Timing and vacuum advance - theory and practice?

[Yellow Truck]

I admit that I've been reading up on the subject and a little knowledge can be a dangerous thing, but I'm seriously confused about timed and manifold vacuum for the timing advance.

David Vizard in his book makes the case that you need a leaner mixture at idle and at cruise, and that a leaner mixture needs more advance to complete the burn and create maximum combustion pressure at the appropriate time ATDC. We can leave cruise out of it because at cruise there will be some throttle engagement and timed vs manifold will be less significant.

Running the vacuum advance off a timed port means that the advance at idle is just the initial timing. Since one of the goals is to get a reasonably lean mixture at idle, it stands to reason that to get as smooth an idle as possible (and best vacuum), if the vacuum advance is run off the timed port it would require more initial timing advance to compensate for the fact that the vacuum advance won't kick in until partial throttle is applied.

It also seems that if I set the initial timing lower, and ran the vacuum advance off the manifold port, it would have the advance it needs at idle without having to wait for the plates to open to engage the advance.

I've read an argument by a retired GM engineer where he argues you should NEVER run the vacuum advance off a timed port. I'm sure you all have seen this argument before. I'm interested in what the experts do, but also why they do it.

[Drew Pojedinec]

Learn how to make your distributor do what your engine needs.

[ericwevans]

To expand on what Drew said, I'd get the base timing and advance curve where the engine likes it with the vacuum advance disconnected completely.  Once that is right then add vacuum advance and the engine will let you know what it wants.  If you have an adjustable vacuum canister that will throw another variable in.  My current combination doesn't like vacuum advance at all.  At idle if you use manifold vacuum it would have random misses, about 1 every 5 seconds.  At cruise you could hear concussions in the exhaust.  So I currently run no vacuum all.

[Drew Pojedinec]

Ok, how I do it......


First,  I make an educated guess where the total and initial need to be.
For my last build I figured 18 initial and 32 total.  In my case, it's a good baseline, prior to testing.  The small amount of advance is due to my BBM heads, which have a faster burn chamber.

Here is the regular slot,


So, being I use a Duraspark, I weld up the advance slot.


I then use a grinding wheel to cut a little slot, then I clean it up with  a die grinder carbide bit and some sandrolls.


Since I was only looking for a little advance, I ground a small slot to start with.
.300 or .350 I forget


Plug vac advance at this time.
I install the springs I think I might need to get the advance in when needed.  Things to consider, vehicle weight, gearing, convertor, etc.  Think about how loaded the engine must be to get things rolling.  In my case I have a loose convertor and didn't see the need for the advance to come in too soon, so I aim for 2800rpms.  My car cruises around there so I figure it's a good starting point.  Plus with a duraspark you can bend the tab for final adjustment.

I then install the distributor and fire the engine up.
First thing I'm looking for is how the vehicle idles at the initial advance.  Hot start should also be a consideration, tho with a mini starter, I was pretty certain I could go a little either way.
idles at 18 just fine, maybe it likes 20, maybe it prefers 16.  Figure it out.

Once I have that number pegged.  I get the total to be where I want to be.  Go for a drive.  Think you need more?  add more, think you need less, add less.  Want it to come in sooner, make it so.  I don't worry about initial at this time.
Once I get a good idea of my total, I take the distributor apart and reweld or grind (it's easier to grind) to get my initial where I want and total where I want.
Sometimes with me, this takes 3-4 times of reinstalling/taking apart (I do it on the engine) to get it all perfect.

At this time, in order to figure and get a baseline for vacuum advance I run the engine.  Vac advance isn't plugged, it's attached to my handheld vacuum pump.
At max advance/cruise rpm I use my vacuum pump to bring the advance in as needed.  If I need to change what I see for a baseline I do so.
Once again, hook everything up and go for a ride.  like it?  don't like it?  make adjustments.

Hope that was a little more specific than my last post.
and sure, there are better ways to do with with a distributor machine.... but I don't have one.

