# Calculating fuel injection rate with no MAF? **URL:** \ **Category:** Maintenance/Repairs\ **Created:** [August 11, 2023, 12:00am UTC](https://community.cartalk.com/t/calculating-fuel-injection-rate-with-no-maf/192215 "2023-08-11T00:00:27Z")\ **Posts on this page:** 14\ **Page:** 1
**Author:** ![George\_San\_Jose1](https://avatars.discourse-cdn.com/v4/letter/g/e5b9ba/32.png) [@George\_San\_Jose1](https://community.cartalk.com/u/George_San_Jose1)\ **Post date:** [August 11, 2023, 12:00am UTC](https://community.cartalk.com/t/calculating-fuel-injection-rate-with-no-maf/192215/1 "2023-08-11T00:00:27Z")
My Corolla has an MAP, but no MAF. Wondering how the fuel injection rate is calculated? Is it just based on the MAP? More negative manifold pressures mean the throttle is more closed, so less fuel needed? Then some diddling of the fuel flow rate for the O2 sensor readings? ---
**Author:** ![db4690](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/db4690/32/2583_2.png) [@db4690](https://community.cartalk.com/u/db4690)\ **Post date:** [August 11, 2023, 12:11am UTC](https://community.cartalk.com/t/calculating-fuel-injection-rate-with-no-maf/192215/2 "2023-08-11T00:11:19Z")
you have what’s known as a speed density system ---
**Author:** ![Mustangman](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/mustangman/32/44130_2.png) [@Mustangman](https://community.cartalk.com/u/Mustangman)\ **Post date:** [August 11, 2023, 2:01am UTC](https://community.cartalk.com/t/calculating-fuel-injection-rate-with-no-maf/192215/3 "2023-08-11T02:01:26Z")
Mapped like this…and small corrections by the O2 sensor ![](https://global.discourse-cdn.com/cartalk/original/3X/4/c/4cbb4196736cfd6a50fdac1495974e0a843922b6.jpeg) ---
**Author:** ![TheWonderful90s](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/thewonderful90s/32/29584_2.png) [@TheWonderful90s](https://community.cartalk.com/u/TheWonderful90s)\ **Post date:** [August 11, 2023, 4:19am UTC](https://community.cartalk.com/t/calculating-fuel-injection-rate-with-no-maf/192215/4 "2023-08-11T04:19:40Z")
It has to be tuned to that exact engine. No allowance for turbo chargers or blowers beyond a very small boost, over boring, intake valve or manifold enhancements, engine swaps to a different engine, nor valve timing adjustments without somehow programming the computer with a new fuel map. When Toyota added VVT-i they changed to MAF. ---
**Author:** ![George\_San\_Jose1](https://avatars.discourse-cdn.com/v4/letter/g/e5b9ba/32.png) [@George\_San\_Jose1](https://community.cartalk.com/u/George_San_Jose1)\ **Post date:** [August 11, 2023, 8:36pm UTC](https://community.cartalk.com/t/calculating-fuel-injection-rate-with-no-maf/192215/5 "2023-08-11T20:36:36Z")
> [@Mustangman](#): > > Mapped like this hmmm … Looks helpful, but the axis labeling is a little confusing. Which is the base idle point? Where that little sphere is located toward the front? If so, I’d expect the MAP reading to be around -670 mbar (-20 inch Hg, typical intake manifold vacuum at idle) at that point. But it isn’t? I presume the vertical axis is injector pulse duration (in msec) , and is directly proportional to the fuel flow rate into the engine. ---
**Author:** ![Mustangman](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/mustangman/32/44130_2.png) [@Mustangman](https://community.cartalk.com/u/Mustangman)\ **Post date:** [August 11, 2023, 8:55pm UTC](https://community.cartalk.com/t/calculating-fuel-injection-rate-with-no-maf/192215/6 "2023-08-11T20:55:45Z")
> [@George\_San\_Jose1](#): > > I’d expect the MAP reading to be around -670 mbar (-20 inch Hg, typical intake manifold vacuum at idle) I think the map is in absolute pressure so the idle pressure would be about +330 mbar absolute which is 670 mbar vacuum. And yes, I would expect the fuel to be in open time in msecs on the vertical axis. Fuel pressure should be constant so flow would be variable only with time. Every engine would, of course, have a different map. ---
**Author:** ![Nevada\_545](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/nevada_545/32/41232_2.png) [@Nevada\_545](https://community.cartalk.com/u/Nevada_545)\ **Post date:** [August 11, 2023, 9:11pm UTC](https://community.cartalk.com/t/calculating-fuel-injection-rate-with-no-maf/192215/7 "2023-08-11T21:11:12Z")
Intake air doesn’t need to be measured; it can be calculated by knowing the engine displacement, manifold absolute pressure, atmospheric pressure, throttle angle and intake air temperature. These values/calculations are stored in the PCM. ---
