I checked the vacuum and the results are interesting. Manifold vacuum at idle is around 17"Hg or about 450mm Hg. That seems to indicate proper operation. The carburetor is a solex 28IBT. The port that would connect to the distributor has 0 vacuum at idle. The vacuum on that port increases with engine speed. That leaves me a bit confused when looking at the vacuum advance curves for the Dauphine. Are the curves based on manifold vacuum and not on the actual port from the carburetor??
Advantages of vacuum advance on a 4cv?
- Thread starter Stan W
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I share your confusion. Those engines sometimes had a factory port on the inlet manifold, which I guess is where you've measured. I've no idea why the carburettor port for the vacuum line to the distributor has no vacuum at idle. I'm thinking others following this thread will know and we'll both learn!!
I did have a vacuum guage in the 4CV when they were fashionable back in the 1970s, measuring at the inlet manifold port, and mostly it showed zero vacuum because the 4CV was usually flat out trying to keep up with the traffic! I probably still have that 2" guage if you want it - easy to post over!
What happens at 8100 RPM. The engine blows up???The centrifugal timing will max out at around 28-30 degrees at about 3000 rpm and remain there until 8000 rpm.These figures are average but you will find more detailed figures in the manual.
The video I posted at #11 explains the difference between ported and manifold vacuum. In my experience ported vacuum became more prevalent with the advent of emission controls.
Not one of Frans' engines......
Point taken and thank you.
@Greenpeace is correct, to get the required CO reading at idle you needed to adjust the mixture screw and retard the ignition, hence the ported vacuum line so no vacuum advance at idle and bob weight springs with no centrifugal advance at idle.
Happily, with 1960s Renaults in Australia, CO readings are a long way from my mind at least.
Here is a link that might explain it better with diagrams. When the throttle is almost closed for idling the port is above the butterfly edge, then as you throttle up the port is gradually exposed to the manifold vacuum till it becomes full manifold -ve pressure. After that the vacuum drops because the butterfly is opening more and letting more air in.
https://www.britishvacuumunit.com/ported-vacuum.html
Jaahn
That was the old common method before the pollution controls came in.
I would not be sure that graph is exactly correct for the vacuum curve ??
and there are other flow effects at the pinch point.
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Getting back to the original query, I would say yes, having manifold (non ported) vacuum advance at idle will (usually) result in a smoother idle and more efficient combustion at a leaner mixture saving fuel (but with higher NOx emmisions). Tradeoff is that if you like to stamp on the throttle at the lights you instantly have no vacuum, advance drops back to the static timing and feels like engine has bogged down and has no power
As @dauphproto writes, his engines like a mechanical 28 Deg BTDC at peak torque (and zero vacuum).
Why is this so you ask? So as to develop its peak cylinder pressure at approx 15 Deg ATDC.
If his spark plugs ignited at 32 Deg BTDC then peak clinder pressure would be at 11 Deg ATDC and knock will occur.
At 24 Deg BTDC then peak clinder pressure would be at 19 Deg ATDC which is wasted Horsepower.
In the same way, if using just mechanical advance then as vacuum increases it takes longer to combust and peak cylinder pressure occurs after 15 Deg ATDC which again is wasted horsepower so vacuum advance is used to get peak pressure to occur earlier. At cruising speeds too much advance can be felt as 'surging'.
Getting back to the original query, I would say yes, having manifold (non ported) vacuum advance at idle will (usually) result in a smoother idle and more efficient combustion at a leaner mixture saving fuel (but with higher NOx emmisions). Tradeoff is that if you like to stamp on the throttle at the lights you instantly have no vacuum, advance drops back to the static timing and feels like engine has bogged down and has no power
As @dauphproto writes, his engines like a mechanical 28 Deg BTDC at peak torque (and zero vacuum).
Why is this so you ask? So as to develop its peak cylinder pressure at approx 15 Deg ATDC.
If his spark plugs ignited at 32 Deg BTDC then peak clinder pressure would be at 11 Deg ATDC and knock will occur.
At 24 Deg BTDC then peak clinder pressure would be at 19 Deg ATDC which is wasted Horsepower.
