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OHV distributor differences, mechanical vs vacuum advance

spock345

spock345

Well-known member
Joined
Nov 3, 2016
Location
Livermore, CA
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.
 
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.
 
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.
 
spock345 said:
wondering, what are the pros and cons.
Click to expand...

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.
 
84B23F said:
IIRC, sometime in 1970s came vacuum retard on different engines...VW...etc.


Nice overview, btw.
Click to expand...

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...
 
JohnMc said:
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...
Click to expand...

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.
 
84B23F said:
IIRC, 1975-244's engine was around 8:5/1 compression ratio...which was very low when compared to 1960s 'muscle' cars.
Click to expand...

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.
 
Thanks for all the information. I am still trying to get a grasp on older advance systems and this has helped tremendously.

142 guy said:
My recollection is that some of the B18s just ran on pure mechanical advance.
Click to expand...

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.
 
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.
 
142 guy said:
10.5:1 compression ratios
Click to expand...

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
 
spock345 said:
At least some just operated on mechanical advance.
Click to expand...

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.
 
spock345 said:
Currently my car is set up with a 0 231 153 003 JFR4.
Click to expand...

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.
 
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.
 
spock345 said:
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.
Click to expand...

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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cwdodson88 said:
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.
Click to expand...

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.
 
cwdodson88 said:
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.
Click to expand...

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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