Help Me With My Math

[Silver Bullitt]

Help me with my math here. I ran a friend with an '18 5.0 M6 PP1. For reference, my car is an '18 5.0 M6 PP2. Both cars are stock right down to the paper air filter. The only difference is my car has a resonator delete w/ H-pipe. He still has the resonator in place. Both cars have active exhaust.

We did a total of six pulls to the top of 3rd gear, four from a dig (one in Sport, one in Track, and two in Drag mode), and two from a 45 mph roll in 2nd gear (one in Sport and one in Track). My car pulled his by at least 1-2 cars every time. I know I might have a slight advantage torque wise over him up to about 4K rpm from the H-pipe, but I was still pulling him past that. We were a little shocked that the res delete/H-pipe could make that much difference.

One other thing occurred to me as I was thinking about this again today. The PP1 has a stock rear tire size of 275/40/19 which equates to a 27.66" tall tire. The PP2 has a stock rear tire size of 305/30/19 which equates to a 26.20" tall tire. Using a 3.73 rear gear ratio for the PP1, my math says that's roughly a 3.94 rear gear ratio for the PP2. I think that explains some of the reason I could consistently pull him over just the H-pipe.

Can someone verify my math for me to make sure I'm not missing something here?

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[ChromaticGrey]

Could be all of that. Also each engine is different, and they don't all make the same power. You engine could have been a bit stronger from the factory.

Also driver makes a difference. Try swapping cars and see who wins then.

As far as the math, cant help you, but would imagine the tire size difference is so minute that it wouldn't matter much. How much tread wear do each of you have and what is your effective tire radius, your getting into that realm.

 

[2017GBGTPP]

You also have the PP2, which is a better handling package with better traction. If you weren't faster than him then you wasted $3,000 on the upgrade.

 

[nastang87xx]

I can promise 110% that the H pipe and resonator delete did absolutely nothing for actual measurable power gains. Frankly I think you're thinking way too far into it. There's a lot of small variables that could add up.

 

[drive_55_not]

I can promise 110% that the H pipe and resonator delete did absolutely nothing for actual measurable power gains. Frankly I think you're thinking way too far into it. There's a lot of small variables that could add up.

I agree, Driver is one variable that's easy to check, Swap cars and see what happens.

 

[Torched10]

Lots of variables,like how much gas in tank,what each car weighs etc.100lbs is 0.1 sec or car length on 1/4 mile

 

[frank s]

Seems to me the rolling radius of the two tires, translated as you say into final gear ratio is plenty to explain the result you describe. That and driver differences. There must be some underlying circumstance that inspires you to make such a fuss about it?

 

[Silver Bullitt]

You also have the PP2, which is a better handling package with better traction. If you weren't faster than him then you wasted $3,000 on the upgrade.

Not in a straight line, and certainly not from a roll.

I can promise 110% that the H pipe and resonator delete did absolutely nothing for actual measurable power gains. Frankly I think you're thinking way too far into it. There's a lot of small variables that could add up.

My car showed dyno gains of 10-18 lb-ft below 3800 rpm pre and post install, so maybe a slight advantage on a dig, but once again, not on a roll.

Seems to me the rolling radius of the two tires, translated as you say into final gear ratio is plenty to explain the result you describe. That and driver differences. There must be some underlying circumstance that inspires you to make such a fuss about it?

No fuss, was just really looking for someone to validate my math on the effective gear ratio from a smaller diameter tire, as I believe that made the primary difference.


I like the idea of switching drivers. We both left reasonably close from a dig, but the rolls are what really surprised us the most. That's pretty much all car and takes the driver out other than the 2-3 shift. As for weight, can't say on gas, but I had just filled up the day before, so my tank should have been pretty full. Both cars are 301A, so I would guess the weight to be pretty close. I've probably got about 50 lbs on him, but that's not a bragging point.

 

[LxMike]

Could be a few small things. The tire height difference would have your rpm's less than 300 higher at 45 mph. Who counted or honked the start? Weight, shift rpms... Maybe try switching cars or let him move first and see if you can reel him in.

 

[GregP27]

The OP said they did "pulls," not a standing-start drag race. So, better traction has nothing to do with it unless they were spinning tires, nor does better handling in a gear pull.

The PP1 and PP2 share the 3.73 gear ratio, so they SHOULD be nearly identical. 2 - 3 lengths in, say, a third gear pull from 2,000 rpm would be nearly identical. You're over 100 mph when you top out in third, and 2 lengths is maybe 25 - 30 feet.

Assuming you have a stock manual transmission, 7,200 rpm will get you to about 87.5 mph on my tires (285-35R-19). 87.5 mph = 12.83333 ft / second. So, 25 feet will be covered (at 87.5 mph) in 0.1948 seconds. Let's assume the 3rd-gear pull took 8 seconds. That means the last 25 feet took about 2.4% of the 8 seconds. That's pretty close to identical, but still a win for your car.

The difference in tires is pronounced. The 275/40-19 has a diameter of 28.66 inches (2.388 feet) and the 305/30-419 has a diameter of 26.20 inches (2.1833 feet). The difference is 9.38%.

That will develop about 9.38% more force at the tire contact point on the smaller tire. That probably accounts for most of the 0.1948 second discrepancy. You could swap tires and repeat the pulls to find out for sure. I would, just to satisfy my curiosity.

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