X-Force v others and header sizes.

[ducker85]

I think that video is worth finding n watching.

IMO the 4 into 1 headers are the way to go regaurdless and then depending on the desired performance n note use a cat back to control this

 

[PIR4TE]

I am thinking of going the 3" with 1 7/8" headers, so firstly is the 1 7/8" headers to large for a standard application (no internal engine mods, cams, head work and what not) or is 1 3/4" better?

Munz ^^ said it best, but to expand upon the detail... a factory VE SS requires an exhaust flow of 126-136 CFM (@ 28in), which is well within parameters of the supplied exhaust. The issue is not restriction / back pressure when running the standard tune, rather the main issues with the stock exhaust are

• The standard "log" headers provide exhaust port velocity (torque) at the expense of HP loss because the escaping gas velocity is almost too restrictive (fast), and flow (CFM) is definitely low.
• The standard "log" headers are generic cast, defined by GM as an integral part of the factory powertrain's tuned volumetric flow, yet is left unchanged whether the vehicle is a boat, airplane, truck or car.

In our application as a fast but heavy road car ideally the headers should be modified to maintain high velocity in the upper 200s FPS (hence torque), reasonably increase flow, and provide equivalent sound pressure waves back to the valves. This results in improved economy, torque, horsepower and basically makes the engine sing.
However the common mistake people make regarding exhausts (and primaries more critical), is supersizing pipe Cross Sectional Area because race car noise... the automotive equivalent of strapping a fire hose to garden tap.

5300	1	1 1/4	1 1/2	1 3/4	1 7/8
Exhaust Port CSA	1.608	1.608	1.608	1.608	1.608
Primary OD	1	1.25	1.5	1.75	1.88
Primary CSA	0.875	1.125	1.375	1.625	1.755
Exit FPS	300	275	217	190	170
Result	Sonic choke with HP loss (too fast)	Recommended HP and TQ	Largest exhaust port exit (slow FPS)	Torque loss + Reversion + Scavenging Loss (too slow)	Torque loss + Reversion + Scavenging Loss (too slow)

The ideal primary pipe size and headers for your car with a stock tune are the factory headers. Go bigger without tune and overall performance will suffer, although it may sound better and feel like it redeems itself at peak rpm.

Also, are people with X-Force systems happy with their's or do you wish that you went with a different brand and if so what brand?

I am happy with X-Force now that I sorted the drone which pretty much all aftermarket catbacks have to some degree. I went for mild steel this time but next set will be aluminised or stainless with a different centre section.

I'll probably hit up a cat back system first due to funds and then move on to headers and cats.

Yeah good first move, a 2.5" catback may increase exhaust velocity (hence torque) slightly by marginally promoting flow, and perhaps HP however the primary restriction to flow (HP) is deliberately in the headers as it suits torque delivery in the standard tune. A 3" with standard headers won't do you any good, at least in performance terms as the pressure drop from skinny stock headers and cats means the exhaust exit speed at lower RPM is too slow.

Lastly just to make sure I don't need a tune with the cat back or am I better off doing the lot in one go so I don't have to have the car re-tuned?

No tune required with catback, not really required until you decide on a plan for better performance / efficiency. CAI & tune starts at $1250, at which point you should also consider replacing the stock headers for 1 3/4 tri-ys like I had. This will cost you an extra $300 installed and will make a big difference.

 

[PIR4TE]

There was a thread on here about exhaust myths.

The test car was a silver 6 speed 317 Senator. The tested every combo of exhaust and the 1 7/8 primary 4 into 1s with 3" 200cpi cats and 3" cat back made the most power before tuning.

They tested it with;
stock exhaust,
HSV front, 2 1/2 cat back
HSV headers, hi flow cats, 2 1/2 cat back
1 3/4 tri Ys hi flow cats 2 1/2 cat back
1 3/4 4 into 1s etc
And finally the 1 7/8 4 into 1s with cats n 3" back and it made the most power on a stock motor (no CAI, no tune)

The video was from a workshop called garage I think

It was OzGarage, and showed which exhaust combo made more peak HP for that car, which is materially different to OP's.

 

[Not_An_Abba_Fan]

The exhaust system after the headers is a restriction on any car. The bigger the exhaust, the more peak power it will make.

