FMU ratios relating to boost conditions(turbo peeps please help me on this one!) - Honda Prelude Forum

Spocko · 06-25-2001, 12:37 AM

Hey guys, I heard a lot of you say that running a 12:1 Vortech will be WAY TOO rich for the Lude, so I bought a recalibration kit for 8:1 and 10:1. However, I came across this on Fmax's site...

"AIR/FUEL RATIO: Be safe in knowing that your system runs an air/fuel ratio, that is in accord with what force induced production cars should run. This H22A4 in particular consistently performs at around 12:1 air fuel ratio under boost. Sometimes its hard to run 12.5:1, especially when the engine loses power at these leaner mixtures. Although the Motec, and other sophisticated air/fuel ratio meters are very accurate, they are only as accurate as what they are measuring. My theory is that due to the overlap,lift and duration of the VTEC, the air/fuel ratio in the exhaust may not necessarily be representative of what truly occurs in the combustion chamber. Suffice it to say that the H22A4 appears to perform much better and produce more power at ratios of 12.0-12.2:1. Even at max RPM and max VELOCITY air/fuel and EGT stabilize to levels that don't compromise combustion integrity."

So along with that, shouldn't I stick with my 12:1 ratio? They are using those additional 440cc Lucas injectors, so I am not sure if that matters or not, but I don't have them on my kit. I DID however upgrade my injectors to RC Eng. 370cc's (Saturated). With these, what would be a good, safe ratio to run 8 lbs. on until I can get to the dyno to tune it? Should I stay with the 12:1 FMU now, or switch the discs to a different, lower ratio? Thanks!

bgod · 06-25-2001, 10:52 AM

every case seems to be different becasue there are still some people running straight drag kits with 12:1 fmu's and running good numbers. i'd leave the 12:1 alone until you can get on the dyno.
remember, it's always good to be rich rather then lean. once you're on the dyno you cna play with the discs and see what works best.

but many feel that the 12:1 for all applications with the drag kits are too much.

Smilez · 06-25-2001, 10:08 PM

Spock, what they are talking about of course is your final AF ratio being 12.5:1 or so. The FMU being set to 12:1 doesn't mean you are 12:1 in fact you could be higher which is why you might want to run like 9:1.

Of course you should do this on a dyno w/wide band o2 so you can tune it properly.

mike2016 · 06-25-2001, 10:46 PM

The FMU has nothing related to the Air/Fuel ratio, different applications have different needs, a 12:1 FMU will raise 12 PSI of fuel pressure per pound of boost. You will be running more than 100 psi of fuel pressure on a 12:1 FMU. if you are boosting at 7 psi, Stock fuel pressure 40 psi + 12 psi X 7 = 114 psi at full boost.

You have to use a very accurate O2 meter to find out what kind of air/fuel ratio you are running, you won't know what kind of condition your motor is running until you go to the dyno. All the Drag kits come with 12:1 FMU because it'll make sure you run rich, which is safer.

Spocko · 06-27-2001, 10:32 PM

That's what I am going to stick with until I get to the dyno