Gentlemen, there appears to be conflicting information regarding STR for torque convertors. If you choose to reply, please refrain from insulting or confrontational statements, my fellow American brothers.

From what I have read and understand, a torque converter with a 3600 stall and a 1.8 STR, would be very " tight ". However, a 3600 stall with an STR of 2.5 would be " loose ". If the 3600 stall using a 2.5 STR is used, when the driver goes WOT from a dead stop, this will in theory and from what I've read, in reality, cause the to be unable to hook.

Please advise.

 

STR is the ratio between the convertors mechanical input and the convertors mechanical output.
Let's use a transmission with a transbrake as an example and compare a 3600/1.8 and a 3600/2.5. If the engine is capable of outputting 400ft.lbs. at 3600rpm,a 3600/1.8 would be putting 720ft.lbs to the transmission's input shaft,a 3600/2.5 would be putting 1000ft.lbs to the transmission's input shaft. Now in this situation,the convertors mechanical output shaft is 0rpm(being held and locked by the transbrake). When the transbrake is released and the convertors mechanical output shaft starts to rotate and used by the transmission and driveline,the STR starts spiraling downward to a lower ratio. Yes,at the moment of 'hit',traction would be much less because of the 1000ft.lbs vs the 720ft.lbs. 1.8 would be considered 'efficient',2.5 would be considered less 'efficient' and that less efficiency also shows up thru the traps(for a non-multidisc lock-up convertor). A 2.5 also generates MORE heat due to the increased slippage and heat kills transmissions. A 2.0 is considered a excellent compromise for street use.

 

For reference, this topic really started with post #45 of this thread:
https://ls1tech.com/forums/automatic...y-stall-3.html

So lets only discuss the more technical details of STR and converter theory here....

As I posted in the other thread, here is an excellent thread of converter theory:

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

 

Yes, you are correct. However, in that same thread, post # 47, the person stated that it was the opposite. Additionally, after further research on another forum, I think the GTO forum, regarding torque converters and STR, there is conflicting information regarding STR, and some nice sarcastic comments were made regarding the LS1tech forum, in general. To top it off, from what I read, most of the comments indicated 3600 was way too much, and too sloppy around town, if under 2000rpm.

 

In my experience, what I posted in #47 of the other thread has been the case. About 15 years ago, Yank sold two different "street" 3500 stall speed LS1 converters; the SY (Super Yank) 3500 and ST (Super Thruster) 3500. The primary operational difference between these two was STR. The SY series had a very low STR (1.6 I believe) and ST was more like 2.1 or so (I don't recall the exact values.) While it's definitely true that the lower STR model was of higher efficiency in the upper rpms and bleed off less trap speed, at the point of rated stall speed it didn't offer as much torque multiplication and thus felt a bit "looser" in this range to me. So I guess it all depends on what point in the rpm range you are referring to "looseness/slippage". The ST3500 felt tighter to me around town cruising with the same engine/same gears, vs. the lower STR SY3500 - which may have also been a product of my driving habits/throttle application to cruising speed. That is the basis for my personal opinion/perception of STR at and below the rated stall speed, but again I don't at all disagree with the statement that lower STR = better top end/high rpm efficiency. Also, STR doesn't seem to translate to a guarantied fixed efficiency when comparing two different brands, so that might add additional confusion/variables to reported experiences.

Sorry to hear that. LS1Tech is the leading source of accurate LS-based information and modification discussion, so perhaps some other sites feel intimidated by our vast and comprehensive database of factual info and knowledgeable members. If so, it's their loss.

And frankly, anyone who makes the blanket statement that a 3600 stall is "way too much" for a street based LS1 application is either not speaking from a position of significant experience, or is a rare and extremely sensitive case which is not indicative of the vast majority of perceptions.

 

Back in the 60s' I had a Buick Skylark Gran Sport,325hp@4200 445ft.lbs@2800,with a Super Turbine 300 auto tranny. The tranny had a 'variable pitch' convertor. This was a factory setup. It had a microswitch on the carburetor linkage that tripped at WOT. I would estimate the built-in 2 stall positions as 1600 & 3500. I set-up switches on the dashboard so I could control it. One switch would activate the high stall whenever I wanted and the other switch would prevent the high stall from being activated by the linkage. Back then the widest slick you could buy was 7" wide('cheater slicks'). The car had HUGE rear drum brakes,Buicks were known for huge drum brakes. I went to 'Velvatouch' metallic shoes on the rear and with the parking/emergency brake locked I was able to apply full torque to the rearend without 'pushing thru'. It acted like a transbrake. Back in the days of 'streetracing',I was able to test the street launch traction in both settings to determine with was best for the conditions. The tranny was a 2 speed(like a powerglide) and rear gears were 3.08. I would run in the high stall thru first(shift occurred at 70mph),continue in drive under the high stall and about 100mph disengage the high stall to go to the lower stall(it felt like a shift) to get the higher mph without the 'looseness' of the higher stall. Learned long ago that a 'loose' convertor hurts high/top mph. Fortunately modern aftermarket convertors are able to be built 'tight' and still have a high stall rating.

 

Loosely speaking, higher str hits harder out of the hole but looses some efficiency up top (higher rpms), lower str hits softer but pulls better on the top end. And somewhere in the middle is a give and take between launch and down track efficiency. Stock converters are low str.