Quote:
Originally Posted by stugray
Ok:
"It's easy to see that the engine reaches redline faster and drops to idle faster with the GrimmSpeed Lightweight Crank Pulley installed"
Umm. NO.
If you look closely at the graph, the rise & fall of RPMs is exactly the same.
The two graphs are only offset ,because they started at slightly different initial conditions.
The SLOPE of the RPM rise falloff is exactly the same for both pulleys which means "no appreciable difference in rotational inertia".
Thank you for posting a graph that so perfectly makes my point.
AND: Mass Moment of inertia Stock 30.42 lbin^2 vs 6.09 lbin^2 for the GS pulley.
Great! SO what is the Mass Moment of inertia of the entire rotating mass?
Assuming that they actually calculated it correctly, see how the mass Moment of inertia of a 4.9lb pulley can be over 30 lbsin^2???
That's a factor of 6X for something so close to the center of rotation.
That means that the mass moment of inertia of the entire ~500 lb rotating mass could easily be in the 3000-10000 lbsin^2 range. (and that is being generous since MOST of the weight of the rotating mas is a LOT further away from the center of rotation than the pulley)
SO changing the pulley makes a whopping:
30.42 - 6.09 = 24.33 lbin^2 in an engine with greater than 3000 lbs in^2?
For a whopping 24.33 / 3000 < 1 % difference? (actually MUCH less as the 3000 is VERY conservative)
Whoo Hoo! I'm sure you can feel that in the butt dyno.
As I have stated before: you will get a bigger benefit by changing the pressure plate bolts from steel to titanium or shed 1 ounce from the timing chains.
Grimmspeed: I apologize. I would not go out of my way to make this type of statement if I had not been called out to do so.
I still think the pulleys look awesome.
If I was building an engine from the ground up I would use these.
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I was the one who did the testing, and the one who made this graph, and I must say that you might be interpreting it wrong. I can get why it might looks like the change is only due to an offset, but I think that's because you're looking at the 2 period moving average line, and not the raw data lines that it is based off of. I normalized all the data to start from the same initial conditions, and then averaged three runs (3 stock pulley, 3 GrimmSpeed Pulley), and then plotted them on this chart. I included the moving average line just to help show the difference, although it can be a confusing way to view the data if you don't know what you're looking at.
The testing was conducted the exact same way, and using an OFT to datalog. The OFT is good for about 10 samples a second, and all I did was log time, RPM, and throttle position. This is how I was able to obtain starting points, and using the average of three runs each specifically to prevent seeing results in the form of ONLY a data offset. I did the same for the decel chart (this are actually two charts added together), to make the time bouncing off the rev limiter negligible.
So all the graph is intended to show is that at each instant in time (we'll use the acceleration side of the graph for discussion purposes right now) The Lightweight Pulley is at a higher RPM value than the stock pulley EXCEPT for idle, and redline, as you would expect. So again, the trendline used is a 2 period moving average, and was chosen ONLY to show a more smooth dataset as the actual datapoints can be hard to understand unless you know what you're looking for (which is rpm values at a given time). It is really hard to argue that the chart doesn't start at idle, and end at redline for both sets of data (no offset there), and that our pulley is always ahead of the stock pulley.
We do both agree though that small changes in weight on the rotating assembly can make a difference. All we're doing here is showing the data that we recorded, showing a change. Sure shedding weight from the timing chain or changing the pressure plate bolts will have an effect too, and people can always pick and choose what they want to do. Me personally, I would probably do the substantially easier thing and just change the crank pulley if I were looking to shed weight.
But also, it does look pretty cool, and that's one of the main reasons I'd put it on my motor too
No need to apologize when your questions and points are perfectly valid. We're simply presenting the data that we observed consistently and averaged. Speculation is fun, but testing is even more fun. So thank you!
Chase
Engineering