Fuel Economy

[albertramsbottom]

Hi
Why does my T25, 1982, 2ltr Aircooled van get less mpg than my 1979 bay window 2 ltr van. I can get at least 30 mpg out of the bay if i drive it carefully, but can only get 22 out of my T25

It make sme want to cry

Oh and just out of intrest where would be the chepest place to buy a starter motor for my Aircooled t25?
Cheers

[Dartmoor]

Well, I think I get a bit more than that out of my T25 Aircooled. I hired a 1600i Danbury in the summer (before seeing the light!), and that was diabolical on fuel - really really bad!

[jason k]

you wont get much more from it but get it set up properly and you can see 25 to 27 mpg but not up hills in the beacons!!!

[HarryMann]

Its bigger

Its heavier

Its squarer

Its probably in worse nick?

and 30 mpg is good for a car of that vintage let alone a big camper, are you sure??

[dave friday]

I think the main differance is the frontal area,much bigger on the t3/25, my 1600 bay did 30mpg ish on a run[55/60mph] 25 around town.

[albertramsbottom]

Thanks folks

Oh and I definatly got 30mpg out of my 2ltr bay. The 1600 I had before was only 23mpg

More power = better fuel economy. Power to weight ratio

Cheers

[HarryMann]

Up to a point...

It's torque (BMEP) that is probably the more relevant figure

[CovKid]

I'll never forget many years ago driving to Cambridge in a T2 in the worst oncoming wind I've ever experienced - foot flat to the floor I could barely make 40mph and the petrol it drank was horrendous that day. No doubt about it, weather does have an effect. I've been far more careful lately, backing the right foot off so theres just enough ooomph to take me where I need to go. That alone can save you a fortune.

[HarryMann]

Aye, like a never ending battle

.. and the T3 in a stiff side-wind has its wind-resistance about doubled I reckon.

[lloyd]

Have to balance displacement to rpm.

Built a 2.0 for early hightop bay I used to have. Builder engine was Type 1 1.6 twin port modified with 78mm crank, 90.5 bore, ported, mild cam, blueprinted, twin delordo carbs, tuned exhaust with a custom 4.57 gearbox. Cruised 70mph at 3400rpm. Around town got 19+mpg and 26+mpg on roadway at 70+mph. Traveled in convoy with an old splitty at 45-55mph and got 38mpg. There was no roadway hill I couldn't cruise up and over at 60+mph. Never ran over 4000rpm .. well almost never as it pulled to 5000rpm Sold it to a mate for his baja bug with 85k on it and he put another 22k on before he threw a rod. I told him when he bought it to replace the bearings and rings, but he didn't.

a 2.0 engine at 4500rpm moves same volume of air as a 6.0 at 1500rpm. American 6.2 v-8s now get low to mid 20's at 70mph at 1700-1800rpm.

Had 87 BMW 535is. First stock, then with a 4.0 bored and stroked, head work, exhaust work, etc. Both engines would give 25-29mpg at 70-85mph... average 65mph on 700 mile trips including fuel and meal stops.. Short ones. Slow down to 55-60mph and get 34-38mpg.

Used to drive a 97 Jeep Grand Cherokee 4.0 (tuned header, tuned intake), automatic, lifted 4", 33" tires.. setup for offroading. With mud tires would get 21-26mpg at 60-70mph. Street tires gave 25-30mpg.

These figures are adjusted from US gallon to imperial gallon. (Imperial gallon = 4.546 lt vs American gallon = 3.785 lt... American is 83% of Imperial - Imperial is 120% of American) So American mileage of 20mpg is actually 24mpg imperial gallon.

Mileage is mostly dependent on how heavy the right foot is.

[syncrosimon]

In 1977 my dad bought a nearly new Wolfsburg Edition T2a 2000cc Microbus Deluxe with full steel sunroof.
He did 125,000 miles on the original engine and gearbox, his rule of thumb was 22mpg, but she would also do less.



The only breakdown I can remeber was due to one of the fuel solenoid wires coming off whilst on holiday.

