Ferocity02

iTrader: (60)

Well, I went to the engine builder today to get some work done, and while I was there a man began to talk to me about how these new generation motors tend to have problems with the exhaust choking up. He wrote out a diagram of the firing order of an LS1. Basically, he said, since cylinder 1 fires 90° after cylinder 3, that this causes a "choke" in the exhaust in the collector. Basically, the exhaust from cylinder 1 has trouble getting out since the exhaust from 3 is already there. He said leads to the "hotrod sound" and the slight stumble of the motor. He said that he installs "a piece of steel" into the collector that helps solve this problem. He said it increases power, torque, and vaccum, while reducing stumble, the grumpy sounding exhaust, and track times. He also said he makes the car more throttle responsive and more sensitive to tuning. He didn't show me what he exactly installs, but he said he has to make the piece different for every application based on firing order and header design. He said doing this improved a guy's Nova from a 10.30 to a 10.06 quarter mile ET.

Has anyone heard of this? He said he's done it for quite a few customers and all of them loved the improvement. Just curious. Thanks!

MadBill

Without crossing over two primaries from each side, any two-plane crank V-8 will have a cylinder pair on each bank firing 90° apart. One way to separate the outlets would be something a little like a 'tri-Y' collector, namely putting an internal baffle down the center of the collector between the adjacernt firing pipes. The pulses would still mingle of course, but only after they had expanded in to their own 'collector half'...

Ferocity02

iTrader: (60)

So has anyone ever performed such a modification to their headers?

turbotacoma

iTrader: (1)

Flowmaster makes this for the end of the tyipcal 4 to exhaust collector. It turns a 4 to 1 design into a 4 to 2 to 1 design. Sport bikes use this to help with mid range power, while keeping a good top end. I believe it is called pulse flow collectors. Check out Summit or Jegs. I used this to build my headers for the tight engine compartment I have.
http://www.ls1miata.net/gallery/miat...ust_008?full=1

http://www.ls1miata.net/gallery/miata/header_004

J-Rod

This is why proper header length and tube placement are important. If you set them up properly, you pair up header tubes so that once tube creates a vaccum in the other which helped scavenge the cylinder. This is why you see talk of 32" primaries ona 346. That is pretty close to ideal primary length. If you pair the tubes up with a good merge collector, you're good to go.

Now, you can do a 4-2-1 you simply need to have you tubes placed properly to do so. You can even do a 4-2-1 with a few mandrel u-bends if you know what you are doing.

Ferocity02

iTrader: (60)

Does ideal primary length change with a change in displacement, such as a 427...?

J-Rod

Absolutely, and it also changes as a result of RPM.


Header pipe length (in inches) = ((850*(360-EVO))/RPM – 3
Header diameter (in inches) = ((cylinder. disp. * 16.38 / ((hdr len + 3) * 25))) * 2.1

EVO is Exhaust Valve Opening

These formulas are from A. Graham Bell’s Performance Tuning in Theory and Practice. For a street engine, the RPM used should be the peak torque RPM. For a race engine, the peak hp RPM should be used. I tend toward using the Peak HP, other wise the headers get REALLY long primaries.

So, on a 346 you'll find ~32" primary for most applications.


Now, that doesn't take into account in an engine when you have a poor exhaust port, or a really good port. As an example. on an LS7 good results have been shown with a 1 3/4 header. This may be due to the fact that the exhaust port is relatively poor in comparison to the intake side, and the velocity of the 1 3/4 over a 1 7/8 does a better job. Time will tell.

So, these formulas give you a good number to start with, but some empirical testing is always in order.

billyb

iTrader: (15)

so slp headers should work good with their scavenger pipe style ,but all i read is that they suck, can anyone shed some light on this.

Zymosis

iTrader: (7)

People say they suck because of their extremely poor fitment and ground clearance.

Ric

Well, all those bends cause turbulence, too. And that costs power.

Ferocity02

iTrader: (60)

Yeah, I think every 90° of bend results in a 3% loss of flow. I think this is the number for FI, probably a little different than free-flowing exhaust.