Kev's Corrado - Project "Stealthy T"

Actually, thinking about this more, pressure is pressure. Even with the intake leak, fuel pressure should have matched boost pressure as the map sensor doesn't know any different really.

The air leak definitely wasn't helping matters, but just to be safe, I've ordered a new fuel pump and a filter!

I was gonna ask that, but I really don't understand turbos (why don't they spin up from 0rpm!!!!!) so I decided to not ask.

An air leak should make you run rich though, not lean.

They do spin up from 0rpm, they're permanently spinning whilst the engine is running I guess you mean why don't they make boost until a certain rpm / load?

Yeah right, an air leak should cause rich running, I got that reversed. I was compensating in the map for the air leak but as intake pressure exceeded, more air leaked out. Makes sense in my head but can't explain it very well, LOL! Mass Airflow measurment is an advantage here.

Thing is, cause I'm not using a load based map (it's TPS v RPM), it's a bit more sensitive to things like this. I really must finish off my MAP v RPM map. I might have escaped this problem if I ran that map!

Yeah, that's what I mean. Is it just the wastegate actuator or the spring or something? :/

yeah, a map measuring pressure should have compensated for the leak. Don't you wanna run a 3d map?

It's time, hassle and money mate..... don't have a lot of either at the moment! But I really must get up to Vince's rollers and let him loose on the DTA and FINALLY get a decent map into it!

RE turbo behaviour, it's all to do with airflow and the motor's capacity to swallow it The 'boost' pressure you see on a dash guage is actually the restriction the turbo sees as it's trying to force air into the engine.

Boost per se doesn't exist as far as turbos are concerned. It's all do with airflow in lbs/min and pressure ratio.

Have a look at this Compressor map - [You must be registered and logged in to see this link.]

Pressure ratio is the exhaust restriction. Imagine blowing on one of those kiddies windmill-on-a-stick things. Your breath is the exhaust gases and the resistance to flow the windmill creates is the pressure ratio. In other words, the harder you blow on it, the faster it will turn the compressor wheel, but there is a little more to it than that in terms of restrictions and what not.

So anyway, as you can see, the smaller the pressure ratio, the less lbs/min 'boost' you get out of it.

Low pressure ratio - part throttle. High pressure ratio - full throttle.

And also, if you look at the top of the 'island', you can see that if you exceed 4 on the Pressure Ratio, the boost you get in return is minimal, so the aim with any turbo engine is to keep the desired boost pressure in the turbo's island, which for this turbo (my 35R funnily enough), you don't want to exceed 3 on the Pressure Ratio. That's what people call the 'surge line'.

Any of this making any sense?

Yeah that makes sense, i don't really get reading those maps though. We have to do it for my Auto Engines module at the mo but it's a headache.

The pressure ratio is the ratio of normal pressure against the restricion in the turbo, so a ratio of 4 is the same as running 4 bar?

I don't fully understand them either mate

Garrett's definition of Pressure Ratio is "Absolute outlet pressure divided by absolute inlet pressure".

I understand that as:

Outlet pressure = The exhaust system.

Inlet pressure = The back pressure in the manifold as a result of funnelling 3 litres of exhaust gas through a tiny gap.

So what the chart is telling you is how much air flow you'll get out of the compressor scroll against this inlet/outlet exhaust ratio.

I guess as garrett call it 'absolute' pressure, it would indeed be in bar.

Thing is though, how many people do you know who've measured their exhaust pressure at the tailpipe?

P.S. I've always found this guy to be very understandable with his explanations of this kind of stuff - [You must be registered and logged in to see this link.]

Good news Kev, amazing how 'just' fitting a DBW throttle has caused this much hassle!

As for the pressure ratio, the unit of measurement is irrelevant as long as you apply the same unit to both inlet and outlet, PSI or bar would work fine in either case.

