Low RPM on Boost or Higher RPM off boost better for economy?

[Didge3]

I'm interested to know whats the better method of accelerating in an aspirated car.

In a N/A car this is relatively simple, lower RPM=less fuel can be injected so it makes sense to change early whilst still maintaining acceleration.

However in a turbocharged car changing up at low RPM means to maintain acceleration you're putting demand on the turbo, whereas you could accelerate gently to a higher RPM without needing the turbo. But which is better?

By my understanding;
More RPM = More fuel injection cycles (plus higher pumping losses)
Lower RPM but using boost = Engine goes richer to make up for more air in the cylinder

 

[StevenRB45]

My understanding with it is, accelerate up to speed reasonably quickly, though not at maximum attack. Although the boost requires more fuel in terms of efficiency it does the most work for fuel used when you do this. Once up to the cruise the highest gear possible and try not take big bites of boost.

Most the small turbo petrol are set up to drive like diesels anyway so there's very little to gain in winding it out. The Citroen we've got generates peak torque in a wide band 1500rpm to 4000 and peak power is maybe 5 but the torque is the more useful thing to drive on. Anything above that and you tend to run out of turbo given it's only a teeny thing anyway.

If it's twin air related though.. the issue with that is that the unboosted engine is so pathetic you're hitting boost alot to maintain speed, the 1.2 in the citroen is capable of being very economical because it can pull itself along at moderate pace without dumping fuel into boost.

 

[Didge3]

It is regarding the TA specifically in my case. I find a can accelerate at a reasonable pace (probably that of a 1.2 8v, so acceptable) whilst only using <5psi. The problem with the TA is its so eager to spool that turbo.
Guess the answer to the question is a bit of both then, not changing up too early and require heaps of boost but using a small amount of boost and changing up when you can still maintain progress at low boost.

Not sure if you've driven a TA for any length of time and if it had a boost gauge but its simply not true that a TA needs boost to maintain speed, once up to 70mph unless you're going up a decent incline it'll happily plod along unboosted.

 

[StevenRB45]

Had the pleasure of an early TA 500 when they first came out for a few days.

May have been the combination of 25bhp less than I was used to, the fat sports tyres and my mainly uphill national limit commute home but I was deeply unimpressed by 34mpg. Only had about 1500 miles on the one I had though.

I will say I didn't drive any differently than I did in the Suzuki it just seemed to empty the fuel tank in about 5 minutes. I definitely could have done better...very much choose not to but in the same circumstances the swift did 38 mpg and was faster.

Not just the TA you can make any modern turbo drink by boosting hard the whole time, just the smaller the base engine is the heavier you hit the boost and the more variable the fuel consumption is with driving style.

 

[Didge3]

Ah well by no means am I defending the TA's fuel economy, I'm asking the question in the first place to see if I can better mine. I drive pretty tamely most of the time and from fuelly I get 40ish out of it... If I drove it like I used to drive my Forfour I'd probably be looking at closer to 30!

This is my 3rd turbocharged car and when on economy runs I usually drive them without using boost as much as possible. Was fantastically easy in my Saab but then it was a 2.3 so could waft along quite nicely without it. My Renault Twingo 1.1 turbo was waaaay better than the TA for fuel economy, turbo spooled up later so was easier to stay out of it and slightly more oomph off the turbo meant I saw 60mpg on a gentle motorway run. Wasn't nearly as entertaining to drive as a TA though...

 

[StevenRB45]

I recall there is a thread in the 500 section specifically about twin air economy and some people did get amazing numbers in the 60s.

I also seem to recall those people lived in Holland though so no hills may be a contributor!

Could be a good place to pick up twin air specific driving tips for economy. For me with any downsized turbo petrol it's just a case of use fairly large throttle applications only to get up to speed as required then try and stay away from it afterwards. Tends to get good results with the Citroen and given peak torque is 1500 rpm the Turbo must be boosting early in the rev range.

