Is there any real benefit to a 3" exhaust

[CalaisV8]

I have a VY2 Calais with a 2 1/2" header/hi flow cat system mated to the original cat back section, and am looking at adding a twin cat back system to match. IS there any REAL benefit to a 3" over a 2 1/2" solution?

What Im after is a general increase in power/torque, a nice sound (without the droaning) and improved fuel economy on the highway.

Thanks in advance for your thoughts and comments.

[255-LS1]

its been covered many times before mate.

Generally, a good dual 2.5" is perfect for bolt ons and small cam, after that the dual 3" takes over. Dual 3" gives you more room to move in the future and not having to do things twice. Plus dual 3" sounds horn :evil:

Think about your goals and then decide from there.

[smithro_6]

hi there i have a full 3" Difillipo sounds great and couldn't be happier

[Daves8]

ON a 5.7 twin 2.5 inch is fine unless you have 300rwkw....

Twin 3 inch will drone more......

i HAVE 1 7/8 headers, twin 3inch to fiff then twin 2.5 to tips

[LS1-5.7]

Bigger is not always better. An exhaust with incorrect back pressure will affect engine performance. The word "scavenging" should come to mind.

[Darkside]

In the process of my Heads/Cam build we experimented with the exhaust. I run 1 7/8 headers and dual 3" exhaust. We felt that it was showing a bit low on the dyno compared to other similar setups. A restriction was put into the exhaust where the collectors are bringing it down to 2.5" then back to the 3". I picked up hp and torque with the restriction. In hindsight, I would have gone with the 2.5" duals. My cam is on the larger side as well.

[Wonky]

I'm far from an expert but do do a lot of reading. The general opinion would seem to be that on a 5.7l twin 2.5" is best until you go large cam/heads. With dual 3" on a bolt-on 5.7l you seem to run the risk of losing power, as someone above described.

On a 6.0 or 6.2l it is arguably a different story. For almost 2 years my SSV sedan with twin 3" KPM was the most powerful bolt-on SS/SSV A6 at Chev's, consistently pulling higher numbers than the mainly 2.5" others and drove very well! I don't even know what the car that beat it had as it was basically a "blow in" that I only saw that once.

[CalaisV8]

Thanks for the feedback, as I suspected full front to back 2 1/2" is the way to go on a non modified GenIII. Now all thats left is to decide on which one, X-Force, Redback, Sureflow etc

[Peter B - CV8]

Thanks for the feedback, as I suspected full front to back 2 1/2" is the way to go on a non modified GenIII. Now all thats left is to decide on which one, X-Force, Redback, Sureflow etc

Or you could go a 3.5 single system from Sureflow (like me). I'm in West Pennant Hills as well - send me a pm if you want to catch up & have a listen & drive.

[WOMBIE]

Or you could go a 3.5 single system from Sureflow (like me). I'm in West Pennant Hills as well - send me a pm if you want to catch up & have a listen & drive.

That's a nice offer :goodjob:

[ben78]

my vy calais has a 3" lukey cat back system and don't have any trouble with drone , it has a nice rumble to it as well. :)

[nomad-kr]

Is there any real performance difference between brands that anyone has noticed?

XForce, Manta, DF, Redback etc?

After reading, I'm prob looking at getting some 1 7/8 headers and twin 2.5in

[SirNemesis]

Bigger is not always better. An exhaust with incorrect back pressure will affect engine performance. The word "scavenging" should come to mind.

Scavenging is a function of the extractors. The cat-back backpressure should be as little as possible to ensure good performance.

[QIKMIK]

After reading, I'm prob looking at getting some 1 7/8 headers and twin 2.5inAny reason for the big headers on a 5.7? I would've thought that the 1 3/4 or 1 5/8 would be fine unless there is FI or stroking going on. I had 1 5/8 Paceys with Redback twin 2.5" on my VTII. No rear boxes either. There was a bit of drone around 1800rpm but otherwise it was great. A man from long ago called Voodoo had the same system on his Caprice with rears and had no drone issues.

