I have a 71 Skylark, and the original 350 in is the machine shop getting a fresh rebuild. I'll be increasing the compression from 8.5 to somewhere around 9.5, and a mild cam, bigger than stock, TA Intake, TA headers. My TH350 is rebuilt with B&M shift kit, and stock 2.56 gears. Should I stick with a stock torque converter or larger. I don;t quite understand exactly what stall speed is/means?? Can some please xplain stall speed, and torque converter advice????? Thanks Jay
Torque Converter stuff A torque converter connects your engine to the transmission, performing like a clutch does in a manual transmission car. Torque converters do more than a clutch though. They actually multiply torque like a gearbox. You get maximum torque multiplication at maximum slip. There is no torque multiplication with no slip. You can think of slip in terms of RPM. If your car is idling in gear but not moving, the engine is running at idle speed but the transmission gears are not turning. The difference between idle speed, maybe 600 RPM, and the transmission speed of 0 RPM is taken up in the torque converter. This multiplies torque, but the car won't move until you take your foot off the brake. Once you do and the transmission starts to turn, the slip is reduced and the torque multiplication is reduced too. If you stand on the brakes and the gas at the same time, the torque converter will eventually limit the RPM speed of the engine. This is called the "stall speed" of the converter because the engine is "stalled" from turning any more RPM's. This is also the maximum slip that the converter can absorb and the maximum torque multiplication the converter can produce. This is why "powerbraking" will allow you to spin the tires more easily. When you powerbrake and then release the brake, the torque is applied suddenly to the tires, sending them up in smoke. Typical stall speed for a smallblock car might be 1200 RPM. Raising the stall speed of the converter can allow you to launch the car at a higher RPM, putting you further into the powerband of the engine, and making more torque available for launch. High stall converters also smooth out the idle torque pulsations that come with big cams. You will see converters rated at 2000, 2500, 3500, 4000 etc. RPM. With the right engine components (cam and heads) and gears the higher stall converters can help starting line performance. The other side of this is that if you do most of your driving in the RPM range below the stall speed of the converter, you will lose gas mileage and increase the transmission temperature because of the increased slip. With your combination, you'll probably have about 325 horsepower and maybe 400 foot pounds of torque. The 2.56 rear gears mean that you will have to be careful with torque converter selection. Your engine will probably turn about 2500 RPM at highway cruising speeds. Around town, you'll spend a lot of time below 2000 RPM. So you cannot expect a 3000 RPM converter to work for you - you'll either need to stay close to stock, or change the gearing to get RPM up quicker and help your combo out. If you gear up, then you will probably want to consider more cam and compression to take advantage of the gears. Pretty soon you've created a monster of a drag car, but it's not so good for cruising. The best advice I can give you is to consider the elements of your combination that affect performance as a group or system. So, if you're talking about accelleration, you need to look at car weight, traction, gearing, transmission, and engine characteristics together. Make sure that all of your choices support each other. Don't put in a "stall converter" just because it sounds like a good idea!
Thanks John for the info, it really does help out. Two questions, 1st if I was going to take a step up in torque converter, it would be maybe the B&M Tork Master in Summit which advertises a stall speed of 1900 - 2100, or with my gears, will I heat up the tranny as you said?? 2nd question, A salvage yard in Texas has a 1971 Skylark wagon complete Rear 3.08 ratio for 400 including delivery. Would this help, or is it more work than I will actually benefit from.
