Piston Slap: A Slippery Stall For Smoother Shifting?

AT Racing World

Roger writes:

Hi there, I am fighting this issue with my 1962 Ford Fairlane Sport Coupe: On a shift (say, first to second gear), if I keep my foot on the gas, it shifts hard but not obnoxiously, but if I let off the pedal it shifts with a … WACK!

It shifted so hard the first time I thought I’d been hit from behind! But this happens on any shift, if I let off on the gas pedal, although not so much on downshifts. My garage diagnosed it as a sloppy differential backlash, and they said it’s all good now ($450 later). But I drove her home, and it was exactly the same!

Here are the specs on my driveline:

  • Ford 302 stroked to 347 cubic inches by a pro builder.
  • Ford C-4 built to handle 450-500 hp.
  • Attached to the C-4 is a Gear Vendors overdrive.

In the springtime, I took her to a garage that only works/builds/repairs classic cars, from Ferraris to Camaros. Other than changing fluids/filters, they told me my problem is the stock Ford convertor (which stalls around 1200-1500 rpm) and that an aftermarket convertor with at least 2400-2800 stall will “slip” enough to stop those too-hard “BANG” shifts. I know they are on the money about a higher stall convertor, because my engine builder also said I should install an aftermarket convertor.

Do you think this will stop my issue with these horrible shifts?

Sajeev answers:

Yes, I think that’s the ticket! The stock converter shoulda been dumped the moment a stroker small-block and a built transmission were planned. That’s an exaggeration, but you see my point. For the transmission, you normally need the following done with any performance engine rebuild:

  • A fresh rebuild.
  • A shift kit, shift improver, or an internal upgrade unique to your transmission (a la Ford J-MOD).
  • A higher stall speed, “looser” than stock torque converter.
  • A standalone transmission cooler, usually plumbed in series with the factory cooler in the radiator (for double cooling).

The fresh transmission rebuild is obvious, since new clutches, gaskets seals, etc. are needed just as badly as a new engine. Not doing so kills the current transmission sooner, and yanking it out while the motor is absent generally saves money in labor costs. Take it from someone who has seen this via building three high-performance transmissions, including a Ford C6 chronicled here at Hagerty. (This may not apply to axles/differentials, especially if you aren’t running racing slicks, and have no interest in doing gnarly burnouts.)

Next is the shift kit: a logical upgrade, as you want quicker shifts to go with your perkier motor, but there’s a catch: Sometimes a shift kit must be specced to work with a specific stall speed in your torque converter.

Installing a looser converter can help holeshot off the line, and fix shifting issues much like what Roger’s experiencing in his Fairlane. Unless the vehicle has a performance-tuned automatic from the factory like a CVPI Crown Victoria, AMG Mercedes, etc., the torque converter stall speed is generally too low for performance driving. Low-stall converters are designed for smoother, less CVT-like performance and more fuel efficiency.

Speaking of factory-perfected tuning, a high-performance powertrain should never be in a situation where one hand doesn’t know what the other is doing. The critical components to discuss with your rebuilders are the camshaft choice, the torque converter stall speed, and possibly the steps to dial-in a shift kit. I reckon Roger’s assertion that the stock converter is 1200-1500 rpm is right, and a 2200-2500 rpm stall converter is needed to fix the shifting behavior.

But I have no clue what cam is in Roger’s new motor, perhaps an even more aggressive stall (i.e. 3000+ rpm) is needed. This is precisely why your engine builder needs to be included in transmission work. Get a torque converter with the blessings of both engine and transmission rebuilders and your throttle off upshifts will be delightful.

Advice from professionals like this is priceless, but it’s also part of their service. I have yet to meet an engine or transmission rebuilder that wasn’t thrilled to share more information with a not-rude customer. So ask them for confirmation, as you have nothing to lose. Rather, you have everything to “loose” in your converter.

Have a question you’d like answered on Piston Slap? Send your queries to pistonslap@hagerty.comgive us as much detail as possible so we can help! Keep in mind this is a weekly column, so if you need an expedited answer, please tell me in your email.


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    I preface this by saying I know nothing of how these Ford trannys work, and this may seem like something that someone shoulda found easily by the pro garage, but…
    On a GM trans of this vintage, there is a vacuum line that runs from the intake manifold to the modulator. It should be a hard line, but sometimes people have replaced them with rubber tubing. If the tube has come off, loose, or possibly has a split – anything that interferes with the vacuum – it causes similar shifting issues as Roger describes.
    Don’t have any idea if the C-4 operates the same way, but thought I’d toss this out as something that would be cheap and easy to investigate and fix.

