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n/a mpg


JIMBOW1
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Can't tell you about NA mpg, but my TT gets around 25 mpg motorway (average 80ish), 18mpg (110ish), less than 12mpg (130ish), and at 150+ you can almost see the fuel gauge drop!!

 

Tend to get about 17mpg average around town.

 

For sub 70mph speeds and around town I wouldn't have thought there’d be much difference between to TT and NA.

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Can someone confirm that the Supe TT follows the theory and should be better fuel economy than the NA? A standard 3.0T will have better fuel economy than a 3.0l but I don't know about the supras fuelling and what-not for NA/TT.

Not sure I follow the logic there... :conf:

 

24mpg from my auto SZ (cats still in) on the daily run. I have had it up as high as 28 on a very leisurely drive across the country, and as low as 18 on a blat with the aircon on full.

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But the main point of forced induction is to get a larger mass of air into the cylinders that its normally aspirated brother. Assuming a roughly fixed AFR, this means the turbo would burn more fuel rev for rev. In fact the fuel consumption on the turbo may be even higher because extra fuel may be added to cool the intake charge on a car without a hee-uge FMIC.

 

Unless there's some weird therodynamic effect I'm missing. What's the resoning behind the theory then? :cool:

 

There's no way that turbo and an NA variants of the same engine could share the same fuelling.

 

*edited for spelling*

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I get about 23mpg out of my NA, that's driving mostly like a grandad (flat cap is in the glove compartment!), with the occasional Heavy Right Foot on most journeys. That's also with a clogged air filter, and knackered spark plugs :)

 

Not sure about the theory behind getting better MPG from a TT. If you stay off the boost, then possibly..... But with boost, it seems like getting something (i.e. better performance) for nothing (no extra fuel used, or even less fuel used), and something for nothing doesn't usually hold true, IMHO.

 

Hope that helps a bit.

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On low throttle there's no 'more' fuel being put into the engine, like at idle for example, but the air flow through the 'idling' turbos is making the engine run more efficiently hence reducing the amount of fuel needed to keep it running. You get no air pulsing problems in the manifolds as you do on the NA.

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On low throttle there's no 'more' fuel being put into the engine, like at idle for example, but the air flow through the 'idling' turbos is making the engine run more efficiently hence reducing the amount of fuel needed to keep it running. You get no air pulsing problems in the manifolds as you do on the NA.

Isn't this confusing the efficiency of getting air into the cylinder (volumetric efficiency) with fuel efficiency (mpg)?

 

A turbo engine does this by compressing the air and blowing it into the cylinders when on boost. An NA car has to rely on intake and exhaust manifold tuning to optimise the airflow at specific RPM and get the peak torque and power in the right places. This is nowhere near as effective as using a turbo.

 

If you raise the volumetric efficiency, then you have to supply sufficient fuel to burn the air with (unless you specifically want to run lean in certain low-load areas of the map).

 

I believe that a small turbocharged engine can produce as much power as a larger displacement NA for less fuel, but I still cannot see how any given can be less fuel-efficient than the same engine with forced induction.

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If you raise the volumetric efficiency, then you have to supply sufficient fuel to burn the air with (unless you specifically want to run lean in certain low-load areas of the map).

 

Exactly, which is why I said I'm not sure on the mapping of the supra, I'm working on the principle of it and I'm not sure how 'real-world' it is!!

 

But basically there must be a point on throttle where the losses in an NA caused by air flow and compression drawing power from the system (and the relative losses in fuel required to account for them), are overtaken by the additional fuel requirements of the forced induction from the turbo. On idle the turbos are spinning and helping the air flow through the engine and so during the induction, compression and exhaust strokes there is no loss in power as there is in an NA, and there is also no additional fuel put in because the compression increase is too low to warrant it (at idle). But as the throttle is increased the air pressure rizes pretty quickly and more fuel is chucked in to keep the charge constant.

Where is this magical point at which the forced induction requires more fuel than the losses of poor induction/exhaust gas flow have to account for? I'd say as high as 10% or more, so keep it below that and the TT is better than the NA.

 

Like I said though I'm going on what I know about 'principles' of the 4-stroke cycle, and am probably totally wrong!!!

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OK, I see where you are coming from now.

 

Considering two identical engines running identical AFRs, but one with a turbo and one without:

 

The losses on compression in an NA will be higher because of the higher compression ratio (typically about 10:1 as opposed to 8.5:1 ish on the turbo version). I'm not sure about the pumping losses on the induction stroke as I don't know what impact the manifold pressure would have on actually forcing the piston down in the bore - my guess is not much. On the exhaust stroke you could run lots of overlap and use the turbo to "blow down" the cylinder but I'm not sure you would want to as the point of blowing down is to maximise available volume for the next charge but you get a better charge with forced induction anyway.

 

So taking the compression stroke in isolation, yes, there will be less losses in the turbo engine, so for a given throttle position (take note) the turbo engine might be operating more efficiently in terms of usable power at the flywheel versus grams of fuel going in.

 

However...

 

Because the turbo engine has a lower compression ratio than the NA, then off-boost it will produce less torque for the same % throttle opening and therefore be making less power overall at the flywheel (even though it might be making this power more efficiently in terms of fuel consumption).

 

Now, put both these engines in a car with the same mass and ask someone to drive them on the same road and you will find that you need to open the throttle on the turbo further to maintain the same road speed. If we can assume that the turbo engine still stays off boost, then you are effectively running an 8.5:1 CR car against a 10:1 CR car so its not difficult to see that you will have to be a LOT heavier on the throttle with the off-boost turbo than with the NA to keep up, meaning lower mpg. If (as in the real world) the turbo kicks in as you start to put your foot down, then the regular rules come into play: On boost = more air = more fuel. Back to less mpg again.

 

In short, I don't think that the reduction in pumping losses on the off-boost TT will anywhere near make up for the lower torque output when compared to an equivalent NA engine at the same throttle position in a theoretical situation. In the real world, at the same load, then turbo engine would either be off-boost and running faster to produce the same power, or on-boost.

 

 

[/waffle] :D

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