Pretty much as the title says.

A given car at a given weight with a given tire and first gear ratio sitting staged against the footbrake at say 3000 rpm. We react to the light and floor the pedal and the motor rpm immediately flashed to let's say 5500 Flash RPM.

At that instant....what is happening to the fluid in the converter and the tire reaction before it goes to the converters optimum slip? And is there a way to calculate/estimate how far the car travels before the full "hit" of the converter,,,or the maximum load transfer...gets to the wheels? And then how track conditions can be adjusted with launch RPM / Converter preload, etc?...other than with suspension mods.

I'm trying to get it in my head how far the car travels down the track and how that is effected by weight and traction....it would be obvious that if the tires unload/break traction there is no lag at that point.

To me this has always been the Voodoo of how a converter works, and how to optimize it.....and also how a higher flash speed often allows the car to hook or optimize the 60' where compared with a manual trans the full load transfer momentarily pulls the engine speed down....this you can see on the tach and feel in the car. I've always been a stick guy...so old habits and conceptual understandings die hard!

I'm hoping the converter guru's can understand what I'm trying to get my head around and chime in...it's just what happens in that first second that I always have trouble visualizing. I've looked on dozens of sites trying to find the 'A-ha!' moment

BTW...I havent had my coffee yet so forgive me if I'm not making sense.... LOL!

Just been looking at some of my racepak logs. I can't tell you everything you need to know but below is info according to a slightly less than typical pass. This is footbraking BTW. Converter now flashes to above 6000rpm due to a rear gear change (3.73) It used to flash / stall : 5200 / 5000 with the 4:10 gears but it was specc'd for a much older combo. Car is 4023lb last time I weighed it W/D.

At the point below (white vertical line) Converter stall 6100 rpm is achieved in 0.456 seconds. Vehicle has moved 2.16 ft, @ 11.1mph and pulling 1.7g's at that point. Note Pre launch RPM is 1850 (part throttle but just activating the 2 step).

Car doesn't actually look like it moves until aprox 0.200 sec after the throttle has been pressed though it's pulling G before the driveshaft begins to register movement (0.39 G)

I don't know if the info is helpful or how accurate the Racepak G meters are, everything is pretty much loaded up at the start line holding it on the brakes. Trans temp sensor is in the pan though I plan to fit an inline one as the temp doesn't change much in the pan much at all on a pass. Also a throttle pos'n sensor and an engine oil temp sensor in the pan will be fitted.

Tig,

This is exactly the kind of data I was hoping for.....how the car reacts at the launch in the instant the converter flashes. It was always explained to me that a high stall converter can actually soften the hit of the line for just an instant allowing the car to move forward BEFORE the full power of the motor is applied. It's just hard to conceptualize in my brain how that logically happens, it always seems counter-intuitive to a "stick guy". So by adding only weight (only variable change) it would delay/increase this 'lag' time, and removing weight would speed up/decrease the lag time.

Now logically the theory behind a transbrake is to make the converter/automatic transmission car "act" more like a Clutch car...so more of an instantaneous "shock load" is applied to the tires....true?

I know there's a lot of experienced bracket racers on here, so hopefully this kind of 'cause and effect" understanding/discussion can hopefully be helpful

During that short period of time while the engine is flashing from 3000rpm to 5500rpm, the engine's rotating assy is actually absorbing some of its torque, which in-turn momentarily reduces the amount of torque applied to the converter's shell. That added flash time gives the car more time to transfer weight onto the back tires before all the engine's torque gets applied the converter's shell.

When you launch closer to the converter's stall, less engine power gets absorbed by it's rotating assy, so less time to transfer weight.

Grant

Glad it is helpful. I have no data prior to the rear gear change but I think what you are saying will be correct. The 60ft times dropped off around 0.03s although on some passes we were trying a ignition retard on launch, so there are only about 4 full passes of data. We have never tried a transbrake and have no plans until we swap out to a th400 maybe. The stall comparison was done off an older more rudimentary data logging tach (G Tech).

Engine acceleration rate plays a big part in the amount of engine torque that's available to do external work. In Tig's graph above, the engine flashes from 1850 to 6100 in .456sec. That's an average engine accel rate of 9320 rpm per second between 1850 and 6100.

My stick shift car's current engine has a no-load acceleration rate of 11,500 rpm per second. If you were to dyno that engine at its no-load 11,500 rpm accel rate thru the heart of it's torque curve, the dyno would show zero torque output. At that accel rate all the engine's torque output would be used internally to accelerate its rotating assy, with no torque left over to do any external work.

