Collector size and exhaust size

[dudeling7]

hey guys jsut have a problem im uncertain about.

Im looking at getting some different headers for my car. at the moment the exhaust is xforce 4-1 (does anyone know collector size), high flow cat, 2.25 exhaust.

now im just wondering if i got a new header with a 2.5 inch collector would my cat back be a large bottle neck and restrict the headers from their potential as they are only 2.25 inch and my cat is also 2.25 inch?

what is the best sizing combination?

1. To have everything the same size as the collector?
2. To have the cat and b pipe larger then collector?

Thanks

[preludacris]

i think xforce collectors are 2 inch. mine was. just cut ur current collector off, and weld a 2.25 inch one. itll flow heaps better.

[dudeling7]

only 2 inch? really? does anyone with xforce header know collecter size for the 4-1?

did u add the 2.25 to youres preludacris? was the gain big? thanks.

[dudeling7]

i think xforce collectors are 2 inch. mine was. just cut ur current collector off, and weld a 2.25 inch one. itll flow heaps better.



Also does a 2.5 inch collector with 2.25 inch catback perform better then

a 2.25 collector and 2.25 inch catback?

or will there be no difference as the 2.5 inch collector is bottlenecked by the smaller exhaust

sorry for being confusing if it is lol

[BlitZ]

2.5 might be abit big for a b18b and might lose some power if its std

[preludacris]

i added a 2.5 inch collector

2.25 should suit yours better. 2.5 u mite see some gains almost at redline, low down would be a biatch.

i've heard having a slightly larger cat than the piping helps it flow better. Can anybody confirm this?

[Mst_Mugen]

that is correct... but it also depends on wat other mods you have... NA cars require a small degree of back pressure.... have a 2" catback, will mean that on a general basis, you should get a cat that is at least a quarter inch larger to decrease the restriction of airflow..

[twilightprotege]

so wrong. backpressure = evil!

what NA cars need is exhaust velocity. you want an exhaust JUST big enough to flow all of your exhaust gas at your chosen rpm limit. and smaller and you'll get backpressure and kill (big time) power.

anyway, after the header you only lose power, so spend the money to get the header right because that's where all the power is made.

most high power NA cars can get away with 1.75" primaries, 2.25" collector and a 2.5" collector back system

[Filo_xtc01]

most high power NA cars can get away with 1.75" primaries, 2.25" collector and a 2.5" collector back system

I thought that most of the high-end headers have 2.5" collectors?

[BlitZ]

so wrong. backpressure = evil!


its just all deifnition really.. we all understand the concept

you need back pressure to optimise flow.. 3.0" = no back pressure.. 1.75" = back pressure.. what would u rather on a NA 1.6L?

People also reccommend mandrel bent over a str8 pipe under axel design as it lacks back pressure... the back pressure creates vacume effect for consistent flow..



most high power NA cars can get away with 1.75" primaries, 2.25" collector and a 2.5" collector back system


why would you want various sizes in your exhaust?

[twilightprotege]

no no no! any backpressure at all is evil! backpressure is when exhaust gases essentially cant get out of the exhaust, thus creating a gas "traffic jam" up the exhaust line. exhaust backpressure is *the* most misused term in the automotive industry. blitz what you are saying by having a 3" exhaust on a 1.6L having no backpressure is wrong, you have no exhaust velocity. you'd have absolutely zero backpressure

think of exhausts like a freeway. your freeway is a 1.6L engine and is required to flow 50000 cars per day for optimum use (aka power). at 50000 cars per day the traffic stays at 110kph. when you change that to say 55000 cars per day, you start getting traffic jams and the traffic speed slows down to say 90kph. this is backpressure. if your freeway is designed for a 5L dinosaur engine and is designed to flow 150000 cars per day and you're only flowing 50000cars per day, you've wasted money on the road....or in other words your exhaust is too big and the majority of it is unused and cars dont keep up with the rest of the cars in the flow and slow down


the reasonons for the different sizes. primaries are easy. the collector size is NOT the entry or exit of the collector.

