Choosing a stall converter

Jump to: navigation, search
Line 1: Line 1:
 
'''How to choose a stall converter?'''
 
'''How to choose a stall converter?'''
 +
 +
I may be not-the-best person to attempt this, but until someone better takes a stab at it, here I go:
 +
If you have an automatic transmission, then you don't have a clutch pedal. You have a torque converter instead.
 +
Most of my experience is with V8s, so that's the focus here.
 +
Let's start with the "stall".
 +
Drive your car to a deserted road, stop, watch the tach, and stand on the gas very abruptly.
 +
Assuming everything is functioning normally, the tach will jump, to at least 1200 RPM, before the car actually
 +
begins to jump forward. Or the tires go up in smoke, depending on your combo.
 +
That is your "flash stall".
 +
Now, repeat, but this time keep your left foot on the brake pedal, hard. But don't do it for more than a couple of
 +
seconds, then do a gentle drive afterward. Assuming you didn't just shred the tires, that was your "foot-brake
 +
stall".
 +
 +
Now that I'm started, there's so much to cover that I'm not sure how to proceed without making it confusing. ( I do
 +
have A.D.D.) It all inter-relates. I want to make it fascinating and educational.
 +
Anyway, back to stall.
 +
 +
You can find all sort of descriptions and explanations, but your first interest is in what your stall does for your
 +
car.
 +
Need mostly centers on your camshaft, heads, compression, gearing, and expectations. Tuning and fuel quality are
 +
your responsibility. For this thread we must assume you got the tune right and run enough octane.
 +
 +
I guess the best way to proceed is to detail 5 different combos I've owned and driven, that really stand out in my
 +
recollections:
 +
 +
Trying to keep this in chronological order, the first combo is a '79 Firebird. 150-HP 301 with a slightly leaky
 +
Q-Jet hurting MPG but not driveability, backed by a TH350, a 2.73:1 axle, and on new Firestone 215/75R15 tires. I
 +
bought it that way, at too high a cost because it was so cherry, and that used up all the money I was hoping to
 +
spend on performance mods. So I paid attention to what I had.
 +
The converter was an 1800-stall. I could, occasionally, chirp a tire from a dead stop. But i soon got curious
 +
about the [b]converter slippage[/b]:
 +
Cruising along at 55 MPH, lift off the throttle, the RPM instantly dropped 200 RPM. Ease back into it, it came
 +
back up by 200 RPM. In later years, I came to learn that this is typical of most stock GM converters, though I
 +
doubted, and still doubt, that that converter was stock.
 +
But then I tried it the other way: stomping the gas, RPM climbed another 200 RPM. Did that mean a total of 400 RPM
 +
of slippage? I lifted off. Yes, a total slip at WOT of 400 RPM. Again, fairly typical it turns out.
 +
[b]Lesson learned[/b]: normal slip of torque converters. Also applies to lockup style when not locked.
 +
 +
Combo 2: '78 Camaro LG3 ( 145-HP 305 ) TH350, swapped stock 1200-stall for B&M TorkMaster 2000 converter, 3.08:1
 +
axle, 235/70R15 tires.
 +
I know this converter was stock, I know it was 1200 RPM flash stall, I know the slippage was the same as my 'bird
 +
already described above, and I know it drove fine for granny. But I decided to swap it. The B&M gave my combo a
 +
1700 RPM flash stall, [b]the launch[/b] felt much better. It felt like I had swapped to a 350, when launching. And
 +
by launching, I mean flooring the gas the instant the light went from red to green. It pushed me into the seat
 +
noticeably more, and I loved it.  I thought I had invested wisely, and I felt I got what I paid for.
 +
But I was in for another surprise. I finally stopped playing in traffic and got on the highway. I was so stunned
 +
to discover my slippage wasn't 200 RPM. It was just 50 RPM! So I stomped the go pedal, the RPM shot up by another
 +
350, for a total of 400.
 +
I didn't bother to check my MPG, this carb was also leaky, and I always did 17 MPG, same as with the 2.41:1 axle
 +
it had before. But that aside, I know it would help MPG. After all, my previous '78 / LG3 / TH350 / 1200 / 2.41:1
 +
/ 205-75R14 Camaro did 23 MPG with a best of 26.
 +
I can't say the 2000-stall drove better than the 1200-stall in the snow, but I can say it wasn't any worse.
 +
 +
Combo 3 was a friend's '65 Mustang. It had an 8.3:1 302 with Roush 200 heads, a Comp Magnum 292 cam, headers,
 +
intake, carb, 3500-stall, C4, 3.80:1 spool. Tires were 25" diameter.
 +
That thing needed 10.5:1 compression, but a full summer of tuning got it driveable. To the point that he swapped
 +
street radials for drag tires. Then we went out to the exact center of nowhere, did up the 5-point race harnesses,
 +
and he let it fly.
 +
If you've ever felt a jet plane on takeoff, this puts them all to shame. Nothing you've ever experienced at any
 +
amusement park even comes close, either. This was like God's own pillow-covered sledgehammer to the heiney.
 +
First experiences are always more vivid than follow-ups, and that's the case here. This wasn't to be my last
 +
experience with race converters, but it was an eye-opener.
 +
 +
