Just curious does anyone know how Cranking VE actually functions?

[GrannySShifting]

Lets see. As in when you change the factory table from 100 to say 130, what are you going to see on datalog

[mbray01]

My understanding and experience is that its a modifier of the normal ve. Therefore, if the target cell in the ve table is say 60%, and the cranking ve is 100%, you will get fueling for 70% ve. If you have 50% in cranking, then you will get 35%ve, in your example 130% would result in 91%ve

[GrannySShifting]

My understanding and experience is that its a modifier of the normal ve. Therefore, if the target cell in the ve table is say 60%, and the cranking ve is 100%, you will get fueling for 70% ve. If you have 50% in cranking, then you will get 35%ve, in your example 130% would result in 91%ve

Seems to be about accurate. I know I asked this years ago and no one knew for sure. I noticed cranking in speed density the VE number would always say 879 for example, but there was a definitely fueling difference if you have 40 or 80 or 120 in the cranking VE table, so its a modifier of the VE value and you can see it in logged injector pulse width when it 70-120 rpm cranking

[Cringer]

Little changes in this table have a big impact on the calculated Cylinder Airmass (g) value.

[blindsquirrel]

I know this is really about how that table works, but remember that at cranking speed everything is happening slowly enough that actual volumetric efficiency will be very near 100%, because not much if any vacuum is generated. Time-between-events at very low speed is long enough to completely fill the cylinder even if the throttle isn't fully open (see also: Hatfield/McCoy-type feuds over whether the throttle should be blocked open or not during a compression test). And what's the TP during cranking - 15-20%? That's a lot of air relative to cranking speed.

[Cringer]

I know this is really about how that table works, but remember that at cranking speed everything is happening slowly enough that actual volumetric efficiency will be very near 100%, because not much if any vacuum is generated. Time-between-events at very low speed is long enough to completely fill the cylinder even if the throttle isn't fully open... And what's the TP during cranking - 15-20%? That's a lot of air relative to cranking speed.

That is not the problem...as much time as you have to fill the cylinder, you also have all the time in the world for the piston to travel past BDC and push air back out while waiting on IVC on that aftermarket cam. And to overcome that problem, you don't need time, you need air velocity (and that comes with RPM, which you don't have during cranking). Air is a spring you know. So no, it is not going to be 100% or that of an OEM cam.

(see also: Hatfield/McCoy-type feuds over whether the throttle should be blocked open or not during a compression test)

Well don't you have quite the memory on you. I suppose in a world of professional mechanics working against book time, it does not behoove them to do thing in certain ways. BTW I still take the time to pre-fill my oil filters with oil during oils changes. I suppose everything falls on a good-better-best spectrum though.

[blindsquirrel]

'Dynamic displacement', then. The swept volume between IVC and TDC.

Why does the engine make barely any vacuum at cranking RPM? What is the volumetric efficiency when manifold pressure is equal to baro? Anything lost between BDC & IVC is relatively small in comparison to the ease of filling the cylinder with atmosphere. Or should I delete my post in the service of not spreading bad info?

So you think the only basis for an argument against holding the throttle at WOT for a compression test is only because some people are lazy and/or working on flat rate? Have you personally tested the difference in readings on an engine you're familiar with? And most important, the purpose of a cranking compression test is to compare all the cylinders, not to get a specific number. If there is any difference between throttle open/throttle closed, it will affect all cylinders the same, which is not an issue with a relative, not absolute, test.

[Cringer]

'Dynamic displacement', then. The swept volume between IVC and TDC.

Why does the engine make barely any vacuum at cranking RPM?

Because there is little to no airflow actually going through the motor! That is my point.

What is the volumetric efficiency when manifold pressure is equal to baro?

Less than 100% since the dynamic displacement is less than the swept volume. That is my point.

Anything lost between BDC & IVC is relatively small in comparison to the ease of filling the cylinder with atmosphere. Or should I delete my post in the service of not spreading bad info?

