Stock Downpipe (Cat) and Front Pipe Question

[Myx]

I've read that the replacing the stock downpipe and frontpipe will allow our Civics to gain horsepower. The downpipe (cat) and front pipes that replace our stock ones are larger in diameter (2.5" to 3") than the stock one (2").

So, looking at the stock downpipe and front pipe, I notice a few things. The stock downpipe (cat) has a large opening and goes down to a smaller opening. Replacing the cat with an aftermarket one I can understand would help you gain horsepower. But why would a larger front pipe gain your horsepower?

I ask because the front pipe is the same diameter as the rest of the exhaust going back to the mufflers. I provided a pic of both the stock downpipe (cat) and front pipe. Look closely at the front pipe. Where is there a restriction? If the diameter of the front pipe is a restriction, so would the rest of the exhaust going back to the muffler as well.

1) So how it is that the front pipe is a 'restriction' and replacing it with a larger diameter front pipe yield horsepower gains?

2) How does going with a larger downpipe and frontpipe gain you horsepower when the rest of the exhaust is a smaller size? Why not go with the same size downpipe and front pipe as the stock exhaust? Or at least the front pipe since the innards of the stock cat can be a restriction?

A lot of folks here are a lot smarter than I so I am just asking a few questions. Some of this doesn't make sense though.

Edit:
I just looked at dyno runs of AFE aftermarket downpipe, front pipe and exhaust tested individually and they all added horsepower and torque. Below are the links.

AFE exhaust: 7whp (This was installed first with stock downpipe and frontpipe). HERE
AFE downpipe: 5whp (On lowend. Topend, they were experiencing hp loss due to hot intake temps/ignition timing being pulled. HERE
AFE frontpipe (Or Rear Downpipe they called it): 10whp. HERE
Blog Details on full exhaust setup: HERE

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[Shankmeyster]

Also remember that you won't need an aftermarket front pipe if you plan on buying almost any of the exhausts out now as most if not all are shipping with a front pipe as part of the exhaust. I think the big boost is on the downpipe and, if you don't upgrade that, then the front pipe/exhaust w/front pipe will net you a slight gain but not the amount you get off the downpipe. Then again it's a crapshoot whether you will be able to get the downpipe to work without a tuner to disable an O2. In the end the front pipe has only been tested by PRL so far as I know.

 

[360glitch]

The catalytic converter is the only true restriction in the stock exhaust if you are running the stock turbo. In this case the issue is the catalyst the exhaust gasses have to flow through, not the diameter.

 

[Shankmeyster]

Need to see a company come out with a cat delete downpipe in stock size and see what happens.

 

[Myx]

The catalytic converter is the only true restriction in the stock exhaust if you are running the stock turbo. In this case the issue is the catalyst the exhaust gasses have to flow through, not the diameter.

I am thinking the same actually. Tomorrow I may decide to go back to stock front pipe and stock exhaust. The only exception is a single exhaust to the muffler of choice. The deep, deep sounding 3" exhaust sounds great in general but my trek back from the track tonight reminded me that the days of this type of exhaust sound are behind me.

 

[17siturb0]

I think the Si tuned with the stock exhaust without a cat would make basically the same gains as one that’s full bolt ons. I’ll say this, most bolt ons are not pointless, definitely not pointless but they should not be prioritized. This platform makes the power from the tune itself, using higher octane fuel, or blends with E85. However, Vits turbo upgrade or big turbo, that’s where the 3 inch downpipe/front pipe and catback exhausts ( and other bolt ons ) will shine the most. Our stock turbo is the reason for all this. It’s tiny.

 

[Green82]

is that the catalytic converter on the front pipe? always thought those were under the car at least they were on all my cars i drove in the past

 

[Myx]

is that the catalytic converter on the front pipe? always thought those were under the car at least they were on all my cars i drove in the past

No! The cat converter is on the downpipe. The front pipe attaches to the downpipe.

 

[Green82]

No! The cat converter is on the downpipe. The front pipe attaches to the downpipe.

ahh thanks

 

[shadow]

The larger diameter front pipe helps with power even if the rest of the pipe going to the muffler is only 2". As exhaust gases leave the combustion chamber, it starts cooling down and thus decrease in volume. By the time it has exited the front pipe, it has cooled sufficiently and the 2" dia. stock pipe is enough for the reduced volume.

 

[5inn]

Has anyone found a dyno chart with just a front pipe change?

I'm wondering if there are any real gains from changing just the front pipe, while keeping the stock downpipe and stock exhaust.

To answer the inevitable question of 'why not change the dp too?'. Changing the downpipe is not legal in 'Street' class autocross, but changing the front pipe is.

 

[Myx]

Has anyone found a dyno chart with just a front pipe change?

I'm wondering if there are any real gains from changing just the front pipe, while keeping the stock downpipe and stock exhaust.

To answer the inevitable question of 'why not change the dp too?'. Changing the downpipe is not legal in 'Street' class autocross, but changing the front pipe is.

I have not seen any. Have been browsing the forum for a before front pipe installation and after front pipe install dyno to see the difference as well. What Shadow said about makes sense but I have seen nothing to back it up. I know of no-one that measured exhaust cooling from downpipe exit to front pipe exit to see if it 'cooled sufficiently'. I don't need to see this but if the info is out there, I'm all about learning and applying it.

 

[Aegean303]

So does the front pipe really make much of a difference? Or net any gain?

 

[Shankmeyster]

No one has tested it or any exhausts so far as we know.

 

[Boss240]

So has anyone figured out if its worth it to upgrade the front pipe and leave the stock down pipe in place?

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