Fuel Economy Throttle Bodies Vs Carbs

[mos 1]

Hi,

 

just curious really.

I know the throttle bodies vs carbs debate has been done to death on here but no really seems to have posted what they noticed interms of fuel economy when moving from carbs to throttle bodies using the same engine running the same spec.

i probably wont be in a postion to even do this conversion for sometime as i just starting my s16 88mmbore/stroke project which will use at least initially my existing dellorto 45s

but particularly with the price of fuel today, if i could justify it to myself and my girlfriend (ok mainly my girlfriend!!!!) that a move to throttle bodies will significantly reduce my fuel bills, recouping some of the cost. coupled to the other benefits of throttle bodies it might just be a goer!!!

 

my calculation below is based on fuel economy increasing by 5 mile per gallon.

 

using shell optimax (the only fuel i ever use) at £1.03 a litre locally (and this is only likely to increase!)

 

£1.03 x 4.54 litres per gallon = £4.68 per gallon

i usually get about 10 gallons in the tank when i fill up so thats £46.80 a tank

i get about 240-250 miles out of the 10 gallons between fill ups meaning about 24-25mpg.

i do about 15k per annum at the moment thats 15,000 /240=62.5 fill ups per year

62.5 fillups per year at £46.80 a go =£2,925 per annum on fuel!

 

well based on my assumtion of an increase in fuel economy of 5mpg using throttle bodies the mileage between fill ups would increase to circa 300.

doing 300 miles per tank the figures drop to 50 fill ups per year and an annual fuel bill of £2,340 respectively.

best part of £600 a year saved, if throttle bodies weigh in at about £2,000, then in three years or so they have paid for themselves!

i might just get away with it!!!

 

on a more serious note, i have been pricing mappable ignition systems, as i will need one for my s16 project and the cost of an ignition only capable system is far cheaper than a system that i can initially use for ignition but ultimately use for fuelling as well.

i am trying to make a decision if i will ever take the plunge and move over to throttle bodies (even if it is in a year or two from now) so i get the right system for me long term.

throttle bodies from my research do not significantly out perform carbs with a mapped ignition on a road car so i can't justify it from a solely performance stand point.

obviously i know throttle bodies are ultamately better for not only performance but also aspects such as smoothness, cold starting, throttle response etc. but that said, from what i have read they are only margibally better than carbs with a good mapped ignition.

which leaves me with only one serious arguement to justify the cost, fuel savings

 

anyone done the carb to throttle bodies conversion?

what do you guys think, worth the money for the other benefits alone and most importantly how much did it improve your fuel economy over carbs?

 

thanks

 

mark

[smckeown 1]

well after reading all that and reading how much u spend on fuel, the answer is simple..a diesel.

 

But seriously, i doubt very much whether you will indeed get the savings you predict over 3 years, will u keep the car for 3 years ? carbs are way cheaper than TB's, want to save money, don't use optimax and use less right foot

[mos 1]

i could remove the throttle bodies etc to another vehicle at a later date if i wanted to i suppose, a new inlet manifold and a remap and off you go!

the fuel thing is really out of curiousity/jest rather than anything deadly serious.

just wondering what sort of fuel economy throttle bodies return on the road so if i ever do get around to doing it i know what i can expect, thats it.

Edited August 11, 2006 by mos

[Sandy 191]

The mapped ignition is were most of the improvement in economy comes from, with either bodies or carbs. But lambda control or slightly lean part throttle mapping can improve things further with TB's. On a standard engine, well designed and set up throttle bodies will improve economy if driven in the same manner, but the temptation to floor it is bigger of course!

 

If you're running a fairly serious cam, having the injectors down in the head can improve economy quite alot over carbs, becuase less fuel falls out of the airstream, but most TB set ups have the injectors in the bodies of course, which is the best compromise between power and economy usually.

 

Mapping is the big issue though, there are alot of mappers seemingly who either don't know how or can't be bothered to set up part throttle well.

[petert 587]

lamda closed loop and fuel cut off on over run, are the two main benefits of TB's over carbs. Well setup carbs will still deliver 14.7:1 on cruise, therefore delivering exactly the same economy at that load. But when you back off for a corner, the carbs will still be delivering unnecessary fuel. Carbs don't have the flexibilty to tailor mixtures for ANY situation.

 

Despite what Sean said, a high octane fuel will be cheaper/Km to run, if your car is mapped correctly. As said, most dyno operators either don't know or don't care about mapping the ignition properly at part loads. You can't do it properly on the road. It takes a lot of time to do correctly on the dyno. Each load/RPM point needs to individually monitored, while watching the torque readout on the dyno. As the torque is increasing, it's also eating up fuel, leaning off the mixture at that point, so you have to go back and do the AFR's again.

[mos 1]

lamda closed loop and fuel cut off on over run, are the two main benefits of TB's over carbs. Well setup carbs will still deliver 14.7:1 on cruise, therefore delivering exactly the same economy at that load. But when you back off for a corner, the carbs will still be delivering unnecessary fuel. Carbs don't have the flexibilty to tailor mixtures for ANY situation.

 

Despite what Sean said, a high octane fuel will be cheaper/Km to run, if your car is mapped correctly. As said, most dyno operators either don't know or don't care about mapping the ignition properly at part loads. You can't do it properly on the road. It takes a lot of time to do correctly on the dyno. Each load/RPM point needs to individually monitored, while watching the torque readout on the dyno. As the torque is increasing, it's also eating up fuel, leaning off the mixture at that point, so you have to go back and do the AFR's again.

