The '2 ton bugswatter' - back for a gentle stroke

[Jamo]

It's not just bhp, its' engine flow, which JP has made better with a better exhaust manifold and a whacking increase in turbine wheel size on the new turbo.

[Rocco]

It's not about bhp, it's about the amount of engine flow

yes like a wide and deep river

[JP]

Nail on head Jamo. Was running a ported stock wastegate on the 3079 with no issues whatsoever as that particular turbo size on a cast manifold was not allowing the engine to perform at its optimum be. Once we went up a frame size and went tubular with tial hotside the problem has gotten highlighted.

If I wanted to run 2bar all day long I could leave as it is. However it means that the egt's would be a lot higher than if I were to relief some pressure using a better flowing wastegate. Secondly part of the knock control process is to pull boost which would leave no safety net when used in a racing environment.

[diesel des]

Sorry, but if your now running a bigger and better wastgate than stock and you couldn't control boost to below 1.8 bar, then that wastgate isn't opening properly. As I say I'm not sure what its effective orifice size is but if its approx the equivalent of a 2 inch hole in your exhaust manifold and its freely venting to atmosphere, or a similar non restrictive exhaust then you would be lucky to get 0.8 bar boost.
Check its fully open or temp mechanically block it open and vent to atmosphere and try again.

An approx rule of thumb is that you will have about the same turbine drive pressure as you do boost on an efficient setup. Best case is you will have a bit less drive pressure than boost, so say 2 bar boost and 1.5 bar drive in an ubber efficient setup. If you have 1.5 bar in your exhaust manifold and no restrictions in the wastgate flow path, a 35mm hole would easily vent all the exhaust drive energy required to get your boost below 1 bar.


I'm fully aware how turbines and compressors work as I have mapped them on a gas testing rig as part of my job..

[diesel des]

Not being cheeky BTW, just trying to save you time and money.

[JP]

Don't worry I hear where you coming from I would've thought the same until doing the maths. I've covered that side already in G60rallye's thread who has the same problem. Likewise Jeffer suffered the same issue as you'll read there (last 2 pages).

[Audi_CQ]

Strange that such a large WG shouldn't keep up. I'm running 600whp with std WG and can easily set boost at 0.75bar if i want to (tested both on street and dyno). My turbo is an old T66 so maybe it needs more ex flow to boost then more modern turbos, but still...... Any restrictions in flow after WG maybe?

[Jamo]

JP reckons his head is flowing 50% more on the exhaust side than a stock head If you take that into consideration with a stroker setup, I can see why it's too small

[diesel des]

Head flow isn't that relevant, don't get me wrong it makes a few % difference, maybe 10 max. The exhaust side still just flows what went in. The mass flow of the engine is the same on inlet and exhaust when you take fuel into account. Nothing's created or destroyed.

As Audi cq says, stock wastgates have been proven to control boost easily at big power levels. So I still think their is an issue with your "upgraded" wastgate. Maybe a downgrade to stock might be interesting.

The more power you want to make the smaller the wastegate you need, untill you get to the point where none is used and all flow goes through the turbine to drive the compressor. That is the most efficient configuration btw, hence why large diesels and even the porche turbo uses none. They use vnt.

[quattro84]

And why does a VNT work like that? It's having multiple A/r turbines for a single compressor. The matching between max compressor flow versus turbine choking flow is 'off' due to the small turbine housing. Because the need for fast spool-up the rest of the engine provided turbine energy needs to be dumped in order to control turbine.

[diesel des]

The vnt is more efficient as all the flow goes through the turbine. Think of it this way, if you have a windmill enclosed in a pipe, you have the option to spin it with some air, or all the air available in the pipe. If you choose to use only some of the air then to give the windmill the same energy it must be at higher pressure than if you took the option to use all the air at lower pressure.

So, ideally all the flow is used to drive the turbine and none is wasted with a wastegate. BUT, you then need to size the turbine so it won't overdrive the compressor at peak power/ max rpm. This would mean loads of lag, then max boost appearing at max power rpm.

A compromise is to use a wastegate and loose drive power and waste energy at high rpm. This setup means the turbine is too small and hurts power as your drive pressure and egt is higher than it would of been with the correctly sized larger turbine were used.

Another, much better comprise than a wastegate is to vary the effective a/r of the turbine housing. Eg vnt. All the flow goes through a turbine so drive pressure and egt are cooler. At lower rpm you use a small ar for response and torque and at high rpm you open up the ar so you reduce drive pressure and don't overdrive the compressor.

This still isn't perfect as the turbine is bigger than optimal at low rpm, and smaller than optimal at high rpm. Still way better than a wastgate though. Not to mention the issue of compressor matching over such a wide operating range.

