Adam W Posted January 16, 2003 Share Posted January 16, 2003 I hope you boys don't mind me hijacking your forum for a Mk3 related post, but I reckon you'll have the best idea what's going on here. Last weekend, I got my stock intercooler replaced with a mahoossive Skyline GTR IC, with all lovely custom stainless pipework. I also have a very free flowing exhaust but that's been on there a while now. Before, with the boost controller off, I'd hit 0.8bar at 3000rpm and it would pretty much stay there to the redline, maybe dropping off slightly right at the top end. Even really loading the engine up at very high speed in top gear didn't affect the boost reading. Now however, boost starts a tiny bit earlier, and I hit 0.7bar at 2800rpm, but as I accelerate through the revs, the boost starts to climb up and up, hitting almost 1 bar at 5500rpm. This wouldn't be such a big problem on it's own, but when I have the boost controller set to high, the boost used to jump to 1.2bar and sit there nicely. Now I either hit fuel cut as the boost creeps way past that level under WOT, or I have to turn the gain of the boost controller right down and I don't get proper high boost until the top of the rev range where I used to get it bang on at 3000 rpm! The car is definitely faster even with this boost problem, but it could be faster still if I could hold a consistent boost level just under fuel cut, as I could last week! I can understand how low backpressure in the exhaust could mean the wastegate would no longer do it's job properly, I don't see how an intercooler could do this! I've checked everywhere for boost leaks, but Ireckon if I had a leak I would loose boost at high rpm, not gain it! Any suggestions gratefully received! Quote Link to comment Share on other sites More sharing options...
THOR Racing Posted January 16, 2003 Share Posted January 16, 2003 Adam, It might be the mass of the intercooler and the time taken to fill it. The turbos are on one side BUT the sensor used to detect how much boost you're getting is on the other side (throttle body input) At least it is on the MkIV, Im sure the MAP sensor with your boost controller is fitted to the throttle body as well). So as the boost increases at the turbo the pressurised air mass fills the vast intercooler but the sensor on the other side (exit of the intercooler) has yet to see this pressure rise due to the time it takes to filter through the IC. By the time it reaches the MAP sensor the pressure at the turbos has rocketed? The boost controller can now act to control the turbos but it's all a bit too late as there is again a delay in getting the feedback from the turbos (due to the IC) and it ends up cycling up and down trying to settle to a fixed boost level. So you have an underdamped control system. Now lowering the gain of the boost controller will allow this boost rise to be slower and hence the time lag of the IC becomes less significant. Hence you appear to be able to control it better. This is maybe an overdamped control system. I could be completely wrong but I'd say the mechanics of the control mechanism for turbo boost control have been altered and that's the problem. Have you tried a full retune of the boost controller? This could allow you to get back to the critically damped control system which is optimum. Regards Pete Quote Link to comment Share on other sites More sharing options...
Ian C Posted January 16, 2003 Share Posted January 16, 2003 I don't know your modded setup, Adam, but I know the MkIII as stock has a MAF sensor on the intake side *before* the turbo. However, if the boost controller has it's own MAP sensor, and it probably does, then Pete's explanation sounds reasonably fair. Also bear in mind that shovelling more (denser) air in means more exhaust gas out, so it can change the behaviour of the turbo system - faster spooling sounds like a plausible (and welcome!) side effect. One way to check is to disable the boost controller and go to stock wastegate control, see how it behaves then. If it creeps, your wastegate ain't big enough -Ian Quote Link to comment Share on other sites More sharing options...
Adam W Posted January 16, 2003 Author Share Posted January 16, 2003 THnaks for the reply Pete. I made a mistake in the post above, it's not the gain I've adjusted, it's the other one! ("Set" I think, or "ratio"). I've turned down the peak boost it's aiming for, not the speed it tries to get there, in order to avoid fuel cut. My boost controller solenoid goes inline between the port on the turbine exit and the wastegate, is that what you meant? The boost gauge reads off the fuel pressure regulator line, and the air is metered by a Karmann Vortex type airflow meter that sits upstream of the turbo (air is sucked through it). I could try turning the gain right up and the setpoint right down, perhaps that would give me a big overboost hit at lower revs which would get me closer to the boost curve I had before. I just want to make sure that there are no "mechanical" issues before I try and solve this with the electronics! Quote Link to comment Share on other sites More sharing options...