this is a collector:
http://www.burnsstainless.com/MergeCollectors/order_collector.gif

the primaries are on the left (or 1.75") and the size of the collector (2.25") as it is known on the right. what you want for optimum collector design is this
http://www.burnsstainless.com/MergeCollectors/order_long_trans.gif
this expands the collector out to 2.5" so together it looks like this:

http://www.burnsstainless.com/MergeCollectors/order_collector.gifhttp://www.burnsstainless.com/MergeCollectors/order_long_trans.gif

doesnt look the best as posted, but you get the idea. this is why i said the above sizes

[BlitZ]

Why would you want to expand the collector out? ...

highway scenario..

cars are moving on the 2 lane road at 2 cars/sec.
then it merges to 1 lane... at 1 lane its 1 car/sec

If the lane was to become 2 lanes again, the rate at which cars will pass will reamin at 1 car/sec..

************************************************** ***********************
Are you also saying that an 3" exhuast on a civic b16a is just as effcient as a 2.25" with 2.25 being the point of optimum flow?

[ginganggooly]

Why would you want to expand the collector out? ...

highway scenario..

cars are moving on the 2 lane road at 2 cars/sec.
then it merges to 1 lane... at 1 lane its 1 car/sec

If the lane was to become 2 lanes again, the rate at which cars will pass will reamin at 1 car/sec..

************************************************** ***********************
Are you also saying that an 3" exhuast on a civic b16a is just as effcient as a 2.25" with 2.25 being the point of optimum flow?

I'd assume that Fluid/Gas flow dynamics are quite different to freeway traffic flow dynamics :)

Merge collectors work very well... You get a venturi effect at the merge, which helps increase gas velocity, thus reducing back-pressure. The larger section on the exit of the restriction works like an anti-reversion chamber... apparently.

I read it online, so it must be true :D

[twilightprotege]

yes, you expand from the collector again for the venturi effect. helps pull the gases out of the primaries even more - makes their velocity much greater in the primaries which is where you want maximum velocity. again, velocity is the key to NA exhausts. it doesnt reduce backpressure because there should never be backpressure in a properly designed exhaust system. again, backpressure is the most misused term in the auto industry.

just with your car analogy, 2 cars per second on a 2 lane road equal 4 cars per second on a 1 lane road if the 1 lane road if the speed limit (aka velocity) on the 1 lane road is high enough. also the 1.75" primaries are merging into a 2.25" area. no bottle neck there. a correctly designed header should have exhaust gases reaching the collector one after the other (ie equal length header), definately not at the same time (unequal length header)

again, i'm not saying a 3" exhaust is best on a 1.6L, you want an exhaust JUST big enough to flow all of your exhaust gas at your chosen rpm limit. work out how much exhaust gas your engine creates, and choose an appropriate size from there.

[BlitZ]

that is the first time i have heard a smaller collector and larger back system will create better power..

Ill research it someone..

thanks for the insight ..;) rep pts up yo

I have yet to see an aftermarket header with a flanging collector.. any links?

[ginganggooly]

that is the first time i have heard a smaller collector and larger back system will create better power..

Ill research it someone..

thanks for the insight ..;) rep pts up yo

I have yet to see an aftermarket header with a flanging collector.. any links?

It's merge collector bro-builder :P
There are heaps of references to these online. All the good, high-end headers run a merge collector of some sort.

[twilightprotege]

merge collectors if properly designed are awesome. unfortunately they arent cheap and 99.9% of off the shelf headers have an incorrectly designed collector

i've seen results where a v8 bogan ran something like 8.9 seconds over the 1/4 mile with a standard run of the mill collector. that figure dropped to something like 8.6 or 8.7 seconds with just adding a properly designed merge collector. for a car that's already going damn fast that's insane gains.

