Ark Posted July 18, 2006 Share Posted July 18, 2006 OK chaps, I've been wondering: why is 1.2 bar considered the maximum safe boost for stock twins? Before anyone leaps in and answers, consider this: In order to raise the twin-boost to 1.2 bar, the first turbo is also raised and makes 0.74 bar by itself. So why isn't the maximum twin-boost 1.48 bar? Quote Link to comment Share on other sites More sharing options...
Ian C Posted July 18, 2006 Share Posted July 18, 2006 Because 0.7bar at 4000rpm isn't the same amount of airflow as 0.7bar at 6500rpm. -Ian Quote Link to comment Share on other sites More sharing options...
chilli Posted July 18, 2006 Share Posted July 18, 2006 beyond 1.2 the blades of a j-spec ceramic turbo are in danger of shattering. when they are both on they share the load anyway equally see what you mean about when just #1 is on (mine makes .8 bar with just #1 online!) but although the pressure is high the flow is lower = lower turbo rpm, lower rpm = lower centrifugal forces on the blades... Quote Link to comment Share on other sites More sharing options...
normore1 Posted July 18, 2006 Share Posted July 18, 2006 Pressure is not additive. In parallel mode each turbo sees the full 1.2 BAR. This means that the blades on the output side are pushing against ~17.6 psi of pressure this translates into a fair amount of force on the blades and quite a high level of resistive torque on the turbine shaft. There is a danger of twisting the shaft of the turbine at higher boost levels at least with the UK/US turbo. I never heard of the issue with the ceramic blades shattering but I am less familiar with J-Spec. Reports indicate that above 1.2 BAr the turbines become quite inefficient (creating more heat than pressure) anyway. Quote Link to comment Share on other sites More sharing options...
JohnA Posted July 18, 2006 Share Posted July 18, 2006 Pressure is not additive. In parallel mode each turbo sees the full 1.2 BAR. while this statement is true at a high level (i.e. boost pipes), it is nowhere near the truth at 'blade' level This means that the blades on the output side are pushing against ~17.6 psi of pressure this translates into a fair amount of force on the blades and quite a high level of resistive torque on the turbine shaft. which blades? turbine or compressor? Anyhow, I can't think of either pushing against this. Exhaust blades see a lot more pressure/heat, and compressor blades see a lot less. .. There is a danger of twisting the shaft of the turbine at higher boost levels at least with the UK/US turbo. Reminds me of "twisting the chassis" in F&F:d This can happen at any boost level, if surge conditions are allowed to develop for long enough. I never heard of the issue with the ceramic blades shattering but I am less familiar with J-Spec. They are glued to the shaft (with special glue ofcourse!) and pop off at will:Pling: Reports indicate that above 1.2 BAr the turbines become quite inefficient (creating more heat than pressure) anyway. Turbines remain efficient at way over that pressure. It is compressors that lose efficiency when airspeed across the blades exceeds their design range. Even then, boost pressure is not the factor, it is shaft speed and inlet temps (after the airfilter) that are the problem. Boost is only loosely connected to the issue, airflow is much more relevant. Quote Link to comment Share on other sites More sharing options...
Jake Posted July 18, 2006 Share Posted July 18, 2006 I never heard of the issue with the ceramic blades shattering but I am less familiar with J-Spec. They are glued to the shaft (with special glue ofcourse!) and pop off at will:Pling: Isn't the wheel and shaft a one piece ceramic design then? I thought the shaft was ceramic too. (not sure, just ISTR) Quote Link to comment Share on other sites More sharing options...
JohnA Posted July 18, 2006 Share Posted July 18, 2006 ceramic shaft? what ceramic would sustain this sort of shearing forces? Alloy wheels are fixed to the shaft via a process of cold welding (a *lot* of pressure all of a sudden) Ceramic wheels use proprietary glues, which are a closely kept secret. This is the real reason aftermarket 'refurbishers' fit steel wheels, they cannot fit the other ones;) . Quote Link to comment Share on other sites More sharing options...
wkdtime Posted July 19, 2006 Share Posted July 19, 2006 Pressure is not additive. In parallel mode each turbo sees the full 1.2 BAR. This means that the blades on the output side are pushing against ~17.6 psi of pressure this translates into a fair amount of force on the blades and quite a high level of resistive torque on the turbine shaft. There is a danger of twisting the shaft of the turbine at higher boost levels at least with the UK/US turbo. I never heard of the issue with the ceramic blades shattering but I am less familiar with J-Spec. Reports indicate that above 1.2 BAr the turbines become quite inefficient (creating more heat than pressure) anyway. Thats a little strange mate, i was always under the impression that runnning the turbos in parallel helps prevent the shaft from twisting....isnt this why big twins run in parallel?? Quote Link to comment Share on other sites More sharing options...
tbourner Posted July 19, 2006 Share Posted July 19, 2006 I hope we don't have any pedantic nit-picking in this thread!! eh Jake? Quote Link to comment Share on other sites More sharing options...
Ark Posted July 19, 2006 Author Share Posted July 19, 2006 Ah-ha! So it wasn't such a stupid question after all! Quote Link to comment Share on other sites More sharing options...
Ian C Posted July 19, 2006 Share Posted July 19, 2006 Pressure is a function of airflow. The turbos flow a load of air down the intake system, and it builds up against the back of the valves to a certain pressure. The faster the engine goes the more often this pressure is alleviated by a cylinder taking in air, so the more air the turbos have to flow to keep the intake system pressurised. When both turbos are running they both 'see' 1.2bar but are flowing half the air volume necessary to generate that pressure. If only one turbo was running it'd have to flow twice as much air volume, and that's beyond the capability of that sized turbo. I haven't heard of a 1.2bar turbo failure for a while, so I think they can take it more often than not. Considering how many people on here are running this level of boost. -Ian Quote Link to comment Share on other sites More sharing options...
osso Posted July 19, 2006 Share Posted July 19, 2006 pussies! you should all be running at 1.4 bar, or 1.6 bar as Dude would say So long you have an uprated fuel pump and adequate cooling you'd be asking for trouble running at high boost in this hot weather! then you'll have to worry about detonation and other things that might cause the engine to blow up lol Quote Link to comment Share on other sites More sharing options...
Jake Posted July 19, 2006 Share Posted July 19, 2006 ceramic shaft? what ceramic would sustain this sort of shearing forces? It was on http://mkiv.supras.org.nz/specs.htm I saw it. It says: The Japanese twin turbo uses ceramic (exhaust side only) shafts which can be replaced with no modifications by the steel CT20A (large version) turbine shafts. These steel shafts are found in 2JZ-GTE Aristo's and some turbo diesel Toyotas. I don't know if it's correct but I remembered seeing it somewhere. Admittedly it doesn't say they are one-piece but it does say ceramic shafts are used. Which seems rather odd, as you said. Quote Link to comment Share on other sites More sharing options...
dangerous brain Posted July 19, 2006 Share Posted July 19, 2006 One of the main non-technical reasons why 1.2 bar is the highest reccomended boost on stock J-spec tubbies is that after about 1.15 bar the extra heat generated by the turbo's operating out of their design bracket starts to actually lose you power. This showed up on my Thor dyno graph. You could se that by the time it was generating 1.2 bar the power started to fall off very rapidly. So in short yup you could operate the turbo's at 1.4 or 1.6 bar but all you are doing is exponentially losing yourself power and overheating your turbo's and possibly your engine Quote Link to comment Share on other sites More sharing options...
JohnA Posted July 19, 2006 Share Posted July 19, 2006 It was on http://mkiv.supras.org.nz/specs.htm I saw it. They really mean 'wheels' by saying 'shafts' Quote Link to comment Share on other sites More sharing options...
JohnA Posted July 19, 2006 Share Posted July 19, 2006 ... So in short yup you could operate the turbo's at 1.4 or 1.6 bar but all you are doing is exponentially losing yourself power and overheating your turbo's and possibly your engine That's very true, the extra heat registers as extra 'boost' but the number of oxygen molecules doesn't go up. Centrifugal compressors are quite sensitive to the temperature and thickness of the medium they compress. Quote Link to comment Share on other sites More sharing options...
Scooter Posted July 19, 2006 Share Posted July 19, 2006 It was on http://mkiv.supras.org.nz/specs.htm I saw it. It says: Quote: The Japanese twin turbo uses ceramic (exhaust side only) shafts which can be replaced with no modifications by the steel CT20A (large version) turbine shafts. These steel shafts are found in 2JZ-GTE Aristo's and some turbo diesel Toyotas. I don't know if it's correct but I remembered seeing it somewhere. Admittedly it doesn't say they are one-piece but it does say ceramic shafts are used. Which seems rather odd, as you said. only thing about the above is that i've got some 2JZ-GTE turbo's from an Aristo and the have ceramic blades but presumably steel shafts as per above. be interesting to compare my turbo's to a J-spec supras......... Quote Link to comment Share on other sites More sharing options...
JohnA Posted July 19, 2006 Share Posted July 19, 2006 ..be interesting to compare my turbo's to a J-spec supras......... UKSpec turbos also have larger A/R ratios, so that alone will make them feel like they boost later, but keep higher boost easier. (not a lot, but measurably) Quote Link to comment Share on other sites More sharing options...
letmeshowyou Posted July 19, 2006 Share Posted July 19, 2006 presumably there's an AFR calculation to be considered aswell... Quote Link to comment Share on other sites More sharing options...
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