In the same way, if using just mechanical advance then as vacuum increases it takes longer to combust and peak cylinder pressure occurs after 15 Deg ATDC which again is wasted horsepower so vacuum advance is used to get peak pressure to occur earlier. At cruising speeds too much advance can be felt as 'surging'.
I'm late coming into this discussion, but I'll give my findings over twelve years of ownership and invisible modification of a R1090 Dauphine. For maximum power with standard centrifugal advance curve, I use 6 - 8 degrees of static advance. Vacuum advance is useful in reducing fuel consumption at low throttle openings. However, it needs to be limited because the engine will become very rough when backing off or cruising at high speed. Both SEV and Deucellier distributors can be adjusted to limit centrifugal advance. A 28IBT carburettor can be 'ported and polished' to increase flow without significantly increasing venturi size. I have used 'seat of pants' and 'road dynomometer' methods to come up with my settings, but they seem to work fairly well. My other significant modifications include 60mm liners with domed pistons, port and polish plus matching of manifolds to head, combustion chamber reshaping with the objective of de-shrouding the valves plus all the other normal tweeks.The compression ratio is about 8.6 : 1. Getting back to the original question, I don't think that there is much performance advantage with vacuum advance, but it will probably improve fuel economy.
Well there you go then,This is the full thoretical explanation of the 28 degree rule. The problem with our less than optimum Engines is that the burn time varies with cylinder pressure and mixture, this is why you may be able to run more advance and gain more power and by default, efficiency. Some of the top A series builders in the UK supply their engines with the dizzy locked at 27-28 degrees, so that customer error cannot creep in and cause a meltdown. Not a happy camper at 28 deg on tickover.. esp. on SU'sGetting back to the original query, I would say yes, having manifold (non ported) vacuum advance at idle will (usually) result in a smoother idle and more efficient combustion at a leaner mixture saving fuel (but with higher NOx emmisions). Tradeoff is that if you like to stamp on the throttle at the lights you instantly have no vacuum, advance drops back to the static timing and feels like engine has bogged down and has no power
As @dauphproto writes, his engines like a mechanical 28 Deg BTDC at peak torque (and zero vacuum).
Why is this so you ask? So as to develop its peak cylinder pressure at approx 15 Deg ATDC.
If his spark plugs ignited at 32 Deg BTDC then peak clinder pressure would be at 11 Deg ATDC and knock will occur.
At 24 Deg BTDC then peak clinder pressure would be at 19 Deg ATDC which is wasted Horsepower.
In the same way, if using just mechanical advance then as vacuum increases it takes longer to combust and peak cylinder pressure occurs after 15 Deg ATDC which again is wasted horsepower so vacuum advance is used to get peak pressure to occur earlier. At cruising speeds too much advance can be felt as 'surging'.
Great to hear from someone with experience in modifications that are effective, but simple and inexpensive.
My engine is a 1959 4cv with 58mm Dauphine cylinders 8.8:1 compression (because the head had already been shaved) an autobleu manifold (used on USA export 4cvs and Dauphines 1956-1959) and the Dauphine 28 IBT carb. Valves and cam are standard for 4cvs and Dauphines in 1959. From all the great information here, I see that the ported vacuum advance will not do anything for the idle (unless I can use a bit more static advance). Do all the Dauphine carburetors have similar ported vacuum connections? I always liked the Zenith carburetor used on the 1094 Dauphines back in the 1970s. Any opinions on the various stock carburetors used on Dauphines?
I just re-read my response. When I wrote that centrifugal advance can be adjusted, I meant vacuum advance, which can easily be limited with both makes of dustributor. Centrifugal advance can also be adjusted, but not nearly so easily.
Many of the older British engines had a little thumbwheel on the outside of their distributors to adjust the vacuum advance.
My understanding is that those thumbwheèls on Lucas distributors of the 1960s were at the end of the vacuum advance shaft and they adjusted static timing only.
There's an adjustment on the vacuum capsule on the SEV distributors used on R8s too. Never played with it mind you.
I used to watch my dad twiddling with them doing tune-ups when I was a kid (he was a BMC mechanic at the time) but can't say I've ever felt the urge to play with them either.
I have to admit to fiddling with them once because one vacuum unit didn't seem to have much effect when I sucked on it and that adjustment did get its action similar to what I thought others were like. But I never really knew what I was doing from a technically correct perspective. Just suck it and see really. 