Well that is your claim. Can you actually prove that?

So you think the only basis for an argument against holding the throttle at WOT for a compression test is only because some people are lazy and/or working on flat rate? Have you personally tested the difference in readings on an engine you're familiar with? And most important, the purpose of a cranking compression test is to compare all the cylinders, not to get a specific number. If there is any difference between throttle open/throttle closed, it will affect all cylinders the same, which is not an issue with a relative, not absolute, test.

I have done a lot of compression tests on small engines, either pull start or kick start with carburetors. I can tell you that wearing yourself out manually powering the crank and watching the needle move is tiring and provides slow and inconsistent results. Go WOT and the needle moves quickly and you get consistency. And if you are lucky enough to have a battery and electric start you are just adding wear and tear to both items. Yes I know the ECM is going to crack open the throttle and perhaps negate most of my concerns.

Sure, if I had 8 compression gauges hooked up then you are correct about throttle effecting all cylinders and I could do one single test. However, I am but a wee peasant and can only afford a single compression gauge, thus I must perform 8 individual compression tests. But at least by loosening the bolts on the TB, for the few seconds I am cranking the engine, I know I can eliminate one variable from my results.

[Cringer]

'Dynamic displacement', then. The swept volume between IVC and TDC.

Why does the engine make barely any vacuum at cranking RPM? What is the volumetric efficiency when manifold pressure is equal to baro? Anything lost between BDC & IVC is relatively small in comparison to the ease of filling the cylinder with atmosphere. Or should I delete my post in the service of not spreading bad info?

So you think the only basis for an argument against holding the throttle at WOT for a compression test is only because some people are lazy and/or working on flat rate? Have you personally tested the difference in readings on an engine you're familiar with? And most important, the purpose of a cranking compression test is to compare all the cylinders, not to get a specific number. If there is any difference between throttle open/throttle closed, it will affect all cylinders the same, which is not an issue with a relative, not absolute, test.

And now it is my turn to cross examine the witness, so Moister "blindsquirrel", if that is indeed your real name...

According to what you are saying, all things being equal...

1) Any cam on any particular engine will yield the same cylinder airmass during cranking.
2) This means that for a cam swap you would (a) NEVER need to tune any part of start up and that (b) OEM and aftermarket cams (stage 1 through stage 69) will all start the same.

And if we accept what you are saying is true...

1) The VE/vacuum of all cams are the same during cranking
2) Yet somehow the VE/vacuum of all cams are different at idle
3) Yet somehow the VE of an aftermarket cam exceeds that of an OEM cam at high RPMs

Can you please explain how that is possible?

[GrannySShifting]

Little changes in this table have a big impact on the calculated Cylinder Airmass (g) value.

That you are correct on, cranking VE 40 or 130 resulted in gms/cyl of .30 to .90

Doesnt really matter much as the fueling is right out of that calc and timing is lock solid out of the cranking table 9 degrees at .30 or .90

[Cringer]

That you are correct on, cranking VE 40 or 130 resulted in gms/cyl of .30 to .90

Doesnt really matter much as the fueling is right out of that calc and timing is lock solid out of the cranking table 9 degrees at .30 or .90

What do you mean "doesn't really matter much"?

Cylinder Airmass controls fueling. MAF and VVE are flow rates (grams/sec) and the ECM takes those inputs into an algorithm to create a final airflow value, which is Dynamic Airflow which is also a flow rate (grams/sec). And then performs a calculation to determine the airmass in the cylinder, which is a scalar value (grams). The stoichiometric ratio is based on scalar masses (not flow rate).

Assuming E0 gasoline at 14.68:1
.30 grams per cylinder = 0.0204 grams
.90 grams per cylinder = 0.06130 grams

So now the ECM determines how long does it need to open the injector to spray that many grams of fuel. Which is why fuel injector characterization is so important and underpinned by the fuel system.


