Author Topic: converter fallback  (Read 312 times)

Offline spookie

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converter fallback
« on: September 09, 2018, 06:47:41 AM »
Running sbc with PG. Shift at 6200 fallback is 5200.Runs 5.60s@117mph 1/8mi. Using Valvoline synthetic dextron VI. is this to much fallback?

Offline bikeguy307

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Re: converter fallback
« Reply #1 on: September 09, 2018, 11:24:46 AM »
Depending on weight it could be. 1000rpm is a base line number, but a lighter cars seem to like less fall back (looser converter). My car is 2080 with driver 5800 stall shift at 7000. The fall back is around 900.

Offline JrFuel Hayden

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Re: converter fallback
« Reply #2 on: September 09, 2018, 11:38:51 AM »
Shifting at 7000, and stall at 5800, isn't that a 1200 fall back ? It could be your converter maker is not right about your true stall.
My RacePak shows my converter flashes right after the shift to 8500 rpm after a 9700 shift. All the converter people I talk to say the RPM  right after the shift is the TRUE stall.
If you shift at 7000, and your fall back is 900, that would mean your true stall is 6100 rpm, Right ?

Just my 2 cents,
Jon
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Offline bikeguy307

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Re: converter fallback
« Reply #3 on: September 09, 2018, 01:41:03 PM »
Depends on who you talk to, as to what is true stall. Some say it is the fallback number, some say it the number from a stall check (This is the number I use). Around 300rpm difference is normal between a stall check number and a fall back number.

Offline JrFuel Hayden

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Re: converter fallback
« Reply #4 on: September 09, 2018, 03:07:17 PM »
That's why I always ask where their lowest RPM goes to after the shift, Great if they have a Data computer , if not a play-back tac, or a camera aimed at the tac, and reply it.
Just trying to keep things consistant. How is a "stall check" is done ?
I have seen many a happy racer once they change converter , trans gear and or rear gears to better match engine power range.
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Offline bikeguy307

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Re: converter fallback
« Reply #5 on: September 09, 2018, 05:07:00 PM »
A stall check is done with the trans warmed up, you go to WOT against the trans brake with no launch chip.

Offline JrFuel Hayden

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Re: converter fallback
« Reply #6 on: September 09, 2018, 09:40:37 PM »
Our converter guy said not to do that, at least not too long on the WOT, he says it can overheat the trans fluid and maybe more important it can bend the blades.
I was helping out a JF team running in Comp at Sonoma and he was having a problem missing , and insearch of the issue he would do a "stall check" to see if it would miss after trying some fixes.
It burned up the converter and made the fluid smell real burnt.

Jon
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Offline bikeguy307

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Re: converter fallback
« Reply #7 on: September 10, 2018, 05:51:52 AM »
Well you should never do it to find an engine miss, and to do a stall check you should never be more than a short 2 count. Its something that should be done once just to have the data and then only if you are suspecting a problem. Stall checks are not as needed today with the use of data recorders and playback tachs but it is still useful data to have.

Offline JrFuel Hayden

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Re: converter fallback
« Reply #8 on: September 10, 2018, 11:55:13 AM »
I agree on the no more than a 2 count
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Offline THE FRENCHTOWN FLYER

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Re: converter fallback
« Reply #9 on: September 11, 2018, 02:12:22 PM »
I once heard a MoTech (Chryseler Corp.) say a SS/AA Hemi Super Stocker sitting on the starting line at full stall sees a trans fluid temperature rise of 100*F - per second!

Offline wideopen231

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Re: converter fallback
« Reply #10 on: September 11, 2018, 03:48:48 PM »
I would not think fall back had anything to do with stall as long as rpm after shift was above stall number. If I shift at 7500 and then at 8500 I would expect it to fall back to same number,say 1000 off each. If it fell back same amount on both shifts I would say thats the number. Thats provide both times the number it fell back to was above stall. Maybe clarify .Shift 7500 hit 6700.then 8500 hit 7700 and both on 6600 stall converter. These are approximate numbers my old nova did I think it was 2400 lbs,xxxx was while back.  My thinking was hp and weight effected it more than stall.Maybe in left field or xxxx might be different ball park.

I have always heard stall is as bikerguy stated in checking stall comments.
« Last Edit: September 11, 2018, 04:09:22 PM by wideopen231 »
Relecting obama is like shooting right foot because it did not hurt enough when you shot left foot

Offline retroboy

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Re: converter fallback
« Reply #11 on: September 16, 2018, 09:39:38 PM »
Wouldn't the gear ratios in the transmission effect this?

Offline Roger

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Re: converter fallback
« Reply #12 on: September 17, 2018, 07:47:15 AM »
Think of it like this. If you had a clutch car with a 2 speed transmission with a 1.76 first gear ratio and shifted at 7500rpm, then the engine would drop to 4260rpm after the shift (7500 /1.76 = 4261). Now if you take the clutch out and replace it with a 1.76 first gear Powerglide and a 6600rpm stall convertor and then shift it at 7500, you would not fall back to 4260 but rather drop back to the 6600rpm convertor stall speed. Raise the shift point to 8500 and it wouldn’t fall back to 4830 (8500/1.76 = 4830) as with a clutch but to the 6600 stall speed. The only way you can get the Powerglide car to fall back to an rpm over the 6600 of the convertor stall speed would be to shift the car at 11,700rpm and watch the engine fall back to 6650rpm. And that’s only IF you can get the engine to reach well over 11,000rpm to begin with.

Offline wideopen231

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Re: converter fallback
« Reply #13 on: September 17, 2018, 03:08:02 PM »
what abot converter slippage?  I know quite a few guys have talked about 10% or more,using daat logger info.
Relecting obama is like shooting right foot because it did not hurt enough when you shot left foot

Offline gregm784

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Re: converter fallback
« Reply #14 on: September 17, 2018, 03:34:46 PM »
My converter guys say i should see 4.5-5%. i'd love to know for sure, and that is a main reason i want a data recorder.
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