VE table numbers what do they mean, question about 83 pound injectors too

I would think 108% volumetric efficiency (VE) may be a little low for a turbo car. Basically if this is true VE it means you are filling the cylinders to 108% of maximum capacity. It is telling you that if your cylinder can hold 1 pound of air/fuel you are really putting in 1.08 pounds due to the efficiency of your combination. That is what a supercharger or turbo is suppose to do right? I would think you would do better than that though. I have cells in the 105% - 108% VE range at wide open and I am naturally aspirated. My VE tables coordinate with actual dyno VE results so I think I have all the parameters in the software correct. Having VE numbers above 100 is not unusual with high performance engines. Not ever running a supercharger or turbo or seeing one on the dyno I have no idea what yours should really be at.

Although I have not had to do it, if your base tune up is fine and you switched injectors and changed your injector size in the computer it should compensate your pulse widths of the ijnjectors to maintain A/F's.

VE numbers, injector size and HP have nothing to do with each other. What it does sound like is your injectors are too small if you have a 98% duty cycle. It sounds that simple. Another option would be to increase your fuel pressure or make sue you are maintaining fuel pressure first. That would lower your duty cycle.
 
Originally posted by JDEstill


are you SURE you have 83 lb/hr injectors?

John

Yeap they were at Kinsler about 3 months ago and got new screens and
reflowed. They flow 84.5 at 45 psi.
 
Originally posted by JDEstill

The VE should hit a maximum at your torque peak.



John Estill

I would think that VE should hit a maximum at your maximum horsepower?
Anyone else have a thought on this? My car is a small 283 motor and
lacks torque but makes big horsepower to redline.

My Peak HP at the tires is 750 rwhp at 7700 rpms thru a C4 Auto and high stall converter.

My Peak Torque is 700 at 6700 rpms.
 
actually, max VE should be at max torque... max torque is the point at which you are pulling in the most air and making the most cylinder pressure per cycle... VE is all about the efficiency of a single engine cycle... hp is related to how many of these cycles you can produce in a certain amount of time... so if there are a lot more cycles (higher rpms) but each one is less efficient (lower VE, less torque) you'd still be doing more overall work (more hp)...
 
Originally posted by JeffB
max torque is the point at which you are pulling in the most air and making the most cylinder pressure per cycle...

Jeff

Thanks for the reply. Wouldn't max cylinder pressure per cycle be
at maximum boost? If I'm making 22 psi at Maximum torque (6700)
but I"m making 28 psi at 8000 rpms wouldn't there be more cylinder
pressure at 28 psi?
 
not necessarily... because at the max torque rpm (highest VE) your motor breaths the best... this is a combination of head flow, valve size, rpm, intake port length, etc. So less boost can still result in more air in the cylinder... basically the VE is a percentage of the air available that the motor actually ingests... so if you have 15 psi of boost (2 bar absolute pressure) and 100% VE you'll pull in twice as much air as your displacement (roughly)... but if you have 20 psi (2.3 bar absolute pressure) of boost and 80% VE you'd only pull in 1.84 times your displacement...
 
because at the max torque rpm (highest VE) your motor breaths the best

That makes me wonder how bad it would be to run a torque converter that actually stalls past your peak torque. My motor wants more fuel around 4800, I'm told that must be the torque peak. Converter stalls @ about 5500.

Dave what kinda timing do you run on top end with that much boost?
 
Originally posted by Bobo


Dave what kinda timing do you run on top end with that much boost?

17 degrees and the plugs are gapped at .025
 
One last question.

I'm still having a problem understanding this. It's still a bit confusing
to me. Let me try to ask this question in a way that its understood.

IF maximum VE=Peak Torque, should the numbers in the VE max out
at peak torque? Doesn't more fuel get consumed at max hp and rpm?
Wouldn't you want the maximum amount of fuel at Maximum hp/rpm/boost?
Or does VE not translate directly into how much fuel is going in.

Other words say I had peak torque at 6800. If I made my highest VE
number@6800 would it give the motor the most fuel at 6800?
 
Originally posted by JDEstill
VE = volumetric efficiency.

So it figures up the theoretical max air flow, and looks up the VE value that you gave it. It multiplies the 2 together to get the actual, real life air flow. Once it knows that, it knows how much fuel to squirt to match that air flow.

John Estill

How does it figure theoretical max air flow? Wouldn't maximum MAP=max mpa?

My car continues to make horsepower as it makes more boost (all the way to 8000)
This is tellilng me the motor is flowing more air till red line, correct?
My maximum torque is 6700 rpms though. Wouldn't I want more fuel squirted
while its making more horsepower and boost than say peak torque? If your
saying maximum VE number should be peak torque, will it squirt more fuel at 8000
than 6700?
 
Originally posted by Dave King


How does it figure theoretical max air flow? Wouldn't maximum MAP=max mpa?

My car continues to make horsepower as it makes more boost (all the way to 8000)
This is tellilng me the motor is flowing more air till red line, correct?
My maximum torque is 6700 rpms though. Wouldn't I want more fuel squirted
while its making more horsepower and boost than say peak torque? If your
saying maximum VE number should be peak torque, will it squirt more fuel at 8000
than 6700?

it doesn't figure airflow and then look up VE... it knows the current rpm and engine load (map sensor value) and uses that to look up VE... with the VE and other params like engine size, RPM, intake air temp, etc. it can calculate the amount of air that's coming in... then since it knows the injector size and your target A/F ratio it can figure how much fuel to add...

if your motor continues to make more power till redline then your boost must be going up faster than your VE is dropping off.... as far as injector pulse widths are concerned (amount of fuel per engine cycle) your max pulse width should be at max torque... but as far as max fuel quantity per unit time that would be at max horsepower (even though each 'squirt' is less fuel, there are more squirts per second so the total amount of fuel is greater)....
 
