Monday, November 16, 2020

Cone Dodgy Numbers

This post is not to be taken seriously. No seriously, I mean that. This post is where I attempt to guesstimate the results of my latest power improvements, and I will be venturing into the world of comparing results from different dynamometers and no such comparison should ever be trusted, but all too often it's the best we can do, so let me make some excuses, and then we can get on with the dubious number wrangling.

Click here to skip my rambling and just see the graph....


In theory the right way to upgrade a car's power is to do a several dyno pulls on dyno A before modification and then after modification calibrate and re-measure on the same dyno A. For further modifications, one would ideally have another before/after pull on Dyno A. Dyno B which is a different dyno altogether shan't be involved and dyno C is right out! 


I didn't do that. Not because it's a bad idea but due to a combination of time constraints, weather and cost. After all, dyno time is not free.
  • Weather: Modifications of major magnitude are often done in the winter, and the decision to proceed often coincides with the onset of cold weather. Once the snow is up, toting around the 20ft trailer becomes a lot less fun. The risk of loosing everything I worked on due to one ice patch or similar is daunting because I'm not maid out of money. 
  • Time: If I would decide earlier in the season this would be less of a problem, but when engines, or major portions thereof are being replaced and the first event is early April, and one pulls the trigger after Christmas, adding a trip to the dyno into the schedule is dubious, especially since the engine builder wanted some key parts off the original to get started.
  • Cost, well dyno time isn't free, but the time off to visit a dyno is potentially even more expensive, and as I said, I'm not made of money.
Ok them's my excuses and that's that. (Well yes it does boil down to Gus is cheap and he procrastinates... shhh!) 

Also in this case there was a necessary switch in dynos because the dyno from last time was down for service, so even though I stuck with my Emtron ECU and my tuner Nick, I had to take it to a different location. 

Funny Math

Even using the same machine, day to day atmospheric difference can cause some variation, but as a rule aside from what the operator might do inputing measured temps/humidity/pressure there's little one can do to ensure equality there so one just hopes that that source of error is not significant. 

Beyond that the different brand of dyno have different overall reading tendencies, much of which is fuzzy despite no end of internet discussions on the topic. Obviously, in the real world a given pull releases X amount of heat energy, and Y % of that winds up being used to accelerate the dynamometer. In theory they should all get the same result but in practice they don't, and the proprietary nature of their software and hardware is such that it's bad business for any Manufacturer to clarify how their numbers relate to this theoretical truth. All that any consumer grade dyno actually guarantees is consistency for runs on that dyno (assuming consistent atmospheric conditions of course). For tuning an engine that's what's really required, and so no shop is going to invest in a system calibrated to a real world standard. Only large OEM's have that kind of budget, and they "simplify" things by measuring the engine outside of the car anyway. ("Brake" horse power).

So in the portion of the world not working with multi-million dollar test labs, we have to wing it. 

The dynos in question are a Dynojet (last build) and a Dyno Dynamics (this build). What we do know from the internet, and from the claims of my tuner is that Dyno Dynamics is among the "heat breaker" dynos. The readings from these dynos tend to be lower than the much more common and nearly industry standard Dynojets. Both the Nick and the world wide web say something like 10-15% lower readings for the same car on a DynoDynamics vs a Dynojet, but that's a really really huge range. For a 190hp reading that's a range of 209 to 219, and for a build that is comming off of a 193 Dynojet reading that's a 60% difference in how much power was added, and saying a range always leads the conversation into hemming and hawing in an unsatisfying way, so I want to pick a number and have some basis for it that is is only partly insane.

Rather than using an internet post with little or no known basis in fact, I'm going to lean on my knowledge of what changes are being made to the car and derive a "possible" factor to convert the new dyno readings to the scale for the old dyno. 

Some Logic?

Things I know about the effects of the changes to the car that are relevant to this exercise include:
  1. The Compression ratio changed from 11.5:1 to 14.0:1, and according to this page, that's supposed to give a 5.7% increase in power. This is based in hard physics including the Ideal Gas Law (PV=nRT) etc. so, unless something else changes, it should be fairly predictable.
  2. The RPM limit is being raised to 9200, this means peak HP is not comparable and we can't use that number for scaling things, we should be using peak torque instead.
  3. The new roller-rocker setup deletes the variable lift function and ignores the lower cam. The lower cam is so named because it functions in the lower RPM's, typically below 6200. Any measurement below the cam switch point in the old build has both cam and compression changes to account for, so it's hard to predict the change under 6200 rpm.
  4. The cold air intake was increased from 2.75" to 3" and the throttle body from 65mm to 70mm, so the upper RPMs are likely benefiting from that change as well as the compression change 
So it sounds like there's nothing left, but that's not quite true. I *think* its reasonable to presume that the air intake is never limiting the developed torque until after the torque peak. Other factors could limit it first, but with the torque increasing, the air is not limiting, and thus below the torque peak and above the cam switch in the old tune is an area that  should be valid for comparison where only the compression change should be having an effect. Conveniently torque peaks are also frequently reported on dyno sheets. That conclusion of course relies on the assumption that the port/polish jobs and valve flow are similar, but I have no reason to suspect changes there. 