I would have though that inlet pressure should not be the result of pressure in the exhaust system as there is a cylinder head in the way, in fact if the exhaust side exceeds in the inlet pressure then you will get exhaust gas coming back into the cylinder when the valves overlap - this is very bad for power and efficiency. Inlet pressure should be measured in the manifold but the pressure generated is the result of the work done by the compressor side of the turbo; the pressure ratio is a measure of how efficient the turbo is (ie mass air flow produced) when compared against the mass air flow provided by the exhaust side of the engine. A turbo is a compressor being driven by a turbine, I would expect the pressure ratio provided gives a comparrison between these two through various conditions.

Another thing that is very important is the intake design, the intake on a Tornado (aeroplane again but hey ho ) gives a compression ratio of 24:1 - so for every 24 litres of air entering the intake they are compressed down to 1 litre! That is a bloody huge amount of compression, this is also why turbo intakes taper - this provides a slight pressure increase (read compression) but more importantly also accelerates the airflow.

Boost, you need to think of a turbo from both sides of the system.

You need airflow over the turbine (exhaust) side at sufficient flow to prevent stalling of the compressor (inlet) side, in this respect a turbo is identical to a jet engine. If you apply too much load to the compression side during low RPM then you will suffer a stall on the exhaust side due to low exhaust gas speed (and temperature).

Turbos are therefore designed so that they produce little boost (compression) below a certain rev range (of the turbo) to ensure you do not stall the exhaust side through excessive work(energy) required.

This is why historically turbos have both suffered from lag and been ineffective below 3-3.5k engine RPM, couple that with larger turbos needing more gas flow resulting in higher engine RPM requirements. Modern turbo design has benefitted massively from CFD modelling of the airflow over both sides of the turbo, better understanding of thermodynamics (heated gas moves faster than cold gas and by running that over a turbo turbine will both remove the heat through work done and increase the pressure) and innovations such as variable vane turbos (think of them like an aircraft propeller, the blade angle changes to provide thrust with reduced engine loading).

We use a 'reverse' turbo system to generate cold air on aircraft, the air source on all aircraft is taken from the compressor stage of the engine, as this has been compressed it is going to be very hot, using large air/air intercoolers is both heavy and in-effective so they are passed through 'cold air units' that make the hot air 'do work' therefore cooling it in the process.

A good book to get hold is called the 'Scientific design of intake and exhaust systems' It is very old and a lot of contains some serious maths that goes way over my head but it does give a good insight into the science behind intake and exhaust systems, you always have to remember that despite the fact that most of us consider exhaust gas to be a constant pressure through the system it is, in fact, a series of rapid pulses.

Yandards wrote:Good news Kev, amazing how 'just' fitting a DBW throttle has caused this much hassle!

Indeed! That's what the missus said. "It was perfectly fine before you put that new pedal in it"

I think it was coincidental though. I'm replacing the fuel pump and filter this weekend, just to outrule them. They're both a good couple of years old now.
I might also put my spare injectors in too, just to start off with a new fuelling slate and move on.

Yandards wrote:I would have though that inlet pressure should not be the result of pressure in the exhaust system as there is a cylinder head in the way, in fact if the exhaust side exceeds in the inlet pressure then you will get exhaust gas coming back into the cylinder when the valves overlap - this is very bad for power and efficiency. Inlet pressure should be measured in the manifold but the pressure generated is the result of the work done by the compressor side of the turbo; the pressure ratio is a measure of how efficient the turbo is (ie mass air flow produced) when compared against the mass air flow provided by the exhaust side of the engine. A turbo is a compressor being driven by a turbine, I would expect the pressure ratio provided gives a comparrison between these two through various conditions.

That does happen though, hence why twin scroll turbos were invented, which greatly reduces exhaust ingestion. Which ever way you look at it, as soon as those exhaust valves open, the gases are going to be met with the mother of all restrictions - the turbine scroll and wheel. It is inevitable some of the exhaust will go back into the cylinders, but not enough to dilute the air/fuel mix in most cases. This also dispels the myth about huge overlap "sending boost down the exhaust". Wasted boost can't go anywhere other than backwards because of the restriction, so if anything, excessive boost will end up back in the intake. Too much overlap would chuck the boost down the exhaust with superchargers though.

