Jim Smart
August 13, 2007

C4 Performance Basics
The best C4 to build is '70 or newer with the larger input shaft. Both the 26/26- and the 26/24-spline, .839-inch shafts, and forward clutch hubs can take more punishment than the smaller .788-inch, 24/24-spline shaft. Professional builders tell us the 26/26-spline shaft will withstand up to 600 hp. Of course, this doesn't always guarantee your C4 will survive 600 hp and the corresponding torque. A good rule of thumb is to go with a hardened aftermarket .839-inch, 26-spline input shaft (both ends). Some factory 26/26- and 26/24-spline shafts were hardened, but apparently these are few and far between. Your best choice is an aftermarket shaft.

Another important point with C4 performance is case selection. The '70-and-later case is considered stronger than '69-and-earlier because the bellhousing bolts to the case instead of the front pump. Transmission professionals also tell us the '82-'86 C5 case is better still thanks to oiling-system circuit improvements.

You can net the same oiling-circuit improvements in an older C4 by doing some of them yourself. Because the No. 9 thrust bearing in back of the C4 tends to be oil-starved and prone to failure, transmission professionals like to modify the case whenever possible to improve oil flow back there. As we understand it, the solution is to drill a 1/8-inch hole into the case that ties this thrust bearing to the oil-cooler return galley. Ford did this with the C5, along with an additional lubrication hole in the thrust bearing. Mention this to your transmission builder and get their take on this modification. Every builder is of a different opinion.

Another suggested modification is to smooth the oil passages by chamfering each of them to reduce fluid-flow resistance and turbulence. According to a few builders we've spoken with, this improves line pressure and fluid flow.

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Flexplate Facts
There are two basic flexplate sizes for the C4: a 157-tooth (left) like we find in most compact and intermediate Ford applications and a 164-tooth (right) more common with heavy-duty truck and full-size Ford applications.

Bellhousing size goes right along with flexplate size. The larger 164-tooth flexplate needs the larger bellhousing.

Also remember offset balance. If you're building an early small-block Ford (before '82), expect a 28-ounce offset (smaller counterweight). After '82, expect to see a 50-ounce offset with the large counterweight. This is common with '82-and-later 5.0L High-Output V-8s.