Jet Power

[fetuhoe]

Well you live and learn - I have never seen one like that before which is really interesting.

Do the injectors screw into the heads?

We have 2 or 3 CIS 2.7 engines and they are all the later type which are 1974 MY.

This looks like a 1973 911T engine - at least from the Workshop Manual.

Sorry for the mistake.

Have you found the Air Corrector size yet?

[hot66]

That is interesting .... so a 1973.5 2.4 CIS motor runs with mfi heads ?

[shoestring7]

Reply from Richard:

You could start of with 36mm venturies and 145 main jets along with your current F11 emulsion tubes and air jets (at the top of the stack – pull off the extended air jet holder).

You might be able to get the venturies from Tuthill.

The rest of the jetting system is the Weber IDF and should be able to get those kind of jets from Webcon. You probably only need 55 idles.

Gah! I was hoping to manage with the car's current 32mm chokes. Anyone know a UK supplier other than my mates at Tuthills?

>C

[shoestring7]

Reply from Richard:

You could start of with 36mm venturies and 145 main jets along with your current F11 emulsion tubes and air jets (at the top of the stack – pull off the extended air jet holder).

You might be able to get the venturies from Tuthill.

The rest of the jetting system is the Weber IDF and should be able to get those kind of jets from Webcon. You probably only need 55 idles.

Gah! I was hoping to manage with the car's current 32mm chokes. Anyone know a UK supplier other than my mates at Tuthills?

>C

Ah.

Google is my friend: http://www.pmocarb.com/dealers.htm

Emails sent (too late to call), answers awaited. Francis.

>C

[fetuhoe]

36mm chokes sound a bit big - unless it is a race motor.

What will the revs be at peak HP?

Basic formula Choke Size(mm) = 20 x Sq Root (V/1000 x N/1000)

V = Cylinder capacity in cm^3 N = RPM at Peak Power

A starting point for the main jet size is to use a jet 4 x the size of the venturi and an Air Corrector which is Main Jet + 50

The idle jet and the idle jet air corrector will affect the start of the transition circuit.

[shoestring7]

36mm chokes sound a bit big - unless it is a race motor.

What will the revs be at peak HP?

Basic formula Choke Size(mm) = 20 x Sq Root (V/1000 x N/1000)

V = Cylinder capacity im cm63 N = RPM at Peak Power

A starting point for the main jet size is to use a jet 4 x the size of the venturi and an Air Corrector which is Main Jet + 50

The idle jet and the idle jet air corrector will affect the start of the transition circuit.

Capacity is 2.7, its running on 'E' cams, so peak power is at 6200rpm. According to the Taiwanese calculator in my hand that'll need a 81mm choke.

C>

[fetuhoe]

Capacity is 2.7, its running on 'E' cams, so peak power is at 6200rpm. According to the Taiwanese calculator in my hand that'll need a 81mm choke. C>

Try again

(450/1000) x (6200/1000) = 2.79

Sq Root 2.79 = 1.67

20 x 1.67 = 33.4

I would think that the range of chokes for cylinder capacity of 450cm^3 would 32mm for a basic road engine such as a T , 34mm for mid-range tune such as an E/S and 36 for a race engine.

If you consider 4 cylinder engines with 40DCOE carbs this is the typical range you would expect to see in day to day use.

In your case a 32 may be a bit small but a 34 would be reasonable alternative and a 36 would mean that the ratio between choke and barrel size is only 1.1.

This will reduce the venturi affect at low air speeds (equivalent to low engine revs) and this will reduce the effectiveness of the fuel metering.

The net result will be that the bottom end behaviour will be less than ideal and the car may stumble when picking up from low revs.

At 34mm the ratio becomes almost 1.2 which is much better.

The ideal ratio is 1.25:1 which is obtained using a 32mm choke.

These numbers clearly show the compromise between bottom end 'pick up' and top end horsepower so the 'duty' cycle of the car is really important when making decisions.

If all you are interested in is WOT then the largest choke is the best idea.

If you are driving in towns then the small venturi may be best.

I would think the 34 would be a reasonable choice.