C4 or C6 behind 351 Windsor

As usual there is some misinformation and or exaggeration

Weight with converter
C4 - 138
C6 - 173
for comparison Turbo 400 185, turbo 350 165

Parasitic drag on stock transmissions
C4 - 28-30 HP
C6 - 55-58 HP
TH-350 36-38 HP
TH-400 44-46 HP
(race prepped C6 with roller bearings, 35 HP)

The C6 is a little bigger, mainly because of the ribbed case to bell housing.

And if he had an FMX, that pig weighed about 195, but only took about 25 HP to turn.

As for the C4's ability to hold up to HP ~ TCi offers a 825HP C4, Mike's builds stuff that will handle 1,000 HP, and Dynamic's Mighty Mite has been in Mustang's running 8.30's. I'm sure there are plenty more

 

One thing about C4's that only had a brief mention:

C4's come in 2 different styles. The small bellhousing/case fill and the big bellhousing/pan fill.

The big bell housing has a stronger case, and the dipstick tube goes into the pan. It uses a 164 tooth flexplate.

The small bell has the dipstick tube going into the case, and uses a 157 tooth flexplate. This small bell allows a smaller trans tunnel and more exhaust room.

There is also the C5, which replaced the C4 '82-'86. It has a different valve body and a lockup torque converter, but can easily be converted back to a C4 by backdating those parts. The case has improved oiling to the rear thrust washer, and larger oil cooler passages with no check ball. Internally, the gearsets have more and larger oiling holes, and the drums will hold more clutches. The C5 is a good core to build a performance C4 on. They also came in case fill and pan fill flavors.

 

I run C6 with 351W in my 56 Ranchwagon. Runs great. You can pound it all day long.
They are not hard to come up with in the older wrecking yards. Easy to rebuild. See the car at ugotlorrie

 

Not quite true - There were big (164 tooth) and small (157 tooth) bellhousing case fills from 64-81. (and the odd ball 143t Pinto version). The 64-69 had the smaller 24 spline input and the 70-81 had the 26 spline. The pan fills were all 164T large bellhousing.

 

At one point in time the C6 was the gold standard for Ford powerplants. Why? Because the factory C4 wasn't up to the task. That was about 30-40 years ago. Much has changed. The aftermarket has brought substantial improvements. Material have advanced a lot. The C4 is now much stronger than Ford probably ever thought it would be. The C4 is just more efficient and smaller.