3 inch exhaust cat back??

[Vindots]

Anyone here has that exhaust? And is there any dyno comparison between the usual JDM 2.3 and 2.5 inch system?

 

[C&S Evo7]

Unless you go turbo , 3" is too big, you will lose power

 

[tricker_luke]

Seems mixed opinions on this, I have been recommended a 3inch system with my n/a setup

 

[Vindots]

Thats why i asked, Because in the USA, they are using 3inch exhaust and proved gains throughout rev range..

 

[C&S Evo7]

my understanding is by increasing the pipe size the exhaust gas pulse velocity is reduced, the speed of the gas exiting the engine is paramount to the engines performance.

i will try and explain my understanding a little but its a hugely complex subject and although i understand some of the theory i am by no means an authority in exhausts, so i suggest doing some research yourselves-
the exhaust gas comes down the exhaust in pulses due to the firing order , the pulses of gas need to flow as quickly as possible away from the valves during overlap and, as there can be an area of low pressure just behind the pulse it can help scavenge the gas out of the combustion chamber ready for the next firing cycle, the manifold pipe length is also important here so the pulses dont collide but fit together in sequence . if they are even in length then one pulse has gone past the collector area when the next pulse arrives and so on.

(as i understand this is why many scoobis sound like they do , the manifolds branches are unequal in length so the pulses are unevenly spaced hence the odd noise they make, if you fit equal length manifolds they sound pretty normal. )

if we fit a 3" system then although we have more area inside the pipe the actual speed of the gas can be reduced. people think by fitting a 3" system they will reduce back pressure which is correct however as they lose the gas velocity and then lose power they assume " these engines must have back pressure to work" actual back pressure would mean it was hard for the gas to escape from the combustion chamber, and as we all know an engine is just a big air pump it is critical the spent gas gets out as quickly as possible. so they actually need zero backpressure with the narrowest pipe possible to keep the velocity up.

 

[eXceed]

The above is correct, however... From a tuning point of view I can confirm that the bigger the diameter the better results I have seen, especially over the other side of the pond. Will be fitting a 3inch to my NA (although cammed maybe makes the results not comparable), so will obviously post back with my results. I also have a standard exhaust but not sure if I can be bothered to try with and without...

 

[b19twg]

Id think its 3" exhaust when your running cams and 2.5" wigh normal bolt ons.

 

[pulpmelon_r]

Internet motor sport science:

my understanding is by increasing the pipe size the exhaust gas pulse velocity is reduced, the speed of the gas exiting the engine is paramount to the engines performance.

i will try and explain my understanding a little but its a hugely complex subject and although i understand some of the theory i am by no means an authority in exhausts, so i suggest doing some research yourselves-
the exhaust gas comes down the exhaust in pulses due to the firing order , the pulses of gas need to flow as quickly as possible away from the valves during overlap and, as there can be an area of low pressure just behind the pulse it can help scavenge the gas out of the combustion chamber ready for the next firing cycle, the manifold pipe length is also important here so the pulses dont collide but fit together in sequence . if they are even in length then one pulse has gone past the collector area when the next pulse arrives and so on.

(as i understand this is why many scoobis sound like they do , the manifolds branches are unequal in length so the pulses are unevenly spaced hence the odd noise they make, if you fit equal length manifolds they sound pretty normal. )

if we fit a 3" system then although we have more area inside the pipe the actual speed of the gas can be reduced. people think by fitting a 3" system they will reduce back pressure which is correct however as they lose the gas velocity and then lose power they assume " these engines must have back pressure to work" actual back pressure would mean it was hard for the gas to escape from the combustion chamber, and as we all know an engine is just a big air pump it is critical the spent gas gets out as quickly as possible. so they actually need zero backpressure with the narrowest pipe possible to keep the velocity up.

Real world facts:

 

[C&S Evo7]

To be fair that graph is so close and so far apart from a time point of view it could easily be the other way around and while it shows gains was the yelling adjusted to suit? IMO its not proof at all.

Ask yourselves what the touring cars used when they were na? Pretty sure they weren't 3" systems

 

[pulpmelon_r]

To be fair that graph is so close and so far apart from a time point of view it could easily be the other way around and while it shows gains was the yelling adjusted to suit? IMO its not proof at all.