Good luck
Drew

[Yellow Truck]

Drew,

That is a very good lesson in how to set up the full timing profile, I am glad I didn't post a smart ass response last night! It doesn't address metered vs manifold vacuum.

I am running BBM heads as well, and I'm running a stock Autolite with Petronix, and on metered vacuum in it's short period where it was working well it seemed to like around 18 degrees of initial advance. I had mine done by a local specialist (who has the machine) and he reported that he had it set up for 13 degrees of initial advance for total advance of 34 degrees, with mechanical advance all in by 2,950.

He said that it would add about 10-12 degrees of vacuum advance when hooked up. So far I've been running mine off the metered port, so the initial advance is all it sees at idle.

I think if I decide to get into adjusting the curve etc. I'll change out to a modern electronic distributor and control box. I don't have the tools or skills to do what you are doing!

[Drew Pojedinec]

I am glad I didn't post a smart ass response last night! It doesn't address metered vs manifold vacuum.

I had just worked an 18 hour day in an engine room doing stuff like pulling an 1800lb gear off a shaft with 2 rosebuds and 8000psi hydraulic pressure, etc.  I was tired and my answer was too brief.  Sorry.

I think if I decide to get into adjusting the curve etc. I'll change out to a modern electronic distributor and control box. I don't have the tools or skills to do what you are doing!

I love the Duraspark, it's super simple and reliable.
The only tools I used for this was a mig welder (you could probably use JB Weld), an angle grinder, a die grinder, and a timing light.

As far as manifold vacuum vs ported, my thoughts are this:
Manifold vacuum is a bandaid for improper initial timing.

If your initial and your mechanical advance are correct you shouldn't need vacuum advance but in one place.  Lightly loaded cruising.

I'm no expert, i just tinker with these things really.  Still finding the perfect "happy place" for my engine (for me sometimes it takes a few hundred miles of tinkering before I get something perfect).

Drew

[Drew Pojedinec]

So, thinking about it.....
easiest thing for you to do, would be to find a bushing for the advance pin.

There is a lil metal piece that sticks up, the advance arm that I have pictured above is pretty much the same as your Autolite and the size of the slot is the amount of mechanical advance you get.
Well, you can use a rubber bushing on that pin to limit mechanical advance.  This would allow you to add more initial with your total staying the same.  I have a couple of those laying around, and I'm sure your local guy does too.  Or take the distributor back to him once you establish where the initial is the best for your engine (I'm sure it's more than 13)

[turbohunter]

X3 on the no vacuum advance.
I tried to make it work on my truck but it didn't like it.
Im sure I could have gotten it to run well but didn't care about spending the time to when it already runs great mechanically.
Then there's the ported/non ported debate. Hurts my little brain.

[Yellow Truck]

Thanks for the very good replies. I am VERY glad I didn't post a smart ass reply.

[FirstEliminator]

   Currently, I run all of my vehicles on ported vacuum. Often, I've thought about trying to run the vacuum advance direct to manifold. Here is why I ponder it: A while back I was working on a 90's T-bird. While watching data on the scanner one of the pids would show timing. The timing fluctuated around at idle. It would go up to 25 or so. So, the computer would move it around to accommodate a smoother/cleaner idle. Have you noticed when you plug in the vacuum advance to manifold vacuum the engine idles higher? It can tolerate more advance and it seems to be  more optimal of a setting.  With manifold vacuum engaging the vacuum advance at idle means a higher rpm, you'd compensate by closing your throttle to be at the same idle rpm. Seems like you'd have better efficiency at idle with manifold vacuum. So why not just set your initial timing 10 or so degrees higher? Because with too much initial it will kick back when hot. I set my initial as high as I can so it doesn't kickback while cranking on a hot day after a 10 minute hot soak. I usually find this to be somewhere around 15 degrees---in a street set-up with an RV type cam. Why would it tolerate more timing at idle than cranking? When cranking there is more time for charge to fill the cylinders. Idle is about 300% faster than cranking, so charge density is less at idle because with the same throttle opening.   I believe ported vacuum was used to reduce certain emissions at idle. Oxides of nitrogen I think.

