Calculating Throttle Body Airflow
Calculating Throttle Body Airflow
(OP)
This is a two part question.
1) Has anyone run across a formula for calulating airflow through an EFI throttle body, e.g. an X-mm throttle body can flow Y-cfm?
2) Is there a way to correlate injector fuel flow to airflow? I have found that the OE puts very little extra fuel flow in stock EFI systems, so if a guy was to install a larger throttle body, it would A) respond as much as it's built in flexibility would allow, B) have zero or negligible effect (related to A), or C) under load possibly cause a lean condition when the injectors run out of flow ewhen the engine has plenty of air. Not all engines would be susceptible to this for various reasons but I have seen a guy with a 2.5L Jeep have a surge and a stumble at about 3000rpm under a heavy load (hill) when he went from the stock 48mm throttle body to a 60mm unit. It sounded suspiciously like a lean condition (potentially damaging) and it went away when he reinstalled the stock throttle body.
Many thanks!
Jim
1) Has anyone run across a formula for calulating airflow through an EFI throttle body, e.g. an X-mm throttle body can flow Y-cfm?
2) Is there a way to correlate injector fuel flow to airflow? I have found that the OE puts very little extra fuel flow in stock EFI systems, so if a guy was to install a larger throttle body, it would A) respond as much as it's built in flexibility would allow, B) have zero or negligible effect (related to A), or C) under load possibly cause a lean condition when the injectors run out of flow ewhen the engine has plenty of air. Not all engines would be susceptible to this for various reasons but I have seen a guy with a 2.5L Jeep have a surge and a stumble at about 3000rpm under a heavy load (hill) when he went from the stock 48mm throttle body to a 60mm unit. It sounded suspiciously like a lean condition (potentially damaging) and it went away when he reinstalled the stock throttle body.
Many thanks!
Jim





RE: Calculating Throttle Body Airflow
engine displacement (in^3)* volumetric efficiency*rpm*pi
The normal air/fuel ratio is 15 pounds of air to one pound of fuel. So you can approximate the fuel flow from that.
The jeep stumble does not surprise me since the computer is programmed for the 48mm body, he may have to have the computer reprogrammed, if that is possible.
Regards
Dave
RE: Calculating Throttle Body Airflow
The size can be skewed in either direction depending what is most important to you. A very rough rule of thumb might be to make throttle airflow velocity similar to cylinder head port velocity, (assuming steady flow). Work out how many cylinders are drawing on the throttle body simultaneously, and proportion throttle area accordingly. A bit crude, but it will get you into the ballpark. Most production car engines fall roughly close to this.
For injector size, you need a minimum of 6cc per minute individual rated injector flow for the Hp developed in one cylinder. For example 300cc injectors would be just sufficient where each cylinder developed 50 Hp.
RE: Calculating Throttle Body Airflow
At full throttle it is quite likely (in fact almost inevitable) that the fuel flow will be open loop, so if the Jeep's system uses the pressure drop through the throttle body for metering purposes then that could cause big problems.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Calculating Throttle Body Airflow
If the air flow is being measured by an air meter then the TB area vs throttle angle will set the airflow volume and speed thru the air meter.
These parameters should be in somewhat a balance.
Adding a large wot TB area and it's attendent rate of change up to WOT upsets this relationship by skewing the air meter transfer function to the ECM and it's attendent output pulse widths to the injectors.
Then add the throttle position sensor to this yet that is an input to the ECM and contributes to the final fueling results and ignition timing.
I have experienced useing a large TB and lost throttle response plus some hesitiation over a long period of time.
Replaced this back to the smaller stock TB and regained the starting and smoothe drivabilty and torque.
The ECM also changes it's tables in response to these attempts to influence the engine power.
No substitute for engineering the total system based on airflow, fuel requirements and hardware capacities.
RE: Calculating Throttle Body Airflow
These amount to errors outside of design limits for the total engine package.
RE: Calculating Throttle Body Airflow
"MAP sensing" IS a speed density airdetermination method.
If you use a larger throttle body then you will tend to underfuel in an alpha/n system. Because of the (crude) airflow versus throttle angle model being miles out.
MS
RE: Calculating Throttle Body Airflow
The larger T/B could have caused things to go beyond the range of the OEM MAP sensor.
RE: Calculating Throttle Body Airflow
Or you can measure the velocity.
At least, that's the terminology where I work.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Calculating Throttle Body Airflow
The point I was alluding to is that an a speed/rho system a MAP sensor is used. In a Mass flow system a MAF sensor is used and in an alpha/n system a TPS is used....
I some how very much doubt that a larger throttle body would cause the MAP sensor to go past its calibrated scale - how do you presume that it could?
Surely a larger TB will not be increasing manifold pressure substantially past atmospheric.
MS
RE: Calculating Throttle Body Airflow