Ask yourselves what the touring cars used when they were na? Pretty sure they weren't 3" systems

Same day back to back dyno test, with around 2 degree IAT variance and no air pressure variance.

Just because you don't like, it doesnt make it false.

3"ers are loud, you will not pass a 105db sound test for the UK circuits.

 

[Vindots]

i've been reading in k20a.org and those people justify their 3inch systems? im confused now?
maybe we can taper 2.5 pipe to 3inch muffler hehe!

I hope TDI north will jump in here!!

 

[tricker_luke]

What about noise limits with a bung?

 

[Rob.]

What about noise limits with a bung?

Wont that just restrict the exahaust an make the 3" pointless though?

 

[C&S Evo7]

Same day back to back dyno test, with around 2 degree IAT variance and no air pressure variance.

Just because you don't like, it doesnt make it false.

3"ers are loud, you will not pass a 105db sound test for the UK circuits.

10 hrs different, who said i didnt like it? i have been looking at graphs like this for many years, more than some of you have been alive and while it shows a couple of Hp here and there, the graph could easily be the other way around, the curve and pattern is very close i just wouldnt consider a dyno printout like that absolute proof but hey you are all entitled to your own opinions .

give the 3" systems a try, i think you will get just as good results with a well designed smaller system, there is no reason a 3" system will be any louder than a 2.5 if the silencers are designed to suit..

if you fit a bung you might as well keep a smaller system without, it always seemed to strangle my car when it was in, fel like someone had robbed 40 hp

 

[pulpmelon_r]

10 hrs different, who said i didnt like it? i have been looking at graphs like this for many years, more than some of you have been alive and while it shows a couple of Hp here and there, the graph could easily be the other way around, the curve and pattern is very close i just wouldnt consider a dyno printout like that absolute proof but hey you are all entitled to your own opinions .

I'm genuinely surprised someone with extensive dyno experience doesn’t understand that time is not a metric that alters a reading.

I repeat: consistent Intake temperature, coolant temperature, air pressure, tyre pressure and humidity. Are you suggesting Paul West doest know how to fairly operate his dyno?

I have purposely posted on this thread after reading your posts C&S Evo7 to provide evidence to the contrary - do you have any counter evidence? Or just stuff you read on the internet?

 

[C&S Evo7]

no hard facts just experience, and my experience shows that going too big can be detrimental to the power with an N/A engine, whats interesting is historically 3" systems havent worked too well, i dont see what can have changed that much, but like i said the dyno printout is so close, while i totally agree it shows a gain at the top, it could just as easily show a loss, there are many factors that can make the dyno read a couple of hp up or down, i just wish people wouldnt get so hung up on a dyno graph.

I know paul very well and know he would never rig his dyno (well he wouldnt rig it much eh Paul ;-) ?)

what i am suggesting is that just because we see one dyno printout that shows a small gain at the higher rev range on one prticular exhaust, it desnt mean that every 3inch system will work,
Im sure it has more to do with the manifold efficiency anyway.

 

[pulpmelon_r]

The reason this thread was posted was due to common knowledge in the states that you can make power NASP with a bigger exhaust.

The UK is about 5-6 years behind the states on the Honda tuning front, with all motor k-series cars running 9 second passes in the outlaw class (drag racing)

But hey I should probably shut up and keep this advantage to myself though right? Also I'll be sure not to share the data on the 3.5" velocity stacked intake i'm working on too :xaiweb4:

 

[C&S Evo7]

the cars in outlaw are not exactly stock though are they ;-) Skunk were running 9's N/A, 5 years ago .


(my last drag car had 980 hp N/A and ran 7's)

 

[pulpmelon_r]

Thanks for pointing that out... More like 10 years then :xaiweb3:

And the fact they run open header might tell you a thing or two.

 

[George K]

Drag cars are turbo or s/c, so need totally different exhausts - this thread is about n/a. I have been surprised over the years by how much bigger diameters can be used - from ealry BMW F2 engines and later (but normally with a lumpy torque curve) - but undoubtedly they will need a perftectly matched header to show results. For most of us we also have noise limitations, so a good 2.5 system without a bung is probably still the best all round solution.