How to convert [units of mass/sec] to [cylinder airmass in unit of mass] (note that 4 will be the constant value used for an 8 cylinder 4 stroke engine)
Cylinder Airmass = Dynamic Air * 60 / 4 / RPM

If Dynamic Air is in grams/sec, then Cylinder Airmass will be in grams
If Dynamic Air is in lbs/fortnight, then Cylinder Airmass will be in lbs

[mbray01]

Despite all th back and forth here, my belief is that the need for this table is a direct correlation of map almost perfectly matching baro during cranking, not due to low vacuum, but due to the low "signal" the map sensor sees, as the intake volume fills just as fast as cylinder volume from slow cranking speeds. Just changing the ve table would correct start up, but would also affect transients during idle to wot transitions.

[abc]

Just changing the ve table would correct start up, but would also affect transients during idle to wot transitions.

I don't believe this could or does happen. This would mean the engine would have to run near the cranking speed or the crank to run number would have never been met. I'm not sure how you drew a line from cranking VE to drivability transients?

The need for the table is that the air behaves different at the very slow speed of cranking compared the much higher speeds of idle and above, which I believe you to know that.

[GrannySShifting]

What do you mean "doesn't really matter much"?

I meant in terms of timing, the gms/cyl value is not affecting what the timing is during cranking, its directly looked up in the Cranking timing table. It was same timing with both gms/cyl being reported

[mbray01]

i don't believe this could or does happen. This would mean the engine would have to run near the cranking speed or the crank to run number would have never been met. I'm not sure how you drew a line from cranking ve to drivability transients?

The need for the table is that the air behaves different at the very slow speed of cranking compared the much higher speeds of idle and above, which i believe you to know that.

im not advocating to do it this way, as this is wrong. Im simply stating this as more proof that the start up map is a modifier. Try it one time, you will see, you can correct an overly rich startup in that firs column , 100kpa cell, however, if you are running a stock idle rpm, it will affect transients.

[Alvin]

I've always taken this as a percentage of GMVE. That's why people who don't tune VVE will need to use numbers like 60-80% and people who do will leave it at 100%

I don't think i've ever seen a stock cal not be 100%

[04silverado6.0]

I have found the magic number to be 92-95 on cranking ve. I see lots of people setting them to 80-85. I use 95 on truck cams and 92 on real cams. I never need to make any adjustments beyond that.

[verlon]

Cranking VE is not a percentage of main VE. It is straight VE.

CrankingAirmass = [Cranking VE] * [Cranking VE Baro Mult] * [Cylinder Volume] * [3484.48 mg*K/L*kPa] * MAP / [Cranking Charge Temp]

[Cringer]

Cranking VE is not a percentage of main VE. It is straight VE.

CrankingAirmass = [Cranking VE] * [Cranking VE Baro Mult] * [Cylinder Volume] * [3484.48 mg*K/L*kPa] * MAP / [Cranking Charge Temp]

Fantastic info! Apparently the forum will not let me thank you again officially.

verlon the man the myth the legend.jpg

Edit I ran some numbers and came up with this result, which seems reasonable.
VE%: 0.8
BaroMult: 1.0
CylVol (L) 0.775 (6.2L V8)
Const (mg*K/L*kPa): 3484.48
MAP (kPa): 98
MAT (K): 299.8 (= 26.6* C = 80* F)
VE Airmass (mg*K/kPa): 706.1941

[daktah]

If I wanted to make my car start a little easier as in less aggressive than factory so I won't disturb my neighbors firing it up at 5:00 a.m. is cranking VE the way to do it? Ideally I would really like it if it starts kind of like old carbureted cars do where it doesn't flare at all and doesn't really exceed 1000 rpm before idling, if at all possible and practical.
I've already tried messing with crank spark and flare control but they don't help in this case because even if you set the whole table to 20* flare control will just clobber it back down low or negative (which is what makes it so loud) and if you zero flare control then the engine will rev up and hang at 2-2.5k for a second or two which is slightly less loud but still loud and far from ideal.