c5blkvette

If someone can answer this question I will be very happy as my brain is hurting.

I have been doing some research on X-pipes and there is one explanation that says there are irregular exhaust pulses and that two cylinders on opposing banks fire within 45deg of each other. When the two header collectors merge (at the X-pipe), the two exhaust pulses “compete” for space and can send a pulse back up the headers that causes backpressure etc. The X-pipe is there to allow enough room for the pressures to equalize and continue on down to the tailpipe.

http://www.mustangforums.com/m_1028601/tm.htm

OK, here is my reasoning: The LSX V8 has firing order of 1-8-4-3-6-5-7-2 and each fires exactly 90deg apart of crankshaft revolution. Further, the exhaust valve for each cylinder opens at a set # of degrees after firing so in theory, these should also be 90deg apart in order 1-8-4-3-6-5-7-2.

If this is the case, how can any exhaust pulse overlap with any other exhaust pulse?

slow67

iTrader: (2)

Actually, the LS1 has a firing order of 1-8-7-2-6-5-4-3

bygblok

iTrader: (4)

yes, the firing order on an old style GM is 1-8-4-3-6-5-7-2........ in recent years you can do a cam swap on the older motors and gain a few HP by swapping to the newer firing order. Thats also why an LS motor sounds a little different as well. With the banks firing at better aspect ratio's to each other it tends to smooth out the exhaust pulse a bit.

joecar

If you look at the FO, you'll see that cylinders on each bank fire 180° apart, with two exceptions, 2-6 and 3-1 which fire 90° apart;

the exhaust pulses for 2 and 6 are closer together than the other pulses (same for 3 and 1), which means the pulse for 6 (and for 1) arrive into the collector too early (the pulses don't overlap), their position in the collector is advanced compared to the other pulses; this changes the pipe harmonics/resonance, reduces the pipe "tuning" effect;

the correct way to fix this is to have the primaries for those cross over the banks (same concept as a 180° dual plane intake manifold) so any two consecutive firings are from alternate banks...

but that may not fit in a F-body or Mustang engine bay; so X-pipe is an attempt to allow the pulses to partly cross over; but now all the pulses cross over, but that's ok because now all the pulses have the same spacing, and the pipes can be tuned for this.

The PAT WS6

iTrader: (14)

if u look on the intake manifold of ur car it will tell u the firing order of the lsx based v8's on the drivers side part of the manifold

c5blkvette

Thanks Joecar, this makes sense. I guess if you had a dual system with no x-pipe, there would be a pulse with more pressure (the two cyls that fire 90deg. apart on the same bank) and then a lower pressure when the other bank is doing the same thing. the x-pipe just evens things out...

Adnectere

I disagree. I think there are many, many reverbations in the exhaust pipe. When the exhaust valve opens, you get a pressure wave coming from that cylinder and that wave is going to travel until it hits something to change it. That's either going to be the end of the tailpipe in an open exhaust or the muffler in a closed one. That wave is then going to flip 180 degrees and go right back the way it came.
So not only do you have waves coming everytime a valve opens, but you also have reflected waves coming back from the end of the system and intermixing. Trial and error will mostly tell you how long the exhaust tubing needs to be to be most efficacious. Check out the Scientific Design of Intake & Exhaust Systems. I forget who it's by. My two cents.

GuitsBoy

iTrader: (4)

As a quick sidebar, are the 2/6 and 3/1 firings the cause of blatting or rasp?

c5blkvette

True, there are many reverberations but the x-pipe is not primarily dealing with the pressure wave pulsations (that travel at around the speed of sound) but rather dealing with the flow of the actual exhaust gasses.

Most headers and exhaust designs deal with first order pressure pulses only. It is too complicated to factor in the second order and then by the third order (ie. bounced back the third time) they are negligible.