On a street car, perfectly tuning the exhaust to the engine and to your driving style and mods is impossible. Anything will give you a gain over stock. It will come down to your budget and what you prefer. Sound is entirely subjective, people like different things. Different exhaust designs affect the sound as well. For example, if you want a definitive V8 note that has a nice bark to it, do not go with a merge or "X" type exhaust, they combine the exhaust pulses and tend to "muddle" the note. An "H" pipe design will give a more defined V8 note and will be less restrictive.

The factory headers and exhaust are made to a budget and to meet emissions as well as giving a good balance of power and economy. However, it IS restrictive.

Putting on a higher flowing system will increase power, torque and economy. Tuning the PCM to suit the system will give even more gains.

The difference between 1 3/4" and 1 7/8" headers on a 6.0L is two fifths of bugger all. Even on a dyno you would only see 1kW if you're lucky. They will flow well and not lose any bottom end compared to tri Y's. The tri Y versus 4-1 argument was in the old carby days, where the flow of the exhaust made a huge difference to the intake velocity. Now, with modern EFI's and engine management systems, the bottom to mid of tri y and 4-1's are virtually the same. It's only when the revs get up that 4-1's will out perform the tri Y's. Oh, and 4-1's sound better.

I am doing Hurricane 1 7/8" 4-1's, 200 cell high flow cats and a twin 2 1/2" system for $2460 shipped. All bolt on and comes with everything.

 

[ducker85]

Oh, and 4-1's sound better.

thats was my point. 4 into 1's give a deeper note and sounds amazing

 

[PIR4TE]

On a street car, perfectly tuning the exhaust [system] to the engine and to your driving style and mods is impossible.

Impractical, not impossible, however bolting on a high rpm off the shelf system on an untuned car which needs more TQ not HP seems backward.

Putting on a higher flowing system will increase power, torque and economy. Tuning the PCM to suit the system will give even more gains.

Agreed. However high flow headers on an untuned 6.0 will get less torque, and reversion, less scavenging because of reduced exhaust port exit velocity.
Sure bolting on bigger headers may get more power revving its tits off way up the rev range... but hardly noticeable really. Also better economy? Surely that depends how you drive (hence at what fuel rate you are measuring it) but generally worse economy driving normally because the engine is less efficient for typical driving.

Anything will give you a gain over stock.

Any performance exhaust after the headers (aftermarket cats and cat back) may promote slightly more flow hence increase the exit velocity of OP's stock headers thereby increasing torque and improving efficiency.

The factory headers and exhaust are made to a budget and to meet emissions as well as giving a good balance of power and economy. However, it IS restrictive.

Agree with all above except they are deliberately restrictive (yes) to maximise velocity across typical driving in stock state of tune. Sure they promote torque at the expense of horsepower, but without modifying and spending a hella lot it suits typical driving. Our two tonne behemoth needs more torque not horsepower.

The difference between 1 3/4" and 1 7/8" headers on a 6.0L is two fifths of bugger all. Even on a dyno you would only see 1kW if you're lucky.

My point is that 1 1/4 to 1 7/8 is definitely a backward step for the OP's stock tune. Further that he could follow a performance path using $300 headers which still won't require a tune and give great results.
I agree there is bugger all difference between 3/4 and 7/8 in the large scheme of things but with our car you are ultimately better erring on the side of torque (velocity) than a slight increase in high revving power (flow).

They will flow well and not lose any bottom end compared to tri Y's. The tri Y versus 4-1 argument was in the old carby days, where the flow of the exhaust made a huge difference to the intake velocity. Now, with modern EFI's and engine management systems, the bottom to mid of tri y and 4-1's are virtually the same. It's only when the revs get up that 4-1's will out perform the tri Y's. Oh, and 4-1's sound better.

That's totally subjective!! Let's introduce some objectivity to the claim of ideal header design for our car, versus hp dyno figures... check the design components incorporated to the set on the fastest NA VE/G8/VF (no point in arguing this again, agreed).

 

[Not_An_Abba_Fan]

Quote Originally Posted by Not_An_Abba_Fan
On a street car, perfectly tuning the exhaust [system] to the engine and to your driving style and mods is impossible.

Impractical, not impossible, however bolting on a high rpm off the shelf system on an untuned car which needs more TQ not HP seems backward.

No, impossible. You can tune an exhaust to a specific RPM, but not to every driving condition experienced by a street car. Removing a restrictive exhaust system and fitting something that flows better as well as having better velocity will see gains in power, torque and economy.

