BPU giving better fuel consumption. Need explanation.

After reading the thread where several members said the their fuel consumption is better after going BPU I have to admit that, due to my limited understanding of erm... everything, I have no idea why.

I thought it would increase fuel consumption. Is the saving true in all cases or just in some/most?

Can anyone give a laymans terms explanation as to why it saves fuel? I'm not disputing anything just curious and eager to understand.

Cheers,

I can only see why everyday fuel consumption (i.e. running below 4Krpm) would be better if one is running TTC.

Could be a cleaner burn due to more free boosting turbos, exhaust manifold pulsing occurs on all cars due to the sequence of the exhaust valves opening and closing, if a 'pulse' of exhaust gas bounces off a restriction and goes up another exhaust exit as the valve is opening; you get less of the burnt air out of the cylinder on the stroke (as air is trying to get back in at the same time), so the next power stroke has a higher concentration of burnt fuel in the chamber.

Free-er flowing turbo air due to quicker spool will have a tendency to remove this gas quicker and keep it out, reducing the above effect.

This wouldn't cause a noticeable difference in MPG though I'm just talking bollox!!!

You mean reduced reversion and improved volumetric efficiency (mainly from the decat effect). Yes, that would reduce fuel consumption a bit.

Another (more measureable!) factor is the introduction of pod filters associated with 'BPU'

In the summer they will force the engine to breathe warmer air, sometimes substantially so. On easy driving this can mean 1-5% reduced air density (easy!) so the corresponding fuel in closed-loop will be less. This will be clearly measurable if one commutes daily in the same car.

Could it be possibly that they have Dyno'd it after the mods and had their fuelling adjusted/corrected?

My car on stock had terrible mpg. After I got it to BPU, I replaced the oxygen sensor and got it tuned and mpg was much better. Probably down to the Oxygen sensor in my case though.

greater efficiency = less throttle for the same speed?

If you keep it sequential and don't change clogged airfilter or such, then a tired oxygen sensor is a logical explanation for such a noticeable mpg improvement.

They don't die abruptly, rather they slowly become lazy, showing leaner than they should.

So the ECU slowly adjusts on the 'rich' side, unknowingly that it isn't stabilising around 14.7 anymore.

Changing the oxygen sensor after 80K miles tends to bring improvement in general. (I'm talking std zirconia sensors here, not the new generation or the titania ones)

i personaly find it hard to accept that decatting (if that is the definition of bpu as used in the first post) will net any significantly better mpg. in my experience i spent a lot more time accelerating, which did not help my economy.

the other factors mentioned esp o2 sensors will have a dramatic effect but hardly attributable to bpu mods.

As your say John, TTC will do it - I rediscovered the page on MKIV.COM which shows you the pinouts to wire in electronic TTC to a Fields harness- looks like it would take an hour tops, and as my new job is going to be a 100 mile round trip commute I want to save as much petrol as possible - plus it will be good going single practice as I will be used to the no boost then lots of boost type picture.

... as my new job is going to be a 100 mile round trip commute I want to save as much petrol as possible - plus it will be good going single practice as I will be used to the no boost then lots of boost type picture.

I have gone exactly the opposite way and have it boosting hard from way down low. That's the supra's distinctive feature, why not build on it:)

Yes, 1 bar at almost 2Krpm doesn't help mpg, but it does feel like a n/a 6lt.

If you have a daily 100mile commute and want to keep costs down, then the supra is probably the wrong car to (ab)use.

Get a turbodiesel runabout, mine (95astra) has exactly *half* the supra's consumption, and cost feck all.

It will pay for itself immediately and will stop you from piling up miles on the supe. Remember that it's tyres, suspension etc that also age, and dumping 2000 additional miles per month is not gonna do it any good.

Plus you won't care if someone bumps on it in the carpark --- it happens often.

After reading the thread where several members said the their fuel consumption is better after going BPU I have to admit that, due to my limited understanding of erm... everything, I have no idea why.

 

I thought it would increase fuel consumption. Is the saving true in all cases or just in some/most?

 

Can anyone give a laymans terms explanation as to why it saves fuel? I'm not disputing anything just curious and eager to understand.

 

Cheers,

You are increasing the engines breathing efficiency, driven the same way as before removing the cats the lesser exhaust back pressure will increase BMEP, a measurement of an engines efficiency.

I have gone exactly the opposite way and have it boosting hard from way down low. That's the supra's distinctive feature, why not build on it:)

Yes, 1 bar at almost 2Krpm doesn't help mpg, but it does feel like a n/a 6lt.

 

If you have a daily 100mile commute and want to keep costs down, then the supra is probably the wrong car to (ab)use.

Get a turbodiesel runabout, mine (95astra) has exactly *half* the supra's consumption, and cost feck all.

It will pay for itself immediately and will stop you from piling up miles on the supe. Remember that it's tyres, suspension etc that also age, and dumping 2000 additional miles per month is not gonna do it any good.

