Vacuum advance on a PI car

[Graze]

Can someone explain to me why the PI TR6 didn't have / need a vacuum advance?

 

Or is it just my car that doesn't have it?

 

Thinking it would help increase the advance at high revs?

 

Standard car, electronic ignition

 

Graze

Edited July 26, 2014 by Graze

[RobinTR6]

It has a mechanical advance, dizzy base plate moves to advance timing. Not sure why tho.

[jean]

As far as I know you can act on the advance curve by changing the springs in the dizzy.They act against the bob weights, stronger or weeker springs will change the timing curve.

123 ignition dizzy does it electronically

[peejay4A]

Peter Cobbold will no doubt pick this up but vacuum advance is there to provide extra advance at cruise speeds when vacuum is at it's highest. Something to do with the speed of the flame front in the cylinder. I imagine the characteristics of the PI system preclude the use of vacuum advance. A PI engine seems to produce less vacuum than a carbureted equivalent for some reason.

Edited July 26, 2014 by peejay4A

[TR NIALL]

Is it not Controlled by the Metering Unit.

Edited July 26, 2014 by TR NIALL

[peejay4A]

Not sure what you mean Niall. The MU fuelling is driven by manifold depression but the MU doesn't affect ignition timing.

[Andy Moltu]

Peter Cobbold will no doubt pick this up but vacuum advance is there to provide extra advance at cruise speeds when vacuum is at it's highest. Something to do with the speed of the flame front in the cylinder. I imagine the characteristics of the PI system preclude the use of vacuum advance. A PI engine seems to produce less vacuum than a carbureted equivalent for some reason.

 

The vacuum is largely a reflection of the cam characteristic rather than the induction system. As I recall the CR camshaft was also used in carburettor cars.

The early PI cam as found in the CP cars does produce less vacuum than the later ones so may have been a factor in in not using the vacuum advance but it may also be because it ran better without it. It would be interesting to know what influenced Triumph at the time.The CP dizzy has got a vacuum advance unit on it, although it isn't used, although I've never sucked on it to see if it could function.

[Peter Cobbold]

Pete is right, The MU is involved in this, so is spark timing.

 

Like all classic engines the spark should be advanced to around 45BTDC at cruise. Why? At cruise the flame takes longer to cross the combustion chamber. Thats because the partial cylinder filling reduces compression and heating and the mixture is usually leaned out.

But the PI engines do not have the vac advance capsule connected up.So the max advance they give is due to the centrifugal swing only, about 30BTDC at crusie rpm. Thats not enough advacne to give best cruise economy: its worth maybe 10% on mpg. So why is that?

The reason is in the PI - it has no faciltiy for acceleration enrichment. There's no facility resembing the throttle pump on Webers or the piston damper on SUs.

That means the mixture from the PI runs very lean for perhaps one or two seconds after suddenly flooring the throttlle. That 'lean spike' burns abnormally: it 'knocks'. That knock is heard as pinking. If the spark is set advanced that pinking would get worse, briefly. The cure for knock is spark retard , too much advance makes it worse,

Since the lean spike cannot be eliminated by enrichening the fuelling transiently the PI cruise mixture is set rich to try to minimise the pinking. This also kills mpg.

 

The reason for the lean spike is due to liquid fuel pooling on manifold floor, explained here, along with data showing the lean spike from the PI:

http://supertrarged.wordpress.com/2013/06/12/the-lucas-pi-lean-spike/

 

The cure to pinking woudl be transient enrichment - tricky to do. Or it could be transient retard of the spark. Once pinking has been solved the cruise spark can be advanced by connecting the v-a capsule**, and the mixture leaned, with improvment in mpg.

More lean spike data recorded by Tim D and some ideas here: http://supertrarged.wordpress.com/2014/05/26/a-possible-cure-for-the-pi-lean-spike-and-pinking/

 

 

Peter

 

** to a butterfly edge take-off whose positioning is critical, and has yet to be determined.

Edited July 26, 2014 by Peter Cobbold

[TR NIALL]

Peter has explained what I was trying to Say but Peter explains it so much better than Me.

[John L]

Is there any benefit in increasing the mechanical advance curve in the distributor, buy taking back the limit stop? The one fitted only allows 10 degrees of distributor mechanical advance. So that should be 20 degrees + the 12 degrees BTDC = 32 degrees all in.

John

[Peter Cobbold]

Is there any benefit in increasing the mechanical advance curve in the distributor, buy taking back the limit stop? The one fitted only allows 10 degrees of distributor mechanical advance. So that should be 20 degrees + the 12 degrees BTDC = 32 degrees all in.

John

John,

No. That curve giving 32BTDCis fine for operation at wide open throttle. If the swing were opened up to 45BTDC - to make it ideal for cruise- the spark is then much too advanced for full load and it would rattle its pistons ( detonate).

Theres no way of adjusting the centrigugal swing on the fly according to engine load.

So the vacuum advance daiphargm pulls the points clockwise in the disy, adding more advance to that on the centrifugal swing.

Except we cant with the PI...

Peter

 

The reason for all these spark timing changes is entirely to do with how fast the mixture burns - at different loads and different rpm. Loads affect cylinder filling and hence pressure and temperature and rate of combustion ignoring turbulence. RPM strogly affect turbulence and that affects how fast the flame spreads across the combustion chamber.