An engine's efficiency is highest at peak torque, which means that the VE numbers will be highest at peak torque. However, I think I now understand the confusing part of the issue.

Remember, there are more things than just the VE number that determine the ACTUAL AMOUNT OF FUEL being delivered to the engine. Yes, Dave, you are correct in that your engine will require more fuel at high RPM and boost levels than it will at peak torque. However, the fact that the RPM and boost levels are higher will cause the ECU to spray more fuel into the motor. The numbers here are garbage, but I think this example will drive the point home:

At peak torque (6000 RPM, 180 kPa) VE=98, ECU calculates a mass airflow volume of 123456 units. 123456 units x multiplier percentage from VE table = 120987 units of airflow. The ECU will now look at the target a/f ratio (10:1 in this example) and determine that it needs 12098.7 units of fuel.

Let's buzz the motor to 8 grand now and crank the boost. Peak hp.

We are now at 8000 RPM and 300 kPa, but VE number is only 80. Because of the high RPM and boost, the ECU calculates a whopping airflow volume of 234567 units! 234567 units x multiplier percentage from VE table = 187653 units. At the same a/f ratio, your engine now requires 18765.3 units of fuel. A lot more fuel, but the VE number is a lot smaller! The VE number is representative of how your motor flows air at a given RPM and boost level, not simply how much fuel it needs. The amount of air you are flowing is what determines how much fuel is needed.

This has been a fun little thread for the last few days!! :D

Craig
 
I agree... your number example is what i meant by "the boost is going up faster than the VE is dropping off"
 
Hats off to you guys! You answered everything I threw at you.
Thank you so much for the time to respond and answer my
questions. You made someone completely understand what
is going on now. We need to add these things to the speed pro
manual!

Thanks again


Dave
 
Ahhhhh, ding the light bulb went on full power for me finally. But, for those of us without turbos a question as it relates to your VE number with a turbo. since I would expect on a dyno a turbo to show greater than 100% VE due to the turbo itself is it because your telling the computer you have a 2 bar map that your VE values are at or under 100%? I assume this si correct but just wanted to make sure.

Great thread.
 
being one of the developers, Craig could best answer how the speedpro specifically deals with this... but here's my crack at it: if you're running 14.7 psi of boost (200 kpa absolute pressure) you should roughly double the amount of air entering your cylinders... so if all your VE entries are 100 then the computer would assume that you'll pull in twice as much air at 200kpa vs. 100kpa... if you increased the VE values as well then it would think even more air would come in (because you're saying the motor breathes better AND there's more pressure)... like if you put 200% VE at 200kpa and 100% VE at 100kpa then the computer would add 4 times as much fuel at 200kpa as it would at 100kpa, which would be too much... is that right, Craig? or is it not quite linear like that?
 
I'm actually not one of "the developers" but I'll take that as a compliment and say thanks! :D This is obviously a topic that is difficult for everyone to have the same understanding of. I'm not even sure that I have the same understanding as Lance (who IS one of the developers :) ) but here's what I can do. When I have a little extra time (as if that might even happen) I will fire up the handy dandy engine simulator and experiment with different RPM, boost levels and those types of things. Terms that we have been using here like "twice as much fuel" or "half the fuel" can be tested here simply by observing the calculated injector pulsewidth. I can play with input levels and VE numbers and let you guys know what I see and we can all form our own opinions from there.

Sound like fun?

P.S. For those who are interested, I ain't leaving this building tonight until I have the Windows help files completed. It cannot be ruled out that I will still be here in the morning smelling VERY bad.

Craig
 
I personally find it is easiest to think of VE as the percent of air that could theoretically get into the cylinder if everything was perfect. The computer can calculate the "perfect" air flow from the sensor inputs. If the boost goes up, it knows that via the MAP sensor and adjusts the "perfect" air flow accordingly. So the real air flow is some percentage of that perfect air flow, and the higher your VE, the closer you get to perfection, and the better your engine is breathing.

Like Jeff said, if you had a VE of 200%, then the computer would think the air flow was double what a "perfect" cylinder fill would be, which already takes into account the boost pressure, and the fueling would be at least double what it needs to be.

A turbo engine's VE tends to be lower than a hipo n/a or supercharged car due to the high exhaust backpressure we have. A supercharged car will have an intake pressure a lot higher than the header pressure, a turbo car is typically the opposite. With header pressure usually higher than intake pressure (example: maybe 30 psi in the headers to make 20 psi in the intake) there is a lot of exhaust left in the cylinder at the end of the exhaust stroke, which takes up room that could have been filled by fresh charge. So a supercharged car should have the advantage of more air flow at the same boost level, but the disadvantage of the power it takes to turn the supercharger being sucked directly off the crank. The power lost due to higher backpressure/lower air flow is usually less than the power lost due to turning the blower.

Anyway, you can use your VE table to evaluate changes to your car. Suppse you have a VE of 40% at cruise and 85% at WOT, and you decide to go to a bigger cam. Once you get your system dialed back in you might see that VE at cruise is now 35% and 90% at WOT. From that you can see exactly how the bigger cam has improved your engine's breathing at WOT while hurt it a little at cruise conditions. If the WOT VE drops from 85% to, say, 80%, then you know you have hurt your WOT performance with that new cam. You can evaluate any part that affects your engines breathing capability: heads, cam, turbine wheel/housing, headers, etc...

John Estill
 
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