Sunday, December 17, 2006

Why tune VE?

This is a post in one of the forums, I usually don't like to duplicate information, but singular forums posts tend to get lost in the noise, so I'm posting it up here as this is something that should be very clear to everyone that's trying to tune their car.

The question was:
Does this (VE tuning) gain me anything performance wise?


This is a very good question, I'm glad someone is trying to understand what it is that VE tuning is actually about, not just how to do it.

1. Without VE being perfect, you'll never be able to find out proper timing. When computer detects the tiniest tendency to run lean, it will be very trigger happy to pull timing with usually no good reason. Thus, if your VE is on the lean side (and it usually is, after all that's what adding better flowing parts is about--flowing more air) you will get a lot of knock in random spots, and no amount of pulling timing yourself will cure it, causing your car run like poo.
2. Sudden transitions are hard to get right. Without VE being dead on, you are making it almost impossible to get right. Bad transitions cause knock, which lingers around, doesn't last just when going over the areas that aren't perfectly tuned. I've seen knock last over 5 secs. If you're a drag racer, that's diminished performance for half of your run. That's why it's also important to tune not just some of the VE, but ALL of it.
3. When your car develops a problem, you will notice it. If your VE is well done. airflow numbers will be down, knock will appear, but you know it's not the fault of the bad tune, but a result of some hardware component failing.
4. When VE is perfect, it is meaningful. If you add a part that supposed to improve engine's breathing, your VE will go up, and if you get it tuned perfectly before and after, you will know just how well the part works, and for what MAP/RPM range.
5. With perfect VE, your airmass and airflow numbers will be meaningful as well. with their close correspondence to torque and horsepower respectively, you can optimize your powerband.
6. Perfect VE enforces other tables to be meaningful as well. For example, to obtain the same proper fueling with wrong VE, you will have to hack either your IFR or PE numbers. With all of them perfect and meaningful, when you want a 12.9AFR, you can just command it in PE and it will happen, instead of taking stabs in the dark hoping that some arbitrary PE will make it happen by accident.
7. Since VE dictates airmass and airflow, everything based off such tables will work better as well. Shift tables for automatics need to know how much power you're really making. If you're lying about VE, then this power estimation is also wrong, making the transmission misbehave.

In general, VE in itself is important. A lot of other things are derived by calculations based on numbers calculated from VE. It really ends up being a domino effect. If VE is meaningful and proper, then it forces other things to meaningful and proper as well. But if you botch/ignore VE, then the bad effects will propagate, making the entire tune a major hack, making the car drive horribly, and sending the tuner chasing his tail. You pick which domino effect you'd rather experience.

In the long run it's really just easier to do it Right.

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