24Chromium

Yes, you caught some of the mistakes I referred to in my posting. Going into 6th gear as I enter T1 was obviously a mistake. I meant to shift into 4th gear. It had been so long since I ran TH, I was very out of practice, so going to 5th gear on the front straight was sub-optimal. After dropping very nearly 7/10's the next morning, I am quite certain that I might even dip into the 55's if I can execute a perfect lap.

I do not subscribe to the scrub and store school of thought. I bolt them on and run them hard straight away. I shoot for 15 HC's or typically 3 days. For example, I was on track at Road America for 6 days straight, but only used 2 sets of tires. Of course, it was my first time there, but I felt the sun did more to slow me down rather than the tires. Hoosiers don't age well. They feel awful if you've got, say, 5 HC's on them and haven't used them in a week or two. It takes a few laps or more to get them soft and sticky again.

24Chromium

I agree the tires can make a big difference, but I've been on track with many GT4s that have the whole compliment of intake plenum, throttle body, tune, headers, exhaust (or lack of), gearing, etc. Those make a huge difference as well. Maybe even more so than tires. Getting dropped like a stone at corner exit onto a long straight is gutting.

Matt, please keep the list as-is.

orthojoe

Thanks for the insight!

I'm gonna disagree with you on this one. It takes a lot of HP to make any sort of difference. There's about a 2 second difference between a GT3 and GT4. Put some R7s on a GT4 and you are running GT3 times. The 90hp differential is negated by R7s. I'm certain that those power modifications on a GT4 don't get you 90hp. Power mods buy you a few tenths, but tires buy you a few seconds.

provoste

Fact. Carrying 3-5 mph exit speed onto a straight never allow power to overcome that delta. Some of the Midwest & NE tracks (NCM, Putnam Park, VIR, WGI) with fresh pavement see a 3-5 second delta just going from RE71R to Hoosiers.

orthojoe

Yup. It's all about exit speed and how soon you are on that throttle

Yargk

My rough analysis finds:

1.8% lap time diff per 10% weight lost
1.3% per 10% power gain

(weight does more than power)

So on a two minute track, if you take 10% of your car's weight off, you'll lose about 2.16 seconds. Even just a second faster would require 5% weight reduction, or 150 pounds..

The 991.1 GT3 has 23% more power than the GT4, but weighs 4.8% more. So it's 3.59 seconds faster from power on a 2 min track, but 1.04 seconds slower from weight = 2.55 seconds faster at Thill, by my guess. (Assuming equal tires, aero, etc.)

orthojoe

Interesting numbers, Keith. I'm pretty close to your calculations in reality. I would not have guessed that weight made that much of a difference.

Yargk

I'm not sure I'd expect it to be that much larger than the lap time change with power either. There's a lot going on: the way a car accelerates with a difference in power is different than a difference in weight (is the power increase "peaky?" A weight decrease helps at all rpms, but won't help top speed), tires work better with less weight on them (grip vs. normal force is actually slightly non-linear), and a weight decrease makes aero do more.

I was trying to figure out how power helped lap times by noting the percentage of a lap spent at full throttle and then using quarter mile calculators and differences in trap speed to see how the average speed between turns would be affected. Once I had this I actually used some of your GT3 and GT4 lap times to check it, but I had to have a term that accounted for the weight difference. Then I found this number on the internet: "Interesting data point: In a recent article discussing the new Ferrari 430 Scuderia a factory engineer is quoted saying that an 80kg (176lbs) weight reduction shaved 0.8sec off the car's Fiorano lap time (1min 25sec total). "

DC640

That's it. I'm going on a diet. Probably easiest way to make things light when I drive my car 😬

Veloce Raptor

Beantown makes a great point: make sure your set up compliments the tires you choose.

Weight is a huge factor. Its effect varies by track. On tracks with a lot of elevation change (like the Glen) or lots of slow corners followed by long acceleration zones (like COTA ), it can be as much as 0.75 seconds per mile per 100-150 lbs, depending on HP as well. Thus..:a 200 pound passenger can have a dramatic effect, for example.

orthojoe

I supposed where the location of the weight loss is can be important as well.

It is true that I'm consistently one second slower when I have a passenger in the car

matttheboatman

Got it posted now.

sdillon

Got a 3:15.60 at Utah Motorsports Campus, the full track configuration last weekend. Matt - the earlier time you posted for me at UMC was for the outer configuration (there are actually four - east, west, outer, and full). Shims up front, toe links in back, and RE-71s on stock wheels. Otherwise stock.


Very interesting to read the discussion re: power and weight. I enjoy my car on the street so I'm not willing to rip too much out of it. On the other hand, there are so many corners I find myself in 3rd at 3500 rpm just waiting forever for it to spin up again. The cheap advice is to use 2nd, but it makes for a lot more shifting and also keeps me so near the top of the power that oversteer becomes a bit of an issue. I'd love some more midrange!

kfmcmahon

loL...until the GT3 is on Pirelli slicks...never ending cycle

kfmcmahon

yes, me too...cheaper to drop 40lbs than swap out CF this and that....unfortunately i like beer to much