Yeah, a quarter....

Of "one MEELION dollars"

 

I was bored this past Sunday and decided to look into creating a thrust chart for my car:



The loss in thrust between each tire size is less than 2% according to my calcs, so not really much at all. I also found a chart made by Doug Mikko (4wd4me) comparing some of the STU offerings and saw identical thrust losses with the increased tire sizes (255, 265). Personally, I didn't think there was much of a dropoff in power/thrust when going from 255s to 265s, but the increased turbo lag out of slower corners was very noticeable.

 

this is 245/40, 255/40, 265/35-18?

 

^Yeah I'd assume so, of course the first two offerings being 17" tiyas.

So Im not really sure how to interpret that graph. The great the thrust (g's) the quicker acceleration, correct? The blue line shows the highest g force and does it at an earlier MPH which I guess would mean the quickest acceleration. But just because it is accelerating quicker doesnt mean it's spooling quicker.

I know Im splitting hairs but each pull wasnt started at exactly 20mph. The slopes of the lines all appear about the same it's just that the 245's accelerate harder which makes perfect sense given the gearing. But of course you give up some MPH at the top end.

What conclusions can be drawn from that chart? Im not trying to degrade what you did because I love charts like that, it speaks to my stable analytic personality trait. Im just not really sure what to make of it. I guess it validates what we know about gearing changes given different tire sizes. Goes to show like almost everything in life it's all a compromise and there isnt one best method.

Thanks for sharing your data!

 

In AutoX, top end speed is never an issue, so the downside is pretty non-existent, which is nice, because it's one less thing to worry about.
The chart is a nice visual representation of what effect gearing has/doesn't have on our acceleration. I love charts LOL

 

While I'd agree that spool is probably more important as all it takes is 1 tight corner where you're out of the powerband to blow the run, I wouldn't go as far to say that top end isnt an issue. In my case my local venue is very open so I'm used to faster courses but I'd love more top end. I have my rev limiter set to 8300 and Im constantly smacking the rev limiter in 2nd and sometimes will grab 3rd depend on the course. When I build my block I'd love to rev the thing to 9000.

 

Because why not, heres a comparison of another STU car I found to an SM evo (mine).

 

Jimmy runs events all over the place. The courses vary enough that I can't imagine it would be too easy to gear your car in a way that works for all of them (local courses are SUPER short and tight). I could be wrong, though, I don't do a whole lot of AXing. I have to imagine acceleration is far, far more important (in general). Again, I don't do much and just go out there and have fun so never give it much thought. I could see someone more into it taking everything into consideration, though.

Jimmy: is gearing/tire selection for top speed something you tune for?

 

Im not saying one is necessarily more important that the other. My only point was that top end also comes into play but of course its venue specific. Oh and something else to take into account is the power the car is making. My car for example is more in line with an SM car in regards to power. So I'd say top end MPH is more crucial for an SM car vs a STU car as they'll be smacking that rev limiter that much more often.

 

I could see that being the case. I'm at 450whp with a 6spd. I use 3rd occasionally, with a 7800RPM rev limit.

 

Yep.


No problem!

Yes, the higher the thrust the better the acceleration. The plots don't all start at the same speed (~20mph) because they are based on RPM, but pulled off the same dyno sheet. That's why you're seeing the 245/255/265 plots start at different speeds. As for turbo lag, I don't have a real way to quantify the reduced/increased lag unless I were to datalog during a run. Here's the resource I used to create my chart - http://www.rhoadescamaro.com/build/?page_id=591

As for conclusions, in my original post I was curious to see if going back down to a 245/255 tire would help increase acceleration enough to where it would offset the extra lateral grip of the 265....4% gain in thrust from switching to a 245 doesn't seem to be worth it. However, my next Excel project is to plot out what that 4% extra thrust means in terms of time on an autocross course


No. Depending on what tire size I use, I'm aware of what my top speed in second is, but it's not a priority to me. Right now, I'm still playing around with the idea of trying 245s or 255s again (most likely 255s).

 

Considering that we spend most of the time cornering while sometimes accelerating, I don't think 4% thrust gain is as useful...wondering what the next chart of yours is going to show!!

 

At yesterday's event at Marina, there was a local taking some pictures and he also made GIFs of various cars through the finish slalom



GIF by Craig Boyle

 

Car seems to be working really well there!!

 

Cool GIF

Lazy course workers; that 2nd to last pointer needs to be addressed, lol!