 

thanks for that and your comments on my 88mm bore thread

i think your right and i should just go for throttle bodies, i will have to start doing my homework on them.

QEP do what appears a great value kms system but i dont know yet as to whether i am better off spending more to buy a better system or if the KMS stuff is perfect for my needs and i will be effectively paying more for a well known brand.

then of couse the really tricky bit, not fitting which of course is tricky, but find a decent tuner/mapper to set them up!

 

thanks

 

mark

[veloce200 3]

find a decent tuner/mapper to set them up!

 

you'd be amazed what you can do with a laptop, narrow band and a performance meter. i've only tuned mine this way 1.9 GTi(ECU + AlphaN)it's basically std 1.9 otherwise. I get good fuel economy over 32 mpg usually except when I drive it hard as I overfuel to be on the safe side without wideband. It's a cheap option but it's not very different to filing su carbs and checking plug readings! I'll get it dynoed soon be interesting to see if it produces 130hp. It is a 58,000 mile 1987 genuine engine.

[mos 1]

you'd be amazed what you can do with a laptop, narrow band and a performance meter. i've only tuned mine this way 1.9 GTi(ECU + AlphaN)it's basically std 1.9 otherwise. I get good fuel economy over 32 mpg usually except when I drive it hard as I overfuel to be on the safe side without wideband. It's a cheap option but it's not very different to filing su carbs and checking plug readings! I'll get it dynoed soon be interesting to see if it produces 130hp. It is a 58,000 mile 1987 genuine engine.

 

it this hometune with a laptop type activity relatively easy then.

i am computer literate but not a whizz if you get my meaning

when i had an audi tt i had a program called vag com which i used to do many things such as reset the central locking features/change the alarm, reset the throttle body an measure my airflow on an excel spreadsheet.

but the software kind of did it all for you, you just followed the instructions on screen.

is it along similar lines or a lot more complicated than this?

[petert 587]

As veloce200 said, it's relatively easy (and fun) to tune the light-mid load mixtures. The easiest and cheapest, is to buy a cheap 0-1mA panel meter, and solder a 10Kohm trim pot in series with it. Calibrate the meter with a known voltage, like a flat AA battery, so you get 1V at full scale deflection. Hook it up to a narrow band O2 sensor and go for a drive!

 

The aim is to get the mixtures between 0.6 and 0.8V. That's approx. 14.7 and 12.5:1 respectively. The panel meter is a lot easier to read and interpret than a digital meter. Have some one drive, while you watch the meter and adjust the mixtures accordingly. You'll get it close enough that it will feel very responsive and only need a minor tweak on the dyno. Time spent doing this will save you $$$ in dyno time. You'll find it near impossible to do high RPM full load mixtures, but your work doing the mid load will give you reasonable numbers to put in the high load. Be conservative and make the high load numbers rich rather than lean.

[mos 1]

have read a few posts that state a few of these systems now possess a certain degree of self mapping on the road.

is this correct or am i missing something?

that should take alot of the work out of it just leaving the fine adjustments for a proffessional on the rollers? or like most things in life, its not quite that simple!

 

thanks

 

mark

Edited August 13, 2006 by mos

[Sandy 191]

Ignition mapping is 50% of the result and it has to be done manually. The fuelling can be brought close by closed loop, but it's only relative to the accuracy of the ignition map, so a roller session should still be undertaken.

[mfield 20]

I used to get a repeatable 300 miles to a tank when using carbs with 3d ignition, the car made good power and was driven at normal road speeds. When given a caining the mpg would drop down to 180 - 200 miles to the tank.

 

If i was to do it again i would defo go with the tb's though just to save the hassle that comes with carbs.

[christopher 5 1 Cars]

I'm sorry but I don't know what everyone is banging on about the "hassle with carbs" if you have a good setup there is a tiny bit of adjustment every 2 years or so. I do it myself now..

 

My engine is small but I am running twin 40's and a high lift cam. I can do close to 400 miles on a tank. If you floor it all the time then of course it takes lost of fuel... An ECU may be a computer but a carb is all about physics. Its simple, reliable and cheap. At the end of the day how much you do you spend idling?

[petert 587]

At the end of the day how much you do you spend idling?

 

Idling is fine. It's more about their ability to cope with a wide power range. Take the BDA for example. Folks who have to run them on carbs for Historical racing classes have to put up with a 6500-9000 power range. No amount of fiddling will get them to work down low. The same thing on TB's could be driveable from 3000-9000.

Edited August 14, 2006 by petert

[mfield 20]

I'm sorry but I don't know what everyone is banging on about the "hassle with carbs" if you have a good setup there is a tiny bit of adjustment every 2 years or so. I do it myself now..

 

My engine is small but I am running twin 40's and a high lift cam. I can do close to 400 miles on a tank. If you floor it all the time then of course it takes lost of fuel... An ECU may be a computer but a carb is all about physics. Its simple, reliable and cheap. At the end of the day how much you do you spend idling?

 

 

So if you want to change your cam or fit a different exhuast the "hassle of carbs" plays no factor ?

 

I for one wasn't talking about balancing or seting the air bypass screws for that rock steady idle and smooth operation, with tb's your have all the variables there to hand in the laptop. No need to change jets and then air correctors to match, gone is the need to make comprimise between a boggy bottom end and out right power at the top end (assuming we're talking 8k+ revs) or a lovely torquey bottom end but running out of puff by the time you get to the high revs.

 

I know tb's still need setting when changes are made but it will be quicker and easier hence less hassle, considering there are less and less carbs specialist but more and more ecu mappers.