Crossflow fuel consumpstion?

[greg]

I've Alpha 3D ignition and it makes a huge difference (older car was a 150BHP Live Axle Xflow).

 

Low down drivability is much better, and hence cruising is smother.

 

On the few really long cruises I've done I get near to 200 miles on a tank, which is nearly 30 mpg using the rarther inaccurate Caterham VDO instruments.

 

During this weekends motoring extravaganza (Rolling Road, Brooklands, Goodwood) - 600 miles on arround 3 and a bit tanks.

 

Greg, Q 86 NTM (Green 185BHP XF)

[greg]

Ah Look PC agrees with me - I must know what I talking about

 

Greg, Q 86 NTM (Green 185BHP XF)

[Dirty Den]

Peter, could you explain 'vacuum advance' in language that a mechanical numpty would understand?

 

Cheers,

 

Den

 

😬 - Self portrait - still unable to remove the smile!

[Peter Carmichael]

The power of an engine is developed from the push of expanding gas on the pistons. The gas expands because it is heated by the combustion process. The timing of the combustion determines the timing of the push on the piston tops and the timing is utterly critical for getting the most power out of the engine under any operating conditions (revs and throttle opening). The timing of the combustion is controlled by two things:

 

1. The timing of the spark

2. The speed of combustion

 

Let's consider full throttle to start with:

 

At full throttle, the cylinders are very well filled with the fuel/air mixture, which gives a consistent fast speed of combustion regardless of revs. At low rpm the piston is moving relatively slowly, yet the combustion process is the same combustion that takes place at higher rpm - ie. it takes the same amount of time. In the time period it takes to burn the mixture, the crank rotates half as far at 3000rpm as it does at 6000rpm.

 

Even at low rpm, the combustion is started while the piston is still rising so that it meets the piston at the right time - this is known as the spark timing and is measured in degrees of crankshaft rotation in advance of the piston arriving at the top of its motion (top dead centre). In order for the push of the expanding gasses to meet the piston top at the same point in its motion at 6000rpm as at 3000rpm, the combustion has to be started more degrees in advance the faster the engine turns. A traditional distributor does this through a centrifugal mechanism that gradually increases advance as revs rise. A 2D mapped electronic ignition system does this electronically.

 

Now let's consider part throttle:

 

At part throttle, the cylinders are only partly filled with fuel/air mixture, which gives a slower speed of combustion. This requires more advance to get the best out of the engine. A traditional distributor does this through a vacuum advance mechanism that senses the vacuum at part throttle in the inlet manifold - the vacuum occurs because the engine is trying to suck air in but the partially closed throttle obstructs the flow. A 3D mapped electronic ignition system does this electronically.

 

When engines are converted to use multiple carburretors such as twin Webers, it is common to disconnect the vacuum advance because there is no convenient point from which to sense the induction vacuum. This gives the same full throttle performance but adversely affects the fuel economy and driveability. 3D electronic ignition can give you back the fuel economy and driveability.

 

[Graham Perry]

3D can give you some really wierd advance numbers as well. I don't have my map to hand but on my cross-flow in some places it is as much as 50-55 degrees advanced at light throttle positions. Mind you it was set up on an engine Dyno so was thoroughly checked in various modes. Try that at full chat and the engine would grenade. With Vacuum advance I doubt you would ever see anywhere above 36 degrees of advance anywhere in the rev-range regardless of throttle position.

[Peter Carmichael]

The range of advance required depends a lot on combustion chamber shape. A pent-roof 4 valve combustion chamber has a spark plug in the middle, so it is equidistant to the furthest corners of the combustion chamber. This means that the flame front is wide and has a relatively short path to travel. A 4 valve per cylinder design therefore can burn its charge quicker than most other designs and needs less advance.

 

A 4 valver will see as much as 30 degrees advance at full throttle and full rpm, but at part throttle this can go up to 46-50 degrees.

 

A crossflow will need more advance generally.

 

The key to the progression is to get the advance to be suited to the characteristics of the engine coming on cam and 3D mapping is the best way to achieve this. A vacuum advance was only ever a useful and convenient opportunity to get closer to optimum. Better than nothign though.

[JKJ]

My god, the sun comes out, you get in it, drive around alot, big smiles, and get out. Fuel consumption irrelevant. If you use the thing seriously, daily, why ???? when there are other workhorses out there.

[Mark H]

''why'' Because i can

 

OOPPPSS!

[Graham Perry]

Agreed, I just put fuel in and drive as cost of ownership is not something I care to find out for my seven . If it was my road car mind and I wasn't getting 52 mpg every week there would be a major public inquiry !

Such is human nature.

[scooby dooby doo]

i wish i could drive the seven to work every morning ❗

I had a job once where it was a 50 mile drive to work - mainly on nice empty A and B roads and it took about an hour and 10. Now I spend 45 minutes on the tube, buses, waiting for the train etc.

 

The former is infinitely more fun ❗

 

HOOPY R706KGU CYCLE WINGS AEROSCREEN K SERIES CUCUMBER

[CHRIS CLARK]

Good explanation Peter C.

So write up one for Low Flying so we get back to some good Tech Spec articles in there!

Please...............

 

A mate has a 1600 Sprint and that does 19-21 mpg at best these days. I'm sure he has a carb problem though as you can remove one of the idle jet/air bleed (had explanation of this over 'phone hence not sure which) screw adjusters on front carb; no. 2 cylinder; and it makes b****r all difference!

 

 

Clamshell Club Founder Member.