Hey Guys Yesterday I bought a 69 Buick GS with a built 455 motor swap. 1. It has a TH400 w/ 2600 rpm stall converter (not verified by me, but confirmed by car builder) 2. It has 225/70/14 tall tires so 26.40" tall (verified) 3. It has a 3.42 posi read end (not verified by me, but confirmed by car builder) GEAR RATIO SPEED CALCULATIONS BELOW I have the following observations after my test drive 1. It does 55mph (GPS verified) at 3200 rpm 2. Based on the correct gear calculations It should be doing 73 mph at 3200rpm.... Could it be possible that the auto trans is slipping to the 33%? 3. Car does seem to shake off that SLIPPING CLUTCH FEELING (you know the one that feels like wheelspin without any wheelspin) 4. Another observation is that RPM drops more than 1000 rpm anytime I let go of the gas... almost instantly. 5. When you step on it it really gets up and goes (no feeling of slippage but has tons of power so its somewhat hard to say for sure) My feeling is that the trans never actually gets full lockup till much higher in the rpm range than its stated 2600 rpm stall.... I've never driven any automatic cars/trucks with a stall RPM above stock/production range (1000-1400rpm rpm is my best guess) Driven a lot of powerful cars some with big cams but all were manual transmissions. What do you guys think is wrong? What should be my course of action? I someone can tell me what to look for and observe I can do that in a second drive today or tomorrow and report back. Thanks Guy!
The 2600 RPM is if the car isn't moving. At higher RPMs, that converter will still slip a lot. It never "locks up". -Bob C.
There is no such thing as a converter that will stall at 2600 RPM behind any engine. Converter stall will vary depending on engine torque, car weight and gearing. You can buy an off the shelf converter advertised to stall in a certain range, but unless the converter is built for the combination, it may not be anywhere near optimal. Driving a car with a converter that is too loose gets old really fast. It may go like crazy when you romp on it, but it is lazy under normal driving conditions. All converters will slip some unless they have a lock up clutch. For stock converters, figure 150-200 RPM above what you calculate with tire height and gearing. For a 3.42 rear and 26.40" tires, it would be 336/26.40 X 3.42 X 55 MPH = 2394 RPM. Add 200 RPM and figure 2600 RPM. Of course your observed results also depend on the accuracy of your tachometer. 70 MPH would be 3047 + 200 = 3247 RPM. You should be able to feel if it is slipping too much because of the converter. Considering the RPM drop of 1000 when you take your foot off the gas, it sounds like you aren't getting any engine braking, and that points to internal transmission problems.
Your axle and converter might be fine for 1/4 mile drags with slicks. Maybe not so great on the street, your description sounds like what I would expect with that converter and that axle ratio. A gear failure with that much slip would burn up the trans pretty fast. I like good mileage and some amount of performance. So its a 2.41:1 axle and a switch pitch transmission here. good luck, Bruce Roe
yeah - something isn't right. My 65 with A 26 inch tire and 3.42's gets to about 65 mph at 3200 rpm. Hell my 71 with a 28 inch tire and 3.73's will cruise at 70 at 3500....more or less. If your converter is slipping that much at cruise though, it'll burn fluid pretty quick. Check it out.
Well you can get a valve body that has no engine braking built into it. At least that's what I read, I opted for one with braking cause I love to rack the pipes.