Well personally I like the 2.56 gears, since the motor is rebuilt the car wont seem as lazy as it did before. There are a lot of guys who have ran both 3.41+ gears and compared their 1/4 mile times to 2.76 rears, the differences have been surprising small. With Buick torque you really don't need a really high numerical gear because we have no problem getting going. Lots of people swear by the High numerical gears but they are usually more devoted to going fast in the 1/4 mile and or they have an overdrive. 3.08 is a good trade off though. For the Torque Converter you would have no problem with using a 2100 stall with your combo, that sounds perfect. The 2100 stall will actually make your 2.56 gears feel more like 2.76 because of the trans is slipping a bit. Because of this slipping at low rpm there is a Torque multiplication which helps give you better acceleration from a stop. Once you are at speed you idealy want zero slipping which is what a torque converter with a electric lockup will provide. With a 2100 stall this isn't a big deal but with a 3500 higher stall the motor is always reving higher than it would with a lower stall speed. This is a good thing if your motor doesn't start making power till 2500+. I have a 80 model yr TH350 that has the electric lockup on it and it is prety cool, I have it hooked up with a switch on the dash to turn it on or off. The stock lockup setup used a sensor of some sort to signal lock up. You can probly get aftermarket converter with the lockup and rig up a simple switch so you can control it. Just an idea.
Jay - I'd suggest that you call TCI or B&M and talk over your application with them and get a recommendation. Make sure they are paying attention to your rear ratio, tire size, transmission type, and intended use. If they just jump to the conclusion that you need a certain "stall" based on engine size you need to look for advice elsewhere. I think the 3.08:1 gears are a great compromise for a street/strip car. I have that ratio in my '68 and it does good even with short 14" wheels and 235 series tires. This combination works out to about the same performance I'd get with 3.42 gears and more typical 15" wheels and tires. Remember that the actual gear reduction of your driveline depends on rear gear ratio, tire size, and transmission high gear ratio (transmission is not a factor unless you have an overdrive transmission). Using a shorter tire increases the driveline gear reduction and increases the available torque at the wheels, just like using a higher numerical ratio gear. There are some good online calculators to work this stuff out. Check out this page for some charts that show the relationship of tire size, speed, and rpm. They also have a good set of charts to help you get the right speedometer gears for your combination. (don't worry about it being a Chevelle page!) http://www.chevelles.com/shop/ratios.html I think you'll find that your car will pull harder with a 2000 rpm stall torque converter and the 2.56 gears, but you'll notice a bigger difference if you go to the 3.08 gears. The more gear you run the more converter you can use. You could use 3.42's for more accelleration but you'll be buzzing the motor pretty good at cruise on the highway. Have fun and good luck!
Thanks so very much for the info. I talked with TCI tech and was extremely dissapointed with their help. I should say no help. TA performance gave me the most help, but after taking your advice with my 2.56, I bought a TCI Saturday Night Special from Summit which advertises 1800 to 2200 stall. I will let you know how it works out. I love this board!!
Just my opinion Jay.....but, I did this with my close to stock 350 Skylark, go with a ~2500 stall converter and add a trans. cooler. You will love it. You will be surprised how much more power you have off the line!
Just to share a bit of experience, as I street drive my car a lot, and I was in your shoes a few years ago: I built a small block with around 325HP/TQ (though you didn't mention cam specs, so I may have had a higher rpm powerband), and had to find a converter that would stall high enough, yet lock up at cruise. I got a 10" PTC that stalled around 3000-3200, yet it had barely any slip under cruise conditions or around town. At the time, I had a 3.08 gear and 28" tire, so 65mph was only like 2450rpm. I had a tranny cooler, and could not get my tranny temp over 170* all day long in the summer. The car ran mid thirteens at the time. The same converter went 3400RPM behind my current combo (just under 500HP/TQ) and still only slipped 2.1% on the big end of the track and at cruise. I was very happy with it, and just traded it in for a super-stock type 8" from PTC (time to let this small block breathe!). I think that with a good custom converter, the lines between a street and strip car can be very easily blurred. There are many great companies out there that have these available in the $300 price range. Incidentally, I also broke the sprag out of an inexpensive off-the-shelf converter before buying the PTC, which meant two tranny rebuilds in a month. I don't know what caused the breakage. Just my $0.02, I realize not everyone is out for track performance as well as street driving and seat-of-the-pants.
