I'm not sure if I'm the only one with this problem, but it's felt like this pretty much since I bought the car. It just feels like the damn torque converter is so loose. Like I have to give it a lot of gas to get the car moving. Even if I just let off the brake while at a stop the car doesn't move anywhere like pretty much all other auto's do and then I have to push on the throttle significantly to get it moving.

I had a Dynotech race tranny in my GTP before and after almost a year of driving the GXP I can't handle the trans anymore. I think the breaking point for me was having Kevin's car from Cartuning for the past day, he has a 2005 V6 GT with stock torque converter and oh my what a HUGE difference it is to drive it. When you let off the brakes the car actually starts to roll forward and I need very little throttle to get the car moving.

Anyone else experience this?

 

I like it, except for the extra gas it eats in stop and go driving. . Reminds me of my 69 Charger with a 10” converter. Out of the hole it would stall to almost 4K and pushed that 4,000 lb beast to a 12.2 ¼ mile blast with no nitrous. It took some getting used to, but having a converter that stalls at say 2K like my truck will leave you wanting a lot more when going from a dig.

 

Yeah, they say it's the stall speed...but don't ask me to explain it.

I agree, it is MUCH different than any other car i've ever driven...

 

OMG...I thought I was just going crazy. Never thought of it being the torque converter. It drives me nuts too. Whenever I come home I have to pull all the way up to the garage or the wife cant get her car in the driveway. I hate when I have to pull up a few more inches cause it feels like I have to gun it to make it move.

 

it's just a loose convertor. less initial shock to the tranny is my guess.

 

Hell, IMHO the loose converter is the only thing that makes an automatic fun to drive! You also have to remember that if these cars didn't have a loose converter with the higher stall speed all they would do is just roast the tires off the line and never hook up.

 

It is not the stall, it is the STR ratio. Probably 1.8 or lower. GM (in my opinion) designed the t/c to mask the DOD operation. Thing would be a lot more fun to drive on the street with a higher STR.

 

Huh? In English please.....

 

STR is the ratio between input torque and output torque. In essense, the higher the number the 'tighter' the converter feels. A low STR will hit the tires softer than a higher one. Excellant for drag racing, sucky for street driving. It is what makes the nice low end torque of your LS4 feel soft. A tighter converter would give a more direct relationship to the amount of go pedal you give and how hard/quickly it pulls. Would feel more connected. That same converter would probably make the DOD switch more noticable. (and annoying)

 

Yep accelerating from a stop or up a hill feels like the car is working to hard from the higher rpms from the loose converter until the clutch locks and it just walks up a hill.

 

how do you fix this? tunning? new converter?

 

To get a new converter in, you basically have to drop the entire front clip pretty much...

 

excelerating lol i've never seen it spelt like that before.

 

Ah so thats why theres an edit button. it's a new form of acceleration that only ls4's can do.

 

You mean ruin the fun the General gave us. It may have been by accident if they used a converter more suited to a 6 cylinder.

Get better tires and enjoy the extra power put out due to the higher stall which is the main reason these do such nice burn outs even when stock. Put a lower stall in and you will be in a lower part of the TQ band. Drive a stick in 1st or 2nd at say 1800 - 2000 RPM, then mash the gas. There is your low stall solution. If you have sticky tires and a TQ converter that puts you in a good solid part of the TQ curve you will launch hard.

I think a converter that stalled in the 2500 RPM range would have been a better compromise to the one they put in, but I'll take the extra stall for the fun of it.

 

I have zero expertise with auto transmissions since all I've ever worked on were M22's and now the T56 but I'm here to tell you that these cars have a 258mm dia torque converter which translates to roughly a 10" diameter converter. A 10" converter tyipcally has a stall speed in the 2800-3200 range.....guess where our cars stall?!!!!!

I can certainly tell you that if these cars had a 12" diameter converter you would have immediate take-off and be roasting the tires with next to nothing for throttle.

So when you say STR ratio, what specifically in the trans is creating this other than the first gear raito of 2.92 which is almost identical in ratio to a 4L60E transmission which uses a 3.06 first gear ratio which means for all intent and purposes the only difference in take off between our cars and an LS1 Z28/Trans Am with the 4L60E trans is the converter?

O6 SS...not trying to pick on you.....just trying to get educated!

 

The t/c will multiply torque to a value greater than the engine puts out. The output is based on how fast the stator spins vs the impeller pump, the curved vanes can give you more output than you put in, sounds counter-intuitive, but essentially you are trading revs for torque. The transmission will put out more torque (instant force) but the output of the transmission is spinning slower than the input at the flywheel. The engine puts out more power (torque over time) and the t/c sorta 'stores it up' and releases it as more torque, but at lower rpm than the input from the engine. You are actually getting less power to the tires (due to inefficiencies in the transmission and the like), but higher torque. Probably as clear as mud, but it works.

Now, maximum torque multiplication takes place when the engine speed is as much higher than the output of the transmission (read: at a stop). For example we can use my '95 LT1 Caprice. 3.73 gears+2800 stall+~350 lb-ft+~2.45 STR means it is hard not to melt the 255 tires when leaving a stop sign. Goes like this: Hammer the gas, stalls up to 2800, say it makes 320lb-ft of torque here. The STR multiplies this by 2.45ish and the tires see the equivalant of almost 800 lb-ft of torque for a moment. (as the speed of the output gets closer to the input, the STR ratio drops closer to 1:1 until they are equal when the input speed = output speed.

Same scenerio with the (questimated) 2800 stall and 1.8 STR of the LS4. At 2800 the little 8 makes ~290 torque. Multiply that by 1.8 and you get momentary torque at the tires of 525 lb-ft. The tires are much less likely to go up in smoke with an agressive launch that uses all of your stall (and using that stall gets you in the powerband faster). Downside is that, the car feels lazy as it takes more throttle to get the same output down low until the speed of the trans output gets closer to the speed of the engine/trans input. My high STR LT1 will creep forward when you take your foot off the brake and it probably has the same stall as the LS4.

Generally, low STR, high stall is good for drag racing because you get into your powerband quickly and can get good traction at the launch because of the lower torque at the rear of the tranny. (rubber tires have a greater static coefficient of friction than dynamic, meaning they grip harder when rolling than when stopped)

Higher STR will make the car feel stronger and have better low end response.

Probably made it worse with the explanation and I might have to go back and edit it a time or two after I reread it and see if I reversed anything.

 

So who's going to pressue slp or w-body store to start making t/c's for these cars.. i think they already have 245 mm performance torque converters for the earlier model years. Although.. since the transmission didn't change much.. can't you use that t/c?

 

Hmmmm....I have never heard of anyone modifying a transmission for better take off qualities by changing the stator or impeller pump to change the STR! I have heard of replacing these parts for higher strength components to prevent breakage but not for a "ratio change".

Every modification I have seen for an automatic trans for better launch qualities focus on the gear ratio and torque converter. Where can I find more specific information on your theory? I've reviewed websites for auto trans modifications and didn't see any mention of modifying the components you mention for better launch....these two sites offer complete trick modified 4T65-E transmissions....why don't they mention STR or what components to buy to change this ratio? http://www.transmissioncenter.net/alto_2.htm & http://www.intense-racing.com/Mercha...ategory_Code=T

Again, I'm not trying to be a hard *** about this ...I'm just trying to learn!

 

Replacing the converter is the only way to change the STR. The fins are permanently set into place when it is built.