Radius Carb Inlet

[cammmy]

Hey Guys

Assuming I haven't got this backwards, would it be worth putting a radius here:



If you look at the barrels in most carbs they have a radius at the entrance:



Also ram pipes have a radius at the entrance but there doesn't seem to be any on the SU's or strommies...

[drofgum]

cammmy,
That is where the stub stack goes.
                               Cheers,
                               Paul

[Deleted User]

The two types of carbs you show operate in completely different ways - fixed venturii vs variable venturii.
You'll get a few % extra flow on an SU with a stub stack, but nothing dramatic.

[James]

At most you get 5% increase in flow with a rampipe or stub stack.

There's a diagram on the board somewhere comparing the different variants of ram pipes and stub stacks.

[Tim Bancroft]

Got to say that I found a dramatic increase of Torque when I fitted some stub stacks to my GT6's HS6s.

[James]

Here's the diagram.

[Richard B]

Interesting theres no difference in flow between Type 7 & Type 8.

Mind me asking, where did this come from?

Type 7 looks very similar to a PI plenum chamber intake.

[James]

Well 7 and 8 are basically the same— just one is longer.

Can't remember where it came from — a Vizard book maybe.

[GT6 M]

Its frae an article Vizard did in the early eighties,
recreated for the mag CCC,    I originally put it up for Brucie Boy, when he was having troubles with his 3 carb set up.

M

[rotoflex]

I've got some like Type 9 & it's good to see that they're in the better 50%, but it really does look like anything but stub stacks & a plenum is less effective.

I'm wondering about just adding stub stacks, using the original air filter box, cutting out a rectangular area that originally had the two inlet pipes, fabricating a rectangle-to-round adapter, & running a round hose down to a K&N filter near the radiator.

Would there be any disadvantage to using the original air filter box as a plenum for the carbs with stub stacks?  Is it not deep enough to be effective?

[James]

The rule of thumb for clearance is something like 1.5x the depth of the ram pipe — so a 100mm pipe would need 50mm clearance — something like that.

[drofgum]

rotoflex,
Try searching "stub stack" in the archive. There is a wealth if information on this subject there, including an earlier post of the table listing the effectiveness of the various entry shapes. I remember that the combination of an air box containing stub stacks, and fed by a trunk leading to a filter near the front of the car, was considered to be especially effective once the correct needle etc. was chosen.
                                                                                                                              Cheers,
                                                                                                                              Paul

[99176]

Roto, that is what I've done. Empty airbox with enlarged feed pipes and tubes to sock filters in cold air at front.
Stub stax inside with the famous 'Fray Bentos' individual pie tins fixed on cover to give air clearance.
Works well and looks the mutts nuts. Slight smell of gravy at full chat !

[cammmy]

3708 wrote:

The two types of carbs you show operate in completely different ways - fixed venturii vs variable venturii.
You'll get a few % extra flow on an SU with a stub stack, but nothing dramatic.

I realise that the SU's a variable and the other is not but I believe the principle is still the same. If you are sucking air into a pipe, the potential flow will be higher if the entrance has a radius.

What are stub stacks? May have to do some searching.

[drofgum]

cammmy,
Number 11 in the table is a good example of a stub stack.  They radius the entry without adding any significant length to the pipe/carb.
                                                                                                                   Cheers,
                                                                                                                   Paul

[cammmy]

Cool, I think my brother would be able to make something that looks like No 10 quite easily.

The difference between 5 and 6 is quite scary. Goes from a loss in flow to a significant gain just by adding a roll back instead of a radius!

[andycon67]

Has anyone rounded off the front of the piston to make the air flow smoother?

Andy

[99176]

All the rage in the late sixties and early seventies but seems to have gone out of fashion.

[JohnD]

The 270 degree roll back may not be necessary, if that makes it simpler to fabricate.
The tuning guru Dave Walker, of Emerald M3D efi fame and closely related to "Rave" in the old CCC magazine, wrote an article for CCC some years ago in which he put stacks on a cylinder head on his flow machine and modelled the rim in plasticene to simulate various degrees of roll back   He found that more than 180 had no effect.

John

[drofgum]

2445 wrote:

Has anyone rounded off the front of the piston to make the air flow smoother?

Andy

Andy,
This subject can be turned into a book. There is a wealth of information about modifying SU carbies going back decades. Not only can the piston be smoothed, but the bridge below it can also be reprofiled, and throttle plates knife edged, throttle shafts thinned... The big question is whether it makes sense doing this work when larger carbs are easily fitted.
                                                           Cheers,
                                                           Paul

[Hogie]

Hi Folks,
              I don't know if this is a dumb question but my 4A has round K&N filters.
On the inside of the filters the inlet to the carbs is the flat rear face of the filter (attached to the carb inlet face).

I take it that the filter doesn't improve things, in that the effect on the air is that of a flat faced carb (no stub/trumpet).!!

Roger

[cammmy]

1317 wrote:

Andy,
This subject can be turned into a book. There is a wealth of information about modifying SU carbies going back decades. Not only can the piston be smoothed, but the bridge below it can also be reprofiled, and throttle plates knife edged, throttle shafts thinned... The big question is whether it makes sense doing this work when larger carbs are easily fitted.
                                                           Cheers,
                                                           Paul

Probably stemmed from competition rules forcing cars to run standard carbs.

[drofgum]

880 wrote:

Probably stemmed from competition rules forcing cars to run standard carbs.

cammmy,
I agree, that certainly seems to be the reason for the original development of these ideas. My bigger carbs comment was more aimed at those who might attempt these things now on a road car.
                                                             Cheers,
                                                             Paul

[cammmy]

That's a good point.

There's always that satisfaction of knowing you modified something to work better though, rather than just slapping on something that's bigger. That just feels so.... American :P ;)