Author Topic: Advise on A112 70HP+40DCOE  (Read 18574 times)

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Offline bibito

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Re: Advise on A112 70HP+40DCOE
« Reply #15 on: September 12, 2009, 06:14:40 PM »
Here is a picture of the manifold I intend to use,
and the head set up.
I guess not much room left for kidney shape?

Grant, finally I'm getting some positive comments ...thank you,
would you please, attach a few pictures of your car with that set up...
Where can I get the cam you are talking about?
« Last Edit: September 12, 2009, 06:15:23 PM by bibito »

Offline grant gauld

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Re: Advise on A112 70HP+40DCOE
« Reply #16 on: September 13, 2009, 03:47:02 PM »
That 30/70 cam works well on the 903cc motors.For 1050cc try 35/75 or the original A112 cam.This is assuming that its just a fun weekend car.For competition a much more applicable cam choose will be needed.Your inlet port size is OK again for fun but not competition.

Offline bibito

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Re: Advise on A112 70HP+40DCOE
« Reply #17 on: September 13, 2009, 05:21:19 PM »
I'm going to stick to fun weekend car,
so hopefully it will all work out well.
Will I be able to tell the difference between the A112 original abarth cam, which I have,
and the 35/75 that you suggest, using the DCOE?

Offline Paul vander Heyden

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Re: Advise on A112 70HP+40DCOE
« Reply #18 on: October 10, 2009, 02:54:39 AM »
Hello all,

There are two principle issues with using a single side draft carburetor (weber/dlellorto/solex/etc).

1.  Fuel/air mixtures do NOT like to make 90 deg. turns.  Almost invariably the fuel comes in and hits the far wall at the 90 deg turn and separates.  Remembering that fuel must be in a well mixed and agitated state in order to properly ignite and burn, this fuel/air separation is not ideal.

2.  Because of the problems associated the necessarily long intake runners with a side draft carburetor, tuning the 4 different circuits in the carburetor becomes much more difficult.  Don't make things too large, as you will actually lower the intake charge velocity.  This will worsen the problem discussed in Point #1.  The longer runners will also mean that the engine will not produce maximum HP over 7500.

If you must, use 25mm venturis, but be prepared for the fact that the engine probably will not be happy above 7500 RPM.  It may rev to more than this, but it will not be making any HP.

Main fuel 125   
Main air 170
Emulsion Tube F11
Idle  50 F8
Accelerator Pump 40
Venturi from 25 to 30 depending on conditions and application.

Here is a good starting point for jets.

There are as many opinions on camshafts as there are camshaft grinds.  However, there is no arguing that the goal is to get as much combustible mixture into the cylinder, as this is what makes HP.  I prefer camshafts that are shorter duration and have larger lift at the valve.  This combination does mean that the valve train must be optimised and the geometry double checked.  As there is a limited time to accomplish all the tasks in 300 degrees for example, if we want to achieve 12mm+ of valve lift the the acceleration of the opening ramp of the lobe is going to be quite radical.  Likewise, if we delay the closing as far as possible, then the closing ramp will also be very quick.  So this means that using the lightest components and better valve springs are indicated.  So the valve opens faster and stays open the maximum amount.  This is primarily determined by the diameter of the foot of the lifter and the maximum surface pressure that the cam lobe/lifter interface will withstand.  The payoff is in having a "greater area under the curve" and thereby getting maximum volumetric efficiency. 

If you can combine this with a well designed exhaust system that does a proper job of scavenging at peak torque RPM, then the volumetric effiency of the engine can be as high as 110%.  Again, if the engine is seeing 1.1 Bar of inlet pressure (instead of 1.0 Bar) more combustible material will make it into the cylinder to be burned.  A note of caution here - If you engine is 110% efficient at peak torque, and your computed compression is 10:1, then the REAL compression at peak torque RPM is 11:1.  This can have devastating effects if you are using a grade of fuel that is marginal.  It goes without saying that most pistons damaged by detonation occur at peak torque RPM (5500-6000).

I hope this provides some of the technical background as to why the single side draft set up may not meet everyones expectations.

Good luck