When me and my brother were little dad used to drive us around Oxford Street, to see the christmass lights, me and my brother sat on top of the seat with our heads pocking out of the open sun roof. I have also seen 100mph on the speedo of this vehicle, on the flat.

My 1600cc T2 did 30mpg once, in France on a deserted stretch of N road whilst coming up from the South. I never went above 50mph. Normally it would do 27mpg.

My 2.1 syncro does anything from 25mpg on the motorway, to 20 ish around the lanes.

I have always found that these dubs will only give good fuel consumption in top gear, which means main roads.

[lloyd]

run in highest gear engine will pull you. I find about 2000rpm in 4th will do most hills. Below 2000rpm is lugging it. On flat runs 30mph in 4th is no problem. Often pull hills in 3rd at 25mph... in traffic and on lanes. The less revolutions per mile engine turns, the less air/fuel is going through engine per mile so better mileage.

[HarryMann]

So the T2 is a fair bit less draggy and weighty than the T3, makes sense... we have some drag figures for T3 somewhere. Think KTRev posted them a while back. Not so sure about T2 figures though, maybe somewhere on t'Internet?

Imperial gallon = 4.546

yes, that'll be 4.54545 recurring then. I find 0.22 is easier to remember and dead accurate. A bit like 3.2808398.... feet in a metre, 0.3048 metres/foot is dead accurate and not recurring.

Optimising the speed and throttle setting for maximum economy has always been a big discussion point in motoring circles, particularly amongst competitors for economy trials. And its well known that throttled Ottos are thermally inefficient due to pumping as well as friction losses, going against the grain of normal reasoning. Here is an example of a fairly typical, modern and efficient injected engine. These are SFC curves developed by cross-plotting the manufacturer's SFC mapping at various throttle settings



The first thing to notice is that the engine is using about 2/3 of the fuel flow per HP when at full throttle(120 lb-ft) than when at low throttle (30 lb-ft)
Then, that this is potentially a 'good' engine across the board, as the curves are quite flat over a broad range...
Then, that 4,000 rpm would be the max. cruising revs as sfc definitely starts to climb seriously beyond that, unless at full bore.

The yellow and light blue lines look worthy of study, 45 to 50 lb-ft, and the rpm band 2500 to 3500... as this might represent the nominal torque to cruise at a normal 'traffic' speed. Note at lower torques, even 5 lb-ft less, the purple curve is showing a fairly big jump in sfc (+10% @ 3000 rpm).

But to determine the optimum , best mpg, one would have to cross-plot these curves, yet again, with the full drag polars for the vehicle in question and this would then tell you what gearing is required (if top gear was the one you wanted to cruise in) to achieve that 'best' mpg...

You've said above, that pretty well the less throttle you use and the slower you go the better the mpg...

I think the latter is generally the case, but not necessarily the former (which we know from economy trials around closed circuits, very small engines at full throttle, sometimes being used in 'burst and coast-down' mode)

Without doing the cross-plot, there would be a minimum speed as well as a minmum throttle opening. My guess is that this minimum would be too low for practicality 30~ 40 mph at a guess), but that an 'optimum' could be found where there was very little increase in consumption for a significant (say 10 mph) increase in speed...

Another conclusion, pretty self-evident, is that the lower the combined rolling and aero drag, the higher the max mpg speed would be, and an optimum point commensurately higher still (we know a small low-drag modern saloon doesn't suffer terribly when going from 50 to 70 cruising speed)

NB. Weight affects rolling resistance, not insignifcantly, but only to the first order.

Food for thought...

[jason k]

run in highest gear engine will pull you. I find about 2000rpm in 4th will do most hills. Below 2000rpm is lugging it. On flat runs 30mph in 4th is no problem. Often pull hills in 3rd at 25mph... in traffic and on lanes.

not in an Aircooled, you must rev em as labouring em in too high a gear on hills creates a heatbuild up and heat is no good for an Aircooled motor. dont be afraid to rev em. more revs= more cooling air.

[HarryMann]

Sounds good advice when hill-climbing Jason, but they were talking economy driving (presumably mostly on the flat)..