Best exhaust diameter for performance?

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FH-5

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Hi all,

It's time I need to get myself a new exhaust system made up.

So I need to know from all you Micra experts what is the best exhaust diameter in terms of performance for my 1.3 Si? I got 104 BHP so what would give the best flow and performance?

I have a stainless steel 4-2-1 manifold fitted already from Matt Humphris, in other words I have no cats :p. Once I know from all of you what the best exhaust diameter is, I will have one made up from the 4-2-1 manifold downpipe all the way to back box (just a straight through pipe :p).

Thanks as always in advance guys :grinning:
 
I've not had to think about exhaust diameters in a while so forgive me if I am wrong but the diameter of your 4-2-1 downpipe all the way back should be the "best". If you have varying sizes in exhaust diameter then the norrowest part will be the most restrictive part. If you had a 4" down pipe 1" mid section and a 4" back box then your exhaust would have more losses through the 1" section. (Of course that exhaust sytem is just for example)

So if the outlet of your 4-2-1 is 2" then I'd go with 2" cross section all the way through... Remember your tip has to be 2" or above too.

On a side note your manifold design is important too. If I remember at 4-2-1 gives more power through the rev range so is best for street / urban driving but a 4-1 manifold gives a higher top end power? I think thats right so if you want to rev mega hard the manifold design also plays a part BUT not so significantly I would recommend you change it.

Pepper
 
You need to make the exhaust bore go smaller at the end. Because hot exhaust gasses cool down at the end of the exhaust wich also means they loose volume so they slow down in a pipe that is to big. If you make the pipe smaller towards the end it will accelerate those gases out of the exhaust :) This is known as the Venturi efect.

btw didnt you buy an awsome amazing super-duper NISMO exhaust?
 
Or put simply big exhaust no back pressure, no low down power, and sounds like a big fart on a little engine
I'd say around 2.5" would be best diameter
 
2.5" is to big. That would be cool if he was running 200hp :) Id say 2" and a slightly smaller backbox. 2" should be good up to 150hp depending on the design and number of bends.
 
Maybe, but I've never had trouble with a drop in back pressure at 2.5", I run 4.5 on my turbo car and 5 at the exit
 
Im runing a Walker original replacement exhaust on mine. Its slightly oversized i think (outside diameter is around 43mm) and it works for now. My car is dynoed at 90hp running a stock unmoded exhaust manifold etc :)
 
I don't believe anything larger than 2" will yield any gains other than a possible increase in system weight and perhaps noise levels depending on choice of muffler.

Dynos will vary but we also ran at surrey rolling road FH-5, so figures are as close as comparable that you can get other than running back to back. We have a 4-2-1 with 2" collector and 2" system and even with 264deg cams, 52deg overlap and throtlebodies, (which most people will tell you immediately loses you low end torque), we had very similar torque output until passing around 4250rpm).
 
Mine runs the full matt Humpris 2" system with janspeed decat silencer with the janspeed 4 branch and is unbelievably fast (not quite but makes 118 bhp)doesn't seem to give any resistance as max power is at max rpm
 
2" dont forget you need a bit of back pressure on a n/a engine which will be given in the bends and even on the bits of straight pipe anyway. but yeah anything bigger than 2" is just a bit daft. on a turbo car is doesn't really matter cause back pressure is a restriction on them thats why big gains can be had just from doing the exhaust on most turbo stuff.
 
sr20bud said:
2" dont forget you need a bit of back pressure on a n/a engine which will be given in the bends and even on the bits of straight pipe anyway. but yeah anything bigger than 2" is just a bit daft. on a turbo car is doesn't really matter cause back pressure is a restriction on them thats why big gains can be had just from doing the exhaust on most turbo stuff.
Click to expand...

Backpressure in itself is a bit of a myth, think about it in terms of exhaust velocity and things will become a lot clearer for both N/A and turbo alike...
 
Wow! Awesome replies and advice form everyone thanks :grinning:! So I should just match my 4-2-1 manifold diameter when making new exhaust system? And yes I have that Nismo back box from Japan just arrived is still in customs, here is a picture of it's diameter (4.5CM is 1.77 Inches):

197983666.jpg


Ok so shall I match the 4-2-1 manifold diameter for rest of exhaust size, but what about if my back box pictured above is bigger or smaller diameter than mid pipe and manifold? I'm confused :(
 
the stock system has a couple of areas where it drops to 33mm dia bore fwn and it seems to handle the CGA 85hp ok.
45mm bore pipe has twice the area of that, so should be more than enough imo :)
 
Ok that is fine, the whole middle section from 421 downpipe to back box will be a straight through pipe to match diameter of manifold downpipe.

