Data Recorder Shows 300 rpm 'droop' After Car Launches???

[Vic Santos]

Don, I use to have the "hump" in my SS car and got rid of it by working on the Q-Jet. The basic problem is the Q-jet bowl. Its too small and makes the secondaries rich during high G's (launch). It takes a lot of detailed testing to solve this problem. Work on secondary rod shapes and hangers. If you don't have a Q-Jet disregard what I've said except for the "too rich" part.
Vic Santos

[Michael Compton]

Having ran a car in traditional Super Stock and now SS/BS (comp style), I have encountered the same exact thing that you are describing. According to everyone I talked to, it is almost like the motor takes a big drink of fuel right off the starting line. If you have AFR meters on the car, you can really see the "rich" issue. Some of it I believe was fuel pressure driven and some of it the jetting we were running.

A couple things that did help my current combo in SS/BS were moving the AFR sensors to the headers and not the collectors to get a better reading and increasing the timing on the starting line and pulling it back down track. Seems that the increased timing on the starting line helps burn the fuel a little better when on the tranny brake and within the first 60 feet.

And I think Kyle might be onto something. My issue has almost gone away since changing to the Quickfuel carb and my new A-1 convertor. Maybe it is a combination of the changes I made over the winter but I still think carb and converter helped tremendously.

[Chris Hill]

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[BlueOval Ralph]

Don't thinks it's converter, the issue is the converter and engine, fuel curve are not happy together as Marv Ripes would say.

[Ed Wright]

Marv would know.

[ss3011]

Torque converter slip is a function of engine torque and turbine shaft rpm. The turbine shaft (input shaft) is at zero rpm on the starting line and a torque converter can be analyzed using a formula k=n/sqrt(T) k is a constant that defines a particular converter n= engine rpm and T = engine torque So when the turbine shaft is zero k= (the stall speed) divided by the square root of the engine torque. This is why if you use the same converter, for example a 280 k converter, behind a big block that makes 500 ft lbs it will stall at about 6260. The same converter behind a small block that makes 450 ft lbs will stall at 5940. This is a balancing act because the torque curve changes with rpm, but that K number stays the same. It is a function of that particular converter. So if you launch the car and the engine is making 500 ft lbs, and something effects the engine and it drops some torque, you get a sag in rpm, on your data logger.
One other comment, is that the K is the same as long as the converter is hydraulically stable inside. If the converter cavitates then the K is not predictable. Cavitation can happen if the torque going into the converter is higher than the converters ultimate capacity. This is more likely to happen when the turbine shaft is at zero rpm. Capacity increases quickly as the turbine shaft picks up rpm after launch. So engine rpm could go higher that the predicted stall speed based on the K factor. This could be another reason you see a sag in rpm.

[Dyno]

Russ, that is what I was going to say! Dyno

[Adger Smith]

It can be worked out with fin angle and fin design (shape) Very small changes. I've had Comp customers make small changes in the stator back cut angle to get desired results. If all the carb/fuel issues are worked out then go to the converter. You will never get a converter tuned in if you have sagging power output. It takes time and a lot of effort and sometimes big money to get the "just right" converter for the conditions and application. I had one Comp customer that had 6 different converters. There were only 2 different degree stators between all 6. Very minor changes to each converter. I tried 5 converters in my own SS/Modified car and carried 2 to the track as well as the one in the car. The reason good comp racers don't have this problem is testing and their correct selection of parts for the conditions. Lately I've had a sharp mud racer that studies his Data and makes converter changes. Keep working on it. It will be faster and more consistant when you get a handle on it.

[Don Whitmore]

Everyone....thanks much for the input, really, really appreciated.

@Aaron Keown....the car is an '88 Firebird with a 350 ci 265 hp fuel injected combo, in SS/JA. The car has run 1.25 sec 60 foot times which I thought was pretty good, but your comments on Jeff Dona's car (1.18 sec) blew me away. IMO, that's an awesome 60 foot time for that kind of Super Stock car.

From what's been said, beginning to think one of the reasons is the AFR is wrong at the starting line. Thinking it through at the starting line, the car is staged and on the rev limiter. The throttle body is wide open, but the engine is only consuming the air required by the limited rpm. The trans brake is released, the engine rpm accelerates quickly and the converter flashes. At converter flash, the engine encounters the load imposed by the car and struggles to accelerate further.

At that point, I would think cylinder pressure would be high, so is a lean or rich mixture better at the given rpm (6100 rpm).

Not sure the converter is right either, it was used for a different (but similar) combo...and may need tweaking...per ss3011's point.

Just thinking 'out loud' ...

[art leong]

Originally Posted by Don Whitmore

Everyone....thanks much for the input, really, really appreciated.

@Aaron Keown....the car is an '88 Firebird with a 350 ci 265 hp fuel injected combo, in SS/JA. The car has run 1.25 sec 60 foot times which I thought was pretty good, but your comments on Jeff Dona's car (1.18 sec) blew me away. IMO, that's an awesome 60 foot time for that kind of Super Stock car.

From what's been said, beginning to think one of the reasons is the AFR is wrong at the starting line. Thinking it through at the starting line, the car is staged and on the rev limiter. The throttle body is wide open, but the engine is only consuming the air required by the limited rpm. The trans brake is released, the engine rpm accelerates quickly and the converter flashes. At converter flash, the engine encounters the load imposed by the car and struggles to accelerate further.

At that point, I would think cylinder pressure would be high, so is a lean or rich mixture better at the given rpm (6100 rpm).

Not sure the converter is right either, it was used for a different (but similar) combo...and may need tweaking...per ss3011's point.

Just thinking 'out loud' ...

I would think you might look into accelleration enrichment function. And program it off rpms and map sensor.
What I know about ECU's might fill a thimble. But I got mine to leave pretty good that way. I also leave at least 500 rpm (2 step setting) before converter lockup.