Optimum exhaust diameter

[Hellraiser]

Hey guys,

I am unsure which exhaust diameter is ideal for our little D14 engines. From what I've gathered so far, it seems as the majority of people with aftermarket exhaust tend to use 50 mms/2 inches or 55 mms.
Has anyone done back-to-back testing which of the two is better in terms of peak power and/or the overall power curve of the engine?
In the future, I am going to fit a custom exhaust to my Civic. So before buying the silencers, tubes etc. I'd like to have some information.

Regards

[saxophonias]

Ι have used 50mm and 55mm. If you go for extended mods you could go for 55mm tube without fear.
These days my exhaust set up is 4-1 dc header, 55mm tube, ek9jdm cat, open mid pipe and dc2 back box. Quite happy with it, torquy from low revs and almost stock sound (no problem with MOT and police etc.) I couldn't imagine how much of an improvement in mid and low revs the dc2 back box could be over an open aftermarket one. As for high end power maybe there are some losses there but nothing major comparing to the low and mid gains.

[hondaNickx]

I've used several different types 50mm , 55mm and 60mm .To be honest 50 or 55mm would be the best .
60mm if you're going to turbocharge the D14,For D14 N/A form 50mm is more then enough.
EK9 and ITR exhaust is 57mm outside and they are around 185-190hp with 1.6 to 1.8L .
So a 1.4L that is going to get 130-140hp with a vtec head and some tuning would work fine with a 50mm exhaust.
That is ,if the exhaust is made without any bottle necks or restrictions.

[Hellraiser]

Thanks for the input so far! But have you found any particular difference when comparing the 50mm to the 55mm exhaust system?
I'm generally learning towards a 55mm exhaust system. BUT, as far as I know, must headers for the D series engines have a 50mm collector. Hence my following train of thought: if the exhaust gases flow through the aforementioned 50mm collector; wouldn't a larger diameter than the collector size of the header reduce the velocity, therefore reducing power?
As I am not an engineer or automotive technician I might overlook some important factors here. Can anyone chime in on this?

[hondaNickx]

Thanks for the input so far! But have you found any particular difference when comparing the 50mm to the 55mm exhaust system?
I'm generally learning towards a 55mm exhaust system. BUT, as far as I know, must headers for the D series engines have a 50mm collector. Hence my following train of thought: if the exhaust gases flow through the aforementioned 50mm collector; wouldn't a larger diameter than the collector size of the header reduce the velocity, therefore reducing power?
As I am not an engineer or automotive technician I might overlook some important factors here. Can anyone chime in on this?

I haven't found any big differences in hp , but i do think the torque is better with a smaller pipe.Although i don't have a dyno comparison of the 2 exhaust sizes.
And you're right ,All oem systems taper down in diameter the more they go to the back of the car.This is because the exhaust gasses cool down and take in less place.
Mostly honda krimps down the size where the rear muffler is connected to the mid pipe.EK9 and ITR exhaust systems are 57mm outside diameter and 55mm inside diameter.
At the rear muffler they taper down to aprox 50mm inside diameter.An exhaust that tapers gradually from the front to the back would be perfect but very difficult to build.
All the D-series exhaust manifolds i ever had all had a 50mm exhaust diameter.