I have a A4 with 3.73 gears and 245/50/16 nittos. On a speed calculator using a 5700 rpm redline and whats above i should top out drive at 117mph but i dont. I hit the rev limiter(5700 w/ msd) at 108-109. Could my torque converter be slipping or is the calculator just not accurate.
this is the site
http://www.f-body.org/gears/

 

I have my own spreadsheet for the 4L60E and I get 117mph @ 5,700rpm in 3rd gear (1:1), with 25.65" (theoretical) tires and 3.73 gears.

You might want to check the actual tire diameter. I've seen them as low as an effective rolling diameter of 25.0", which would explain away about 1/2 of your difference. An inaccurate tach would also explain part of the difference.

But what is your rev limiter set at? I thought stock programming was more like 5,800rpm, which would be closer to 119mph.

 

Stock rev limiter is 5750. I have a MSD 6al that is set at 5700 so it will cut spark before the computer cuts fuel. My speedo is off do to the 3.73 gears but they shouldnt effect engine rpm`s. At the track i hit the rev limiter right as i cross the line at 108-109. This was with 245/50/16 Sigma HP tires that were brand new. I know they arent that good of a tire but a friend of mine runs a tire shop and gave them to me when my old tires were dead and i didnt have the money for new ones yet. I am going to go to the track tonight with the nittos which are also 245/50/16 and see if my mph is different.

 

Your car doesnt shift into OD? Right when i cross the traps im about to go into OD and sometimes it seems like it shifts into OD just before i trap... Im thinking its because my stock converter slipping, I want to rig up the lock up switch and make a pass and see what happens.

 

Hmmm I have a thought.

Could it be due to aero drag??? I don't know if it will make a difference at that speed but I do know that if you try to figure your top speed by calculating it you will NEVER reach that due to the fact that drag increases with the SQUARE of velocity!! I.E. 10 mph increase adds a factor of 100 to the drag.

 

Ok smart guy, how come he is hitting the rev limiter and other people with his setup like me are not hitting the rev limiter... Maybe my car is more aerodynamic

 

umm beats me

 

You say you hit the rev limiter at 108-109, but you say your speedo is off. If you are gauging your speed by the trap speed in the 1/4, that is your problem. Your trap speed is not the instantaneous speed at the end of the 1/4. Its the average speed of the last 60 feet (I think thats right). When you hit the rev limiter, you are pretty much dead on at 117-119. I'll double check the trap speed calc, but I'm pretty sure thats it.


Found it - its the last 66 feet.

 

I thought G-Tech made that up because their 1/4 mph readings were always off... so its true?

 

No, its not true. It is very easy to show that for a 120mph car, pulling 0.25 G's at the end of the track, the difference between the average speed over the last 60-ft of the track, and the actual velocity at the exact end of the 1/4-mile is LESS THAN 1 MPH.

Do you realize that in order for there to be a 10MPH difference between the average over the last 60-ft, and the actual speed at the 1/4-mile, you would have to pick up 20MPH in the last 60-ft? Not realistic, when it has taken the car 1260ft to accomplish the 1st 100MPH. It can be demonstrated that even a Top Fuel Dragster does not gain that kind of MPH.

 

YES, it IS true. G-Tech didn't make anything up, but he didn't mention anything about a G-Tech in his first post. My impression is that he has the car in 3rd, preventing it from going to OD and he hits the rev limiter at the end of the track. The calc says 117, so he happily goes to get his 117mph timeslip, and low and behold, it says 108-109. I'm sure Fred is correct on the math behind the average and instantaneous, but his post appeared misleading to me, suggesting that your trap speed is NOT an average, which it is. He stated his speedo was off, so he apparently wasn't using that as a gauge.

 

OK... lets' go through this one step at a time, so no on misunderstands.

Yes... it is true that your trap speed is the average over the last 60-ft.

Yes... it is true that the average over the last 60-ft will be less than the actual speed at the end of the track (assuming you don't have your foot on the brake... so I don't get accused of missing that point ).

Yes... The g-Tech manufacturer mis-states the numbers to make it appear the g-Tech is very accurate.... it is not accurate with regard to actual speed at the end of the 1/4-mile.

No... it is not true that the difference between the average speed over the last 60-ft, and the actual speed at exactly 1,320-ft can explain away a 10mph difference.....

Not sure how you came to the conclusion I was "suggesting that your trap speed is NOT an average" since I used the words "the difference between the average speed over the last 60-ft of the track, and the actual velocity at the exact end of the 1/4-mile"

Hope that clarifies the points, to everyone's satisfaction

 

All i know is at 5700 rpms i`m not doing 117 mph. I cross the line at 13.2 (2.1 60ft) @ 109 with hitting a 75 shot at the top of 2nd through 3rd. I will hit the revlimiter right as i cross the line. And yes the car is kept in drive. I know how the track calculates the MPH and it is not that far off. I will be getting a stall soon so we will see if a new stall helps.

 

You could always try a lockup switch, i think i'll rig one up and see what it does for the track

 

Well, going by the f-body.org gear chart, at 104 - 106 mph, it shows engine RPM around 5100 - 5200, which is about what my car would be at (speed and RPM) when I had 3.73's for awhile last year. SO, that says to me that the chart "should" be accurate, so either A) your T.C. is slipping too much, B) your car is the least aerodynamic Camaro evar!!, or C) maybe it's possible that the ignition system is cutting the spark before it actually hits 5700 rpm, thus limiting you to less than the "theoretical" 117 mph??

Just FWIW, my car ran the exact same time and speed as listed in my sig even with 3.73's, as compared to with 2.73's............so if you're running .2 seconds faster than me, 108 - 109 mph sounds more realistic than ~117 anyways.