PmD percentage correlations

[Anonymous]

A discussion with Rusty and some very precise PmD posts by "Lars" on SpyderChat suggested the following thoughts on correlation between dyno whp, PmD seconds and 1/4 mile times.

 m http://www.spyderchat.com/phpBB/viewtop ... 45#4144645 m

` hope you find it interesting ...

[MRMike]

Interesting indeed!   s:lol: :lol: s:lol:  

I was pondering the magnitude of dyno errors, stated to be insignificant within +/- 3%. Thus, the potential dyno errors within the power range of a normally aspirated MR2 are:

3% of 120 whp = 3.6 whp
3% of 140 whp = 4.2 whp

The 4 whp dyno error window interested me, as it is in the range of gains from so many normally-aspirated mods i.e. it's consistent with all the uncertainty regarding the dyno results from headers, mufflers and air intake systems.

That got me to wonder how far percentage relationships correlate between PmD times and dyno whp. What would a 3% PmD stopwatch error be?

8.2 seconds * 3% = 0.25 seconds
6 seconds * 3% = 0.18 seconds

... say a 2 tenths of a second error window ... That sounds conservative, especially, if the measurements were done carefully and averaged over 6 runs.

No statistically I don't think it will be that low.  The 8.2 figure is not a 'real' number.  It is the arithmatic mean of the 6 runs.  So you could have recorded 8.6, 8.4, 8.2, 8.0, 7.9, 7.8 for arguments sake. The range therefore is from say 8.6 to 7.8.  So it's not possible to simply multiply by 3% to get the sampling error.  

The total sampling error would include the time taken to stat/start the stopwatch each run, which evidently will be more than the overall 3% you attributed earlier.

Even best case scenario we take the 3% sampling error you suggested, the sampling error observable in the results of 0.25 is significant. As you said yourself "tengths were significant in this methodology". If the sampling error produces an anomoly the same as what you deemed a  'significant' event how can we have confidence in the methodology?

We've seen posted PmD's of say ~7 seconds for MR2's with normally-aspirated mods (e.g. header, muffler & cpu for A/F ratio correction). We also know (from PmD tests) that showroom stock MR2's take 8.2 - 8.5 seconds for a PmD.

7 seconds / 8.2 seconds = 85% ... or ~15% different.

120 whp * 1.15 = 138 whp ... about what dyno tests show for cars with those normally-aspirated mods.
Hmmm ....

I wonder why you find this surprising? It's a directly proportional relationship, if I lose 15% of my body mass, it stands to reason that all other factors being equal I will be able to run 15% quicker?

For instance a quick 0-60 calculation can be acheived by using Kerbweight(Kg)/BHP*0.9

Standard MR2===== 975/138*0.9 = 7.9
A 15% lighter MR2==828.75/138*0.9 = 6.6

Roughly 16% difference.  Again if I add 15% HP wouldn't you expect the PMD to decrease accordingly?  

You have identified a definite correlation here, and its a simplistic one that has nothing to do with the PMD methodology. It s a simple question of changing the independent variable to change the dependent.

Whilst this holds true, the numbers that you use as the base in this relationship ie the 7 seconds/8.2 seconds are still IMO not statistically sound.

I enjoy reading about the PMD method, and will continue to do so.  I also really respect what you're trying to do with this method of assessing the performance of different mods etc.  And as a guide as I have said before it's a good tool.  But in regard to proving a relationship between the PMD results and 1/4 mile and the like I think you're some way off. Again this is only my opinion, and Hell who am I to say i'm correct. I hope this is taken in the friendly/inquisitive manner its intended!    s:lol: :lol: s:lol:

[Anonymous]

I hope this is taken in the friendly/inquisitive manner its intended!    s:lol: :lol: s:lol:

... absolutely, MRMike ... lively debate ... all in good fun and sport ... in the pursuit of "the truth" (the hardest thing in life to identify).    s:) s:)  

Your points are well taken.  You are obviously aware of the statistical significance of these relationships.  Nothing's perfect (including dynos and 1/4 mile runs) ... but, as a cheap (non-destructive) way to record performance differences, I haven't discovered a more precise measurement tool than PmD's.

Trying to get others (the mass of members) to adhere to good "scientific method" has been like trying to herd cats!   s:D s:D    I don't mean to be a hardass to/at anyone.  I'm just trying to collect accurate info (i.e. more meaningful than butt dyno data, accompanied by exaggerated adjectives) ... by which we can plan modifications and make informed purchasing decisions.

I've logged all my runs in a spreadsheet i.e. it contains the raw run data without/before averaging (as well as many other tidbits, I've collected in my readings).   I'd be happy to send the spreadsheet as a email attachment, if you (or anyone else) PM me with an email account which can accept attachments.

I just wish we could get/encourage others to carefully log PmD data before and after each of their modifications ... and post them - to benefit all!

[MRMike]

I hope this is taken in the friendly/inquisitive manner its intended!    s:lol: :lol: s:lol:

... absolutely, MRMike ... lively debate ... all in good fun and sport ... in the pursuit of "the truth" (the hardest thing in life to identify).    s:) s:)  

Your points are well taken.  You are obviously aware of the statistical significance of these relationships.  Nothing's perfect (including dynos and 1/4 mile runs) ... but, as a cheap (non-destructive) way to record performance differences, I haven't discovered a more precise measurement tool than PmD's.

Agreed  s:lol: :lol: s:lol:  I'd be most interested in seeing what you have collected thus far. I have a quite a lot of GTECH data, if that could be of any assistance.  I imagine it could be useful because you have the ability to plot RPM v time v HP v torque which could be useful in establishing a relationship.