Mercedes C63 - V8 Mileage troubles
 

Going from a 4cyl car to a V8 has been a shock in many ways. For instance My current mileage for past 2300km street driving is around 19.5L/100km or roughly 12mpg. This is at average speed of 28km/h or 20mph....about 70/30 city/highway.

http://i.snag.gy/q7mzU.jpg

My mileage is now actually worst than it used to be after I started experimenting with quicker acceleration to speed and then just cruising rather than babying it. I figured I will accelerate at better brake specific fuel consumption if the throttle is more open?! According to my scangauge, TPS is only at 18-20 TPS when accelerating casually (idle TPS is 14)...so i'm assuming the throttle is only open 5-10%? the pumping losses must be massive. However larger throttle opening for acceleration did not work. I think this car has been tuned to run rich. I have not been able to find a BSFC dyno or graph for C63, but from looking at EFI converted domestic V8s, they all produce best BSFC at peak torque. which is what I target for acceleration (3500 RPM). In street driving, I turn off engine at lights, try to drive without brakes and perform mini pulse & glides. Also the car engine-brakes heavily at low speeds closed throttles and dropping to neutral makes unhealthy sounds.

currently not sure if the mileage is normal or due to poor driving habits. Highway driving, best I could do by cruising at 65mph speed limit is 13 lhk or 18 mpg. Should I give up or are there certain tips/tricks for V8s ?

FWIW, the EU NEDC City Cycle average speed is 27km/h, if you're averaging 28km/h that basically means your driving route is 97% City 3% Extra Urban.

That kind of extreme City driving will of course kill MPG, some C63's on Fuelly are doing a little better but probably doubling your average speed.

Mercedes-Benz C63 AMG Mileage | Fuelly

ok thanks, I do drive a bit slow on the highways usually 60-65 mph. Still fuelly mileages are much better than mine. I think i'm doing worst than city EPA.

You got rid of the Subaru?

No Impreza will be kept as I cannot drive the C63 in winter here in Canada.

I found that both the air-filters were extremely dirty and one was nearly clogged with small leaves and bugs! This was only after 3 months of driving! So I changed the filters. I also installed a vacuum gauge, this was i think the most helpful. I also ran some top cylinder lubricant in the tank along with ethanol free gas. I managed to beat EPA highway today on drive to work. I turned off engine at every redlight and during highway driving i pulsed to 30km/h above target speed and coasted in neutral until target speed. I just rev matched the gears to neutral->drive was smooth. Pulsing was done at highest gear possible at around 1800 RPM with only ~5in-hg vacuum. Quite happy with 12.2L/100km highway. It's a vast improvement and if I keep it up then I will save $30 of gas a week. enough gas and money for the weekend auto-cross!

When I had the 02 Q45 Sport, I never really worried about the mpg around town.
Just tried to maximize the freeway drives.
It's a V8, just not going to get good mileage around town.

Although I haven't been posting it I am getting about 15 mpg with my Econoline, It is a barn door, loaded, pulling a 3000 pound trailer. It has the 5.4 v8, Auto transmission. I don't drive in town much but the ultraguage is usually 12 mpg or less.

The maximum mpg speed is about 54 mph. I generally cruise at 55-60.

I spend a mile or more getting up to 55 mph. Faster acceleration kills mpg.

I think you got your numbers wrong. The engine isn't tuned to run rich, at least not more than any other car, you just underestimated the throttle position that gets it into rich AFRs. At lower rpm the throttle position does not need to be very high to load the engine up. Try to get an AFR signal from the OBDII port to see how much throttle you can use before it starts running rich.

Also 3500rpm is way too high. Small 4 cylinder engines often have lowest BSFC point in the 2500-3000 range, the bigger bores on your engine mean the charge takes longer to burn and so for efficiency you want to run it slower, my guess is 2500rpm shifts would do the trick.

All that said I am not sure you will be able to do much better, everyone these days thinks big V8s get okay gas mileage since they are good freeway cruisers but they guzzle down the fuel almost just as fast in the city while you are going a lot slower.

For almost all city driving in a v8 amg benz thats excellent fuel economy. Are you using premium fuel or the cheap moose piss?

Im sure you have no problems trading it for a 240 d - 300 td for more mpg and find one with 4matic for your winters. :thumbup:

sorry I think I said it wrong. What I meant was the engine seems to run rich at cruising/idle loads even. it's also a short-stroke engine which loves to rev (in this case peak torque at 5000 RPM). The engine seems to stay stoich/closed loop below 3000 RPM no matter what throttle position. but ya 3500 is too much and i'm targeting 2500 RPM now.

I think it's mostly about smoothness with this car. Gently rolling on to the throttle and increasingly opening the plate but being careful not to spend too much time under high vacuum or crossing 2500 RPM. Accelerating in 6th or 7th gear or at low RPMs sometimes causes the torque converter to slip or unlock and i'm not sure if its a good thing for efficiency. it seems to accelerate in a locked state if I leave it in automatic and accelerate with traffic speed starting from 3rd gear. i think I will just use the torque app to monitor engine load and target about 70% MAP. Throttle position sensors don't seem to be too accurate on these cars.

i used to average 12 mpg at 20 mph avg speed. Starting from yesterday i'm averaging about 18.4 mpg at 30 mph avg speed (70% highway). I run 91 octane ethanol free gas. 94 here has ethanol.