But if my backbox is smaller diameter to middles pipe and manifold is it ok? Also the backbox exit pipe is 90mm, that does not matter does it?

Thanks :grinning:
 
All you need is a good flowing system. As proved from EBBDUDE who created a full system at 40mm there wasnt much gains to be had
 
EBBDude made a 2inch exhaust.

FH-5 i would do a 2inch midsection and just put your NISMO exhaust at the end of it to keep the velocity of the gasses up!
 
Ok thanks. I will do that - just a 2" mid section and put the 1.77" back box on if that will "keep the velocity of gasses up". Or I may match the diameter of 421 downpipe, I think it is around 2" anyway. Anyone elso feel free to add any advice :p.

Thanks very much everyone!
 
Mandrel of course, matching the manifold. But I don't know if my backbox is mandrel? Anyone know any cheap mandrel bent exhaust places in or around London (even Birmingham as I'm up there many times a month), just need mid pipe connecting from downpipe to backbox.

Thanks
 
The idea is that the bore gets slightly smaller towards the end....so your setup would be great. I did have a chance of looking at a Spoon exhaust on my friends integra. The bends are actually a bigger bore than the straight bits to keep the flow up but the backbox is slightly smaller bore tho ;)
 
If you look at the backbox photo the minimum diameter is at the flange, regardless it will still be fine. Most of the gains are seen in the manifold, until you start pushing things further you're not going to need anything radical, besides no-one on here pushes things anywhere near enough to warrant anything major.

Come spring time next year we'll see what a 2" system will support, so I will bookmark this thread for future reference along with one from back in 2007 when people were claiming that 150bhp could be achieved from an N/A CG13DE for £1500 ;)
 
The backbox pictured above is mandrel formed bends BUT if you look at the mounting flange you can see the pipe is necked down to a smaller diameter!

But it isn't gonna be a compromise at 100hp imo

If it were me I'd just use the size (45mm) that matches the backbox but get the necked down section + flange replaced for 45mm flange. Also you want t'other end to blend smoothly to the same diameter as downpipe. You don't want any steps in the system because steps cause turbulence.

My car has rover k series 4 branch reworked to fit, with xr2 crossflow downpipes. The downpipes exit is 45mm. Centre section is 50mm with mandrel bends also cherry bomb centre and tailboxes. 65mm tip.

The exhaust is far from ideal, but not bad really!

My car makes 98.1hp

Cg13
Cga3 inlet
Sr20 throttle body
Qg injectors
Ga16 airbox
Cg10 ecu
Lightened flywheel

Next up Iam trying cams and higher compression :D
 
Ok, I measured the 421 manifold (pictured below) primary pipes on my K11 and it turns out they are smaller than 2" at 4cm which is 1.57 inches!

dscf1397f.th.jpg


So should I still have a slightly larger bore mid section at 2 inches rather than sticking to size of manifold pipes which are 1.57 inches? In other workd, it would go like this:

Maniflod downpipe 1.57" - Centre pipe 2" - Back box 1.77"

fwn???
 
airmike said:
Measure the pipe that connects to the cat. Where 2 branches join into 1.
Click to expand...

I know that is what I want to do but cannot get car up or go under car to do that. Will try take it to garage and measure the Y pipe tomorrow.
 
Low Rider said:
http://www.micra.org.uk/threads/20068-Hitting-150bhp?highlight=150bhp

Thread from the dead lol ;)
Click to expand...

That takes me back haha. I wouldnt mind knowing if you manage it / what you actually make for your £1500 Low Rider. (you buy new though dont you?)

Mine has made 150+ without breaching the £1500 (engine cost) but then its not running N/A...so doubt Ed would give me £50 :D

Btw iirc the exhaust on mine is 1 3/4 inch diam with cats and a silencer included, pretty sure its not any bigger but i can double check, just incase im making it up.
 
back pressure is a total myth, I went from a stock system to a 2.5" and noticed gains everywhere, I then went up to a 3" and noticed even more gains this was with a 3" manifold too, albeit it was on a different engine but the principle is the same.

I would go with a full 2.5" catback on mine if I was given the chance with a 2" downpipe, I would be confident it would see gains through the entire rev range.
 
TheSam said:
back pressure is a total myth, I went from a stock system to a 2.5" and noticed gains everywhere, I then went up to a 3" and noticed even more gains this was with a 3" manifold too, albeit it was on a different engine but the principle is the same.

I would go with a full 2.5" catback on mine if I was given the chance with a 2" downpipe, I would be confident it would see gains through the entire rev range.
Click to expand...