**Author:** ![George\_San\_Jose1](https://avatars.discourse-cdn.com/v4/letter/g/e5b9ba/32.png) [@George\_San\_Jose1](https://community.cartalk.com/u/George_San_Jose1)\ **Post date:** [August 11, 2023, 9:18pm UTC](https://community.cartalk.com/t/calculating-fuel-injection-rate-with-no-maf/192215/8 "2023-08-11T21:18:42Z")
> [@Mustangman](#): > > bsolute pressure so the idle pressure would be about +330 mbar absolute which is 670 mbar vacuum. Ok, thanks, that makes sense. Atmospheric pressure is 1000 bar (absolute, above a total vacuum), so a 670 mbar vacuum from atmospheric pressure would be 1000-670 = 330, which is where the sphere is located on the diagram. One more thing that’s confusing however, at wide open throttle , wouldn’t the MAP be more or less at 1000 mbar (atmospheric pressure)? Wondering why the diagram goes all the way to 3000 mbar? Does this seem correct? If you increased the engine rpm with no engine load (parked in driveway), you’d move along a diagonal on the rpm/MAP plane, towards the green area in the far corner, fuel flow would only gradually increase with rpm. But under load you’d move to the left of the diagonal line, towards that red area, less rpm, higher MAP, more fuel. ---
**Author:** ![George\_San\_Jose1](https://avatars.discourse-cdn.com/v4/letter/g/e5b9ba/32.png) [@George\_San\_Jose1](https://community.cartalk.com/u/George_San_Jose1)\ **Post date:** [August 11, 2023, 9:21pm UTC](https://community.cartalk.com/t/calculating-fuel-injection-rate-with-no-maf/192215/9 "2023-08-11T21:21:24Z")
> [@Nevada\_545](#): > > it can be calculated by knowing the engine displacement, manifold absolute pressure, atmospheric pressure, throttle angle and intake air temperature The Corolla’s 4AFE engine computer knows 4 of those, but doesn’t know the throttle angle. It only knows whether the throttle is at the idle position, wide open, or somewhere in between. It has a TPS, but the “s” means “switch” for this engine. ---
**Author:** ![Mustangman](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/mustangman/32/44130_2.png) [@Mustangman](https://community.cartalk.com/u/Mustangman)\ **Post date:** [August 11, 2023, 9:24pm UTC](https://community.cartalk.com/t/calculating-fuel-injection-rate-with-no-maf/192215/10 "2023-08-11T21:24:53Z")
Clearly 3 bar is a boosted engine. Turbo or supercharging. ---
**Author:** ![Mustangman](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/mustangman/32/44130_2.png) [@Mustangman](https://community.cartalk.com/u/Mustangman)\ **Post date:** [August 11, 2023, 9:31pm UTC](https://community.cartalk.com/t/calculating-fuel-injection-rate-with-no-maf/192215/11 "2023-08-11T21:31:42Z")
The more inputs you have the better control you can create. Throttle position can be used for short term enrichment… an electronic accelerator pump… or to manage blow off controls for turbo applications. The switch provides some useful info although a sensor would be better. Coolant temp is also useful for fuel management. Race cars generally don’t use MAF sensing. It is an intake restriction. ---
**Author:** ![George\_San\_Jose1](https://avatars.discourse-cdn.com/v4/letter/g/e5b9ba/32.png) [@George\_San\_Jose1](https://community.cartalk.com/u/George_San_Jose1)\ **Post date:** [August 11, 2023, 9:34pm UTC](https://community.cartalk.com/t/calculating-fuel-injection-rate-with-no-maf/192215/12 "2023-08-11T21:34:42Z")
> [@Mustangman](#): > > Coolant temp is also useful for fuel management. I’m presuming that diagram presumes normal coolant temperature; i.e. warm engine. ---
**Author:** ![Mustangman](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/mustangman/32/44130_2.png) [@Mustangman](https://community.cartalk.com/u/Mustangman)\ **Post date:** [August 11, 2023, 10:27pm UTC](https://community.cartalk.com/t/calculating-fuel-injection-rate-with-no-maf/192215/13 "2023-08-11T22:27:54Z")
I would assume that, too ---
**Author:** ![neader32](https://avatars.discourse-cdn.com/v4/letter/n/ecc23a/32.png) [@neader32](https://community.cartalk.com/u/neader32)\ **Post date:** [August 17, 2024, 2:15pm UTC](https://community.cartalk.com/t/calculating-fuel-injection-rate-with-no-maf/192215/14 "2024-08-17T14:15:48Z")
Great post, learned a lot.