[Gmecm] ALDL MPG Information

[Gary Evans]

You can calculate MPG quite accurately from the values available in  
the ALDL stream.

Since you have a vacuum referenced fuel pressure on TPI cars, the  
injector flow rate is fairly constant, and the following calculation  
should be accurate enough for casual use. You're not going to create  
a lab instrument with this, but it will be the equal of typical  
factory fuel economy computers.

You need three variables from ALDL:

BPW
RPM
MPH

and two constants

Injector flow rate (volume per ms)
Number of injectors on engine

WIth this information, you can calculate as follows:

BPW * injector flow rate * # of injectors = fuel per rev

then:

fuel per rev * RPM = fuel per minute

finally:

fuel per minute / MPH / 60 = fuel per mile.

Cram it all together and flip it over to get miles/gal and you get:

MPH / (BPW * injector flow rate * # of injectors * RPM * 60) = Miles  
Per Gallon

You will need to make sure the units (ms/sec/min/hour, etc)  all line  
up when you do the calculation, but that is how it is done. You will  
probably also also need a fudge factor to dial in the calculation and  
account for variances in the fuel system components.

In the 80's Jaguar used a purely analog version of this idea for the  
in-dash display. It didn't even receive data from the computer,  
rather it was wired directly to the injector harness. Aside from  
normal car-to-car variance in manufacturing tolerances, it was  
acceptably accurate. You could theoretically do the same thing with  
your micro-controller box by tapping the injector harness, VSS and  
TACH leads to accomplish the calculation thing without ever touching  
ALDL at all.

-Gary



On Dec 1, 2005, at 2:08 PM, Greg Etchell wrote:

> You might be able to modify the ALDL stream to output the  
> calculated airflow
> or injector pulse width or other internal variables.
>
> Injector pulse width and speed might give a Rough mileage value.   
> Probably
> not to accurate but with some tuning it might be close....
>
> g
>
> -----Original Message-----
> From: gmecm-bounces at diy-efi.org [mailto:gmecm-bounces at diy-efi.org]  
> On Behalf
> Of lostrecep tion
> Sent: Thursday, December 01, 2005 1:01 PM
> To: gmecm at diy-efi.org
> Subject: RE: [Gmecm] ALDL MPG Information
>
> those are very interesting articles is there anyway to derive mpg  
> with a TBI
> computer 1227747  being as it does not have MAF ?
>
>
>> From: "Greg Etchell" <Greg at WildoaksRanch.com>
>> Reply-To: gmecm at diy-efi.org
>> To: <gmecm at diy-efi.org>
>> Subject: RE: [Gmecm] ALDL MPG Information
>> Date: Thu, 1 Dec 2005 09:20:01 -0800
>>
>> Check out
>>
>> http://www.circuitcellar.com/avr2004/wabstracts/A3805abstract.pdf
>> http://www.circuitcellar.com/avr2004/wentries/A3805.zip
>>
>> It is kind of what you are looking for and it might give you some  
>> ideas.
>> Let us know what you figure out!!
>>
>> G
>>
>> -----Original Message-----
>> From: gmecm-bounces at diy-efi.org [mailto:gmecm-bounces at diy-efi.org] On
>> Behalf
>> Of Kristoffer M. Hain
>> Sent: Wednesday, November 30, 2005 5:57 PM
>> To: gmecm at diy-efi.org
>> Subject: Re: [Gmecm] ALDL MPG Information
>>
>> I meant TPI not TBI.  It's a Tuned Port Engine
>>
>>
>> Kris
>>
>>> Hi!
>>>
>>> I have a 350TBI with a 1227730 computer and would like to know if
>>> anyone has any information about gathering information for MPG  
>>> from it.
>>> I want to connect the stream to a microcontroller to build an  
>>> outboard
>>> display.  Ideas?  Thoughts?
>>>
>>> Thanks in advance!!!
>>> Kris Hain
>>> _______________________________________________
>>> Gmecm mailing list
>>> Gmecm at diy-efi.org
>>> http://lists.diy-efi.org/mailman/listinfo/gmecm
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>
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