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Tech Forum : M20 timing - need explanation

- BMW E21 Community
   - Tech Forum
      - M20 timing - need explanation
Max   Posted Saturday, May 15th 7:12am [Edit] [Quote] [IMS] [View car]
Member
Post nr. 63
   
OC, CA, USA
85 635CSi euro, 89 325iA
Hello,

I need some engineering/physics help on this one. I don't quite understand what's happening and how I should deal with the timing so I won't end up with the burned valves or even a piston(s)
Here's the deal. M20B23 with e30 L-Jetronic and eta intake to match the ports in the head. e30 bottom end ( 9.8:1 CR) and e21 323i head with e30 325i cam and dual valve springs
I know mix and match kind of thing, but I like the results and everything except the ignition timing which I can't figure out is fine.
The car in question is e30 323i which got e21 323i head with new e30 325i cam. Obviously I can't time the engine to the old specs because of the different cam.
Power is good regardless how the timing is adjusted ( I'm not talking too retarded that it's dying or too advanced that there's a missfire)
Before the head rebuild I had problem running with the stock timing advance using US 91 octane.
When I did the rebuild I cleaned the carbon and used e21 head with 325i cam, that's all I did.
Now I don't know how to set the timing, I've been doing it by ear for years and really know how to do it. But in order to do it by ear I have to get it too advanced so it pings and back it up. On this engine right now it's impossible to do. I can move distributor and advance it to the point where the misfire starts to occur- still no pinging at all. If I go back, retard it, some loss of power, but nothing problematic too ( should become slow , but it doesn't change much).
I assume it has to do something with the cam timing, but for me it seems that the timing is retarded and I know it's not a good thing. Gas mileage is not that bad, but I want to figure outwhat is the proper timing adjustment is, so I'd have it timed as it should be.
Also I can run 87 US octane on the engine with 9.8 CR and still have no pinging at all. It's not right I'm sure.
Everything is checked, marks, distributor position, etc. Everything is working properly and motor runs great too.

Thanks for any recommendations and info.
Max

Marquis_Rex   Posted Saturday, May 15th 10:31am [Edit] [Quote] [IMS] [View car]
Member
Post nr. 83
   
UK
BMW 323i 2.7-as featured in Total BMW Nov 2002,Porsche 911/993TT
Max, I understand your questions, and can answer them somewhat- or go some way to answering them, but I'm very busy this weekend (I'm contemplating spending silly amounts on a Porsche 911!)- I can't give you exactly numerical answer though.
I'll get back to you with an involved answer after this weekend if that is OK.

Max   Posted Saturday, May 15th 11:45pm [Edit] [Quote] [IMS] [View car]
Member
Post nr. 64
   
OC, CA, USA
85 635CSi euro, 89 325iA
I knew you'd know the answer!
I appreciate any input from you and take your time, whenever you feel calculating that to me it's great.
Anyway some info you probably wanna hear. Stock distributor I'm using specifies 19 BTDC at 3000 RPM with vaccuum disconnected. I went as far as 25 BTDC or so and did not see any ill effects ( as I mentioned in previous post)
The camshaft I'm using is from late 325i ( the one with low CR and cat converter, so the valve timing is somehow retarded).
I would go ahead and replace the cam if this is necessary, I like how it runs now too, but afraid of ill effects. BTW with 30+ C outside temp I have no any issues with cooling or anything like that.
Thank you very much for your input.
Max

Marquis_Rex   Posted Monday, May 17th 1:48am [Edit] [Quote] [IMS] [View car]
Member
Post nr. 86
   
UK
BMW 323i 2.7-as featured in Total BMW Nov 2002,Porsche 911/993TT
Max, again, I'm a little pre occupied with air cooled flat sixes at the moment so can't get deep into this one yet, but I'll re-post a personal post about distributors and ignition curves I posted to someone on this forum. It doesn't answer your qustion directly, but it may give some pearls of wisdom an insight.

