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De-siamesing inlet ports

I extract this image from the excellent "Colin Chapman, inside the innovator" by Karl Ludvigsen. (I hope I don't infrige any copyright Mac? )

Would anybody here have tried that since?
Renaud

Location: Brittany

Re: De-siamesing inlet ports

renaud
I extract this image from the excellent "Colin Chapman, inside the innovator" by Karl Ludvigsen. (I hope I don't infrige any copyright Mac? )

Would anybody here have tried that since?
Renaud


You are entering a new world. I hope you have a lot of free time.

Re: De-siamesing inlet ports

Someone tell me the advantage please. With this method you still have two ports fed from one input. Isn't this similar to using two carbs? ie. Twin SU's. Have I missed something.
Vince Leek

Re: De-siamesing inlet ports

Frank Hernandez
renaud
I extract this image from the excellent "Colin Chapman, inside the innovator" by Karl Ludvigsen. (I hope I don't infrige any copyright Mac? )
Would anybody here have tried that since?
Renaud


You are entering a new world. I hope you have a lot of free time.

I certainly will have enough time and propriety to read what will be told Frank!
Renaud

Location: Brittany

Re: De-siamesing inlet ports

Vince leek
Someone tell me the advantage please. With this method you still have two ports fed from one input. Isn't this similar to using two carbs? ie. Twin SU's. Have I missed something.
Vince Leek

Hi Vince,
I'll quote Chapman:
"The A7, like most production 4 cylinder side valve engines, has inlet ports 'siamesed' on cylinders 1 & 2 an again on 3 & 4." (...) "Under racing conditions that system is not efficient, because the ingoing charge of fuel/air mixture to the inner cylinders (2&3) of each pair is robbed by its neighbour as each induction stroke takes place. The valve on N°4 opens before the inlet on N° 3 is shut (...) on a 1342 firing order. This is an important factor at high engine speed because it prevents complete filling of the inner pair of cylinders." (...)
"The modification permitted a clean alternate 'pull' on each induction stroke which reduced the 'changes of direction' inevitable with an ordinary two-port system with only one carburettor. This resulted in a worthwhile power increase and fgor the season that I raced the car, no other 750 formula car could beat it" (op cited)

Renaud

Location: Brittany

Re: De-siamesing inlet ports

Didn't he also use the twin choke solex from the small V8 ford on this?
I think I read his biography or whatever and he claimed when the engine was running badly,on 2 cylinders he could still achieve 62 mph!
On de-siamesing when I was interested in tuning minis about 20 years ago, with the same porting a 5 port would achieve about 125 bhp and an 8 port only about 5 more.
I think they then got more with "scatter" cams any one know more.

Re: De-siamesing inlet ports

Austin in the shed
Didn't he also use the twin choke solex from the small V8 ford on this?
I think I read his biography or whatever and he claimed when the engine was running badly,on 2 cylinders he could still achieve 62 mph!
On de-siamesing when I was interested in tuning minis about 20 years ago, with the same porting a 5 port would achieve about 125 bhp and an 8 port only about 5 more.
I think they then got more with "scatter" cams any one know more.

You're right, that was the carb he used.
Renaud

Location: Brittany

Re: De-siamesing inlet ports

The goal of this mod is to even out the mixture variation that you get when 1+2 and 3+4 are paired up. You get a
--21--21-- pulse on the front pair and
34--34-- pulse on the rear pair.
after 4 sucks there is a slow down while 2 and 1 fire, 3 then sucks and the air has to speed up, straight away 4 sucks and the droplets moving towards 3 have to U turn towards 4, you get the idea, whereas
If you link 1+4 and 2+3 you get
1-4-1-4-1
-3-4-3-4-
The even draw means that the fuel mixture separation is even on all the cylinders so you can tune the carb mixture for both cylinders,whereas on the first example you set a compromise mixture which is a bit weak for one cylinder and a bit rich for the other. You get a bit more power. But it is only a bit.

Re: De-siamesing inlet ports

Does it still make as much difference if the engine was supercharged?

Re: De-siamesing inlet ports

Austin in the shed
Does it still make as much difference if the engine was supercharged?

Don't know, never tried supercharging.
All I can say is that it is a lot of work for a small return.
I would have thought that running a pair twin choke carbs would be far simpler.
If you are trying to go faster in competition I suggest you do concentration exercises and yoga, then you see an immediate and marked improvement and not even have to get your fingernails dirty.

Re: De-siamesing inlet ports

Rather than de-siamesing I made my ports slightly larger, the same, and made them match a standard manifold gasket....

I also relieved the radius round the corner.


Photobucket
Photobucket
Making the ports really big, the dremel would take too long plus this way i could machine the ports symmetrical and both the same




And only had to use a powerfile / die grinder to blend / polish rather than to remove the huge volume of cast iron.

Regards

Rob

Location: The 3D shed, Tewkesbury

Re: De-siamesing inlet ports

Well, you can do like Bill Cowley did all those years ago and use de-siamezed ports and 2 twin-choke carburetters.
I don't see the point in 2 twin-chokes with siamezed ports.

Mixture separation isn't the only problem that's overcome; there's also the question of each carburetter (assuming only 2 chokes) getting an even flow. Siamezed ports give a suck-suck-pause to each choke, with the sucks overlapping to some extent, depending on the dwell of the inlet cam. De-simaezed ports give suck-pause, suck-pause to each choke.

Location: Richmond, Texas

Re: De-siamesing inlet ports

Rob Beck
Rather than de-siamesing I made my ports slightly larger, the same, and made them match a standard manifold gasket....

I also relieved the radius round the corner.

And only had to use a powerfile / die grinder to blend / polish rather than to remove the huge volume of cast iron.

Regards

Rob


Of course to de-siamese Chapman had to enlarge the ports too.
Renaud

Location: Brittany

Re: De-siamesing inlet ports

On my old special 'COG' (due to reappear soon in VSCC trials), I araldited pieces of 16G aluminium into the inlet ports to desiamese them. The aluminium protruded about 1" into the inlet manifold (usual Raeburn aluminium type with single SU).
This was done purely as an experiment. There was no significant increase in power but it did pull better at low revs (standard cam).
I suspect that any improvement was down to gas flow speed into the inlet valve rather than mixture distribution.

Re: De-siamesing inlet ports

Malcolm Parker
On my old special 'COG' (due to reappear soon in VSCC trials), I araldited pieces of 16G aluminium into the inlet ports to desiamese them. The aluminium protruded about 1" into the inlet manifold (usual Raeburn aluminium type with single SU).
This was done purely as an experiment. There was no significant increase in power but it did pull better at low revs (standard cam).
I suspect that any improvement was down to gas flow speed into the inlet valve rather than mixture distribution.

I suppose de-siamesing with a single carb could not give much improvement?
I'm still interested to hear on that topic though!
Contrary to what Frank implied above the can of worms seems empty...

Location: Brittany

Re: De-siamesing inlet ports

Rob, what is the significance, if any, of the lower front face of the block that looks as if it has been milled?
Flats machined on either end of block to take extra feet

Location: Farnham

Re: De-siamesing inlet ports

That's to fit the clamp that bolts the block to to the crankcase.

Re: De-siamesing inlet ports

David,

Ruairidh is spot on. There is a small machined area either end with holes drilled and tapped to take bolt on lugs providing one extra block holding down stud at the rear of the block and two extras at the front. There are two at the front because the block is on a mag crankcase and I didn't want to interfere with the dynamo housing.

Regards

Rob

Location: The 3D shed, Tewkesbury