Injection metering unit timing with large overlap cams.

[2.5piman]

Hello all,

I have always timed the metering unit as per the manual.

I came across an internet poster who suggested that it is better to retard it if a performance cam is fitted such that it injects after closure of the exhaust valve, both from the point of view of economy and power and cut down on unburnt hydrocarbons in the exhaust. He suggests, and I must admit it makes sense, that some of teh fuel will go straight down the exhaust.

I have a 40 80 70 30 cam so timing would have to be at least 70 degrees after TDC,.

Before I go to the trouble of retiming my metering unit and find that it upsets the running, I would like some feed back from those of you running Lucas injection?

Alec

 

 

[2.5piman]

Hello all,

 

an error in my first post, 70 should of course be 30 after TDC.

 

Alec

 

Alec

[Nick Jones]

That makes more sense!

Interesting one. In theory he is correct of course. In practice I’m not sure it makes a huge difference. 

By the sound of it, your cam has similar timing to the one now in my Vitesse. However, my EFI is relatively primitive batch-fire so I have no way of altering the injection timing to experiment.

In practice, my fuel economy at cruise and working it moderately hard is pretty much the same as with the original 308778 cam. It does need to be richer at low rpm/load (sub 2k, very light throttle) to maintain nice manners (improved a bit by going from plenum inlet to ITBs), so is less economical when pottering about in traffic. It also uses rather more at full chat, but then it has about 25 more horses.

Going back to yours, something to maybe bear in mind is whether delaying the start of injection will leave enough time to get the “full shot” in before the inlet valve closes? Though again in practice you probably don’t need to delay until the exhaust valve has actually closed - 15-20 deg would likely do it.

Will be very interested to know if it makes any detectable difference if you do try it!

[2.5piman]

Hello Nick,

Theres approximately 180 degrees of inlet open so I would expect that is sufficient?

I just wondered if anyone had actually run like that.

I plan to take my metering unit off and check it's calibration so I'll give it a try. However if I need to adjust the unit's settings I won't try both together.

Alec

 

 

[JohnD]

I would refer you to the page "NOTES ON INSTALLATION AND CALIBRATION, RACING CARS" on the Lucas Injection website: Notes on Installation and Calibration - Racing Cars (lucasinjection.com)

And in  particular, to the paragraph,"Due to the injection delay period, the required beginning of injection may vary between about 60 deg. and 200 deg. B.T.D.C. on the inlet stroke, according to the type of engine.  A recommended starting point for injection timing investigations would be of the order of 100 deg. B.T.D.C.   Having determined the optimum timing for maximum power, there will usually be a tolerance of about + or - 20 deg. without significant loss of power."   My underline.

Lucas Pi is not like modern, direct injection or injection systems that squirt into the inlet valve.  Lucas injectors are about six inches from the valve!   It is therefore a intermediate stage, a missing link, between continuous injection systems like Jetronic and the moderns.   There is no 'timing' with continuous, and not much with Lucas!

John

[Nick Jones]

5 hours ago, 2.5piman said:

Theres approximately 180 degrees of inlet open so I would expect that is sufficient?

3000 rpm is 50 rev/s or 18,000 degrees a second…… so you have 1/100s available….. or 1/200s at 6000rpm.  For port injection, with its low injection pressures, sequential injection only really works at lower rpm and is mainly to minimise HCs and maximise economy.

For direct injection (petrol or diesel) one of the reasons for very high injection pressures is to be able to get sufficient fuel in within the time window available.

As regards Lucas PI, contrary to John’s remarks above, I would consider it one of the more effective timed port injection systems existing - but limited within the above time constraints and approx 50psi differential across the injector nozzles.

Do you have any HC measuring equipment Alec?

[2.5piman]

Hello John\Nick,

the timing in that article is at odds to that recommended for road cars. The article I mention in my initial post was specifically for Lucas injection.

Is your Vitesse on injection or carburettors? If the former what timing do you use, I know the timing can only be within a band rather than accurate to a degree or so, nor does it need to be.  Interestingly Kas Kastners claims that his experimentaion with moving the injectors further from the intake port gave some power increase.

 

No, I have no such equipment although many years ago an MOT tester remarked on 'high hydrocarbons'.

 

Alec

[JohnD]

My Vitesse uses Pi, Alec.   I 'time' it according the Brown Book, with the spindle orifice showing at the injector port, opening, not closing.    I'm not sure where that is in relation to TDC, or how many degrees of crank  rotation  that window represents, I would guess within the 20 quoted in the 'racing cars' article.   I'll work it out next time I 'time' it.

John

[Nick Jones]

Meant to say before, on PI (as I’m sure you are aware), throttle balance is critical as the fuel is metered on the basis of the average vacuum across the 6 ports so any variation in airflow will cause cylinders to run rich/lean accordingly. This effect is most marked at idle/small throttle openings. The oversized TBs don’t help with this and neither does the very crappy factory linkage, though even with better aftermarket linkages it’s still hard to achieve and maintain perfect balance.

 

[2.5piman]

Hello John\Nick,

Yes, that is what I normally do, I'm happy if I get it so the port is just visible at the top, so it will be BTDC by a small amount.

I'm happy with my home made throtlle linkage and any difference is due to the relative sealing of each disc, but I took care in fitting them. I have infinite adjustment for each pair.

 

Alec

 

 

[hoffman900]

This might be useful:

Really the answer is “just depends”

r