GT6 Mk3 Recommended Timing with Strobe

[Spit1972]

Hello all.

I've replaced my dizzy with an electronic ignition system but need to do the timing on my '73 GT6.

I set the timing to 13 degrees BTDC using a strobe, but its running very slow and barely ticking over.  On various sites I've seen 10 degrees BTDC and also 6 degrees ATDC recommended.

Has anyone any recommendations as to a better angle? 

I also don't fully understand the whole advance/retard requirements so explanation and or advice welcome.

 

David

 

[NonMember]

I think 13BTDC is the Mk1 Vitesse/GT6 timing for 100 RON fuel (5 star). For 97 RON (4 star) it was retarded to 10BTDC. I don't recall the Mk3 GT6 figure offhand but I suspect its 10.

Interestingly, Mk2 Spitfires were 13 and Mk3 were 6, but when my brothers owned one of each we discovered the POs had set them up the other way round, and neither car seemed to object.

I don't think you'll find an ATDC idle setting on a UK car. That would be the US emissions spec with the vacuum retard capsule to slug the idle and force higher air flow.

[dougbgt6]

David,

We've been here before, as NM says, 100 RON fuel, 5*,  is no longer generally available. 10 degrees is a good starting point, but to be honest do it by ear. Turn the dizzy till you have maximum revs then back a gnats. Take it for a run, if it pinks, back a gnats more. I have no idea what my mk3 is set to, but pretty sure it's lass than 10.

Doug

[Mjit]

1 hour ago, Spit1972 said:

I also don't fully understand the whole advance/retard requirements so explanation and or advice welcome.

In theory you'd fire the spark when the pistion hit TDC on the compression stroke, there would be an instant burn and the force of the expanding gas would push the piston down the cylinder, driving the crank.

In reality petrol doesn't burn instantly and as soon as you go past TDC you have the crank pulling the piston down.  Fire the spark plug at TBC and chances are the burning gases will just end up chasing the piston down the cylinder, rather than pushing it.  As a result you need to 'light the fire' while the piston is still coming up on the compression stroke, retarting when the spark plug fires from the TDC to BTDC.  Go too far though and you're lighting the fire too early and the expanding gases will actually reach, and start pushing down on the piston while it's still coming up the cylinder on the compression stroke.

If you're driving up a hill you need to put your foot down, pumping more fuel in to the cylinders and raise the RPM, making the crank and pistons move faster.  This means you both have more fuel to burn and a piston trying to run away faster from the expanding gases so you need to retard the spark, firing it further BTDC so the fuel has time to burn, expand and push the piston back down the cylinder.

When you crest the hill though you just want to cruising down the other side so you take our foot off the gas, reducing the amount of fuel going in to the cylinders.  The engine's still turning at the same RPM though.  With less fuel to burn you don't need to fire the spark plug so early so you can advance the ignition back towards TDC.

 

With a classic distributor you have some weights on springs that get thrown outwards by the spin of the drive shaft and are hooked up to a retarts the ignition more the further they are thrown out (the higher the RPM).  When you close the throttle you're not just limiting the amount of fuel going in but the speed at which air can get in to the cylinder, which creates a vacuume - and this can be linked to a vacume advance unit on the distributor which does the opposite to the interal weights, advancing the ignition.

 

You say you've replaced your dizzy with an electronic system.  What that means will vary with the system.  If it's just a points replacement system then it's still the weights controlling retardation/the vacuum controlling off-throttle advance.  If it's a full electronic system like a 123 dizzy or MegaJolt you have a speed sensor to tell RPM (on the crank for things like MegaJolt/from the dizzy drive shaft for 123) and a vacuum sensor to tell engine load.  There's then a table of RPM/load points, with an advance/retard value for each.  The code checks the RPM and load, reads the advance value and fires the spark plug at that time.

[Waynebaby]

Hi Spit1972,

Here's a link to a video by Moss Motors which explains the principles of vacuum and mechanical advance quite nicely (if you don't mind the references to American football in part 1 of the video!)

 

[Spit1972]

Thanks all. Some great information here. I’ll give it a go.