Rotrex M42 Build - 285bhp little update

[keri-WMS]

Stonking upgrade there Matt, I like!!!

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GeoffBob, there's one bit I'm unsure of...

I thought the reason for bigger inlets was volumetric efficiency, due to the differing pressures the inlet/exhaust charges see when being shoved through the ports and valves. Assuming naturally aspirated for a quick theoretical example:

Inlet = 14.5psi of pressure pushes the air into the cylinder in a simplified engine, ie atmospheric pressure (which is also why engines lose power at altitude) which is fairly weedy and why a bigger valve helps fill the cylinder more quickly. If the valve remained closed it would see 14.5psi trying to get past it.

Exhaust = The air is forced out of the engine by the rising piston, with many times the force/pressure. In fact if the exhaust valve is closed on a 10:1 compression ratio engine at TDC the valve sees 145psi from the charge (minus the 14.5psi of the atmosphere in the exhaust manifold) trying to get past it, so 9x the potential pressure differential is available to overcome a small port.

So as the inlet is the weak link the best bet is a bigger inlet valve, which all engines I know or have. The fun part comes when you add boost, if you had 9bar of boost on the same 10:1 ratio engine (theoretical) you might need the same size inlet and exhaust valves?

A few notes:
- All the above ignores the heat / pressure / density changes caused by combustion, it's a cold-air engine!!!!
- In a real exhaust stroke the charge is hotter, with more pressure and less dense which makes it "runnier", which is another reason for a smaller exhaust (your original point in fact).
- In a real engine the valves of course open as soon as they get a chance, so there's far less of a pressure differential at the valve, I was only making the point about the potential pressure if the ports were flowing really badly or similar.

Slightly off topic but on a "boosted" theme!

[GeoffBob]

Hi Keri,

I follow your argument, but as I understand it, this is not the case.

Assuming a basic normally aspirated engine then, to a first approximation, the pressure in the cylinder is at atmospheric pressure while either the inlet valve or exhaust valves are open. There is, therefore, no pressure drop across the valves.

We know, of course, that this is a very simplistic view, and that in reality there is indeed a pressure drop over the valves. What is important to realise, however, is that the valve sizes are selected in order keep the pressure drop across the valves as small as possible while open (as a function of the amount of air that the engine is required to pump) in order to keep our simplistic view of the engine as true to reality as possible.

However, since the exhaust gasses are at lower density than the inlet gasses (since they are hotter), the exhaust valves need not be as large as the inlet valves in order to maintain an identical pressure drop over the valve during the induction and exhaust strokes respectively.

Something to keep in mind here is that the pressure drop over the valves has nothing to do with the compression ratio of the engine or the absolute pressure of the inlet or exhaust manifolds, it has to do (as described by Bernoulli’s equation) with the density and velocity of the gas being pumped through the valve, as well as the absolute dimensions of the valves themselves. And quite obviously, the larger this pressure drop the less air is inducted, or the less exhaust gas is expelled. The absolute pressure in the cylinder is, however, affected by these parameters, but not the pressure differential across the valves.

And indeed, yes, it is the valves (as a result of the fact that the pressure drop is a function of the volume of air pumped through the engine) that (in part) define the volumetric efficiency of the engine.

[GeoffBob]

... if you had 9bar of boost on the same 10:1 ratio engine ...

You'd have a machine for launching cylinder heads through bonnets

Sorry, I couldn't resist

[keri-WMS]

Fair enough - it's been ages since I had a read of an engine book!

[appletree]

Rich, ill measure them for you when I'm next home, think they 23mm but not sure sorry.

[appletree]

Just thought i would update this with a little project that ive been on with.

When i got the car mapepd as it was a ll a bit of a rush towards the end i ended up running wit hno IACV but with the throttle stop adjusted to make the car run perfect at a hot idle,this worked quite wel with the ECU set up to auto advace/retard the ignition to maintain the idle.

The above setup worked well but i alway wanted to add an IACV just to make the running more factory so ive beenworkin on the below set up.

I first made a bracket that would bolt on to the inner wing and then bought a E46 318i IACV and fitted it to the bracket, next welded to bosses on to my inlet manifold and inlet pipe and piped in the IACV.

The Bracket, few extra holes in it as it was made from a bit of a old bracket i made to mount my recaros.



Fitted to the car





A bit of a mapping session in the garage doing a few cold starts and tuning the opening of the valve and i now have a car that idles perfectly weather hot or cold!