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AFR table for LPG Vapour injection

Understanding AFR

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hi guys and girls,

I have installed a LPG Vapour Injection system on my stock 454 Big Block

here is my projectpage:


Some info on my setup:

GMC C15 Suburban, see projectpage in my signature

engine: 454 Chevy Big Block low compression about 8:1

single fuel: only LPG injection

ECU: Megasquirt MS3X (see tune in attachments)

Reducer: Tomasetto AT13XP set at 150 kPa gas pressure (rail pressure - MAP)

(replaced this reducer: Femitec FR400)

Injectors: Valtek 30 BFC, 3 Ohm

(replaced these injectors: Femitec GIAC-01 (2 Ohm)

Peak&Hold boards (4A/1A) from SECU-3.org

I have extra sensors for LPG connected in one itegrated unit:

- gas pressure (mpxh6400a)

- gas temperature (10 kOhm NTC)

- external MAP sensor (mpxh6400a)

Ignition is:

FAST dual sync distributor for crank and cam signal

8x LS D585 coils at 3.5ms dwell for spark

My idle is at 700rpm and ~45 kPa MAP

I cruise at 1800-2000rpm and ~55 kPa MAP

max torque seems to be around 3500rpm

max power around 4500 rpm and then it feels like it rolls over

Never had it on the dyno yet. I want to road tune it a little more

I have been looking and looking online for information on generating a AFR map for LPG injection. I use Tunerstudio but that offers a AFR map generator thats focusses on wet fuels like gasoline. I have read that LPG injection dont like rich mixtures (

Attached Files

Anybody reading this?

Yes, but it isn't something many will be familiar with and so may be hestitant to comment, as was I. I did have something to do with CNG decades ago, and a couple of comments that may be relevant.

1/ forget ALL about AFR with this application, think in terms of lambda - this is good advice for everyone. With lambda the fuel isn't directly relevant, it is the raw data the AFR gauges and charts use to give a figure that is going to be different for ANY fuels.

Lambda gives the percentage, or amount, of unused oxygen in the exhaust gases and 1.0 is stoichometric representing complete combustion using all the fuel and oxygen - less than 1.0 represents a rich mixture and higher than 1.0 represents a lean mixture.

2/ with LPG and CNG, there is no evaporative cooling and richer mixtures will run hotter, not cooler as is normal with liquid fuels. A common result of rich running is cracked exhaust manifolds and high fuel bills - if you have the option, an EGT (Exhaust Temperature Sensor) would be a vbery good idea, especially for heavily loaded engines. For those reasons, a slightly leaner mixture may be preferred, perhaps 1.05 to 1.1, depending an testing, of course.

3/ one of the biggest advantages of LPG is it has, technically, a high octane rating and so a compression increase and/or more timing may be advantageous - however, it can be harder to ignite, especially at low temperatures, and so the ignition system may need to be looked at, with the spark plugs a grade or two colder.

Not sure if that's much help, and I STRONGLY advise getting as much information on this as you can, as the advantages may be outweighted by the problems - not least making sure you can get the fuel, as in some countries the vehicle fill networks aren't very extensive.

hi Gord, thanks.

Thats the info I have found online so far. And I have succesfullly road tuned it where my 0-100kph time went from 13 seconds to 9.5. Thats a big gain for a car of my size ;-) . I know there should be sa shit ton of info out there, cause installers are installing advcanced LPG injection system all the time. But its in their plug-n-play computers and they dont share a whole lot of ifo sadly.

Guess its back to road testing and just seeing and trying what works.

fow now I run stoich, so lambda 1 mostly and AFR 15 for WOT and about 16.5 for cruise.

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