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Engine Output - Required Airflow?


Anthony

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Trying to take a slightly more scientific approach to my engine build, I've been reading up on airflow and volumetric efficiencies

My thinking is, if I can calculate my engines maximum airflow I can then estimate its maximum power potential

From this, I can then identify the bottlenecks and restrictions in the system and improve on them

This will help the engine work more efficiently, and free up more power with less stresses


Does anybody have any airflow figures for various engine components?

Maximum engine flow?
Cylinder head flow?
Inlet manifold flow?
Carburettor flow?

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Some initial assumptions.....

100% VE (volumetric efficiency) can be calculated as follows.....


100%VE = (Cubic Inch Displacement X RPM) / 3456

100%VE = (122 x 7000) / 3456

100%VE = 854,000 / 3456

100% VE = 247CFM

So, a standard 2.0 litre engine will flow 247cfm at 7000rpm if volumetric efficiency could be made to be 100%

I've heard and read that it takes approximately 1.5cfm to produce 1hp

So, if
1.5cfm = 1hp
247cfm = 165hp

So, these very rough calculations show a 2.0 litre engine will flow upto 247cfm, and can produce 165hp

Since these engines don't produce this power they obviously provide less than 100% volumetric efficiency, and have bottlenecks/restrictions in the system

Who's worked out where and what these restrictions are, and how they can be overcome?

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unless you have countless hours on a free flowbench i would go with recomendations from this and other forums,any engine will only produce its maximum potential at a given rpm with a combination of the correct parts(intake system,head mods,cam choice and timing and exhaust system.this is not exhaustive though so s'cuse the pun)
it is widely regarded that to obtain over 80hp/ltre from a 2 valve engine is a good result.

'freeing up power with less stresses' is a contradiction in that to produce more power the engine will be under more stress.

these engines like many others were a compromise of power over tractability and fuel consumption so by design they are what they are.

100% ve is not the ultimate goal,with carefull modding and matching of components you can obtain more than this but at the price of driveability,do you want an engine that produces high power at say above 5500rpm or slightly less power but more torque through a wider rev range?

even some of todays modern? 2valve engines only produce around 75hp/ltre,the latest ls v8 series in the corvettes is around this,although emissions play a big part in the final power output.

how often are you going to be driving at 7k rpm? :)

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I've heard that a 150 stromberg will flow upto ~84cfm

So, fitting three of them would provide a maximum of 252cfm, meaning they would no kinder be the restriction

But, I've read that the manifolds aren't very efficient
Also, I've heard people say the firing order of the 6 cylinder engines isn't good for triple carbs, as the intake pulses wouldn't be evenly spaced out

ie;
Firing order 1,5,3,6,2,4
Would give carb pulses at
Front, Rear, Middle, Rear, Front, Middle

I can see that's not evenly pulsing, but it is an issue of any kind?

I note the Jags have the same firing order, and they use triple carbs

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Anthony,
Whatever it is you are having, i'll have 2 kg please  ;D ;D

You have to get your 252CFM of  uncompressed air to the engine, not easy if you take into account all the restrictions you have under the bonnet,
Try to have a unrestricted even flow of laminated air without turbulences to the carbs would be your first priority.

Dirk

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;D I get a little carried away at times

What would you recommend to get air to the carbs?

What's wrong with the original airbox as a starting point?
Turbulence, restriction, or both?

Obviously I couldn't use that with triple carbs, but I could use that as a basis

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The lemans cars, more out of necessity because they used the longer webers, took the air from inside the wheel well.

Have a look under the bonnet and try to imagine how the air has to travel to reach the carbs when you are traveling forward.
Obviously you want to try getting the air from the front where it is helped by the car speed and you want as little obstructions as possible. A nice straight ducting will do. Turbulence eats up energy and thus slows down the air, with a result of less reaching the carbs.
It is a bit like a low grade turbocharger really.

Original air filters are also an obstruction, the air has to travel upwards first then sideways before it can enter the carbs, you can improve this by different filters and or ram pipes...

Not easy but all little things help,

Cheers
Dirk

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