Tri Y, why or why not?
 

The complete reasoning behind Tri Y headers is a mystery to me, but it still seems like a good idea due to fewer pipes and simpler plumbing. Specifically two fairly small diameter pipes poking out past the constraints of the firewall area instead of four similar ones or one larger pipe with all the combining in the engine bay.

Firing order is: 1-3-7-2-6-5-4-8
Looking from front of engine to the rear cylinders are numbered:
Passenger side: 1-2-3-4
Driver side: 5-6-7-8

720 degrees in a full two revolution cycle of the engine, each cylinder's events 90 degrees apart. What seems ideal is to merge cylinders 360 degrees apart with equal lengths of pipe, the length being tuned so exhaust events have effect in the desired rpm range. As I understand it the high pressure burst from an exhaust valve opening travels down the exhaust pipe to the merge where it creates a low pressure wave in the other pipe that reaches the other cylinder exhaust valve somewhat prior to its closing reducing the amount of exhaust left in the cylinder. Being 360 degrees apart the exact same thing happens in the opposite direction.

Trouble is ideal matching is 1/6, 3/5,7/4, and 2/8, wrong side of the block for easy plumbing. Enter my reckless limited understanding of Tri Y, and consider initially pairing 1/2, 3/4, 5/7, and 6/8, which seems benign as the cylinder events are 270 degrees apart, no flow by both at the same time. Now combine those four pipes in an equal length fashion behind the firewall and clutch merging 1/2 with 6/8, and 3/4 with 5/7, and we more or less get close to the ideal matching.

I don't have a clue about the final merge or X, but somebody smart would know how to figure out where to put it etc. Should work shouldn't it?

The 4 cam 928 engines have a tri-y arrangement, I believe. They use all stainless material and a bellows to allow for the differing expansion rates and to prevent stress cracks. I put a set of Martin Schneider tuned headers on my '81, had the cams ground to a cheater (European spec?) and lost power, WTF!!! It did sound like a NASCAR stocker at Charlotte with open headers, but no more power in any configuration than stock. Quite a disappointment after the financial investment. I still have the headers and they were on this site for sale. Would possibly fit up to and including an '86, but '87 has a 3 bolt flange.

Your power loss was probably more perceived than actual. Its pretty typical to lose a little low end when opening up the exhaust and adding more overlap/ duration on the cams. You effectively moved your torque curve up the RPM band. (HP is just a function of torque and RPM) In my case, my 4.7 did get a little soggier on the bottom, but really sings to 5800 RPM. (It used to start falling off above 5100 RPM) Naturally, Mike will mention that the euro engine makes more power... granted I wish I had a 4.7 euro S engine under the hood. My next engine will use the Euro S top end as a start point.
-Kerry

32v 928 use a stepped "log" with good flow, but no tuned aspects.

Had this pounded into my head from my days on the fox Mustang forums, HCI needs to match, heads, cams, and intake all need to work together to make power over a wide band.

I posted the same thread starter on Rennlist and it was linked to a thread two years ago I started on the same topic, so more details now in the longer thread. I'm going to take another swing at getting all the data I need into the header modeling software I have, pipemax or maxpipe something like that.

I was talking with Greg Brown a couple weeks ago, came in mind made up on how I want to proceed on a next motor, got new things to think about. With the new cnc cranks he is planning on building a 16v 6.5L stroker soon, but he has had amazing results with his new exhaust system on a stock Euro S motor.

RE:my power loss,
Stone stock the 175,000 mile '81 charged right up to 140 mph down the back straight at Charlotte and held it thru turn 3 & 4 with an instructor in the car on street tires. After the refurbishment, it hit the wall at 135 (maybe) and wouldn't push past it. WTF?? Later it just sorta started fizzling out, ran great at low speeds and was finally out of breath around 110. It was really safe for my friends son to drive to school, but we checked every single thing and found nothing out of specs. ??????

RE: Tri Ys. There is so much room up in there to install the 4 tube headers, it's unbelievable. No jacking nor knuckle busting/cursing like a camaro or mustang. Just need spacers for the long studs to take up the space of the thinner flanges.

Stock I think the 81 is supposed to do 140, the later 928S bumped that to 145 mph. 110 mph indicates a major loss of top end power, more than just a messed up exhaust unless it was plugged up at some point.

The hard part is getting four tubes past the steering linkage and the pinch at the firewall. Much easier i am thinking to get two tubes through.

I even opened the headers at Charlotte and besides everybody thinking "they've got one of the Richard Petty nascars out there!" No difference.

Open headers don't do much good if the rest of the system isn't tuned for them.

No idea what was wrong, but it sounds like more than can be associated with headers.