back pressure

[sean1380 0]

i have a g60 but iv heard that your able to fit any size bore u'd like on a turbo as u dont require the exhaust to give u back pressure is this correct??

 

also if it is does the same apply to a charger??

 

i was going to fit a straight throught system to my car but does anyone know of the regs on cats as i no there 1991 on but my cars a 1991 so does it need one?or would it be a case of just see if my cars got one and refit if necessery??

 

also is it true magnex no longer exist as im needin one of there boxes as i hear they give a good raspys/poppy sound

[Kevin Bacon 5]

i have a g60 but iv heard that your able to fit any size bore u'd like on a turbo as u dont require the exhaust to give u back pressure is this correct??

 

Yep, pretty much. The turbine wheel itself is all the restriction a turbo engine needs. The rest of the exhaust is purely for silencing and always a compromise between noise levels and performance.

 

also if it is does the same apply to a charger??

 

It's not as critical since superchargers don't rely on exhaust pressure to provide the boost. You might get a small gain stepping up to 2.5" bore though.

[sean1380 0]

cheers

just iv got a length of stainless that i got from a old factory,think its about 3" ill av to check but was thinking of puttin a straight through system on with just aback box

 

would it be defening tho??

iv got the same set up on mini n thats load but would the rado be as loud or louder??

[Philmo 0]

I've heard of the need for "backpressure" before, but never seen a logical/engineering dynamics explanation for the need. I'd have thought the expanding exhaust stroke flow would want all the space possible to move into?

What is the explanation?

[Kevin Bacon 5]

cheers

just iv got a length of stainless that i got from a old factory,think its about 3" ill av to check but was thinking of puttin a straight through system on with just aback box

would it be defening tho??

iv got the same set up on mini n thats load but would the rado be as loud or louder??

 

Yeah, a 3" bore single box system on a G60 would indeed be LOUD :D

 

I've heard of the need for "backpressure" before, but never seen a logical/engineering dynamics explanation for the need. I'd have thought the expanding exhaust stroke flow would want all the space possible to move into?

What is the explanation?

 

Me neither and it's a good question / debate!

You sometimes see turbos with 4" downpipes now (3" outlet on the turbo) because as you say, it gives the very hot exhaust space to expand into.

 

Now, with my limited knowledge / understanding of thermal dynamics, I would guess that as the hot exhaust expands and therefore cools, it must create a vacuum that pulls more exhaust out of the cylinders in the same way "tuned" exhausts do on 2 stroke engines. So yeah, the sooner the exhaust can expand and cool, the better...in turbo applications.

 

In normally aspirated applications, I think they use "scavenging", which kind of uses the exhaust and intake pulses to help evacuate spent gases (afaik?!), which is why the exhaust configuration is more important :shrug:

 

I think Superchargers fall between those two ideals....

[Philmo 0]

So with Karl's old 6 branch and Shaun's old 2.5" two box - the flow on mine should be about right? Hope the volume's reasonable!

 

[i've decided to re-name her Second-Hand-Rose, as well!]

[sean1380 0]

right well i think iv binned the idea of a straight through system as making all the bends would be a right bitch i didnt realise untill i had a look underneth before :cuckoo:

 

think iv found a system im gonna buy on ed38 so hopefully alls gonna be well :clap: for half the price of one i got quoted today

[Philmo 0]

Me neither and it's a good question / debate!

You sometimes see turbos with 4" downpipes now (3" outlet on the turbo) because as you say, it gives the very hot exhaust space to expand into.

 

Now, with my limited knowledge / understanding of thermal dynamics, I would guess that as the hot exhaust expands and therefore cools, it must create a vacuum that pulls more exhaust out of the cylinders in the same way "tuned" exhausts do on 2 stroke engines. So yeah, the sooner the exhaust can expand and cool, the better...in turbo applications.

 

In normally aspirated applications, I think they use "scavenging", which kind of uses the exhaust and intake pulses to help evacuate spent gases (afaik?!), which is why the exhaust configuration is more important :shrug:

 

I think Superchargers fall between those two ideals....

 

So ideally the exhaust config & cams you use should be tailored to suit the induction method used?

I haven't yet come across any cams which purport to support natural or blown applications specifically. Is this something for the future?