I could go on.... but its hot. Some other time, if anyone wants the long version. Its worth getting to grips with combustion 'cause its common to all petrol engines.

Edited July 27, 2014 by Peter Cobbold

[Peter Cobbold]

Peter has explained what I was trying to Say but Peter explains it so much better than Me.

Niall,

Thank you. Its nice to know my efforts are read !!

Peter

[Graze]

Read and appreciated Peter

 

Graze

[RobinTR6]

Yep read and appreciated Peter, thanks.

[hot-growler]

hats off to Peter, he has always put a reply on my threads with great knowledge and encouragement, a credit to the tr-register.

thank you

[Peter Cobbold]

Thanks guys for the thanks. I enjoy trying to understand the whys and wherefores of engines, as well as the dos and donts. And assume its useful to others.

 

Cant always be right so, as always, dont always believe everything you read on the web !!

 

Peter

[pgee]

Hi, I installed a Powerspark electronic ignition, the one that replaces points. Problem is when pulling up the car stutters, so at higher revs. At low revs it runs fine, even better than with points.

Looks like the advance at higher revs go's wrong somehow.

Put back in the points and the car runs fine at higher revs so the mechanical advance from the distributor is working.

Any ideas?

Peter

[John L]

I fitted an Accuspark unit, and now the firing seems to be at the limit of the rotor arm, its burned on the heel, not in the mid position, the base must be advancing, but it would seem that the accuspark unit cannot be fitted to the base plate at the same place as the points, or the magnets in the pickup are not in the right place, compared to the cam underneath. So thinking about it the magnets must be between the cam points as there isn't enough room at the cam point, am I right in thinking this?

I have tried to mount the accuspark in a slightly different position on the base plate but I still haven't got it right yet.

Is there some reason why the spark is happening so late on the heel of the rotor arm? As its on the heel of the rotor arm, there is a bigger gap to the cap connect points, and perhaps this is what is giving me the occasional miss fire?

John

[BlueTR3A-5EKT]

Hi, I installed a Powerspark electronic ignition, the one that replaces points. Problem is when pulling up the car stutters, so at higher revs. At low revs it runs fine, even better than with points.

Looks like the advance at higher revs go's wrong somehow.

Put back in the points and the car runs fine at higher revs so the mechanical advance from the distributor is working.

Any ideas?

Peter

[BlueTR3A-5EKT]

If you had vacuum advance working ......Screw that attaches the electronic gizmo may be too long and touching the lower part of the base plate. Thus stopping or restricting the rotation of the baseplate and its controlled vacuum advance.

 

Peter W

[pgee]

Thanks, checked it, but no.

Peter

[Mike C]

I'd put a timing light on the engine and check that the distributor was smoothly advancing/retarding as you rev the motor.

[TriumphV8]

We frequently changed PI ignition to a full mapable timing.

The reason is that MegaSquirt can be adapted easier

if you first hook up, than connect ignition and at last add fuel.

 

We did not notice pinking on part throttle advance with the PI system.

Also other carb system did not react with fuel pinking.

The reason is simply that within milliseconds the timing is at the new

point at the table and that has less advance.

 

So if you drive at 2000 rpm with part throttle and 34 degrees advance

and press the pedal there will be the same timing of 21 degrees the

conventional ignition would offer just after a few milliseconds and before

something starts pinking.

You immediately will have the same timing as with PI distributor!

 

123tune did something very worse and that might be the reason for the

noticed fuel pinking if part throttle is engaged:

They gave a heavy delay to manifold pressure timing,

what is fully separated from rpm timing at 123tune.

 

The reason is that they wanted not to run into trouble with

pulsations from a single cylinder what might happen if you grab

the signal from the brake booster hose or a single cylinder.

 

When you push the pedal you see the advance moving slowly

back to only rpm advance. This can last a second or longer.

That will create fuel pinking and must be compensated by

a very small amount of part throttle advance.

 

We gave a maximum part throttle advance between 10 and 15 degrees

and the engine feels much better and fuel comsumption drops.

To be honest the reported EFI fuel consumption values would not

be possible without part throttle advance.

[pgee]

The TR6 PI has a mechanical advance without vacuum. The mechanical advance stops (standard) at about 2500 rpm, so I read. My TR6 works properly with the normal points, also at high revs. So, it should work.

 

The electronic ignition I got is from Powerspark https://blog.simonbbc.com/faq-helper-page/quick-look-at-the-powerspark-ignition-kit-k1-lucas-22d6-25d6-6-cylinder/

I cannot do any different setting on this one. Just put it in and it should work, so they say.

 

Could it be the HT leads?? I have original copper ones and is it possible there's a (magnetic) interference with the electronic module?

 

Edited June 24, 2018 by pgee

[Steves_TR6]

Various people, including me, have used the simonbbc ignition with success, bit if your car runs fine on points but not with the simonbbc unit then :

 

- it could be faulty

- it might not be installed quite right, i had an issues with earthing inside the distrib causing intermittent rough running

- the black hexagonal ring that installs on the cam could be wonky/cracked etc, i had a cracked one

 

I’d connect up a timing strobe to check the advance is working correctly

 

Steve