My 4cv has a Dauphine carburetor with a port for vacuum advance. Would a distributor with vacuum advance make the car run "cleaner" (less unburned fuel and less odor) at low power levels? I have the idle mixture set as lean as possible, but I think it could burn more of the fuel than it does. I still have the tiny SEV jr. distributor. The timing is set properly and the car runs well. The engine has Dauphine size cylinders, standard 4cv/Dauphine head, autobleu manifold, standard camshaft and runs 8.8:1 compression. Any advice?
Advantages of vacuum advance on a 4cv?
- Thread starter Stan W
- Start date
Stan, the shortest and most basic explanation I can give you, is that at full throttle there is no vacuum and the car runs on the timing dictated by the centrifugal weights, and the mixture dictated by the main jet and the venturi suction. As you lift your foot the vacuum starts coming back because of more vacuum due to the butterfly closing but still doing the same speed. Now the vacuum is advancing the timing more to burn the new and different air/fuel ratio coming in. So at cruising speed the timing will vary at different degrees, more than what the centrifugal can give you, as per the throttle opening, and may have better combustion and less backfiring if there is any.
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 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.
Hi Stan, what Frans says is correct, but my understanding of it from the good old days is that vacuum advance gives an improvement in fuel economy for part throttle running, for example around the burbs. So my take on that is it must improve the burning or there would be no improvement.
Possibly when the power was low then part throttle running was minimal, but as power increased in later models part throttle running was more common and justified the extra 'complexity and cost'.
The advance at part throttle is needed partly because the combustion is slower and not as vigorous and complete due to the throttling and lesser air/mixture intake reducing the compression pressures and turbulence. It may be possible that you could lean off the part throttle running 'a bit' as the advance would help with the slower leaner burning.
Go for it I say. And report !
Jaahn
Possibly when the power was low then part throttle running was minimal, but as power increased in later models part throttle running was more common and justified the extra 'complexity and cost'.
The advance at part throttle is needed partly because the combustion is slower and not as vigorous and complete due to the throttling and lesser air/mixture intake reducing the compression pressures and turbulence. It may be possible that you could lean off the part throttle running 'a bit' as the advance would help with the slower leaner burning.
Go for it I say. And report !
Jaahn
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Thanks for the explanation - that is how I thought it worked. Since my 4cv has a slightly modified engine and I don't run it very fast, it is almost always run at a low power setting. Any suggestions on which distributor is best? SEV, Ducellier? Are the chances good that the rubber diaphragms still work? I've never had a bad one in any make of car, but they are getting old.
Vac unit primarily for Cruising where the part open throttle means low compression, so you can run more advance than normal without detonation. Worth upgrading if you do a bit of Main road driving as it should improve fuel economy (which is why it was added)
All the points made above are also valid. Never had any bother with Ducel dizzys the vac unit pulls the points to give a wiping (self cleaning effect. SEV bits were rarer on UK stuff
All the points made above are also valid. Never had any bother with Ducel dizzys the vac unit pulls the points to give a wiping (self cleaning effect. SEV bits were rarer on UK stuff
Can you find a Dauphine distributor with a vacuum advance? I know that they are out there but you'd want to be sure it was OK in bushings and centrifugal advance if it were to give an improvement. Stating the obvious I guess. I'm not convinced you'd detect much difference for your car but I'll be really interested to hear how you get on.
I have a new Ducellier large body unit without vacuum advance that I bought about 20 years ago. I ran it in the 4CV for a while and it made precisely zero difference except making #2 plug near impossible to remove. So now I run a reconditioned SEV Junior and a 6V transistorised ignition.
I have a new Ducellier large body unit without vacuum advance that I bought about 20 years ago. I ran it in the 4CV for a while and it made precisely zero difference except making #2 plug near impossible to remove. So now I run a reconditioned SEV Junior and a 6V transistorised ignition.
I am sure I could find a Dauphine distrubutor. I have the advance curves (centrifugal and vacuum) for both the 670-01 (Dauphine) and 670-05 (Gordini, Floride) engines. Can anyone tell me what the vacuum is at idle for a 670-01 engine? It might be a little more than the gordini engine due to the cam. I am trying to figure out if the vacuum advance will be of any benefit at idle (cleaner running). I understand that the primary purpose is better efficiency at higher speeds.