Following that, I was playing with an '85 Camaro with a mildly modified L69, 700R-4, 3.42:1, and sticky 215/60R15s. I decided on a B&M TorkMaster 2400. It drove pretty stock, until I swapped the heads. Finally it all came together:
 +
No traction in first gear, very very fun!
 +
Driveability? Not annoying, not irritating, not aggravating. Snow? no clue. Rain? easy to drive, but stomp the fun
 +
pedal, and no traction in first or second gears.
 +
But I took the car apart, saved some parts, and a year later I was putting that converter in a '95 Z28 convertible
 +
with the LT1, 4L60E, 2.73:1, and 235/55R16s.
 +
How to describe this? My flash stall was now 2400, twin black stripes became effortless, from a rolling 5 MPH
 +
start, and even starting with the 4-wheel-disc-brakes applied enough to keep the car stopped, getting both tires
 +
turning took like half throttle.
 +
But the driveability? It was hell. My slippage was always 1200, maybe because someone or something had caused it
 +
to fall off a workbench some weeks prior. But no denting was found. Still, lockup no longer worked. I replaced the
 +
trans, and still no lockup.
 +
One late night, taking dad home, I was at a light, and along came a Porsche Boxster.  Killed his fool junk!
 +
Great converter for playing, needs some rear gear.
 +
Great tragedy I never tried it in any snow, in either car.
 +
 +
So, the combo is everything.
 +
 +
If you're studying the COMP CAMS catalog, for example, you'll note that once you look past the smallest cams, they
 +
begin noting stall speeds. But as with all things camshaft, these are for the popular engine. 
 +
If you're gonna pick a cam for a lesser-displacement version, read the notes for the next larger cam.
 +
 +
For racing, you want a stall about 500 RPM below the RPM at which your engine makes peak torque. For most mild
 +
SBC 350s, torque peak is typically 3500 RPM. With Vortec heads and such, 4000 is more like it. AFR heads should
 +
raise that to 4500.
 +
Choosing converters is also about [b]size[/b]. Stock SBC converters are about 300 MM, which is nearly 12". They're
 +
referring to the outer diameter of the whole converter.
 +
Why it matters?
 +
My TM2400, above, was a loose 12". That looseness is most of why it was able to slip 1200 RPM all the time. Had it
 +
been a tight 10.5", still 2400 flash stall, it would have been much less slip, and much less irritating.
 +
So, this bit about [b]loose or tight[/b]:
 +
Loose converters are great when your static compression is way low, or way high. Way high, a loose converter helps
 +
reduce detonation. Way low, it covers the soggy off-idle. And a cam that's way too big is the same soggy feel as
 +
way low compression.
 +
 +
[b]Efficiency[/b]: This is difficult to discuss. Look back at my '78 Camaro. The TM2k was much more efficient than
 +
the stock converter it replaced, and was also more efficient than what was in my '79 Firebird.
 +
But how to tell, the reduced slip at cruise? Partly.  That it had that plus the better launch? Mostly.
 +
So my TM2400 wasn't efficient? Correct.
 +
Are all converters [b]rated[/b] by the same standards? NO!
 +
Watch some vids on You-Tube where guys have recorded their gauges while driving around. I watched mostly LS1 F-car
 +
guys' vids, but that's enough to illustrate the point. Yes, their results are combination-dependant, but I watched
 +
a 4000-stall drive tighter than  my 2400 stall. And at 4000 stall, it's not just the smaller diameter.
 +
[b]Gas mileage[/b]: My 2400 most definitely did hurt my LT1's MPG, in town and on the highway. But would that 2000
 +
have helped my '78 in town, had I fixed the carb, and acted mature when lights went green? Probably.
 +
It's not as simple as less stall = more MPG.
 +
Engines can lug to the point it's un-mistakeable, but like detonation, there are lesser levels where the driver
 +
really can't tell. And if your converter is too tight, it will hurt your MPG.
 +
[b]Performance[/b]: Is it all in the launch? Nope. The instant after an upshift is called the shift recovery. And
 +
with an automatic, how much the RPM drops from the upshift isn't controlled by gear ratios alone. Yep, we're back
 +
to the converter. And it's effect on performance after the upshift is more pronounced with a wide-ratio 700R-4
 +
than with a close-ratio TH350. With the TH350, the launch is what the converter helps most.
 +
But with more gear, you need more stall.
 +
Torque converters are much more sensitive to [b]load[/b] than to engine torque.
 +
More gear reduces the load that the converter feels. Same with taking weight out of the car. But not only that,
 +
having more gear also takes load off the transmission, helping it survive stoplight antics.
 +
 +
[b]When to buy what[/b]?
 +
Should this be an early mod? Or later, after everything else?
 +
The first mod in the drivetrain should be making sure you're not running a 2.73:1 axle ratio with 26" tires. With
 +
taller tires, a bit more than 2.73.
 +
  
 
[[Category:Transmission]]
 
[[Category:Transmission]]
 
[[Category:Undeveloped articles]]
 
[[Category:Undeveloped articles]]

Revision as of 18:29, 22 December 2011

Retrieved from "https://www.crankshaftcoalition.com/wiki/index.php?title=Choosing_a_stall_converter&oldid=19951"
Personal tools
Namespaces
Variants
Actions
Navigation
Categories
Toolbox
 
Terms of Use
Copyright
Privacy Policy