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Engine Vac at idle too low, how to fine tune MS and B230

dl242gt said:
Also with the small intake pcv hose on the B230FT. The fitting in the manifold has a different size hole than the n/a one. Hiperfauto pointed that out.
Click to expand...
culberro said:
This. It's not a 3mm ID barb, there's a restriction in it to limit the boost leak. The restricted orifice is key.
Click to expand...
Thanks guys, always nice to learn something new! Out of curiosity, since I don't have that small hose on my B21FT, what are the normal NA & Turbo barb inner diameters (roughly), and which position on the manifold is used?
 
TLDR: @thelostartof - I looked at your MS setup & idle log in more detail, and I flat out don't understand how it works.

Long winded version:
I'm not an engine guy and don't have a good feel for fluid dynamics of engine airflow, so this may be confused.

The specs on your V16T cam from the enem web site are:
"Volvo B21/23 V16T: 268/112/12.2/1.5
268/268/110/114/12.2/12.2/1.6/1.2/ 0.40
Sport cam for turbo stage 2/3"

This is 268deg duration (at ??? lift&lash), 110deg intake/114deg exhaust lobe centerlines (in crank deg), and 1.6mm intake/1.2mm exhaust valve openings at TDC. Calculating gives an overlap of 268/2 + 268/2 - 110 - 114 = 44 degrees (crank).

While Wikipedia doesn't have an entry for big ass cams, it does have one for: https://en.wikipedia.org/wiki/Variable_valve_timing , which I think has similar behaviors. Quoting parts of the wikipedia entry:
Early intake valve opening - ... is another variation that has significant potential to reduce emissions. In a traditional engine, a process called valve overlap is used to aid in controlling the cylinder temperature. By opening the intake valve early, some of the inert/combusted exhaust gas will back flow out of the cylinder via the intake valve, where it cools momentarily in the intake manifold. This inert gas then fills the cylinder in the subsequent intake stroke,

Late intake valve closing - ... involves holding the intake valve open slightly longer than a traditional engine. This results in the piston actually pushing air out of the cylinder and back into the intake manifold during the compression stroke. The air which is expelled fills the manifold with higher pressure, and on subsequent intake strokes the air which is taken in is at a higher pressure.
Click to expand...

With this, I'd expect reduced idle vacuum (aka higher MAP pressure in MegaSquirt) and low MegaSquirt VEs around idle since part of the air/fuel charge has been replaced with inert exhaust gases. For the MegaSquirt VE table, I'd expect much lower than normal VEs on the left, and idle operating at higher than usual MAP values. TLAO's VE table looks nothing like I'd expect (note: IncorporateAFR and MAP_Multiply are both turned on):

tlao_VE_table.png

The idle log is even more confusing to me:
tlao_log_idle.png

This shows idle at ~815rpm at 81kPa MAP, with a ~13.9 AFR and a VE of ~72. From the MS config, it's running huge 1700cc/min injectors, so the 2.465ms PW (with a 1.0ms deadtime) results in the idle fuel flow of ~37cc/min. <<< This is unbelievable.

I checked my B21FT logs (LH2.4), and another somewhat standard B230FT log (MS2), and found that idle fueling was around 7.5cc/min. I'd expect most any redblock to have pretty similar idle fueling, not the 5x larger fueling. I don't know how an engine would even idle with 5x fueling, so I think I must be missing something basic.

For additional discussion, here's a picture of a traditional vacuum gauge:
vacuum gauge.png
Can an engine expert explain the "Late Ign Timing" red zone slight loss of vacuum? I'm guessing it's due to late burning of the fuel, resulting in higher residual pressure when the intake valve opens, and higher exhaust-to-intake reversion.

On a related vein, one old school method for adjusting the carburator mixture screws was to use a vacuum gauge and adjust the mixture for highest idle vacuum. How does this work?
 
bobxyz said:
TLDR: @thelostartof - I looked at your MS setup & idle log in more detail, and I flat out don't understand how it works.

Long winded version:
I'm not an engine guy and don't have a good feel for fluid dynamics of engine airflow, so this may be confused.

The specs on your V16T cam from the enem web site are:
"Volvo B21/23 V16T: 268/112/12.2/1.5
268/268/110/114/12.2/12.2/1.6/1.2/ 0.40
Sport cam for turbo stage 2/3"

This is 268deg duration (at ??? lift&lash), 110deg intake/114deg exhaust lobe centerlines (in crank deg), and 1.6mm intake/1.2mm exhaust valve openings at TDC. Calculating gives an overlap of 268/2 + 268/2 - 110 - 114 = 44 degrees (crank).

