J&S versus MSD

I've been driving around with the J&S for about a year and a half now.  I'm still not entirely satisfied with the way it works.  An other option to control timing is to use an MSD.  Both units have their pro's and con's so I decided to combine the best of both.

J&S: This is a device which just listens to the engine through a microphone.  When it hears knock, it takes out timing.  When there's no knock, it takes out no timing.  However, there are some problems with the J&S.

  1. Knock exhibits hysteresis, so this means that once there is knock, a lot of timing must be taken out before the knock stops.  In my car, the J&S took out up to 18 of timing!
  2. The sensitivity is very difficult to adjust.  The device often takes out timing when it's not needed, even when cruising. 
  3. A lot of people have reported big power losses with the J&S, supposedly up to 30hp!  I haven't measured the exact amount on my car, but I can feel a huge power drop when the J&S kicks in...

The J&S should therefore only be used for what it was designed, as a backup in case there's knock.  And not as an active device to prevent knock.  These 2 things are very different!

MSD: Takes out timing in function of boost.  It takes out 2 of timing for each psi of boost, with a maximum of 6.  The problem with the MSD is that it always takes out timing, regardless if it's necessary or not.  Problem is that at low RPM, there's no need to take out timing, knock mostly only occurs above, say 4000 RPM.

Solution: Make the MSD RPM dependent, use it as primary retarding device and disable the J&S under 4psi.  The MSD offers enough protection at those low boost levels.  This way the J&S doesn't take out timing unwanted.  It's just a backup in case knock still appears (definitely so with the 12psi I'm running...).

msd_outer.jpg (39476 bytes)Plan is to use Dick Bipes' mod on the MSD.  Then I'm going to use a pressure switch to disable the J&S under 4.5 psi.  I'm also going to experiment with a resistor on the thermo sensor line.  Not that I'm running lean, but I have about 120psi fuel pressure, and I hope to be able to lower that a bit with the resistor... 

msd inner.jpg (46075 bytes)msd with bipes mod.jpg (48928 bytes)pressure_switch.jpg (36947 bytes)Here's the internals of the MSD.  As you see, there's enough space in there to put in the board.  With a few threaded studs, the board can be easily installed.  Everything isn't connected yet, but you should have a pretty good idea of what things are going to look like.  This is the 4.5psi pressure switch I'm going to use for the J&S.

 

I don't think the order in which the 2 devices are hooked up matters, so I'll be installing them like this:

ECU - MSD - J&S - CAS

Stay tuned for the results!

Apparently, the order does matter!  The MSD alters the amplitude of the incoming timing signal, so it seems wise after all to use the order I mentioned above.  That way the altered signal is sent to the ECU and not to the J&S.  Should you swap the MSD and the J&S, then the J&S would see the altered signal of the MSD.  I don't know if the J&S would mind, but I feel it is best not to try this.  It's possible that you would damage the J&S by doing so ...

July 2000:
I've found out that the MSD, which I bought second hand, appears not to be functioning...  After browsing the archives, it appears that this is not uncommon with these things.  Sometimes they don't even work brand new!  Probably the pressure sensor is the culprit here.  I suspect that there might be a lot of people driving around with defective units without knowing it themselves!  There's no indication that the unit is functioning or not.  Only with Dick Bipes monitor you can effectively monitor how much timing the MSD is taking out, if any.  My MSD doesn't do anything at all.  The first owner probably newer new his MSD wasn't functioning ... he bought a Link ECU because he didn't get his Sebring tuned right.  

So this means no modified MSD for me than ... anybody who needs a Dick Bipes mod and monitor?  I've got one for sale!
I can say however that the modification works.  I bought 4 of these things, for me and for a few other members of the Flanders MX5 club.  On their cars, the units work perfectly ... but of course they have a functioning MSD.
 

As luck would have it, only a few days after I found out that the MSD was defective, I came in touch with John Pizzuto.  John is de J in J&S, the designer of the J&S.  He is planning to rewrite the software of the J&S to make it do the same thing as a MSD with Bipes mod, although with fixed settings for base and rate.  You can read about it here.  It's a bit incoherent, but you should be able to make out the most of it.
To make a long story short, the plan is to make the J&S listen only above a certain boost and rpm (probably 3800rpm) and to let it take out a predetermined amount of timing.
Since the device cannot see boost directly, it uses the output of the AFM instead.  Currently, de J&S activates when it sees 0.9V (for the 1.6l), which in practice is the transition into boost.  At 3psi it has about
0.75-0.80V, which in my opinion is much better to make it start listening.  And even this is a bit too soon, because at 3psi it's virtually impossible to induce knock.  My car for instance will only have knock under 0.6V, so using a trip point of 0.65-0.70V should be safe enough.
Also, from a certain rpm (4500 for instance) the unit will take out 1 of preventive timing per 100 rpm, with a maximum of 6 (thus up to 5100 rpm) or 10 (5500 rpm), a number yet to be determined.
Disadvantage here is that the settings are hard coded and not adjustable with a trimmer like the Dick Bipes mod.  This means that the settings must be chosen very carefully, or else it might be necessary to use an MSD after all ...

Results as soon as John has finished the new software!