the low down on a/f ratios

[sprattzvx]

Well this might shit you to tears a/f threads are most likely all over the shop.
Im abit confused about the right a/f ratios to use at wot. Speaking to a few yanks who are well advanced in the l67 scene, and the general consensus is if your intercooled should be looking at running at least 11.5-11.9. Ive been under the impression thats dangerously lean, but these guys say anything richer and it will start to bog down, anything under 11.0 is loosing major power. So whats the real story from those who have real experience with boosted applications?? Krusty? Ryan? Anyone popped an engine too lean? 202 you popped one?
looking at dyno printouts of other l67s, the a/f are often around 12.0-12.1. Thought 12s where na territory? Infact ive never seen a printout of an l67 making good power with a/f in the low 11s i dont think.

[antus]

11 is the rich end of the scale from 14.7. Its true that < 12 (more fuel) is pretty rich and it will bog down and loose power, boosted or not. Lean cruise is the other end of the scale @ 16->18 depending, and you dont wanna be there under boost.

[sprattzvx]

11 is the rich end of the scale from 14.7. Its true that < 12 is pretty rich and it will bog down and loose power, boosted or not. Lean cruise is the other end of the scale @ 16->18 depending, and you dont wanna be there under boost.

Cheers antus, im still having trouble keeping a/f in first gear out of the 10s which i think is really slowing down the get up and go of the line and the soure of my frustrations as when i hit second all is good but im sure i could wipe at least a second maybe more off my 0-100 time by cleaning up first gear a/f without upsetting the rest of the gears it bogs down badly. If i cant sort this soon i might end up with the kalmaker software pack for more control.
Ive seperated this thread from my usual as there seems to be a lot of different opinions on safe boosted a/f ratios.

[VL400]

There is generally 2 schools of thought with WOT A/F ratios. On one side you have rich A/F (11s or less) and more spark advance, and the other leaner (12s) and less advance. Which one is correct, well that could start a war.

Essentially your looking to run MBT (minimum best timing), that is the minimum timing needed to get the most torque. This happens when peak cyl pressures occur at 14deg ATDC - it takes awhile for the flame front to build pressure after ignition. The richer you are the slower the burn so to get the peak pressure at the right time your going to need more advance. However running on the edge in terms of advance and AFR gives less room for error and variance. Running way too rich (10s or richer) can also cause issues with carbon build up and hot spots causing pre-ignition.

There is a heap of info on the net but really spark advance and AFR cant really be treated as two different steps in the tuning process.

[Holden202T]

no i have never popped a motor, ever *touch wood*

and yeah i generally tune to 12.5-12.8 afr's at WOT for N/A but i haven't yet to get into the boost side of things, delcowizzid or jayme might be able to shed some light, i know they have both done plenty with boosted motors.

it also depends on the engine too, i know some motors like to be a bit leaner than others, sometimes its just a matter of trial and error.

also like my dyno mate says, 12.5 = rich safe WOT, 12.8 = max power WOT .....

so if i was tuning a drag car i'd probably go for 12.8 cause it doesn't really have enough time to get hot from being that lean, but then if it was a speedway car that was going to be going flat buiscut for 15 laps at WOT 90% of the time then you'd want to go 12.5 to keep it safe and help keep engine temps under control a bit.

for a street car you can nearly go the max power setting cause your not at WOT for more than 5-10% of most drives.

having said that these are all naturally aspirated numbers.

[delcowizzid]

some tuners go as lean as 13.5:1 on L67's to make bigger power numbers but you would want some serious intercooling happening before going anywhere near that lean 12.5 would be my leanest supercharged fuel ratio for modest boost turbo i stay round 11.8-12:1 max and even richer on boost over 14psi