installed wider tires and got much slower trap speed

[Mr. Y]

Hi guys.

My setup is 7M-GTE (internally stock and tired) on R154 with PT67DBB, AEM and other goodies. I was running 225/50R16 tyres on sawblades and got 117mph trap speed. Recently I've installed 235/45R17 fronts and 275/40R17 rears and now my trap speed is only 110mph. ET became worse also.

So now the questions:
1) does my trap speed sound right for pt67@17psi and pump gas?
2) is that trap speed drop adequate? I knew I become slower, but not by 8mph!

 

[Rennat]

im thinking that you mean 275? cause if you put on 175, then im not suprised... lol

 

[Mr. Y]

ofcourse 275, my bad, sorry

 

[rtrdpenguin]

More unsprung weight maybe? Tires are actually pretty heavy. But I don't know much about drag racing =\

 

[supramad77]

did you get your calcs right for the side wall profile? The wider the tyre the slower you go unless you drop the side wall down enough to compensate.

 

[Slow66]

Hi guys.

My setup is 7M-GTE (internally stock and tired) on R154 with PT67DBB, AEM and other goodies. I was running 225/50R16 tyres on sawblades and got 117mph trap speed. Recently I've installed 235/45R17 fronts and 275/40R17 rears and now my trap speed is only 110mph. ET became worse also.

So now the questions:
1) does my trap speed sound right for pt67@17psi and pump gas?
2) is that trap speed drop adequate? I knew I become slower, but not by 8mph!

Old tire diameter = 25"
New tire diameter = 25.8"

1) No it doesn't sound right, and that is evidenced by your previous trap speed. Did the car feel sluggish? Felt like it lost power? If i lost 7mph from one track day to another, the first thing i'd do is a compression/leakdown check to make sure the motor is in check, and then i'd go from there.

2) The trap speed should have come down slightly, (probably more due to a resultant increase in traction/drop in 60's, as opposed to the actual diameter itself), but not nearly that much. Refer back to # 1.

How far apart were the track days? Weather conditions from both days? 60ft times? Clutch gone bad? Stuck caliper? Could be a myriad of things but the tires wouldn't be where my attention was directed at first....

 

[Mr. Y]

Thank everyone for input, especially Bryan
I'll start from comp. test tomorrow.

 

[Mr. Y]

Inspected datalogs more carefully and found out that on "weak" runs boost increases too slowly after shifts. Inspected WG and found that it was closing too slow... (So it was opened a bit after shift)
Damn chinese replica... It's time for real tial now.