   One thing I can't see is how a street driven engine would run better with no vacuum advance. The optimal timing at part throttle operation is going to be different than full throttle. Race cars don't need vacuum advance because they either idle or are WOT. Race cars don't care about part throttle efficiency and emissions. Load, environment, and throttle are all variable with a street engine. The vacuum advance helps accommodate optimal tuning for this application------if the vacuum advance is tuned correctly. I find vacuum advance tuning is usually between 8 and 16 degrees. If you plop a vacuum advance on that pulls 25 degrees because it was set to run on a 1975 EGR engine with 7.7:1 compression would be like swapping around guitar strings and expecting it to sound good when playing the same old chords.

     So, why do I still run ported? I don't know, I guess just 'cause.  But, I'm talking myself into it.  Think I'm going to try it to see if there are any drawbacks and if the theoretical benefits actually exist.

   I have a different method of tuning advance range and curve speed in the Duraspark. I wrote about it in this posting: http://fepower.net/simplemachinesforum/index.php?topic=308.msg2388#msg2388   The procedure may not be as complete as I'd like. But the gist of it is there. Someday I'd like to detail my set-up a little bit better.
     

   My life is overloaded with work at work, work at home, work on my house, work raising kids, work helping friends and once in a blue moon work on my car projects. So, if you can explain to me in detail why my theory won't/can't/doesn't work, please tell me. I don't have enough time left in my life to do all I want to once. So, doing anything twice really sucks.

   thanks,
     Mark
 Berkshire Transmissions
North Adams, Massachusetts
     

[chris401]

I run ported vacuum advance for more engine braking. After you set your base and mechanical you can safely test and set your vacuum advance at idle with a mighty vac and a timing light.

[BattlestarGalactic]

I've never had any luck putting any vacuum advance on anything I own.  Even my stock '89 F150/302.  I converted it to carb, put Duraspark ignition on it.  I tried both manifold and ported and it didn't like either.  Ran like crap no matter what I did.  Left it all unhooked.  All the hot rods are mechanical since they don't have enough vacuum to worry about.

Like mentioned I'm sure if you spend some quality time tinkering you can make it work.  Me?  Nope.  Does it idle?  Yes.  Does it go like stink at WOT?  Yes.  Perfect, no other work needed. 

[Autoholic]

This is one of those loaded questions, where you will get 1,000 different and specific opinions, all circling the same drain. There will likely never be consensus as there aren't two identical, old school engines out there. Just changing a few things from stock in a group of 100 owners can result in tens of thousands of different variables, and that's assuming the engines were all closely built to spec at the factory. Some food for thought, some engine management systems measure manifold pressure (MAP sensors) to help control timing. Something else to think about, a lot of vac cans have advance and retard functions.

I'm not an expert, nor am I a veteran of seeing what works. The theory behind manifold advance, is that it will detect idle and idle circuit only cruising load reasonably well. Many if not most carbs are still on the idle circuit under 2,500 RPM's. When only the idle circuit is being used, or you are in transition, port vacuum will be zero or poor. When you suddenly let off the go fast pedal at 5,000 RPM's, port vacuum will drop to zero. Port vacuum cannot differentiate between starting, idle, near idle cruising and suddenly going off throttle. There most definitely will be a difference in manifold vacuum in relation to RPM as throttle position changes. 5,000 RPM at WOT is different from 5,000 RPM off-throttle. This is where manifold vacuum comes into play and remember that it is still used today in some EFI systems (self learning EFI often uses it). Using a lot of initial timing and zero manifold vac advance means that the engine's timing won't differentiate between WOT and off throttle at a specific RPM, and what the load is. Using ported vacuum will give you advance at WOT and no advance at off throttle, but it mostly gives your timing an indication of throttle position when you think about it, aside from starting when you really aren't drawing enough air in to register a lot of vacuum. It really depends on how you setup your timing. The point about too much initial timing when doing a hot start is also a very good point to make.

