Outer Limits 4x4 Board

Posted: **Sun Oct 16, 2005 11:34 pm**

Hi all, ive got a Rocsta(1.8ltr 4 engine petrol) and have bent up the exhaust a lil bit.

im thinking of going for a 2 inch straight through with muffler.
currently got 2inch -> muffler -> 1.5inch

im hearing conflicting stories saying if you go bigger;
1.)no problems just tune her right to the exhaust, nps
_or_
2.)you'll reduce backpressure which will fuk up the valves

now the exhaust im thinking about should be sweet as anyway, BUT can i go bigger, and does it effect engine life in any way?

(reason i wanna go bigger is just to ease up the breathing and a lil more go and a nice engine note wouldnt go astray)

Posted: **Sun Oct 16, 2005 11:38 pm**

I'll be fine. IMHO engines setup like what we have dont need back pressure. I doubt very much it'll stuff the valves, thats what the EGR is for.

Posted: **Mon Oct 17, 2005 12:19 am**

IMHO engines setup like what we have dont need back pressure.

Can you explain this??

Posted: **Mon Oct 17, 2005 12:24 am**

IMHO engines setup like what we have dont need back pressure.

Can you explain this??

Posted: **Mon Oct 17, 2005 1:04 am**

We dont run drag engines that have tuned length pipes etc that rely on pulsing in the headers to help pull the exhaust out, perhaps extractors at the most, but even then there is no need problems with big exhausts that still allow scavenging. As for the rest of the system, there is nothing to be gained by reducing/constricting exhaust flow from the engine apart from making it quiet.

The only engine I can think of that would like some back pressure is the Zook LJ50.

Posted: **Mon Oct 17, 2005 8:27 am**

Just go 2 1/2 inch and enjoy the sound

.....what eva sound a Rocsta makes

Just make sure it will fit above that bent cross member of yours

Posted: **Mon Oct 17, 2005 9:51 am**

yer..
i got a 2 1/4 zorst on my zook (x tractors - 2 1/4 - lukey muffler- 2 1/4 out)
sounds sweet. makes them rev alot smother and easyer! more power than standard! becasue it will let the engen breathe
Philby

Posted: **Mon Oct 17, 2005 9:53 am**

Back pressure helps with torque though.

LudaCris

Posted: **Mon Oct 17, 2005 10:02 am**

Yeah i'd go bigger. Look at it as a big air pump, the easier the air can escape the engine the better it will work. But quite often once you open up the exhaust the air is restricted on the intake side. Its still better IMO to have the exhaust running as free as possible.

Posted: **Mon Oct 17, 2005 11:44 am**

Back pressure helps with torque though.

wanna enlighten us ludacris?

Posted: **Mon Oct 17, 2005 11:21 pm**

We dont run drag engines that have tuned length pipes etc that rely on pulsing in the headers to help pull the exhaust out, perhaps extractors at the most, but even then there is no need problems with big exhausts that still allow scavenging. As for the rest of the system, there is nothing to be gained by reducing/constricting exhaust flow from the engine apart from making it quiet.

The only engine I can think of that would like some back pressure is the Zook LJ50.

Cheers, Thanks slunnie sounds interesting

Posted: **Mon Oct 17, 2005 11:22 pm**

With a naturally aspirated engine, backpressure is needed for some type of flow scavenging from the exhaust valves, or torque is lost. When revving high it will flow well, but if you have no torque it will never start moving.. lol

In a turbo application, backpressure is not a worry due to the turbo allowing exhaust to pass through according to its needs.

Most fatory exhausts are too small in diameter due to cost, emissions (both gas and noise) and to increase drivability.
Too large of a pipe on a naturally aspirated engine will reduce torque and reduce economy. You need to find a happy medium where torque and power is optimal

Posted: **Mon Oct 17, 2005 11:49 pm**

Look at it as a big air pump, the easier the air can escape the engine the better it will work.

that was deffinatly my impression when i was young however engines such as twostrokes suffer everything including power and burn out your exhaust ports.

i just wanted to know what the deal was EXACTLY interms of petrol 4 stroke car engines.

drag cars are a whole different story since they get serviced after every run up the quater and are totally worked for no exhausts etc.
the only reason they have pipes to start with is to keep the flames from burning anyone and everything around it heh

but thanks for the advice everyone
hehe Draven, im gonna either get another off a wrecked rocsta and keep old one as spare _or_ winch the old one back into postion.

