Question about exhausts, headers etc

[Wing Commander]

No real point behind this, other than I was wondering about it today whilst making silly V8 noises to myself in my head.

If you have a V8 engine running on open headers, how does that work? Does the engine need to be tuned for the fact that none of the exhaust ports are now connected together.

I first started thinking about this when thinking about the V8 interceptor used in Mad Max. In the film, he has 4 exhaust tubes exiting each side, so one per cylinder. If these pipes are not connected together, surely the car will not run correctly? Can this be tuned out?

Also, how would the length of the headers/exhausts affect things? In the case of the interceptor, the pipes will be quite long, but on some drag cars, they have relatively short pipes in the "engine bay".

Anyone have any basic reading material on this that would give me a quick intro? Nothing too complex is required as I really have no idea about this, but feel free to chip in with stuff!

Thanks

[GreigM]

From my simple understanding headers come together in the collector simply to aid packaging (i.e. single cat, silencer and exit)..separate exits would be better as there would be less back pressure. Lengths would be irrelevant as the only reason lengths are kept identical on normal header pipes is to ensure the gasses don't "collide" in the collector causing too much back pressure.

[bob_g]

Can't help with the question without dredging the very bottom of the memory banks, but just wanted to give a big thumbs up for making V8 noises - It's good to see I'm not the only one!

(was doing it today after a C63AMG burbled past me at work )

[paulrussell]

Exhausts exist to take harmfull gasess away from the engine bay. Engines would run better without headers as there wouldn't be any restriction of the exhaust gasess. I hope that's simple enough for you lol.

[chevy-stu]

From my reading and experience mainly with small block Chevy's, the common thinking is shorter headers (primaries) will suck lower and mid-range BHP and torque.. Fully open headers is no issue on high revving engines like drag racers, as they're just trying to get the gases out as fast as possible, and these are high compression engines not needing any back-pressure..
Most people run long tube headers if possible, which are usually about 36" in length from the cylinder head to collector, as this gives the best compromise of power to packaging size...

[TheEnd]

As short and open as possible for high speed, but for low speed, you can connect them together and tune the resonance to help clear the cylinder.

[BlueMR2]

I was making a V8 rumble sound earlier. Well the 928's V8 was .

As its older thus requires and has no cats i've been wondering if it will spit flames.

[berni29]

Hi

I thought that as each cylinder fires the exhaust gasses produce a pressure wave which travels down the header. In a well designed system all these pressure waves meet at the same time where the headers go into one (or two then one) and all the pressure waves add up and you get a strong single wave which keeps the gas moving quickly and improves performance.

Berni

[Life Saab Itch]

berni29 said:

Hi

I thought that as each cylinder fires the exhaust gasses produce a pressure wave which travels down the header. In a well designed system all these pressure waves meet at the same time where the headers go into one (or two then one) and all the pressure waves add up and you get a strong single wave which keeps the gas moving quickly and improves performance.

Berni

Exactly the opposite.

The pulses are tuned, via the length of the headers, to be regular in occurance at the collector. This means that each exhaust pulse pushes the last one along and helps pull the next one along.



That's why Subaru's have their distinctive engine note, because the exhaust headers are not the same length.

[Mastodon2]

BlueMR2 said:

I was making a V8 rumble sound earlier. Well the 928's V8 was .

As its older thus requires and has no cats i've been wondering if it will spit flames.

I would not have thought the fuelling would be aggressive enough to spit flames. I've seen naturally aspirated rally cars (without anti-lag, obviously) and highly tuned sports cars do it, but I would imagine that without deliberate turbo anti-lag enabled most cars could not do it.

You can get pops and bangs from the exhaust as unburnt fuel ignites deeper inside the exhaust system and shoots a fantastic noise from the exhaust, if the exhaust is built to let you hear it. Case in point, a friend of mine own a Fiesta ST; the exhaust it came with was pretty quiet, and it was changed out for a stainless steel Powerflow system. With that came pops and bangs on the overrun when driving hard, as the trailing throttle exhausts a rich fuel/air mix which ignites on the hot metal as I'm sure you well know. However, as a 2.0L making only 160bhp, the fueling is not really aggressive enough to put enough fuel/air mix into the exhaust to have it burst into flame at the tip of the exhaust. Having a shorter exhaust system with a straight-through exhaust will help to obtain those fantastic pops and bangs, as would a decat.

