4AGE small port --> making the ports smaller again

[ed]

stew - go easy, no vtecyo commets required - its a rasonable discussion...

[Hokey]

To overcome the problem of fuel coming back into the ports wouldn't it be smart to set your cam timing to have the valve shut as soon as the piston starts its compression stork or is it important to have a bit of overlap?

[Joshstix]

If you have enough velocity in the ports then the cylinder will continue to fill on the start of the compression stroke. When it's working perfectly this can lead to greater than 100% VE.

Closing the intake valves early can stop reversion but it is not going to help make power that's for sure.

[Hokey]

oh ok so it is trying to get a high velocity and spot on cam timing. sounds like something to muck around on a dyno with.
Cheers

[myne]

And what might suffer chronic reversion below 5000rpm might truely rock above 7000rpm.
That's the problem with fixed cam grinds. If only they could be constantly changed with total precision throughout the rev range.

[Darren]

To overcome the problem of fuel coming back into the ports wouldn't it be smart to set your cam timing to have the valve shut as soon as the piston starts its compression stork or is it important to have a bit of overlap?

We spent hours on the dyno playing with cam timing to reduce the reversion but we coudn't tune it out. Something, or maybe a combination of everything between the tip of the ram tube and the tip of the exhaust wasn't working. The little test with the ram tubes made a big difference and since then we've built a much bigger set of extractors to test if the restriction was there.

By the way, i picked up the new rams with the venturi machined into them tonight. Mucho blingo.



And just to clarify what i was saying about what reversion did to the power curve, heres a pic of one of the dyno graphs.



I really do like the idea of building the venturi into the port closer to the valves, maybe one day...

Darren.

[mic*]

And what might suffer chronic reversion below 5000rpm might truely rock above 7000rpm.
That's the problem with fixed cam grinds. If only they could be constantly changed with total precision throughout the rev range.

I agree but i think that it has more to do with static plenums than static cams. Electric soleniod style valves are the futures answer to getting rid of cams altogether... Cam shafts are archaic. Electrically controlled valves should have been acheived for mass production a long time ago.

My understanding of reversion is basically the relationship that the intake or exhaust flow has with the sound of the valve seats slamming. (EDIT: Reversion being a word used to describe a bad relationship - probably poor use of the word really. Anyway too late now) The sound pulse travels up the runner of a fixed length, then after reaching the open end is reflected back towards the valve. Given that sound is a compression waveform, there are areas of higher pressure and lower pressure. If the inlet valve for example opens as a high pressure "peak" of the sound wave comes back up the runner, filling of the combustion chamber can be significantly improved. The dynamic part is, speed of sound is constant (for simplicity) but the time between an inlet valve opening, and then opening again next time decreases as rpm increases.

Thus generally long runners are "tuned" for low rpm, and short runners for high. Dynamic runners would be better.

I have an idea that i've built a VERY crude "nonfunctional-demonstrational" prototype of. It dynamically varies the X-sectional area of the intake runner at a point to improve intake velocity at low rpm's. I am thinking the best "point" for this thing to be shoved in a intake runner would be right before the port, but i have little means to see how twould affect flow properties of the intake charge and reversion.

[myne]

Reversion is what happens when a valve closes too late.

Be it inlet or exhaust, there's an exact moment at the exact RPM when either the induction or exhaust flow stalls. At that precise moment is when the appropriate valve should be closed before the vacuum(exhaust flow) or pressure(induction flow) causes the gas to reverse direction "reversion".

It's more or less the same with opening events too. At whatever RPM you're at, there's an exact time to do everything. Sadly engines are a huge compromise and we're limited to fixed cams and fixed compression and fixed manifold tracts.

Reversion is partly what makes vehicles with aftermarket cams 'lope' on idle.

Picture this, a 304* cam with 110* centre at idle. I'll start at the combustion stroke because it should make more sense.

1) Mixture is compressed, ignited, and the piston is forced downwards.
2) Around 60* BBDC the exhaust valve opens, the exhuast starts rushing out, the piston hits BDC, and then begins its' ascent. (You lose power at this RPM because the exhaust starts to flow out before it has finished pushing the piston)
3) The piston nears 60* BTDC, the inlet valve opens. The exhaust is mostly gone, but a little bit of it starts to exit into the inlet. (you've just contaminated the inlet charge with inert gasses. It also upsets the inertia of the inlet charge)
4) TDC is reached, the exhaust is still being pushed out both ways. The inlet charge is stalled. What inertia it had is basically gone.
5) The piston starts its induction, vacuum builds up, the inlet charge begins flowing in, the exhaust flow stalls and starts to reverse into the chamber.
6) At 60* ATDC the exhaust ceases flowing into the inlet as the valve closes. The inlet charge is really gathering momentum now
7) At BDC the inlet charge is still entering.
8) Shortly after BDC the inlet charge stalls as the pressure equalises. It starts to exit via the inlet.
At 60* After BDC the inlet closes. The mixture, less than it should be, and contaminated with already burnt gasses is compressed, and finally ignited.

At higher RPM these timings make a lot more sense. The induction charge's inertia actually pushes more air in even as the piston is rising.
The exhaust charge starts leaving even while it's still pushing on the piston, and free power is realised by not having the piston hit a huge pillow of gas before the valve opens. As it leaves its' inertia creates a vacuum that starts the inlet flow happening. There's no contamination and no reversion. But that's at more like 8500rpm, not 850.


As you menitoned above, Varying the crossectional area of the inlet runner (Something TVIS does crudely) allows the inlet charge to flow FASTER. Since it's flowing a lot faster, there's a lot more inertia in it. It's harder to stop, it's harder to stall, and it's harder to reverse. It's like the narrow part in a river. It's still flowig the same, but it's bloody hard to swim against because it's flowing twice the speed.

[mic*]

Heard the word "reversion" used to describe the intake charge being stalled / partially reversed coz of aucoustics in the runner obviously in combination with nature of 4 stoke timing as you described. As per edit, not really good use of the word but it stuck in my head.

Just goes to show how far humanity is from building a truly effecient IC engine...