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[Marks DTM Calib]

The next article will be about about cam duration, quite a short easy one to once you can understand the above.  

[Marks DTM Calib]

Do we have any requests for a topic anybody would wish covering?

[Entwood]

If you have nowt else to do ..... ( and the stuff so far is brilliant ..    )

I've never understood the theory behind the multi-ram system .. ie . how simply changing the length of the air-intake changes the torque curve .....

In the days of carbs the idea was as short and unobstructed intake as possible to get as much air in as you could !!!

 :-/ :-/ :-/ :-/

[Marks DTM Calib]

If you have nowt else to do ..... ( and the stuff so far is brilliant ..    )

I've never understood the theory behind the multi-ram system .. ie . how simply changing the length of the air-intake changes the torque curve .....

In the days of carbs the idea was as short and unobstructed intake as possible to get as much air in as you could !!!

 :-/ :-/ :-/ :-/

The multiram is a spin off from the above.

We mention in the above how the mass and momentum of the inlet gases help to fill the cylinders.

So, if we have a short large inlet (i.e. a motor bike) we have slow moving air at low rpm and fast moving air at high rpm.....this results in low torque at low rpm and lots at the top revs.

If we now make the inlet long and smaller (i.e. the Rover V8) we have fast moving air at low rpm giving lots of torque and at high rpm the air flow is restricted.

If we then look at the multiram and plenum setup we have a few possabilities

1) Both Valves shut

We have each cylinder feeding off half a plenum and a single long medium size inlet tube. The result is lots of torque at low revs

2) Both valves open

We have each cylinder feeding off a full plenum and the inlet tube is now the two in parralel. The is result torque at the top end.

Plus two other which are switched in at around 3000-4000 rpm to smooth the transition (so its not sudden like on the old straight six boat anchor from the Senator)

So, we are doing little more than varying the size and length of the inlet to keep the air velocity up throughout the rev range.....and the reasons why we do this are as per the valve timing above

[Entwood]

Ahh ... penny drops !! so its the mass/velocity of the air AFTER the throttle bodies that matters .... in simple terms .. when we have a partially open throttle (restricted air in) we want to keep the velocity high to let the cylinders "breathe" easier  ...

[Marks DTM Calib]

Ahh ... penny drops !! so its the mass/velocity of the air AFTER the throttle bodies that matters .... in simple terms .. when we have a partially open throttle (restricted air in) we want to keep the velocity high to let the cylinders "breathe" easier  ...

Pretty much, if you read about the valve timing above, we want to ensure we get as full a charge in the cylinders as possible with max atomisation of the fuel........and that does not work at all well with slow moving air!

[Bandit127]

How about inlet pressure?

This could be used to discuss (K&N type) filters, port polishing and turbos?

Does the 'resonance' of the air box fit in here? It is certainly important fo highly tuned motorbike engines.

[Mr Skrunts]

Do we have any requests for a topic anybody would wish covering?

Just thinking out loud to a degree,  but at some stage I would be interested in your thoughts on Blue printing, Balancing and Gas flowing.

The reason I ask. in the days of reading the car mags like CCC there were plenty of articles on the like of the Blydenstein engines and the differences of the Satge 1.2. and 3 cams, big valve heads etc.

These words dont appear very often these days, is it that todays motors are more efficiently produced and this work is not required, or is it something that motorsprt just take for granted.

Would any of the above be beneficial whilst work was being done on a Miggy.

[Kevin Wood]

Ahh ... penny drops !! so its the mass/velocity of the air AFTER the throttle bodies that matters .... in simple terms .. when we have a partially open throttle (restricted air in) we want to keep the velocity high to let the cylinders "breathe" easier  ...

Pretty much, if you read about the valve timing above, we want to ensure we get as full a charge in the cylinders as possible with max atomisation of the fuel........and that does not work at all well with slow moving air!

There's also a resonance effect with intake tract length to each cylinder (which the multirams vary by isolating the 2 banks at differing lengths). In addition to the pressure pulse that Mark mentioned with regard to the exhaust system, the same thing happens in reverse in the intake. As a cylinder takes a "gulp" of air a pulse travels back down the intake away from the valve, is reflected from the entrance to the tract, and then travels back up it again. If its arrival at the intake valve coincides with the intake valve opening again you have a convenient high pressure pulse behind the valve which has a supercharging effect on the engine.

This only works in a couple of very narrow RPM bands, of course, but if you can tune it to happen where the engine has a "flat spot" for other reasons, such as cam characteristics or poor exhaust scavenging you can make a more tractable engine with a flatter torque curve. Alternatively, for a highly tuned engine, you can arrange it to happen at the top end of the engine's characteristics for more power.

Ideal intake length depends on RPM (the frequency of the pulses), so you have F1 engines revving to 18K RPM with little stubby trumpets over each valve (high resonant frequency), and intake manifolds with great big long ducts on lazier engines (low resonant frequency).

Kevin

[Bandit127]

What's going on with the EXUP valves fitted to Yamaha bikes then? Honda adopted these for the 'Blades too. It's a butterfly valve fitted in the exhaust that is virtually closed at low revs and open at high revs.

I understand that they increase low rev torque and work on resonance but don't know how.