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Old 09-21-2020, 11:42 AM   #1
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Default OHV distributor differences, mechanical vs vacuum advance

I am still learning my way around the B18/20 motors and have been wondering, what are the pros and cons of vacuum advance and mechanical advance on these Bosch distributors?

Currently my car is set up with a 0 231 153 003 JFR4.
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Old 09-21-2020, 11:50 AM   #2
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To break it down simply:

Mechanical advance gives you a little bit smoother performance at high RPM (so might be alright on a race engine, but I'm very doubtful you'd find this particularly useful on the street)

Vacuum advance is supposed to give you better fuel economy at lower RPMs, but as you rev up it'll retard the timing a little bit for some extra power when you need it.
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Old 09-21-2020, 12:12 PM   #3
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It is not a Pro / Con question.

The time to complete combustion to achieve peak cylinder pressure 'about 20 deg' ATDC is 'relatively constant'. As engine speed increases you need to start combustion at earlier crank angles to complete combustion in time to achieve peak pressure at the desired crank angle. No getting around that requirement to advance timing as engine speed increases. That is what the mechanical advance mechanism tries to do.

The function of the vacuum advance is different. The ignition timing at wide open throttle is pretty much set by the mechanical advance curve. At smaller throttle openings (when there is a vacuum in the manifold) the fuel mixture density in the combustion chamber is reduced and combustion takes a little longer. At less than wide open throttle the vacuum advance mechanism starts the combustion at an earlier crank angle to try and account for the fact that the lower density fuel mixture takes a little longer to burn. The vacuum advance mechanism does not affect peak horsepower production; but, may improve fuel economy and improve drive ability (when done correctly).

Too much vacuum advance may contribute to detonation if the curve is incorrect. It has been my observation that on my B20E the part of the engine operating map that seems to be the most likely to incur detonation first is the zone around 2700 - 3300 RPM and 65 - 85 kPa (absolute MAP). I have been able to mostly tune out detonation by reducing advance in that region of the engine map (I am running MSExtra with spark control) and leaving the wide open curve untouched. That is much harder to do with a mechanical / vacuum advance system.

Volvo has used just mechanical advance, mechanical advance with vacuum advance and mechanical advance with vacuum retard on its B18 and B20 engines. The mechanical advance with vacuum retard was used on the B20E (and F ??) engines; but was a ported vacuum signal so not quite a straight manifold vacuum signal. On the B20E the vacuum advance was an emission control feature, not a performance enhancing feature.
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Old 09-21-2020, 11:52 PM   #4
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Quote:
Originally Posted by spock345 View Post
wondering, what are the pros and cons.
There are timing charts in one of the older Volvo manuals. Google: TP30432-2 file:pdf

Overview on this topic here

RE: vacuum advance and mechanical advance

For years, vacuum/mechanical was integrated into distributors. Mechanical side used springs, which are used for timing advance, and vacuum pot for vacuum side. Both must be working. Springs and pots were specific to a given engine...springs are not springs, each spring set offered (different tension) could affect timing curve.

At minimum, a timing light and RPM gauge/meter is required...along with specs for a given engine.

Here is a company that offers an electronic distributor.
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Old 09-22-2020, 12:15 AM   #5
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Originally Posted by 142 guy View Post
...mechanical advance with vacuum retard on its B18 and B20 engines.
IIRC, sometime in 1970s came vacuum retard on different engines...VW...etc.


Nice overview, btw.
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Old 09-22-2020, 12:20 AM   #6
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Originally Posted by spock345 View Post
Currently my car is set up with a 0 231 153 003 JFR4.
If using a carb setup, I would suggest reading this info
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Old 09-22-2020, 08:32 AM   #7
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Originally Posted by 84B23F View Post
IIRC, sometime in 1970s came vacuum retard on different engines...VW...etc.


