exhaust manifold design question . . .

[mikedc]

Why do OEM cast iron manifolds neck down so small, so quickly after leaving the cylinder heads?

Let's say a hypothetical car's main (dual) exhaust pipes are 2.5" diameter. Why not shape the iron manifolds to have a 3" collector opening at the bottom, and then reduce the exhaust pipe size from 3" down to 2.5" a bit farther down (perhaps 1 foot)? It seems like that would give the exhaust system a far less brutal bottleneck. It would mimic the overall stepping-down effect of a set of headers better.

Granted, what I'm proposing would cost the OEMs a couple more dollars per-car. But uncorking the exhaust manifold area is very low-hanging fruit for a production drivetrain. Wouldn't that be worth doing, at least on hi-po models?


I know, there is a penalty for using an exhaust pipe any larger than necessary. It kills the gases' velocity and hurts the engine's low/mid torque. But there's just one problem with that argument here - the hot rod world is full of cars running 3" exhaust systems the entire length of the car. It's pretty clear that going 1 pipe size bigger than the OEM system is not enough to cause a practical drawback.

[Rod]

the manufactures have done that look at an LS1 exhaust and then look at an LS7 exhaust, GM tripled the opening size

[Zspoiler]

probably for emissions .gas mileage and packaging for a number of chassis are a number of reasons ,There have been a number of performance factory exhaust manifolds over the years as well.

[67-LS1]

The way I understand manifolds and headers is as part of a “system”. When exhaust gas leaves the side of the head it’s as hot and compressed as it’s going to be in the exhaust system. As it travels down the system it expands as it cools. As it cools it also slows down. So a system would benefit more from a larger pipe system more so than a larger manifold.
A “manifold” gathers all the gases together and directs them to the pipes with not a whole lot of emphasis on maximum flow.
A “header” on the other hand is designed to make the individual cylinder pulses arrive at the collector at different times. As they travel down the individual tubes the gas is cooling and expanding and when it gets to the collector and the exhaust system the temp and velocity drop even more.
I’ve not see the manifold from an LS7 but it’s probably way better than a standard truck LS log type manifold but still not as good as a header. But headers are not near as durable as cast manifolds so the factories aren’t keen on them.

[CamaroAJ]

The way I understand manifolds and headers is as part of a “system”. When exhaust gas leaves the side of the head it’s as hot and compressed as it’s going to be in the exhaust system. As it travels down the system it expands as it cools. As it cools it also slows down. So a system would benefit more from a larger pipe system more so than a larger manifold.
A “manifold” gathers all the gases together and directs them to the pipes with not a whole lot of emphasis on maximum flow.
A “header” on the other hand is designed to make the individual cylinder pulses arrive at the collector at different times. As they travel down the individual tubes the gas is cooling and expanding and when it gets to the collector and the exhaust system the temp and velocity drop even more.
I’ve not see the manifold from an LS7 but it’s probably way better than a standard truck LS log type manifold but still not as good as a header. But headers are not near as durable as cast manifolds so the factories aren’t keen on them.

gases expand when they heat up, not when they cool.

[mikedc]

I'm thinking of cases like this. Would it have killed the OEMs to make the manifold's collector outlet an extra 1/2" bigger diameter? FFS, at least use a notch bigger exhaust pipe tube for the first 5-10 inches.

I know they weren't gonna do it on a station wagon. But the OEMs have sold tens of thousands of sporty hi-po cars with exhausts choked that severely. It seems like a matter of low-hanging fruit. They sometimes did spend the money to tool-up better versions of the iron manifolds that flowed a couple extra HP. But even those typically necked down into the same tiny collector outlets.

https://static1.pt-content.com/images/pt/2020/12/uhttps3A2F2Fsthotrodcom2Fuploads2Fsites2-1.jpg


Look at the difference when it's a custom-built header to do the same job. What jumps right out at you is not the header's primary diameters or tuned lengths. It's the gigantic increase in total airflow past the first few inches.

https://static1.pt-content.com/images/pt/2020/12/uhttp3A2F2Fwwwonallcylinderscom2Fwpconte-1.jpg

[dhutton]

Fifth gen Z28 manifolds are a nice design. Stainless tri-y. I’ve used them a couple of times where space was tight.
182200182201

They have similar manifolds for the LT.

Don

[pitts64]

I noticed Harleys have a step down choke right were the exhaust leaves the manifold then it gets big again. My motor cycle machinest buddy said this adds a lot of torque..