World's first FT86 Anti-Reversion,(A/R) Header

[RobertPaulson]

Originally Posted by KoolBRZ

It works! My theory is correct. 1 or 2 mpg difference could just be my imagination, but I just went to Portland and back and got 6 mpg higher than usual. This proves that compensation to the AVCS valve overlap should to be made when changing to an unrestricted exhaust, so that the power and economy can be brought back up.

I immediately noticed better acceleration, and low-end torque, particularly in 5th and 6th gears. Normally my car bogs down as soon as I shift to 5th, but now it accelerates smoothly through all 6 gears. Now bear in mind, my car is an auto, and both 5th and 6th are over-drive.

I now have an unrestricted intake and exhaust, so I have some more work to do to adjust the valve overlap to compensate. This does point me in the right direction though. I'll post AVCS tables if anyone's interested, just ask.

1-2 mpg could be because you drafted people longer, drove slower or had better gas. Moreover, its not normal for your car to bog down when shifting into 5th and 6th. My car has never had this problem, pre or post tune.

and please, post the tables. i'd love to see them.

[zooki]

Originally Posted by KoolBRZ

It works! My theory is correct.

I immediately noticed better acceleration, and low-end torque, particularly in 5th and 6th gears. Normally my car bogs down as soon as I shift to 5th, but now it accelerates smoothly through all 6 gears. Now bear in mind, my car is an auto, and both 5th and 6th are over-drive.

But does it turn the traction control light on in a corner bro? That's the real test? Right?

[KoolBRZ]

The modded tables are the safe tables that are used when the IAM is at or above .95. The unmodded tables are the normal tables used when there is an IAM below .95. So if it becomes unsafe to run the modded tables, it will automatically use the unmodded normal tables. The tables labeled safe are the modded tables, first Intake, then Exhaust, and the tables labeled normal are the unmodded tables.

If you run an after-market exhaust, your valve-timing might not be correctly compensating for the change in back-pressure. It depends on your tune of course.

[wparsons]

Sweet jesus you're WAY out of your league. Have you looked at what real professional tuners are doing with custom header tunes?

[burdickjp]

It seems to me that you're working from an incorrect understanding of how these things work, which is understandable, as a lot of reputable people in the industry don't understand it.
So let's look at an incorrect, but informative model of what is going on between the intake, cylinder, and exhaust, specifically during the induction phase. The induction phase begins when the intake valve opens. For engines without variable valve timing this happens some few degrees before top dead center, and the specific angle is a compromise between providing peak VE where it's wanted and having acceptable driving characteristics otherwise.
When the intake valve opens the piston is still moving up the bore and the exhaust valve is still open. This is called overlap. The intention is that the pressure in the exhaust port is lower than the pressure in the cylinder, which is lower than the pressure in the intake. This difference in pressure between is the motive of induction; it is what causes the charge to move from the intake to the cylinder. The larger the difference in pressure, to a point, the more charge is moved.
Here's where our model must make a compromise for the sake of being more informative at the sacrifice of accuracy: we're going to look at the charge in the intake as if it behaved like a solid. So imagine the intake was a Play-Doh factory.
The pressure on the atmosphere side is greater than the pressure in the cylinder, and so the atmosphere is producing a force on the intake charge. The charge has mass, and therefore inertia. It takes a small amount of time for the charge to gain velocity and begin moving into the cylinder.
What happens next depends on a few things. One of those things is how much longer the exhaust valve is open. If the pressure in the exhaust port is lower than the cylinder then there is motive for that intake charge to keep moving and go out the exhaust. In some ways this is a good thing, as it ensures there are no exhaust gases in the cylinder, but you don't want to much of this, as that's precious fuel and air. In some situations the exhaust is open long enough that the pressure in the cylinder becomes less than the exhaust port and exhaust reenters the cylinder.
All of this overlap dance depends on the pressure in the intake, cylinder, and exhaust, when the intake valve opens, and when the exhaust valve closes.
Nothing special happens between the exhaust valve closing and the piston reaching bottom dead center. The pressure in the cylinder continues to decrease which increases the difference in pressure between the atmosphere and the cylinder, which increases the motive force on the intake charge, which increases the velocity of the intake charge. When the piston reaches bottom dead center it begins rising, and pressure in the cylinder begins rising. Eventually it begins exerting pressure on the intake charge, trying to push it back out the intake, but it has velocity and therefore momentum. The pressure is slowing down the charge. If the intake valve stays open long enough the charge will stop, and then the piston will start pushing it back up the intake. If the valve is timed precisely correctly it will close when the charge stops, but before any charge is pushed back out the intake.
Some of this strange behaviour is used intentionally. Large overlaps causing exhaust gas to revert into the cylinder is how EGR is done on modern engines. Keeping the intake valve open long enough for charge to revert into the intake is how the modern Atkinson cycle is implemented.

