2001CamaroGuy

iTrader: (6)

It seems to me that an x-pipe, by it's very nature must inherently reduce total cross-sectional area.

Examples:

Burns Stainless x-pipe (Burns says their X-pipe is basically a pair of 2-1 merge collectors back-to-back):


U-bend constructed x-pipe (these seem to just be a pair of 90degrees "overlapped" via "windows"):


Kooks Vette x-pipe (looks like a seamless version of the U-bend constructed x-pipe):


They all, in one way or another result in a reduction in total cross-sectional area at their merge point. How then does this not hurt performance (obviously it does not, but WHY?)? How does one judge how much "merge" is good? At some point, the reduction in total cross-sectional area would inherently result in a bottle neck. Is there some "rule of thumb" (say 1/3 individual pipe size overlap?)?

I'm trying to fab up an x-pipe for my Z4 swap and I don't want to inhibit flow capacity.

Thanks,
Chris


edit:
I found the attached "how-to" which says, 1/3 overlaps. But does that 1/3 reduction become a bottle neck at some point? Any idea how to estimate this loss in flow capacity?

On the surface (over simplifying), that would seem to reduce the total capacity by 1/3. Thus, if using 3" exhaust, it would be equivalent to something like 2" (perhaps 2.5") at that point and thereby limited to that HP limit?

rsz288

The exhaust pulses travel through one at a time so the "x" section opens up more flow space to each pulse as they start to cool and slow and does provide a degree of scavenge effect/mild pressure reduction on the opposite bank where the pulse is yet to arrive.

Dyno tested they would all have slightly different effects at different rpm's with different cam/head setups.

Generally their benefit is improved low/mid range but restriction really only starts to take place once higher rpms's are reached and the pulse arrival rate from each bank starts to see pulses get very close together toward the tail of one pulse and the head of the next pulse overlapping. That might take 10k rpm though to have a substantial negative effect.

As with any thing on internal combustion engines, its always a compromise: gain here, lose there.

SS10Tech

Also note the location of the X pipe in reference to the headers does in fact matter. Where the cross section intersects has a direct impact on what RPM range is effected most.

2001CamaroGuy

iTrader: (6)

I would THINK the further from the headers, the lower in the RPM band the x would "help" due to longer timing of pulses. Correct?

imma_stocker

iTrader: (19)

A well designed X-pipe will still scavenge and accelerate the exhaust gases as they push through. They are supposed to have a reduced cross-sectional flow at a proper angle to do this.

Dunno what the rule of thumb is for perfect sizing but the oil down the exhaust pipe trick will show you where your EGT is low enough to end an exhaust without performance decrease. Figure the gases will lose X% of volume with Y% temp drop, can't remember the rate for exhaust gas but I found a rough estimate online years ago.

stevieturbo

It will have less area compared to two 3" pipes.

But compared to two individual 3" pipes that are not linked, the X merge does open up more area for each pipe.

slowpoke96z28

iTrader: (2)

i think what seems to be your main concern is the exact reason why they make their "magic".

the grease pencil also works as a good trick same as the oil on the pipe imma stocker mentioned

2001CamaroGuy

iTrader: (6)

Got to thinking about it again and I guess the point that always gets missed (by myself included) is not total area but area/pulse. Because pulses come in order (not all at once), the "reduction" is not really an issue because the pulse still sees 3" (or more) for the entire length of the travel.

Wnts2Go10O

iTrader: (12)

i think the idea of the X-Pipe is a sort of manipulation of the helmholtz resonance. the previous pulse helps pull the next:

"When air is forced into a cavity, the pressure inside increases. When the external force pushing the air into the cavity is removed, the higher-pressure air inside will flow out. The cavity will be left at a pressure slightly lower than the outside, causing air to be drawn back in. This process repeats with the magnitude of the pressure changes decreasing each time."

the other more likely way is the venturi effect:
"The Venturi effect is a jet effect; as with a funnel the velocity of the fluid increases as the cross sectional area decreases, with the static pressure correspondingly decreasing. According to the laws governing fluid dynamics, a fluid's velocity must increase as it passes through a constriction to satisfy the principle of continuity, while its pressure must decrease to satisfy the principle of conservation of mechanical energy. Thus any gain in kinetic energy a fluid may accrue due to its increased velocity through a constriction is negated by a drop in pressure. An equation for the drop in pressure due to the Venturi effect may be derived from a combination of Bernoulli's principle and the continuity equation."

25thhawk

iTrader: (3)

What puzzles me is why every x pipe uses a different angle. Before I bought one, I talked with Burns stainless and they use a 15 degree angle. They told me it was the best angle for flow and scavenging. If you look at some of the others, they are made from 90 degree elbows. I cant see those helping very much.

Ryan

2001CamaroGuy

iTrader: (6)

The smoother (shallower) the angle, the smoother thrasition. A straight pipe flows the best and a 90 is the worst, so the close to straight the better. I forget the exact number but some engineering paper said a single 90 was equivalent (flow reduction) to something like 10feet of straight pipe (don't quote me on the numbers...but it was staggering!)!

Wnts2Go10O

iTrader: (12)

this will really throw u for a loop then:


and that actually works better than an x in some combos

2001CamaroGuy

iTrader: (6)

Basically a collector rather than a balance pipe...

Wnts2Go10O

iTrader: (12)

sort of. its function scavenges by manipulating the exhaust pulses but because of that it also balances the 2 sides as one pulse will help pull the other through. its more like the concept behind the 180* headers (where instead of one bank on one collector, each banks primaries are matched to a collector).

2001CamaroGuy

iTrader: (6)

Any point to having a "balance pipe" with 180degree headers (just curious)?

Wnts2Go10O

iTrader: (12)

i was making a comparison between the "box" and the 180* headers. but there is still a point to it, i think (i could be wrong or be correct), the 180* headers evenly divide the pulses (instead of it being 90, 180, 270, 180, 90 of the standard setup, hence the "180*" moniker) BUT i would think the x-pipe/box would still work well and do their intended job.. maybe even better than the standard 90* headers. the H-pipe is more of a balance pipe but not so much of a "merge"

wildcamaro

iTrader: (15)

I have done some CFD analysis comparing x pipe and y pipe systems, and the x pipe does nothing for extra flow, like someone else said the exhaust pulses are helping pulling the next one through thus helping overflow considerably over the alternatives. I didn't get the chance to model an h(balance) pipe system but I know it wouldn't have beat the x pipe. The x pipes position in relationship to the engine's exhaust ports also had a lot to do with improving flow.

2001CamaroGuy

iTrader: (6)

Umm.... you said:

then turned around and said:

Perhaps you misstated something?


Sounds like you have some good data. Just think your fingers got away from you....

Wnts2Go10O

iTrader: (12)

the x itself doesnt provide more or less flow. what it does do is manipulate the exhaust pulses to pull the exhaust through more efficiently.

wildcamaro

iTrader: (15)

After reading what I typed I can see how it would be confusing lol, but I guess I meant to say the x pipe doesn't increase cross sectional area but it does improve flow if that makes sense. The momentum of a previous pulse pulls the next one through, sort of like the vacuum of a piston moving down on the intake stroke helps to pull fuel and air in. By the way if you are more interested the program I used was ANSYS Fluent, I don't know if you have access or not but I would be willing to share my findings