Choosing a new exhaust manifold

[Brit01]

I need to replace my old original manifold and would like to make some improvements.
The primaries are long and not of equal length.

Some friends were testing various systems and had success with these dimensions (they found the primaries made most differences to low-mid torque):

45mm primaries - 65cm long (1.75" / 25.5")
Y connector on each side

50mm secondaries - 25cm long (2" / 9.75")
Y connector

57mm to end the manifold. (2.25")

Engine is a 1.7 standard Alfa boxer, 8v.. twin dellorto carbs (32 chokes, 142 mains, 55 idles, 33 pumps).
This is for road use and I do enjoy low-mid range torque but keeping it's original high revs power.
It still retains has a nice pull in 5th gear.

Any suggestions would be appreciated.

Thanks,

Chris

[Guy Croft]

Chris - if those are pipe internal diameters they are a bit on the big side.

PY generally wants to be as ex valve diameter (which is about 3mm bigger than the throat which is the controlling sction) and maybe up to 5mm bigger. Or make pipe dia as port outer section diameter (or fractionally larger than the port) if the port is larger than ex valve). If the port outer section is not circular work out the equivalent pipe diameter to give that port X sectional area.

SY bore should be 1.25 x bigger than PY on diameter. There seems to be benefit of having the PY pipes bigger than the port but if the port is massive compared with the ex valve size anyway - no.


PY and SY lengths and ratios are 'spot-on'. Tailpipe a bit massive, 50mm ID would be plenty big enough but should be same as SY pipe bore, certainly on your motor 45mm would do. All an overly big pipe does on an atmo unit is slow down the movement of ex gas to atmosphere at the rear end.

PY pipe an SY pipe chord lengths MUST be equal to within a few millimeters. Collectors are NOT part of the pipe length and should have an included angle of 15 deg OR LESS. This sets the length of them, does it not? Bends are critical and nowhere must the ratio of chord radius to pipe diameter fall below 2:1

G

[Brit01]

Many thanks Guy for the informative feedback.

I was just reading DeltaDave's thread about the chord radius and calculation as you mentioned it should not fall below 2:1.

I need to measure the mid section to see what the current diameter is also. That was replaced fairly recently.

From memory I believe the original primaries do expand at the ex. valve probably by about 5mm.

I need to find out the exact diameter of the ex. valve.

PY pipe an SY pipe chord lengths MUST be equal to within a few millimeters

This is also good to know. The original Alfa design is way off. They vary in length by the distance between the 2 ex.valves. Just straight out and backwards. Must be about 15cm difference between the ex.valves. on the primaries.


I'll keep you updated on the progress of this project.

Thanks,

Chris

[Guy Croft]

What is the firing order? 1,3,4,2?

That governs the pairing. In this case you should pair 1&4 and 2&3 primary pipes.

G

[Brit01]

What is the firing order? 1,3,4,2?

Correct.

In this case you should pair 1&4 and 2&3 primary pipes.

So a crossover design would be required instead of a straight line as my 8v has pairing 1&3 and 4&2.
Alfa made the crossover for the 16v version of the boxer.

A friend here recently had one of these made but not well designed as the primaries were different lengths.

Regards

Chris

[Brit01]

This is my friend's manifold (crossover for an 8v boxer).

As you can see the primary(#2 in particular) is substantially different in length. Maybe tricky due to the boxer design and sump layout etc to get the primaries of the same length.

I think this is why others went for the non-crossover design.
Sorry about the quality. These were taken with his mobile phone!

[Guy Croft]

yes,

crossover it must be then, thanks for updating me Chris. Could not fit a 4-1 and other pairing would be an utter disaster.

And before anyone says you can pair differently and it doesn't matter - don't!

The reason you pair 1&4 for example is because that gives the widest angular & time separation you can get betw 2 cylinders on a 4 pot unit - thus minimising the chances of adverse cylinder-cylinder exhaust wave interference and at the same time capitalising on the benefit of wave reflection off the closed valve of the linked cylinder. You can only do this on a linked system like a 4-2-1, on a 4-1 you lose it.

Another good reason for going to Specsavers.

G

[Brit01]

Following DeltaDave's formula it comes to more or less 1.5" for the primaries.
So 1.875" for the secondaries. (1.25x)

The ex.valve throat measures 27.5mm on the 8v boxer.

It has the 4-2-1 but 1&3 with 2&4 straight down either side of the heads. No crossover.

[Guy Croft]

well done - valve throat 27.5mm plus 5mm = 32.5mm (1.28") I would go to the nearest bigger pipe size whatever that is. 1.5" is 38mm



G

[Brit01]

Thanks Guy.

Now the 'easy' job of designing it around the engine/sump/driveshafts and box with those primaries staying the same length without sharp bends!

[Brit01]

This is a design from a friend's crossover. Looks fairly neat and primaries appear to be of the same length. Trying to get the dimensions from him.

[Brit01]

Hi Guy,

This is a photo of the original manifold. Very long primaries as you see and also have a wide diameter and are of unequal length.

Regards

Chris

[Brit01]

Guy,

I made a mistake with the firing order.
The boxer fires 1-3-2-4.

Basically clockwise around if looking from above.

Thanks

Chris

[Guy Croft]

OK

1-3-2-4 F/O - full firing cycle all cyls takes 720 deg

interval betw cylinders is equal and is 720/4 = 180 deg crank

The furthest apart you can separate individual firing events is 360 deg of crank rotation which on a 4-2-1 header means pairing 1&2, 3&4

Obviously it help if paired cylinders are on the same bank, are they?

On a 1-3-4-2 FO you would pair 1&4, 2&3

G

[Brit01]

Let me triple check this.
Been a tough week at work and I'm not thinking straight.

I will route out my manual on my hard drive.
Could have been right in the first place my apologies. I should know as I've removed an re-installed the HT cables numerous times.

1-3-4-2, (It's not quite a clockwise movement - jumps across from 3-4)

4-1
2-3