Quote Originally Posted by Not_An_Abba_Fan View Post
Putting on a higher flowing system will increase power, torque and economy. Tuning the PCM to suit the system will give even more gains.

Agreed. However high flow headers on an untuned 6.0 will get less torque, and reversion, less scavenging because of reduced exhaust port exit velocity.
Sure bolting on bigger headers may get more power revving its tits off way up the rev range... but hardly noticeable really. Also better economy? Surely that depends how you drive (hence at what fuel rate you are measuring it) but generally worse economy driving normally because the engine is less efficient for typical driving.

Do you even know how extractors work? The reason they are called extractors is because of the way the collector scavenges exhaust gases and "extracts" them from the exhaust port. Better economy because you are using less throttle for the same result. You don't require extra fuel for a bigger bang to push out the exhaust gases banking up in an inefficient exhaust system.

Quote Originally Posted by Not_An_Abba_Fan View Post
Anything will give you a gain over stock.

Any performance exhaust after the headers (aftermarket cats and cat back) may promote slightly more flow hence increase the exit velocity of OP's stock headers thereby increasing torque and improving efficiency.

The stock cats are a restriction. The stock exhaust is designed to keep the car quiet, in doing so, it is also a restriction. Fitting aftermarket extractors to the stock cats and cat back will also see gains.

Quote Originally Posted by Not_An_Abba_Fan View Post
The factory headers and exhaust are made to a budget and to meet emissions as well as giving a good balance of power and economy. However, it IS restrictive.

Agree with all above except they are deliberately restrictive (yes) to maximise velocity across typical driving in stock state of tune. Sure they promote torque at the expense of horsepower, but without modifying and spending a hella lot it suits typical driving. Our two tonne behemoth needs more torque not horsepower.

Deliberately restrictive? Maybe, maybe not. It's more cost effect to mass produce a cast manifold than it is to fabricate, jig and hand weld a set of 4-1 tuned length (and I use that term loosely), extractors.

Quote Originally Posted by Not_An_Abba_Fan View Post
The difference between 1 3/4" and 1 7/8" headers on a 6.0L is two fifths of bugger all. Even on a dyno you would only see 1kW if you're lucky.

My point is that 1 1/4 to 1 7/8 is definitely a backward step for the OP's stock tune. Further that he could follow a performance path using $300 headers which still won't require a tune and give great results.
I agree there is bugger all difference between 3/4 and 7/8 in the large scheme of things but with our car you are ultimately better erring on the side of torque (velocity) than a slight increase in high revving power (flow).

Stock tune or not, the modern PCM is somewhat adaptive in that it can "learn" certain things and draw from a range of stored maps to give the best power and economy for any given situation. Changing the tune just alters the maps the PCM can use, therefore giving better results.

I stand by my statement that just by changing the headers, cats and system, you will see an increase in torque and power as well as economy.

Quote Originally Posted by Not_An_Abba_Fan View Post
They will flow well and not lose any bottom end compared to tri Y's. The tri Y versus 4-1 argument was in the old carby days, where the flow of the exhaust made a huge difference to the intake velocity. Now, with modern EFI's and engine management systems, the bottom to mid of tri y and 4-1's are virtually the same. It's only when the revs get up that 4-1's will out perform the tri Y's. Oh, and 4-1's sound better.

That's totally subjective!! Let's introduce some objectivity to the claim of ideal header design for our car, versus hp dyno figures... check the design components incorporated to the set on the fastest NA VE/G8/VF (no point in arguing this again, agreed).

Is the fastest NA VE/G8/VF untuned? Custom tuning a car to suit the system it has will see better performance than loading a generic tune to a car that has a better flowing exhaust. All the bolt ons in the world will not make a car as fast as it can be if it is not tuned to suit. I agree that a tune will see optimum gains, but fitting an exhaust to an untuned car will still see gains.

 

[PIR4TE]

No, impossible. You can tune an exhaust to a specific RPM, but not to every driving condition experienced by a street car. Removing a restrictive exhaust system and fitting something that flows better as well as having better velocity will see gains in power, torque and economy.

No, you were saying driving style and I said driving style suited to car, not every condition the car will meet. Yes you can have an exhaust better suited to your car and driving style?! More torque near stock torque convertor stall speed versus mang.