Sorry, don't know Stan. Might be best to measure yours?
Great idea, I should be able to use my refrigerant gauges.
The Carb is going to be the limiting factor here. Most production vehicles have issues with Carb quality. This never got sorted as Manufacturers never wanted to spend money on Good stuff. Think Stromberg, VV, Pierburg, the list of Dodgy Carbs is practically endless. I wonder if there is anything good quality that could be adapted. Thinking single choke Weber here. Something that can have it's jetting altered.
This is an interesting video. He's only a young guy and the production comes across as a bit "backyard", but he thoroughly explains and demonstrates vacuum advance on ported and manifold sourced applications. I guess the thing you would need to do is find out your manifold vacuum at idle and then check at what reading the canister on the proposed distributor begins working at before you do anything else? I've played with both styles of distributor on VW engines and found no tangible difference. The first thing you normally do is set the static advance on a mechanical distributor to pretty much the same timing as the vacuum type has with the canister connected to its manifold vacuum source anyway.
PS. I found the product at the end of the of the video fascinating. It's an electronic distributor that you can tailor the entire advance curve in (in fractional increments) off your phone. No weights, no vacuum canisters, ahhh, modern technology.
PS. I found the product at the end of the of the video fascinating. It's an electronic distributor that you can tailor the entire advance curve in (in fractional increments) off your phone. No weights, no vacuum canisters, ahhh, modern technology.
I've a fully electronic distributor (a 123 TUNE unit) on the R8, including a vacuum sensor. Wonderful. No discernable difference driving, but hugely improved starting from cold (not that it was ever bad). Nothing mechanical and no points etc. Marvellous. They work on 6V too so earlier cars can use them too.This is an interesting video. He's only a young guy and the production comes across as a bit "backyard", but he thoroughly explains and demonstrates vacuum advance on ported and manifold sourced applications. I guess the thing you would need to do is find out your manifold vacuum at idle and then check at what reading the canister on the proposed distributor begins working at before you do anything else? I've played with both styles of distributor on VW engines and found no tangible difference. The first thing you normally do is set the static advance on a mechanical distributor to pretty much the same timing as the vacuum type has with the canister connected to its manifold vacuum source anyway.
PS. I found the product at the end of the of the video fascinating. It's an electronic distributor that you can tailor the entire advance curve in (in fractional increments) off your phone. No weights, no vacuum canisters, ahhh, modern technology.
There'll be significant vacuum at idle, so the advance might come off as the throttle is opened I'd have thought, as the centrifugal advance kicks in. Need to measure the actual car and look at vacuum and rev changes compared with the observed actual advance with a strobe light.
There's a bit of room to move with the Solexes fitted to our Renaults, in the main jet and emulsion tube areas. I've no idea how much good can be done though.
I noticed the vacuum canister was ruptured on the points distributor in the Datsun engined Austin I just bought so I plugged the line. It works on ported vacuum so does nothing at idle. I can buy a new electronic distributor for $69. But the car starts easy, runs great, performs great and is fantastic on fuel. So I'm trying to find a reason why I need to replace the old one.
That $69 is surely something like a Hot Spark kit for the existing distributor? I run my 4CV with a reconditioned points distributor and a transistorised ignition so the points switch precious little current.I noticed the vacuum canister was ruptured on the points distributor in the Datsun engined Austin I just bought so I plugged the line. It works on ported vacuum so does nothing at idle. I can buy a new electronic distributor for $69. But the car starts easy, runs great, performs great and is fantastic on fuel. So I'm trying to find a reason why I need to replace the old one.
Sorry for unnecessary extra post. Stan, my point about measuring your car now is so that you have the current baseline data to compare later if you fit a vacuum advance distributor, so that you know what has actually changed regarding dynamic ignition timing.
No the whole unit. I bought a VW one several years ago off the same vendor. My brother still has the car and it hasn't missed a beat. The billet HEI distributor in my 351 (they have the ignition coil built into the cap) was $99 and has provided flawless service for 5 years now.
Wow. And WOW!
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You have to be careful where the Vac take off is placed, some Vehicles have it on the manifold, but most Renault's I have encountered have it on the Carburettor where the signal is slightly different and weaker than on the actual manifold. Any engine will make most power and by default best efficiency at the correct point.
