Somebody Please Take Some Header Measurements for Me

[Longitudinal]

I have decided to build a set of stainless headers for my 1982 GS1100GL before putting it back together. To that end, I have ordered a set of laser cut stainless steel flanges and have designed a set of machined pieces that will mate to the head, seal to the exhaust gaskets, and connect to the header primaries.

What I need are some measurements. Can somebody give me primary OD and length (centerline length to the best of your ability) and tailpipe OD and length from a reputable system such as Kerker or Vance & Hines? I know that there are websites that give formulae for calculating header specs, but there is information that I don't have access to, such as cam timing.

[mmattockx]

The 'reputable' systems are not necessarily particularly great for anything but easy manufacturing. They often have unequal length primary tubes and other performance sins. Your service manual should have cam specs in it.

If you want ballpark numbers for a 4-1, use 1.5"-1.625" diameter primary tubes, 28" long to the collector with a 2"-2.25" diameter mid pipe, ~24"-28" long. Cap with the muffler of your choice.


Mark

[Longitudinal]

A true equal length header isn't going to happen anyway. The extra turns on the right two primaries that are necessary to compensate for the longer paths of the left two cylinders are way more work that simply isn't worth the effort on a street bike. 1.5" primaries and a 2" tailpipe both seem reasonable based on other header building experience I have.

Thank you for the input.

[mmattockx]

A true equal length header isn't going to happen anyway. The extra turns on the right two primaries that are necessary to compensate for the longer paths of the left two cylinders are way more work that simply isn't worth the effort on a street bike.

Are you building a drag pipe? The collector for a 4-1 should go dead center under the engine so both the outer cylinders (#1, #4) and inner cylinders (#2, #3) are paired mirror images of each other. You run #2 and #3 to the bottom of the collector and tuck #1 and #4 into the top two inlets to the collector. That gets you quite close to equal lengths on the primary tubes, certainly close enough for a street bike. My Kerker K system is set up like this and is as close as any street 4-1 of the era to having equal length primaries. You can sort of see how they are arranged in this pic:




Mark

[Longitudinal]

Mark,

What I am really looking for is some measurements. Will you provide some? Primary OD, primary length, tailpipe size.

[Steve]

I have two header systems here that I can measure, but they are not the "reputable manufacturers" that you request, and they are not 1100s.

My son has a MAC Tri-Y header on his '80 1000G. Might be interesting to see those lengths for comparison.

I have a 4-into-1 system from an unknown manufacturer on my '80 850G.

.

[mmattockx]

Mark,

What I am really looking for is some measurements. Will you provide some? Primary OD, primary length, tailpipe size.

The K system is close to the numbers I gave you but the mid pipe is on the shorter side and is 2.5" OD. The bike is out at my acreage so I can't measure it right now. I can get some numbers this weekend and let you know.


Mark

[Longitudinal]

Thank you. I ordered materials for a header today. I went with 1.5" primaries and a 2" collector and pipe. So I have committed to those figures. Now I need to get some lengths figured out.

[Steve]

OK, I have finally taken the time to get some measurements. A total of three systems.

My 850G. 4-into-1 of unknown origin. Primary tubes are 1.5", collector and mid-pipe to muffler are 2.5".

Primary tube lengths:
1 and 2 are 30"
3 is 28.5"
4 is 27.5"
From the front of the collector (the 4-pipe end) to the first baffle plate in the muffler is 19".
The length of the baffle assembly is 11".
What makes this baffle somewhat unique is that it is NOT straight-through, neither is it the side-by-side arrangement in many street baffles. It appears to be the standard perforated metal tube, but has a solid disk plugging the center of it, forcing all exhaust to go through the perforations into the outer chamber, than back through the perforations in the rear half to go out the exit port.

My parts bike came with a 4-into-1, also of unknown origin. 1.5" primaries and 2.5" mid-pipe.
Primary tube lengths:
1 is 28.5"
2 is 29"
3 is 28"
4 is 26.5"
From the front of the collector to the first baffle is 22.5"
The length of the baffle is 9".

My son's 1000G with a MAC Tri-Y header. 1.5" primaries, 2.5" mid-pipe.
Primary tube lengths:
#1 and #4 are 25" long before coming together into their 2-into-1.
#2 is 23", #3 is 22" long before coming together into their 2-into-1.
From the front of the two 2-into-1 collectors to the next collector is 7".
In stock configuration, there would be 11" from the last collector to the first baffle plate, but we have added an extension to put the muffler under the saddlebag. That extension adds 20" to the mid-pipe length.
The muffler is 10" long.

I don't know if any of those might need to be tuned for your 1100. Probably not, but that's all I have here.

.

[Longitudinal]

Thank you, Steve. A 2.5" mid pipe seems so large considering engine displacement. I can't help but wonder if the mid pipe size is being determined by the dimensions of collectors/technology to form them rather than what is best for flow.

[mmattockx]

Apologies to OP for the delay, it has been a frantic couple of weeks combined with a bunch of crap weather that kept me away from the shop.

Dimensions from my Kerker canister system:

Primary pipes are 1.5" OD, ~26" long from the exhaust flange to the collector.
Mid pipe is 2.5"OD, ~17" long from end of collector to the start of the canister.


Mark