Vacuum & Timing

[bpossel]

Hi All!

With my later '71 280sl w/emission "stuff", when I set my timing per the book (5-8 ATDC w/vac), my vacuum measures steady at 10".  Today I decided to advance my timing to ~5 BTDC, which increased my vacuum to 15.  My c0 is at ~2.8 at idle.  Idle in Park is ~1100 rpms, in gear drops to ~800 rpms.  As I test drove the car it seems to be more responsive & happy!  No ping, no cough....  idle in Park is a bit high...

 ???Question...  what is the opinion of this group on timing vs vacuum?  What is the correct way to setup the engine?

Note that with the timing set per spec and the lower vacuum, I had to enrichen my ip pump on the main rack by several turns to eliminate a cough through the intake.  With advancing the timing (& increasing the vac), I have now been able to set the ip pump back to the same setting as when I received the ip pump back from Pacific Fuel Injection last year....
Thanks,
Bob

[Cees Klumper]

I recall tinkering with the vacuum some years ago by changing the timing, to get the vacuum more into spec, and the engine ran less well then. Then Dr Benz advised to pay less attention to the vacuum, and focus on the correct timing.

If the engine runs better with more advance, I would leave it like that, even though it may not be as per the book. I've heard more mechanics say they set timing more 'by feel', i.e. by seeing how the engine responds rather than by looking at the exact timing vis-a-vis the book specs.

You probably need to adjust the CSS so that the idle RPM is the same in gear as in Park. At least on my car I can get them to be exactly the same, which is what the CSS is for in the first place.

[glenn]

Hey Bob, The old 'saw', an engine is just an air pump applies.  Most books say 18 to 20" vacuum for tuned, well running engines.  Knock off a couple inches for late timing, late valves, wear, etc.   Reference books will tell you what the various vacuum gauge readings mean--bad guides, manifold leaks, head gaskets, etc.  Vacuum should approach 25" on fast butterfly open/close/open etc.  If it goes over 31" ----??? run for office??

[bpossel]

Thanks Cees and Glenn,

So what should vacuum be at idle (idle ~800) on my '71 w/emission stuff?

I read an article today that talked about turning the distributor until you get max vacuum and then back it down just a bit.  If I did this on my rebuilt Metric engine, my timing at idle would be ~15BTDC or more....  My engine seems to love to be advanced and it doesnt ping... but the idle is then way too high.  As I lean out the ip to lower the idle, then my c0 drops below 1.   Any ideas?
Thanks!
Bob

[JimVillers]

Bob .... I have found that the only "accurate" way to time the engine is at over 3,000 RPM with the vacuum advance disengaged (centrifugal advanced at the max).  The Technical Data Manual specifies 30 degrees BTDC but I set my 230SL at about 37 degrees.  I agree with the other comments, so long as there is no pinging, advance it a little more.  On the other hand, increased advance will give you more top end power but it may sacrifice a little low end torque.  So find the setting that compliments the way you like to drive.

[bpossel]

Hi Jim,

Thanks for your reply and suggestions!

I want to advance a bit more, but the issue is then at idle...  with advanced timing (which really makes the engine perform well), idle is then too high, which also causes a "clunk" when coming to a stop or slowing down...

Any further advice is greatly appreciated....

Thanks!
Bob

[glenn]

Bob, Are the cam and crank in-sync or out a few degrees?  Dist. timing will be effected by how far off the cam/crank are.  Is cam/crank '0' and 30BTDC equal to cam/crank '-9' and 21BTDC?  or sometning like that?