Discussion
At 850-900rpm and using a strobe set your timing between 10-12 degrees (it will bounce between the two a bit). Keep your strobe on the the timing marks and raise engine speed, you will see at 3,500rmp timing is all done at 28-30 degrees, you can rev the engine higher but the mark will stay steadfast fixed at 30 degrees at best.
So 30 less 12 shows us the distributor is only capable of adding 18 degrees of mechanical advance, this is why the vacuum advance module on the side of your distributor is so important. Now check your vacuum advance module is actually working by sucking on the pipe to the unit while watching your timing marks, you should be able to add as much as 15 degrees of advance.
On the Rover V8 with a plenum chamber (14CU & 14CUX) the vacuum advance is not connected to full manifold vacuum, it is connected to ported vacuum and this is important. Rover used ported vacuum because (unlike manifold vacuum) it is quite low at idle only becoming strong enough to overcome the spring resistance in the vac advance module at cruise (small throttle opening high engine speed).
Why did they do it this way? well for one reason and one reason only... Emissions!
The truth is the RV8 engine actually wants surprising 25-27 degrees of advance at idle, especially one running the 14CUX which constantly tries to nail idle AFR at an inappropriately lean 14.7:1 (emissions again!), lean mixtures take longer to burn so you need to light the fire lot earlier earlier (more advance).
Running 10-12 degrees of advance at idle creates hotter combustion which helps burn off some unwanted elements in the exhaust gasses and brings the catalytic converters up to operating temperature on later cars typically running the 14CUX. But 12 degrees gives poor economy and a ragged idle especially on those with a lean 14.7:1 idle.
On cruise if you could look at your timing marks you would see your 10-12 of idle advance plus the 18 degrees of mechanical advance from the distributor plus the 15 degrees from the ported vacuum advance, so that's 12 + 18 + 15 = 45 degrees of total timing but only under cruise conditions.
You can make your Rover V8 idle much more smoothly, run cooler, give better throttle response (especially when pulling away from junctions) & deliver better fuel economy simply my moving the vac advance vacuum reference point from ported vacuum to manifold vacuum. Manifold vacuum is a lot stronger at idle than ported vacuum, strong enough to activate the vac advance at idle.
With manifold vacuum at idle you get your 12 degrees idle advance plus around 10-15 from the vac advance module, so you can expect to see 22-25 degrees at idle.
The first thing you'll notice is your nice 850-900rpm is now around 1150-1200rpm so you will need to correct this using the base idle screw on the throttle body, but once you've done this you should see the 22-25 degrees at a 850-900rpm idle using your strobe. The next thing you'll notice is a much smoother idle, cooler running, better throttle response as you pull off idle... and better fuel economy too.
Ported vacuum was only really used in the mid 80's and very early 90's as a crude emissions strategy, before this proper manifold vacuum was typically used, after the 90's everything went to ECU managed ignition systems so distributors and their short lived ported vacuum connected vac advance modules were never seen again. Try switching from the current ported vacuum system which you'll see is right at the butterfly (ie neither before or after it) to full manifold vacuum and you should enjoy the benefits I've described with no risk.
There's no risk of detonation because as soon as the throttle buttery is opened manifold vacuum drops dramatically, dump the throttle open and manifold vacuum disappears completely. So connected to manifold vacuum your vac advance module will only advance your timing at idle (throttle butterfly all but closed) and when you are cruising (throttle butterfly 90-95% closed and engine speed high). Neither are conditions where detonation is a risk, all you get running the addition advance is a smoother idle and better fuel economy when your cruising (typically between 2,500 & 3,000rpm on the motorway or fast A road).
So 30 less 12 shows us the distributor is only capable of adding 18 degrees of mechanical advance, this is why the vacuum advance module on the side of your distributor is so important. Now check your vacuum advance module is actually working by sucking on the pipe to the unit while watching your timing marks, you should be able to add as much as 15 degrees of advance.