Mick

[nomad-kr]

Any reason for the big headers on a 5.7? I would've thought that the 1 3/4 or 1 5/8 would be fine unless there is FI or stroking going on. I had 1 5/8 Paceys with Redback twin 2.5" on my VTII. No rear boxes either. There was a bit of drone around 1800rpm but otherwise it was great. A man from long ago called Voodoo had the same system on his Caprice with rears and had no drone issues.

Mick

Interesting, my knowledge is mainly in RB30et's... Idealy I'm aiming (at the moment) for a daily that's around 250rwkw. Now I understand I may not get to that figure with just an exhaust and tune, but i do want to do things once and right...

[feistl]

So is it worth upgrading from a dual 2.5" system to a dual 3" on a stroked LS1 (383/6.3L) with large cam and bolt ons?

Im getting the car dyno'd in a few weeks, so ill know the power output then, but it should be around 330rwkw.

Sorry to hijack, but is it worth going a FAST 102mm manifold with large throttle body over the standard LS6/throttle body? Ive heard these are ok with strokers, but not as much power gain on standard engines.

Cheers

[255-LS1]

Depends on the brand also, i changed from a xforce dual 2.5" to DF dual 3" (headers and cats stayed the same), no change to tune and the car felt stronger right through the range.

[255-LS1]

So is it worth upgrading from a dual 2.5" system to a dual 3" on a stroked LS1 (383/6.3L) with large cam and bolt ons?

Im getting the car dyno'd in a few weeks, so ill know the power output then, but it should be around 330rwkw.

Cheers

It will def make a differance.. however $ per kw may not be that impressive.

But yes dual 3" would be beificial with that combo

[feistl]

So is it worth upgrading the extractors as well? Ive currently got pacemaker 4 into 1s, although im not sure on the size.

What would my system be worth second hand? Its basically pacies, 4.5" sureflow racing cats and a fully welded stainless steel exhuast with rear resonators?

Im not sure at this stage whether to go twin turbo, supercharged or just a nice N/A setup. :S Confusing...

[255-LS1]

if your in 2 minds id hold off, you dont want to go spending $$$$$ on new headers etc if your considering going turbo. Make your mind up and then commit.

[feistl]

if your in 2 minds id hold off, you dont want to go spending $$$$$ on new headers etc if your considering going turbo. Make your mind up and then commit.

It just sort of depends on prices etc. There was a nice kit in the for sale section for ~$5500... The car is used for track work, so im thinking N/A is a better way to go. Id rather get faster by learning to drive, not adding more power.

That said, a L98 conversion might be a better idea going forward... as 6.3L is too large for racing :(.

[ebbett21]

I got 4>1 pacies DF twin 3'' car from a performance view the cars feels very strong thru the entire rev range

[255-LS1]

If racing.. BRAKES and SUSPENSION and TYRES should be your priority, you can have as much power as you like but if these aren't up to scratch forget getting faster.

ahhh the joys of cars :lol:

[Phizzle]

Dual 2.5" is the go up until 300rwkw or so. For my money I'd rather see more money spent on the extractors and making sure I got the right set for the intended purpose as there are a few combinations and sizes etc to consider. EG racing/drags would require large primaries like 1 7/8ths, good street torque would see you looking more at the 1 5/8ths and then there's tri-y, 4 into 1.

Also getting the headers ceramic coated or at least heat wrapped will do wonders for under bonnet temps and gas flow velocities. Do it once, do it right :)

[feistl]

If racing.. BRAKES and SUSPENSION and TYRES should be your priority, you can have as much power as you like but if these aren't up to scratch forget getting faster.

ahhh the joys of cars :lol:

Yep, dont worry. Ive spent heaps making the car reliable (EG Trutrac Diff, Sports Cover, 3.91s, all new nuthalane bushes, whiteline swap bars, 6 pot brakes, semi slick tyres, 25% UPD, 10mm leads, platinum plugs, PWR radiator, engine/power steering coolers, all new hoses/bearings, new high end fluids, supercar front bar/spoiler etc etc).