Again, I'm not an expert but I do know that there is no one way to setup your timing, regardless of engine. It depends on what your engine needs to run happy for your application. Any blanket statement out there about which is better, can easily turn into bad advice. Again, some swear by manifold vacuum, some swear by ported vacuum, some swear by using one to advance and the other to retard, and some swear by mechanical only. How many ways can you skin a politician? Do a crap ton of trial and error and see what works for you, no amount of reading will provide you that insight into your vehicle.

Something else to think about. When you are engine braking, advancing the spark will cause the combustion to help resist the engine from rotating. This can help slow down the engine. Ported vacuum won't help with advancing the spark for engine braking. Manifold vacuum will, because suddenly going to zero throttle at say 4,000 RPM will register a lot of vacuum. Food for thought, just be careful of how early the spark happens. Modern EFI systems control spark and fuel injection duty cycle involving RPM, load and throttle position, in some form or another, for various situations, including engine braking. How the engine's ECU computes load depends on the system, utilizing MAP, MAF or both (Evo X used both).


In an over-simplified nut shell...

Manifold indicates load, port indicates throttle position.

[Yellow Truck]

I have been re-reading some of these posts and mulling. I had some issues (described in my other thread http://fepower.net/simplemachinesforum/index.php?topic=4350.0) where it suddenly began running like crap after changing the headers, plus I had noticed some strange movements in my initial timing from start to start (jumped from 16-18 to 28 over night).

Took the distributor apart and had a good look and all seemed fine but I noticed some oil on the intake, and the coil had puked. Had a new Flame Thrower, so put it on, swapped out the plugs (it seems sensitive to fouled plugs and mine were a bit dirty with only 2 hours of running time). Net result was got it restarted after putting it back together and it ran very well again.

We messed with the distributor and timing, moving from a timed port to manifold, and with about 10 inches of vacuum at idle, we had 13 degrees of initial advance, 21 degrees with the vacuum pot connected to the manifold port, and at 3,500 we had 41 degrees advance. So far so good (I think), but then we tapped on the vacuum canister to see if reacted. Advance at idle dropped to 3 to 5 degrees and the engine stumbled badly. We adjusted the connecting rod as it seemed to be bent a little and on a strange angle. Haven't run it since because I need to adjust the lifter pre-load (had a tick, and in getting rid of it I over did it and have a hanging valve again).

Any suggestions on a replacement vacuum canister?

[machoneman]

I'd try a Rock Auto or NAPA supplied cannister although they may all be Chinese made at this point. Be sure it is an adjustable unit with a small Allen key through the nipple....usually clockwise for less advance, counter for more.

Summit btw has Drake's units but I'm not sure they are for FE's or adjustable.

https://www.google.com/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#tbm=shop&q=adjustable+vacuum+advance%2C+Ford+FE&spd=1164009194894188099
 

[chris401]

Something else to think about. When you are engine braking, advancing the spark will cause the combustion to help resist the engine from rotating. This can help slow down the engine. Ported vacuum won't help with advancing the spark for engine braking. Manifold vacuum will, because suddenly going to zero throttle at say 4,000 RPM will register a lot of vacuum. Food for thought, just be careful of how early the spark happens. Modern EFI systems control spark and fuel injection duty cycle involving RPM, load and throttle position, in some form or another, for various situations, including engine braking. How the engine's ECU computes load depends on the system, utilizing MAP, MAF or both (Evo X used both).

That is a good point I haven't thought about. I tried it both ways and felt more drag on ported. Next time I look at a late model I will look at the advance/retard values under engine breaking. In the past the only thing I have really watched for is full timing retard to spot a jumped timing chain. I have noticed advance in the low 60's under cruise make? model? year?, more than likely it was a 4-6 cylinder.

