Poor fuel consumption from HIF 44 carbs + 123 ignition unit

[CliveRothwell]

OK here's the real question:

3.4 litre Series 3 1985 Automatic car.

1 What MPG would this car have returned when new?
2 Now fitted with NEW SU HIF 44 manual choke carbs to overcome the problem of old worn out carbs and useless auto choke unit. New leads and plugs and ignition wiring harness.

Also fitted with new 123 ignition unit, set to position F

Ignition set to 10 degrees BTDC giving CO figure of 2.3 HC figure of 280

these setting achieve the best set of figures for unburnt fuel.
The car starts very well, does not run on and pulls well.
HOWEVER, even with a very gentle right foot, it only returns a dismal 20.5 mpg. Myself and the garage who tuned it are quite surprised at how poor this MPG figure is. Super unleaded has no improvement to MPG. The car does not have the heated air intake manifold unit that goes across the top of the engine. The engine, a factory replacement unit, has covered 43,000 miles.

So give all of the above, do you think I am expecting too much? I was hoping it would return about 25 mpg?

Any advice / experience of / knowledge of fitting these carbs and ignition units would be gratefully received. I am sure there is somebody out there who must have this set up and can quote better MPG figures than this. With fuel at over £5 a gallon I am very keen to squeeze evry mile from my fuel!

Yours in anticipation

Clive

[almcl]

Clive

I think 20+ mpg from a 3.4 is very good.

Car magazine reporting on a road test carried out in 1983 gave an overall test figure of 18.4 mpg. Several journalists at the time commented that the 3.4 wasn't up to the job of lugging nearly 2 tons of car and was less economical than the 4.2

For best economy you do need the heated duct but getting one and a fully functioning thermostatic valve might be a challenge.

My 4.2 FI returns 18 - 20 (when the weather is favourable) and my 5.3 manages a bit over 20 when used on long runs, but both these benefit from the FI's trick of shutting off fuel injection completely on the over run and more accurate fuel metering than is possible with carbs.

[J44EAG]

I think we perhaps have to put fuel consumption, engine design, age and expectations into perspective.

As I remember from my aircraft engineering course, there is a formula for calculating fuel consumption given the number of Pistons, Length of stroke, Area bore diameter and a constant factor. The equation was known as PLANK but that is as much as I can recall. Probably just as well!
Andy Stoddart is the Club expert with this type of thing. You may recall his article in a recent Club magazine when he wrote about his ignition modifications on an efi car fitted with his redesigned ignition timing sensor which helped with fuel and ignition issues. Ken Jenkins, is of course another guru in this field. Perhaps they may be able to help with questions regarding any possible fuel consumption improvements.

The bigger the engine, the more fuel used. Obviously in service factors that govern consumption are driving style, engine tune,ignition timing, state of ignition condition, type and dimension of carb needles or efi mapping, valve overlap and cam design, head compression and flow characteristics and induction and exhaust restrictions together with fueling perameters. Overall gearing also has a bearing on consumption. More about that in a moment.

Other factors can influence. A small engine can sometimes be more uneconomic than a similar chassis fitted with the larger engine. As an example, a Rover 2600 used more fuel on the motorway than a later Rover 3500 that I owned. The 2600 was however better around town.

We run big, heavy, relatively highly tuned cars with fairly big, lairy camshft profiles, that were designed at the time for performance that was almost unequalled in the industry. Fuel consumption until the crisis in the late sixties was never realy an issue was it?. Now it is!

Modern Jaguars of similar capacity until recently have quite frankly, faired little better in 2010 than similar capacity cars did in the sixties. my V6 S-type could just manage 30mpg on the motorway at 60mph. Tom Tom speed over ground and mileage covered readings in this case. My ecu modded 4litre V8 returns around 26mpg under similar conditions.
If the car were to be equipped with a mythical 5litre engine, I`d expect fuel consumption to increase by another 4mpg given the same design perameters. Again it goes back to the PLANK formula above. Add age and wear factors and that does not help either.

