I've been wondering something lately. If you had, for instance, an exhaust system that started with good flowing manifolds and then went down to say 2 1/2" collectors and then into a 3" Y-pipe, would it do any good to go up to a 3 1/2" pipe after that?

Basically what I'm asking is: is there any point in increasing a pipe size after it's been choked down to a smaller size previously? I'm thinking not, but then again I tend to wonder whether the reduction in friction for the rest of the way back would help a bit.

What do you think, guys?

 

Not really. If anything, you can go down in size further back in the exhaust and not have any more restriction. The reason is the further the exhaust travels, the cooler it gets and the mass gets smaller, so there is less volume to flow. So no real benefit from staying the same let alone going bigger...

 

Sometimes its better to have a smaller pipe because it increases the velocity of flow which these motors like at higher RPMs.

 

are you figuring out a exhaust for the truck in your sig? if so i would think that setup would need to flow, if your manifolds are 2.5 inch i would open it up to 3" right at the collector, one thing i learned was anything that produces somewhat good horespower needs a good flowing exhaust. you dont wanna down size pipe anywhere in a setup. anytime you down size, it becomes a restriction and forces gases back towards the motor which in terms causes to much back pressure which chokes the motor..your exhaust is just as important as picking a cam

 

At the collector the pipes are only flowing exhaust from half the engine so I wouldn't think they'd need to be that big. I think the y-pipe would be a squeeze point in mine.

I'm really just asking for curiosity's sake. I see guys going to 4" pipes but I'm wondering if the Y piping ahead of that flows so well as to make it worthwhile. I guess that if it was a double 3" into a 4" single then the Y would be perfect.

The theory of cooling exhaust and smaller mass makes perfect sense. Thanks guys.