question about the downpipe

[erotomania]

so I've noticed that the efi downpipe has a wider diameter at the flexpipe than the carb'd one, but the efi one is constricted by the smaller diameter in between the collecter and the flexpipe anyway

so my question is this. What's the point of having a larger diameter pipe after a smaller one when the flow is only going to be constricted later on in the exhaust system anyway?

what if you were to have a constant diameter all the way through would that improve exhaust velocity or just restrict flow?

hope this makes sense

[A20A1]

A larger diameter would increase FLOW, but lose VELOCITY.

Don't measure the outside of the pipe, it is deceiving. Use the inner diameter.

The stock pipe has two layers, the outer pipe diamter is more like a heat sheild perhaps... the inner pipe diameter is the actual pipe diameter. I don't know how far the two pipe layers extend inside the downpipe but I notticed the two layers when I cut the down pipe in half.

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Some things like EXHAUST DAMS or ANTI-REVERSION DAMS where a smaller pipe opens up into a larger pipe to help hold off exhaust going back up the pipe. The same reason they say it's not always better to port the exhaust port to match the port on the header flange. The added lip from the smaller exhaust port on the head acts as a dam.

Also there are restrictions like EXHAUST VENTURIES to help speed up flow but where and how effective they are in a stock system is beyond me right now. I do plan to incorporate a 1.75" VENTURI in my next 4-1 header, though the exhaust system will be 2" from the header back. I'm pretty sure though the venturi is most effective when used with and placed before an open header collector.

One more thing is an EXHSUST VALVE or EXHAUST GATE these are valves inserted inbetween the exhaust flange and is supposed to help increase power at a certain rpm by adjusting the amount of exhaust flowing through the system. In my opinion if it was that effective it would be installed on more cars but I haven't heard much about it... I assume these valves are also the same as EXHAUST BRAKE systems used on large diesel trucks, busses, etc and they are not for increaseing power... rather they are there to cut power.

Here is a little thread explaining exhaust flow.
https://www.3geez.com/showthread.php?t=352

Here is a little thread on header construction.
https://www.3geez.com/showthread.php?t=6466

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[smufguy]

i dont know if you have actually taken the flex pipe apart, behind all that metal weave, there is a rippled flexy pipe. its diameter is what is under consideration, not the flex pipe (the outside) itself since the outside diameter changes with the distance between the weave on the outside and the rippled inner tube. different companies make different kinds of flex pipes.

[erotomania]

so it looks like in making exhaust systems for a particular engine they try to strike the right balance between flow and velocity for that particular engine, rather than just maximize one or the other?

yes i have noticed the flexpipe underneath the metal weave... The specimen I got off the carb'd car has a pretty worn out metal weave so I can see the pipe underneath it pretty good. But it still looks like the efi one is a bit larger regardless of the outer layer (I'm pretty certain but not 100% sure)

let's say I were to cut off the section from the header flange to right after the collector.. then weld that on the carb'd downpipe with the smaller diameter...

would that improve performance by increasing velocity, or would that just do harm by gases backing up into the engine and also cutting down flow?

[A20A1]

I wouldn't go smaller then the stock pipe... which is around 1.75"

[smufguy]

the thing is, you are a 4cyl 2.0L motor with no engine mods. As in high compression, higher revving engine or anything. you do not have forced induction which will be resulting in higher and greater exhaust gas mass. You do not have a V8 with a 5.0L pushing 500whp either. Thus talking about performance in corralation to switching stock exhaust pipes is irrelevant.

You will only see a difference in power in relation to exhaust pipe sizing if you have a better flowing engine head, as in an aftermarket performance exhaust manifold, and matched up exhaust piping (for ex. DC sports header and 2.25" cat and pipe).

or the better assesment would be having two cars with a turbo and one having a 2" exhaust piping and a 3" exhaust piping, the one with a 3" exhaust piping will yield more power since the turbo is free-er to push more exhaust gases sooner and hence spooling up faster with a bigger exhaust pipe. But DO NOT USE THIS CORROLATION to a NA motor which is anything but stock. for our stock motor, with aftermarket cams, cam gear, fuel injectors and pump, and headers and intake, a 2.25" is perfectly a good sizing. if you have N2O maybe you can get away with 2.5"s but it all depends on what is actually being done to the motor.


PS: the exhaust performance notes are all been discussed in previous threads, so if you are looking for something particular, its better if you run a search first.

[lostforawhile]

the only thing honda wanted to do with the stock exaust is make it as quiet as possible,i am also into brittish imports and the 2.0 is a very common size for many of them over the years, yet many of them use larger pipes then honda does. many people run two inch pipes on civics with the 1.5 and have good results, I have been running a two inch pipe for years on my car and have had good results with no apparent drop in low end power, i do have internal engine mods but this is not too bad of a size for a stock motor.