The catch is that there's a lot more going on in this whole thing. Primarily, there are pressure waves bouncing around which can change everything. As an example, lets look at the intake valve closing. During a specific engine configuration (RPM, load, throttle position) the pressure in the intake as the valve is closing is higher than the cylinder. There is still motive force there, so we could potentially close the intake valve later. But if we change the length of the intake runner it will actually change the pressure, as the pressure in the runner is dictated by the pressure waves in the runner. What's going on when the intake valve closes then becomes dependent on the geometry of the intake and the timing of the camshaft. They must compliment each other.

So what can we do about it all? We can change the timing of intake and exhaust events. The bad news is both intake events are critical, so balancing the intake timing is difficult. The good news is that the angle when the exhaust valve opens is least important, so the exhaust timing can be used to compensate and provide the overlap necessary for a given intake valve closing timing.

Does that help at all?

[KoolBRZ]

There is a lot to understand about VVT, and I don't doubt a word of what you've said, but, my point is that by changing back-pressure with a new exhaust, that affects the dynamic efficiency of the engine. I have proved this to myself, and I believe that this change can be offset by increasing the valve overlap in areas that already have higher valve overlap in the AVCS Intake and Exhaust tables. No, I don't have any dyno runs to prove that, yet.

As you mention above, the pressure difference between the exhaust and the intake helps move the charge from the intake into the cylinders. And, this pressure difference helps move the exhaust out of the cylinders. I'm saying this pressure difference has changed, and the way to compensate for that change is to make changes to the Intake and Exhaust tables used by the ECU to control the AVCS system.

[burdickjp]

Here you are correct:

Originally Posted by KoolBRZ

I'm saying this pressure difference has changed, and the way to compensate for that change is to make changes to the Intake and Exhaust tables used by the ECU to control the AVCS system.

but "dynamic efficiency" isn't a thing, and "back pressure" doesn't correctly describe the way exhausts work on naturally aspirated engines.

[wparsons]

Originally Posted by KoolBRZ

No, I don't have any dyno runs to prove that, yet.

Until you have third party validated dyno charts, don't expect anyone to believe any of what you're talking about, especially with so much contradicting well respected tuners that are getting great results.

[burdickjp]

Originally Posted by bur****jp

It seems to me ... intake valve closing timing.

Does that help at all?

I just realized I did not cite my sources for this.

The "let's treat the intake charge as if it were a solid with inertia" model was presented to me by Kurt Stuart, a mechanical engineering doctorate student at Southern Illinois University Carbondale for an internal combustions class. It's a novel perspective which is very useful for quick-and-easy cylinder filling modeling.

When I need a starting point for anything internal combustion, A Graham Bell's "Four Stroke Performance Tuning" is a good starting point. He doesn't do a very good job of explaining things, but I'll use his numbers to plug into my models and go from there.

Gordon P. Blair's "Design and Simulation of Four-Stroke Engines" might as well be the Bible of Internal Combustion. It is that comprehensive. It also practically requires a bachelor's of science to understand, and says so.

John Heywood's "Internal Combustion Engine Fundamentals" should be in every tuner's library. It's much easier to digest than Blair's, but also isn't very comprehensive.

Charles Fayette Taylor is the father of our modern understanding of internal combustion. His biography is worth reading because it's that damned awesome. A proper library on the subject is not complete without his "Internal Combustion Engine in Theory and Practice". It is dated and most of the information is superceded by the above two authors, but fully understanding the topic should include his works.

To wrap it up; buy Heywood's book and Bell's book. If you can't get enough of those two, and have an understanding of calculus, thermo, and physics buy Blair's book. If you like old-school graphs and descriptions of how dynos were made before computers, buy Taylor's book.

[wparsons]

He's also forgetting (or thinks he knows better) that you can't effectively tune any of this for power if you're not on a dyno.

None of the theory matters in the slightest until you're actually testing the results with something more accurate than being able to break the tires loose.

[burdickjp]

Originally Posted by wparsons

He's also forgetting (or thinks he knows better) that you can't effectively tune any of this for power if you're not on a dyno.

None of the theory matters in the slightest until you're actually testing the results with something more accurate than being able to break the tires loose.

Sure, you can. You're maximizing volumetric efficiency. Don't need a dyno for that.

[Dipstik-sportech]

Stop feeding the troll. On another note, what happened to the Porsche trans axle thread?

[cdrazic93]

Originally Posted by Dipstik-sportech

Stop feeding the troll. On another note, what happened to the Porsche trans axle thread?

Under development? I really hope it bares some fruit.

[wparsons]

Originally Posted by bur****jp

Sure, you can. You're maximizing volumetric efficiency. Don't need a dyno for that.

And how do you know when you've made an improvement or made something worse?