any cylinder) with cap terminal contact and compare it to the net effect of changing the cam/distributor advance by 1 tooth = 1/total no. of sprocket teeth X 360 degrees. If you put a light on the engine before and after skipping a tooth you can determine not only if the baseline timing changes but if total advance/advance curve is affected because of the shift in duration the rotor is in contact with the distributor relative to how much you change the initial position of the distributor by moving the cam sprocket one tooth.
George