Speedo jump with gears?!?

[amerikan]

Tried my car out with all the new mods last night.. I honestly dont know whats up. when i shift from 1st to 2nd the speedo jump.. and there is no chirp or anything. Do you know what can explain this? also.. i did a cheesy way of testing the 0-60.. with all those mods i only got a 6.8 even with a break stand.. is this normal? i know all cars are different but sergs got a 6 0-60.. i know that was with the fordchip guys tuning it.. but still. any comments?

[SergntMac]

If the speedo is jumping, you're spinning your tires. Why they aint screaming too, I don't know.

[CRUZTAKER]

No brakes on a launch. Just crush the accelerator. Mine patches short every time. I am however unable to get the 1-2 chirp....even manually shifting. No biggie, as it does it on it's own occationally.

[looking97233]

the tires can spin quietly.

[jgc61sr2002]

Originally posted by looking97233
the tires can spin quietly. I agree. There could be foriegn matter such as water , antifreeze, or oil on the roadway causing your tires to spin quietly. IMHO.

[amerikan]

guys this is really weird.. because i dont even feel any loss of traction and the speedo jumps at least 20 mph.. then gets back on track.. o well..

[TripleTransAm]

20 mph? Wow, that's a bit...

In 2nd gear, 20 mph must work out to a fair RPM jump, and I doubt that the RPM after the WOT upshift is below the stall speed, so while maybe not mechanically locked-up there probably isn't a HUGE amount of torque converter slippage.

My point: if you see the speed jump by 20mph after an upshift at WOT from 1st to 2nd and then it eventually comes back down again, you should see the RPM fall back down as well, once the speed returns to normal... IF it's wheelspin.

If you upshift and the speedo goes up 20 mph, then comes back down without the RPMs dipping back down as well at that point, then it's most likely an electrical/electronic issue. If you do see a 1 to 1 relationship between the speed coming back down and the RPMs coming back down as well, then.... DAMN, you got yourself some nasty 1-2 wheelspin, dude!!! :D

[SergntMac]

Dude...WTF! Everyone told you to wait until the mods broke in, but noooooo...Now your u-joints are slipping...

[LincMercLover]

:lol:

[amerikan]

lol serg.. very funny..im blaming this on all of u guys :P;)

[looking97233]

TTA I beg to differ with you. This is diffuclt to explain easily but here goes: The following happens in like .5 to 1 sec. Okay a hard 1-2 shift happens, the rear tires break traction, the tourque converter not under a heavy load due to the break of traction, spins almost 1:1, it's not locked but since the load is light there isn't much torque multipication, thus the big jump in MPH without the corresponding jump in RPM, then the tires start to bite increasing the load on the torque converter, which causes it to slip and multiply torque, the speedo starts to come back down, the rpm's start to climb, the tires bite fully and everthing is right in the world. As I said it's kinda hard to explain hope I did a good job of it.

[amerikan]

that makes a lot of sense.. thanks for your time.. greg

[TripleTransAm]

Looking9723,
I think I understand what you're trying to describe, but I honestly can't grasp how it can happen like this in real life. The following is my reasoning why, and I definitely would appreciate more details from you if you still disagree.

Whenever the tires break traction on an upshift, it's the result of one of two forces acting on the tire winning over the other. You've got the entire driveline under stress in one direction, from torque converter to tires, as the engine is delivering its maximum twist. On the other hand, you've got the tire contact force acting at the contact patch... without this force, the engine would rev freely.

During this upshift, the transmission gearing has suddenly changed, meaning the engine speed no longer corresponds to the tire rotation speed. So one of two things could happen: the engine can get violently slowed down to match the new ratio, or the tires can spin up instantaneously to match the engine speed.

99.9% of the time, the engine gets forcibly slowed down, hence the RPM drop during the upshift. It's still struggling against the torque converter and all the load it sees beyond the tranny, but there's nothing it can do about it... it has to slow down to match the new mechanical ratio.

However, on some occasions, the upshift can be violent enough to deliver a violent shock to the driveline and break the tires loose, as the driveline attempts to spin up to match what the engine is currently revving at. As mentioned in the 99.9% case above, the tires can usually take this shock and resist breaking traction, so it's the engine that loses out. But if the violent shock is enough to overcome the tire's static friction coefficient (static, because the tire is not initially sliding), then there is a break in traction and since the coefficient of dynamic friction is numerically lower than the static friction number, the tires have a harder time gripping once they're spinning.

Now, the tires won't spin on their own (and that's why I don't understand your comment about the torque converter being unloaded at that point)... the engine has to keep the driveline under stress, so the torque converter has to be under load. You're right about it not multiplying torque (at least not appreciably) since the speeds where this happens (the RPM in 1st versus RPM in 2nd) are high enough to be beyond the stall speeds... torque converters only multiply torque when the output is below stall speed.

In other words, if it's not the engine that's forcing the tires to spin, I don't know what external force could make the tires spin faster (even momentarily) than they were before the upshift.

So since the engine is always straining against the driveline, if the tires let go as much as 20 mph during the upshift, the engine speed will race up accordingly in the new gear, until the tires grab suddenly and the speed goes down, bringing down the engine RPM the same as it does during a non-tire-spinning upshift.

You're right, it isn't an easy thing to describe. The best way to visualize it is to consider the torque converter as a solid link (which it mostly is at that speed under load, with a little bit of loss thrown in), and imagine the whole driveline as if it was a manual transmission drivetrain.

I hope what I posted makes sense. Let me know what you think.



side note: this sharp change in tire acceleration during an upshift is one of the reasons why dynos generate unusually high peak numbers when the car is allowed to upshift during a run. The immediate tire 'jump' in acceleration makes the dyno report an amazing (although false) torque number...

[amerikan]

Thanks for your time trans. this is probably one of the longest posts i have read :p, and one of the most meaningful. i found out that the road that i was on had little pebbles which is what caused the 20mph speedo jump. last night i was on a "normal"road and the speedo barely jumped. Your explanation makes a lot of sense, i appreciate your time.-greg