 |
 | Home | Contact Us | FAQ | Search & Site Map | Link to Us |
 | Sign In | Join | Other 45 Sites in Network |
Car Forum / Volvo Cars / May 2004Vacuum controlled ignition on B20E
On our Volvo 140 '72 (B20E) there is a distributor which has centrifugal weights and a vaucuum system (as usual) to control the ignition timing under the different speed and load conditions of the engine. How and why the centrifugal works is pretty straightforward. When the engine speed is higher the air/fuel mixture must be ignited earlier because the piston is moving faster. But I've never quite understood why the vacuum system works as it does on our B20E. The theory says that when there is a lean mixture (and also low pressure) in the sylinder, the burning prosess takes longer time. This is the case when the throttle is (almost) closed. Vacuum builds up in the intake manifoil and also in the sylinder. With the throttle closed there is a lean mixture of gasoline in the sylinder. The reason why the vacuum capsule was invented is (from what I've read) that for best possible burning, the ignition timing should consider the amount of fuel (and pressure) in the sylinder to "calculate" when the spark on the plug should occur. If the mixture burns slow, the spark should occur earlier and vice versa. The point is always to get maximum pressure when the piston is at top. Since the pressure in the intake manifoil represents the pressure and fuel amount in the sylinder, the ignition is partly controlled by this vacuum. Shouldn't it be so that a high vaccum in the intake manifoil (means low sylinder pressure and low amount of fuel) should give an advanced spark? This is not the case on our volvo. The hose for the vacuum capsule is conected between the throttle and the engine so it "feels" the intake pressure. Higher vacuum retards the ignition in our case. This is wrong? If the throttle then opens (lets say at a low engine speed) then suddenly the ignition would advance (go earlier) which is not right. It should retard (be later) since a richer fuel mixture need less time to burn. Is it suppose to be like this? The Haynes manual also says that "an increase in engine vaccum results in a retarded igntion" (not directly from the book) for the B20E and B20F engines. It also says that "in this case the vacuum capsule is connected to a point between the throttle and the engine". Could it be that the distributor for the car is wrong? Since the car is fuel injected (which is only on the 'E' and 'F' engines, i think), the distributor has an extra connector so the control unit can measure engine speed. If it's the wrong distributor it probably can only come from an 'F' engine. It is a vacuum "retard" system, to retard the ignition at idle only.......helps minimize exhaust emissions. Timing should be set to 10 degrees BTDC with the vacuum hose off. This was a common bandaid solution to vehicle emissions in the early '70s. > On our Volvo 140 '72 (B20E) there is a distributor which has centrifugal > weights and a vaucuum system (as usual) to control the ignition timing [quoted text clipped - 37 lines] > connector so the control unit can measure engine speed. If it's the wrong > distributor it probably can only come from an 'F' engine. > It is a vacuum "retard" system, to retard the ignition at idle > only.......helps minimize exhaust emissions. Timing should be set to 10 > degrees BTDC with the vacuum hose off. This was a common bandaid solution to > vehicle emissions in the early '70s. I've seen this setup before too, but I thought the B20Es all had advance (pretty sure the B20F had the retard system you describe, I know the B21F had it over several years). My B20B ('74) had ported advance. The vacuum line from the distributor went to a nipple on one of the carburettors, right behind the throttle. >> It is a vacuum "retard" system, to retard the ignition at idle >> only.......helps minimize exhaust emissions. Timing should be set to 10 [quoted text clipped - 9 lines] > My B20B ('74) had ported advance. The vacuum line from the distributor > went to a nipple on one of the carburettors, right behind the throttle. This cleared some of my confusion. Seems that vacuum retard has nothing todo with fuel economy. But how can a retarded ignition on idle minimize vehicle emmisions? The distributor also need new centrifugal springs because they're not strong enough anymore. So the basic timing isn't as should (must compromise to not get too early ignition on high RPM), so right now I can't set it to 10 degrees BTDC for normal running. One other thing that is "strange" with the vacuum retard is that when you start the engine the vacuum is obviously zero and that yields for an earlier ignition. This makes the starter go very heavy in the beginning because the engine is pushed backwards by to early ignition. You can also hear a diesel like knocking when starting. Sounds something like when the big american V8 engine stops. This also happens if you force the engine to a low RPM at same time you throttle it, (e.g. starting in 2nd gear). Wish I had the opportunity to go back in 1972 and try a brand new 140 with B20E :) Strange if it should be like i described above originally. But I as I mentioned, I haven't checked that the distributor is correct for the car. The one that is on is called something like "Bosch J-FURX". TWC: Either Advance or Retard the engine timing had nothing related to "exhause emission". To eliminate the "engine knock" is the purpose. Stig Hornang 寫入： > >> It is a vacuum "retard" system, to retard the ignition at idle > >> only.......helps minimize exhaust emissions. Timing should be set to 10 [quoted text clipped - 31 lines] > as I mentioned, I haven't checked that the distributor is correct for the > car. The one that is on is called something like "Bosch J-FURX". > TWC: > Either Advance or Retard