How to tune engine to drink less fuel?

[classicswede]

Also as we know, modern engine management can do much better than carbs, because of ignition and injection can be adjusted based to temperature and load, quite well, also knock sensing allows us to try to get that last efficiency % out from engine.

This is actualy not completly true. It is possible to tune a carb to give better fuel consumption than injection. The problem is keeping it consistent.

[jtbo]

Also as we know, modern engine management can do much better than carbs, because of ignition and injection can be adjusted based to temperature and load, quite well, also knock sensing allows us to try to get that last efficiency % out from engine.

This is actualy not completly true. It is possible to tune a carb to give better fuel consumption than injection. The problem is keeping it consistent.

Isn't better fuel economy in carb caused by the fact that there is only this 34mm hole (Cisac 1st throat) where from air is sucked to engine limited by throttle butterfly under normal running conditions?
So designing similar limitations to FI (two small throttle butterflies that open in staged manner) we would get similar gain in economy in NA engine?

Of course 130hp, here we will come to turbo again, off boost situation should not still differ a lot from NA engine.

[pettaw]

I agree with Dai that is possible to tune a carb to give better economy, and, actually better power than injection, because its an analogue system and therefore has infinite resolution, whereas engine management is a digital system and therefore has a limited resolution.

However in practice its engine management and fuel injection that wins, because its easy to tune it in and infinitely variable, whereas a carb has to be very carefully set up, with emulsion tubes, progression drillings etc to alter the fuelling characteristics, its very much a fine art.

[Chris_C]

They spent four years at uni telling me that analog was always "better" than digital for that very reason, a good digital designer knows how to make the problem un noticable to the majority

[jtbo]

This is what puzzles me right now, as with injection we do have one large throttle butterfly, when you push throttle pedal it is rather easy to open so much that to get as much air passing to engine with carb you would need to open 2nd throttle butterfly too.
This will naturally give more feeling from power, but at expanse of more fuel consumption or that is how I think it is.

So here at left is FI throttle butterfly and on right two smaller that would be set up similar way as in carb, 2nd will open after 1st has been opened enough.

[Chris_C]

The reason you don't need the dual pick up in a FI system is the petrol isn't lifted by Benuli (sp?) effect.

In a carb, the speed of airflow through the venturi's is super important, just as much as the amount of airflow, this is why nova's with 1 litre engines sticking 45's on are asking for trouble.

Air speed * Venturi width == Vol of air per sec

So, for the same volume of air, it pays to have 2 venturis, each with a fuel jet on the carb. This also means you can run less fuel on idle, than at full throttle (unless you have the 2L carb fitted to 360's, which is lame, and has a smaller jet on the second venturi)

In a FI car, as the fuel is pumped in by the injectors, not picked up by the benuli effect, the air speed is irrelivant, therefore the simplest mechanism possible is best! I do however like the progressive feeling of the second butterfly however...

[Carl]

I have to admit I'm kinda lost when it comes to carbs, but the BX has a 2 stage throttle butterfly, very similar to a twin choke carb (ie one opens later). My take on it is that the smaller opening at low engine speeds keeps the air speed up to give the engine a bit more torque, but when you floor it both open to maximise air flow, and all hell breaks loose.

The mi16 was (is?) considered one of the best of the old school engines (ie no VTEC, VVC etc etc), so I guess it must be a good thing.

My mk1 MR2 worked on the same principle, but there was a main throttle butterfly, then each inlet had it's own secondary butterfly at the end of the inlet manifold. Gave good low down torque, but muchos rev-ability and therefore good top end power. I've got a power/torque curve from my car somewhere which demonstrates it really well.

Not sure how relevant that is, but I guess it might help in future engine planning in some way!

[classicswede]

The BX twin throttle body is basicaly the same as the early 440 injection one.

The reason for using the twin choke linked (common port) carb is for the low end torque and improved low end fuel consumption. It is a good set up and works well on carbs and FI (the astra 16valve has a tini little primary flap with a huge secondary).

From a tuning for econemy point of veiw the single SU takes a lot of beating and is the prefered choice for many engine tuners (for both performance and economy) of the A series engine. Talking of A series engines I recently converted a Mini to gas - its doing an amazing 42MPG on LPG condidering you are normaly see a 5% increase in fuel consumption due to lpg's lower calorific value.

[jtbo]

So I think that twin throttle body is what this project will need as some manufacturers have been using it too.

Making one from old carb would be of course one option, but there might be some leaking problems then.

As a side note, owners manual of my 360GL promises that car would get 42.5MPG on highway at 56 mph or so.