Adam W Posted January 16, 2003 Author Share Posted January 16, 2003 Originally posted by Ian C I don't know your modded setup, Adam, but I know the MkIII as stock has a MAF sensor on the intake side *before* the turbo. However, if the boost controller has it's own MAP sensor, and it probably does, then Pete's explanation sounds reasonably fair. Also bear in mind that shovelling more (denser) air in means more exhaust gas out, so it can change the behaviour of the turbo system - faster spooling sounds like a plausible (and welcome!) side effect. One way to check is to disable the boost controller and go to stock wastegate control, see how it behaves then. If it creeps, your wastegate ain't big enough -Ian The boost controller is a Blitz DSBC (older 1/4 DIN type). I've tried turning it off, and the same thing happens really. External wastegate needed? Quote Link to comment Share on other sites More sharing options...
THOR Racing Posted January 16, 2003 Share Posted January 16, 2003 I've tried turning it off, and the same thing happens really. External wastegate needed? Yes. Sounds like it. Although why adding the IC caused this I don't know. Boost creep is normally associated with the resistance within the exhaust system. Less resistance means more creep. It might be that the cooler denser air from the IC is making the engine more efficient and hence more boost than stock. But, quote "boost creep", is not normally solvable by the boost controller as it's a mechanical thing. So it may be that the IC presents a low resistance to the turbos (the exhaust is already non-restrictive you've said) and hence they spin up quicker and get to boost far earlier. The volume of the IC comes into play now as in time the IC fills up and it's resistance to air flow becomes greater (until it's full of pressurised air when it satuates to flow a constant rate of air) Now the turbo system is brought under more normal control and the boost creep as such reduces? Just thinking out aloud now...... I'm just wondering if (with Fuel Cut removed) it would overboost slightly and settle itself back down to a level below fuel cut (without electronic boost control)?? i.e. the time lag of the IC is really playing a factor ins all of this. It's got to be something to do with colder denser air, greater volume and greater time to fill it? Regards Pete Quote Link to comment Share on other sites More sharing options...
Adam W Posted January 16, 2003 Author Share Posted January 16, 2003 If I have the boost controller with a set of 1 (out of a possible 100) it's almost the same as having it switched off, and the boost can do it's thing without getting near fuel cut. What happens is, it starts at 0.7bar, and climbs to 0.9bar as you load the engine up, in a really high gear, high speed pull it almost hits 1 bar. Could it be that whatever is happening now (lets say boost creep, caused by the exhaust) was always happening before, but was negated by the breathing restriction the piddly stock intercooler caused at high boost and high rpm? Ie, the turbo wanted to spin faster and faster cos the wastegate couldn't bypass enough exhaust gases, but it couldn't force that much more air through the IC, and the two cancel out. Quote Link to comment Share on other sites More sharing options...
THOR Racing Posted January 16, 2003 Share Posted January 16, 2003 Could it be that whatever is happening now was always happening before, but was negated by the breathing restriction the piddly stock intercooler caused at high boost and high rpm? Yes that makes sense. This is sort of what I was saying but in reverse. Are the In/Out ports of the Skyline IC larger than the Stock MkIII one? To be sure I guess you'd have to measure the flow rate of both. So restricting the exhaust might be the key, after all that. Pete Quote Link to comment Share on other sites More sharing options...
Adam W Posted January 16, 2003 Author Share Posted January 16, 2003 Yeah, the skyline is 3", the Mk3 is 2". Plus the things 50% wider and 50% thicker - the reason I fitted it is because it's less restrictive! I'm going to try turning my adjustable wastegate right down (mechanically), running a really high gain and a similar set point. This should give me a nice aggressive boost curve, keep me under fuel cut on high boost, and still allow me to have a meaningful low boost setting for when it's slippery. Failing that, I'll get a restrictor ring made up or plumb in a Tial wategate or something. Quote Link to comment Share on other sites More sharing options...
Ian C Posted January 16, 2003 Share Posted January 16, 2003 All sounds about right to me A restrictor ring may be your only hope! Are you getting anything sorted for Saturday's dyno sesh, or are you just going to boost creep all over the place? -Ian Quote Link to comment Share on other sites More sharing options...
Adam W Posted January 16, 2003 Author Share Posted January 16, 2003 Nope, it's going to be a very half arsed affair I'll have a play with it and see if i can improve things a bit but I'll see how it goes. It's not gonna be a record breaking, dyno breaking full power run anyway as it's only mapped to 1.2 bar or so when the turbo and the fuel system will bo 1.6 no problem. Quote Link to comment Share on other sites More sharing options...
Ian C Posted January 16, 2003 Share Posted January 16, 2003 You can't use that as an excuse, I'm only running 1.3bar and I can do 1.6 too! I still haven't got my EGT gauge fitted so I'm not going above that - apart from when my shonky boost controller decides to go to 1.43bar like last night! Finally changed the feedback speeds and me oh my, it can hold the boost better now... So, that's two Supras going with half-arsed boost control - cool! -Ian Quote Link to comment Share on other sites More sharing options...
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