924Board.org

[ptheskil]

OK I'll have another go at this. I posted some results a while ago using data from an iPhone and it got quite a cold reception (unjustly I thought). I think some people thought I was claiming some sort of bragging rights with the numbers so please read carefully...

Anyway, this time round I'm using an Android phone (Samsung Galaxy Ace) and an app called AccDataRec. This does not calculate power, torque, 0-60, 30-50 etc.- it calculates nothing at all. What it does do is log the internal data from the registers of the 3-axis accelerometer (in my phone this is a BOSCH BMA220 in case anyone wants to interrogate the datasheet). The instantaneous "g" forces in all 3 axes are exported to a .csv file approximately every 10ms with a time stamp to the nearest ms. This gives you most of the raw data you need to calculate power, torque and acceleration times to a reasonable degree of accuracy. I'm going to stick my neck on the line here and say that accuracy is in the region of +/- 5-10% for power and torque, and +/- 2-3% for acceleration times. This is NOT a comparable test to an iPhone/Android app which claims to calculate power and torque directly. I have tested a number of these and they are useless: if you get an answer close to the one you are expecting it is most likely to be luck. Some (few) do a reasonable job of measuring acceleration times as this is 100 times easier and less influenced by other things.

I have done a number of runs in 2 directions in a selection of gears. Not a structured set of tests but a handful of data to get a feel for repeatability etc. I have made no attempt to capture any data on the 1001 parameters which will influence the results: air temperature, humidity, barometric pressure, engine oil temp, gearbox oil temp, tyre temp, air temp, wind speed and direction, road gradient, road surface and so on. The vehicle was not fully stabilised from a temperature point of view so engine and gearbox oils will have been increasing in temperature throughout the tests. Lots of variables then, but the measurement and calculation itself is pretty accurate. Please note what I am saying here: the measurement and subsequent calculation of results from the data taken IS ACCURATE but the results are not robust to the uncontrolled variables. Road load is calculated from rolling resistance (generic value) and CDa (from tech data). This is one of the reasons that the power and torque results are less accurate.

There are 2 sets of data: in gear acceleration times and power/torque.

https://picasaweb.google.com/118033499298158047258/Desktop#5660314261677841218
https://picasaweb.google.com/118033499298158047258/Desktop#5660314251386577890


My main conclusions are:
1. car is in reasonable shape with no significant engine mechanical problems and performs as it should.
2. top end performance is stronger than bottom end compared with factory figures and contemporary performance tests. Slow responding wastegate? I haven't done any work on this yet.
3. recent work on the fuel, ignition and intake/hoses etc. has been worthwhile and I can be pretty sure there aren't any problems with these.

Qusetions/comments/disagreements?
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1981 931 series2 Euro spec

[Khal]

[morghen]

or you could have done a couple of 0-60 runs and some 1/4mile runs to find out if the engine is making about the same power as it should from the factory

so you say 10-15% error on the power guessing test...
well if my car makes 210hp as "it should" 15% of that is...pffff....a lot..wouldnt you agree?
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Supercharger and EFI kits
https://www.the924.com

[ptheskil]

take the power/torque numbers as an interesting curiosity and don't get all hung up about the numbers. What was more interesting to me were the in gear acceleration times. This is a much more reliable/repeatable measurement and is exactly intended to be compared with factory (or contemporary roadtest) measurements. Personally, I find this more useful than 0-60 tests or anything else involving launch or gear changes as there is too much variability unless you practise (do it regularly). My skills with the clutch and gear change are not good enough to be as consistent as I would want and I tend to err on the side of sympathy towards the drivetrain. But even I can manage to plant my foot and keep it there in a single gear. Also, this gives you an idea of engine strength throughout the rpm range and doesn't just focus on power.

This is where I found an interesting result in my recent testing: if you look at the 20mph increments in 3rd gear they are close to the factory numbers at low engine speed but towards the top of the rpm range they are significantly quicker. Don't get hung up about absolute numbers again as there are many differences between my test conditions and somebody else's 30 years ago but the relative figures are significant. It seems (to me at least but am I the only one to read this?) that my car is running stronger at the top end than the bottom. Assuming that this is true then my question would be: why is that? My thoughts are that it could be caused by a "sticky" wastegate which is allowing higher manifold pressures than intended. I think that the difference is too big to be explained by AFR variations and I take the simplistic view that as long as the DITC is working at the bottom end and idle then it's probably OK at the top end. Don't like the word "probably" but...
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1981 931 series2 Euro spec

[morghen]

i'd say your car runs better than factory (if) at higher RPM because you are running a tad leaner than the factory setting and you use better fuel than it was tested on in the 80's.


Also, are you using an intercooler? is your car an S2? if yes/yes then the DITC does not back-up the advance as much at higher RPM because of the colder charge...therefore more advance..therefore more powwaaaaahhh
_________________
Supercharger and EFI kits
https://www.the924.com

[ptheskil]

It's totally stock ROW S2 - at least it is now that I've removed all of the previous CIS "fixes" and "improvements" installed by others

If anything I would say I am running on the rich side - but enough to be at the end of the AFR curve where power starts to drop? I'm not sure. My idle is a touch rich and I have done a lot of work on the WUR to set control pressures and adjust the lower control pressure under boost. No I/C so still suspecting that I'm running slightly higher boost at the top end. AFAIK (guess I should check this though) it is running a stock turbo and W/G although the turbo was replaced by the PO. I'll get some photos...
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1981 931 series2 Euro spec