Timing and Octane Questions

grey528i · Post by **grey528i** » Sun May 15, 2016 9:28 pm

I am relatively new to E12s and pretty naïve about them, but have a question I am hoping I can get help with.

I have seen posts by some who have advanced timing by setting the 22 degrees at slightly lower RPM (1700). What I want to know from those who have advanced their timing this way, or some other way is: whether there are issues with detonation when running US premium (92/93 octane) and about how much extra advance does this put into the timing?

The car in question is a manual transmission 5 spd 1981 528i.

Thank you in advance for any help you provide.

T.Hanson · Post by **T.Hanson** » Mon May 16, 2016 1:49 pm

Me and my '79 and '81, owning for 20-25 years. Box stock, timing using the book specs...

Pull in to any station, insert filler nozzle and select 89 octane, least expensive. Not once have I heard any pinging, seen evidence of anything but clean burning inspecting the plugs. Six months, two years, to only change them thinking fresh is good.

grey528i · Post by **grey528i** » Sun May 22, 2016 2:35 pm

Thank you for the response. I now believe I have the timing set correctly, but I would like to verify this because I have typically set timing on older American cars where there is a tab with the degrees of advance marked.

I have drawn a sketch of what I see through the timing window as the engine rotates and there are two timing marks (neither my eyes nor my son's are good enough to make out the faintly scribed stampings for O etc... We have set the timing to the dot marked A on the diagram below at the specified RPM.

I believe it is the right dot because it is the first dot that will show given the direction of rotation and I assume this means it is the 22 degrees BTDC mark.

There are also a bunch of bumps on the flywheel some single and some paired that I sketched in as grey dots on the outer ring of the diagram. It looks like the total timing goes to the leftmost of the two adjacent to mark A. (What is the purpose of all of these bumps?)

http://imgur.com/iuTcFw1

onovakind67 · Post by **onovakind67** » Mon May 23, 2016 7:56 am

I got tired of trying to peek at a BB flying by a little hole so I put my timing marks on the front of the engine in a manner I'm used to seeing. It is now visible through many degrees of adjustment, and I can set the timing at any rpm. It's also helpful in checking the effect of the VA can.[/img]

grey528i · Post by **grey528i** » Mon May 23, 2016 7:04 pm

That would work.

Is your engine comparable to the m30b28 engine from 1981. If so, What total timing do you run? and how does it compare with the settings recommended for the timing at 2200 with the dot (or 1700 with the dot)?

I apologize for the naïve question. I am new to BMWs and learning as I go.

onovakind67 · Post by **onovakind67** » Tue May 24, 2016 10:33 am

It's an M30B35, so it's not much different. I run 36° total mechanical timing with another 14° of vacuum advance added in. It's not the stock distributor, it's one for a 2800CS, but it was brand new and $75 on eBay.
I've never checked the stock timing mark once I got my new one going, I just run the engine up until the mechanical advance stops, set it to 36°, then plug my VA into manifold vacuum and it's done.

grey528i · Post by **grey528i** » Tue May 24, 2016 6:42 pm

Thanks for the response.

I just checked the timing using your approach, and it looks like my total mechanical comes out pretty close to 36 with the 22 degree dot set at 1700 RPM.

I will see how this works.