Awesome, will go for 2.5" system. I don't have any cats mate so just a straight through pipe from manifold downpipe to back box. What are your thoughts on back box being smaller diameter TheSam?
 
the back box will be a restriction, theres no point putting it on if your getting a 2.5 midsection made up get a 2.5" back box too, it you want to keep the back box then get the rest of the exhaust made up to match it.
 
Ok, can you get backboxes custom made up though? Do you know any good mandrel-bent exhaust places in or around London?

Thanks very much :p
 
TheSam said:
back pressure is a total myth, I went from a stock system to a 2.5" and noticed gains everywhere, I then went up to a 3" and noticed even more gains this was with a 3" manifold too, albeit it was on a different engine but the principle is the same.

I would go with a full 2.5" catback on mine if I was given the chance with a 2" downpipe, I would be confident it would see gains through the entire rev range.
Click to expand...



Everything you just wrote is against common knowledge of engine and exhaust building. You butt dyno must be out of calibration....
 
What should I do airmike?! HELP :laugh:!

Maniflod downpipe 1.57" - Centre pipe 2" - Nismo Back box 1.77" OR Maniflod downpipe 1.57" - Centre pipe 2" - New Back box 2" OR SOMETHING ELSE. PLEASE ADVISE!
 
yet another MSC concensus lol :p you,re best to do some research FH (and bear in mind that an option for all out top-end hp may be different to an option for a daily)
 
I allready explained it a few times in this tread allready. You manifold downpipe isnt 1.57" that are your primaries. They are smaller because then need to extract gasses from 1 cyl only. Your downpipe will be around 2". Then you exhaust should go slightly smaller towards the end. Not because of backpressure but because of EXHAUST GAS VELOCITY. As the gasses cool they become smaller in volume hence no more need for a big pipe. In fact a pipe that is to big will cause the gases to slow down. Using a smaller pipe will take advantage of Venturi efect (google it!) and keep the velocity up. Same thing but in reverse is used on the intake side. Big intake, slightly smaller TB, smaller runners and even smaller valves (not talking about a stock micra sistem but a better sistem that is factory tuned to be the best for example one of a Integra type r).
 
solarice said:
That takes me back haha. I wouldnt mind knowing if you manage it / what you actually make for your £1500 Low Rider. (you buy new though dont you?)

Mine has made 150+ without breaching the £1500 (engine cost) but then its not running N/A...so doubt Ed would give me £50 :D

Btw iirc the exhaust on mine is 1 3/4 inch diam with cats and a silencer included, pretty sure its not any bigger but i can double check, just incase im making it up.
Click to expand...

Hahaha, this thread is cool.........lots of views and plenty of discussion :)

We should be able to top 150bhp but NOT for £1500 and cost is the whole package, (blew that much way back), and I certainly wont be making my exhaust system any smaller ;)
 
Ok this is final, I am getting from manifold to back box 2" straight through pipe. Then back box will remain at 1.77". Case closed. Would I need a silencer or will sound be OK?
 
Low Rider said:
Hahaha, this thread is cool.........lots of views and plenty of discussion :)

We'll top 150bhp but NOT for £1500 and cost is the whole package, (blew that much way back) ;)
Click to expand...

LoL, ah right gotcha.

Tbh its gonna be near on impossible to get a 100% this what you should do...for what its worth the current average (based on thread responses) is looking at near 2" :D
 
Ha ha. 2" it is then going into slightly smaller diameter back box. Would I need a silencer or will sound be OK with 2" straight through pipe (no cats :p)?
 
Depends on you rear silencer really. If its in good condition then it will be ok but if the "sound deadning" or the filling (whatever its called) is burned through it will be loud :)
 
FH-5 said:
Ha ha. 2" it is then going into slightly smaller diameter back box. Would I need a silencer or will sound be OK with 2" straight through pipe (no cats :p)?
Click to expand...

Im guessing you'll hear it lol, you could always test it and get a silencer fitted later if you need to.
 
5 years ago on the AOC everyone would have told you a 2" catback was the biggest you should go for when having an exhaust made, any bigger and you would loose 'back pressure' now people will tell you not to go any smaller than 2.5" its been proven on several of our club dyno days the few people that ran with 3" exhausts made great figures on near stock engines compared to heavily modified engines that had smaller exhausts, engines love to breath.

I had the generic Ebay stainless manifold on my previous car which too over from a jap manifold that came on it, it was then replaced with this

DSC_0035.jpg

DSC_0031.jpg


you didnt have to be a genious to feel the difference in the car after fitting it.

When I upgraded from the 2.5" catback to the 3" catback it felt even better.

IMAG0479.jpg


real world experience > internet info
 
Im guessing astra so its either a 1.6 or a 2.0 which proves my point and also yours. Bigger engine---->more gasses----->bigger exhaust needed.
 
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