C4ProjectCar

While tuning my '90 Corvette's 383, I've run into a bit of a wall. At 2800-4000 RPMs and 70+ kPa, my BLMs are indicating that I should have a VE of over 100% (which I've read some people saying is very unlikely for a N/A engine, and others have said that tuned runners as are found on TPI can give efficiencies over 100%).

I didn't think this was a big problem, but I just noticed that when I commit the changes to the VE table, if I open the table back up I can see that the values over 100 are all rounded down to 99.61, giving me an irreparable lean condition in those cells.

What's my recourse here? Is there a modified XDF that allows larger VEs? Should I be adjusting something else in the BIN to compensate? Or, do VEs over 100 indicate some other underlying issue?

EDIT:
Looks like decreasing the injector flow rate will work?

bjankuski

In this case reducing the injector flow rate is one possible fix that will allow you to reduce the VE under 100%

C4ProjectCar

Is there another, potentially better fix?

RBob

iTrader: (1)

Can either make the engine displacement higher or the injector flow lower. I prefer to change the injector flow as that is what is likely off.

Note that the highest VE possible is 100%. Seems that the conversion equation is off a smidgen in the XDF file.

RBob.

C4ProjectCar

Thanks, RBob.

Is the injector flowrate necessarily off, or is it possible that the tuned intake runners coupled with my moderate cam and fairly free-flowing heads truly results in a VE greater than 100% in that range? (I've read conflicting things about this: some say VEs over 100% are only possible with the most modern engines, while others say they are easily achievable in certain ranges with older engines.)

If I'm not mistaken, I recall that changing the injector flowrate will throw off the dash MPG display. Would adjusting displacement instead have the same effect?

My best guess is that it goes to 99.61 rather than 100 because of rounding error. I've noticed input values in other scalars and tables will similarly slightly shift when changes are committed. Not sure how familiar you are with programming, but due to the way numbers are commonly stored (in binary scientific notation) an exact binary representation of a decimal number often doesn't exist, requiring slight rounding.
Note that if you enter 99.22 (the same distance from 99.61 as 100.0 is) it rounds up to 99.61.

UltRoadWarrior9

I've got a 383 fuel injected engine in the works. I'm just curious what the flow rate of your injectors are? I've chosen the 42lb/hr EV1 high impedance injectors based on some research, but have yet to start the engine or do the tuning.

C4ProjectCar

I'm still using the stock 22lb/hr injectors.
In order to get my VEs below 100, I set the flow rate in the tune to 19lb/hr (19.8 or so would have been low enough, but I wanted some headroom).

UltRoadWarrior9

I realize Corvette guys favor the TPI setup, which is very restrictive, however necessary for the hood clearance. But still, maybe your injectors are not flowing enough fuel for the bigger cubes? or is it really an XDF problem like RBob stated. I'm no expert, so I'm curious as to the fix as well... Good luck

RBob

iTrader: (1)

What you misunderstand is that the VE value is not relative to atmospheric pressure. But to intake manifold pressure. This makes it nearly impervious to barometric pressure changes. It does have a very minor affect on the VE, but only as it affects exhaust back pressure.

The instrument panel has it's own injector flow rate:


I opened a public $8D XDF and they use this for the VE value conversion:

=0.390625 * X + 0.000000, (which is 100 / 256)

If you multiply 255 by 0.390625 the result is 99.609375, same as what you are seeing. This also explains the 'rounding', which is really the resolution of the table values. It can only change in increments of 0.390625

A more accurate VE value is had by using the conversion value of 0.392157, which is 100 / 255

RBob.