One thing I'm leaving out is the effect of the lighter pistons, and the lighter crank, which probably provide a slight gain beyond the 5.7% because less energy is stored as kinetic energy in the rotating assembly, but I would also expect the belt driven external oil pump to have a little more drag than the O.E.M. internal pump, so I'm going to guess that those are both small and mostly offsetting each other.

So that seems to imply that I can have a solid expectation based on a very inflexible bit of physics for the change at the old build's torque peak. The old torque peak was 142.1 ft/lbs at 6750 RPM (luckily this is solidly above the cam switch) and the expected result is 150.2 flt/bs at 6750 RPM. Since the new build's dyno graph reads 132.5 at 6750 RPM, this says that the scaling factor to convert from this particular Dyno Dynamics to that particular Dynojet is approximately (150.2 ÷ 132.5 = 1.134 a. k. a. 13.4% ) Thus the new torque peak of 134.6 ft/lbs @ 7000 rpm translates to a Dynojet number of 152.5 ft/lbs @ 7000 rpm. 

The Result

This result is nicely reassuring since the scaling factor is nicely within both my Tuner's claims range and the range of what you find on the internet. Overlaying the new chart on the old chart, lining up the axes and scaling things to match the above 13.4% factor gets me a comparison chart like this:

Factored comparison of dynamometer results. Note that MPH marked at RPM are only valid for a 4.8 final drive setup. The stock final drive is 4.5, so the 61mph top on the stock trace is more like 67mph in a stock vehicle. Thus I pack this power into a 7% smaller MPH range to begin with.

So the theory of this build was that the top end would gain power and revs, and the bottom end would experience competing effects with CR changes having positive effects and cam changes having negative effects. The hope was that these two would offset each other and lead to an overall faster car. 

I'd say it kinda, mostly worked. 
  • 0-4000 rpm we lack good data from the prior build but what is there suggests a definite loss, but this loss is only at launch and only poorly designed courses spend significant time under 30mph. It does seem likely that launch bogging will be a difficulty, but that is < 5% of the course...
  • 4000-5000 RPM has smoothed out substantially for a small net improvement which is interesting because my scouring the internet for dyno graphs led me to associate that shape with the PPE Engineering exhaust header I'm using. the fact it went away without changing the header is interesting.
  • 5000-6200 RPM has lost some oomph, to the tune of about 10 tq. This is sad, and I did notice it when driving. It's hard to say what impact this will have. 37 to 45 mph is a common acceleration range so this is a bit worrisome. I'll be pondering any ways to improve this. Moving down to a stage 2 cam might help since stage 3 tends to be a peak hp cam. 
  • 6200-9200 RPM shows gains are strong and clear and on par with expectations. My build lacks an ITB intake and is clearly running out of breath at the very top, but hits a dynojet corrected peak of around 216 whp, and the ITB setups built by DRS post 220 to 225 whp. I had dared to dream that the big bore throttle body and bigger intake might allow me to capture all of that, but that was just dreaming and I know it. I suspect I did get ~10 hp out of that switch in any case since the hp peak is still around 8k and the compression alone can only account for 205 of the 216 (dynojet) hp. The fall off is faster however so I wonder if the rising intake velocity isn't compressing the foam filter or something. The intake is sold for 400hp supercharged cars but something is up... since the torque peak moved but the hp peak didn't.
So the net result is that my car has become a bit more course dependent, where faster courses should be much better, and slow stuff hurts slightly more. At the end of 2019 I drove 2 courses where I was riding the (then 61mph) rev limiter for 8-10 seconds on a 50 second course, raising the limiter alone is likely worth a half to a full second on such courses, and the power gains in the upper range hopefully worth .25 or .5 on faster courses. I'm hoping I only gave up a tenth or so on launch and no more than 1-2 tenths in the mid range. The only two courses I managed to run in 2020 were both pretty much a worst case scenario with very little time over 50mph (same course designer). I look forward to some faster courses in 2021 to get a real look at what it can do. 

The positive side of that is that most national level courses are faster courses... so it will be at its best where it counts the most. 

So now it's time for you to shoot holes in my assumptions in the comments section if you can :)

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