[Yellow Truck]

I spoke with the guy that set up my distributor - the pot I have on it is a Standard VC192. He is not a big fan of the Petronix Ignitor stuff and said it doesn't make sense to run an MSD box with it. If I go MSD box go with a new distributor too.

[Autoholic]

You either go the Pertronix route or you go the MSD route. That would be pointless (pun intended) to do both. Some swear by the Ignitor method of going electronic. The Ignitor 3 has a programmable redline. You have to be careful with an Ignitor, they warn about jump starting the vehicle with one. You'll fry it.

[Autoholic]

That is a good point I haven't thought about. I tried it both ways and felt more drag on ported. Next time I look at a late model I will look at the advance/retard values under engine breaking. In the past the only thing I have really watched for is full timing retard to spot a jumped timing chain. I have noticed advance in the low 60's under cruise make? model? year?, more than likely it was a 4-6 cylinder.

Well, an ECU won't specify engine braking. What it will specify is timing based on RPM and load or RPM and throttle position. There will be a fuel map related to this as well. At high RPM and suddenly going to closed throttle, that will have different timing values than high RPM at WOT. The RPM can be identical, what changed is throttle position / load.

The difference between a carb and EFI, is how things are controlled. One uses mechanical controls, the other electronic. You can theoretically run a carbureted vehicle identical to a vehicle with EFI, in terms of what is being controlled. The problem is, you reach a point where mechanical controls for certain things become a Da Vinci invention that makes German engineering look sane. And in the end, the EFI system would still control things better. I'm trying to imagine VVT with mechanical controls and I know it would require some sort of connection to RPM, so a mechanical advance. How that would work is beyond me right now.

[FirstEliminator]

I spoke with the guy that set up my distributor - the pot I have on it is a Standard VC192. He is not a big fan of the Petronix Ignitor stuff and said it doesn't make sense to run an MSD box with it. If I go MSD box go with a new distributor too.

    The Pertronix is a trigger, the MSD box is a spark enhancement. Why wouldn't they work together? I think they both have instructions on connecting one to another. If you like a stock look, a polished MSD distributor with a red cap is plain wrong. The only difference with a Pertronix is one extra wire.  Being I drive my cars long distance, I prefer to have something that uses parts that can be found at every auto parts store in the nation, which makes the Ford Duraspark my #1 choice. Yeah, I run an MSD along with it, but if there's a problem it can be converted over to a stock Duraspark box if needed.

[FirstEliminator]

    I've never picked sides on ported versus manifold vacuum for the distributor advance. Yet, I usually just run ported. Recently, I found a drawback to running the vacuum advance on manifold vacuum.
  The vehicle was an 82 Ford truck that had a 347, trick flow heads, I'd suspect around 220 or so of duration at .050--overall a good street build. Trans was a manual 5 speed--ZF 5S42. The truck had a bad driveline vibration that we fixed. Engine had a misfire and wasn't quite up to par on power. I checked out the timing and noticed the vacuum can had 20 degrees of advance and the initial was set at 12. Being hooked up to manifold vacuum there was actually 32 degrees at idle. It did idle, but was sensitive. If you let out the clutch a little fast and there was a dip in engine rpms where the vacuum would also dip, then the timing would go down to 12. The carb was closed up pretty tight to accommodate 32 degrees at free idle. With this carb setting, puling out 20 degrees of timing would then cause the engine to stall easily. 
    The best idea was to run ported and adjust the idle speed and mix with about 16 initial. Now, base throttle opening and base timing wouldn't mismatch when load was applied. The idle would maintain much better when engaging the clutch. Stalling was reduced significantly which made the truck easier to drive.  I changed to an adjustable vacuum advance and put about 10 degrees of vacuum on top of 34 total (initial and mechanical).
     In this application ported proved to be the better choice. The idle stability issue may not have been apparent in an automatic trans application. If I get time I may try changing to manifold vacuum on one of my cars to see if there is any improvement.
    My opinion on ported versus manifold is do which ever is best for the application.  With manifold vacuum I'd suspect the greatest benefit would be in the form of idle speed fuel efficiency. Which in a mild daily driver type car is important. When you're standing still, idle is just wasting gas, so use the least amount.

