FastKat

iTrader: (10)

I've heard torque converters referred to as torque multipliers. I've also heard them described as getting the engine into it's peak power band as fast as possible.

With that being said, let's say an engine hits peak torque of 400 lb/ft at the crank at 4,000 RPMs. Will the converter:

1) get the input shaft of the transmission to 400 lb/ft of torque much faster but never exceed 400 lb/ft, or
2) get the input shaft of the transmission to 400 lb/ft much faster AND exceed 400 lb/ft of torque

Or maybe something else? I guess the question is whether the torque converter multiplies torque only up until peak crank torque (just gets the engine to its peak torque faster) or whether it increases all torque including peak crank torque.

01ssreda4

iTrader: (96)

Yes, the converter can multiply existing torque.....to a degree. Meaning, if you feed it 100 ft/lbs @ 1500 it could multiply it to 200 or greater. Feeding in the input on one side from the engine, the converter multiplying and outputing on the other. It does have limitations and it has the capability of less multiplication as rpms rise. So its magical abilities are greater at lower rpms, and I believe it is fairly linear in nature.

Then it does flash, which gets the engine to a higher power/torque output much faster. Imagine, the karate kid trying to chop a board with only 1 foot to rare his arm back. Then give him another shot where he can use all his strength and start his swing over his head. Theoretically the exact same amount of strength from Mr. Miagee's student, but the strength applied better in the second example and yielding far better results.

mrvedit

Here is a "rough" explanation:
The stall speed (or flash) of the converter is the maximum RPM the engine can be with the output RPM of the converter much lower. The stall speed is generally realized at launch from a dead stop, assuming engine at WOT and tires hooking.

With the engine at the stall speed (RPM) and the converter's output RPM much lower, the converter does indeed multiply the torque, up to about 2.2 times. As the output RPM reaches the stall speed (RPM) of the converter, the torque multiplication goes down to just straight 1-to-1 and even to e.g. 0.95 as the converter slips a bit.
The maximum torque multiplication is called the STR. While a converter with a higher STR will have higher torque multiplication at low output RPM, it will drop off faster; i.e. the torque multiplication occurs in a tighter RPM range. In contrast, a converter with a lower STR will have less maximum torque multiplication but over a broader RPM range.
Note: This is as I (barely) understand it, and it certainly subject to correction.

FastKat

iTrader: (10)

Thanks for the replies guys. That definitely helps me understand torque converters a little better, but I still don't really have an answer to the question. Will a converter increase torque over the engine's peak torque at the crank... in my example, 400 lb/ft?

LS-ONE_DAY

https://ls1tech.com/forums/automatic...str-guide.html

FirstYrLS1Z

iTrader: (4)

No increase of power at the crank,increase of torque to tranny's input shaft.
Couple examples:
M6-400ft.lbs coming out of crank and because of M6 clutch(1:1),400ft.lbs to tranny's(M6) input shaft.
A4-400ft.lbs. coming out of crank and because of convertors's 2:1 torque multiplication when it's at FULL STALL,800ft.lbs to the tranny's(A4) input shaft.

slow67

iTrader: (2)

A converter's maximum STR is when at stall speed (input shaft is not turning) and drops off (fairly linearly) as the converter couples.

DANOZ28

iTrader: (1)

theres a Q & A on yanks website i found helpful.

LS-ONE_DAY

Good lord people read the thread I linked above. It has pretty much everything other than a mechanic engineers perspective

DANOZ28

iTrader: (1)

dont tell me what i can & cant post ya @@@@.

FastKat

iTrader: (10)

Thanks for the link - great info.

So to answer my own scenario, only if the converter can flash to 4,000 RPMs where the engine makes 400 lb/ft of torque at the crank and the vehicle is launching from a stop, the input shaft of the transmission will get more than 400 lb/ft of torque - it will have the 400 lb/ft x the STR.

Once you start changing those variables, the STR calculation doesn't exactly hold up, but the input shaft can still get over 400 lb/ft of torque under the right circumstances, just not necessarily peak crank torque multiplied by STR.

mrvedit

I think you are getting the idea.
However I would suggest getting a stall somewhat below your peak Torque; otherwise your engine will be loosing power immediately after launch. You probably want to stay in the power band of 1500 (or so) RPM below and above your peak Torque.
The hotter the cam, the higher the stall you would want for best ET.