Exhaust size

[LukasGTi]

Hi guys, I´ve just bought custom made exhaust "2.25" inox, 2 mufflers, very nicely done and for good price. But after searching some forums i don t know if this size is for my engine the best, engine is pretty stock just cold air intake. I know that 2" exhaust would suit stock engine best but as I said, it was a bargain.. It´s loud af but i don t care about this, i just don t want to loose Hp.

What do you think ? Sell it or keep it ?

Anyway it is 100% better than the 46mm stock one.

[LukasGTi]

Anyone ?

[BUMMERS]

What engine? Assuming it's the 1.6 16valve engine going off your username I wouldn't worry about loss of backpressure. The noise will probably make it feel faster anyway!

[KamRacing]

are you happy? if so keep it.
You will loose exhaust gas speed and maybe a little bit of power but if you are not worried then theres no point in selling.

[LukasGTi]

Yes it´s 16v, as I said before, I care about hp, not noise. From my subjective feeling, in full thorttle I tried to ignore the sound and just watch speedo and rev gauge and I tried to compare it with the old one. I felt it like the car is a little bit slower in low rpm´s, but is revving faster +5k rpm to the limiter than before. But it´s just a feeling.

I would really like to see some dyno sheets with similiar setups before/after that, but I can not find anything.

From my own research in exhaust gases I found out that backpressure is nothing to worry about, the important thing is gas velocity. I have found some dyno sheets from different cars (but 1600ccm), and hp and torque increased across whole spectrum.

[Chipwizards]

Use a manifold with a 2" final collector outlet then make a tapered section with a 3.5 degree half-angle (7 degree total angle) to meet up with your system the whatever distance downstream that is needed to make them meet.

DON'T deviate from the angle suggested, it's the optimum, but much steeper and it is worse than having a 90 degree step.

Do that it and it should be the best of both worlds.

Like this:-

http://www.ebay.com/itm/NASCAR-Washbon-Stainless-Steel-Header-Tri-Y-Collector-10069-/311659543354

[axsaxoman]

you really want to understand inlet +ex sizes+lengths
you need to do some reading
lots of good books -some may be a little old --but physics has not changed and movement of gas through an engine is all fluid dynamics .

all you need to know about modifying a 4stroke engine
a good cheap book thats not too involved for normal mortals
four stroke performance tuning by A graham bell

haynes book ISBN 0-85429-275-6

[LukasGTi]

Well I have done some research. Really read a lot on internet forums.

I found one great thread on some honda forum, about exhaust systems. I have seen a lot of dyno charts before/ after bigger exhaust, unfortunately not saxos, but now I know how it works.

Backpressure is totally useless and it´s unwanted. Best backpressure is ZERO backpressure, all that matters is gas flow velocity and it´s temperature. So I know that am OK with 2.25", maybe I have lost a little bit of low end torque but I definitely haven´t lost anything at the top end. I have propably gained something. So I think that 2" system are good for increasing complete spectrum a bit, but with 2.25" you will gain slighty more in the top end.

If anybody wants a link to that thread I can post it here.

[LukasGTi]

Actually only thing where backpressure helps little bit is fuel economy, but for performance is totally useless.

[axsaxoman]

please explain in detail how restricting the flow of your exhaust ,to cause "back pressure" and therefore making it use more of the power to push gas down the exhaust system is good for economy
smaller dia ex-pipe will increase gas speed --at the point where it causes a measurable"back pressure" ,as you call it it, is reducing your economy

do some proper reading and not just forum post trolling if you want to understand the subject +then you can make proper judgements backed up by physics

[LukasGTi]

Well I can' t explain it in detail, because my english is not on that level. Fuel economy is better with little backpressure, because you don't need so much fresh oxygen for combustion, because little bit of exhaust gases is returned back to the combustion chamber (thx to backpressure) , and then yo do not need so much fuel to burn effectively. It ' s pretty logic if you think about that.

[axsaxoman]

what a load of nonsense.
I will stop replying to this as you obviously know a lot more about this subject than I do
i,ve only been doing race tuning for 40 years

[KamRacing]

forget the backpressure. Its not the primary focus.
No-one wants backpressure. The best scenario would be that gases exit the exhaust as fast as possible at all engine speeds but this is impossible
No exhaust gases are pulled back into the combustion chamber. The pistons are pushing it out.
What you want is a high gas speed. The trick is to optimise this for the engine and the driving characteristics you want as a driver.
High gas speed pulls gases away from the engine with an effect called scavenging. It also helps the cylinders fill with clean air from the inlet.
As gas speed increases you get the thing you dont want - backpressure. With smaller diameter exhausts you get this bottleneck at higher rpm, but the exhaust is tuned to give better low and mid range.
Go bigger and you have low gas speed at low rpm so the car can take a bit more to pick up. At high rpm the gas speed is more optimal, pushing power up the rev range.

Its obviously is more complicated than that but its a good start. The pulse tuning is done by the manifold so thats another critical part to look at.

[LukasGTi]

I am just writting my own opinion.. You asked me to explain that so I did. Not telling that I know more than anybody. Stop doing a drama, It s just a theory. It's starting to be OP now.. My actual question was If 2.25" is ok, what do you think about that?

[KamRacing]

You have too big a diameter exhaust in my opinion.

[LukasGTi]

forget the backpressure. Its not the primary focus.
No-one wants backpressure. The best scenario would be that gases exit the exhaust as fast as possible at all engine speeds but this is impossible
No exhaust gases are pulled back into the combustion chamber. The pistons are pushing it out.
What you want is a high gas speed. The trick is to optimise this for the engine and the driving characteristics you want as a driver.
High gas speed pulls gases away from the engine with an effect called scavenging. It also helps the cylinders fill with clean air from the inlet.
As gas speed increases you get the thing you dont want - backpressure. With smaller diameter exhausts you get this bottleneck at higher rpm, but the exhaust is tuned to give better low and mid range.
Go bigger and you have low gas speed at low rpm so the car can take a bit more to pick up. At high rpm the gas speed is more optimal, pushing power up the rev range.

Its obviously is more complicated than that but its a good start. The pulse tuning is done by the manifold so thats another critical part to look at.
Thanks for this reply! Agree with everything.. that fuel economy part may be bullshit if i thunk deeper about it.

[LukasGTi]

Think that 2" pipe would be better at low and mid range, and 2,25" in top end. Just my opinion.

[LukasGTi]

I have4-2-1 raceland

[Chipwizards]

What do we know John?
I bet you have you lost count of championships won like I have?
'Scientific design of exhaust and intake systems' is a good read and of course there's Prof Blair's works and the Lotus simulation package which is really good if you've got the time to run the numbers.