    Great Comment! For his application I would definitely have used an FMX tranny. I had one in a 69 Mustang Fastback with 351 Windsor engine. I owned many Fords back in the day. Every C-4 turned to junk quickly.
    The C-6 was a good tranny for heavy cars. The FMX was like a hybrid. Lighter than C-6 but built to handle 3-400 hp easily.

    Second that. This sounds exactly like the symptoms of a bad vacuum modulator, or the line going to it. Or, perhaps the line isn’t connected to the intake manifold properly. I’d check this before doing anything else.

    So my friend Mr. Google tells me the C4 does indeed have a vacuum modulator. I would take the above advice and check the line. These things are also apparently adjustable. I do now own a Ford, but I am far from a Ford man.

    In general, I would take it to the shop that did the transmission work and get their prognosis. If it came built from a company, I would get on the line with their tech support. Most of the worked transmissions I have had over the years bang harder with more pedal consistently.

    The high stall converter may do the trick, and it may be needed anyway, but you may end up covering up a symptom of another problem. If you aren’t going to actually drag race yours, you probably don’t need all the lump.

    I personally hate high-stall converters, which is why there is nothing with more than a mild cam in my stable.

    A failing torque converter is commonly indicated by sluggish acceleration, RPM flare between shifts and the fluid having a burnt smell or being dark in color. As far a a rough or hard upshift is concerned, the vacuum modulator and vacuum plumbing is the most likely culprit. Even a high-stall torque converter has no effect on shift quality. Our 8.90 Super Comp dragster is proof of that. It leaves the starting line at 6100 RPM but when the ‘glide goes into high gear at 7200, there’s no shock to the tire due to shift programming.

    Looks like I got this one wrong! The vacuum modulator needs to be checked first.

    Looser converter might help, but it will likely provide a more enjoyable drive.

    Well you may not be wrong, exactly – in fact you are entirely correct that the careful matching of parts in the powertrain will result in better performance and extreme mis-matching will really hurt things. As a few others have pointed out, a torque converter change may indeed ultimately be in order. The thing you kinda missed (and should get a slap on the wrist for, since you’ve often espoused it) is to go through the potential problem/fix list from SIMPLEST to most complex – in that order.

    A higher stall speed torque converter will mask the problem, but it’s not the correct fix. Hard shifts indicate excessive or improperly modulated hydraulic application pressure to the relevant clutches/bands. Since the poster indicates the problem occurs with all upshifts, the issue likely involves either the vacuum modulator or the governor (and their related valving) which work together to control line pressure. Other possibilities involve issues with the shift accumulators or modifications made by the transmission builder to increase performance and durability. Automatic transmission “shift kits” come in various versions calibrated to the vehicle’s intended use, and going too far toward a full-race configuration can negatively impact normal street driving. Finally, there is the possibility that the proper kit was used, but the transmission builder made one or more mistakes in assembling the gearbox. Proper diagnosis in this case will require a set of pressure gauges and someone who intimately understands the inner workings of a C4. Installing a higher stall speed torque converter without first determining the cause of the harsh upshifts is simply throwing parts at a problem and is unlikely to result in a satisfactory outcome. Plus, it will reduce fuel economy and generate excessive heat if you ever attempt to tow with the vehicle.

    Doesn’t sound like a converter problem to me. FWIW, the typical “hot street” converter generally has around 2500-3500 stall speed. The idea is to have the engine in/near its torque peak for best acceleration. Power output, gearing, and car weight all affect stall speed, even if the stock converter is used: in a sense, you’re using leverage, either as a load or as power input. I’d suggest that the owner gather his car’s info – power output & curve, cam specs, gearing, & car weight – and call a couple of converter makers. There are several TC companies that can provide solid recommendations and products – expect to pay $500 for a tailored performance converter, and a lot more for a race unit. Money can be saved if there’s an OEM unit with the stall you need, from one of many rebuilders.

    A really great article, Sajeev, with solution(s) almost certainly found!
    Our Community pitched in where needed, and it looks like all will be well.

    For some “uninitiated”, it would have helped if it was disclosed early-on, that this is an AUTOMATIC transmission.

    I seem to remember that I had an FMX tranny in my ’71 351ci-Cleveland-engined Mustang, and that it DID was once remedied by replacing the Vacuum Modulator, (I’m still thanking God for that less-expensive cure.)

    Also — I saw that very craft, from the first photo, somewhere above New Mexico…

    First of all if you’re driving a car with performance modifications it’s not going to act like a normal car. I put a b&m shift kit on a ’67 th 400. It involved taking out the accumulator spring and throwing it away. This alone gives hard shifts. A high overlap cam
    affects the vacuum profile so the modulator will work differently. You may benefit from a higher stall converter from a performance standpoint but it won’t necessarily make the car drive better.

    Back in the day novice transmission builders would loose the throttle valve or modulator pins, or just forget to put them back in.

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