Microtech What AFR reading when under boost
12-10-11, 05:27 PM
#1
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What AFR reading when under boost
I am running the microtech lt10s with x4 box on my stock ported 13b-re running near 14lbs on the twins. I have my innovate wideband and i have been tuning with my laptop to meet the 14.7 ideal afr reading (read that on the innovate manual) and under boost I have been tuning to hit anywhere from 11's to 13's ish. I was wondering if the correct afr is 14.7 under regular vacuum and under boost pressure. I ask because I am scared of going lean if 14.7 is an incorrect ratio to shoot for under boost and for gas mileage. I got really horrible gas mileage and was thinking my afr was too rich at where I am at right now. So should I stick to tuning my engine under boost to 14's to stay on the not-lean side and for better gas mileage? thanks
12-10-11, 06:37 PM
#2
AFR is not everything,
You have to look at your timming also, And there is no GOLDEN AFR for boost.
but aprox.
0-5 psi 12.3-12.6
5-10 psi 11.9-12.3
10-14psi 11.0 to 11.9 AFR
check intake air temps, and see if they are super hot, might to run richer.
For cruising, in vaccum , what ever your car likes, if it does 14.7 go for it.
Post your timming map depending of boost and rpm
You have to look at your timming also, And there is no GOLDEN AFR for boost.
but aprox.
0-5 psi 12.3-12.6
5-10 psi 11.9-12.3
10-14psi 11.0 to 11.9 AFR
check intake air temps, and see if they are super hot, might to run richer.
For cruising, in vaccum , what ever your car likes, if it does 14.7 go for it.
Post your timming map depending of boost and rpm
12-11-11, 10:10 AM
#3
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Whoa! STOP RIGHT THERE! DO NOT TUNE FOR 14.7 UNDER BOOST!
Ratios under boost need to be much richer than stoich, primarily for chamber cooling and to prevent preignition and/or detonation.
Typically I'll tune an engine at these starting ratios:
0-4 PSI: 12-12.5
4-8 PSI: high 11s
8 - 15 PSI: mid to low 11s
16 PSI +: high 10s
Those are just a start. Some engines (high overlap ports) like richer ratios in the 0-4 PSI range. But the ratios listed above are going to be safe even with fairly aggressive timing.
Now, as for you off-boost ranges.
You can go a bit leaner than 14.7 in the cruise areas of the map. Mid 15s usually does the job at around 33-35 degrees of timing and 15 degrees split. Everywhere else can be 14.7 until about 6 or so inches of vacuum, because then you get into the high load area. You want to smoothly taper down to about 13.8. As you approach 0" and as the RPMs rise up, taper smoothly to about 12.8.
Again, these are just generic numbers. Your specific engine will need to be tuned to what it likes. High overlap ports tend to want richer ratios. Street/stock ports tend to make more power at slightly leaner ratios (mid to high 13s).
Under boost, you always need to tune richer to stay safe.
Ratios under boost need to be much richer than stoich, primarily for chamber cooling and to prevent preignition and/or detonation.
Typically I'll tune an engine at these starting ratios:
0-4 PSI: 12-12.5
4-8 PSI: high 11s
8 - 15 PSI: mid to low 11s
16 PSI +: high 10s
Those are just a start. Some engines (high overlap ports) like richer ratios in the 0-4 PSI range. But the ratios listed above are going to be safe even with fairly aggressive timing.
Now, as for you off-boost ranges.
You can go a bit leaner than 14.7 in the cruise areas of the map. Mid 15s usually does the job at around 33-35 degrees of timing and 15 degrees split. Everywhere else can be 14.7 until about 6 or so inches of vacuum, because then you get into the high load area. You want to smoothly taper down to about 13.8. As you approach 0" and as the RPMs rise up, taper smoothly to about 12.8.
Again, these are just generic numbers. Your specific engine will need to be tuned to what it likes. High overlap ports tend to want richer ratios. Street/stock ports tend to make more power at slightly leaner ratios (mid to high 13s).
Under boost, you always need to tune richer to stay safe.
12-11-11, 07:40 PM
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I run even the lower boost cars in the high 10's, the extra fuel keeps em cooler and they seem to last longer for me that way. Especially for a track day car/race car. Plus the difference between high 10's and mid 11's is maybe a 3% difference in power output.
The only time your car should be at 14.7 is at idle and crusing rpm under about 15% throttle position. It should be to around 13.0 by the time to reach 0 manifold vaccum.
Zack
The only time your car should be at 14.7 is at idle and crusing rpm under about 15% throttle position. It should be to around 13.0 by the time to reach 0 manifold vaccum.
Zack
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