Hi All, I recently converted my 308 GT4 from twin dizzys to a single dizzy black stallion ignition system. The BSM ignition system lets you program electronic advance at 800 rpm intervals. The old points system retarded the ignition curve by 10 degrees when you came off the throttle. The problem I am having is getting the idle to settle, sometimes it'l idle at 1500 sometimes 1100, it's not stable. Other than that the car runs well. My question is what should I set my ignition curve to? I have set the static advance to 6 degress but have added 7 degress of electronic advance a 800 rpm, hence at idle I'm running 13 degress total advance. The new BSM system does not use the throttle position switch to retard timing. You can only set a static curve with software. I'm assuming at idle (800-1000rpm) I should not be using any electronic advance just a static advance of 6 degress. I this correct? I am about to try the following curve: RPM Static + BSM = TOTAL Advance 0 6 0 =6 deg 800 6 0 =6 1600 6 12 =18 2400 6 16 =22 3200 6 20 =26 4000 6 24 =30 4800 6 27 =33 5600 6 28 =34 6400 6 28 =34 Any suggestions or comments? I want the car to idle at around 1000 rpm but I dont want to set the 1600 rpm point to 0 deg of electronic advance as this will affect the curve up to and just beyond 1600 rpm marks. So at 1000 rpm (idle) based on the above curve the electronic advance will be about 3 degrees so total advance at idle wil be 9 degrees. Cheers George
George: You didn't say what year your GT4 is, so my comments will apply to a 1975 USA edition. The setup for the distributors calls for a 5 degree retard on the R2 points, which is enabled when the idle microswitch is depressed. Static timing (with the R2 points disabled) is 6 degrees BTDC So what happens is that at idle (800 RPM) with the microswitch and R2 points enabled the = 6 -5 =1 degree BTDC I think 13 degrees is way too much advance at idle. I would set it up at 0 degrees electronic at 800 RPM and then set the static timing at 3-6 degrees BTDC Then match the curve in the manual for the rest of the RPM range.
My 1975 USA gt4 loves all the advance I can throw at it. I am at about 18 btdc at idle, and 36 all in. It runs 10million times better than stock curves. The factory compression ratio is not only very low, but has a poor quench. My idle is 850. I bet I could run even more advance, and it would like it. I am 133 on mains, and I forget the airs (210?) Jay
You'll definitely have a better shot at a more stable idle with less advance at idle (as this requires allowing a little more air in thru each carb barrel to get the 1000 RPM idle so the little mechanical differences in the throttle plate locations when closed have less of an effect on idle RPM) - but, at 9 deg BTDC, you'll still be running more than the guys running on just the R1 points at 7 deg BTDC. What are your programming rules/limitations?
Thanks Steve, Limitations of the BSM system are you can only set electronic advance at 800 rpm intervals. So if I set 0 deg advance at 800 rpm I will only be running 6 deg of static advance. The next interval is 1600 rpm, if I set 0 deg at 1600 then I'm guaranteed 0 deg (6 deg static) at 1000 rpm but I think setting 0 deg at 1600 will be two low. The BSM system interprets the advance between 800rpm intervals hence if I set 1600 to 0 deg its a steep and big jump to the next advance setting at 2400 rpm. I guess I could use the values in my original post but then adjust the distrubitor static advance at 1000 rpm to 6 deg. Cheers George
OK, but what about the value chosen at each point -- is that also restricted? Are you saying that 28 deg is the maximum possible amount at any point, and, if you set the static timing to 3 deg at 1000 RPM (to get a total of 6 deg BTDC at 1000 RPM), then the upper end RPM gets limited to 31 deg BTDC total advance?
Hi Steve, You can set any value at any 800 rpm point from 0 to 10400 rpm. What I have tried to do with the values originally posted is to keep the 1000 rpm total advance as low as I could but beyond 1000 rpm I tried to match the R1 curve in the 308 GT4 manual. If I had the option to set electronic advance at 1000 rpm I would set it to 0 and get only 6 deg of static. But I have chosen 0deg@0rpm, 0deg@800rpm and 12deg@1600, this gives me approx 3deg at 1000rpm + 6deg of static, hence 9deg. If I set the 1600 rpm value to low to keep the 1000rpm (interpreted) value low then there's a big jump from the advance at 1600 to the advance at 2400 needed to match the original R1 curve. Cheers George
Got your point about needing to account for 800 vs 1000 RPM if the slope of the advance curve is not 0 between 800 and 1600, but if you can pick "any" value at each 800 RPM point why not: RPM Static + BSM = TOTAL Advance 0 3 0 =3 deg BTDC 800 3 0 =3 (1000 3 3 = 6) 1600 3 12 =18 2400 3 19 =22 3200 3 23 =26 4000 3 27 =30 4800 3 30 =33 5600 3 31 =34 6400 3 31 =34
Thanks Steve, I'll try that option, This all came about when I tried the Birdman proceedure for balancing the carbs. I forgot that I had 7 deg (electronic) + 6 deg static set at 800 rpm hence my advance would have been between 13-15 deg total. As such when I was trying to set the idle air sync the carbs were pulling a lot more air than in the proceedure (ie 7-8 on my meter). I can only put this down to having way to much advance at 800-1000 rpm.