Nice overview, btw.
AFAIK that was purely for emissions. The testing in the early 70's wasn't very realistic, it was all done on an idling engine, they weren't trying to test them at higher outputs. So the vac retard was there just to delay ignition at idle. This reduced NOx production without needing to reduce compression ratio (as much). Of course, as soon as you put your foot down, it would make as much NOx as ever. But since that wasn't part of the test...
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Old 09-22-2020, 12:42 PM   #8
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AFAIK that was purely for emissions. The testing in the early 70's wasn't very realistic, it was all done on an idling engine, they weren't trying to test them at higher outputs. So the vac retard was there just to delay ignition at idle. This reduced NOx production without needing to reduce compression ratio (as much). Of course, as soon as you put your foot down, it would make as much NOx as ever. But since that wasn't part of the test...
Indeed. When I deleted my B20E's original distributor and went to a more traditional ignition map using MSExtra I noted that the fragrance of the exhaust became decidedly acrid at idle. I have thought about retarding the timing just in the idle cells to see if that reduces the eye watering effect of the exhaust.
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Old 09-22-2020, 01:49 PM   #9
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without needing to reduce compression ratio (as much).
IIRC, 1975-244's engine was around 8:5/1 compression ratio...which was very low when compared to 1960s 'muscle' cars.
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Old 09-22-2020, 07:02 PM   #10
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IIRC, 1975-244's engine was around 8:5/1 compression ratio...which was very low when compared to 1960s 'muscle' cars.
The 1971 B20A had an 8.7:1 compression ratio. The 1971 B20B had a 9.3:1 compression ratio and the 1970 and 1971 B20E had 10.5:1 compression ratios. The 1972 B20F had an 8.7:1 compression ratio. The B20A had vacuum advance, a total of 10 deg max at the crank. The B20B and B20E both had vacuum retard (negative control). The B20B had a max of -6 deg at the crank and the B20E had a maximum of -10 deg at the crank. I believe the 1972 B20F had the same distributor arrangement as the B20E even though it had a lower compression ratio.

Direct comparisons between the ignition systems is difficult. The B20E mechanical advance comes on much faster than the B20 A or B. The E achieves full advance (25 deg) at about 3000 RPM whereas the B20B achieves full advance (24.5 deg) at 5100 RPM. Depending on where the B20B takes its vacuum signal from that may explain the difference in the advance requirements - just a guess. The B20A is really different because you set the basic timing at 1500 RPM rather than around 800 RPM like on the B20B, E and F.

My recollection is that some of the B18s just ran on pure mechanical advance.
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Old 09-22-2020, 07:38 PM   #11
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Thanks for all the information. I am still trying to get a grasp on older advance systems and this has helped tremendously.

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My recollection is that some of the B18s just ran on pure mechanical advance.
At least some just operated on mechanical advance. As far as I can tell some B18D and B18B motors got just mechanical advance. Nothing seems to be consistent though. My car came to me with a Bosch 0 231 153 003, only mechanical advance.
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Old 09-22-2020, 07:41 PM   #12
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Any impressions I ever have about what Volvo used back then always turn out to be based on really small sample sizes.

I.e. my 1963 122 I used to have, the 1963 PV I have now (which has very little original motor stuff anyhow). A couple of 1800E's. A couple of 145s.
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Old 09-22-2020, 10:29 PM   #13
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Quote:
Originally Posted by 142 guy View Post
10.5:1 compression ratios
Some 1960s B18s required "ethyl" (premium)

IPD sells a head gasket for converting a high compression to a Low Compression - B18 B20

RE: pure mechanical advance.

Dual Point Distributors I recall, but a vacuum advance is needed at low throttle settings for best performance. 1939-1962 GMC Pickup Trucks had Distributors with Vacuum Advance. Any engine running a constant RPM would not need a vacuum advance
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Old 09-22-2020, 11:47 PM   #14
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Originally Posted by spock345 View Post
At least some just operated on mechanical advance.
I don't know...but based upon other automotive engines say since 1950s, a mechanical/vacuum advance would have been common.

Now, in marine applications, where constant RPMs are expected, inboard boat engines may have used mechanical advance only. Engine load is dependent upon propeller design and RPM...so the springs used for mechanical advance would reflect propeller/RPM dynamics.