Gearing does make a difference. For a while I ran my V8 on 19" wheels fitted with X350 tyres which were 4.25" longer in rolling circumference than the correct tyre for my car. Performance was reduced. gear changes were "interesting" but I could manage 30mpg on the motorway. The car was at that time very, very highly geared but was a slug around town. A classic case of not being able to get something for nothing.

This could become an interesting topic for debate. Can we keep this thread going between us?

Mike Kennedy

[almcl]

This could become an interesting topic for debate. Can we keep this thread going between us?

I'll try, Mike.

Another factor which may be important is weight. The V12 cars are marginally heavier than the 6 cylinder ones and of course it depends how and where they are driven, but the mpg isn't desperately worse than the 6 cyls (in my experience.)

I am not sure if highly tuned is how such engines would be described today but one measure of this is the bhp per litre. Now the 3.4 and 4.2 come out pretty similar (roughly 48 hp/ltr) but the v12 (in its HE form) with its 12:1 compression ratio is about 20% more efficient (or stressed) at 56 hp/ltr. Of course it's a heavier lump, by about 90kg, but still arguably more efficient.

If you can dig out any more info on that formula, I'd be interested, does it take account of boosting or is it for normally aspirated engines only?

[J44EAG]

Hi Al,

Thank you for the questions. So nicely presented and in need of an answer.

Here is your evenings homework.

Although couched in a more advanced formula than the one that was knocked into me, you will find all you need to know in the attached links - somewhere!

I shall be setting test questions for you in the near future and failure of the exam will not be tolerated. Although degree level stuff, as a JEC member I know you will have few problems with the concept of the formula or its application.

The Constant reference that you will find is something to do with multiplycation by a factor 33,000. Some bright spark found that out and apparently it works!!

I remember that the formula was based on readings taken at 15degrees C at sea level with a barometric pressure of 14.9psi/one atmosphere or one Bar. Any increase in turbo or supercharging inlet manifold pressure would therefore have to have that factored into the equation. Similarly a drop in air pressure due to altitude would also have a considerable bearing as mixture would enrich as height is increased due to a decrease in barometric pressure.

Note that a modified Otto cycle engine (No Al, not a motor bike, but the name of the internal combustion engine`s original designer, Herr Otto) is no more than 27% efficient. It cannot be more as its design dictates that it has one power stroke and three to wear the engine out! Not a lotta people know that!

http://www.eng.warwick.ac.uk/staff/av/courses/ice.html

http://en.wikipedia.org/wiki/Specific_fuel_consumption

http://www.search.com/reference/Specifi ... onsumption

http://www.hyperflite.co.uk/fuel-consumption.html

http://en.wikipedia.org/wiki/Combustion

http://en.wikipedia.org/wiki/Internal_combustion_engine


I think we need some help with this topic. Is there a member using the site who can simply explain the topic of Specific Fuel Consumption to us?

Mr Stodart are you there in our moment of need?

Mike

[CliveRothwell]

We ceril, full of itainly get technical answers in here don't we! Excellent detail and full of interest, justt the sort of answer I thought might turn up. Going back to the original query though for a moment: does all this mean that I can't really expect any further improvement than the 20mpg that it is returning at present?

I think I was secretly thinking that somebody had already done this sort of conversion and knew what sort of consumption figure might be achievable. (surely I'm not the first to fit these to a 3.4)

Many thanks for the fascinating replies.
regards
Clive

[J44EAG]

I just want to touch on a point Al raised earlier regarding bhp/ltr.

Now this is a thorny subject. Output figures published by car manufacturers can often cause us a headache to fully equate when it comes to comparing a specific power plant against a competitors product and performance figures. This is often because of differing measurement standards used to produce such figures. A wide varience in measurements occurs across the motor industry and there appears to be no set universal measurement standard.

I`ll try to explain.

For the UK, Jaguar for instance, quote the output of an 02 V8 4 litre S-type at 281BHP. In the US, the identical powerplant is quoted at 271BHP. This assumes measurements recorded with a core engine run devoid of power sapping ancilliaries such as altenator, waterpump and PAS pump. Other common manufacturers may or may not adhere to similar test proceedures or test measurement standards. See the S-type Forum pages in the July 09 copy of our magazine for a fuller explaination.