Posted: **Mon Oct 17, 2005 11:58 pm**

fnukle wrote:
Look at it as a big air pump, the easier the air can escape the engine the better it will work.
that was deffinatly my impression when i was young however engines such as twostrokes suffer everything including power and burn out your exhaust ports.

i just wanted to know what the deal was EXACTLY interms of petrol 4 stroke car engines.

I wasnt talking about two strokes

You will find quite differing oppinions ever between muffler/exhaust shops....some same smaller for better back pressue otheres say larger for better flow. I dont think you will get a definative answer so you'll have to make the decision for your self.

Posted: **Mon Oct 17, 2005 11:59 pm**

2 strokes are a totally different ball game in the exhausts (expansion chambers), and a specialised field in itself. These things are critical and finnicky and it will have a big effect on power, power delivery and engine longevity.

4 stroke extractors if designed properly where all of the manifold pipes converge into a suitable sized flange still should not require backpressure apart from the area immediately after the convergance (which produces the scavenging effect).

If the 4-stroke just has headers, then the less backpressure the better for the engine.

Posted: **Tue Oct 18, 2005 12:22 am**

With a naturally aspirated engine, backpressure is needed for some type of flow scavenging from the exhaust valves, or torque is lost. When revving high it will flow well, but if you have no torque it will never start moving.

What hotforok is saying here is correct. There is an overlap with inlet and exhaust valves. When the exhaust valve is just about to close on the exhaust stroke the inlet valve starts to open and scavenges some exhaust due to some back pressure (or airflow restriction) in the exhaust and puts a bit more mix into the cylinder which helps with torque. (This sounds contradictory as the piston is still travelling up and almost at the top of it's stroke but things are happening so fast that it just works.
All this mainly applies to stock engines anyway as that's the way theyr'e designed.)However torque usually suffers at high revs because at high engine speeds there is too much exhaust restriction but there is usually too much set by manufacturers in the first place so going a bit bigger shouldn't make much differrence.

Posted: **Tue Oct 18, 2005 7:04 am**

JUST STICK THE EXHAUST ON AND STOP WORRYING ABOUT IT, ITS NOT A HIGH PROFORMANCE ENGINE AND WONT REALLY AFFECT IT........ IF ANY THING IT WILL GIVE IT A BIT MORE TOP END POWER ( LIKE 3KW) BECAUSE THE EXHAUST WILL FLOW OUT EASIER

Posted: **Tue Oct 18, 2005 6:31 pm**

If you go to large a pipe , the exhaust gas flow, slows down as it pores out of the headers, killing power. 2'' should be fine with a straight through muffler for your 1.8 ltr.

Posted: **Tue Oct 18, 2005 8:29 pm**

Go bigger exhaust and headers

I've got 4-1 lukey performance headers and full 2 1/4 inch exhaust on my feroza which is a 1.6
It has improved down low torque and has made it sound like a WRX

I wouldn't go any bigger then 2 1/4 inch

Thats my 2 cents worth.

Posted: **Tue Oct 18, 2005 8:50 pm**

HotFourOk wrote:With a naturally aspirated engine, backpressure is needed for some type of flow scavenging from the exhaust valves, or torque is lost. When revving high it will flow well, but if you have no torque it will never start moving.. lol

In a turbo application, backpressure is not a worry due to the turbo allowing exhaust to pass through according to its needs.

Most fatory exhausts are too small in diameter due to cost, emissions (both gas and noise) and to increase drivability.
Too large of a pipe on a naturally aspirated engine will reduce torque and reduce economy. You need to find a happy medium where torque and power is optimal

Now if only someone could tell this to all those wankers in their little rice burners with the big mufflers ! - as if a spoiler and a big muffler make a Hyundai into a race car !

Posted: **Tue Oct 18, 2005 9:48 pm**

I reckon... all these buzzboxes with huge exhausts have LESS power than the stock ones.. LOL

But it sounds cool

Posted: **Tue Oct 18, 2005 9:51 pm**

LOL saw the spoiler/5" zorst on a kia the other day, now thats gota be worse than an excel.

Any way to work out the optium ratio for 4wd/racing exhaust size by ne chance?

Posted: **Tue Oct 18, 2005 9:55 pm**

Didn't you know:

Buy ricer , Add 200kg of "aero" kit, 2000W of doof-doof, walk in exhaust, wish list stickers (incuding Type-R, Nismo, HKS, Turbo), lay seat down & fit 17" rims with 45 profile tyres where 13" 75 profiles go and you have an instant fullly sic racecar, hey bro.

