Rpm drop at the shift

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supernova
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Rpm drop at the shift

#1 Post by supernova » Sun Aug 08, 2010 7:57 pm

Is a 800rpm drop ok or should it be more for a glide from 1st to hi gear? I shift at 7200 and it drops to 6440 or so. That on a 250 shot of nitrous. I thought it should drop lower.
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1972 Nova SS
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Best to date: 1/8
et: 5.28
mph: 134
new wt. 3340 lbs

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Re: Rpm drop at the shift

#2 Post by Marty Chance » Mon Aug 09, 2010 5:24 am

the power curve of the motor dictates how much drop we want on the shift, but 800 is ok as long as you do not run out of rpm before the finish line
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Re: Rpm drop at the shift

#3 Post by supernova » Mon Aug 09, 2010 7:45 am

It rpm's at 7240 or so out the back with a .078 nitrous jet set up. The reason I'm asking is we tested with a .082 nitrous set up this past weekend. When it wasn't spinning the tires in hi gear it was spinning in low. It's a 275/60/15 DR tire. I can tell that it will rpm out the back once hooked all the way down the track a lot higher than with the smaller jet. Like around 7400 and up. Would it be better to have just a little tighter?
Blackhoodmafia!!!! :mrgreen: :mrgreen:

1972 Nova SS
572 C.I. BBC

Best to date: 1/8
et: 5.28
mph: 134
new wt. 3340 lbs

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Re: Rpm drop at the shift

#4 Post by John_Heard » Mon Aug 09, 2010 7:58 am

Might consider a taller rear gear than tighter converter on those tires...

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Re: Rpm drop at the shift

#5 Post by supernova » Mon Aug 09, 2010 8:26 am

John_Heard wrote:Might consider a taller rear gear than tighter converter on those tires...
It has 4.30's in it now. 4.10's?
Blackhoodmafia!!!! :mrgreen: :mrgreen:

1972 Nova SS
572 C.I. BBC

Best to date: 1/8
et: 5.28
mph: 134
new wt. 3340 lbs

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Re: Rpm drop at the shift

#6 Post by John_Heard » Mon Aug 09, 2010 8:43 am

No wonder...

Uh, I'd drop down to 3.7 range - what tranny do you have and what 1st gear?

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Re: Rpm drop at the shift

#7 Post by supernova » Mon Aug 09, 2010 9:13 am

PG with a 1.80 low gear. I would think that going to a lower gear in the rear would cause the converter to be loser with maybe even less rpm drop.
Blackhoodmafia!!!! :mrgreen: :mrgreen:

1972 Nova SS
572 C.I. BBC

Best to date: 1/8
et: 5.28
mph: 134
new wt. 3340 lbs

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Re: Rpm drop at the shift

#8 Post by John_Heard » Mon Aug 09, 2010 9:33 am

It will, although it might not be as much as what you'd expect.

I'll let Marty explain more as he's the expert on this subject.

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Re: Rpm drop at the shift

#9 Post by supernova » Mon Aug 09, 2010 7:09 pm

How is this?
And can someone explain the humps in 1st gear?
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Blackhoodmafia!!!! :mrgreen: :mrgreen:

1972 Nova SS
572 C.I. BBC

Best to date: 1/8
et: 5.28
mph: 134
new wt. 3340 lbs

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John_Heard
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Re: Rpm drop at the shift

#10 Post by John_Heard » Mon Aug 09, 2010 7:32 pm

The smaller humps are tire spin. The first big hump where it rolls over is the converter locking up.
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Re: Rpm drop at the shift

#11 Post by supernova » Mon Aug 09, 2010 7:35 pm

So at the the end of the big hump were it starts to go up is were the converter stalls at?
Blackhoodmafia!!!! :mrgreen: :mrgreen:

1972 Nova SS
572 C.I. BBC

Best to date: 1/8
et: 5.28
mph: 134
new wt. 3340 lbs

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Re: Rpm drop at the shift

#12 Post by supernova » Mon Aug 09, 2010 7:36 pm

And the little hump, are they caused by the car porpoising?
Blackhoodmafia!!!! :mrgreen: :mrgreen:

1972 Nova SS
572 C.I. BBC

Best to date: 1/8
et: 5.28
mph: 134
new wt. 3340 lbs

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Re: Rpm drop at the shift

#13 Post by John_Heard » Mon Aug 09, 2010 7:39 pm

Yes that first one is the converter loading the engine down. Also notice in 2nd gear it's a nice smooth line - that means it was hooked. I don't see spin at the shift in the engine rpm, if you had driveshaft rpm data, you can see that better.

I wouldn't call that proposing, when it does that it's very easy to spot, very cyclic looking.

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Re: Rpm drop at the shift

#14 Post by supernova » Mon Aug 09, 2010 7:42 pm

This gragh is on the .078 nitrous jet set up. I did not get a good pass on the .082 nitrous jet set up.
Blackhoodmafia!!!! :mrgreen: :mrgreen:

1972 Nova SS
572 C.I. BBC

Best to date: 1/8
et: 5.28
mph: 134
new wt. 3340 lbs

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Marty Chance
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Joined: Fri Sep 19, 2008 6:34 am
Location: Cheney, KS

Re: Rpm drop at the shift

#15 Post by Marty Chance » Tue Aug 10, 2010 6:29 am

have you dynod the motor? where is peak hp? I like to keep as much gear in the rear end as possible so you may need to tighten the converter. In a perfect world we would just drop the first gear in the trans from 1.8 to 1.5 and leave the rear end gear alone and tighten the converter. Problem is there is no such thing as a p/g 1.5 planetary. Are you 1/8 mile or 1/4?
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