Calculating Throttle Body Airflow 2

[4xWriter]

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

 

[CESSNA1]

4XWRITER: Interesting question but difficult to resolve. The fuel flow through the injectors is controlled by the computer based on many inputs. To approximate the airflow through the throttle body use the follwing

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

 

[Warpspeed]

There is no real optimum throttle size that can be calculated for all situations, it has to do with pressure drop at full throttle, and drivability at small throttle openings.

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.

 

[GregLocock]

I find it unlikely that the fuel system as such would cause a problem at 3000 rpm, because it at 6000 rpm it would have to deliver twice as much fuel per second (roughly). it may be that there are hard wired limits if it is sequential, which seems a bit unlikely on a jeep.

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.

 

[OP800]

Another way to look at this is the throttle body is sized for max airflow and rpm of the engine displacement with some tolerence.
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.

 

[OP800]

On the other hand, if MAP sensing is used in place of mass airflow measurement (in Speed Density engine control) then the larger throttle body will reduce intake vacuum at a larger rate and tend to overfuel the actual engine needs as well as the effects of using the same throttle position sensor signal vs throttle angle on the larger TB area.
These amount to errors outside of design limits for the total engine package.

 

[mattsooty]

Huh???

"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

 

[Fabrico]

The larger T/B could have caused things to go beyond the range of the OEM MAP sensor.

 

[GregLocock]

Hang on, don't confuse MAP and MAF. MAP is Manifold Air Pressure, and is just the relative pressure between the plenum (typically) and ambient. MAF is some kind of mass air flow sensor, such as a hinged sprung flap.

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.

 

[mattsooty]

MAP is actually Manifold Absolute Pressure but I know what you mean Greg.

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

 

[4xWriter]

Gents, thanks for all the input. You all gave me lots to chew on.