Advise on a 4-2-1 header please

[ace124]

Recently i bought a header for my 124 couple sight unseen.
Today it arrived and i took some measurments:

Primarys: 38mm bore OD, lengths:
#4: 250 mm long
#3: 330 mm long
#2: 350 mm long
#1: 360 mm long

Secondarys: 41mm bore OD, 500mm long
The rest of the system is 51mm bore OD

Refering back to Guys book, i think the manufacturer of this system has got it all wrong!
Also i feel there is too much difference in the length of the primary pipes especially number 4 being a lot shorter than the others.
They are for a 1.8 TC with standard 131 2L cams on a head prepared as per GC's Fast road spec with twin 45DCOE's.

My question i guess is an obvious one in relation to the pipe lengths, and how do other 124 manifolds measure up, not having measured any others myself.
Are they all this varied?

Thank you

[Guy Croft]

"I think the manufacturer of this system has got it all wrong"

Y'all got that right!


The ONLY thing that has changed since I published my manual is that simulation & dyno test now indicate that LONG primary and SHORT secondary are BEST and in almost every case 4-2-1 on 4 cyl units is BETTER than 4-1.

Every engine and required powerband is different (not to mention every inlet system and cam setup) and unless you have pretty robust facilities for testing it can be better to settle for a non-interference 4-1 system rather than an interference 4-2-1 system rather than try and guess the optimum relative length of primary and secondary pipes.

For your n/a TC you want something like:

4-2-1
32 - 34" total length of pipe.

For example:
Primary 1.5" bore, 25"
Secondary 1.75" bore 8"

Altering the relative lengths of Py & Sy within the overall length constraint and changing the rampipe/inlet tract length will simply alter (but not really harm) the powerband (area under the torque curve and its position)
Py pipes MUST be linked 1&4, 2&3

4-1 (for ease of fit only)
31.5" long 1 5/8" bore

Tailpipe min length 3ft and bore 1.75-2"

The lengths are PIPE length and the length of the collector joints is additional to that not part of it. You can make the collector as big or small as you like because the reflected/refracted waves that make a 4-2-1 work emanate from the change of section that they see at the pipe end.

The silencer MUST be one or two large capacity straight thru units with virtually no back pressure.

What can you get away with? The truth is with engines like this is it is SUPRISING how well they can run with systems that are technically ALL WRONG! I'm not advising on that! The data above comes from actual advanced simulations and dyno test (so you are VERY lucky to share it). All free/low-cost on-line exhaust 'design' software works off known configurations and presets and use it at your own risk.

Anyone who thinks of writing in and telling me about 'other' primary pipe linking configurations on n/a engines with 1342 firing order - DON'T!!!



GC

[mickwood]

The very odd and mismatched primary lengths you describe match pretty exactly with the original Abarth tubular manifolds for the 124 - I still have one under the bench (take a look at photo 1 below - also note the odd angle that the manifold assumes for no 2 cylinder interface with head).
Interestingly, i had a new exhaust made up by a "specialist" when i fitted the 16 valve engine - Guess what - he said the same as Guy - you need quite long (and matched) primaries and short secondaries - I will measure them this weekend, but i think they match Guys measurements almost spot on - to fit that into a 124 aint easy and he came up with the solution you see in pic 2 below. only problem was the exhaust runs very hot on 124 16v and burned the glassfibre engine cover. hence we initially fitted a heat shield but subsequently also wrapped manifolds in ceramic.

[ace124]

WOW Mick,
If they are original 124 Abarth extractors, then this maker has definitely copied them cause they look very similar in design...Id very curious as to the lengths of the original exhaust header.
Please post them when you get a change to measure them up. i did it by shoving the tape measure thru each pipe.

thank you