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Marius850R

Upgrade N/A Cams , Only Intake Cam Or Upgrade Both Int/Ex On My 850 T5-R

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Exhaust can't go back into the chamber because its getting forced out. Also, remember the turbocharger is getting pressed on by the exhaust gas but it also makes it a one way trip for exhaust flow.

It does sound like you're speaking from a theoretical perspective on these posts rather than a practical, personal experience perspective.

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In my experience more overlap in a stock or lightly modified car is not undesirable because of filling losses but rather from reversion.

The pressure in the ex manifold is higher than the pressure in the intake manifold so exhaust reverts back into the cylinder effectively creating EGR anytime there is overlap.

Moderate to heavily modified turbo engines have greatly reduce ex to intake manifold pressure differential (commonly referred to as EBR or exhaust back pressure ratio). This comes from higher flowing exhaust, freer flowing intake, larger turbine wheels, etc..

As the EBR gets closer to 1:1 more overlap can be used as the intake manifold pressure is not 'over powered' by the ex manifold pressure and cylinder back filling is minimal. This means some overlap can actually contribute to dynamic cylinder filling since the engine's fluid dynamics start to behave more like a NA system, that being small amount of exhaust back pressure relative to the manifold pressure.

Consider the following example in a modified turbo engine at WOT. Ex manifold pressure is 16 psi, and intake manifold is 14 psi. The pressure relative to atmospheric has little bearing on the equation because it is the relative differential that is important. The example here is somewhat like an N/A engine at WOT where you have atmospheric pressure in the intake manifold (14.7 absolute) and 2 psi back pressure in the exhaust (16.7 absolute). The same differential in pressure as the turbo example. Granted flow does certainly change with a pressure change but I'm only showing the correlation between the two, not an exact match. Point being, the more modified the engine the more overlap you may find helpful for increased power.

Each situation is different but the real answer is gather data. What is your engine EBR? Then you can make a proper selection on advance or overlap. Tap a line to the EGR port or manifold and run it to a boost gauge, compare it to the intake manifold boost gauge. You'll be surprised what you find. In some situations you can get what the bonneville guys call cross over, where the ex manifold pressure is lower than the intake manifold pressure, this is the holy grail of engine tuning and represents where the engine is running at it's peak VE.

For what it's worth my 18T setup (at it's worst point) has 20psi ex manifold pressure when the intake is at 14.7 psi (1.36 EBR, at best it's 1.28 EBR). That's with turbo back, intake, and angle flange 18T, stage 1 tune. car is otherwise stock.

Exactly what I was trying to get at.

Interesting results, where did you tap the manifold? post a pic if you got it. stock t5 manifold I take it?

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I don't know how much air you're going to be pushing out the exhaust side with the high exhaust manifold pressure from such restrictive manifolds/turbines. If anything, you're going to get more reversion. This is probably what kills a lot of the engines on here... exhaust gas goes back into the chamber, things get too hot and detonation starts. Not to mention the small compressors are usually out of their efficiency range, adding to the heat.

The only time I've heard of exhaust going back in is when the intake opens when the exhaust isn't closed. This can be found here

"Reversion is the flow of exhaust gasses back into the combustion chamber when the downward movement of the piston creates a vacuum in the cylinder. As we mentioned in engine tuning basics, the exhaust valves are still open when the intake stroke begins. This presents the potential for exhaust gasses to be drawn back into the combustion chamber when the piston moves down the cylinder."

http://www.custom-car.us/exhaust/header.aspx

Exhaust can't go back into the chamber because its getting forced out. Also, remember the turbocharger is getting pressed on by the exhaust gas but it also makes it a one way trip for exhaust flow.

Sorry but your wrong :blink:

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Pulled off the EGR system but left the port and connected a line to the EGR port on the ex manifold. Ran that to a boost gauge.

Done and done.

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The only time I've heard of exhaust going back in is when the intake opens when the exhaust isn't closed. This can be found here

"Reversion is the flow of exhaust gasses back into the combustion chamber when the downward movement of the piston creates a vacuum in the cylinder. As we mentioned in engine tuning basics, the exhaust valves are still open when the intake stroke begins. This presents the potential for exhaust gasses to be drawn back into the combustion chamber when the piston moves down the cylinder."

http://www.custom-car.us/exhaust/header.aspx

Sorry but your wrong :blink:

Sure, on a N/A car..

I am not even going to waste my breath until someone else can conclude how a motor works..

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Sure, on a N/A car..

I am not even going to waste my breath until someone else can conclude how a motor works..