Plus you won't care if someone bumps on it in the carpark --- it happens often.

I know its off topic, but I worked out the fuel will cost 1700 quid for the 6 months I will be at this post - I cant see being able to buy, insure and fuel a beater (even though it does double the mileage) and trust it to start every day. Therefore making the supra more fuel efficient seemed a reasonable alternative.

*EDIT*

Plus, my supra (assuming it wasnt horribly clocked) when I bought it only had 25000KMS on the clock - nothing about my inspection when I bought it led me to believe it wasnt genuine, and I wonder if I bought a car that was just polished for most of its life before I got my hands on it In my three years of ownership I have done 9000 miles only.

Everyone mentions decatting as the main improvement for BPU so we will see in a few weeks. I have monitored my MPG since Novemebr and am decatting this weekend so will tell all once I have a few results

I know its off topic, but I worked out the fuel will cost 1700 quid for the 6 months I will be at this post - I cant see being able to buy, insure and fuel a beater (even though it does double the mileage) and trust it to start every day. Therefore making the supra more fuel efficient seemed a reasonable alternative.

But the beater will cost less than half that, so about £800. My Renault 5 was £250 and £350 to insure - £200 saved in 6 months.

If you know a bit about cars, you can buy something with £50-£200, fit a new set of economy tyres, maybe even new battery and you're set for a year.

You'll probably sell it next year for the same money, if you feel like selling (which you may not do, once you get used to it!)

People pay this sort of money for a set of front *brakepads* for the supra, lol...

hmmm.... I did reconsider this john, but I dont know that much about cars that I would be confident buying at this end of the scale. Having said that I suppose it isnt much of a loss if it doesnt last that long either - I knew a guy who never paid over 100 pounds for a car, when it was shot, he just scrappedit an got another. Dont forget I would still have to pay for insurance on that 'beater' vehicle.

Also there is something to be said for having the enjoyment of driving a Supra to work and back--- it still makes me smile. I never intend to sell the car and would happily have it rebuilt in the future if required, in fact I think it runs better for being used regulalrly nowadays - before it used to sit for weeks at a time, not the best thing for it I know, but its still low mileage

hmmm.... I did reconsider this john, but I dont know that much about cars that I would be confident buying at this end of the scale.

Auctions mate.

You might even get a 'performance' car for that money, but avoid them, you want a sissy high-mpg unmolested vehicle for reliability/dirt-cheap motoring

If it's private you might even get a long MoT.

Also there is something to be said for having the enjoyment of driving a Supra to work and back--- it still makes me smile.

I guess it depends if it's sitting in heavy traffic, doesn't it?

not the best thing for it I know, but its still low mileage

It won't be low-mileage for long with 2000 extra miles per month, will it?

Rain, ice, salts, idiots everywhere --- the supe is in constant danger here in the Southeast.

Maybe it's different where you live though:zen:

Auctions mate.

You might even get a 'performance' car for that money, but avoid them, you want a sissy high-mpg unmolested vehicle for reliability/dirt-cheap motoring

 

If it's private you might even get a long MoT.

True.. my previous mtotor was a 1.8D fiesta 450miles on 35 quid of diesel- pretty good until I killed it with... you guessed - it lots of high speed commuting.

I guess it depends if it's sitting in heavy traffic, doesn't it?

Oh totally cant stand traffic whatever car I am in, though I suppose my left calf would be bigger than my right after 6 months of that sort of thing.

It won't be low-mileage for long with 2000 extra miles per month, will it?

Rain, ice, salts, idiots everywhere --- the supe is in constant danger here in the Southeast.

 

Maybe it's different where you live though:zen:

Hmm, well I am driving early so its pretty quiet, but I take your point - I think a case of suck it and see might be the best approach (then looking for a diesel chugger if that looks like the way forward)

James

I just sold a perfectly good Rover100 with 12 months MOT and no faults last week. Vicki had it for 8 years. Went for 400 quid.

It was sat on the drive going no where for the last 18 months and I considered using it for commuting, but then what's the point in having two performance cars sat at home doing nowt? I bought the Supra to use and enjoy every minute of it...so the 100 had to go.

James, yours is a 6sp as well, so not as nice to drive in heavy traffic as the auto (which is *bliss*)

I understand Pete's point as well.

Hell, what's the point of having a Supra if you drive in a shitebag every day?

Tough one

I do only clock up 100 miles a week commuting though.

Wouldn't it be nice to think of your supra as a car you could enjoy for a long time, and then when midlife crisis time comes, or you are trying to rediscover your youth, have enough money packed away somewhere to get the whole thing done - bare metal, parts laid out on a bench, so it was better than new....

*EDIT*

Then you could enjoy it now.. and again later

Hulk Hogan must be what nearly 60.... he likes his Supras!

..Hulk Hogan must be what nearly 60.... he likes his Supras!

doesn't need an engine crane either:ecstatic:

I'm building a GT40 for my mid life crisis.

I'm set on an Ultima for the mid-life crisis lol