While Wikipedia doesn't have an entry for big ass cams, it does have one for: https://en.wikipedia.org/wiki/Variable_valve_timing , which I think has similar behaviors. Quoting parts of the wikipedia entry:


With this, I'd expect reduced idle vacuum (aka higher MAP pressure in MegaSquirt) and low MegaSquirt VEs around idle since part of the air/fuel charge has been replaced with inert exhaust gases. For the MegaSquirt VE table, I'd expect much lower than normal VEs on the left, and idle operating at higher than usual MAP values. TLAO's VE table looks nothing like I'd expect (note: IncorporateAFR and MAP_Multiply are both turned on):

View attachment 26637

The idle log is even more confusing to me:
View attachment 26638

This shows idle at ~815rpm at 81kPa MAP, with a ~13.9 AFR and a VE of ~72. From the MS config, it's running huge 1700cc/min injectors, so the 2.465ms PW (with a 1.0ms deadtime) results in the idle fuel flow of ~37cc/min. <<< This is unbelievable.

I checked my B21FT logs (LH2.4), and another somewhat standard B230FT log (MS2), and found that idle fueling was around 7.5cc/min. I'd expect most any redblock to have pretty similar idle fueling, not the 5x larger fueling. I don't know how an engine would even idle with 5x fueling, so I think I must be missing something basic.

For additional discussion, here's a picture of a traditional vacuum gauge:
View attachment 26639
Can an engine expert explain the "Late Ign Timing" red zone slight loss of vacuum? I'm guessing it's due to late burning of the fuel, resulting in higher residual pressure when the intake valve opens, and higher exhaust-to-intake reversion.

On a related vein, one old school method for adjusting the carburator mixture screws was to use a vacuum gauge and adjust the mixture for highest idle vacuum. How does this work?
Click to expand...
Just a guess at the carb tuning, the best vacuum would be the most complete combustion? Too much or too little fuel will choke or starve an engine. However, I have no direct knowledge of this.
 
VB242 said:
Just a guess at the carb tuning, the best vacuum would be the most complete combustion? Too much or too little fuel will choke or starve an engine. However, I have no direct knowledge of this.
Click to expand...
I don't think combustion matters at all. If you had no fuel and could spin the motor at idle speed with only the starter, I think vacuum would be there without combustion. No changes.

Vacuum is there from the mechanical intake stroke of the piston as long as the rings are good and the throttle body is closed. So as the intake valve opens during the intake stroke, air is pulled into the cylinder and negative pressure (vacuum) happens in the intake manifold. A cam with a long open duration can spoil the vacuum and less is created. That's what causes a rough or lopey idle. But a cam with a long duration allows more air/fuel to come in, which works out well for high rpm power.
 
The lopey (irregular) idle comes from the exhaust gases diluting the air/fuel charge resulting in inconsistent burn rates. If diluted too much, misfires and leftover O2 in the exhaust will occur. Part of the challenge is tuning megasquirt when the WBO2 reading may be bad due to misfires.

TLAO - can you post some normal logs showing a mix of driving - idle/cruise/acceleration?

https://www.motor.com/magazine-summary/mastering-the-basics-reading-a-vacuum-gauge/ (download pdf at bottom of page to see pics)
 
bobxyz said:
The lopey (irregular) idle comes from the exhaust gases diluting the air/fuel charge resulting in inconsistent burn rates. A RESULT OF A CAM WITH LONG DURATION. If diluted too much, misfires and leftover O2 in the exhaust will occur. Part of the challenge is tuning megasquirt when the WBO2 reading may be bad due to misfires. A cam with short (OEM) duration will not be an issue.
Click to expand...
Clarification added.
 
bobxyz said:
View attachment 26639
Can an engine expert explain the "Late Ign Timing" red zone slight loss of vacuum? I'm guessing it's due to late burning of the fuel, resulting in higher residual pressure when the intake valve opens, and higher exhaust-to-intake reversion.

On a related vein, one old school method for adjusting the carburator mixture screws was to use a vacuum gauge and adjust the mixture for highest idle vacuum. How does this work?
Click to expand...

This is how I always used to adjust my carbs (Qjet, Demon, etc) but I would then back the idle screw out (richer) maybe a quarter turn or so, as it helped with off-idle transition with longer duration cams.

I've never really thought about the why until just now. My thought on why an overly retarded ign timing would cause low vacuum is that it makes less power. If it's making less power, you may have to open the throttle blades a bit more to get an acceptable idle speed. This is really me just trying to sound the problem out. If you increase timing, from say 4 to 12 tdc, idle speed picks up. Why? Well, if you increase timing from 24 to 32 degrees at 3000 rpm, power increases. I'd imagine it's the same at idle, and there you're only trying to overcome the engine's internal friction, plus accessories. Even if you gain only 1 hp, that's a big percentage increase. So if you suddenly are making an additional 10% power or whatever, engine speed is going to increase, and you can close the throttle blades a bit to bring it back down. Same idle speed and smaller aperture=more vacuum.

But this is just a guess.
 
