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

The data recorder is showing a significant rpm 'droop' (300 rpm - 400 rpm) after the car launches (converter flash)...sort of like a 'camel hump' in the run trace.

The car is a 350 ci SS/JA weighing about 3100 lb. The 60 ft times are good for this type car but wondering if they can be improved if the 'droop' is reduced???

I've noticed the Comp cars don't seem to have this issue, so is it something we're doing or just a characteristic of a Super Stock car???

What can be done to reduce or eliminate the 'droop?' Converter swap? Gearing? You just gotta live with it???

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

That is the converter loading. It is normal to see that in a car like yours.

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

Getting the fuel curve right will help that!

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

Mine shows a couple hundred if it turns the tires a little. Dead hooks don't show it.

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

All...thanks for the input.

@KRatcliff....what is different that the Comp cars don't see to have the 'droop?'

@BlueOval Ralph...would you go leaner / richer in this area? I forgot to mention it is a fuel injected combo.

Thanks for the help!!

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

I cannot answer the question on the Comp cars due to zero experience with them. I can only surmise that the ones you may have seen may be leaving at a significantly higher RPM, the converter may be smaller (ie 7"), and the engine isn't pulled down as much or at all when the converter fully engages.

I have looked at the graphs of many fuel injected stockers and super stockers. Every one of them had the same hump. Even our super stocker with a carb combo has a hump. Most were 2 step cars, but a few were foot brake stockers.

Any tuning that you do for this event is basically behind a curtain and you are not exactly sure what the A/F ratios are due to the action of the 2 step. I have a certain pattern of attack for that event, but that is just for my stuff and I have a lot to learn before I come close to getting these tunes right.

Listen to the others with more experience than me on this. I was just giving you my experience.

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

IMHO you don't see it is much in comp simply because of the nature of the beast, which is light, lighter, and lightest. The lighter cars with higher H.P. are naturally not going to be as susceptible to theese things.
But you should deffinately believe these guys are pointing you in the right direction, but in all honesty there are hundreds of things that if not exactly the right pieces or setup correctly that can cause this. I am assuming you are running an LT-1 in either a camaro or firebird from you info (and please correct me if I am wrong and that is not the case) and I have a little bit of experience with those engines and what you are talking about can make or break the way the car performs. Just for example, one of my best customers Jeff Dona was able to go a 1.18 sixty foot at bella rose. Now given, he is in I and you are J but I am guessing pretty close to the same basic combo, now I am not you so I don't know what you consider a "good" sixty foot time, but in my humble opinion I think Jeff's car is one of the best real super stockers off the line that I have ever seen.

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

listen to Keowen if its who I am thinking it is he has built some fast stuff--Comp cars will/might/could have either a tiny flat line or a VERY small droop when the car first leaves the line usually its a rapid upward spike or you hope it is---- A Comp engine regardless of size is making at least 2.6 HP per CU IN and then the really good ones make all the way up to almost 3 HP per CU IN.possibly more!!! The starting line RPM 's ,( gearing , car weight etc.factored in) that they are leaving at too are such that the phenomonen(RPM drop) you are seeing is so small or does not even exist.
I think it would be unwise to try to compare a Super Stock engine such as yours with the graphs from a Comp engine based solely on the tremendous performance differential here... My suggestion would be to find and talk to a similiarly classed car owner/driver for their perspective on this... Also if you have a data recorder you should be talking to the manufacturer for his input on what you are seeing. They are constantly talking to racers so they might be able to figure out a bit what it is you need too. FED387

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

A little richer at peak torque if EFI give it a little pump shot

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

Mine used to do it when it was too rich "down there". It was simply pulling the engine down. Now only if it turns the tires. 1.3X 60's there's a hump, 1.2X 60's, no hump.

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

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

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

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.

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

..

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

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

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

Marv would know.

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

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.

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

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

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

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.

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

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' ...

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

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.

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

Acceleration Enrichment is like accelerator pump shot with a carb. Won't happen with a 2 step unless you don't have the pedal all the way down on the 2 step.
If using a 2 step, ignore the wide band O2 data there. It won't settle down until the car has moved. All you can do is add or remove fuel there to see what it needs. I would add fuel first, rather than pulling fuel from a lean condition. It's likely to look lean there, but that data won't be reliable with a 2 step. All you can do is test. I find most cars to be rich there, however. Get it too lean there and you can stick a ring, or worse.

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

One thing to remeber when looking at your data is to be sure all data loggers have same sample rate! Most of the O2 sensors are some what slow in response time and don't operate in very wide bands. I just finished working on a project for one of the major Auto Company's on a engine development program that used 3 different bands of O2 sensors.

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

The humps I was originally getting were to 6100/6150, then pulled down to 5800. If it spun the tires then the humps would be to 6300 or so, then pull down to about 5800.
After fixing the air/fuel there normal dead hooks go to 6100/6150 and stay there a few frames then go up. Now if it spins the hump will still be 6300 or so, then to 6150 or so. Doesn't pull down to 5800 any longer. Shows in my 60's & ETs too. I at first thought I had a 5800 RPM converter. A few passes at a track rental fixed it.