To come to the point of this post. The power outputs quoted by Jaguar rarely if at all are ever seen when our cars are run on a rolling road. Apparently Mercs and BMW cars do. Why?

Perhaps this is because they quote figures at the flywheel with ancilliary components running on an engine whereas flywheel figure for a Jaguar is often many BHP lower than Jaguar quote. That might disappoint a few of us but that was as I found the situation when my V8 went to Paramount for ecu, filter and exhaust mods. Upon arrival I was warned about this fact before the initial run up commenced.

Sure enough, the first run as a stock engine produced a flywheel output of just 208bhp at a road speed of 87mph in third gear. The rev limiter then cut in. This equates to 52BHP/litre.

The second run commenced with a new K&N filter, remapped ecu and sleeved down, resonatorless 60mm Milltek sports exhaust. Under the same perameters 258BHP was achieved which brought the power output up to 64BHP/litre.

Without larger exhaust manifolds and sports cats matched to reworked big valve ported and polished cylinder heads, little more can be done to increase that power output from the engine as it now stands. Improved volumetric efficiency has resulted which makes for an easier running and performing car but no improvement (in this case) has resulted in regard to fuel economy. I suspect this is due to the fueling mapping curve being set to provide more fuel to the engine to gain performance.

Supercharging would obviously improve power output where an engine is force fed with extra charge air at something around 3psi above ambient air pressure. There are pluses and minus factors to be considered as driving a supercharger from the engine crank leads to more power sapping bhp but improves the overall power out put over and above that of a naturaly asperated equivalent engine. Increased power BHP/ltr figures will consiquently be seen.

I think you might be hard pressed to achieve much improvement on your consumption figures Clive, although a decent K&N oiled cotton filter will help volumetric efficiency by allowing the engine to breath slightly deeper.

Comments please Gents.

Mike

[CliveRothwell]

Hello Mike

You raise some good points in your last response. Whilst many would like the idea of K&N filters, I am more inclined to try a stab at the warm air feed pipe route, keeping the car looking original, even if it actually isn't standard and original. My thinking is that the Jaguar engineers back then used the notion of warmer air for a good reason; presumably, that still stands today as some new cars still have hot air pick up pipes. I have sourced a full set of cross over plumbing and boxes so I'll report back with the results when I get them calculated. I suspect it won't make much measurable difference in terms of MPG but might alter the emissions PPM figures very slightly.
Thanks for the very interesting response.

Regards
Clive

[almcl]

Cilve

You might find this portion of an article by Roger Bywater (of Jaguar V12 development fame) of interest:


The rule about air temperature is simple, cool for power (maximum charge density), hot for economy (minimum charge density to reduce losses due to throttling). In this respect the standard arrangement is much better than many people think. Sure, the under-bonnet air temperature at idle can easily get up around 70 C but the faster the car goes the lower the air temperature falls - simply because the radiator is passing its heat to a much larger quantity of air per second - so at 80 m.p.h. the engine is breathing air at around 45 C. That's still a bit higher than the ideal but not nearly so bad as many people think. Obviously the arrangement helps to maximise economy in moderate speed urban cruise without compromising top end power too much.



The full text is here:
http://www.jagweb.com/aj6eng/v12_performance.php

There have also been interminable discussions on the J-L site, some even came to useful conclusions.

[J44EAG]

With some luck Clive and a bit of poking around on the net, I`m sure there would be a K&N type filter available for your car. This would hopefully be a direct swap for your standard paper filter and once installed would be impossible to detect from outside your filter box. Originality for all intents and perposes would be preserved from the appearance point of view.

Try Motorsportworld on the net. Their prices are the best I`ve found and they are extremely helpful guys running the business.

Paper filters do stiffle an engine and a decent filter does make a difference.

Mike

[CliveRothwell]

This looks like good news then - documentary proof that warmer air will improve mpg figures and a second option of the K&N paper filters which will fit without looking non standard and which will improve air flow. I look forward to sourcing and fitting the same. I assume the emissions will have to be rechecked whilst the car is hot?
We'll get 40 mpg yet!!
Many thanks for the help, Al / Mike