Transient enrichment

[Bruce Plecan]

Subject: Transient enrichment

Is this race car, or street car, how much cam?.  Lots of things come into
play.
I'm addressing a street engine, as opposed to streetable or racing.

Snip

| I run across bazillions of different theories, all of which I've
| decided are crap (feel free to correct me though).

Everyone is entitled to their opinion, weither you call it crap or not is
immaterial.
| e.g.
| 1) The flow change lags behind the pressure change.
| Bollocks. That would make it run rich, and besides the
| pressure change is CAUSED by the flow.

The further from street toward race this does matter, more and more.

| 2) The intake runners have columns of air that need to be
| accelerated to the new conditions.

| Bollocks. That would also make it run rich, and besides the
| flow always tops between intake cycles anyway.

Why then is cross sectional area so important to throttle response?.

| 3) There is a delay getting the fuel in the port to the cylinder.
| Bollocks. Its squirted directly on the back of the valve, at low rpm
| it'll just run in under the influence of gravity, when the valve is
| opened.

Gravity ain't fast enough for cylinder filling of puddled fuel.

| 4) Manifold wall wetting effects
| Bollocks. This is port injection, the walls don't ever see fuel.

Look again, where does the fuel go when sprayed against a closed valve?.
The back of the valve, and splashs the walls, not the entire lenght like a
carb set up, but still some.

| 5) The vapourised fuel in the port condenses because of the pressure
| increase.
| Maybe, but it seems that this would not cause a major change in fuelling,
| as its hard to see how the fuel in the cylinder will not be burnt.

If the droplets are large enough it won't..

| 6) A richer mixture (i.e. more powerful) is needed to accelerate the
engine.
| Bollocks. Running a seriously rich mixture at say 1300RPM/0" vac, does not
| help
| the problem. Neither does serious amounts of dMap/dt. dTPS/dt helps a
| little,
| but not much.

Then try going very lean, ahh, see, problem gets worse

| 7) Ram Air effect of the flow coming into the manfold.
| Perhaps, but hard to see how this will have more than about 10% percent,
| running
| that much richer steady state doesn't seem to help.
| ----------------------
| Having stated what I think its not, here's what I think it is:
| 1) Fueling delays. The ECU not responding to changes in MAP as
| quickly as the engine. The engine will respond on a cycle by cycle
| basis, whereas an ECU and MAP sensor most likely will not.

If you've changed cams, or got too much volume in the MAP sensor line, then
the filtering rate may be off.  The ecm can play a chess game between
ignition cycles.  Time/response not an issue or no high performance engine
could run one.

| 2) The fact that the fuel is injected before the IVO (intake valve open)
| event, but the engine "samples" MAP at IVO. Therefore it may not be
| predictable what the fuel will need to be, when the fuel is squirted in,
| because the MAP can change significantly then and IVO.

For vaporization, (ie street) you want the fuel to hit the back of a closed
valve,
also, the shearing action against the fuel as the valve opens.

| A carb will also experience delays in fueling, mainly due to distance
| from the port, so hence the need for the throttle pump. Q. How do TBI
system
| deal with this?

About the same as TPI...

| Weber DCOEs throttle pumps will squirt seriously far and fast, and these
| carbs are known for
| being able to acheive exceptionally good throttle response.

Only when calibrated correctly, and the same for EFI...

| SU carbs combat this by momentarily restricting the airflow, thereby
| bypassing
| the problem. Plus the excellent vapourisation at low flows, will make the
| engine more resistant to lean misfire.

At the expense of poor throttle response (compared to weber whenboth are
right).

| I've heard rumours of some OEM ECUs, doing multiple injections per
| revolution
| to effect their enrichment, based upon dTPS/dt, this would certainly help.

Yes.
Grumpy

| Obviously the other idea that may help, it calculating the fuelling and
| injecting at
| late as possible in the cycle, i.e. to coincide with the closing of the
| valve.
|
| Open to the floor for comments/flames.....