This is why I always give my Engines a run on the Dyno to establish what is going on before fitting. In theory an Engine should run 28 deg max advance at full cry. This is all well and good, but if the engine is not totally up to scratch on all fronts eg. cylinder filling, compression, it will produce more power with more advance.
If you check any Mega money Motor you will find it around 28 deg Max, which proves the Builder knows his stuff.
Three examples I can give you on the same type motor
My first 1550 built by Salv which he spent a lot of time on Ran peak power on 31 Deg advance, but was backed off to 29 for a little safety margin. Almost what theory dictates, a Hot engine.
My own 1700 built by Me which peaked at 33 deg, but is set at 31 again for the sake of the head gasket. This tells me I must do better!!
Finally the little 1400 currently in the Dauph which is the most interesting example, as it has a few shortcomings esp. in the compression department, so it actually runs 36 degrees Max. If you set it late then it lacks go, if you have it too advanced it wont rev.
All my Hot Renaults actually had great fuel economy on the road giving over 35mpg on Freeway type driving
The point I am trying to make badly, is that each engine is individual and the manufacturers figures are more for reference than some sort of absolute best place.
I always set the timing the same way. Get it properly warmed up, if you have vac, then disconnect it and plug off the source. Then I dial in the amount of full advance I require on the light (a function well worth paying extra for) then in the case of 5G type motors take the engine up to 5000 rpm and check the marks align. Full mech advance on these is pretty high up compared to other engines. What ever the timing is doing at idle is whatever it is doing. It's much more important to have the upper end set correct than the bottom.
A good rolling road session is invaluable to find out what you may be missing, but the problem nowadays is finding someone who knows how to meddle with something 40+ years old on a Carb or Carbs. One of my retired friends now has a little sideline with our local rolling road where he goes in to play with the old stuff on Carbs or dial in new side draught installations for them.
Makes you feel old!!!
This is why I always give my Engines a run on the Dyno to establish what is going on before fitting. In theory an Engine should run 28 deg max advance at full cry. This is all well and good, but if the engine is not totally up to scratch on all fronts eg. cylinder filling, compression, it will produce more power with more advance.
If you check any Mega money Motor you will find it around 28 deg Max, which proves the Builder knows his stuff.
Three examples I can give you on the same type motor
My first 1550 built by Salv which he spent a lot of time on Ran peak power on 31 Deg advance, but was backed off to 29 for a little safety margin. Almost what theory dictates, a Hot engine.
My own 1700 built by Me which peaked at 33 deg, but is set at 31 again for the sake of the head gasket. This tells me I must do better!!
Finally the little 1400 currently in the Dauph which is the most interesting example, as it has a few shortcomings esp. in the compression department, so it actually runs 36 degrees Max. If you set it late then it lacks go, if you have it too advanced it wont rev.
All my Hot Renaults actually had great fuel economy on the road giving over 35mpg on Freeway type driving
The point I am trying to make badly, is that each engine is individual and the manufacturers figures are more for reference than some sort of absolute best place.
I always set the timing the same way. Get it properly warmed up, if you have vac, then disconnect it and plug off the source. Then I dial in the amount of full advance I require on the light (a function well worth paying extra for) then in the case of 5G type motors take the engine up to 5000 rpm and check the marks align. Full mech advance on these is pretty high up compared to other engines. What ever the timing is doing at idle is whatever it is doing. It's much more important to have the upper end set correct than the bottom.
A good rolling road session is invaluable to find out what you may be missing, but the problem nowadays is finding someone who knows how to meddle with something 40+ years old on a Carb or Carbs. One of my retired friends now has a little sideline with our local rolling road where he goes in to play with the old stuff on Carbs or dial in new side draught installations for them.
Makes you feel old!!!