So basically spent $20k without actually make more power.... Going forward though i just need to decide which way to go....

[255-LS1]

better book some driving lessons then and spend your coin and fuel and tyres hehe

[Dreamerdan63]

Regardless that 3 inch sounds better than 2.5 inch and we all know inches count in life EXCEPT with exhausts. No commodore needs a bigger exhast than 2.5 inch regardless of what ill informed retailers tell you. The secret to a good exhaust is scavenging the cylinders and this relies on a large degree on exhaust pulses caused by the correct size exhaust matched to the motor size etc. If you go too big you lose the scavenging effect and will lose lowdown torque which is what you want in a street car. You will also use way more fuel. If yoiulike the look of a 3 inch then put 3 inch tailpies but stick to 2.5 inch from the extractors to the tailpipe start. I run a supercharger on my VY SS ute that puts out 440hp at the rear wheels and uses less fuel than stock. I run a Difillipo stainless 2.5 inch system with stainless extractors. Sounds great but never drums and never attracts attention of police.

[SirNemesis]

Regardless that 3 inch sounds better than 2.5 inch and we all know inches count in life EXCEPT with exhausts. No commodore needs a bigger exhast than 2.5 inch regardless of what ill informed retailers tell you. The secret to a good exhaust is scavenging the cylinders and this relies on a large degree on exhaust pulses caused by the correct size exhaust matched to the motor size etc. If you go too big you lose the scavenging effect and will lose lowdown torque which is what you want in a street car. You will also use way more fuel. If yoiulike the look of a 3 inch then put 3 inch tailpies but stick to 2.5 inch from the extractors to the tailpipe start. I run a supercharger on my VY SS ute that puts out 440hp at the rear wheels and uses less fuel than stock. I run a Difillipo stainless 2.5 inch system with stainless extractors. Sounds great but never drums and never attracts attention of police.

No, wrong. Scavenging is a function of the extractors. You want the least restrcition/backpressure after that. I've dyno'd back to be with a good twin 2.5" system and dropped after the extractors and seen almost identical results (1rwkw more with no exhaust at the top end). The curve had not changed, so the arguement of lost low end also doesn't occur in the real world.

[smw757]

my vy calais has a 3" lukey cat back system and don't have any trouble with drone , it has a nice rumble to it as well. :)

I'd agree with this statement - the 3" seems to rumble whereas the 2.5" has a tighter sounding, more sedate bellow (if that makes sense).

[Wonky]

my vy calais has a 3" lukey cat back system and don't have any trouble with drone , it has a nice rumble to it as well. :)

I had a Lukey 3" on my original VY SV8 and agree it did sound quite good! :yup: However, I'm sure the original question is asking whether a twin 3" has any benefit over a twin 2.5". I'm pretty sure Lukey don't make a twin 3", only the single I had.

[Dreamerdan63]

The effect is very noticeable in the real world and scavenging is not created by extractors. The only reason extractors are ever an improvement is because they replace an innefficent factory exhaust system. Nowadays the effect is less noticeable due to manufacturers doing everything possible to increase engine efficency for better fuel consumption. Even the crossover pipe has a great affect on your vehicles scavanging of the cylinders and recent work by some companys have found surprising results from just a change in those. And exhaust pulses run and are produced like a sound wave from a fair way back in the exhaust system which was extensively researched by racers in the 60's and 70's. A lot of the principles used today to increase volumetric effiency in engines were in fact known about from late 1950's and some a lot earlier. I read quite a few books in my youth about these principles and used them to huge advantage in a HQ ute running a 308 with a 2 barrell 500 holley TQ20 cam that would blow away engines with 3 times the gear in them. And I used to get 23 mpg too. I tried all combinations of bigger exhaust and bigger extractors but it was all about intake air speed and getting the right exhaust pulse from the exhaust system. And loud don't mean nothing...just means noise.

