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

[boredom.is.me]

Woah woah woah! Slow down! I don't think the thread can handle this rapid increase of intelligence.

[Koa]

Originally Posted by himbo

Hey, can't knock this guy. He had an idea, experienced with it, and reported. At the minimum, we all learn something from the discussion in this thread. Kudos for trying and getting ideas flowing. There's something to be said about experimenting and learning things first hand rather than from a book or a forum.

I guess.. Except when he has a group of very informed individuals trying to help him from fruitlessly wasting time experimenting a concept that is already proven to be adverse to the system being experimented on.

In this case, the concept was anti-reversion and the system is our internal combustion engine.

Also note he didn't "report back" like he originally championed about during the start of the thread until I prodded him and forced his hand to admit his defeat (which we all knew would happen. Weird.)

There's a fine line between experimenting the unknown and wasting ones time.

Scientists walk that every day.

[Dimman]

Originally Posted by wparsons

So much mis-information...

First, dual variable valve timing has been around for well over a decade.

Second, without knowing the valve timing in relation to BDC, knowing how much overlap you have doesn't tell the full story. If you get overlap by advancing the valve timing you'll get more scavenging (assuming a good header), but because of the timing you won't be losing compression.

How exactly does the exhaust push the intake charge out?

You're more likely to lose intake charge (or compression) by keeping either the intake or exhaust open as the piston passes too far past BDC than by having good overlap.

Some overlap is good because scavenging (high exhaust velocity creating a vacuum in the cylinder) will pull more intake charge into the cylinder. On a turbo or supercharged engine you don't need (or want) as much overlap because you're already forcing the intake charge in under pressure and will just blow it right out the exhaust. If anything, more overlap gives worse economy and emissions because you'll lose some intake charge out the exhaust.

You might want to take a look at what the intake and exhaust cam timing actually looks like in a datalog compared to throttle and rpm...

http://www.datazap.me/u/wparsons/jul...6&zoom=193-430

http://www.datazap.me/u/wparsons/jul...7-680&mark=611

Depending on your timing and overlap, positive pressure waves from the exhaust can enter the chamber when the intake is open due to the overlap. Depending on existing pressure in the chamber the difference can be enough that flow momentarily reverses either pushing a bit of spent gas back into the chamber, or all the way to pushing fresh charge out of the chamber. This is the 'reversion'.

But conversely since waves cycle between positive and negative, with proper exhaust tuning engineers design for the opposite. Having a negative pressure wave arrive. This has the effect of exhaust moving into a lower than normal pressure which helps extraction. With overlap that leads to better intake charge filling.

So what his 'antireversion' inserts did was partly what they were designed to do which is interfere with returning high pressure waves by cross sectional change (although I didn't look to deep at them to see if the shape is actually good for that). But the two negatives it did was also interfere with the good low pressure waves as well as reduce standard flow from choking the tube down.

As people have already figured out, valve open/close/overlap timing is the better way to handle this. Opening the door to the good and slamming it shut to the bad. Tuners have dealt with this since DOHC became common (technically even on OHV cam in block design by looking at lobe separation angle when picking cams). The limit was you could tune for a power band that was good in one area but not another. VVTi/AVCS etc... have gloriously removed that limit.

[wparsons]

^^ I fully realize that, and my post was poorly worded. My real question to him was how does overlap automatically mean it's going to push the intake charge out. He makes so many gross over simplifications that end up just being flat out wrong, and other ideas that are always wrong (like overlap being better for economy).

[wparsons]

Originally Posted by Turdinator

Cam timing does have an effect on the dynamic compression ratio. It is why on NA engines when you go to larger duration cams higher static compression ratio is recommended.

This is the first link that came up in google and will give you an idea about it.
http://cochise.uia.net/pkelley2/DynamicCR.html

I don't agree with a lot of what OP says but don't automatically shoot him down because of who he is and potentially add to the misinformation. I think @Dimman was on the money.

Duration and timing aren't the same thing... it doesn't matter what the AVCS system is doing, the valve duration is fixed (by the cam lobes).

You can lose intake charge if you do dumb things with the cam timing, but that's irrelevant on a good tune.

[Tcoat]

Originally Posted by Koa

I'm reviving this thread to see where you're at on this @KoolBRZ

Thanks a lot there Koa. I had not seen this one before and just read the whole thing. I feel I am at least 8% dumber than when I started simply through thread osmosis!

[Dimman]

Originally Posted by wparsons

^^ I fully realize that, and my post was poorly worded. My real question to him was how does overlap automatically mean it's going to push the intake charge out. He makes so many gross over simplifications that end up just being flat out wrong, and other ideas that are always wrong (like overlap being better for economy).

Well it's also an oversimplification to say you will lose intake charge by keeping the intake open past BDC. With enough intake velocity (higher rpm) the charge has enough inertia to continue filling even as the piston starts moving up in the compression phase. But too slow (lower rpm) it will push out charge.

The point being, like overlap, it's a matter of when you do it making the difference between good or bad.

[SirBrass]

Originally Posted by Dimman

Depending on your timing and overlap, positive pressure waves from the exhaust can enter the chamber when the intake is open due to the overlap. Depending on existing pressure in the chamber the difference can be enough that flow momentarily reverses either pushing a bit of spent gas back into the chamber, or all the way to pushing fresh charge out of the chamber. This is the 'reversion'.

