Timing v. A/F ratio.

Big_G

Active Member
Open to opinions on the relationship of timing and a/f ratio at full power levels (max boost), in an effort to prevent detonation and generate the most power. Case in point...my combo. 8.3 scr and approx. 18 lbs boost and 30* timing at 6,000 rpm. No signs of detonation...using a fairly cold plug. Visible smoke (expected) at full throttle. I believe I have the v/e and desired a/f tables set to 11.0:1 in the high boost cells. Now the best power seems to be made at about 12.5-13.0:1 a/f ratio, so I am leaving a good percentage (5-6%?) on the table. It seems to me if I lean out the full throttle mixture a point and a half to get to 12.5, my risk of detonation will increase. How do I figure the appropriate timing with the new a/f ratio to maintain a safety margin? Keep in mind I can't here the detonation...engine too loud. I can, of course, read the porcelain on the plugs..Thanks, gents..
 
theres two common routes, the older school way is to slowly lean out the mix in small test stages and carefully inspect the plugs at each step.
this takes experience and knowledge of what your looking at, and understanding that along with the A/F ratio changes your more than likely going to need to make ignition curve changes as the F/A ratio leans out at each stage.
yes this does work and work rather well but it takes time and testing

or you can go new tech install temp sensors in each exhaust primary and O2 sensors in each exhaust collector, to read exhaust temp, and a knock sensor to monitor detonation and fuel air ratio and get a decent oil and water temp gauge , try to keep the piston temps under about 600F. theres tricks like oil spray jets to cool pistons. and adding alcohol or water injection to keep the combustion temps down.
youll need a lap top and soft ware and the sensors and wiring of course
yes your supercharged engine DOES tend to show a slightly richer plug color and fuel/air ratio meters do tend to indicate your effective mixture is a bit richer for the exact reason you stated.
thats one reason super charger cams tend to have a wide 112-116 LSA, so that theres a bit less overlap time where compressed boost could enter and partly exit into the exhaust port while both valves were open
116_0403_basic_10_z.jpg

if you do a search theres some info on both

viewtopic.php?f=50&t=3515&p=9263&hilit=+data+software#p9263

http://www.thesensorconnection.net/cate ... egt-probes

http://www.innovatemotorsports.com/prod ... gworks.php

http://www.phormula.co.uk/KnockMonitor-KS-4.aspx

http://tunertools.com/proddetail.asp?pr ... CMSCAN-KEY

viewtopic.php?f=44&t=469&p=579&hilit=software#p579
 
Last edited by a moderator:
Thanks, Grumpy. I am running FAST EFI, so I have a wide-band o2. I will set up a knock sensor so I can data-log for knock and tune accordingly.
 
Back
Top