In 1969, Volvo car engines were using a mechanical/vacuum advance...PERIOD.
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Old 09-23-2020, 09:08 AM   #15
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Originally Posted by spock345 View Post
Currently my car is set up with a 0 231 153 003 JFR4.
Bosch Classic site suggests

0 231 153 003 was replaced by 0 231 170 085
=============

Skandix site suggests

Distributor, Ignition 0 231 153 003 Bosch JFR4
Volvo 120 130 220, P1800, PV
applicable for:
Model
Volvo 120, 130, 220 restricted on: model years, engine type
Volvo P1800 restricted on: model years, engine type
Volvo PV544 restricted on: model years, engine type
=================

My guess was this distributor was for "high performance" engines...pedal to the metal applications...myself, I'd replace it with a vacuum/mechanical based distributor if this one failed. Those timing charts I cited in a Volvo manual gives a good idea where timing advance should be. I don't have time to review them, but I'd look them over. Chances are, your engine is not upto OEMs specs today. Somebody may have installed your distributor years ago for some reason. Matching OEM timing specs for a given engine is what is relevant.
==========================

On another note, anyone using a point/condensor setup should consider pitching them, and consider these two eBay ads HERE and HERE

I have not researched this topic, but if I owned an engine with points, I would pitch them for an electrical device like these. One seller says, 12 Volt only, and another ad says, with right-hand points only, so do your homework.
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Old 09-23-2020, 09:28 AM   #16
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Bosch does have a Bosch to Volvo part number conversion HERE

Problem here is knowing what was Volvo's part number was for your Volvo.
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Old 09-23-2020, 10:26 AM   #17
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Originally Posted by spock345 View Post
Thanks for all the information. I am still trying to get a grasp on older advance systems and this has helped tremendously.



At least some just operated on mechanical advance. As far as I can tell some B18D and B18B motors got just mechanical advance. Nothing seems to be consistent though. My car came to me with a Bosch 0 231 153 003, only mechanical advance.
My 67/68 b18D was mechanical only, and I believe it was the same "003" distributor. They work just fine for stock applications. I would have no concerns running it on a stock engine or mildly warmed over engine if you get rid of the points and go to something like a pertronix or crane ignition module, unless you are keen on points and want to take the time to deal with points gap, dwell, and whatnot. The only thing to really think about is max advance. I noticed that with the mech only system, the engine made better power and fuel economy if I set the timing at 3k rpm to something like 32-35 btdc, this put 800 rpm idle in the neighborhood of 15-17 btdc. In order to do this, you take off your pulley, and measure the current marks, then add additional marks up to 40*BTDC, then grab a friend, tach, and timing light and go to town.
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Old 09-23-2020, 12:35 PM   #18
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My 67/68 b18D was mechanical only...deal with points gap,
....I noticed that with the mech only system, the engine made better power and fuel economy if I set the timing at 3k rpm to something like...l.
B18D had 8:7/1 compression ratio...when B18B had 10/1...

Points - Considering what they cost today, from prices I've seen, it would be cheaper in long run to convert to a non point system.

Timing - The distributor's internal springs controls amount of timing advance. Springs get old, and someone may have installed non OEM springs. Scanning Volvo's specs for 4 bangers, about 34 BTDC was the max for a specific engine...most were lower.
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Old 09-23-2020, 12:40 PM   #19
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Originally Posted by spock345 View Post
0 231 153 003 JFR4.
See this pic HERE
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Old 09-23-2020, 12:40 PM   #20
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My 67/68 b18D was mechanical only, and I believe it was the same "003" distributor. They work just fine for stock applications. I would have no concerns running it on a stock engine or mildly warmed over engine if you get rid of the points and go to something like a pertronix or crane ignition module, unless you are keen on points and want to take the time to deal with points gap, dwell, and whatnot. The only thing to really think about is max advance. I noticed that with the mech only system, the engine made better power and fuel economy if I set the timing at 3k rpm to something like 32-35 btdc, this put 800 rpm idle in the neighborhood of 15-17 btdc. In order to do this, you take off your pulley, and measure the current marks, then add additional marks up to 40*BTDC, then grab a friend, tach, and timing light and go to town.
Your observations are consistent with comments from Phil Singher that in his dyno testing they never went beyond 35 deg total advance and that depending on the engine best performance was in the 32 - 35 deg range. This was for engines with correctly set quench depths. Given the typical deck height issues on production B18 / B20 engines, it may be unrealistic to try operating with that much advance.

The more interesting issue is that although the total advance is the same, the curve on the E/F engine comes in much faster (consistent with your number) than on the later B engines which needed to reach 5000 RPM before maximum advance was reached.
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Old 09-23-2020, 12:46 PM   #21
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http://www.vclassics.com/archive/ignition.htm
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Old 09-23-2020, 08:51 PM   #22
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B20E...MSExtra
Have you attempted to mount a knock sensor?
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Old 09-24-2020, 10:13 AM   #23
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Have you attempted to mount a knock sensor?
flat tappet cam... good luck.

