Interesting to see that the mfa provides relatively accurate readings after all, as the rumours I had heard were that it was a bit vague. Have you any thoughts on how fuel economy could be helped by the use of a warm air feed (possibly with a thermostat-operated flap to vary the amount of warm air in relation to cold air), to maintain a constant inlet temperature? My car has no such flappery, and achieves 40 mpg on a motorway run in the summer at +30C, but this will drop down to 25 mpg during the winter at -30C. Urban consumption goes from 30mpg summer down to 17mpg winter. I assume this is due to a temperature-related drop in combustion efficiency. Admittedly the UK winter doesn't usually get so cold, but for the sake of a couple of flaps it could represent a worthwhile saving? Don't some airboxes already have a flap for warm air inside?
In theory the 1.6 engine should be capable of better FE but then you have to factor in poor fuel vaporising mixing from the carb, increase pumping loss from carb manifold and "toilet bowl exhaust manifold. Also the actual state of tune of the engine, i.e. global spark curve for part load and so on... So 36mpg still good IMO for what it is. MFA readings depends on being leak proof and engine state to tune i.e. spark and so on at part load. Gertie, the K-Jet car in the "How to ABF your Mk2" thread has achieved 50 mpg as read on the MFA gauge over a 45 miles distance by its owner. However a study for FE was not conducted to validate. WUR mod may actually cause the car to run richer than stioch at part load so more than average of CO generation. Run part binding brakes would indeed put more the engine under more load and cause more fuel flow, worsened by the modified WUR. Still in spite of that over 40mpg is not bad IMO. My car is STD and all that is intact as far as I can see. The flap starts switching to exhaust heating from under 20C. So this would have been active in my car. @ -30 you do not worry about FE, more above chucking as much fuel that will vapourise to get the thing to start! I guess you point of the temperature related combustion efficiency means it would be beneficial for the air flap to switch-over to exhaust heat as the engine warms up and is running at these ambient. As you have a distinct delta between summer and winter FE it maybe worth retrofitting the goblins and observing what is happening. You would still have to address fuel issues related to density though. So I am not surprised at your reported FE in that kind of winter.
I have a 8v GTI manifold and downpipe and 16v exhaust system, so I'm assuming I should obtain higher MPG.
Mine neither - My airbox is gutted and while not totally relevant my ABF pushes a lardy MK3. I regularly and easily get 43MPG+ on the 20 mile trip to work in summer but struggle to get 37 on the same journey in Winter and can be as low as 34 when it gets colder still. That's driving conservatively as Toyotec outlined above with speed <=50MPH. MPG does however creep up on long journeys to 40+ even when cold and can only assume that this is due to the engine reaching equilibrium in terms of running temps which is doesnt get a proper chance to do on my work run. Makes for very interesting reading though
Great write up Edd My MK2 isn't that bad on petrol really! Would be interesting to see what mine actually does. Also would be good to see how acurate the MK4 1.8T's MPG guage is ?
Still good for a carb'd car. You would need to look at carb state/settings and dizzy settings. A bit more of a challenge to FE compared to EFI or even SPI.
Maybe I could disable the hot air path and leave the flap permanently towards cold air and attempt the boring drive for 175miles again[:^(] See if I could get it to read worse by a significant percentage.
MK4 as per MK3. ECU FE in the MFA is done by the ecu as it calculates fuel consumption from injector flow, VSS and influences the fuel level. A bit more complicated and pretty good as I demonstrated with my Ankor early last year.
Please dont... I drive like that when I'm going home to see my parents... and it is a very boring drive
I seem to be getting 35mpg with mixed driving/heavy right foot at the moment with a 2.0 8v. I remember getting more in the summer though so might mend the warm air flap if temps are making such a difference to people.
FE update "nominal driving" As an update to this and in the light of some of the commnets about winter/summer MPG difference. Nominal Driving Further from refilling with 18.27 litres, I have been driving the car normally. By nominal I mean: Less anticipation in traffic. Allowing the car to cold idle more when first started as I faffed about the sat nav, cleaned glass etc By being less economical with the throttle, engine speeds and loads. Still applying light throttle ( injectors active) on declines. The final result. 18.27 litres = 4.02 gallons and 140miles. Calculated mpg = 34.8mpg. I am more of the opinion that some of the lower mpgs reported in the wintrer have to do with driver action in response to the colder weather and less to do with not having the air heater disabled. My VR6 FE study was done in similar weather around the same time last year. That airbox is optimised and has none of the air heater components installed whatsoever yet it still achieved around 35mpg in a similar study. Until I disable the air heater to a cold air feed default on this 8v, I am not convinced about it's effect.
There is another effect that is mentioned but its full ramifications are not fully recognised.. Cold air is denser, which is why we like cooler intake air temperatures.. However if the ambient air is also cooler it means the engine is also trying to push the car though denser air, I have often wondered how much differnce this will make, for aircraft it makes a lot of differace (it's why we pressurise airliners and fly them high), but then we are talking a much more significant pressure varation!
one thing that i'm still unsure of and it bothers me.. (being a motorsport student) when decelerating, with the injectors unactive (cant hear them chattering) is there any fuel being injected and burnt still? or is engine braking to a stop gently using less fuel than the engine at idle? (afr of about 10.6 afaik)
Brainstorming is always good! Good to know we could go to that level of detail. As you hinted we are tallking about 0-70mph speeds, 1010mbar, 10 deg C and about 75% humidity. It would appear the same car in the similar conditions as the first test, my method of operating the vehicle seemed to have the most influence.
How long was it at 10.6 AFR and where was the sensor placed when taking that measurement? Purge active?
excellent research there Eddie - I'm now thinking i should not have swapped my 8V for a ABF, but life will always change and kick you in the butt.... Now i need to fit a TDI, or get new car !!!
Over the Xmas/New year break, we travelled from North Kent up to Lancaster to visit friends for New Year. We brimmed the tank in my girlfriends 1989 8 valve Gti and checked the tyre pressures to make sure they were all at 30psi. The car had two of us in it and was pretty loaded up with food, beer, clothes, hiking gear etc!!. On the way we avoided most of the worst traffic and travelled at around 65-70MPH on the motorway. We got all the way up to Lancaster (around 330 miles) and still had around a third of a tank left. Used the car a small amount up there on two short journeys and when we travelled back we left it till around 470 miles before getting a bit nervous and pulling in for fuel!! We didn't want to fill her up again as the motorway petrol station was charging 139.9 per litre!! However, I reckon there was about three or four litres left in the tank!! By my calculations, I think we could have got another 30 miles before running out so that would equate to 500 miles. This works out as around 41.6 MPG Not too bad really for a 23 year old car! I reckon with an engine undertray, ultrasonically cleaned injectors and the valve guides done we should be looking at around 45 MPG on a run!! Alpenweiss2
