I've recently added headers/cats/3" catback to my car. Now from another thread on the exhaust board, I understand that since I have more exhaust flow capability, it will shift my torque and horsepower peaks up in the RPM range.

I started tuning fuel by changing my Injector Constant to 21.4# in order to get a 128 average BLM. I'm getting 128 all the way to 1800 RPM where I'll go more and more rich as the RPMs climb, across all the kPa range. Looking at the extended VE table, I see a peak in the efficiency around the same point the stock engine will produce peak horsepower.

Is it safe to assume that the VE peak coincides with the horsepower peak, and that I should shift that peak up a little in the upper RPM table? I.e., an engine is the most "efficient" pump it can be at the horsepower peak?

Thank you. I just want to understand why I'm changing this other than to lean it out a bit.

 

Actually, the shape of the VE curve should closely approximate the torque curve. Opening up the exhaust can be just as beneficial at low RPMs as high so a global shift of the curve to the right is too simplistic an approximation, particularly for a longer runner TPI that's intake restricted above 4500 RPM.

 

Now wait a minute here, The Volumetric Efficiency of the motor rises as RPM increases. The TPI system is known for getting VE over 100% between a certain RPM range, hence the Tuned Port designation. When it is over 100% VE one can see that the extra % acts as an artificial supercharging effect. I have heard that this effect is about equal to 2lbs of boost.

So if the VE follows the Torque curve which builds quickly and falls off gradually as the engine spins higher, then how can the VE curve coincide with the Torque curve? It in fact follows the hp curve, which builds gradually and drops off abruptly once the TPI manifold goes past peak VE and starts the descent.

Which peak are you trying to shift? You cannot shift the VE peak as that is not computer controlled and you cannot shift the hp peak.

Please clarify.

 

VE follows the torque curve. You need the most fuel/air at the point of highest torque ... not at the point of highest horsepower.

Tim

 

Thanks for the replies.

RedIrocZ-28 - If you look at a 3D graph of high rpm VE vs RPM vs MAP table, there's a defined peak (it's rounded) at around 3600 RPM and 90 kPA where the values surrounding that area are all in the 87-93 range (on my stock AUJP). I shifted the table from 2000 RPM up by 400 RPM, and filled in the 2000 RPM line with figures that'd keep a gentle curve. I did this assuming that a change in exhaust or intake flow would have an effect on the actual VE of the engine. It might have, but nothing as easy as shifting the table in the PROM.

I tested this shifted table last night and it helped 1600-2400 RPM below 40 kPA, but anything above 2400 RPM and 40 kPa is still very rich, in some places more than before. So shifting the table didn't really work, but it was worth a try to see what happens.

This is all for the purpose of learning and practice. Once the Stealthram is installed in a couple weeks, I'll have to start all over again.

 

No, you have max VE at peak torque.
Someone is playing with the numbers, if they are trying to pass off the TPI getting anywhere near 100% VE. A NASCAR engine is lucky to get 103-105% VE.

Changing the exhuast changes the self egr'ing of the engine, and that's the primary reason for having to change the fuel and timing.

You tune to what the engine likes, and you have to test for that. The buttometer is easily fooled. The AFR in a stock cal is 2/3s about keeping the EPA happy, and keeping the engines intact for warranty.