Ask Away! with Jeff Smith: Cam Size and Its Role in Torque Converter Selection

I have a ’69 Chevy C-10 pickup with a 383 stroker small-block and a TH350 transmission. We recently put a bigger hydraulic lifter cam and headers on the engine to make a little more power. Before, the cam was really mild and idled like a stocker. The rest of the engine is Vortec iron heads, headers, and a Performer RPM intake and 600 cfm Holley carburetor. Now, with the big cam, when I pull the transmission into drive from park or neutral, it clunks really badly. I’ve been told my problem is a bad accumulator spring, but we checked that and it’s okay. Is the torque converter bad? Thanks for your help.

J.R.

Jeff Smith: We talked with our friend Jimmy Galante, who runs a shop called Racetrans in Sun Valley, CA. 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. 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.

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. Let’s say for example that the engine idles at 900 rpm with roughly 10 to 11 inches of idle vacuum. 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. 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.

Older automatic transmissions like the TH350 and TH400 (and most other early domestic automatics) use manifold vacuum as the indicator of engine load. So an engine with a long-duration cam will generally suffer from lower engine idle vacuum. 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. 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.

This problem is compounded by the fact that the long duration camshaft requires more throttle opening to maintain a decent idle speed. Stock converters are designed to offer crisp throttle response by reducing the converter slippage. We have to jump into a short course in torque converters to understand how this works.

Torque converters can be a bit like black boxes as they incorporate so many variables. Basically, a larger diameter converter will be somewhat tighter than a smaller converter, all other things being equal. Stall speed is affected by numerous variables beyond just diameter.

A torque converter is essentially a fluid coupling between the engine and the transmission. It also functions as a torque multiplier at low vehicle speeds. 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). When we turn on the switch to energize Fan A, the air coming off the spinning blades induces the second fan to begin to spin – although somewhat slower. Now imagine that Fan A is connected to the engine (called the impeller) and Fan B (called the turbine) is connected to the input shaft of the transmission and both are enclosed in a round case. This two-piece setup is called a fluid coupling. This is how very early converters were built, but they did not work well because they suffered from very poor acceleration and massive slippage. Then a third small fan was placed in between the impeller and the turbine to redirect fluid into the turbine. This third fan is called the stator. It multiplies torque when the vehicle is just starting to move, dramatically improving acceleration.

Stock converters, as we mentioned, are very tight, intending to increase throttle response and improve light throttle acceleration. But this efficiency also reduces what is called the converter’s stall speed. There are essentially two ways to measure stall speed. The first is to place your foot firmly on the brakes and with the transmission in drive and the engine running, slowly increase the throttle opening. The point just before the car begins to move forward is considered foot brake stall rpm.

The second way to test is called flash stall, which is perhaps a bit more accurate but more difficult to measure. Placing the car in a safe location, launch the car hard. As you hit the throttle, watch the tach. There will be an rpm set point where the engine revs to just as the car move. A better way is to data log the launch and then call up the rpm trace. On the launch, you will notice a little dip in the rpm curve as the car is launched. The high point before the slight dip is the flash stall rpm.

Stock converters offer very low foot brake or flash stall rpm, electing to reduce slippage in favor of better fuel mileage and crisp throttle response. The problem comes in when big cams are matched with a stock converter. The stock converter tends to load the engine harder and since the idle quality is already suffering from low idle vacuum, the engine struggles against the converter, lowering the idle vacuum. This is the double whammy because the lower idle vacuum then signals the vacuum modulator that load is being applied – slightly increasing the line pressure which could increase the load on the engine, lowering the idle vacuum even more. This is one advantage to the later overdrive transmissions like the 200-4R or 700-R4 because these transmissions use a TV cable that senses the engine is still at idle so it maintains lower line pressure – reducing the load on the engine.

A performance torque converter will help your situation because a slightly higher stall speed converter will tend to offer more slippage at idle. This reduces the load on the engine, improving the idle vacuum and reducing how much the vacuum modulator increases the pressure. Where this can get confusing is in selecting a performance converter because there are literally hundreds of different converters for a TH350.

The first big variable is the diameter of the converter. Stock converters are often between 11 and 13 inches in diameter. The larger the converter, the more efficient it is relative to stall speed. So the quickest way to increase stall speed is to reduce the diameter of the converter. Then stall speed can be modified by changing the angles of the blades on the turbine and impeller along with the design of the stator. For a street-driven truck like yours, you should stick with a relatively mild street converter.

Stall speeds are difficult to estimate because they are directly related to the engine’s torque output. That’s why TCI claims 1,500 to 1,700 for its Sizzler converter for a small block and 1,700 to 1,900 rpm for the same converter behind a big block. This occurs because the big block makes more torque based strictly on its larger displacement. This or a similar converter from another company will help reduce that load on the engine at idle and should reduce the tendency for the engine to lurch into gear.

Not to make this answer even longer, but it’s possible to reduce this “lurch” issue by careful engine tuning. Try adding more initial timing to the engine. This will improve idle vacuum. Plus, accurate idle mixture tuning will also improve idle vacuum. We had a small block Ford recently that a friend asked us to look at. The engine was idling at 10.5 inches and seemed to be very unhappy – especially in gear. By careful adjustment of the idle mixture screws, we were able to increase the idle vacuum from 10.5 inches to nearly 13 Hg. This helped the engine in gear and also improved the off-idle throttle response.

So you can see that you have lots of options – many of which can be attempted without spending any money. A slightly looser torque converter will certainly make life better and might just improve acceleration at the same time – assuming, of course, you don’t just light up those tires!