measured the 7a cams again

[rash]

after Audi _CQ and an other guy in germany posted that the 7A cams have other timings than often posted (inlet opening 6deg before OT rumor) I decided to measure the 7As on my own to prove it...

so i bought a second pair of 7A (because I am to lazy to disassamble my engine) and assembled them to one of my spare cylinderheads... i also bought tools to measure..

and here is the result:

Audi_CQ your result is confirmed!

I measured:

exhaust closes 2 deg before OT
inlet opens 8 deg after OT

measured at 1mm valve lift

so there is a angle of 10 deg between inlet opening and exhaust close

the difference is caused by measuring tolerance and stretched gear chain...

[JP]

Interesting so what you reckon is the full cam event timings for int and exh? As currently the most widely used is those that are identical to the MK2 16v KR cams. Which have the same published valve events but lower lift than the 7A.

[Billman]

so no overlap?

[rash]

so no overlap?

not at 1mm valve lift (as posted in many postings)

but little overlap at 0mm lift... as I said, when the inlet valve begins to open, the exhaust valve is still 0.3mm open...

[Billman]

is this crucial on a turbo engine? Do we want something like this or should we avoid it?

[quattro84]

More overlap = more energy flowing out of the cilinders to power the turbine.

[repo]

More overlap = more sensitive to backpressure
More overlap = less hp per cfm of indulged air = less efficiency
More overlap = less dynamic compression ratio

And some..

[Nik0las]

So don't change your stock cams with 7A or ..... ?

[Audi_CQ]

More overlap = more energy flowing out of the cilinders to power the turbine.

Based on what?

More overlap = more sensitive to backpressure
More overlap = less hp per cfm of indulged air = less efficiency
More overlap = less dynamic compression ratio

And some..

[quattro84]

Based on what?

Higher flow speed of fresh charge air straight into EM resulting in higher exhaust gas flow out of cilinder. So even more fresh air to take place in new combustion cycle, (scavenging effect) so exhaust energy rises.
The extra amount of fresh (dense) air spooling into EM is able to expand before turbine (mass = energy) enhancing spoolup, but would affect temperature and pressure up to a point that it'll become of a negative role.
Next to that the greater exhaust flow is reducing reverse manifold pulsation by dampening it. New BMW two stage turbocharging setups use it.

[Audi_CQ]

Higher flow speed of fresh charge air straight into EM resulting in higher exhaust gas flow out of cilinder. So even more fresh air to take place in new combustion cycle, (scavenging effect) so exhaust energy rises.
The extra amount of fresh (dense) air spooling into EM is able to expand before turbine (mass = energy) enhancing spoolup, but would affect temperature and pressure up to a point that it'll become of a negative role.
Next to that the greater exhaust flow is reducing reverse manifold pulsation by dampening it. New BMW two stage turbocharging setups use it.

Yes, but what if exhaust backpressure causes exhaust to flow into inlet instead? A lot of small turbo cars have higher backpressure then boost.

[repo]

Higher flow speed of fresh charge air straight into EM resulting in higher exhaust gas flow out of cilinder. So even more fresh air to take place in new combustion cycle, (scavenging effect) so exhaust energy rises.
The extra amount of fresh (dense) air spooling into EM is able to expand before turbine (mass = energy) enhancing spoolup, but would affect temperature and pressure up to a point that it'll become of a negative role.

I have heard this explanation before, too. Audi CQ is right, this theory wouldn't work, if there was even a little backpressure. Also, not being an *** or anything, but I find the whole explanation pretty far fetched.

I mean, If You retard the ex cam, You open the ex valve later, closer to the bottom dead center, when combustion gases have already lost their energy to turn the crank, and of course expanded to their maximum.

When You have higher degree cams, the valve is opened earlier. More early You open it, more energy You loose from crank turning in advance of turbine. If You only add overlap You simply open the valve later, loosing the most important driving force of the turbine; the tip of the exhaust pulse.

Higher degree cams naturally have more overlap. I think this is the reason for the misconception, and the fresh air expanding theory.

[JP]

There is a crossover point where overlap becomes beneficial on a FI engine setup.

At low rpm exhaust back pressure will exceed intake pressure and will be detrimental in initial spool up. Once intake pressures start to exceed exhaust pressures it becomes beneficial. As the excess intake charge that is going straight out the exhaust will help drive the turbine.
The other advantage is that there is improved fresh air quantity in the combustion chamber as more of the spend gasses have been forced out. Like pouring fresh oil into your engine whilst still draining out the old. The end effect being improved combustion, lower EGT's and being able to hold peak boost to redline on a smaller back housing than traditionally required. The spool of the smaller back housing compensating for the back pressure losses at low rpm.
Other advantages lie with big turbo application which can suffer from surge on the initial onset of high boost on an engine which is not efficient enough at low rpm to cope with it, where the overlap is stabilizing the rate of boost increase resulting in smoother power delivery without surge.

As a general rule of thumb so long exhaust pressure does not exceed more than half of the intake pressure overlap is not a bad thing on an FI motor assuming that correct exhaust port and manifold configuration is being used for the cam profile in question. i.e. a stock I5 head will not benefit from it due to the nature of the highly restrictive stock exhaust ports. Remember it doesn't matter how good the exhaust manifold is if the bit before it is the bottle neck.

[quattro84]

About working or not, it is currently used on newer engine designs with different flame front propagation, squish areas to enhance exhaust scavenge flow, high CR, and adjustable camtiming to reach highest VE's from idle RPM (that are not found on the IL5).
The reason why is obvious and previously mentioned, wanting instant boost means small turbines and therefore high back pressures and sick temperatures.
Far fetched or not, it works. It's umluft. Currently tested by different OEM's without the extra fuel.
Sure these turbo setups are a complete different ballgame compared to our IL5, but it's all possible and coming. Even umluft.

[Rusty]

This reminds me, I've got a 7A inlet to try