I am aware of some turners who apparently can "map in" pops and bangs while remapping an ECU, by altering the fueling map, ignition timing etc. Not sure how much of that is voodoo and black magic but some cars do seem to bang a lot, unfortunately none that I have seen making flames unless they were turbo. I'm sure someone will youtube a flame spitting, naturally aspirated street car.

Most cars simply muffle the pops. On both my Fiesta (not an ST unfortunately but a humble 1.3L) and the Focus Zetec my brother used to own, if you revved the car hard you could not hear anything other than engine noise inside the car, but if you stood directy behind the cars while being revved hard, on the lift-off you would get little pops and burbles from the exhaust. Unfortunately the standard exhausts that Ford fitted to these cars are simply strangling the noise from the exhaust with their extensive baffling. My car is in for a new exhaust, half as a test run to make sure a local exhaust builder is up to scratch to do an exhaust for my next car, which really will be my pride and joy instead of a car I look after fastidiously while yearning for something faster, and half because as I will have the car for at least a few more months, I might as well have a loud exhaust which puts a smile on my face, and hopefully will let the pops and bangs be heard!

[The Wookie]

BlueMR2 said:

I was making a V8 rumble sound earlier. Well the 928's V8 was .

As its older thus requires and has no cats i've been wondering if it will spit flames.

Not unless there's something wrong with the MAF, or ECU, or injectors, or ignition... So being a 928 it probably will

[300bhp/ton]

GreigM said:

From my simple understanding headers come together in the collector simply to aid packaging (i.e. single cat, silencer and exit)..separate exits would be better as there would be less back pressure. Lengths would be irrelevant as the only reason lengths are kept identical on normal header pipes is to ensure the gasses don't "collide" in the collector causing too much back pressure.

I'm sorry to say, but that's completely wrong, on each count and point you make.

[300bhp/ton]

Wing Commander said:

No real point behind this, other than I was wondering about it today whilst making silly V8 noises to myself in my head.

If you have a V8 engine running on open headers, how does that work? Does the engine need to be tuned for the fact that none of the exhaust ports are now connected together.

I first started thinking about this when thinking about the V8 interceptor used in Mad Max. In the film, he has 4 exhaust tubes exiting each side, so one per cylinder. If these pipes are not connected together, surely the car will not run correctly? Can this be tuned out?

Also, how would the length of the headers/exhausts affect things? In the case of the interceptor, the pipes will be quite long, but on some drag cars, they have relatively short pipes in the "engine bay".

Anyone have any basic reading material on this that would give me a quick intro? Nothing too complex is required as I really have no idea about this, but feel free to chip in with stuff!

Thanks

Exhaust tuning is a black art. If you want some heavy reading Google David Vizard, there are some good books also.

Exhaust tuning does have some bearing on engine type, a traditional V8 with a dual plane crank, i.e. most American, German or Jap V8's (but not Ferrari, TVR AJP or Lotus as these are flat plane cranks). These traditional V8's have a firing order which means you get uneven firing across the cylinder banks, i.e. one bank fires twice in succession during the combustion process.

This complicates exhaust setups.

But the reason you want to merge the pipes is for a thing called exhaust scavenging. This only really works on n/a engines, but it introduces a fifth process into the combustion cycle, where an exhaust pulse from one cylinder helps to draw in/create a vacuum in another cylinder giving a higher charge density.

As you can see, having two cylinders fire after each other on the same bank means matching all the correct primaries together is a pain. Hence some high power setups have pipes connect each bank of cylinders too.

Ideally you need something like this:


Cost and packaging are limiting factors with setups like these. Flat plane crank V8's don't require this cross over section for optimum exhaust tuning.

But scavenging is very important. On something like a tuned LS1 V8 in a Corvette correct scavenging can see upwards of 30bhp gains.

The length of the header primaries is also very important too.

Essentially the longer the primary the lower the rpm scavenging will occur. So as most road V8's of this type don't rev past 6000-7000rpm often and require good mid range performance too, most will go for a "long tube header". Shorter primaries will still promote scavenging, but at far higher rpms. I've heard it said that "shorty" headers on an LS1 won't scavenge until around 10,000rpm. Which is pretty useless in this application.

A shorty header for a LS1:


A Long tube header for a LS1:


The shorty will typically make 3-5bhp gains while the long tube 20-40bhp gains depending on setup.


Exhaust scavenging goes further than just headers. Most V8's will also run a merge collector in the exhaust also, typically either a Y, H or X design. This does the same sort of thing too.


