Great article in one of my recent magazines. They tested, on a dyno, the power loss due to a header pipe dent. They used a 372CID, 530hp, 7000rpm, 10:1, 242/250 cam, 1 7/8" header primaries.

They dented a different depth and different # of tubes and measured power.

1/4" dent x 4 pipes --> NO change
1/4" dent x 8 pipes --> NO change
1/2" dent x 4 pipes --> NO change
1/2" dent x 8 pipes --> NO change
3/4" dent x 4 pipes --> -3HP at peak
3/4" dent x 8 pipes --> -5.4hp at peak, -0.8hp average
3/4" dent x 8 pipes x 2" long dent --> -15.7hp peak, -4.5hp average

So how much do you really think that single dent in the Jeeps downpipe is really costing in terms of power. Sure it's after the collector, but at no more than 5000 rpm, it costs what 0.5hp?

 

The downpipe dent is little more the the restrictions inherent in every bend of the exhaust system anyway (unless someone has a 100% mandrel-bent exhaust system). It is my opinion that people give that dent too much credit, and it is my experience that it has virtually no effect on any system with the *possible* exception of a finely-tuned engine and exhaust system where every single bend is a mandrel bend.

Looks like they had to put a 3/4 x 2" deep dent into each of all eight 1 7/8" pipes (that means they crushed every primary down to 60% of its original size for an unknown percentage of each primary's length--primary length can be very important and is not known here) and lost less than 3% of their power.

Let's say for the sake of argument that the factory downpipe dent is 40% of the pipe since that looks roughly accurate to the eye. That's not the same as denting every primary, but since it is after the collector, we can say that it is in some way similar to a 40% dent in each primary. A 3% loss of power from the 177-190 hp peak a 4.0 is like 5 hp. That peak is at 4500-4750 RPM, and honestly, how many of us EVER realistically run our 4.0 up to 4500 RPM? The loss of power at the lower, more realistic daily RPM's is likely 1-2 hp at most if not less.


If the opportunity to get rid of the dent comes along and is possible for someone, go for it, but don't freak-out and go out of your want to remove it. Heck, I still needed a small depression in my new downpipe because, yes, the front driveshaft can and will hit that downpipe if the suspension compresses far enough. I plan to extend my front bumpstops then, SOMEDAY, remove the dent, but for now it remains.

 

Apples and oranges.


4.0s and 2.5s make their torque on the bottom end.