At a 9320 rpm per second accel rate like in the initial part of Tig's graph, about 81% of my engine's torque potential would get absorbed internally, with only about 19% of it's torque potential available to do external work. Say my engine makes 500ftlbs steady state, at a 9320rpm accel rate it would only have 19% or 95ftlbs left over to do external work.

WOT at a zero rpm acceleration rate, all of an engine's torque potential is available to do external work. Where Tig's graph knees over to a flat line at .456sec, that's the point where the engine's maximum torque output suddenly gets applied to his converter shell.

The points on his graph where the engine is getting pulled down against WOT after the shifts, that's where torque applied to the converter shell actually exceeds the engine's maximum torque output. That's because energy absorbed/stored within the engine's rotating assy as it gained rpm then gets discharged as the engine loses rpm. That discharge of stored inertia energy is what causes the tires to chirp on the street during a firm automatic shift.

Being a stick guy myself, I find it's easier to think of the converter itself as a self-contained load sensitive variable ratio transmission. The converter basically "converts" shell rpm hydraulically to increase torque applied to the slower spinning transmission input shaft.

Grant

Wise you might be interested in this data log I pulled for you. This is a zoomed in view of two things. Engine RPM and Acceleration. (Through the first gear shift).

This is a foot brake launch from about 2100 RPM with a ~6400 RPM flash converter. So I was NOT up on the converter very hard.

But what you'll see is that the INSTANT the RPM start to climb... so does the acceleration of the car. So there is absolutely zero delay in throttle vs torque input to the car and getting things moving.

If you are thinking about "lag" per se... the g's pretty much skyrocket through the first 1.5g's which is over a 0.28 Second time period. So if there is any coupling delay you're thinking of, it happens very very quickly.

Just goes to show how fast things are happening.

Yeah, I think I need to reset my zero point. Like dizuster says it is almost instant and the car is pulling .36 g before the drive shaft registers so the car must be moving. The G curve is added on this one, peak g is about 0.6 sec in or .35 sec from the track start point. The wobbly bit on the G trace just before 1-2 gear change is where the front wheels touch down. Wheelies do cost et

It's weird to me, to see data logs, and the Green, and Red lines, don't match up in high gear. LOL.

Yep converter is wayyy off. 19% slip. We have a new one and we will be able to see how it compares when we get back out. Also nipped a ring land on #3 sometime during the last few passes, performance was off but we just put it down to the rear gear change.

As far a zeroing your file you cannot break the beam until the drivesaft is moving so thats the best place. As for "premature" readings on the G meter coule be the way it is mounted, where it is mounted, number of magnets on the driveshaft sensor etc.

Here is a file from the heads up car. For those who don't know this is a 2800lb naturally aspirated small block on a 29.5x10.5 M/T slick. As for the Op's questiom yes it happens but it is almost instantaneous. But it also depends on gearing, converter, trans gearing etc etc. We launch at a relatively low RPM to give the converter a "run" at the car. In our case we are talking 1/2 second before we have movement from the DS to flash of the converter. In the case of this screen shot on that day in St Louis with the conditions was 9300, a tad lower than I would like it to be and it got short shifted so in theory it was really tight for what we like. We see max G's about .710 into the run which is typical for us. My main concern is where the wheelspeed is at .5 into the run. Show me that number and I can tell you if it was a good run or not. FWIW the weather for that run was a baro of 29.3, 61% and 90 grains wasn't a bad run at 7.90/174 but it could have been better with a stator swap or more gear that run. But we have to make a judgement call on that stuff early as its just two of us working on the car.

Didn’t have data logs on my car but the 9” vert was flashing right at 6500, way to loose. Thru traps engine was on 7000 rev limiter 100 feet from finish line and drive shaft was 5800 in 1/8 mile. 4.56 gear and 15/33 Goodyears. wish now I could’ve had some data in that first 60 ft other than the 60’

When I was running a 727 you could see the rpm change on the graph when it hit the flash rpm. The rate of rpm gain slowed as the converter started to lock up. You can see a pronounce change on the graph. Running a low first gear set 904 now. There isn’t the pronounced change in rpm gain. It’s a smooth arc now. The low gear set will “show” as a reduction in flash rpm, because the car will move easier/sooner. I have to say logging is very helpful. My logging system is not ideal and you need to figure something’s out, but I can log AFR,throttle position,driveshaft rpm, engine rpm and soon g force with about $600-$700 invested. It’s not plug and play like the high dollar systems, but it is way cheaper.