-
The Z Club relies on the support of it's paid members - join the club or renew your membership here.
Car myths...
- Thread starter Markp.com
- Start date
steven lack
Well-Known Forum User
driving slower, saves you petrol
not necessarily , depends on how heavy your foot is , you can still drive fast with minor pedal travel once your rolling and its imperative to drive without out calling on torque , ie : dont use 4th when 2nd is best etc
i drove a fiat tipo once in italy on holiday it had a gauge showing you how economically you were driving , if you floored it gauge would show very bad consumption if you were clever with the throttle you could keep it in the eco area of the gauge
hope this helps
cheers steve
not necessarily , depends on how heavy your foot is , you can still drive fast with minor pedal travel once your rolling and its imperative to drive without out calling on torque , ie : dont use 4th when 2nd is best etc
i drove a fiat tipo once in italy on holiday it had a gauge showing you how economically you were driving , if you floored it gauge would show very bad consumption if you were clever with the throttle you could keep it in the eco area of the gauge
hope this helps
cheers steve
Russ
Club Member
I've always wondered, say my throttle position is the same, would I use the same fuel at 3000 rpm in fourth as I would 1500 rpm in fifth? Or would I use half?
What I'm trying to say is "is the fuel directly affected by thottle position or is your throttle position just indicative of how much fuel per revolution you use"?
What I'm trying to say is "is the fuel directly affected by thottle position or is your throttle position just indicative of how much fuel per revolution you use"?
Gio
Well-Known Forum User
The throttle pedal is connected to the butterfly valve in the carb - or throttle bodies. The valve position throttles the flow of air (hence the name) so if your foot is off the pedal, the valve is shut thus reducing the flow of air to almost nil and consequently the fuel drawn into the flow of air is also v small thanks to Bernouilli. Some (all?) carbs have idle jets to allow proper fuel flow at idle.Russ said:
FI is similar except the fuel flow is controlled by the electrickery in the ECU. But the air flow is still controlled by butterfly valves in the throttle bodies - hence the need for a MAF to tell the ECU how much air is flowing (from which it can work out how much fuel is needed).
So the fuel flow (using carbs) is dependent on the air flow which is dependent on the throttle pedal position plus how much air was flowing already before you floored the pedal - or lifted off.
Russ
Club Member
So its a combination then, throttle position affects air flow, but if the butterfly is part open at 2000rpm its drawing less fuel than at 6000rpm?
I have sat down and thought about all this, but I'm still not entirely certain. I'm thinking at 6000rpm with a perfect intake and exhaust system you'd use 3 times as much fuel as at 2000rpm.
I have sat down and thought about all this, but I'm still not entirely certain. I'm thinking at 6000rpm with a perfect intake and exhaust system you'd use 3 times as much fuel as at 2000rpm.
Gio
Well-Known Forum User
OK here's two things to think about. And, to keep things simple, just imagine a single carb setup (mutlipe carbs and FI are too complicated for me at this time of night.)Russ said:
1) It's not steady state it's dynamic. Imagine pootling along and then snapping the throttle open to 50%. The vacuum produced by the engine sucking air/fuel can then increase air flow (the hole is bigger cos you just opened the valve to half way open) and a more air will flow so it sucks more fuel so more fuel/air goes into the engine which produces more power so it rotates faster so it creates more vacuum so it draws more air which... and so on until the engine is spinning fast enough that it can't such any more because the throttle is restricting the air flow. At the beginning of this adjustment, the fuel flow is much less than at the end but your throttle position has been the same throughout.
2) It also depends on load. Imagine tooling along at 50% throttle opening on a flat road and then there's a hill. You will slow down because the engine power output (depends on the amount of air/fuel flowing) won't be enough to maintain the same speed cos you're going uphill so the engine slows down so it draws less air and hence less fuel. Again, same throttle position but different air / fuel flow. You will have to open the throttle more to reduce the restriction of air flow so more air/fuel can be more easily sucked into the engine and hence it can produce more power to maintain your speed.
Phew!
Russ
Club Member
So what you're saying is I need to fit a single carb????
Just kidding
Thanks for the reply. What this comes down to is if I use a TINY bit more throttle I sit at 85mph doing about 3000rpm instead of 70mph at 2500 rpm or something, and I wondered if this is really hurting my fuel economy.
Just kidding
Thanks for the reply. What this comes down to is if I use a TINY bit more throttle I sit at 85mph doing about 3000rpm instead of 70mph at 2500 rpm or something, and I wondered if this is really hurting my fuel economy.
Gio
Well-Known Forum User
That depends on the torque curve produced by your engine. Why? The efficiency of your engine varies throughout its rev range. Maximum efficiency is where it produces the most energy out for the fuel in. This is torque hence your maximum efficiency is exactly where your maximum torque is.
So for any given gear, your most efficient speed is wherever peak torque is. At least it would be if it were not for aerodynamic effects where drag increases (IIRC) as the square of the speed.
So for any given gear, your most efficient speed is wherever peak torque is. At least it would be if it were not for aerodynamic effects where drag increases (IIRC) as the square of the speed.
Gio
Well-Known Forum User
No really, honest guv, would I lie to you?
The torque curve is exactly a measure of the efficiency. And so your maximum torque is your most efficient speed.
At 2,000 rpm you are using less fuel per minute than at 4,000 rpm that's for sure. But your question was about fuel per distance travelled not fuel used over time. So - assuming your max torque is at 4,000 and also leaving aside aerodynamic drag - you will use fuel more quickly than at 2,000 but will go further for the same quantity of fuel.
The torque curve is exactly a measure of the efficiency. And so your maximum torque is your most efficient speed.
At 2,000 rpm you are using less fuel per minute than at 4,000 rpm that's for sure. But your question was about fuel per distance travelled not fuel used over time. So - assuming your max torque is at 4,000 and also leaving aside aerodynamic drag - you will use fuel more quickly than at 2,000 but will go further for the same quantity of fuel.