Garrett stated the PR is a ratio of inlet and outlet, but not which wheel/s! I agree though. Both wheels will see a restriction and the PR should be the two forces divided. When there is equalibrium between the two, that must be the 'efficiency zone'? This is what people talk about when running huge turbos on small engines + low boost. The turbo never gets into it's efficiency zone.

Yandards wrote:you always have to remember that despite the fact that most of us consider exhaust gas to be a constant pressure through the system it is, in fact, a series of rapid pulses.

Yep, because of the valves. It'll be interesting to see what happens to these pulses when cam-less engines finally go mainstream. Looking forward to that. Constantly variable lift and duration. Huge potential for super lean cruise and high power.

Have you noticed how noisy engines are becoming? I heard a TFSI running the other day and it's tappey as hell. I think most of that is the high pressure injection. Imagine the racket valve solenoids are going to add to that!!

Yeah there are a few folks at work with some higher mileage mk 4 goofs thay have the high pressure injection - they sound like they are spitting bottom bearings!

Efficiency in turbos is just a way of calculating the optimum range of operation - the higher the boost pressure the more heat and work the turbine has to do. This works in much the same way as accel times from 0-60, most supercars sit at the 3-4s point, the real work is done from 60-100 and there you can see the extra power that a Veyron has really making the difference.

Motor manufacturers have been chasing CV valvetrains for ages, I am not even sure that sort of setup is in use in F1 yet, I remember reading something years ago about the development hell of trying to perfect reliable CV valve gear.

You will still retain the pulses despite the valve gear being CV based as there is still a requirement to have a fully closed valve for the compression stroke - the only way to eliminate the pulse of an engine is to change the 4 stroke cycle!

Interesting! It must need a hell of a strong solenoid to push a valve spring down? I assume valve springs are still used, in case of solenoid failure?

Did approx 250 miles testing over the weekend and I think cylinder 1 overheated and scratched the bore during it's near death experience. Possibly 6 too. The usual suspects!

It's fine when cold, oil is too thick to pass through the scratch on the bore. It's fine when hot too, when the pistons have fully expanded.

It's just that intermediate warming up temp where it will intermittantly misfire, but getting the piston speed up smooths it out again.

I'll live with that until the 24V is built up. It's still fast as hell in boost though, which is the main thing, and doesn't blue smoke at all

kevhaywire wrote:Interesting! It must need a hell of a strong solenoid to push a valve spring down? I assume valve springs are still used, in case of solenoid failure?

Did approx 250 miles testing over the weekend and I think cylinder 1 overheated and scratched the bore during it's near death experience. Possibly 6 too. The usual suspects!

It's fine when cold, oil is too thick to pass through the scratch on the bore. It's fine when hot too, when the pistons have fully expanded.

It's just that intermediate warming up temp where it will intermittantly misfire, but getting the piston speed up smooths it out again.

I'll live with that until the 24V is built up. It's still fast as hell in boost though, which is the main thing, and doesn't blue smoke at all

Heh exactly the same symptoms my 16vG60 had, fine when cold rough on the edge of compression when warmed up. A quick hone would sort it out but that is a lot of work for an engine that is only ropey on the edge of idle.

As for CV valve trains, I don't know how they would implement it at all, the advantage to a valve that has variable lift of tenths of a millimeter over its operating range when coupled with good managment of that valve train are huge. I suspect some form of spring would be retained but then again there must be other wierd and whacky ways of getting the same result!

Yeah I did consider whipping the engine out and rehoning it, but as you say, it's a lot of work for an engine that's probably not far off expiration anyway. The piston slap over winter was loud enough to wake the dead

I am actually looking forward to moving onto 24V world, so this is the excuse I needed really.

And speaking of the 24V. It's done 138K. The bores look to be well within tolerance but the exhaust valve guides are periously close to VW's 1mm wear limit.

I can't be **** to source another engine, so I will just throw it together and see what it runs like. I will just drop an R32 head on it if the guides are past it and benefit from more flow.