"In terms of distributor, I used a late E30 323i dizzy. According to my listing there have been quite a few dizzys for the M20 over the years"

E21 323i ("77-82) distributor no# 0237 302 006
with quoted timing at 22 degs at 1500 rpm
E21 323i late- 0237 302 032 same timing quoted as above
323i 1982-1983 dist no 0237 302 038
19degs at 3000 rpm
323i 1983-1984 dist no 0237 302 040
16 degs at 5000 rpm
323i 1985-1986 dist no 0237 304 025
16 degs at 5000 rpm
320i/6 1982-1983 dist no 0237 302 037
26 degs at 3000rpm
320i/6 dist no 0237 302 039
23 degs at 5000 rpm
320i/6 dist no 0237 304 024
23 degs at 5000 rpm

I may have missed some dizzies.
Evidently from the above the 2.0 litre 6 pot needs more advance then the longer stroke 2.3l. The distributor I used was the ###304-025 dizzy. After reviewing ALOT of engine data base stuff (I've touched upon this somewhere else) I still don't know WHY the the E30 323i is specced at needing such a low amount of ignition advance at high speeds. Even a relatively fast burning combustion chamber needs a good 24-25 degrees plus at 5000 rpm and looking at the BMW hemi chamber it looks like it has very little motion (BAD for burn rate) so it should need more.
This is certainly true and when I flipped from my E21 ### 032 dizzy to the E30 ### 025 dizzy I found the engine utterly lacking top end go. In this country with our fuels and ambient temps and humidity even with 95 octane fuel there isn't a tendency to knock- its probably different where you are, so please be aware. I timed the #025 distributor to give me about 27-28 degs advance at 1900 rpm and the low to mid range was spot on- I did various 30-50 mph and 50-70 mph runs to varify and compared it back to back in terms of times achieved with the older 032 distributor. In the end I ended up taking the distributor apart- inside you'll find two springs and arms which open with increasing engine speed. I modified the "high rpm spring arm stop" to give me more top end advance. I wouldn't reccomend anyone do this unless they know what they're doing - you must use a protractor and bend the cetrifugal spring arm stop! I aimed for about 26- 29 degs advance at above 5000 rpm. [ Currently the E30 #025 gives around 16 degs and the E21 # 032 around 34+ at those heady speeds!]
The e30 engine is strange in that it has a dizzy curve that after a point actually RETARDS the timing with increasing engine speed! It was this that lead me to wonder if BMW were protecting the engine for some reason!

The thinking behind me using an E30 distributor was that when you modify a car with a long duration cam- you'll lose low end engine breathing or Volumetrc Efficiency and gain top end. When you have poor VE you need more ignition advance and when you have more you need less. So the E30 dizzy seemed ideal. In retrospect perhaps a 320i/520i/6 distributor would be better as it already has more top end advance. With the small 32mm E21 ports you're probably going to need as much high speed ignition advance as you can get -probably somewhere between the original #032 distributor figure ( around 34+ degs at high end) the #038 figure. I would reccommend you find out what your distributor number is first of all."


Hope it helps a little....

Max   Posted Monday, May 17th 11:32pm [Edit] [Quote] [IMS] [View car]
Member
Post nr. 67
   
OC, CA, USA
85 635CSi euro, 89 325iA
Mine is 038 with 19 deg @3000

What I'm getting from your post is that the use of smaller intake ports and lower duration cam will require more advance. So my feelings are correct in that the timing is retarded.
I'm not going to touch the distributor because I don't have a spare one and because I know how complicated it is to adjust things in there.
So what should I set the timing ? I believe when I was checking the numbers ( trying to make it ping) I was running about 25-27 degrees at the same 3000 RPM's. Seems I can't go much further because of zunfolge change- missfire.

Thanks
Max

g323   Posted Thursday, Dec 11th 7:56pm [Edit] [Quote] [IMS] [View car]
Member
Post nr. 324
   
California
323i 1982
what is the difference between these distributor? i have a 040 vacuum unit i would like to put on a 032, would that change the timing specs?


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