Exhaust Questions

[Fugi]

ok here is the sitcho..

CAR:R32 gtst

i recently had to get a 2 inch temporary cat back little system cauz nobody could help me with stock catback.. but anyways.. after getting the system made up the guy at the place started telling me that my 3" is too big and how every zorst system needs back pressure.. im currently runnin 3" all the way through 3" cat and a cannon on the back..

is he just talkin shit to sell me new exhaust or should i buy a muffler??

BTW: anyone needing to get a EPA done and has 3" cat and needs to borrow 2" catback for r32 gtst pm me..

[murrayis]

The turbo produces enough back pressure, i have run mine with no exhaust before just the turbo and it ran fine, bit lud and smoky but fine

[BAMR33]

Back pressure helps for a smooth idle and response when driving around the street, but for gaining mid range and top end power the bigger (sensibly) the better and 3" is pretty much right on for the rb20. The muffler shop guy is probably just use to dealing with NA cars. The cat and mufflers will give you all the back pressure you need.

[mr. kirisaki]

Hmm I got told the complete opposite at an exhaust shop a while back, saying my exhaust was too small, too much restriction and I needed a 3 inch. Now I'm just confused lol.

[Fugi]

yea thats what i thought murrayis and bamr33 thanks for the info... and yea mr.kiri i was told that aswell a while back from racepace when i had a 2.5" cat.. seems to be the 3" is the way to go..

[abu]

Just go a 3" turbo back system, with a high flow cat.

If thats to loud for you get a resonator fitted.

Thats the system most people run on here and make very good power with.

Edited September 11, 2007 by abu

[Zerrun]

im just curious has anyone had their car dyno'd before and after getting a new exhaust system fitted? if so what increase in performance did you get? i would assume other work done to the car would affect this figure aswell so anyone with a near stock gtr done this?

[TTOBES]

How does 'back pressure' help performance...??

[R31Nismoid]

It doesnt on a turbo car, even on a N/A car, its just that people with N/A cars feel the "loss" more because they put on the exhaust and dont get a tune to compensate for the cars altered behaviour

Backpressure & turbos = bad.

[Nismo_God]

i replaced stock exhaust with turbo back exhaust, no other mods and made an extra 25kw

[TTOBES]

Yeah I agree, although I would go as far as -

ANY backpressure (Turbo or NA) = BAD!!!!!!!!!!!!!!!!!!!!!

A lot of the 'exhaust experts??' arn't quite up with the play yet...

[redevil]

i work for an exhaust company that designs exhausts for power stations and earth moving equipment boats and trucks.... and i'll tell you first hand the less backpressure u can get the better.... all engines have a maximum allowed backpressure per h2o pressure drop..... but yeah anyone thats says you need back pressure is miles off..... put it this way the less backpressure u have the easier it is for a turbo to spool.... the reason they say there is too big is because once you go beyond a certain point there is too much area and not enough flow to move that amount of air so in cause starts causing backpressure.... so yeah 3" would be ideal for an r32.... the most ideal is 2.75" or 70mm tube work.....

Edited September 13, 2007 by redevil

[Oblivion]

Nice facts Brenno..

as said before 3" is a good upgrade for your car.

i am running a 3" straight through system...i have no probs with idle or power.

[TTOBES]

There is a real science to exhaust tuning and the whole thing is a bunch of compromises...

Here is a blurb from another forum that is interesting reading...

'Ok, this explanation may be MORE information than some want to consider but...

First of all, I MUST say that backpressure is NEVER a good thing in the exhaust. By backpressure I mean resistance to flow creating more pressure for the exhaust exiting the exhaust valves to work against. Ideally you would have a vacuum at the exhaust valve and in fact that can be created with a properly tuned exhaust system. Our exhaust manifolds are NEVER going to accomplish this so my statement is for illustration only. But, consider each phase of an exhaust. If the manifold pumps into a restrictive catalytic converter (backpressure) is it going to flow more or less? Easy, less. If the cat flows into a restrictive Y pipe, resonator, silencer(s) kinked pipes, chambered muffler, obstructions, air damns, etc... each part adds up and the engine cannot exhale efficiently.

