Essentially a torque converter is designed like one electric fan (we’ll call it Fan A) facing in close proximity to a second fan (Fan B). It also functions as a torque multiplier at low vehicle speeds. Stall speed is affected by numerous variables beyond just diameter.Ī torque converter is essentially a fluid coupling between the engine and the transmission. Basically, a larger diameter converter will be somewhat tighter than a smaller converter, all other things being equal. Torque converters can be a bit like black boxes as they incorporate so many variables. We have to jump into a short course in torque converters to understand how this works. Stock converters are designed to offer crisp throttle response by reducing the converter slippage. This problem is compounded by the fact that the long duration camshaft requires more throttle opening to maintain a decent idle speed. In a stock application, manifold vacuum of 15 inches or above signals the modulator that the engine is not under heavy load so it reduces the line pressure to reduce engine load. So if the engine now idles at 10 inches of vacuum, the vacuum modulator senses that the engine is under load, so it raises the line pressure generated by the front pump. So an engine with a long-duration cam will generally suffer from lower engine idle vacuum. Older automatic transmissions like the TH350and TH400 (and most other early domestic automatics) use manifold vacuum as the indicator of engine load. For the record and for the sake of clarity, idle vacuum is measured in inches of Mercury (Hg”) not in pounds per square inch (psi) which is a measurement of pressure. This is 5 to perhaps 6 inches of idle vacuum lower than a stock engine with idle vacuum of 15-17 inches of vacuum in neutral. Let’s say for example that the engine idles at 900 rpm with roughly 10 to 11 inches of idle vacuum. We’ve had experience in the past with stock, tight torque converters and big camshafts where the lower engine idle vacuum directly affects the converter. You didn’t mention what idle speed or manifold vacuum level your engine idles at, but we’ll hazard a guess that with the longer duration cam that the engine now idles at a slightly higher engine speed along with a lower idle vacuum. He says that the accumulator spring, even if it was broken, is not the source of the problem as it functions during second gear apply on a TH350 transmission. Jeff Smith: We talked with our friend Jimmy Galante, who runs a shop called Racetrans in Sun Valley, CA. Is the torque converter bad? Thanks for your help. I’ve been told my problem is a bad accumulator spring, but we checked that and it’s okay. Now, with the big cam, when I pull the transmission into drive from park or neutral, it clunks really badly.
The rest of the engine is Vortec iron heads, headers, and a Performer RPM intake and 600 cfm Holley carburetor. Before, the cam was really mild and idled like a stocker.
We recently put a bigger hydraulic lifter cam and headers on the engine to make a little more power. I have a ’69 Chevy C-10 pickup with a 383 stroker small-block and a TH350 transmission.