I took a vehicle I owned to a performance orientated dyno shop about 15 years ago. It had a worked 400 CID small block Chevy in it. The engine had previously made 445 BHP naturally aspirated with a 750 vac secondary Holley. I picked up a Powerdyne centrifugal supercharger for a good price so I fabricated some brackets and a drive pulley, made a carb hat, and set it up blowing through a 850 double pumper Holley. The engine would obliterate the tyres at low rpms in any gear, but as soon as it hit 3K it would fall on its face. I couldn't find anything clearly wrong (it wasn't my first boosted rodeo) so I was forced to let the moths out of my wallet and engage a "professional". The dyno guy assured me he had lots of experience with blow through Holleys, and indeed the showcase in the foyer had a range of blow through carby hats for sale. First pull it made 240 odd HP at 2500 rpm, flatlined to 3000 rpm then fell off a cliff. AF ratio was in the high 11s until 3K then it plummeted to 5 to 1 (at this point the exhaust pipes just poured out thick black smoke) and the engine wouldn't rev any further. (I couldn't see the black fuel smoke when I was road testing it because of the tyre smoke
). Over the next 4 hours, multiple times old matey would say "I know what it is" and proceed to make changes. Jetting, power valves, spark plugs, ignition timing etc, and with each change it lost power.
At the 4 hour mark (after a 140 BHP pull) I decided enough was enough. I asked the guy to take it off the dyno and said I would go home and put my tune back in it and do some research. He wasn't keen, and said "I'm pretty sure I know what......", I cut him off and said "I want to take it home, If you lose any more power it won't get up my driveway".
Anyhoo he said he'd do me a deal, and "only" charge me $100 an hour, seeing how he'd made it run worse.
I got home, put it back how I had it, (which as far as I was concerned should have been a close tune) and jumped on the net. I came across a Turbo Mustangs forum in the States and most of these guys' turbos were blowing through a carby. I found a thread where the guy had pretty much the same problem as me. He had a side entry carby hat too and worked out that the boost air blowing across the top of the float bowl vents was actually siphoning fuel out of the float bowls once the air speed was high enough. He cured his setup by pushing rubber hoses over the vents and running them down into.the boost pipe facing the incoming air. I did the same mod, pulled out the gate, nailed the throttle, and the car instantly stalled and wouldn't start again. I must of had a bit more boost than the internet guy as the incoming air had instantly forced all the fuel out of both of the float bowls via the main jets, and drowned the engine.
I pulled the hoses off, fabricated a couple of little brass elbows and fitted them with restrictors (a couple of old jets) and mounted them to the vents tube facing the boost pipe. Hey presto, problem solved, the engine would now cleanly hit the redline in the blink of an eye.
As all of my subsequent builds have also run carburettors, that was also the last time I've taken a vehicle to be dyno tested. I'm not in the habit of spending good money to go slower.
As daughproto said, make sure the dyno guy knows what a carby is before you let him anywhere near it.
). Over the next 4 hours, multiple times old matey would say "I know what it is" and proceed to make changes. Jetting, power valves, spark plugs, ignition timing etc, and with each change it lost power.At the 4 hour mark (after a 140 BHP pull) I decided enough was enough. I asked the guy to take it off the dyno and said I would go home and put my tune back in it and do some research. He wasn't keen, and said "I'm pretty sure I know what......", I cut him off and said "I want to take it home, If you lose any more power it won't get up my driveway".
Anyhoo he said he'd do me a deal, and "only" charge me $100 an hour, seeing how he'd made it run worse.
I got home, put it back how I had it, (which as far as I was concerned should have been a close tune) and jumped on the net. I came across a Turbo Mustangs forum in the States and most of these guys' turbos were blowing through a carby. I found a thread where the guy had pretty much the same problem as me. He had a side entry carby hat too and worked out that the boost air blowing across the top of the float bowl vents was actually siphoning fuel out of the float bowls once the air speed was high enough. He cured his setup by pushing rubber hoses over the vents and running them down into.the boost pipe facing the incoming air. I did the same mod, pulled out the gate, nailed the throttle, and the car instantly stalled and wouldn't start again. I must of had a bit more boost than the internet guy as the incoming air had instantly forced all the fuel out of both of the float bowls via the main jets, and drowned the engine.
I pulled the hoses off, fabricated a couple of little brass elbows and fitted them with restrictors (a couple of old jets) and mounted them to the vents tube facing the boost pipe. Hey presto, problem solved, the engine would now cleanly hit the redline in the blink of an eye.
As all of my subsequent builds have also run carburettors, that was also the last time I've taken a vehicle to be dyno tested. I'm not in the habit of spending good money to go slower.
As daughproto said, make sure the dyno guy knows what a carby is before you let him anywhere near it.
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