Race Engine Forum UK

Hi guys!

I have been offered two different dual carb manifolds for Lada SOHC engine. Normally you don't have two totally different designs to choose. This got me little confused, so i'll ask your opinion about these manifolds.

Other is this type (ignore dirty Dellortos, using Webers only). Straight one, approx. 10-12 cm long and has slight downdraught angle

Other is this, we call it "swan neck" - manifold, obviously by its shape. It has 15 cm long channels and very generous curves.

Pros and cons in my opinion:

Straight type:

+ obviously better flow due to straight line into intake channel
- carbs sit so low on the engine bay that you have limited space for heater pipes and air filters
- carbs are very close to exhaust manifold


Swan-neck manifold

- "S"-curve robs flow but adds manifold lenght?
+ rises carbs higher where is much more room for them and air cleaners
+ longer distance to exhaust manifold

I'm not looking for 100 hp/litre, maybe 70 hp/litre in future. Comments and opinions are welcome.

Tommi

hi Tommi

Shooting from the hip a bit here: strictly from a flow point of view I would prefer the straight shot manifold. But how much better will it be? Only a flow test can tell you for sure.

If you can fit a shield over the exhaust and build a good cold air feed to the carburettors then you would probably overcome the heat from the exhaust manifold.

How much space will you have between the trumpets and the inner wing with the carbs low in the engine bay? You need at least 25-30mm there.
Packaging is always something that makes you do things you would rather avoid. You optimize one thing and then the second and third want to live in exactly the same space. Oh dear... ;-)

regards
Tom

Any other well-versed members care to comment?!

G

Uhmmmm... actually I was hoping for you Guy.
Could you give some indication on the sort of flow loss that the double bend in the swan neck will give?

regards
Tom

OK - let's be semi-scientific about this, give the swan-neck a fair trial.

Tommi, can you do a measurement for me?

On the swan-neck item measure the pipe internal diameter at both ends and give me best possible measurement of the pipe chord radius at both ends. This may need a card template and pair of compasses.

See diagram below.

The optimum ratio of chord rad/pipe dia is 2:1 but bigger is better, ie: the bigger the bend rad the lower the loss due to separation and turbulence on the short side of the bend radius.

That is one feature of a swan-neck we must consider, the other I will discuss later.

GC

Excuse my ignorance, but wouldn't it be beneficial for readers like me, that don't know the Lada head, if the angles of the inlet in the head itself would be taken into account as well?

Good idea.
Actually there is a nice drawing already on this site over here: viewtopic.php?f=5&t=2631&start=15
I'm taking the liberty of putting it up here as well. Nice downdraft in the port, could match up with the swan neck better than expected at first glance.

regards
Tom

Are all these Lada engines SOHC, siamese twin port style engines? Or did they produce twin cam versions as well? How about horizontal plane angles? I doubt you can do much with the siamese port setup about that, but it could be of influence?

The exit section of the swan-neck device obviously matches quite well to the head but unfortunaely just outboard of the mating section there is a big bend.

If the manifold had a straight run into the port it would be great, yes.

Compare lines in attached dwg, for some reason MS Paint would not let me use colors.

Don't underestimate:

1. The losses in the bend per se (and there are two)
2. The impact of the changes in direction imposed on the airstream.

Kinda leads me into what I was going to say after receiving the radius data, hope I see it soon,

G

Reading this thread and thinking about the air intake of my Alfa.

Don't underestimate:

1. The losses in the bend per se (and there are two)
2. The impact of the changes in direction imposed on the airstream.

The Alfa boxer has a long 90 degree air intake from the airbox.

Normally racing alfas remove these and install straight trumpets and socks/filters.

But to get torque at the lower/mid rev range Alfa designed the 90 degree intake to make the trumpet longer, otherwise it wouldn't have fit under the bonnet.

How much loss from a 90 degree bend I wonder.

One day I will do a test run with straight air intakes albeit shorter.

Thanks for all your comments, this is certainly an interesting thing.

TomLouwrier: thanks for posting diagram, i forgot it totally.

WhizzMan: Lada SOHC engine is copied from Fiat SOHC engine in some parts. It has similar combustion chambers, similar port desing etc.
There was some twincam-heads for Lada engine, not in mass production though, but they are almost non-existent.

Guy, here's the measurements:

I tried to measure radiuses as carefully as i could and after couple trial and errors i got reasonable readings. They may have tolerance of +- 1 mm, but i believe that's adequate tolerance.

Inlet side port radius: 20,0 mm

Carb side port radius: 24,0 mm

And then the port measurements:

Then i took a piece of plasticine and made a models for both SSR's. Maybe these will give you an idea how these ports are shaped.

And lastly, visited to friend in other town and took pictures of his Lada engine bay. This has exactly the same straight-shot manifold as my first post.
I remember little wrong, there is room even 115 mm high air cleaner box.

There is a down side certainly with this type manifold, if you ever have to remove carbs, better pray first. They are real PITA to remove.
Usually this type of manifold is assembled with carbs bolted in all ready. It's easier to measure vibration rubber or thackeray washer clearances when manifold sits on work table. But then manifold bolting to head, oh-no. Couple nuts are nearly inaccessible.
Swan-neck manifold has loads of room to nuts.
Little comparison to OE setup:
Hope these pictures help.

Tommi

If you're right about chord rad (see below) the bend losses alone in the swan-neck will be very high, likely reducing the flow into the head by 20% or more.

Just confirm I have marked up right in the dwg please?

G

Hello,

It look like there should be the halve port diameter added to it to get the the real chord radius ?
They look not very so very tight from the plasticine mould ?

Klaas

Guy, your drawing is right.

If im correct with term radius:

"The radius of a circle is the length of the line from the center to any point on its edge."

My 20 & 24 mm readings are half of that i measured, i believe that's and radius of a circle. And the circle is in this case, the manifold channel inside a curve.

Correct if im wrong. There might be a chance for some misunderstanding.

Plasticine moulds have no errors, that's for sure. They show only the short side radius, nothing else.

By looking at the curve in the plasticine and your thumb, I feel Klaas may be right.
If the chord radius is 20mm and the port diameter is 35mm, then the SSR would be about 3-4mm.

Just to take away any misunderstandings: Tommi, could you confirm Radius and Diameter as indicated on this picture, please? regards
Tom
(edited; I mixed up the ends of the manifold)