Choosing a stall converter
From Crankshaft Coalition Wiki
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| + | ==Introduction== | ||
| + | If you have an automatic transmission, then you don't have a clutch pedal. You have a torque converter instead. | ||
| − | + | ==How to choose the stall speed of a converter?== | |
| − | + | You can find all sort of descriptions and explanations, but your first interest is in what your stall does for your car. | |
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| − | Let's start with the "stall". | + | Converter stall speed needs mostly center on your camshaft, heads, compression, gearing, vehicle weight, vehicle use, and expectations. Tuning and fuel quality are your responsibility. For this we must assume you got the tune right and run enough octane, etc. |
| − | 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 | + | 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 related, these are for the 350 SBC engine. If you're gonna pick a cam for a lesser-displacement engine, read the notes for the next larger cam. |
| − | begins to jump forward. Or the tires go up in smoke, depending on your combo. | + | |
| + | ===Flash stall=== | ||
| + | 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". | That is your "flash stall". | ||
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| − | Now that I' | + | ===Brake stall=== |
| − | + | Now, repeat, but this time keep your left foot HARD on the brake pedal. But don't do it for more than a couple of seconds to avoid over heating the converter and transmission. Then do a gentle drive afterward to cool down the parts. That was your "foot-brake stall" unless there was tire spin. | |
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| + | ==Efficiency== | ||
| + | This is difficult to discuss. The B&M TorqueMaster 2000 was much more efficient than the stock converter it replaced, and was also more efficient than what was in my 1979 Firebird. But how to tell, the reduced slip at cruise? Partly. That it had that, plus the better launch? Mostly. | ||
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| + | Are all converters rated by the same standards? NO! Watch some vids on YouTube 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 rpm stall converter drive tighter than my 2400 stall. And at 4000 stall, it's '''not''' just the smaller diameter. | ||
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| + | ===Fuel mileage=== | ||
| + | A 2400 RPM stall may hurt MPG, in town and on the highway but a LOT depends on how often the vehicle is driven hard. But would that 2000 rpm stall speed converter have helped the 1978 Camaro with a 305 in town, had the carb been repaired? | ||
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| + | It's not as simple as less stall = more MPG. Engines can lug to the point it's unmistakeable, but like detonation, there are lesser levels where the driver really can't tell. And if your converter is too tight, it might also hurt your MPG. | ||
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| + | ===Performance=== | ||
| + | Is it all in the launch? Nope. The instant after an upshift is called the shift recovery. With an automatic, how much the RPM drops from the upshift isn't controlled by gear ratios alone. The converter and it's effect on performance after the upshift is more pronounced with a wider ratio TH700R4 | ||
| + | than with a closer ratio TH350. With the TH350, the launch is what the converter helps most. | ||
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| + | Torque converters are sensitive to load as well as 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. | ||
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| + | ===Gear ratio=== | ||
| + | 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 gear ratio. | ||
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| + | ==Examples of five different combinations== | ||
| + | ===Combination 1:=== | ||
| + | 1979 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. | ||
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| + | The converter was an 1800 rpm stall. I could, occasionally, chirp a tire from a dead stop. But I soon got curious about the converter slippage: 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? Yes, a total slip at WOT of 400 RPM. | ||
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| + | Lesson learned: Normal slip of torque converters. Also applies to lockup style when not locked. | ||
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| + | ===Combination 2:=== | ||
| + | 1978 Camaro LG3 (145 HP 305) TH350, swapped stock 1200 stall for B&M TorkMaster 2000 converter, 3.08:1 rear axle, 235/70R15 tires. This converter was stock, it was 1200 RPM flash stall. | ||
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| + | A change to a B&M converter gave my combo a 1700 RPM flash stall. 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. 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. After all, my previous 1978 Camaro LG3, TH350, 1200 rpm stall, 2.41:1, with 205-75R14 tires did 23 MPG with a best of 26 mpg! | ||
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| + | ===Combination 3:=== | ||
| + | A 1965 Mustang. It had an 8.3:1 compression 302 with Roush 200 heads, a Comp Magnum 292 cam, headers, intake, carb, 3500 stall, C4, 3.80:1 spool. Tires were 25" diameter. | ||
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| + | 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. | ||
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| + | 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. | ||
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| + | *Combination 4: A 1985 Camaro with a mildly modified L69, TH700R4, 3.42:1, and sticky 215/60R15 tires. I decided on a B&M TorkMaster 2400. It drove pretty stock, until I swapped the heads. No traction in first gear, very very fun! | ||
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| + | ===Combination 5:=== | ||
| + | A 1995 Camaro Z28 convertible with the LT1, 4L60E, 2.73:1, and 235/55R16 tires. 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. Great converter for playing, needs some rear gear. | |
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| + | ==Racing converter application== | ||
| + | For racing, you want a stall right at or not more than about 500 RPM below the RPM at which your engine makes peak torque. For most mild SBC 350s, torque peak is typically in the 3500 RPM range. With Vortec or aftermarket heads, the torque peak might be marginally to quite a bit higher, depending on the intake port volume and intake manifold size and type. | ||
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| + | Choosing converters is also about size. Stock SBC converters are about 300 MM, which is about 12". They're referring to the outer diameter of the whole converter. | ||
| + | Why it matters? My B&M TorqueMaster 2400 converter mentioned 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. | ||
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| + | 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 too low compression. | ||
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| + | ==Resources== | ||
| + | *[[Torque converter]] | ||
| + | <br><br> | ||
[[Category:Transmission]] | [[Category:Transmission]] | ||
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