Fuel Economy Whilst Cruising

[Space Goat]

Howdy

just a quick question, which may not turn out to be quick

in regarding a turbocharged car (skylines in general), when your sitting on 110 - 120 ks on a straight flat road doing 2500 -3000 rpm, if the car is making boost will it use more fuel than a naturally aspirated car of the same displacement, say for arguments sake an rb25det vs rb25de... what if you could hold the waste gate or a plumbed back blow off valve open so that it makes no boost... will fuel consumption figures increase or decrease... if this is possible what is the best way to stop an engine making boost the BOV or the waste gate...

Thinking out aloud i would think it would be better to hold the waste gate open so that the turbo wont spin as fast and recycling the air through a plumbed back BOV might result in weird air recycling in turn the AFM may play up

Please remember im only talking in generalization and for all intensive purposes of the question the engines are standard

Cheers

Rick

[Guilt-Toy]

next

[Fixxxer]

I'd be thinking that to get from Point A to Point B requires a set amount of work/power. How you get from A to B is another thing relating to efficiency. (also the time it takes).

To get there the most efficient, you would like the least amount of losses, including engine, drivetrain and wind resistance, and keeping the engine in it's most efficient zone of operation.

Off-boost all the time may result in poor efficiency of the engine (it's not operating at it's peak efficiency versus on-boost).

5th gear gives better economy than 4th gear (if the engine isn't struggling in 5th gear), less frictinal losses, less revs, etc.

Anyways, that's my two cents and may or may not be correct....

Fixxxer

[Krishy]

i find that if you dont hit boost then the car uses less fuel then driving the same distance and hitting boost

[Rezlo]

If you had to take an RB25DE and RB25DET both bone stock on the same flat section of road doing 100kph (where the DET would make zero turbo pressure) the DET should be slightly lighter on fuel due to less pumping losses (cylinder) due to slightly lower compression, this is taking into account that the VE at a specific BSFC at low load points is usualy better on a turbo car BEFORE it comes into boost and the AFR's drop to compensate. BUT there are many veribles that can contribute to this going both ways.

Best way to save fuel on a turbo car is to drive it out of boost and ensure you mapping (if its aftermarket) and the rest of your supporting bits are in good codition such as spark plugs, O2 sensor, MAF clean ectr.

[paulr33]

a 30 minute on road wideband AFR tune can sort out your cruise AFRs. once that is done, you should be able to get 400kms+ to a tank each time, with city/traffic driving too.

ive done this on my r33 and manage 400kms every time. on highway i managed 550kms to tank and it wasnt empty (was close).

[Warpspeed]

There are many factors, but assuming similar gearing, similar engine capacity, and best possible engine tune, the higher compression ratio n/a engine should be more fuel efficient.

Why?

Engine thermal efficiency at very small throttle openings will be greater with a higher compression ratio, and with a lower total exhaust back pressure (lower exhaust pumping losses).

[Rezlo]

There are many factors, but assuming similar gearing, similar engine capacity, and best possible engine tune, the higher compression ratio n/a engine should be more fuel efficient.

Why?

Engine thermal efficiency at very small throttle openings will be greater with a higher compression ratio, and with a lower total exhaust back pressure (lower exhaust pumping losses).

Thermal efficiency between the compression split of this size would be minimal, less change than the change in pumping losses or near zero exhaust back presure in this situation.

The 2.0fsi and 2.0Tfsi from VW are a classic example of how the turbo version of the same motor is far more efficient out of boost, at 100kph one can return 7.1L/100km on the turbo motor and the N/A will see around 7.7 to 7.8L/100km, when driving spiritedly you will see around 16L/100 and 11L/100 respecively, So you can see just how fast it can turn around.

Fact of the matter is youll hardset to see a noticible change at cruize, 99% of the time its in the way you drive a turbo car that results in poor fuel economy.

[Space Goat]

hey sorry for bringing up an old thread... but still on topic...

ive deisgned a system that holds open the BOV (plumb back system standard rb25 s2 stagea) even when you have your foot down... ive designed it using ball valves for now but for the next few weeks i will be testing it and if it helps out with the fuel economy i will hook it up using solenoid valves... there is a noticeable difference in power and i will find out about the fuel economy in the weeks to come... its good on the highway and up small hills but you'll need to pull it back a gear if you're driving up big hills... eventually i will have a switch in-car which will basically switch the turbo on and off

my main concern with this is will the computer sense that the car is running off boost and adjust the air/fuel ratio to suit??? or at 3000 rpm the computer thinks usually there is X ammount of boost therefore it needs x ammount of fuel... there is a boost sensor which i think controls this but im not 100% sure... i can easily install a bypass that will hold this boost sensor signal under vacuum but once again im not sure if it will work

[Guilt-Toy]

Please tell me this is a joke.

hey sorry for bringing up an old thread... but still on topic...

ive deisgned a system that holds open the BOV (plumb back system standard rb25 s2 stagea) even when you have your foot down... ive designed it using ball valves for now but for the next few weeks i will be testing it and if it helps out with the fuel economy i will hook it up using solenoid valves... there is a noticeable difference in power and i will find out about the fuel economy in the weeks to come... its good on the highway and up small hills but you'll need to pull it back a gear if you're driving up big hills... eventually i will have a switch in-car which will basically switch the turbo on and off

my main concern with this is will the computer sense that the car is running off boost and adjust the air/fuel ratio to suit??? or at 3000 rpm the computer thinks usually there is X ammount of boost therefore it needs x ammount of fuel... there is a boost sensor which i think controls this but im not 100% sure... i can easily install a bypass that will hold this boost sensor signal under vacuum but once again im not sure if it will work

[Haines]

What AFRs would be good in a cruising range

[Guilt-Toy]

14.7 works good for me