Now, reducing backpressure is one component of a complex system and is in fact interdependent with the other variables. Velocity is very important. Heat, accoustics, aerodynamics, shape, volume and transitions are all important parts of the equation.

To "simplify"... let me attempt to illustrate what happens in a multi-cylinder exhaust system.

As an exhaust valve opens it "vents" the hot under pressure exhaust gases from the cylinder. These gases have mass and speed and therefore enertia. The speed is dependent upon many things not the least of which is heat energy. If you look at the outside bend of a header on an engine under load you will often see the "glow" of this energy being "lost" as it makes the transition.

The purpose for ANY performance exhaust is to reduce the pressure at the exhaust port especially during the end of the exhaust event for that port. A really GOOD system can actually create a vacuum. This is called scavenging and it improves removal of unburnable exhaust gas from the cylinder allowing more fresh air/fuel charge to enter during the intake event as well as reducing the pumping loss of a piston working against port pressure.

There are several ways to reduce this pressure including "tuned" length runners which "tune" the pressure waves to create the above effect... but only for a given RPM window.

One of the MOST confusing realities about modifying exhaust systems is the usual loss of low RPM power/torque. Looking at a stock exhaust manifold or system logic would state that increasing capacity "HAS TO" increase flow and continuing this concept... POWER. So WHY would a better LOOKING part hurt low RPM power/torque? The answer is that the stock stuff flows POORLY both WAYS!!! Better flowing pipes flow better BOTH WAYS!!!

When the exhaust event first starts (when the valve first opens) there is very high pressure in the chamber. This pressure flows out the POOR flowing manifolds just fine at low RPM. As the exhaust event is nearing its end the cylinder pressure is much lower and the poor flowing manifold acts to reduce reverse flow from other cylinders firing into the manifold. Installing a better flowing manifold CAN (not necessarily will) allow exhaust pressure from an adjacent port to flow back into a port at the end of its event and therefore charging that cylinder with additional exhaust pressure and reducing its potential to breath in a fresh charge.

Think of it this way. With 4 exhaust ports collected into one exhaust manifold, if one of those ports is at the end of its exhaust event and has relatively low pressure AND one of the other ports is just starting its exh event and has very high pressure, the high pressure will try to back flow into the first port.

This same idea applies to X and Y pipes. They act to create a high flow one way check valve that reduces reversion and therefore pressure before them. Properly designed they increase scavenging at low RPMs without restricting high RPM flow. This is why they help low RPM torque. This also applies to header merge collector theory/design

With a large volume collector the gasses slow dramatically, pressure spikes and YES it can and does definitely flow backwards from the high pressure collector to the low pressure (relatively speaking) ports at the end of their exhaust event or at overlap.

With a precision merge collector you can actually create a vacuum under the same conditions as the flow moves quickly through the small volume collector and "pulls" on the other primary. These conditions occur mostly at low to mid rpm and contribute to the old wives tale of an engine needing backpressure. Engines need vacuum in the exhaust!

People think they need huge pipes but in fact large pipe is the biggest mistake made by most. Consider that the GS400 is rated 300hp/310tq and has a single 2.35" center pipe... and that my LS400 is rated 290hp/300tq and has a single 1.95" center pipe... it stands to reason that a single 2.25" pipe can support 300hp. So, using dual 2.25"s mean that you could support 600hp, not optimally but it would do it.'

Ok, got all that?? I had to read it a couple of times too!!

[Oblivion]

yes yes well and good, but does that relate to turbo charged cars?

i understand that, but dont turboed cars work slightly differently..

also i was thinking one dark and stormy night....what if you could create a vortex in your exhaust then that would create less turbulence and more vacumm.

but maybe thats been done already

[TTOBES]

The same principals still apply...

The optimum amount of back pressure is none.

Tube sizes are still a trade off between between maintaining gas velocity and restriction(back pressure).

Turbo engines only differ in that they have a restriction in the exhaust system called a turbo.

Creates significant back pressure in the exhaust manifold and chops the exhaust pulses up finely so you can't use the scavenging effect.

A 3" is tried and proven.

Redevil is on to it.

As the man said, "a 2.25" tube will support 300hp...