[ginganggooly]

merge collectors if properly designed are awesome. unfortunately they arent cheap and 99.9% of off the shelf headers have an incorrectly designed collector

i've seen results where a v8 bogan ran something like 8.9 seconds over the 1/4 mile with a standard run of the mill collector. that figure dropped to something like 8.6 or 8.7 seconds with just adding a properly designed merge collector. for a car that's already going damn fast that's insane gains.

SMSP sells very nice collectors :)

This is the article I was looking for... more or less illustrates the point we're trying to make :thumbsup:
http://www.musclemustangfastfords.com/tech/0202mmff_kooksheader/

Actually... thats not the one i was looking for. There is another site where they dyno the two style collectors against eachothother, on an engine dyno. There was a large difference between the two styles.

[twilightprotege]

www.burnsstainless.com <--drool worthy collectors

[preludacris]

if the zuast is too big, at the centre of the pipe, the gases will be flowing thru faster than the gases at the outta edges of the pipe. and the gases at the outside can even go backwards. not good.

so u want the optimum size to not cause a traffic jam, but not so big that the gases are flowing at different rates.

So does the merge collector create some kind of a vaccuum?
edit: it also seems that these collectors are only beneficial to 4-1 style headers. 4-2-1 still use regular straight pipes, yes?

and can anybody else confirm a bigger cat is beneficial?


btw, rep points for u twilight, good info.
thx

[DynoDave]

So does the merge collector create some kind of a vaccuum?
edit: it also seems that these collectors are only beneficial to 4-1 style headers. 4-2-1 still use regular straight pipes, yes?

Merge collectors work on any design headers they just look more prominent on the 4-1 headers,from my research there is only a handful of guys in Australia using this type of collector when designing and building headers.If you do a search for DTR/SMSP/Whitfields/RAGE/MFactory all there 4-2-1 use a small merge collector on the last collector where the headers go from 2 pipes to the outlet.
Regards Dyno Dave

[twilightprotege]

dave i know from talks i've had with the alcorn boys and from talks they've had with you that you prefer 4-2-1 over 4-1 for honda engines as they said that you said they tend to work better for honda's.

can you explain why? benefits over a stepped 4-1? thanks.

[DynoDave]

dave i know from talks i've had with the alcorn boys and from talks they've had with you that you prefer 4-2-1 over 4-1 for honda engines as they said that you said they tend to work better for honda's.

can you explain why? benefits over a stepped 4-1? thanks.
Hey mate
From my own research I found that 4-2-1 makes much better under the curve power than a 4-1 and the best part now is most of the 4-2-1 design are also a stepped primary so you get the best of both worlds.Now looking at it as to why the 4-2-1 is better it comes down to cam profiles and cylinder head flow (as in the percentage of exhaust flow vs intake flow).
Regards Dyno Dave

[twilightprotege]

yeah figured it was an area under the curve thing - i guess you have the secondaries rather short?

[DynoDave]

yeah figured it was an area under the curve thing - i guess you have the secondaries rather short?
The size and length of the secondries comes down to each engines desired RPM band,same as the size of the merge and the length of the transition before and after the merge.
Regards Dyno Dave

[VT3C]

so wrong. backpressure = evil!

what NA cars need is exhaust velocity. you want an exhaust JUST big enough to flow all of your exhaust gas at your chosen rpm limit. and smaller and you'll get backpressure and kill (big time) power.

anyway, after the header you only lose power, so spend the money to get the header right because that's where all the power is made.

most high power NA cars can get away with 1.75" primaries, 2.25" collector and a 2.5" collector back system


FINALLY !! another person who knows the TRUE evils of backpressure on NA 4-cyllinder engines. gas velocity is your friend - the faster you get the exhaust out of the cyllinders the better.

YES you may loose some bottom down power but if you're talking about performance and race, then top-end is your only concern unless you're driving your grandma to the shops to get the milk.. even THEN she'll want tak ;)

you'll find any percieved 'loss' of power down low in the RPM range will be made up for with the engines ability to rev through and beyond the low-RPM range a lot faster. This allows you to get onto your powerband (higher in the rev range for honda motors) faster which is better.