Dwell settings for 2.7 with MSD question
 

As some of you have seen, I have had some issues lately and the boards have instructed me to get the points, dwell and timing set.

Wayne's article is a good one and defers to the Porsche spec book for settings, which is fine. The book says 35 for my '77 911S with a 2.7 and a new MSD 6425. Postings have advised me to get the dwell to 65 - 70.

Which one should I shoot for?

I am hoping to wrap this up tonight.

Thanks

where in WC are you? im over by the montecito area in case you need someone to troubleshoot with. i have a 2.7 and MSD set up.

I am down by Rossmoor, unincorporated WC. My son and I are taking a shot at this tonight, probably about 6:30.

Set to the factory recommendation, that's the only way the points opening will be co-incident with the optimal rotor/contact position.

60-70 is the % of dwell, ~35 is the number of distributor rotational degrees of dwell.

If the factory says 35 degrees, then 35 degrees it will be.

Dwell is just a way to set point gap, there is a range here it is acceptable

the lower the dwell # the wider the point gap, so when I used to set it I would shoot for the widest gap(lowest dwell) in the range on the theory that as the point block wears the gap narrows and the dwell goes up.

But another way to do it is to set it in the middle and then accept that it will need to be done more often.

I did it my way for 100k mi on my '76 C3, it never made a bit of difference performance wise as long as it was somewhere in the spec range

the spec for a '76 depends on the distributor
for Bosch it is 38+/-3°
for Marelli it is 37+/-3°

Fixed that for you.

Thanks wwest.
Still learning as I go.

As Bill infers, dwell is not that critical. That's especially true for a CDI system,
e.g. a MSD on a Porsche. It's critical for an inductive discharge system where
energy storage time is critical at high RPMs, necessitating the proper dwell setting.
With a CDI system as long as the points open properly, dwell is less critical.
One can set the points to .016" and be O.K.

No, the distributor was designed such with the points set to the proper dwell they would open at the optimal time with regards the rotor position "conducing" the spark to the proper spark plug. That will vary quite a bet with vacuum and centrifugal advance.

As close to 38 degrees as you can get. I don't know where wwest comes up with this stuff....... 60 %!!...

the points setting is not that critical. in the 10yrs i owned my 77 i always set them by eye balling them. part of this WAS just to prove the points setting is not that big a deal.
i think i only changed the points once in that time too. never any issues.

the points only provide a ground to trigger the CD unit. no hi voltge or current to burn them up. just make sure the points close completely and they open. i did try to set them so they wer closed more than they were open.

Easy....I haven't had a set of points to set for maybe 34 years now, but always used my trusty Simpson multi-meter set to the ohms range.

Distributor, 360 degrees of rotation divided by 6 cylinders = 60 degree period for complete "point" cycle.

60% of 60 degrees allocated to dwell, point closure period, = WHAT?


36 degrees!

That's been my view, followed by use of a timing light to verify correct timing.

Checking the dwell number just verifies your skill with a feeler gauge. Once you get the hang of the proper drag on the feeler gauge and the points, your dwell will be dead on. All in all, a fun little exercise that provides transferable skills.

The reason i like to use dwell even though i am very capable of using a feeler gauge is that the feeler gauge is really only accurate on new points. As soon as they are used metal starts to transfer and the point halves are no longer the two flat, smooth surfaces that feeler gauges require. Using a dwell meter eliminates these factors and gets you to exactly where you want to be, a certain dwell time.

Which is also why the dwell should be rechecked within ~500 miles, by that time any "high" spots on the contacts and the rubbing block will have worn smooth.

If the CDI only triggers on the opening of the points (0-12vdc edge) then the dwell (how long the points are closed) really should not matter. But as mentioned you should always re-check timing after messing with points because you don't know when exactly they are opening after you reset the points.

Again, the distributor rotor MUST be in the proper position, at, or at least closely nearby, the distributor "connection" post for the plug to be fired currently. That's why the rotor is so tightly keyed. The design of the distributor is such that with 60% dwell, 36 degrees of rotation, the points will open co-incident with the rotor's "arrival" at the correct rotational position for firing the next plug in order.

So, if the points open just a smigen, say only 59 degrees of dwell, the CDI will fire, but which plug, if any, will get the JUICE?

who you gonna believe the factory or .....???

factory says
for Bosch it is 38+/-3°
for Marelli it is 37+/-3°

my experience as an old fart that grew up w/ points on everything is that it doesn't matter as long as it's in spec, so I always set it at the low end as wear on the rubbing block makes it get larger.

This is an identical reply from another string. I am looking for troubleshooting input.

Last night we made progress on the dwell and timing. The max we could get the dwell to was 29, the timing at idle is fine and we are 34 at 6000.

Setting the dwell is a pain and I will be looking up a Pertronix very soon.
In the trip to the gym and back it ran OK, but I didn't push it and on the way back at over 3200 RPMs I sensed it missing a little.

Could this be arcing in the old spark plug cables? so it that I need to replace the plugs and go to a wider gap? I was not having these issues before the new MSD and coil so I am trying to work my way through the system.

I am running the old coil since the MSD fried and I have not received the replacement yet.

Any thoughts?