If things were simple and just worked, I think I would lose interest

Forgot to say, the plug oiling at low piston speeds used to happen quite a lot for the first 20K of it's life, and then it sorted itself out. Maybe if I use it often enough it will do the same again, but I doubt it

Oh and I fitted the new fuel pump and filter (Audi 20V) at the weekend and the pump is a lot quieter, so I suspect the filter was getting a little clogged.

Perhaps the CV valvetrain could be ferromagnetic? Alloy heads, with Fe components? Might make them too heavy, not sure.

I still don't really understand why the VR has a filter housing thing. What ever happened to the old 4-pot stylee of a screw on one?

I'm gonna be throwing my research&developments AAA up on the forum for pennies soon. Not really sure what it's worth, think I paid more for delivery Aberdeen to Oxford than I did for the parts!

By ferromagnetic, I meant actuating the valve position with electromagnets. That kinda thing.

I think that's the problem... "unsprung mass" applies to valvetrain too, which restricts and engine's ability to rev.

Are you familiar with bike engines at all? Namely, the Ducati L twin and it's desmodronic (f'ck knows how you spell it) valves. Ducati found that springs limit the rpms too much because of valve bounce, so they did away with springs altogether! Very nifty, but apparently very un-nifty to setup and adjust.

Anyway, yeah, for revs, you need light valvetrain, but a good shout with the magnetic method!

RE the filter housing, VW were big into greenness and recyclables in those days, so the dash was made of recycled wood pulp, seat padding was made from recycled materials and the oil filter had to be a renewable element, blah blah.... only in the case of the Corrado, the filter element is still metal and paper, so what's the feckin difference?!?!

Only the Golf got a proper paper only filter element.

Went out again last night to do some datalogging and it didn't misfire once on my usual test route.

I think I am rather optimistically hoping it's healing itself. You never know. Metal rubbing on metal sometimes sorts itself out, providing the scratching isn't too severe.

Anyway, gonna drop the oil tonight and shove some fresh 10W/60 in and see how it goes. I've noticed my block is sensitive to oil quality because of the bore clearances. fuel diluted oil tends to get past the rings easily. Fresh oil seems to cure it.

I've got a few guys in line already for picking juicy scraps off my 12v engine, such as the Shcimmel head and cams etc. Honestly!

LOL! this is classic Corrado stuff - intermittent faults! I hope you do get it all fixed though and it lets you know one way or the other so you can set things up for the future

Fuel diluted oil aka blowby gases would be removed by a dry sump with three good scavenge stages...! lol

Dropped the oil last night and sure enough, heap big metal crumbs! Given the bottom of the oil container looked like I had been panning for gold, I am amazed it's A) actually running at all and B) running so well!

Looking at the size of the crumbs, imminent failure is due very soon though, but should hold up OK until I've got the 24V back together.

I'm guessing that piston will keep losing chunks until the ring grooves go, or a crack jumps through the the gudgeon pin and the whole thing implodes

either way, pics!!

Yeah, I'll get some pics up when it's stripped. I was tempted to wind the boost up a lot and give the block a great send off, but then I realised I need to keep the gearbox for the 24V and someone wants the head!

I have the head from Ziderapple's old 12V (AAA) in my garden! Stuck it up in classifieds going free. In fact, if i had some VR pistons and ancillaries I'd have a whole engine!

Righto, finally found some time around the gardening to inspect the damage.

The block itself is fine, no wear lips at all, honing still perfect, but piston 5 broke up. The other 5 are fine, but just look scruffy!

[You must be registered and logged in to see this image.]

Head took a beating too, but inconel valves are a lot harder than ally, so the pitting is just at the carbon level. Zero guide play, valve seats still in great shape. The head is in superb condition otherwise, for 60K miles. A testament to a GOOD oil. Note how clean the internal casting is. You will never see that on a poxy VW 'longlife' serviced engine.

[You must be registered and logged in to see this image.]

And in more news, the 24v I've got is just such an example of longlife servicing and it's f'cked really. Big wear lips in the bores, exhaust guides shot. Not worth bothering with.

So currently car is without engine and I have zero enthusiasm / motivation to do anything about it. Might just scrap the lot to be honest and move on.