the engine timing had nothing related to > "exhause emission". > To eliminate the "engine knock" is the purpose. Well, the following pages says it has todo with fuel economy (most effective combustion, as described in my first posting). http://www.2quicknovas.com/happytiming.html "Its function is to provide extra advance at high vacuum to increase fuel economy." http://www.tpub.com/content/construction/14273/css/14273_67.htm "The vacuum advance increases FUEL ECONOMY" http://www.leeric.lsu.edu/bgbb/7/ecep/auto/g/g.htm "...To understand the relationship between vacuum and ignition timing and reduced fuel economy in computer assisted engines." Most pages I've seen says that the vacuum mechanism exists because it improoves fuel economy. You may be right with your theory, but can you explain how it could avoid knocking? The pages I've referenced only talks about vacuum advance not retard. You say it's not exhaust mission. I still wonder what vacuum retard is good for since it actually does the opposite thing (makes fuel ecenonmy worse?). Set the engine at the optimum BTDC timing ,means just BEFORE the engine knock take place. This Optimun timing vary depends on different engine speed,engine load,and engine temperature. If the optimum timing can be achieved,the car will run fast and smooth. That's will improve fuel economy.. I've to put an extra ignition advanced on my B200E, this make it runs like rocket.. Stig Hornang 寫入： > > TWC: > > Either Advance or Retard the engine timing had nothing related to [quoted text clipped - 22 lines] > for since it actually does the opposite thing (makes fuel ecenonmy > worse?). > Well, the following pages says it has todo with fuel economy (most > effective combustion, as described in my first posting). [quoted text clipped - 17 lines] > for since it actually does the opposite thing (makes fuel ecenonmy > worse?). Retarding the timing any time decreases fuel economy, assuming that the timing is set at the optimum place. What normal vacuum advances is set the timing to the optimum for light loads, to maximize fuel economy. Then the compression ratio is set a little too high (to maximize economy and power) for this timing at full load causing ping. So a normal vacuum advance retards the timing at high load for this reason. A better designed the combustion chamber will promote a faster burn and will require less ignition advance, and will produce more power since there is less pressure inside the combustion chamber approaching the top of the compression stroke. Anything that speeds up combustion will have the same effect, such as installing multiple spark plugs. I did a little reading last night on emission theory. Setting the timing later will lower the peak cylinder pressure and temperature which will reduce the formation of nitrogen oxides (NOx). And quoting directly from my 1971 140 series workshop manual: "The vacuum regulator on the B20A engine raises the firing when the load on the engine reduces. On the B20B and B20E engines the vacuum regulator lowers the firing below the basic setting during idling and engine braking. Reducing the firing is part of exhaust emission control and prevents the engine from emitting excessive, noxious exhaust gases at idling and engine braking."  SignatureMike F. Thornhill (near Toronto), Ont. NOTE: new address!! Replace tt with t (twice!) and remove parentheses to email me directly. (But I check the newsgroup more often than this email address.) >> Well, the following pages says it has todo with fuel economy (most >> effective combustion, as described in my first posting). [quoted text clipped - 39 lines] > is part of exhaust emission control and prevents the engine from > emitting excessive, noxious exhaust gases at idling and engine braking." Thanks a lot! The last part is very interesting and acutally explains why the B20E among others got a vacuum retard system which was my main question from the begining. The first reply to my posting (from normalini) actually was correct. (Thanks normalini, you're right). Engine braking is actually also an important part of the electronic fuel injection system. It makes sense that retarding ignition makes the engine more unefficient. That's what I've always thought, but since the reason has nothing todo with the engine performance, but with exhaust emission preventing, it all makes sense! > >> Well, the following pages says it has todo with fuel economy (most > >> effective combustion, as described in my first posting). [quoted text clipped - 50 lines] > with the engine performance, but with exhaust emission preventing, it all > makes sense! Yes, to sum it all up, in my old 240 with a B21A, there are two vacuum mechanisms: Retard AND Advance The retard hose goes to manifold vacuum and the advance hose goes to ported vacuum. This means that at idle (when manifold has the highest vacuum) ignition is retarded by some 10 degrees keeping the absolute timing at about 0 deg BTDC. At part throttle, the ported vacuum overrides manifold and ignition timing is advanced. One personal experience of my car is from the delay valve on the advance hose being clogged. That meant that the absolute timing never exceeded 15-20 deg BTDC! After removing that little gizmo, ignition advance started working again, and both the fuel consumption dropped and the car responded a lot quicker from idle. That made me a happy(ier) Volvo owner! :-) > On our Volvo 140 '72 (B20E) there is a distributor which has centrifugal > weights and a vaucuum system (as usual) to control the ignition timing [quoted text clipped - 37 lines] > connector so the control unit can measure engine speed. If it's the wrong > distributor it probably can only come from an 'F' engine. I had a '71 142E (B20E) and I noticed the same thing. When I had it, Canada was going through the withdrawal of lead in fuel and a