     Mark

[unclewill]

I stole this from the Corvette forum (http://www.corvetteforum.com/forums/c1-and-c2-corvettes/647366-vacuum-explained-manifold-venturi-and-ported-new-article-by-lars.html)

The most sensible explanation I've read:

The timing advance curve requirements for an engine will vary a bit from one engine to another depending on cam, compression ratio and other efficiency factors. But in general terms, most V8s will produce peak power at WOT with 36-38 degrees of ignition timing. Peak fuel economy and drivability at cruise is achieved with about 52-54 degrees of advance. Best idle quality has a much wider range depending on cam & engine, but tends to be in the 12-24 degree range. Lowest emissions usually occur with timing in the 4-8 degree range.

When tuning, it is important to realize that the upper limits on timing are determining factors for how to set things up: You want the total WOT timing (the maximum timing the engine will see with vac advance disconnected and with the centrifugal advance fully deployed) to be not over 38 degrees. 36 is the best setting for most applications. Once this has been set, it automatically determines what your initial advance ends up being unless you physically alter the length of the advance curve. In most cases, once total advance has been set to 36, the initial advance will end up being about 12 degrees-or-so. And, since most vacuum advance control units pull in about 16 degrees of vacuum advance at cruise speed (where the full centrifugal advance will also be deployed), the 36-degree setting will produce 52 degrees of total combined advance at cruise with the vac advance fully deployed.

But what if your engine/cam combination idles best at 26 degrees advance? Radical cams often require quite a bit of advance at idle. If you simply bump the initial timing up from 16 to 26, your total WOT advance will go from 36 to 46. The total combined timing at cruise will go from 52 to 62. This is not acceptable, and can result in severe engine damage from detonation at WOT, and the car will chug and jerk at cruise from the over-advanced condition. An appropriately selected vacuum advance unit, plugged into manifold vacuum, can provide the needed extra timing at idle to allow a fair idle, while maintaining maximum mechanical timing at 36. A tuning note on this: If you choose to run straight manifold vacuum to your vacuum advance in order to gain the additional timing advance at idle, you must select a vacuum advance control unit that pulls in all of the advance at a vacuum level 2 in. Hg below (numerically less than) the manifold vacuum present at idle. If the vacuum advance control unit is not fully pulled in at idle, it will be somewhere in its mid-range, and it will fluctuate and vary the timing while the engine is idling. This will cause erratic timing with associated unstable idle rpm. A second tuning note on this: Advancing the timing at idle can assist in lowering engine temperatures. If you have an overheating problem at idle, and you have verified proper operation of your cooling system components, you can try running manifold vacuum to an appropriately selected vacuum advance unit as noted above. This will lower engine temps, but it will also increase hydrocarbon emissions on emission-controlled vehicles.

If, however, your engine idles best in the 12-16 degree range due to a mild cam, plug the vacuum advance control unit to a ported vacuum source to eliminate the vacuum signal at idle. You will still obtain the 36-degree WOT total, and you’ll still have 52 at cruise. Also, if you need to pass an emissions test, use the ported source to reduce your hydrocarbons.

By playing with the total length of your centrifugal advance curve, selecting between ported or manifold vacuum, and carefully selecting a matched vacuum advance control unit that meets your specification requirements, you can achieve an optimum idle, excellent off-idle throttle response, and the best fuel economy possible.

[machoneman]

Thanks for posting that 'steal'. It is an excellent representation of what actually occurs.

Interesting too the earlier statement on the mismatch between manifold vac. and idle, or idle transition. Some heavy trucks exclusively use manifold vac. primarily because the speed of movement (i.e. clutch engagement, then rpm rise) is considerably slower than say a hot street car. Put another way, they can get away with manifold vac. and not suffer a lean, off-idle stumble as noted.       

