FrontEngineDragsters.Org Forum
Technical => Dan Dishon's Transmission Den => Topic started by: spookie on September 09, 2018, 06:47:41 AM
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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?
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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.
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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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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.
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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.
Cheers
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A stall check is done with the trans warmed up, you go to WOT against the trans brake with no launch chip.
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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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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.
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I agree on the no more than a 2 count
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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!
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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.
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Wouldn't the gear ratios in the transmission effect this?
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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.
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what abot converter slippage? I know quite a few guys have talked about 10% or more,using daat logger info.
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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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10% is very high, 4% is real tight, in the real world I think 5 to 6% is the norm.
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OK, this is my experience with converters, the more power you make the higher the stall/flash, and Roger is mostly right , the higher you shift to a point the higher the flash [ what the RPM drops to right after the shift].
So the power can be affected by weather, tune-up ,and changes in your combo. After any combo changes I check my converter flash, also the looser [ higher flash] the converter the more it will show slippage, unless you step-up to a lock-up type converter, and that means it will slip less not really lock-up. With a lock-up it will pull the RPM down in the middle so the suggestion is to go to more gear.
My numbers in 2018 ; highest shift 10430 rpm, flashing at 8773 rpm, with 5.7 slippage at DA 1807', lowest shift was 9672 flash at 8624, slip at 5.6% at 1566 DA, running 7.03 @186 mph
At a hot event [ lower HP] 10137 shift flash at 8528 at DA 4000' and 102° air, and 7% slippage.
I prefer we shift more like 9700-9800 rpm, flashing at 86-8700 rpm , I just bought a looser converter because I want to try 8800-8900 flash.
I can share more real numbers if anyone wants, but I hope this helps clear-up any confusion.
Again our car is a JRFuel "B" all iron SBC on pure alky , PG on 12" wide tires weighing 1435 with driver.
Jon