I wrote a pretty long page on mostly just ignition timing. It may seem obsessive to some, but it discusses why power increases or not with ignition advance. I won't write a bunch of paragraphs here. You can read my page if you like.
https://www.240turbo.com/ignition.html

Max power based on spark timing is found by getting the best crank LEVERAGE in the combustion or power stroke. We know in pretty much all engines that occurs around 12° to 15° ATDC. If we understand that, then we need to move spark timing ahead to compensate for fuel burn rates associated with engine speed, cylinder filling, temperatures, various air/fuel mixtures, and more. Whatever timing value we come up with to get this "perfect" formula is called MBT Timing (Maximum Brake Torque Timing), because it results in the the spark firing in a position that makes the best possible torque, or leverage, when pushing the crankshaft DOWN during the power stroke. A few degrees either side of that and you have less power.

A "proper" way of finding MBT is on a dyno. If it's a boosted engine, this is done N/A and no timing advance is allowed. A series of dyno runs to redline are done with a number of static timing settings, maybe starting at 10 degrees BTDC and moving up a few degrees for each pull. Then you can see the highest power result for each rpm increment you measured. You can set your base timing map from that. It'll surprise most that the most power is made in most N/A engines at around 26 degrees from torque peak to redline. But if you don't know that and set it at 30-something or 40 degrees, you aren't getting the most out of it.

Most of us never do the dyno part of that, so we just do the butt-dyno and it works ok.
 
I found a detailed write up for Holley carbs on how to use a vacuum gauge to set the mixture:
www.holley.com

Here Is The Correct Procedure For Setting An Optimized Idle Mixture And Speed

Setting the idle mixture and speed on a carburetor isn't the most exciting or elegant subject, but it is a necessary adjustment that often is overlooked or worse, set up incorrectly. Here's the proper way to handle this basic task.
www.holley.com www.holley.com

Paraphrasing, the full procedure adjusts the mixture screw(s) by 1/8 turn at a time for highest vacuum which is also highest RPMs. As the RPMs go up, the air screw is adjusted to get back to target idle RPM. Once running at highest vacuum with the mixture on the lean side, the final tweak is to turn the mixture screw in ever so slightly to improve emissions.

The part that I'd forgotten about was that the RPM would go up and needed to be dropped back to target idle RPM. With this in mind, adjusting for best vacuum is equivalent to adjusting for best power&efficiency at the target RPM. Best power/most efficient means that the engine needs the least air/fuel to keep the idle RPM. Least air is highest vacuum.

The final emissions tweak makes sense when looking at a mixture vs. power graph -- scroll down a bit here:
Best lean power is at ~13:1 AFR, but best emissions is at ~14.7:1.

For MS idle tuning with a big cam and suspect WBO2 readings, I'd think that using a vacuum gauge and adjusting mixture (i.e. the VE entries), would work OK.
 
If you look at old (60's-early 70's) Chrysler engine tune-up tags that were provided in the engine bay, they actually specify a certain air/fuel ratio at idle. I saw this for the first time a few years ago and found it pretty interesting. Though I grew up a Chevy guy and I'm small block till I die, I have learned and accept that Chrysler engineers were way ahead of the other two Detroit guys back in the day.
 
Any more info on the vacuum pump you found?
I'm curious if you're tested it and measured what kind of vacuum it can pull.
I have the same brake booster and I'm getting enough vacuum for my brakes with 12-15 inches at idle without AC and under just 10 inches with AC. So if you're getting nearly 8 inches I don't think you need much to get in the range that the booster will like.
 
shoestring said:
If you look at old (60's-early 70's) Chrysler engine tune-up tags that were provided in the engine bay, they actually specify a certain air/fuel ratio at idle. I saw this for the first time a few years ago and found it pretty interesting. Though I grew up a Chevy guy and I'm small block till I die, I have learned and accept that Chrysler engineers were way ahead of the other two Detroit guys back in the day.
Click to expand...
Back in the 60s Chrysler also had the transistor ignition setup available on their cars. That was a few years before GM HEI and Ford.
 
dbarton said:
To the OP if you haven't seen it:
T8fanning posted info recently about his installation of a Volvo/Hella vacuum pump and sensor. It's in his thread beginning at post 821.
https://turbobricks.com/index.php?threads/not-mediocre-242.326370/page-42#post-6404258
Click to expand...
Right now I am as normal lacking time in life to get back to checking out the car. I have a ton of good info here that I need to test. If i can find some time tomorrow i will see if I can find a log to post with some other driving around. I also want to try a leakdown test and then also messing with the cam timing to see if vac changes also, but again the time to just sit and work on this car vs everything else around here requiring my attention.

I have looked at those few vac pumps but I have not settled on which one to buy, where to mount (I do like that low mount on the engine mount) and how to trigger on. Besides this mechanical problem i am also debating on ripping out the last 20 years worth of my wiring and try to start from scratch on the car and rewire everything. That might also include the MS box which means going down the rabbit hole of cutting out this harness and building another one from scratch maybe to upgrade to something newer.

I did notice in the log i posted that it has that fuel fuel reading while in other older logs it doesn't. NO idea what changed besides maybe a tuner studio update as nothing on my MS has changed since it was installed.
 
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