Toerag

We all know that our cars can usually gain power by 'leaning out' the fuel at the top end. My question is this:- Can you overfuel at low'ish revs by mashing your foot to the floor and thus not actually accelerate as fast as someone pressing the pedal just enough? Am I making any sense?

brickboy

It will not make much difference, the ECU map will sort out the fuelling required for the airflow, boost pressure, rpm, fuel quality, exhaust gas temp etc.

Now if you've got twin Weber DCOE pumpers fitted, different matter

Pavlo

used to be the case on old carburettored cars, as the wide open throttle reduced air speed through the carb body, meaning crap fuel atomisation.

It's much better on a fuel injected car, and with standard cams probably fine, saying that I don't mash the throttle on my car at low revs.

If you get juddering when you do it, it's not great for the bearings, so best to avoid.

Paul

mark@wrx

I sometimes find that if i'm too heavy on the loud pedal at low revs, the engine kind of holds back until I lift my foot slightly then it takes off. Not sure what causes it though...

mutant_matt

Until someone that knows what they are talking about comes along, I'll proffer a theory.

Perhaps it's because at low rpm with the throttle wide open, the air speed is slow and fuel atomisation is not very good. This is a big problem on Motorcycles that switch from carbs to FI. It makes the pickup from low revs slow and jerky. Suzuki have solved this problem on their bikes by adding in a second butterfly upstream of the throttle butterfly to restrict the airflow at low revs and hence speed up the flow. As revs increase this butterfly returns to the fully open position (i.e. no restrictioin). Perhaps this is the problem you are talking about? The only way to solve it is to not use quite so much throttle at low revs and as revs pick up, increase the throttle opening until you're on full throttle?

....waiting to be shot down in flames....

Matt

hades

Matt - I like the theory! I can say that all fuel injected cars I've driven seem to work better if you feed the throttle in rather than mash it.

I have once in my life tweaked a map on a normally aspirated kit car on the road, and I have (very vague) memories that some of the real time readings were happier if you didn't mash the throttle until about 3500 or so. However, at >300bhp/tonne, you didn't get much time to assimilate the data. Please note I am not "one of the experts" by any means!

Brickboy - I agree it will be worse with big carbs!

Toerag

Can one of the guys with logging software do any tests for us?...

Pete Croney

Air speed past the injectors is determined solely by the vacuum from the open inlet valve that sits just below them.

If you give it WOT this just means that the air can accelerate in more easily.

Once on boost, the air is pushed in.

On a bike engine, injectors that can pump enough fuel for WOT at 14,000 rpm will struggle with the tiny fuel requirements required to make it idle.

On the WRX, tumble generation valves were added to direct the airflow past the injector nozzle and aid misting, when more fuel is being introduced on cold idle. Once the engine starts to warm up, these remain fully open. They are there primarily for cold start emission control.

mutant_matt

Bikes idle fine Pete It's new FI bikes that have problem in the transition from closer to open throttle at low revs, especially if it's full throttle.

I'm fresh out of ideas BTW

Matt