I'm running my B20 off ms2extra, full sequential with LS COP setup, and tried a few methods of knock detection. DET cans were the only way to tell. Even then, its hard to differentiate between cam noise, valve/lifter float, and knock.

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Old 09-24-2020, 12:32 PM   #24
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flat tappet cam... good luck.

I'm running my B20 off ms2extra, full sequential with LS COP setup, and tried a few methods of knock detection. DET cans were the only way to tell. Even then, its hard to differentiate between cam noise, valve/lifter float, and knock.
I had some success.

I am running a mostly stock B20E using a Bosch flat response sensor mounted on a stud on the left side of the block just below the head / block mating surface between cylinders 1 and 2. I used a Digilent analog discovery USB scope and the FFT in the Waveforms software to examine the frequency content of the signal. Yes, the B20 generates lots of noise; but, I was surprised to find that most of that noise occurs outside of the characteristic knock frequency of the B20. I will put a caveat on that comment because I did not test for noise in the characteristic frequency range beyond 4000 RPM.

The signal processing board I fabricated used a fairly common architecture. The knock sensor is connected up front to a differential input op amp to reject common mode noise. The output of the op amp is connected to a Max 7490 tuneable switched capacitor filter. The filter is configured as a band pass centered at the what I think he characteristic knock frequency is; but, I can't remember what the band pass roll off is. The output of the band pass filter feeds into a precision rectifier based upon a Max 4253 Op amp. At one point I was experimenting with using a switched integrator on the output of the precision rectifier; but, when I look at my prototype board it looks like I did not use that feature. I think I decided that it was not necessary with the count feature in the MSExtra software. The output of the precision rectifier appears to be connected through a final op amp which provides some adjustable output gain from the precision rectifier. The clock frequency for the switched capacitor is provided by a trinket overclocked board. The trinket allows me to adjust the clock frequency (center point of the band-pass) in software to experiment with different tuning for the knock frequency. The trinket also does double duty as I use its on board power supply to provide isolated power for all the op amps.

I believe I have the clock frequency set to give a tuned band pass somewhere around 3000 - 3300 hz with fairly sharp roll off. I am doing this from memory because I have the proto board in my hands; but, I am not sure where my design notes are.

I did a little testing of the board by cranking up the advance just in the ignition map cells around 3000 RPM 60 - 70 kPa such that I was able to detect audible detonation. The board generated a knock output under these circumstances and I did some initial fiddling with the knock set up in MSExtra to suppress knock. In my non exhaustive testing, the board did not generate extraneous knock outputs.

My MS board is set up with the knock sensor input and the knock sensor has been wired up to the terminals in the MS case; but, I have not implemented the knock sensing feature. My prototype board will not fit in the MS case slots, too narrow and too tall because of the use of breakout boards. The prototype board is also ..... clumsy. I used breakout boards to mount the the op amps and a proto board with a lot of surface wiring. I wanted to have a proper circuit board fabricated to allow direct mounting of all the SMDs with no green wiring required and design the board so that it would fit in the MS case slots.

I returned the ignition map to its pre detonation inducing condition and have not been super motivated to do any further work. The original thought was that a knock sensor might be a benefit in the event of some variable fuel quality. However, that has yet to present itself as a problem. With the current ignition map running 91 or 92 R+M/2 fuel, knock is pretty much a non audible event. If we get another COVID 19 lock down boredom might motivate me to go the next steps this winter - if I can find my original design notes and schematic.

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Old 09-24-2020, 12:45 PM   #25
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Originally Posted by cwdodson88 View Post
flat tappet cam... good luck....I'm running my B20 off ms2extra, full sequential with LS COP setup
If this info is true below...then indirectly, this distributor is detecting knock

123 Ignition Explained

Spark Balancing - The 123 ignition analyzes engine performance on a per cylinder basis and adjusts the spark timing and energy levels of each cylinder in order to balance the power levels - optimizing engine operation. This spark balancing function, derived from Formula 1 engine management systems, has not been available for classic cars until now.

I'm not plugging 123, but they do list timing degrees for various engines with a given distributor...scroll down this page to see them....with a cross reference to a given Bosch distributor.

PS Pricing wise, I would shop around, in US and Europe
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