Driven that way it'll get over 50mpg though my wife hangs on to gears which then canes it down to 42..she does cruise at 40 in 3rd though which causes me great internal wincing.

Engineering Explained has a good video as to why the downsizing concept usually produces poor results on YouTube.

 

[AndyRKett]

Isn’t this on a Punto so it’s already heavier than other Cars with the twin air engine, I think the weight of the twin air makes it something like 76hp per ton. Compare that to your old Saab which probably had about 100+ Hp per ton.

Have you fitted a boost gauge ? I would have thought something like the twin air would be boosting constantly even at crushing speeds a couple of psi higher than ambient pressure would be expected.

 

[Didge3]

I recall there is a thread in the 500 section specifically about twin air economy and some people did get amazing numbers in the 60s.
Could be a good place to pick up twin air specific driving tips for economy.

Thanks I will have a look

Driven that way it'll get over 50mpg though my wife hangs on to gears which then canes it down to 42..she does cruise at 40 in 3rd though which causes me great internal wincing.

I know that feeling, I have on occasion had my Mum drive my cars when I've had a few drinks. She drives an automatic and has been for the last ~10 years. I wince every time she forgets to change up and we're bumbling down a dual carriageway in 4th.

Isn’t this on a Punto so it’s already heavier than other Cars with the twin air engine, I think the weight of the twin air makes it something like 76hp per ton. Compare that to your old Saab which probably had about 100+ Hp per ton.

Have you fitted a boost gauge ? I would have thought something like the twin air would be boosting constantly even at crushing speeds a couple of psi higher than ambient pressure would be expected.

Yes tis a Punto, I believe Fiat's laughable official figure is 2-3mpg down on the 500's rating. Horsepower is obviously relevant but looking at small turbocharged engines I always look at the torque figure. I had a GP as my first car with the 1.4 8v, technically only made a tiny amount less than the TA in horsepower but the TA has a lot more 'punch' than that did and would easily walk away from it on a m/way in top gear. My old Twingo GT made 99hp but a lower torque figure although it was faster 0-60 than the TA the torque which wasn't available as low down as it is in the TA so 'felt' slower the majority of the time. The Saab had 240hp so yes it didn't need boost to make reasonable pace, although it did get 19mpg around town during winter...

I have fitted a boost gauge yes, usually sits in a very low amount of vacuum when cruising at 70-75mph, if you press onto 80 its usually in a few psi constantly though. At these speeds even the gentlest acceleration does creep into boost too. Under 50mph its entirely possible to not use the turbo at all so long as you're not in a hurry

It does sound like I'm singing the TA's praises a lot, I definitely know its foibles and its not the most fun to drive car I've owned, or the most economical but it does make a good noise

 

[StevenRB45]

I know that feeling, I have on occasion had my Mum drive my cars when I've had a few drinks. She drives an automatic and has been for the last ~10 years. I wince every time she forgets to change up and we're bumbling down a dual carriageway in 4th.

It does sound like I'm singing the TA's praises a lot, I definitely know its foibles and its not the most fun to drive car I've owned, or the most economical but it does make a good noise

She doesn't even have that excuse tbf always driven manuals. But she's never used a tacho in her life, always by ear. The turbo muffles the engine noise so if you're rolling down a hill in second with 5.5k rpm on it's quieter than her old diesel at idle (ask me how I know!!!). Only time it gets vocal is on full boost.

TA is reasonable engine it was just that it was sold as something it could never be and the configuration means it's more sensitive than most to driving style.

 

[DaveMcT]

Low to medium range boost will give much more useful power at the expense of losing some oomph at full revs. After all, the only people who red-line it everywhere are racing drivers.

It's the same with valve lift. You get the most benefit by improving flow at zero to 50% valve lift as that's a much wider area under the graph than the 50% to 100% range. When racing of course you want the whole lot.

Check out the videos by David Vizard. He's discusses the Classic Mini A-Series engine but the basics (especially port and valve gas flows) are the same on any engine.