hernanca

If I recall correctly (IIRC), the 944 1-4 (non-bellows) pipe cracking is due to the difference in expansion and contraction during heat cycles between the aluminum block and the material of the (non-bellows equipped) pipes. Probably the same reason that we have those bellows on some of our (cast iron?) manifolds (I have them on the 86.5).

I am not a 944 guy*, but that is what I remember reading as the cause. I don't know if an in and out thermal coating would help.

(*Not a 944 guy, but I clearly remember the "Wow!" of seeing my first one, in 1982, outside the Omni in Atlanta, Georgia - a beautiful, gold-brown metallic one!)

Imo000

You have cast manifold on an '86.5 928?

hernanca

I have these (pic, below - spare from 86.5 with Devek headers). I was guessing cast iron because of my memory of their weight (thus the "?"), but I just looked at these closely again - what are they - steel pipes with cast iron flanges?

danglerb

No, those are welded stainless I think, welded anyway. The 86 Euro is I still the old style cast, not sure even if different from earlier non S.

danglerb

What I am wondering is which does the greater expanding, heads or headers? I would think headers so and inside and out thermal coat might reduce the expansion enough to skip the bellows.

hernanca

I don't know which does the greater expanding - would be good to know for sure.

Here is an interesting discussion about what the 944 manifolds are made of, and perhaps applicable to our stock manifolds?:

https://rennlist.com/forums/944-turb...loy-800-a.html

Thread deteriorates around the end of page 3 through the same infighting we see here, so not much relevant info after page 3.

And this thread shows a guy that makes an aftermarket 944 Tri-Y exhaust (FRF, pic below) without bellows and indicates it is because he uses thicker material:

https://rennlist.com/forums/944-turb...fr-st-1-a.html

Rob Edwards

Carlos-

FWIW, I haven't forgotten the request for measurements, have just been up to my butt in house-moving fun for the past 2 weeks. This shot is emblematic of the state of my parts collection right now:

Will try to figure out which tube is which, and measure, hopefully this weekend.

hernanca

Thank you, Rob. Looks like you have your work cut out for you!

Rob Edwards

Carlos-

Only took 8 months....

I pulled some measurements off of Anderson's zombie headers last night, using a tape measure. All numbers are from the mounting flange on the head to the end of the pipe.

Holler if you want to see additional pics of particular pipes. Clearly no one was worried about equal lengths, but they were pretty effective.


Primary tube length:

1. 25”
2. 20.25”
3. 23.75”
4. 24.25”
5. 28.25”
6. 24.5”
7. 19”
8. 18.25”

The first merge pipes are as follows:

1&4: 12.5”
2&3: 13.25”
5&6: 13”
7&8: 13.75”

Second merge pipes:

1-2-3-4: 13.5”
5-6-7-8: 13.75”

Once assembled, the total path length from the flange surface to where the 2nd merge pipes end as shown in this linked pic are:




1: 46-7/8”
2: 43”
3: 44-3/4”
4: 41-3/4”
5: 52-7/8”
6: 48-1/4”
7: 42-1/4”
8: 43-7/8”

Moar pics:

1-4:






5-8:

hernanca

Very cool, Rob! Thank you for doing this. Must have been a fun puzzle!

ptuomov

Since this is a good thread about random exhaust speculation, here's another "header" idea for a very space constrained situation:
- Combine 1 & 3 and 5 & 6 primaries as close to the flange as you can while still keeping the merge angles nice
- Run a 1.25x larger diameter secondary pipes from the merged 1&3 and 5&3 to a non-standard 3-1 collectors on each side. Make those as long as you can.
- Run as long as possible primaries as you can from 2, 4, 7, and 8 to the non-standard collectors.
- The 3-1 collectors take two small pipes and one 1.25x diameter larger pipe and bolt on to the stock cat element.

If this looks like a can of crazy to the header fabricator, then you understood it correctly!

You'd never do space unconstrained headers this way.

However, if you have an extreme space constraint and for example need to use the stock cat pipe completely unmodified, this might have some gains for you. This is a copy of the Ford Coyote header system. The idea is to merge the 90-degree separated pulses as early as possible such that they blow on top of each other. There's no room to keep them completely separate with long enough tubes, and this is a case where the long tube separation is the best, almost no separation is the second best, and the medium amount of separation is the worst. There is probably enough length to keep the 180-degree separated pulses away from each other, though, which will help starting at the mid range rpms (how low depends on the length of the pipes).

The cross plane V8 bank wants to run boom, boom, BOOM, boom, boom, BOOM, and the Ford Coyote header lets it do what it wants instead of trying to turn it into something that it isn't. I think there were some very clever men designing the Coyote motor.

ptuomov

The above tri-Y configuration works well if you can keep the secondary pipes longer than a critical threshold.

The secondary pipe length, or more accurately the total pipe length from cylinder #3 to the collector and then up to cylinder #1 on the passenger side and the total length from cylinder #5 to the collector and then up to the cylinder #6 on the driver side is what determines the minimum rpm at which this style header starts working. Longer the total distance, lower the rpm at which the header can push the 90-degree exhaust blowdown interference that cylinders #3 and #5 impose on #1 and #6 past the exhaust valve closing events of cylinders #1 and #6.

It's going to be hard getting that sort of header or exhaust manifold to work if one wants to retain the stock cat pipe completely unmodified. Believe me or not, but I think that tri-y the "wrong" way (combining 1&3->large diameter secondary, 2&4->small diameter secondary, 5&6->large diameter secondary, and 7&8->small diameter secondary) will work better one can't get sufficient total length to the system before all the pipes are merged per bank.

You probably won't believe me, but that's ok! I'm not sure I believe myself yet...

hernanca

Interesting thoughts, ptuomov!

I have been meaning to tidy up my notation, so you gave me a good motive to do so. Below is a diagram of how I understood your description of the Ford Coyote exhaust, as applied to the 928 setup. The new notation helps better illustrate the 90 degree overlap of the exhaust pulses in relation to each other in the firing order. Crank degrees are specified now starting with the first cylinder of the firing order. It is not as good as it could be, because the exhaust valve open/close events do not occur precisely at TDC or BDC, but it is a better depiction of what is happening than what I had before.

Is it undesirable to pair exhaust pulses that are 180 degrees apart? I read one source saying the ideal pairing is 360 degrees apart, but it was only one source and I wasn't sure about its credibility. Does pairing 180 degrees separated pulses tend to cause undesirable pressure at a bad time to the non-firing cylinder?

I realize this thread has been dormant for 20 months, but I do intend to revive it soon with my improved notation, and a few new wild ideas along the lines of the crossed over tri-y!

ptuomov

You'd also want to have a cross-over H, Y-pipe, or or X-pipe after that under the car.

What pulses you should combine depends on the pipe lengths and exhaust cam duration. If you combine 180-degree separated pulses, the pipe lengths have to be at minimum such that the exhaust blowdown pulse from one cylinder doesn't arrive at the other cylinder before the exhaust valve has closed. This will obviously depend on the rpm, so longer tubes will start working well at a lower rpm. (Too long pipes will start hurting at high rpms because of pumping losses, so you don't want to over do it.)

With our rpm range and no constraints on the pipe length, we pretty much know that conventional tri-y manifold with a cross-over or x-pipe under the car works the best. All these crazy alternatives only become practically relevant once you don't have the space for long enough pipes.