Autosport Labs

Quick question about writing a new map for my car. I want to start with a close match to the original distributor and work from there. I have the test bench curves from the distributor, which show "Advance at the Distributor Shaft" over "Vacuum mbar".

Advance lowest value is 20deg ATDC and goes to 20deg BTDC.
Vacuum mbar goes from 0 to 300.

How do I convert these values to to something useful in the MJLJ world? mbar to kPa is a simple conversion however I am confused by the ATDC/BTDC degrees issue. The EDIS system has maximum retard of 57 or so degrees and advance of 10 degrees, so do I offset the original values? There is no "negative" advance when setting up the MJLJ so how would I simulate degrees ATDC as opposed to BTDC?

Thanks for your help.

A timing range of 20BTDC to 20ATDC is pretty unusual, and as you say, EDIS only does 57BTDC to 10ATDC

Are you sure you're not looking at curves for some sort of delta change on the basic advance curve due to vacuum effects and there is some more normal distributor advance curve that this delta is acting on?

BTW, I don't actually know how to set the MJLJ to indicate any ATDC values even if you really wanted to - the configurator only allows 59 BTDC to zero BTDC in a bin and I'm pretty sure that the MJLJ software doesn't cope with ATDC 'advance' values

Ok I guess that was the info I was after, I couldn't imagine youd want a spark ATDC either. There are identical curves above and below the TDC line on the graph so I think your idea of it being a general delta indication is correct.

Ill load it up and see how she runs! Cheers.

Its a 1979 Porsche 924 Turbo btw.

Good thing you asked. Megajolt's timing is adjusted (in every map bin) from 0 degrees BTDC to 59 degrees BTDC. ATDC is not used, ATDC means spark to fire after TDC, it isn't used in neither performance nor fuel efficiency applications.

Your distributor curve doesn't tell you much at all. You must remap your ignition yourself.

First thing to do is figure out your engine's operating pressure. Is it an N/A motor? If so, then pressure is probably 25 to 35 at idle, and 100 kPa at full throttle (at the sea level). Space your 10 MAP bins equally, for example, 25, 35, 45, etc... Keep in mind how you going to use your engine, is it cruising at light throttle, or racing at full throttle, space your MAP bins accordingly, the more definition you have, the less chance of detonation.

Second you will do the same for the RPM bins. From idle, to the rev limit, space them equally, but allow more definition for areas where you'd be driving the most.

Up next is the part where you can blow your engine if you don't take it easy. Gas up with good gas, or the gas you'd be using all the time. Start tuning. Put all advance bins into 10 degree positions, that means no detonation is possible if you're N/A. For 1000 rpm bin you'd put 18 at your lowest kPA bin and go down decreasing that value, 18, 17, 16, 15, etc... Test that location, does it detonate? If yes, decrease timing by a few degrees, if no detonation, advance a few degrees until detonation again, then retard 2 degrees for the max power. Etc, etc... Do your tuning while engine is warm. If you don't have cold air intake, now is a good time to install it, timing deference between burning cold air vs hot air is pretty big.

Here is a starter map for N/A engines, should work on most. <a href="http://picasso.org/mjlj/?q=node/413">ht ... ode/413</a>

Please try both Brent's Java tuning program, and my Windows tuning program. You'll find Java tuner in Software section, you can download my tuner on the right hand side of the screen, under 10 most recent files section. Download file branded (installer) if you have Windows NT, 2k, XP, if you have Windows 95, 98, ME, download version branded (archive). In Windows tuner you'll find very useful runtime display, that will make your task so much easier.

What ever you do, always remember not to go full throttle until all is tuned and tested. If this is your every day driver, drive to work and places using a 10 degree on all the bins map, while tuning, for safety.



AL

<<Is it an N/A motor? >> No, it's a turbo so the upper MAP value needs to be determined. Note that the MJLJ MAP sensor has an operating range of 20kPa to 250kPa

I'm not keen on a "drive it, increase the advance until it pinks/detonates, then back it off 2 degrees" tuning method on something like a Porsche Turbo. I'd thoroughly recommend investing in a rolling road session with someone who knows how to tune this particular type of car properly. It might cost a bit, but you've made loads of savings on the MJLJ to offset the cost!

When I had my (very noisy) rally car RR-tuned we couldn't hear the pinking that was presumably happening - we just held the car at each rpm point and advanced the ignition from 10BTDC in 2 degree steps until the power flattened off and then we went back 2 degrees (*). Maybe it never got into the aurally-detectable pinking level, but than that is a Good Thing (tm). It was also a very quick process, so I was in and out of the RR session in about 45 minutes - luckily it was FOC for me though, anyway!

However, in a N/A motor, constant throttle positions always give MAP values close to 100kPa so, for me, it then seemed to become a bit of a black art on how to account for lower MAP values across the rev range. We just used the tuner's (secret!) black magic rule for adding advance at different rpms and different MAP values - it certainly seemed to work just great, but I'd be interested to know the proper way of doing it on a RR...

(*)Top Tip - when doing this, adjust the rpm point in question AND the two adjacent (ie either side) points to the same values when doing the tuning...this removes any interpolation errors that the MJLJ might be introducing if you're not running at exactly the rpm point that you are trying to tune at

Great info there - I was playing around with a nicer map last night and all the information I have received is starting to make sense.

When you talk about tuner black art, do you mean smoothing out the map in between known values?

Maybe the text you posted should be in the files section, im sure this info would be useful to newbie megajolters.

4600CC's description was really great, I agree it should be in the documentation.
Thanks Man!