[Dreamerdan63]

Because the firing pulses in a V8 engine overlap to some extent (unless the engine has a single plane crankshaft), not every pulse has the whole exhaust system to itself, but the principle is still effective.
So in effect the first pulse helps create a low pressure point on the exhaust side which when the next exhaust valve opens helps scavage the cylinder of exhaust gas and also creates a low pressure inside the cylinder so when intake opens more air/fuel mixture than normal atmospheric pressure is drawn inside. This pulse can be affected even by the type of muffler you run.
As a rule of thumb, a 2 inch dual exhaust is fine for a mild 215 engine, a 2 1/4 inch diameter is good for up to about 300 cubic inches, while a 2 ½ inch exhaust is good for a highly tuned 350.

Also we like to think we increase the volumetric effiency of engines to a huge degree with whopping camshafts etc but even the most bombed street engine if expertly tuned is running max at 85 percent volumetric effiency...and volumetric effiency is what we are trying to increase when we all 'modify' motors for more power...'More Bang for your Buck ' so to speak.

Good info herehttp://www.nsxprime.com/FAQ/Miscellaneous/exhausttheory.htm

and here
http://rantsrus.com/content/true-dual-or-cat-back

[rosey]

single plane crankshaft

please explain what a single plane crankshaft is? :confused:

[SirNemesis]

please explain what a single plane crankshaft is? :confused:

A crankshaft with crank pins 180 degrees from each other, instead of using counterweights.

[markone2]

Regardless that 3 inch sounds better than 2.5 inch and we all know inches count in life EXCEPT with exhausts. No commodore needs a bigger exhast than 2.5 inch regardless of what ill informed retailers tell you. The secret to a good exhaust is scavenging the cylinders and this relies on a large degree on exhaust pulses caused by the correct size exhaust matched to the motor size etc. If you go too big you lose the scavenging effect and will lose lowdown torque which is what you want in a street car. You will also use way more fuel. If yoiulike the look of a 3 inch then put 3 inch tailpies but stick to 2.5 inch from the extractors to the tailpipe start. I run a supercharger on my VY SS ute that puts out 440hp at the rear wheels and uses less fuel than stock. I run a Difillipo stainless 2.5 inch system with stainless extractors. Sounds great but never drums and never attracts attention of police.

Cam only 293rwkw SV8 .
After a bit of back to back exhaust testing....11.001 with Mr Muffler Custom Twin 3"

11.134 with 2.5 off the shelf system. Not much in it imo and the disclaimer *...give me the 2.5 cat back for street duties any day of the week :)...if you search around the forum you'll find a couple more tried and tested units applied to one ultra consistent car for comparison

and then theres .... err ummm ..the not so good result for one system that has already had a passing mention above :1peek:

Please go here for test review .

http://www.ls1.com.au/forum/showthread.php?p=671462&highlight=2.5+twin#post671462

.

[Dreamerdan63]

Yes but in a 'street' car you arent after top end torque for drag racing at the track. Its low end torque that wins the race on the street and big 3" exhausts kill that and fuel economy. And unless you are racing at the drag strip every week the big 3" exhaust just drains your wallet faster. But hey I'm an old street rodder and what do us guys know huh? Nobody even reads books these days so how can I be expecting people to do research on these things?...lol
I have had every V8 that holden has ever produced and EVERY time even with big cams the smaller exhaust won on the street. My biggest failure was a 350 that was fully bombed with 1 3/4" primary extractors and it never even felt like a V8 until you got it up past 3000rpm. I also had a 454 in a dodge dual cab that had street exhaust and altho the vehicle weighed 3 tonnes it was a blast to drive.
Trust an old bloke...(well 46)...big exhausts do not make driving on street more fun!!