6-2-1 Header Secondary Length?

[GreasyNRusty]

I'm going to build a 6-2-1 exhaust system for my primarily street '37 (and no, not an early '37 Mustang design study neither).

Dual 3-1 headers are pretty much easily commercially available, but I haven't yet found dedicated I6 6-2-1 secondary length calcs.

Years back (way, w-a-a-ay back) (when it was Clifford R&D) Jack Clifford always advised using dual 3-1 long-tube headers, then tying the two collectors to one tailpipe for a street tractor motor, which basically describes my Ferrari wannabe.

I am trying to get to a length for these dual pipes (secondaries) before tying them together for the single outlet at rear.

Rule of thumb, longer generally works better at street RPM, so I'm thinkin' secondary collectors/X or H-pipe/over the axle/dual-inlet single-outlet muffler, but: can someone refer me to info/equations re post-collector exhaust system system diameters?

Thanks!
Eddie

 

[pmuller9]

Which I6 ?

 

[GreasyNRusty]

Ummmm
Plymouth Flathead I6
6 Individual exhaust ports. Factory peak torque: 145 lbs.-ft@ 1,200 rpm but with the expertise found on this BB and 80 years of progress I believe I can raise that fairly easily.
Eddie

 

[Wesman07]

An engine simulator I use shows the secondary’s in that type of exhaust system to have a good amount of influence. Generally speaking the longer the secondary the sooner you will feel the benefits in the rpm range.

 

[bubba22349]

I'm going to build a 6-2-1 exhaust system for my primarily street '37 (and no, not an early '37 Mustang design study neither).

Dual 3-1 headers are pretty much easily commercially available, but I haven't yet found dedicated I6 6-2-1 secondary length calcs.

Years back (way, w-a-a-ay back) (when it was Clifford R&D) Jack Clifford always advised using dual 3-1 long-tube headers, then tying the two collectors to one tailpipe for a street tractor motor, which basically describes my Ferrari wannabe.

I am trying to get to a length for these dual pipes (secondaries) before tying them together for the single outlet at rear.

Rule of thumb, longer generally works better at street RPM, so I'm thinkin' secondary collectors/X or H-pipe/over the axle/dual-inlet single-outlet muffler, but: can someone refer me to info/equations re post-collector exhaust system system diameters?

Thanks!
Eddie

Hi Eddie, I can give you some info on the old tried and true Tri Y headers which really work great on street cars up into the 4000 to 4500 RPM ranges. That would be about right for your 1937 Plymouth 201 Flathead six since they made their Max Factory Horse Power at 3600 RPM and Max Torque at 1200 RPM. The Tri Y's have been in use for over 60 years. These specs would probably would also work for the 6-2-1 design you want to do. On these type headers the the primaries should be just long enough to not get reversion, at about 12 to 18 inch length range. The secondary Pipe lengths would then end up as the remainder so that the Total Header length to the end of the collectors is in the 34 to 36 inch range but you can also go more if you have the room to fit them like maybe up around 40 inches. On the Tri Y's the Cylinders get paired together by the Firing Order so for your six that's 153624, so you would pair 1 & 5, 3 & 6, 2 & 4 these type headers are not a tuned length type. I had my first set of these Tri Y's on my 27 T build in 1966 and they work great. Best of luck on your Header build.

Below are some pictured examples of these Tri Y type Headers

 

[Engine Fan]

So primary and secondary length for 3/2/1 headers have been given a range to work with. Taking the big six as an example, we see primary pipe diameters on aftermarket headers most commonly at 1 1/2" and 1 5/8" on long tubes.
Knowing that none of the exhaust pulses interfere with each other(3 into 1) and the stock 300 EFI uses 1 3/4" secondary's what would be an appropriate secondary pipe diameter range on our performance builds?

 

[aussie7mains]

So primary and secondary length for 3/2/1 headers have been given a range to work with. Taking the big six as an example, we see primary pipe diameters on aftermarket headers most commonly at 1 1/2" and 1 5/8" on long tubes.
Knowing that none of the exhaust pulses interfere with each other(3 into 1) and the stock 300 EFI uses 1 3/4" secondary's what would be an appropriate secondary pipe diameter range on our performance builds?

Your possibly getting carried away here, if this vehicle is for street use, it doesnt matter what lengths they are as long as they are LOW RESTRICTION, so for a 5litre six its going to need 1 3/4 primary pipes, 3 into 2 as you've described, length doesnt matter much, just so long as they are smooth and the collectors are cone over pipe and neat. The secondary pipes need to be about 2inch each, they can remain separate all the way to the bumper if you wish, early Jags are like this. If you want a single system then make the secondary pipes longer to make connecting them together convenient. Then into a no smaller than 2 1/2 single system and turbo muffler, this might be a bit raspy when you stick the boot into it, so a resonator (glasspack) will deaden that. All the stuff about tuned pipes is for race cars without a silencer. Look around at some of the really high performance sixes like BMWs and Merces, and even our Barra, they get really good power and torque with a well designed cast iron manifold and system which is quiet as a grave. The Barra has better than 1bhp per ci like this. 190kW, thats 254bhp and that to DIN. I have a 2.5 single pipe on my blown six (200ci but thinks it a 300ci) and that is just fine.

 

[Wesman07]

I believe there is more to pulse tuning than length, rpm and displacement. It needs to be timed correctly for the valve events.