Magic gas goes bang and the car goes vroom. :rolleyes:

Plain and simple, the right amount of overlap is good. Too much is bad, too little isn't as bad, but isn't optimal. Too much WILL result in reversion. How much is too much? As Lucky took the time to explain, it depends on your own setup, tune and operating conditions. You say that exhaust is being forced out? It seems to me that you're treating the exhaust as an incompressible fluid. Flow depends on pressure differential across the valve, not the cylinder motion. If the intake valve opens while there is more pressure inside the cylinder than in the intake port regardless of N/A, turbo or supercharger, guess what happens? :lol:

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Sure, on a N/A car..

I am not even going to waste my breath until someone else can conclude how a motor works..

OK, you're wrong and here's why. You are usually right, this is disappointing though.

Simplistically:

The piston travels up in the exhaust stroke (4th stroke of a 4 stroke motor), the exhaust valve is open and it's pushing exhaust gases out into the exhaust to meet the turbine wheel. As it reaches TDC, the exhaust valve starts to close and the intake valve starts to open. As the piston starts going down on the intake stroke, there's a point where both valves are open (overlap).

In a *turbocharged* engine, the air in the turbo manifold in most OEM-ish setups has a higher pressure than the intake manifold (as Lucky effectively described and demonstrated with firsthand observations). So, do you think the air filling the cylinder is going to come from the intake valve which has less pressure on the other side of it? Or do you think the higher pressure in the exhaust is going to cause the cylinder to be filled from the exhaust valve?

This is one reason why turbo cams, NA cams, and supercharged cams all have different profiles for similar power output goals. Turbo cams designed for OEM-ish applications pretty much always have low overlap for this reason. More extreme turbo cams where very large turbine housings and better headers are being used will have more overlap because the EBR (exhaust backpressure ratio) will be closer to 1:1.

Lucky, that is awesome information. Do you have an R manifold? I wonder if someone could check on a conical-outlet housing setup with everything else being equal.

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This thread is helping me out A LOT. I'm really glad I'm getting NA cams now, it seems like with my set up, they're going to be pretty effective.

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Lucky, that is awesome information. Do you have an R manifold? I wonder if someone could check on a conical-outlet housing setup with everything else being equal.

I do not have the R manifold on this setup now, R never had EGR so there is no existing port to tap.

It's the stock ex manifold with EGR port. Hence the easy hook up.

You could use an R and measure but you'd have to drill and tap it, not exactly a big deal.

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I do not have the R manifold on this setup now, R never had EGR so there is no existing port to tap.

It's the stock ex manifold with EGR port. Hence the easy hook up.

You could use an R and measure but you'd have to drill and tap it, not exactly a big deal.

Doh! Yeah forgot the EGR port part. I bet with an R manifold and a nice sized turbine wheel you're in the 1.1:1 range...

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This thread is helping me out A LOT. I'm really glad I'm getting NA cams now, it seems like with my set up, they're going to be pretty effective.

In short...the major fear of a real sport cam is that boost will blow through the cylinderhead without ever entering the combustion cycle. But with cams as mild as the N/A cams are, this is not even a factor. You could run double the cam that the N/A is and I still wouldn't be worried about it. You'd still see power gains, not losses.

Theses aren't even real "sport" cams anyway, they are simply "a little less suck" than the T5 cams. Nothing at all to worry about.

Plenty of the VR6 guys on boosted engines make good results from Schrick cams that have WAY more overlap and duration than the stockers.

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yeah, that's what my vw friend said, that he aftermarket cams as well and had some pretty big gains from them. with a 19T, tune, and injector, I think the cams will really add to it.

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Doh! Yeah forgot the EGR port part. I bet with an R manifold and a nice sized turbine wheel you're in the 1.1:1 range...

hmmm... just what I need - another gauge :D

I would like to do this, though - it would provide useful information when playing with the cam timing.

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The quick path to power is with data to analyze and use for adjustment.

Not that seat of the pants tuning can't be successful, just slower usually.

They used to make a single gauge dual reference (two needles) boost gauge. I ran one pre/post intercooler to watch the efficiency vs. pressure drop in my last car then switched it over to calculate EBR.

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http://digital-auto-gauges.com/dual-display-psi-tractor-and-turbo-boost-gauge-p-32.html

dual-display-tractor-turbo-boost-pressur

$250 - I'd just head to plxdevices.com and pick up two boost modules and see if I couldn't get two 'needles' to work together - otherwise it'd be the same as this setup, one number up top, one down south.

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