Exhaust flow talk

[NoHachi]

You are still suffering to much from the myth...don't think backpressure..think in terms of flow and resistance..resistance costs hp...full stop. It does it at high rpm as well as low rpm..resistance is the enemy.

Knowing that, 99% of the exhaust parameters have been decided..the ideal system is a trumpet right after the header..(search around, I think that they are even mentionned in Bell, along with a picture of them on a drag VWbug), but that would leae the neighbours deaf and you in jail. THATS why we have exhaust, we need to dampen noise, and the best way of doing that, is adding as much pipe under the car as we can (exactly because of your observation that the gasflow loses energy as heat as it progresses through the exhaust). Now, the header has a diameter, pipes come in constant diameters, so what to do? Keep weight to a minimum (small diameter)(weight is the no1 performance gain from aftermarket exhausts!), maximise power.
In practice that means that you keep the entire exhaust path at the diameter with which it exits the header..inc the perforated tube in the fartcan. The further back you get, the lower the velocity, the less trouble you have from discontinuations. Thats why fartcans, when done poorly, dont gain you any hp, but also cost you almost no power or torque..show me a dyno that proves me wrong. A fartcan done well (inner diameter equal to the rest of the system) gains you power, it reduces resistance along the entire revband (and you pay for this through police tickets and eventually a hearing impairment.)

[banpei]

I think there is a misunderstanding: two stroke engines need the back pressure for functioning properly due to having only two strokes which will effect in escaping fuel mixture during the short period both inlet and outlets are open. Four stroke engines don't need the back pressure due to having valves instead of big gaping holes.

http://en.wikipedia.org/wiki/Two-stroke_cycle
vs
http://en.wikipedia.org/wiki/Four-stroke_cycle

[Ivan141]

@ Banpei, you're not entirely correct. NA tuning involves cams with long opening times and thus bigger valve overlap. Just read in the link you posted... a little quote:

Quote:At high rotational speeds, consistent flow through the intake and exhaust ports is maintained by allowing the intake and exhaust valves to be open simultaneously at top dead center (known as valve overlap). The momentum of the exhausting gas maintains the outward flow and creates a suction effect on the cylinder known as scavenging, helping to draw the intake charge into the cylinder

The theory is much the same as 2-stroke technology, just less critical.
Think about it...what would be the use of tuned 4-1 or 4-2-1 headers if the pulses didnt aid in scavenging as it is called a few posts back.
A turbo engine on the other hand benefits from having very little overlap.

[banpei]

Ivan141 Wrote:@ Banpei, you're not entirely correct. NA tuning involves cams with long opening times and thus bigger valve overlap. Just read in the link you posted... a little quote:

Quote:At high rotational speeds, consistent flow through the intake and exhaust ports is maintained by allowing the intake and exhaust valves to be open simultaneously at top dead center (known as valve overlap). The momentum of the exhausting gas maintains the outward flow and creates a suction effect on the cylinder known as scavenging, helping to draw the intake charge into the cylinder

The theory is much the same as 2-stroke technology, just less critical.
Think about it...what would be the use of tuned 4-1 or 4-2-1 headers if the pulses didnt aid in scavenging as it is called a few posts back.
A turbo engine on the other hand benefits from having very little overlap.

Ah, I didn't read that part.

[BenR]

I think there is alot of confused information here.

Backpressure itself, in performance terms, is not wanted. That is a given for all engine tuners strive to reduce pumping losses.

However, the use of back pressure is beneficial when you take it out of context for peak performance as JTA is speaking of.

When you run a larger exhuast diameter you reduce back pressure and restriction, however there is a fine line between optimum and too big.

As previously stated, the exhuast manifold is used to tune an area of efficiency in the exhaust range. With any exhuast manifold you need to design it to operate at a specific rpm range, but you will always loose in another area, and good exhaust manifold design will put this loss in an area which is used least. And the idea between true equal length headers is that you want each exhaust event to correspond with the next one 180 degrees later, and every event is 180 deg apart from each other.

Each exhuast event travels in the exhaust system as a 'slug' which can be broken down into 3 parts. The front is a high pressure area, middle, and rear most is a low pressure area. As this slug travels you need to time the rear of the slug to correspondingly meet the front of the next slug, and this timing depends on rpm, EGT, bore size, cam timing, cylinder head specs etc etc. This is the basis of manifold tunning

Anyway, i'm going completely off topic.

The discussion about back pressure. In regards to bore size, if you have too big a bore you will simply generate non linear flow which will tuble over itself, velocity is needed. Too small and there is plain straight forward restriction. In regards to back pressure reduction resulting in lost torque at lower torque, its quite simple. Larger bore exhausts gain peak power as back pressure and pumping losses are reduced, however at lower rpm where events have a longer timeframe, this low back pressure is not beneficial as it is 'too efficient'. At lower rpm the window of time during valve overlap is too long, and with high specific output engines you always run generous overlap. Where at high rpm the overlap helps, at low your simply reducing the dynamic CR by bleeding off cylinder filling, and this reduces torque. The lower back pressure allows the bleeding to happen more efficiently in the same time frame, hence you feel less torque.

As with all tuning, if you gain one area, you always loose in another....unless you start supercharging.

[surfingdrums]

I just read this entire thread...my head hurts...

[banpei]

surfingdrums Wrote:I just read this entire thread...my head hurts...

LOL!

@BenR: thank you for explaining! I think I get it now.

[adomz]

hmmm...that's what i was looking for!

[novocaine]

Hi guys, I have a 20valve Blacktop, what would the ideal dimensions be for a manifold to fit my motor? Looking to optimize power in the high revs... 8000rpm is truly beautiful!
The motor is completely stock standard.

[oldeskewltoy]

great discussion!!!

BenR Wrote:At lower rpm the window of time during valve overlap is too long, and with high specific output engines you always run generous overlap. Where at high rpm the overlap helps, at low your simply reducing the dynamic CR by bleeding off cylinder filling, and this reduces torque. The lower back pressure allows the bleeding to happen more efficiently in the same time frame, hence you feel less torque.

What is best described as a peaky engine....... why I argue for street power as opposed to all out peak hp...