Choosing a stall converter
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'''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. | Let's start with the "stall". Drive your car to a deserted road, stop, watch the tach, and stand on the gas very abruptly. | ||
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begins to jump forward. Or the tires go up in smoke, depending on your combo. | begins to jump forward. Or the tires go up in smoke, depending on your combo. | ||
That is your "flash stall". | That is your "flash stall". | ||
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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 | 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 | seconds, then do a gentle drive afterward. Assuming you didn't just shred the tires, that was your "foot-brake | ||
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Anyway, back to stall. | Anyway, back to stall. | ||
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You can find all sort of descriptions and explanations, but your first interest is in what your stall does for your | You can find all sort of descriptions and explanations, but your first interest is in what your stall does for your | ||
car. | car. | ||
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I guess the best way to proceed is to detail 5 different combos I've owned and driven, that really stand out in my | 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: | recollections: | ||
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Trying to keep this in chronological order, the first combo is a '79 Firebird. 150-HP 301 with a slightly leaky | 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 | Q-Jet hurting MPG but not driveability, backed by a TH350, a 2.73:1 axle, and on new Firestone 215/75R15 tires. I | ||
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spend on performance mods. So I paid attention to what I had. | 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 | 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]: | + | 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. |
| − | 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 | 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. | + | of slippage? I lifted off. Yes, a total slip at WOT of 400 RPM. Again, fairly typical it turns out. |
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| + | Lesson learned: normal slip of torque converters. Also applies to lockup style when not locked. | ||
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Combo 2: '78 Camaro LG3 ( 145-HP 305 ) TH350, swapped stock 1200-stall for B&M TorkMaster 2000 converter, 3.08:1 | 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. | axle, 235/70R15 tires. | ||
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I know this converter was stock, I know it was 1200 RPM flash stall, I know the slippage was the same as my 'bird | 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 | + | already described above, and I know it drove fine for granny. But I decided to swap it. |
| − | 1700 RPM flash stall, [b]the launch[/b] felt much better. It felt like I had swapped to a 350, when launching. And | + | 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. |
| − | 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 | 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 | 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. | + | 350, for a total of 400. |
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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 | 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 | 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. | + | / 205-75R14 Camaro did 23 MPG with a best of 26. |
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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. | 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. | ||
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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, | 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: | 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. | 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 | 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 | [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]] | ||
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