Do you even know how extractors work? The reason they are called extractors is because of the way the collector scavenges exhaust gases and "extracts" them from the exhaust port. Better economy because you are using less throttle for the same result. You don't require extra fuel for a bigger bang to push out the exhaust gases banking up in an inefficient exhaust system.

Enough to know that's a simplistic explanation, and only one of several key forces affecting header design... of course I know how they work. The reason for exhaust gases "banking up" right back into the fricken intake is that too large an exhaust exit CSA doesn't work for stock CFM because the exit velocity is too slow.

The stock cats are a restriction.

Not noticeable at OP's stock engine exhaust CFM.

The stock exhaust is designed to keep the car quiet, in doing so, it is also a restriction.

Yes, by its nature, but still flows adequately for stock engine's exhaust CFM.

Fitting aftermarket extractors to the stock cats and cat back will also see gains.

Possibly, not because of removing restriction to his current weak CFM but because it promotes flow thereby increased velocity (within the parameters of stock tune). Definitely not up scaling from 1 1/4 to 1 7/8, that's backward.

Deliberately restrictive? Maybe, maybe not. It's more cost effect to mass produce a cast manifold than it is to fabricate, jig and hand weld a set of 4-1 tuned length (and I use that term loosely), extractors.

Mate it's the headers which are suited to the stock tune! Putting on 1 7/8 is for a totally different state of tune!? Remove the torque ring at the flange (like I did) and see what happens.

Stock tune or not, the modern PCM is somewhat adaptive in that it can "learn" certain things and draw from a range of stored maps to give the best power and economy for any given situation.

Not quite like that with AFR feedback from the bungs, put the 1 7/8 on and see how a stock tune likes dem apples!!

I stand by my statement that just by changing the headers, cats and system, you will see an increase in torque and power as well as economy.

Not 1 7/8, definitely not without a tune that increases the engine's volumetric flow (which infers increased induction).

Is the fastest NA VE/G8/VF untuned?

Haha no but it doesn't run 4-1s, sound better too (Tri-Ys).

Custom tuning a car to suit the system it has will see better performance than loading a generic tune to a car that has a better flowing exhaust. All the bolt ons in the world will not make a car as fast as it can be if it is not tuned to suit. I agree that a tune will see optimum gains, but fitting an exhaust to an untuned car will still see gains.

Well yes, if it will even run untuned without limp mode, warning lights etc., and ECU compensates AFRs ideally for new bung placement then it may produce a smidge more with tits screaming... ie not the most practical improvement for the OP who is careful about cost, no scavenging gains nor torque gains in his stock CFM tune every day driving.

 

[TAS1981]

Aren't we a little off topic and a bit over done on the science of exhaust scavenging?!

OP - ultimately it's sound and cash. I think in your case a 3" full xforce on a daily commuter might be a bit too much. As a daily with a lot if miles that you want to extract a decent amount if power out if I probably would go with something like NAAFs full Manta just because it's probably a bit more grown up than the xforce. I don't commute in my car so I quite like the fact mines a bit shouty. It's a mild steel 2.5 xforce cat back. I just wanted some sound.

All this talk of 4-1s versus tri-ys for a daily commuter if you are not going to go full cam blah blah is just a bit....pointless....while these two are still arguing you could have already ordered your exhaust and be blaring down the road......vroooooom!

 

[PIR4TE]

Aren't we a little off topic and a bit over done on the science of exhaust scavenging?!

OP asks should he go 1 7/8 or is that too big - and our replies are off topic?
There is a relevant technical discussion going on here relating to a very common mistake and you've had enough - this is a forum and thread for exactly this purpose?

I probably would go with something like NAAFs full Manta just because it's probably a bit more grown up than the xforce.

What crap. So the catback you and I have is more childish than a manta? Speak for yourself: "you could have already ordered your exhaust and be blaring down the road......vroooooom!"

All this talk of 4-1s versus tri-ys for a daily commuter if you are not going to go full cam blah blah is just a bit....pointless

NAAF was pointing out that there is hardly any real difference even in style of header design (except he prefers a certain sound). Nothing to do with cams, everything to do with OP keeping stock tune. Read it or no, it's not pointless FFS.
Sure our comments would be pointless if you didn't read what the OP asked, and can't contribute anything but a glib remark, however we were discussing an argument central to what the OP asked.