I have been thinking different aspects of 1.4 and 1.7, I believe that 1.4 would be better for this kind of project as engine has no belt and valves can be adjusted without buying some spacers etc. Also car is bit lighter and smaller engine will consume in theory less fuel and it still would have enough power with turbo.

Car needs to be with 5 speed gear box naturally, but it would be perhaps needed to get diff from 4 speed model (3.64) or one from France version as those had 3.45 diff.

[foggyjames]

I used to get a measured 45-50mpg from my 360 at 60-70mph. The owners manual for the 89 model says 52.something MPG at 56mph.

As for how to make a fuel efficient 300 (keeping a standard shell...aerodynamics being a problem here)...

For starters, you wouldn't be able to use an 8v redblock. The combustion chamber design is old technology and inefficient by modern standards. It'll do the economy (on a run), but not the power (without spoiling the economy). The B200K is about as good as it gets for economy on a redblock - and it massively suffered on power as a result.

A generic 2.0 16v would get you 130 bhp easily, and frugal tuning of EFI would get you the economy. The trick for a 360 would be to pilfer one of those 1.8 Mitsubishi GDI engines they put in the later S/V40s. 122bhp stock...a bit of tweaking and I'm sure it'd make 130bhp, and the clever ultra-high compression lean-burn stuff makes for efficient motoring when cruising.

I think EFI will always get you the best all-round performance (and I say this as a carb enthusiast!), as it has far greater range of control over the engine, and knowledge of what's going on. I bet if you compared EFI and carbs absolutely like for like, EFI would always win on power vs economy. It's able to cope so much more accurately with any circumstances you might throw at it - hence why you can get over 40mpg cruising at 60mph in a 940 estate which has over 200bhp. Accuracy of fuel metering is key to efficiency, whether we're talking power or fuel economy.

To be truly efficient in town, we need to be losing weight...and preferably burning diesel. It's also worth considering that diesels are very efficient when cold. Dropping a D5 lump into a 360 would be very sweet - you'd easily beat your economy targets, and have 163bhp and 250lbft to boot.

Strangely, the new BMW 2.0d unit has exactly the same power and torque specs as the (old) D5. Bet it doesn't do it at such low engine speeds though!

cheers

James

[jtbo]

At highway speeds drag is taking less than 10% from max engine power in Focus that has 0.32cd (100hp engine), drivetrain losses are 15-20%. Removing passenger side side view mirror can reduce drag even 20% on some cars, I think that in 300 that is quite high factor too as mirrors are quite boxy and large, but I have no real data from 300 mirrors.

300 has cd of 0.37 or 0.38, 200 series has cd of 0.42, granted 200 has more frontal area too, which is not directly shown in these numbers.

So changing mirrors is thing that is needed to do for highway mileage. 3 door shell is perhaps most aerodynamic so that is what project needs.

Weight is not going to be great issue as 1.4l model is only 960kg, any modern car weights more.

GDI engine would be great, but getting one and putting it under to hood and still get it trough MOT would be impossible, it would require 2000 euros at minimum, probably much more + all parts, engine etc.

Some engineers have told me that smaller engine with turbo will be always better in fuel economy than bigger engine without turbo, when comparing two engines that pushes similar hp.
So that is why I like to stick with 340 3dr 1.4l model, does it sound too crazy? Some errors in conclusions?

I'm thinking that I would take 2007 Ford Focus 3dr 1.6 as my target, so that I have to get better fuel economy than that has on typical use. Of course more below is always better. That car weights almost 300kg more and gets overall mileage of 42.2mpg

[classicswede]

If you are going turbo then I would use fi as carbs are hard to set up well with turbo.

on na engine carbs are better for power/economy comparing like for like technology.

I would say it is only with the very latest injector and ecu technology that injection has caught up with carbs tbh. The down side with carbs is they need regular adjustment as where modern fi does not.

[jtbo]

If you are going turbo then I would use fi as carbs are hard to set up well with turbo.

on na engine carbs are better for power/economy comparing like for like technology.

I would say it is only with the very latest injector and ecu technology that injection has caught up with carbs tbh. The down side with carbs is they need regular adjustment as where modern fi does not.

Definitely turbo + FI and that is most likely Megasquirt ECU with direct spark as that is most efficient for least amount of money, have to think money too Also Megasquirt is very much of like any modern ECU.

I do know one design of variable length intake manifold that is perhaps possible to make, but not sure if that would be worth the trouble in turbo engine?

[classicswede]

In a turbo engine I doubt that it is worth experimenting with. On na engine intake lengths can make a big difference.