Skip navigation

 

 

Its no secret that I’m a big advocate of Virtual Dyno…..I use this software pretty much daily .

vd_logo_image
With todays modern vehicles equipped with EFI systems that have high speed processing that are able to provide your  datalogger of choice very high sample rates, this software generates reliable, repeatable results.
But what happens if you have an older car, say an OBDI equipped car…….or maybe your classic car doesn’t have EFI at all…..what do you do then?
This was the predicament I found myself in several years ago when I brought home “Project 1991 Firebird Formula”.  This car is equipped with an OBDI EFI system, and while the datalogs I recorded with the tuning suite I use to burn chips  “worked”, the comparatively slow sample rate  of the cars computer (average 5-6 hz, or 5-6 samples per second) made the results somewhat erratic.  This is by no means any shortcoming of Virtual Dyno, rather the car simply not being able to generate enough data for Virtual Dyno to work effectively.
In the back of my mind I’ve also always been wondering how I could use this awesome software on older classic cars…..the carb & distributor guys.  Without being able to record an rpm signal, a throttle position & a timestamp (the 3 pieces of data Virtual Dyno requires to function), that seemed unlikely.
During the last couple of years I’ve also been experimenting with another piece of software that I have come to trust implicitly…..or more accurately, an App for my smartphone.
My initial impressions were that it functioned very well, but the accuracy was somewhat questionable due to a typical smartphones GPS sample rate of 1 hz.   I was intrigued enough to purchase an external GPS receiver which Track Addict easily links to via Bluetooth…..this brought the GPS sample rate up to 10 hz.  Accuracy improved monumentally!
So how does this relate to Virtual Dyno?
One of the nice features of Track Addict is that once you’ve done a lap at your favourite track, done an 1/8 or 1/4 mile pass or run down you favourite segment of road,  you can export all the data in .csv format (the format Virtual Dyno uses) to view in your choice of spreadsheet.  It was while I was looking at the data from a 1/4 mile run that the light bulb went off for me.  The data I was looking at had a timestamp on each row of data, vehicle speed updated at 10 times a second (which I could then do the math to determine engine speed)….all I needed now was a Throttle Position & maybe, just maybe this could work with Virtual Dyno!!!
Intrigued by the possibility of being able to use my smartphone & GPS to dyno vehicles I set out to determine if it actually works.  I headed out to my usual stretch of road I use, put Track Addict in Segment Mode & proceeded to do a full throttle pull through second gear.  Once home, I sent the data to my laptop & starting editing.  First I needed a formula to convert vehicle speed to engine rpm.  A little bit of tinkering led me to the formula of:
Engine rpm = Vehicle speed * Tire revolutions per mile * Differential ratio * Transmission ratio / 60
So if I take 60 MPH as an example,
Engine rpm = 60 * 808 * 3.23 * 1.63 / 60
Engine rpm =  4254 rpm.
Armed with that formula I proceeded to add an Engine RPM column to my Track Addict data, & then added a Throttle Position column & put 100 in the rows that I wanted Virtual Dyno to look at as my dyno run…..theoretically I should be good to go as I have all three things Virtual Dyno looks at.
After saving my new spreadsheet and loading it into Virtual Dyno the resulting graph was noticeably shifted from what it should have been…..something was not accounted for.  Having the luxury of having lots of data from datalogging the car as much as I have I went back through some datalogs and came to the realization that the rpm my formula was generating for a given speed was not matching the rpm I was seeing in my datalogs at a given speed.  The torque convertor……that’s what the missing piece of the puzzle was!!  The torque convertor is not locked in this car at full throttle in 2nd gear…….doing some quick math I determined that I needed to add an 8% torque convertor correction factor onto my formula in order for it to generate the correct engine rpm at a given vehicle speed.  This step should be unnecessary with manual transmission cars, and for those vehicles that don’t have datalogs to rely on, noting engine rpm vs vehicle speed you should likely get your correction factor pretty close.
Engine rpm = Vehicle speed * Tire revolutions per mile * Differential ratio * Transmission ratio / 60 * 1.08
With this new formula plugged in I was hopeful that should take care of things.
Capture
So…..now the big question….did it work?
Apr 18 GPS Dyno
 It actually worked magnificently!!  Truth be told,  compared to using the datalogs from this car with its slow sample rates, this method actually generates much more consistent results……so much so, that I actually use this method with this car now rather than datalogs.
So while this method of using Virtual Dyno does necessitate some spreadsheet editing (about 5 minutes worth per run), it now has opened the door for me to use the software on literally ANY vehicle, EFI or carb does not matter anymore!!

 

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: