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The two middle columns are data, ignore the other columns as they are random numbers from sensors that aren't hooked up.
These are absolute manifold pressures in bar.
At the peak power around 6400 rpm and little under 10 psi boost, the absolute pressure ratio is 1.19x exhaust/intake which is pretty good. It's not too different from a garden variety normally aspirated production car with stock muffled exhaust. So that's good.
Interestingly, at lower rpms and boost levels, this system is now operating at the cross-over pressure ratio, that is <1. Only when the wastegate starts bypassing significant flow, then the exhaust manifold pressure has to increase above the intake manifold pressure. My hope and expectation is that as we increase the boost level, we'll by bypassing less from the wastegate and hopefully bringing down the high rpm exhaust/intake manifold pressure ratios significantly below the current 1.19x.
Here's the graph from 552hp 6200rpm run. The graphs show torque and hp, then pressures and the pressure ratio, and then torque and hp scaled by the intake manifold pressure (to 1 bar intake manifold pressure):
Don't you like the flatness of these torque curves? Especially the one that is normalized to 1 bar manifold pressure?
We now have some data on turbo blow-by flow rates.
Basically, the turbo center housing rotating assembly is sealed from the compressor housing and turbine housing with rings that resemble piston rings. At least for the dual ball bearing turbos, they are designed to work only in situations where the center housing oil drain pressure is below the compressor housing and turbine housing pressures. That's why unobstructed oil drain is really important. If the pressure builds up, the turbo starts pushing oil out to first compressor and then to turbine.
The center housing drain being at low (basically atmospheric) pressure and the compressor and turbine housings being above atmospheric pressures means that some gas wants to flow past the sealing rings to the center housing oil drain. The sealing rings do this very well, but not perfectly. This is commonly called turbo blow-by. The magnitude of the turbo blowby depends on the turbo center housing and the intake and exhaust pressures.
With some measurements, we've calibrated a very simple model for the quantity of this blow-by: Turbo blow-by per turbo in liters per minute is approximately equal to 4.1 sqrt(exhaust manifold pressure in psig). At 13 psi exhaust back pressure, we're looking at about 15 l/min turbo blow-by per side.
What this means is that if you are actively scavenging the turbo oil sump, then you better vent the sump because what comes out of the turbo oil drain is mostly air (by volume). By our guesstimates, it's in the above case 1.4l/min of oil and more than ten times that in blow-by gas flow.
Strangely, I'd kinda figured this when I put mine together. The turbo "sump" has an 8 mm hose venting up to a plastic inline fuel filter and thence to atmosphere.
You are making me wonder if 8 mm hose is enough though?
Strangely, I'd kinda figured this when I put mine together. The turbo "sump" has an 8 mm hose venting up to a plastic inline fuel filter and thence to atmosphere. You are making me wonder if 8 mm hose is enough though?
Since you're not draining oil down that line at the same time, it should be large enough. Unless the sump construction is such that oil foam can make it to the line; in that case, you might want to step it up to a larger size.
This is supposedly from an old factory effort to power a 962 with a twin-turbo 928 engine. Copied from a Pelican Parts BBS thread:
I am wondering whether they were trying to run a short-stroke flat plane crankshaft in that motor, with a cross-plane crankshaft the two independent intake plenums and single-scroll turbos aren't what I would've selected given all that space.
Twenty plus years ago, I was involved in a project to duplicate this effort (Porsche V8 in a 962). The person taking on the effort did research and said this was an effort started by Kremer (935 fame) to "modernize" the 962 platform using a Porsche V8 based drivetrain.
From the copy cat effort we started, we got so far as to get a 928S4 engine fitting behind the 962 cockpit bulkhead while retaining the 962's wheel base. It was a tight fit length wise. There was plenty of room for the turbos and plumbing. Weight was a concern...
Engine was to be fitted with a turbo on each side feeding a plenum on each side, very similar to the flat six used in the 962's.
I'm not so sure that the factory was always as smart as we give then credit for.....I for one would never use long runners to the turbos as they have done. The plenums look to small and I would have thought the optimum inlet runners would be slightly off vertical?
As John says on the video, the hot idle hasn't been tuned yet. All the sensors aren't hooked up yet, either.
The next step is to change the injectors, button up the crankcase breather to the planned configuration, and tune the idle for the 80 lbs injectors.
This exhaust doesn't have enough muffling for the higher-overlap cams and free-flowing turbines. It was fine by John's taste with smaller hot sides and stock cams, but even he says that with this lower restriction engine we're going to need to do another exhaust. The brain trust is on it! ;-)
Twenty plus years ago, I was involved in a project to duplicate this effort (Porsche V8 in a 962). The person taking on the effort did research and said this was an effort started by Kremer (935 fame) to "modernize" the 962 platform using a Porsche V8 based drivetrain. From the copy cat effort we started, we got so far as to get a 928S4 engine fitting behind the 962 cockpit bulkhead while retaining the 962's wheel base. It was a tight fit length wise. There was plenty of room for the turbos and plumbing. Weight was a concern... Engine was to be fitted with a turbo on each side feeding a plenum on each side, very similar to the flat six used in the 962's.
Good to hear from you, Jim.
The space for the turbo system there in that 962 chassis is truly luxurious. Those 962 turbos had some interesting features. For example, some versions seem to have used a first-stage liquid intercooling that run on the same coolant (based on pictures) that the cylinder heads used. And for a six-banger, the single scroll and separate plenums made all the sense in the world. A cross-plane V8 is a different animal though.
There's so much to learn about these turbo systems. Every time I think I get it, something new comes up and I realize I didn't really fully get it. For example, right now I'm trying to understand whether the turbine downpipe length has any tuning impacts. I thought it wouldn't matter at all, but maybe it does affterall make a difference to at what rpm the turbine spools?
Originally Posted by Roy928tt
The idea worked for Sauber Mercedes.... I'm not so sure that the factory was always as smart as we give then credit for.....I for one would never use long runners to the turbos as they have done. The plenums look to small and I would have thought the optimum inlet runners would be slightly off vertical? Geez it would have sounded good though!
As Jim said, this I think was originally a Kremer effort and I think factory didn't have the budget and/or desire to get involved. Did Sauber Mercedes run a cross-plane V8 with a twin turbo?
Originally Posted by V2Rocket
yikes... mind if i borrow your torque curve? i have a cylinder head to check for straightness.
I don't think those "levels" aren't being made anymore... Too bad.
Just wondering if you've had to deal with headlift and if so, what precautions you've taken on this build against it? We have to employ all sorts of expensive modifications in our racecar motors and still every 4 cylinder version that I know of (ie under much longer sustained max peak cylinder and boost pressures) sustains headgasket and headlift issues. I suspect block flex to be one of the key causes and therefore assume that you guys with the V8 blocks don't suffer this to any great degree? I've seen you on some of the high hp i4 threads in the 944turbo section and thought you might have your theories.
Are you using stock headstuds & headgaskets? We certainly are not. I say 'we' to represent the broader modified boosted i4 collective. I'd say most of us have issues boosting much over say 22 -24psi on the racetrack. Also very impressed that you've seen ~ 540hp at what, 11psi?? Is that at the wheels? That seems amazing to me. I suppose we need to double our boost levels for the similar power figure despite a lot of other modifications to our motors. I don't get the impression that you've gone overboard with massive ports, valves, cams etc on your motors? Anyway, I'd be interested in your experience with these issues.
Just wondering if you've had to deal with headlift and if so, what precautions you've taken on this build against it? We have to employ all sorts of expensive modifications in our racecar motors and still every 4 cylinder version that I know of (ie under much longer sustained max peak cylinder and boost pressures) sustains headgasket and headlift issues. I suspect block flex to be one of the key causes and therefore assume that you guys with the V8 blocks don't suffer this to any great degree? I've seen you on some of the high hp i4 threads in the 944turbo section and thought you might have your theories.
Are you using stock headstuds & headgaskets? We certainly are not. I say 'we' to represent the broader modified boosted i4 collective. I'd say most of us have issues boosting much over say 22 -24psi on the racetrack. Also very impressed that you've seen ~ 540hp at what, 11psi?? Is that at the wheels? That seems amazing to me. I suppose we need to double our boost levels for the similar power figure despite a lot of other modifications to our motors. I don't get the impression that you've gone overboard with massive ports, valves, cams etc on your motors? Anyway, I'd be interested in your experience with these issues.
This is Tuomo.
The answer is that there's no replacement for displacement! Well, actually there is: the turbo. But once you got the turbo... there's no replacement for displacement! The 928 S4 has a 5.0 liter motor with about 622cc cylinder, and we've got two turbos on it. At ten psi, our motors are probably less stressed than some 951 motors from the factory. A 500 rwhp 16v 951 would be about as stressed as 1000 rwhp 32v turbo 928, and were not there yet.
The blue engine has stock thicker replacement head gasket, stock head bolts, stock open deck, and stock 4th casting revision head with some porting. We don't anticipate any problems with this setup as long as we control the knock and temperatures.
02-05-2017, 06:58 PM
Intake and exhaust manifold pressures