PS I hate the wet-fart-in-tunnel sound of these sh!tboxes

Posted: **Tue Oct 18, 2005 11:55 pm**

i fully understand 2 strokes are totally different, i used to do motocross back in NZ.
My point was that in some cases bigger is not always better and was wondering if and how that applied to 4stroke 4x4 petrol engines.

If you go to large a pipe , the exhaust gas flow, slows down as it pores out of the headers, killing power. 2'' should be fine with a straight through muffler for your 1.8 ltr.

thank you very much, thats the kind of constructive answer im after :) and thats exactly what i'll get.

also the info to back up Zute's response

Some self-proclaimed engine gurus claim too large of an exhaust tube on a car can cause problems; engines need a certain amount of backpressure to run correctly.

Although the statement about not running too large of a tube is correct, the assumption about engines needing backpressure is not. A vehicle needs the lowest backpressure possible to produce the maximum power by keeping pumping losses low.

Too big of an exhaust pipe causes power loss, especially in low-end torque, because a big pipe has less exhaust stream velocity than a smaller pipe. Velocity is essential to get the best scavenging effect from tuned headers.

In simple terms, if the exhaust gas flow is kept high with good velocity, a vacuum can develop behind the closed exhaust valve allowing even better scavenging when the exhaust valve opens on the next exhaust cycle. Good scavenging is even more critical on valve overlap, the part of the four-stroke cycle where both the intake and exhaust valves are open.

Some basic exhaust pipe diameter guidelines are as follows:

1500cc-2000cc engines- 2 inch
2100cc-2500cc engines- 2.25 inch
2600cc-3000cc engines- 2.5 inch

taken from here

Posted: **Wed Oct 19, 2005 12:32 am**

Well said (or quoted) fnukle...

Heat also aids in flow scavenging as a hotter gas travels with a higher velocity, hence using a smaller pipe.
This is also one reason for people to heat wrap thier exhaust components, it keeps the gas hotter for longer and it flows faster. (Also less heat in engine bay)

There is no set rule for calculating pipe sizes as every application is different but your estimates are fairly close to most peoples recommendations.
Many people go through lots of different exhaust setups until they find which is right for thier engine and what they are using the car for.

Remebering this is only for NON-Turbo engines.

Posted: **Wed Oct 19, 2005 5:11 pm**

Its like this thread rolled into one cuddly ball.

Posted: **Wed Oct 19, 2005 10:50 pm**

Thanks HotFourOk
Wow i had no idea about the heat, thats awesum, heat wrapping will also keep the driveline a lil bit cooler aswell i suppose, mainly my gearbox heh.

thx for the info :)

edit:i supose the only problem with heat wrap is you cant tell if you have a ding in it

Posted: **Thu Oct 20, 2005 8:26 am**

Mainly extractors are heat wrapped to help with this.. In my Corolla the extractors couldn't use the factory heat shield, so i cooked an alternator coz of the excess heat before i realised to heat wrap them. Now its all well and good. And looks nice too

Wrapping any further down probably isnt beneficial, and if you scraped on rocks and stuff the wrap would just rip and get roughted i think. Also probably not the best if your going in mud and water and stuff.. it may keep the moisture in and cause rust.. just my 2c

Posted: **Fri Oct 21, 2005 8:02 pm**

fnukle wrote:Thanks HotFourOk
Wow i had no idea about the heat, thats awesum, heat wrapping will also keep the driveline a lil bit cooler aswell i suppose, mainly my gearbox heh.

thx for the info

edit:i supose the only problem with heat wrap is you cant tell if you have a ding in it

if got a 13bt and i wasnt sure if 2.5" was enough or 2.75" or 3" iam going for 3" i did alot off asking question in here and exhaust shops.one guy said 90%off the liter/dia-inch is a good rule off thumb for turbo diesels.well thats my input anyway have fun making up your mind

Posted: **Fri Oct 21, 2005 9:22 pm**

frp88 wrote: one guy said 90%off the liter/dia-inch is a good rule off thumb for turbo diesels.well thats my input anyway have fun making up your mind

This is a good reason why you shouldn't believe everything people tell you. By this rule a 1.5L Turbo diesel (VW golf, etc) should have a 1.3" exhaust. They come with a bigger exhaust than that from the factory!!!

Outer Limits 4x4 Board

bigger exhaust, more wear?

bigger exhaust, more wear?

exhaust