On the Rover V8 with a plenum chamber (14CU & 14CUX) the vacuum advance is not connected to full manifold vacuum, it is connected to ported vacuum and this is important. Rover used ported vacuum because (unlike manifold vacuum) it is quite low at idle only becoming strong enough to overcome the spring resistance in the vac advance module at cruise (small throttle opening high engine speed).
Why did they do it this way? well for one reason and one reason only... Emissions!
The truth is the RV8 engine actually wants surprising 25-27 degrees of advance at idle, especially one running the 14CUX which constantly tries to nail idle AFR at an inappropriately lean 14.7:1 (emissions again!), lean mixtures take longer to burn so you need to light the fire lot earlier earlier (more advance).
Running 10-12 degrees of advance at idle creates hotter combustion which helps burn off some unwanted elements in the exhaust gasses and brings the catalytic converters up to operating temperature on later cars typically running the 14CUX. But 12 degrees gives poor economy and a ragged idle especially on those with a lean 14.7:1 idle.
On cruise if you could look at your timing marks you would see your 10-12 of idle advance plus the 18 degrees of mechanical advance from the distributor plus the 15 degrees from the ported vacuum advance, so that's 12 + 18 + 15 = 45 degrees of total timing but only under cruise conditions.
You can make your Rover V8 idle much more smoothly, run cooler, give better throttle response (especially when pulling away from junctions) & deliver better fuel economy simply my moving the vac advance vacuum reference point from ported vacuum to manifold vacuum. Manifold vacuum is a lot stronger at idle than ported vacuum, strong enough to activate the vac advance at idle.
With manifold vacuum at idle you get your 12 degrees idle advance plus around 10-15 from the vac advance module, so you can expect to see 22-25 degrees at idle.
The first thing you'll notice is your nice 850-900rpm is now around 1150-1200rpm so you will need to correct this using the base idle screw on the throttle body, but once you've done this you should see the 22-25 degrees at a 850-900rpm idle using your strobe. The next thing you'll notice is a much smoother idle, cooler running, better throttle response as you pull off idle... and better fuel economy too.
Ported vacuum was only really used in the mid 80's and very early 90's as a crude emissions strategy, before this proper manifold vacuum was typically used, after the 90's everything went to ECU managed ignition systems so distributors and their short lived ported vacuum connected vac advance modules were never seen again. Try switching from the current ported vacuum system which you'll see is right at the butterfly (ie neither before or after it) to full manifold vacuum and you should enjoy the benefits I've described with no risk.
There's no risk of detonation because as soon as the throttle buttery is opened manifold vacuum drops dramatically, dump the throttle open and manifold vacuum disappears completely. So connected to manifold vacuum your vac advance module will only advance your timing at idle (throttle butterfly all but closed) and when you are cruising (throttle butterfly 90-95% closed and engine speed high). Neither are conditions where detonation is a risk, all you get running the addition advance is a smoother idle and better fuel economy when your cruising (typically between 2,500 & 3,000rpm on the motorway or fast A road).
adam quantrill said:
Interesting idea about changing the vacuum reference point, however my manifold runs up to 6psi above atmospheric, with wide open throttle. Might this give an unwanted retard on the timing?
Hi AdamI was concerned as regards to the timing or more so the efficiency of the mechanical advance/Retard when installing the blower...I did initially go for the Amethyst system which turned out to be total pants...However it was reverted back to the mechanical advance/Retard but with the addition of the Saab Turbo vacuum diaphragm fitted to the ford distributor as per power engineering's original fitment of the S102 to the V6 Cologne...
My only view on it was that for a supercharged car you would benefit a lot from having an upgraded ignition system/ECU...Purely because of the limited tolerances for error on a mechanical system and especially for a supercharged car...Everything bad occurs a double the speed...
.Look what Mr Hart has done...I personally thinks thats the way forward for any SX...
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