[unclewill]

"Radical cams often require quite a bit of advance at idle."
This is the biggest takeaway for me - if you run a mild cam, try the ported vacuum, wilder cams may require straight manifold vacuum at idle.
I run 12/38 degrees, all in at 2500 on a 482 with Comp 292H.  Later this week I am going to tune using manifold vacuum and the Pertronix adjustable advance canister to ~20 at idle (or wherever the engine is happiest with the new EFI) IF I am too lazy to use the EFI timing function with mech/vac advance locked out.
I'll report back.

[chris401]

Tobbemek or others,
Do you use a scanner? I am curious about timing values when engine braking. If you downshift with throttle closed what are the timing values? If I ever noticed I do not remember, this discussion got me to thinking if anything after 2003/4 would change timing to assist in engine breaking.
Thanks

[Autoholic]

Tobbemek or others,
Do you use a scanner? I am curious about timing values when engine braking. If you downshift with throttle closed what are the timing values? If I ever noticed I do not remember, this discussion got me to thinking if anything after 2003/4 would change timing to assist in engine breaking.
Thanks

The ECU will have a spark advance and a AFR table that correspond RPM to load. Load will be measured in kPa (MAP and / or MAF sensor depending on the ECU) and / or IgnLoad % (throttle position sensor?). So yes, timing will change according to the table. I'm not an expert on ECU controls, so I'm not sure how IgnLoad % is measured and calculated, but I do know the tables are setup for RPM vs Load for spark advance and AFR. If you pull up the spark timing and AFR tables, you can see what the values will be at any given RPM and load. A closed throttle would be the lowest load. Put a vehicle on a dyno and have the ECU connected to a computer, you'll see all of it in real time.

[Heo]

Modern Fuelinjected engines cut of the fuel during engine braking
over 1500 rpm so i guess the ign timing is not important then

[chris401]

Tobbemek or others,
Do you use a scanner? I am curious about timing values when engine braking. If you downshift with throttle closed what are the timing values? If I ever noticed I do not remember, this discussion got me to thinking if anything after 2003/4 would change timing to assist in engine breaking.
Thanks

The ECU will have a spark advance and a AFR table that correspond RPM to load. Load will be measured in kPa (MAP and / or MAF sensor depending on the ECU) and / or IgnLoad % (throttle position sensor?). So yes, timing will change according to the table. I'm not an expert on ECU controls, so I'm not sure how IgnLoad % is measured and calculated, but I do know the tables are setup for RPM vs Load for spark advance and AFR. If you pull up the spark timing and AFR tables, you can see what the values will be at any given RPM and load. A closed throttle would be the lowest load. Put a vehicle on a dyno and have the ECU connected to a computer, you'll see all of it in real time.

I left a well paying carrier in automotive with two certs shy of being a master. I understand how it works but somehow I cannot get my question across the board. You seem to understand engine management better than some who do just test out but that still does not answer my question.

[Autoholic]

Looks like Heo has the solid answer.

I think carb'd engines would be far more interesting to look at under engine braking. You'd need to be on a dyno, with O2 sensors to measure lambda and a timing light. Then either a person or a camera to watch the timing light. I bet very few people outside of manufacturers have studied this for a carb'd engine. This sounds like a project for Jay to study a SOHC on an engine dyno. He could compare engine braking for manifold vs port vacuum to see what is happening. He could then publish it in Hotrod. If the timing is advanced too much under engine braking, I could see it being harmful. Lean mixture detonating too soon. What if peak combustion pressure ended up right before TDC? A study on engine braking becomes a study in engine breaking?

[chris401]

Looks like Heo has the solid answer.

I think carb'd engines would be far more interesting to look at under engine braking. You'd need to be on a dyno, with O2 sensors to measure lambda and a timing light. Then either a person or a camera to watch the timing light. I bet very few people outside of manufacturers have studied this for a carb'd engine. This sounds like a project for Jay to study a SOHC on an engine dyno. He could compare engine braking for manifold vs port vacuum to see what is happening. He could then publish it in Hotrod. If the timing is advanced too much under engine braking, I could see it being harmful. Lean mixture detonating too soon. What if peak combustion pressure ended up right before TDC? A study on engine braking becomes a study in engine breaking?