As for back pressure. Well you don't want it ever!! You want the least back pressure possible while retaining maximum exhaust gas flow.


Forced induction setups are slightly different and from my knowledge don't gain as much in exhaust tuning as you are already forcing intake air into the combustion chamber at pressures higher than 1 atmosphere already. So it becomes more about flow rate than scavenging.


Aero engines have a different take on it too, as in they might be happy to sacrifice some potential exhaust tuning in order to use the exhaust gases as additional trust such as on the Supermarine Spitfire.


Although I freely admit I don't know how much affect they had.

[V88Dicky]

^^^^WHS^^^^

[Life Saab Itch]

Yeah, what 300bhp/ton said.

[Wing Commander]

Guys, this is all brilliant stuff - kudos

So, sticking with the example of the V8 interceptor, what with it being forced induction via supercharger, the fact that it appears to have a separate exhaust for each cylinder matters less as the need for scavenging has diminished?

As you have probably guessed, I would love to tinker about with something suitably crass and loud, such as a blown V8 with straight through exhausts. At the moment, it is firmly on the "would love to do" pile, but theoretically I could have a play with the idea in the summer.

Thanks

[300bhp/ton]

Wing Commander said:

Guys, this is all brilliant stuff - kudos

So, sticking with the example of the V8 interceptor, what with it being forced induction via supercharger, the fact that it appears to have a separate exhaust for each cylinder matters less as the need for scavenging has diminished?

I don't know the exact setup of the V8 Interceptor, but I'd guess it's real setup (the blower was fake and powered by an electric motor), was it used the stock log style cast iron headers with a custom exhaust that just happened to have 4 tailpipes per side.

If making one for real, then you'd be best of running some good flowing shorty headers (assuming you used a supercharger) or some long tube headers if n/a. Running 4 separate exhaust per bank would offer no performance gain and would be a routing nightmare to install and locate under the vehicle.

Wing Commander said:

As you have probably guessed, I would love to tinker about with something suitably crass and loud, such as a blown V8 with straight through exhausts. At the moment, it is firmly on the "would love to do" pile, but theoretically I could have a play with the idea in the summer.

Thanks

V8's are quite accessible as a 2nd car.

When I was 19 I bought a Jag XJ-S V12 (ok not a V8 ) for £700. Removed the rear silencers and it made a great noise.

I've also got a TR7 V8, which I've used for many years as a daily driver.

And for around the £4-6k mark you can get yourself a 4th Gen Camaro V8. Very usable and easy to live with.

This is what mine currently sounds like:
http://s132.photobucket.com/albums/q11/300bhpton/D...


But there are lots of other options such as a XJ8, S-Type, Lexus LS400, V8 Discovery, etc.

[The Black Flash]

300bhp/ton said:

Wing Commander said:

Guys, this is all brilliant stuff - kudos

So, sticking with the example of the V8 interceptor, what with it being forced induction via supercharger, the fact that it appears to have a separate exhaust for each cylinder matters less as the need for scavenging has diminished?

I don't know the exact setup of the V8 Interceptor, but I'd guess it's real setup (the blower was fake and powered by an electric motor), was it used the stock log style cast iron headers with a custom exhaust that just happened to have 4 tailpipes per side.

The side pipes on the Pursuit Special (incorrectly referred to as the v8 inteceptor in film 2 ), are fake, IIRC.

Is it not the case that open headers can help scavenging, as you get a standing wave in the pipe - if you tune the length right, you get a low pressure at the valve. I don't think they have to be connected.

[300bhp/ton]

The Black Flash said:

The side pipes on the Pursuit Special (incorrectly referred to as the v8 inteceptor in film 2 ), are fake, IIRC.

Is it not the case that open headers can help scavenging, as you get a standing wave in the pipe - if you tune the length right, you get a low pressure at the valve. I don't think they have to be connected.

My understanding is they do indeed need to be connected, as it's the pulse from a previous combustion cycle that makes the difference.

[Huff]

300bhp/ton said:

Aero engines have a different take on it too, as in they might be happy to sacrifice some potential exhaust tuning in order to use the exhaust gases as additional trust such as on the Supermarine Spitfire.


Although I freely admit I don't know how much affect they had.

A quick Google suggest it was (very roughly) equivalent to an extra 10% of engine output, from 70lbs thrust on early merlins to over 200 on the later ones. It translates to 70 to 200 'free' hp at about 300mph.