Anyhow, I still think there must be a issue with the wastegate used here. Or its connection onto the collector of the exhaust manifold is poor for flow.

[diesel des]

Jamo, I'm not sure what is ment by the head flowing 50% more than stock. Let's go back to basics. I'm not being patronizing by the way, bear with me here. The engine is just an air pump. The reason we port a head is to increase volumetric efficiency. The stock engine minus turbo, is maybe what 80 to 85% efficient at breathing at say 6k rpm. So what that means is that the restrictions in the head, cams, and manifolds is hurting power and flow by max 20%. So even the most perfect head job (LOL) could maybe give us 10%. The best manifolds maybe another 5%, and cams 5%. Remember, 100% is full breathing efficiency, and can only be better by pulse tuning etc, which only works over a very small rpm range and usually hurts ve at every other point in the rpm band.

So what I'm saying is that jp looks good at porting heads. The heads he does look great to me, and if I needed one done he would be at the top of my list. BUT, no porting job is can increase volumetric efficiency over 100%, so is likely to only increase power and flow by about 10%. So that means turbo sizing is effected, but only by about that 10%.

[JP]

@ Diesel Des, yes and no on the headflow front.

During the combustion cycle process not all exhaust gasses make it out on each cycle. The less overlap on the cams the more exhaust gasses are left in the combustion chamber so the cylinders cannot take in as much fresh air. The result being that boost builds quick with little exhaust flow as you are measuring what is not being used by the engine. Build it too quick and you suffer from surge as the intake can't buffer the flow enough.

If we now improve the flow on the exhaust more of the gasses can exit the combustion chamber, this in turn will allow the turbo to spin harder for more airflow. However as the combustion chamber has more space to fill this time round the turbo has to shift more air to achieve the same boost pressure level. So everything starts to work harder to achieve the same pressure levels. If there is then any form of restriction on the wastegate side of things at this increased level of flow when target boost is achieved this becomes a rapidly accelerating downward spiral in boost creep. Which will only get worse with increased rpm and flow up until the point the turbo exceeds its map and can no longer flow enough to fill the cylinders at which point it starts to drop of again unless you force it to overspool.

If we were running a supercharger the effect would be more obvious. For example on G60 engines running on a stock head we would see 18psi. Change the head for a correctly ported one leaving everything else the same we'd see the pressure drop to 7-9psi. To regain the 18psi charger to spin the charger faster by fitting a smaller pulley. With a turbo this effect is hidden by wastegate regulation and all people see is 1bar. The fact is to achieve that 1bar we are allready shifting airflow volumes lower down the rev range that is normally not seen till the higher end where the engine can swallow the extra air self regulating the boost creep. If we were to set the boost target to 2bar there wouldn't be any issue other than that there is no safety net if the ecu detects knock and tries to pull timing and boost. As it would only be able to pull timing leading to a potentially very interesting mess.

As for volumetric efficiency on a forced induction race engine these values can hit 130-140% in the mid range of the rpm band, whereas on a naturally aspirated engine figures of around 110% can be expected on a full race setup.

[Rikki Kitto]

[diesel des]

Jp, some of what we are saying is the same thing but written differently. When you are talking about exhaust gas trapped in the combustion chamber, most engine designers call that end gas. The scavenging effect you get with the correct overlap and pulse tuning exhaust and inlet doesn't exist in a turbo engine for the following important reason. On a normally asperated engine, at full throttle, on cam and "on the pipe" inlet manifold pressure and exhaust pressure are almost exactly the same, eg atmospheric pressure. But on most turbo engines at max power rpm we have way more turbine drive pressure than compressor boost pressure. This means when both inlet and exhaust valve are in overlap, you can't push out the end gas. So exhaust gas reduces inlet charge.

The only way to push this out is to have the same pressure in both manifolds and relay on scavenging or have less drive pressure than boost. That requires no wastegate and a big big turbine.

Again, without taking the turbo into effect and the effects of running at elevated pressure ve can't exceed 100% over much of the rev range, and with higher exhaust drive than boost will be below 100%.

The effect you talk about with a supercharger is because a supercharger is define as a fixed displacement machine, and a turbo is defined as a speed machine. With the supercharger, as soon as ve is increased the boost will drop as the charger can no longer keep up due to its fixed displacmemt. The engine and supercharger are both fixed displacement machines connected mechanically, so when you tweek one, eg ported head, then you need to tweek the other eg speed up the charger or run a bigger displacement charger.

The map on a turbo compressor is different. For a compressor it can turn at the same speed with a lower flow higher pressure ratio, than at a higher flow lower pressure ratio. Eg look at the speed lines on a compressor map.

I would fit a gauge to measure turbine drive pressure for some interesting results.