I hate to say this on a Ford forum but if you don't have manufactuer specific software I would say they best scenario would be a Modis on a GM. Take it out on the road and test it. Maybe the Verus is better now?

[Autoholic]

I'll start by saying I'm not a mechanic, so you made me do some research to understand what you wrote about modis and verus.

My last comment was mostly directed at carb'd engines though. So were you commenting on what Heo wrote?

[chris401]

No but I would like to see what the fuel is doing as well. Like he said timing probably is not important if it is not pulsing the injectors.

Verus: My last couple of years I was looking to buy my own full grown scanner. The tool man was trying to peddle off the new Versus. I used it for a couple of weeks and it did have more modes and graphs but was still slow on the Asians and would often lock up. OTC is a well laid out tool but was not complete. Some pids were not there and would not read one of the Ford cam banks. Other than factory specific equipment the Snap On and OTC are the only tools I remember working with. My tool peddler is now a Cornwell dealer so I will be posting what I find there in the NON-FE Discussion Forum.

[Autoholic]

well, on a EFI vehicle I'd just hook up a PC with the software needed to tune the ECU if I really wanted to see everything and forget a scan tool. Focusing on engine braking is a bit beyond what a scan tool is designed for IMO. If you really want to see all the information, you need tuning software and ideally an aftermarket ECU.

[chris401]

well, on a EFI vehicle I'd just hook up a PC with the software needed to tune the ECU if I really wanted to see everything and forget a scan tool. Focusing on engine braking is a bit beyond what a scan tool is designed for IMO. If you really want to see all the information, you need tuning software and ideally an aftermarket ECU.

Moved to the nonfe section so Jay does not throw us out on our keyboards.

[Tobbemek]

Tobbemek or others,
Do you use a scanner? I am curious about timing values when engine braking. If you downshift with throttle closed what are the timing values? If I ever noticed I do not remember, this discussion got me to thinking if anything after 2003/4 would change timing to assist in engine breaking.
Thanks

Sorry to much going on right now my bookkeeper is hunting me with a torch and my wife left me yesterday morning with horses and kids and and and but unfortunately she is coming back Monday evening and then all the real troubles start again    she heights dose kind of yokes.
I don't use a scanner and i really  don't know in what direction the ECU takes the timing on a modern car when engine braking.
I do believe its retarded fore emission causes , everything today is about emission Look at VW trying to sheath manipulating the ECU   
The 70 boss 429  boss 302 and 428 Cj have dual vac canisters for retarding on engine braking to get better emission.

Modern Fuelinjected engines cut of the fuel during engine braking
over 1500 rpm so i guess the ign timing is not important then

Another good reason to not have over exposed transfer slots on idle, when engine braking having super high int. vac. sucking a lot of fuel that is washing the cylinders. Independent car manufacturer have shown cylinder /ring wear on injected vs carburetted engines be almost half.

I left a well paying carrier in automotive with two certs shy of being a master. I understand how it works but somehow I cannot get my question across the board. You seem to understand engine management better than some who do just test out but that still does not answer my question.

  I hate working on new cars don't do it fore a living. I do have a VAG COM program fore  my own cars we have 3 AUDI A6 TDI
in the family its more a necessary evil to fix them and they shore can fowl you. A tiny crack in a vac hose almost impossible to find can throw tha MAF sensor in limp  mode     A healty 427 so serenade is what its all about, guess now one on this forum could have missed to watch the 68 Holman MoodyMustang at you tube winning the 60 corvette anniversary. Yheaaa seen it many times.

[Tobbemek]

A tiny crack in a vac hose almost impossible to find can throw tha MAF sensor in limp  mode 

Should have been can throw the ECU in limp mode, hooked up on OBD II show a faulty MAF 
was late last night and i sure hate my bad spelling.