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T 400 random question

trevmountain

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Basic Turbo 400 question.

Can a transmission that's not functioning properly be the cause of a loss in low end get up and go power.

Common sense tells me first gear is first gear and it's just up to the motor and gearing to how much go fast you've got off the line. But I got to thinking about stall speed and how this plays in.

I don't know squat, need some serious transmisson learn me.


Thanks
 
Sure, it can do it in one of three ways.
First, if the engine is revving up but you are not going anywhere fast, then its slipping and causing the problem.

But, you almost certainly would have realized that, so I'm guessing that the engine is bogging down.
In that case, we are left with two possibilites.

First, realize that in an automatic tranny, you do not actually shift gears. All the gears are engaged all the time.
Instead it applies clutches and/or brake bands to apply the different gears into and out of the drive line.
If one brake or clutch fails to release all the way, then you are basically in two gears at the same time.
Which will suck the heck out of the power. When it shifts out of first, then the problem goes away and the truck runs normally.

Since for it to move in that condition at all, one or the other of the clutches or brakes has to slip.
Pull the dipstick and look for burnt fluid or a bad odor.

The other way concerns the torque converter. In a way that would take too long to explain, it actually multiplies torque up to the stall speed.
You get more power at startup than you would if you just had a clutch in there.

Stall speed is not the rpm that the converter stalls the engine, its the speed when it "stalls" and becomes a direct drive.

If it develops a problem and quits multiplying torque, the engine will feel underpowered at low rpms.

However, low power off the line is way more often due to a timing, vacuum leak, or an EGR valve malfunction, or other engine problems than transmission.

Unless you have other reasons to suspect the transmission, strange noises, rpm not matching speed, then all I would do at this point is pull the dipstick and check the fluid.
If its nice and pink, concentrate on the engine first.
 
Great explanation Fordum, I would add it is not uncommon for the sprag in a 400 converter to fail and thus eliminate any torque multiplication the converter stator produces. This will kill off the line performance for sure in this application.
 
Is that the same thing as the stator not working in the torque converter??...I had 2 '66 Buicks with the "switch pitch" converter,and one had the stator switch fail (that was located on the carb) and it felt like it was in third gear,even in first,especially at takeoff from a stop...once you got going about 30 it drove normally...after I figured out the switch was bad and replaced it,it'd do a 300 foot burnout!...couldn't scratch the tires on sand before!.....
 
Is that the same thing as the stator not working in the torque converter??...I had 2 '66 Buicks with the "switch pitch" converter,and one had the stator switch fail (that was located on the carb) and it felt like it was in third gear,even in first,especially at takeoff from a stop...once you got going about 30 it drove normally...after I figured out the switch was bad and replaced it,it'd do a 300 foot burnout!...couldn't scratch the tires on sand before!.....

Yes, but even though your stator blade angle was not changing via your switch the sprag was still functioning so you were getting torque multiplication just not as much
 
Talked to my engine builder today regarding my situation and it sounds like my problem lies in my gearing and stall speed. K30 with a brand new built to the nuts big block, 440 hp and 500 ft lbs at 4000, peaks @ 540 hp and 567 ft lbs at 6500, 37" tires, 3:73 gears, Turbo 400 with stock stall and a gear vendors.

He is saying no wonder I don't got no low end power. He recommends 4.88 gears, with a 3000 stall. According to his calculations I should still be able to cruise the highway at 80 in overdrive no problem and the motor will be happy.

I just want the motor to be the happiest. I mostly use the truck for tooling around. I pull very little but do like to have the ability to tow from time to time. Other than that it's my toy and I like bottom end go fast power.

Does his recommendations sound correct?

Thanks
 
Yes. you really need to re-gear the truck, the 4.88 and gear vendors will put you at 2400 rpm at 70 mph with 37 inch tires. The problem is, your cruise rpm needs to be a few hundred rpm higher than stall speed to avoid
excessive heat build up in the trans/converter. If your idle in gear is still OK with the stock converter, move up to a 2000-2200 stall speed such as the factory high stall TH-400 converter. This converter will probably "flash stall" to over 2500 in your big block powered, heavy truck, but at 13 inches in diameter has a lot of fluid capacity and is still a pretty efficient design. This converter will have over 1000rpm higher stall than the factory low stall 400 converter and with the gears give you lots of good low end and still keep things comfy on the interstate
 
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