general drop in octane ratings, (premium fuel became lead free first). I noticed the same thing - higher engine load retarded timing. The way the original vacuum control was set up, more vacuum retarded timing. I noticed a dual action vacuum control on a 164 in a junkyard (two vacuum connections, one on each side of the diaphragm). This vacuum control bolted right on to my distributor, and with only the "back" side connected to a different spot on the intake manifold (I believe I "t"ed into the line for the manifold pressure sensor), my fuel economy improved and the engine didn't ping anymore. SignatureMike F. Thornhill (near Toronto), Ont. NOTE: new address!! Replace tt with t (twice!) and remove parentheses to email me directly. (But I check the newsgroup more often than this email address.) >> Shouldn't it be so that a high vaccum in the intake manifoil (means low >> sylinder pressure and low amount of fuel) should give an advanced [quoted text clipped - 10 lines] > drop in octane ratings, (premium fuel became lead free first). I > noticed the same thing - higher engine load retarded timing. Well, the case is that higher engine load advances timing not retards in my case! That means that low engine load retards timing. Remember that high load creates low vacuum, and low loads create high vacuum. > The way the original vacuum control was set up, more vacuum retarded > timing. Yes, same is in my case. > I noticed a dual action vacuum control on a 164 in a junkyard (two > vacuum connections, one on each side of the diaphragm). This vacuum > control bolted right on to my distributor, and with only the "back" side > connected to a different spot on the intake manifold (I believe I "t"ed > into the line for the manifold pressure sensor), my fuel economy > improved and the engine didn't ping anymore. Which side of the diaphragm is the hose connected to? Does high intake manifoil vacuum result in retarded or advanced ignition relative to the ignition point when the hose is disconnected? > Well, the case is that higher engine load advances timing not retards in > my case! That means that low engine load retards timing. Remember that > high load creates low vacuum, and low loads create high vacuum. Oops, there's a good reason to proof read a little better. Higher engine load indeed advanced timing - which made the pinging problem I had much worse. > Which side of the diaphragm is the hose connected to? Does high intake > manifoil vacuum result in retarded or advanced ignition relative to the > ignition point when the hose is disconnected? On the vacuum control from the 164, there are 2 hose connections. The one on the outside, that is the same side that your existing advance is on, I left disconnected. The one on the inside I connected to a "T" in the hose to the manifold pressure sensor. Then I set timing by trial and error - retarded until there was no more pinging. At part load and idle this resulted in more advanced timing. This modification had the side benefit of greatly increasing fuel economy. SignatureMike F. Thornhill (near Toronto), Ont. NOTE: new address!! Replace tt with t (twice!) and remove parentheses to email me directly. (But I check the newsgroup more often than this email address.) > On the vacuum control from the 164, there are 2 hose connections. The > one on the outside, that is the same side that your existing advance is [quoted text clipped - 3 lines] > idle this resulted in more advanced timing. This modification had the > side benefit of greatly increasing fuel economy. Thanks! That makes sense and works like a vacuum advance system as on many other cars where the goal is better fuel economy. Did you try running without either hose connected? It could be an idea for me to just disconnect the vacuum capsule. One other volvo owner with a website did that. The question is why volvo made the vacuum system the way it is originally. Haven't found someone who can explain that. "The question is why volvo made the vacuum system the way it is originally. Haven't found someone who can explain that." read original answer! > "The question is why volvo made the vacuum system the way it is > originally. Haven't found someone who can explain that." > > read original answer! Yes, you've right! Just one other guy said it had nothing todo with reducing exhaust emission. "TWC: Either Advance or Retard the engine timing had nothing related to "exhause emission". To eliminate the "engine knock" is the purpose." > > On the vacuum control from the 164, there are 2 hose connections. The > > one on the outside, that is the same side that your existing advance is [quoted text clipped - 10 lines] > that. The question is why volvo made the vacuum system the way it is > originally. Haven't found someone who can explain that. It's been almost 20 years - I sold my 142E in 1985. But the way I remember, with the high compression of the engine, and the too low octane fuels that were available at the time running with no vacuum advance wouldn't have helped my problem. I went straight to the setup with the 164 mechanism. As I recall the reason for the system being set up the way it is was indeed for emission reasons, as mentioned. I can't explain why it does anything to reduce emissions, but I remember reading that in the manual. SignatureMike F. Thornhill (near Toronto), Ont. NOTE: new address!! Replace tt with t (twice!) and remove parentheses to email me directly. (But I check the newsgroup more often than this email address.) |
Reply to this Message |
 Sign In
 Join
 My Latest Posts My Monitored Threads My Blog My Photo Gallery My Profile My Homepage Start New Thread Enable EMail Alerts Rate this Thread
|
©2008 Advenet LLC Privacy Policy - Terms of Use
This website includes both content owned or controlled by Advenet as well as content owned or controlled by third parties.