But conversely since waves cycle between positive and negative, with proper exhaust tuning engineers design for the opposite. Having a negative pressure wave arrive. This has the effect of exhaust moving into a lower than normal pressure which helps extraction. With overlap that leads to better intake charge filling.

So what his 'antireversion' inserts did was partly what they were designed to do which is interfere with returning high pressure waves by cross sectional change (although I didn't look to deep at them to see if the shape is actually good for that). But the two negatives it did was also interfere with the good low pressure waves as well as reduce standard flow from choking the tube down.

As people have already figured out, valve open/close/overlap timing is the better way to handle this. Opening the door to the good and slamming it shut to the bad. Tuners have dealt with this since DOHC became common (technically even on OHV cam in block design by looking at lobe separation angle when picking cams). The limit was you could tune for a power band that was good in one area but not another. VVTi/AVCS etc... have gloriously removed that limit.

Posts like this make the OP so very worth it. Out of stupidity comes intelligent response.

You could say I'm here for the comments .

[wparsons]

Originally Posted by Dimman

Well it's also an oversimplification to say you will lose intake charge by keeping the intake open past BDC. With enough intake velocity (higher rpm) the charge has enough inertia to continue filling even as the piston starts moving up in the compression phase. But too slow (lower rpm) it will push out charge.

The point being, like overlap, it's a matter of when you do it making the difference between good or bad.

I didn't say you will, I said you could. Subtle difference. Also note the "too far past" isn't quantified, because it varies depending on rpm, load, etc.

Originally Posted by wparsons

You're more likely to lose intake charge (or compression) by keeping either the intake or exhaust open as the piston passes too far past BDC than by having good overlap.

[KoolBRZ]

All the fools out there can be April fools today, but I'd rather learn more about my car.

OK, here's what I'm basing my overlap theories on. Just open the attachment and read what it has to say about what overlap is used for. I'm still trying to find out what degree before or after TDC each number in the table ends up being. Without that information, it's hard to determine at what point to set both exhaust and intake to stop/start at TDC. I also think the economy mentioned for overlap loses a lot to an unrestricted exhaust. I think the overlap becomes a lot more economical with the stock exhaust because the stock exhaust is so much more restricted than the stock intake that the intake charge is rolled back on itself by the exhaust being pushed up and out by the piston. Then, because the pressure in the exhaust is higher then the pressure in the intake, as the piston starts back down, some of the exhaust flows back in before the exhaust valve closes. In the PDF they call this action "internal EGR" or Exhaust Gas Recirculation.

I think this "Internal EGR" function should be eliminated in cars with FI and/or with unrestricted exhaust setups. I think that without the exhaust restriction of a stock exhaust that the economy just isn't there. I think it just doesn't have enough economy to justify the power lost by pumping losses. Furthermore, in FI uses overlap will actually allow the intake charge to be pushed right out of the cylinder into the exhaust. So, for FI uses, the Intake timing shouldn't ever open before TDC, and the Exhaust should only close after TDC at high RPM's.

[wparsons]

IMO, before trying to understand vehicle specific things, learn the fundamentals of how an engine works. You're making a lot of assumptions based on your comprehension of parts of it, without fully understanding the full system(s) at play.

You're never going to understand what the valve timing is going to look like from a single table. There are so many variables at play. Did you look at the two logs I posted? They show throttle/rpm/intake vvt/exhaust vvt. If you look at the second log you'll see that the VVT angles are different for the same rpm/throttle after a shift because engine load is different.

As for the EGR, the ONLY thing any sort of EGR system is good for is economy/emissions. For power, you don't want any partially burnt intake charge back in the cylinder, just clean fresh fuel and air every cycle. Doesn't matter what the exhaust is, or if the car is FI or NA.

[Koa]

Kool broke my trolldar.. seriously good lord. I'd have to meet the guy in person.. we're not that far away, you know. @KoolBRZ, we could drive each others' cars and compare?

[Tcoat]

Originally Posted by SirBrass

Posts like this make the OP so very worth it. Out of stupidity comes intelligent response.

You could say I'm here for the comments .

The problem arises though that the intelligent can get so mixed with the stupid that anybody that doesn't already know the right answer can pick the wrong side.
Even totally wrong info can be made to sound like the most profound facts you ever heard and can lead people astray. Ubersuber for example was great at that and could make even the most ridiculous of his "facts" seem to be correct. The results of following those instructions could however lead to disaster!

[KoolBRZ]

From the pdf, it looks like the Intake has 60 degrees of motion, and the Exhaust has 53. In RR it looks like there are negative numbers for the Intake tables only. The highest numbers I've seen are 40 for the Exhaust, and 42 for the Intake. From this I believe the Exhaust adjusts from -9 to +40, and the Intake adjusts from -20 to +45, with 0 being used for the Exhaust closing at TDC, and 0 being used for the Intake opening at TDC. If this is true, then whenever both these tables are above zero there is overlap. I believe this overlap should be reduced for unrestricted exhaust setups, and should be virtually eliminated for FI setups with an unrestricted exhaust.