[sean1380 0]

do they not allready do turbo specfic cam etc??

 

as the back pressure created on a n/a engine needs to be greater as to create that vacum like u say so fitting a oversized system on a small car it looses the pressure and power??

 

but liike u say its not needed as much on turbos and chargers as they are forcing in the mixture into the combust chamber so its kinda got pressure allready then the exhaust valve opens it expands out into the zorst and also gets sucked out by the different in pressures?? :scratch:

[dr_mat 0]

It's all about achieving steady flow. If the bore is too wide, you will only get turbulence in the exhaust gases and the exhaust will be no more efficient than no exhaust at all. If you can get a steady laminar flow of gas through the exhaust then you do get the scavenging effect Kev mentioned above, which can improve cylinder flush out the exhaust stroke (which in turn improves cylinder fill later in the cycle).

For any system with deliberate obstructions in the outlet (turbos!) then clearly this no longer applies between the outlet and the turbo, but even on the exhaust side of the turbo this effect can still be helpful.

A G60 engine has no outflow restrictions so you'll still benefit from a well designed exhaust with a suitable flow rate and diameter.

 

FWIW the exact same principles apply to good inlet manifold design, and for a graphic demonstration of this look at the VSR/VGI manifold - all that torque comes about because of the narrower effective inlet at lower revs. "Widen" the inlet for high rev throughput (higher overall gas flow rate).

[sean1380 0]

aaahh i see now

 

so is there anyway improving the flow on the inlet manifold on a g60?

 

so whats a good size bore size for steady flow as im guessin the common myth is that the bigger the exhaust the better

[boost monkey 0]

I've heard of the need for "backpressure" before, but never seen a logical/engineering dynamics explanation for the need. I'd have thought the expanding exhaust stroke flow would want all the space possible to move into?

What is the explanation?

 

Try 4-Stroke Performance Tuning by A. Graham Bell, well worth the read.

 

So ideally the exhaust config & cams you use should be tailored to suit the induction method used?

I haven't yet come across any cams which purport to support natural or blown applications specifically. Is this something for the future?

 

They definitely do exist, and have been mentioned on this forum! As you/someone has already mentioned, it's to do with valve overlap.

 

It's all about achieving steady flow. If the bore is too wide, you will only get turbulence in the exhaust gases and the exhaust will be no more efficient than no exhaust at all. If you can get a steady laminar flow of gas through the exhaust then you do get the scavenging effect Kev mentioned above, which can improve cylinder flush out the exhaust stroke (which in turn improves cylinder fill later in the cycle).

For any system with deliberate obstructions in the outlet (turbos!) then clearly this no longer applies between the outlet and the turbo, but even on the exhaust side of the turbo this effect can still be helpful.

A G60 engine has no outflow restrictions so you'll still benefit from a well designed exhaust with a suitable flow rate and diameter.

 

FWIW the exact same principles apply to good inlet manifold design, and for a graphic demonstration of this look at the VSR/VGI manifold - all that torque comes about because of the narrower effective inlet at lower revs. "Widen" the inlet for high rev throughput (higher overall gas flow rate).

 

To break that down a little, laminar flow is what you would refer to as normally flowing, so if it were water it would have no choppiness or bubbles. Turbulence is white-water basically, where the "fluid" (gas in this case) is going all over the place because the ratio between velocity and pipe diameter is too big.

 

And yes, the torque generated at low revs on the VSR are a function of the mass (or volume) flow rate of the fluid, as pipe diameter and velocity are inversely proportional. For example, leave a hose running at a set speed. Put your thumb over the end of it (reducing pipe diameter) results in an increase in water velocity as the mass flow rate must remain the same at all times, up until there is a severe blockage or the pipe breaks!

[dr_mat 0]

so whats a good size bore size for steady flow as im guessin the common myth is that the bigger the exhaust the better

 

I don't know what the "best" result would be, but the exhaust bore should be tuned to match the air flow rate through the engine - which will be more or less proportional to the power output (within reason). So your best bet is to look at what diameter tubes e.g. porsche spec for cars with similar ower output.

 

Yup, bigger ain't always better. The boys with the phat exhaust pipes are very much mistaken.. They're swapping power for noise ..