Ian C Posted June 5, 2003 Share Posted June 5, 2003 Just a thought. With this talk of replacing intercoolers etc I was wondering - keeping all variables the same and changing only the efficiency of the intercooler (e.g. by putting in a big FMIC or replacing a knackered SMIC), would the pressure level in the intake manifold be lower as a result of the cooler air charge? After all, cooler air is denser, so the same amount would be at a lower pressure. I'm thinking that the turbos produce x amount of airflow due to y amount of exhaust, so if y amount of exhaust is unchanged but the air shifted by the turbo is cooler, the 'boost' pressure would be lower and yet the same amount of air would be shifted? Discuss -Ian Link to comment Share on other sites More sharing options...
Steve Cargill Posted June 5, 2003 Share Posted June 5, 2003 I think the turbo will boost to a pressure, so if the charge is denser you will get more air at the same pressure so a better charge. Same boost, more power, at least that is how I understand it. I think that is the discussion with hybrids, they are more efficient so you get more power at the same boost. Possibly....... Link to comment Share on other sites More sharing options...
eyefi Posted June 5, 2003 Share Posted June 5, 2003 wont the wastegate stay closed a little longer to keep the pressure the same? y amount of exhaust is more than enough to provide the required pressure. Link to comment Share on other sites More sharing options...
Nathan Posted June 5, 2003 Share Posted June 5, 2003 Correct. Many people cannot comprehend the fact the the same car can make more power on less boost. You seem to have cracked it. Cheers, Nathan TDI PLC edited to say I was actually agreeing with Steve....eyefi slipped one in while I wasn't looking... Link to comment Share on other sites More sharing options...
Guest Terry S Posted June 5, 2003 Share Posted June 5, 2003 Originally posted by Nathan Correct. Many people cannot comprehend the fact the the same car can make more power on less boost. You seem to have cracked it. Cheers, Nathan TDI PLC edited to say I was actually agreeing with Steve....eyefi slipped one in while I wasn't looking... High praise indeed;) Link to comment Share on other sites More sharing options...
Ian C Posted June 5, 2003 Author Share Posted June 5, 2003 The reason I ask is because I prototyped some ducting on my FMIC this week and tested it last night. The bottom of the FMIC is already pretty much sealed as it's flat on the undertray. The top has a narrow and convoluted path past so I didn't worry about that yet, but the sides merrily vent to the hollow areas behind the bumper/under the wing where the brake cooling ducts vent to a low pressure area (the wheel arch). So, rather than going through the FMIC the incoming air can cheerfully go left and right as a path of least resistance. I have ducted the driver's side only so far, just to make sure it stayed in place at, ahem, high speed, which it successfully has. However, I found that the maximum boost dropped to 1.2 to 1.24bar! Now, the only thing I've change is this one bit of ducting. I've been reliably boosting 1.3 to 1.35bar of boost previously to this, over an extended period of time and even in the recent hot weather. The drive home was much cooler than the drive out and still boost stayed at 1.2 to 1.24, even under "load" So I can't blame ambient temperatures. I'm too cynical to really believe that a simple bit of ducting, only on one side of the IC at that, has somehow given me the same airflow at 0.1bar less pressure due to improved intercooling. I guess the best test would be to test the car, then remove the bit of ducting and re-test immediately after... Take this example - two exactly the same setups, wastegates wide open (so the ECU or boost controller doesn't compensate for anything), no boost creep, same exhaust flow quantity, the only difference is that one intercools the air more efficiently than the other. Do they both have the same *manifold* pressure? -Ian Link to comment Share on other sites More sharing options...
Digsy Posted June 5, 2003 Share Posted June 5, 2003 I thikn you have to think of it in terms of mass rather than volume and pressure. Its a mass of air going in and it is mixed with a mass of fuel, then burnt to make a mass of exhaust gas. I'm pretty sure the only way to destroy mass is in a nuclear reactor so if you ignore leaks then same mass comes out as goes in. If you cool the air more in the second scenario, you could have the same exhaust mass flow with the less volume of intake air under the same manifold pressure, because the density has increased. Link to comment Share on other sites More sharing options...
eyefi Posted June 5, 2003 Share Posted June 5, 2003 its funny u should mention this. i was looking at the front mount this weekend and how to seal the sides, cos like u say the air will just exit left and right mainly. i was thinking of making an allu cover for underneath (using the stock spoiler cover as a template) and mounting two uprights on this. have u sealed the top as well? Link to comment Share on other sites More sharing options...
Ian C Posted June 5, 2003 Author Share Posted June 5, 2003 Originally posted by Darren Blake I thikn you have to think of it in terms of mass rather than volume and pressure. Its a mass of air going in and it is mixed with a mass of fuel, then burnt to make a mass of exhaust gas. I'm pretty sure the only way to destroy mass is in a nuclear reactor so if you ignore leaks then same mass comes out as goes in. If you cool the air more in the second scenario, you could have the same exhaust mass flow with the less volume of intake air under the same manifold pressure, because the density has increased. Why is the intake air the same pressure despite being cooler if the turbo is spinning no faster? If, that is, I've understood your post, because after all i'm apparently thicker than you -Ian Link to comment Share on other sites More sharing options...
Nathan Posted June 5, 2003 Share Posted June 5, 2003 Originally posted by Ian C Take this example - two exactly the same setups, wastegates wide open (so the ECU or boost controller doesn't compensate for anything), no boost creep, same exhaust flow quantity, the only difference is that one intercools the air more efficiently than the other. Do they both have the same *manifold* pressure? Yes. You would still have to have a wastegate operating to some degree though but I'd be nit-picking. Say Car 1 has a manifold pressure of 1 bar at a temperature of 100 deg. C. And say Car 2 has a manifold pressure of 1 bar at a temperature of 25 deg C. Car 2 has many molecules of oxygen available to burn than car 1, hence the possibility of more power, even though they are at the same pressure. The nitpicking comes in because if you welded the wastegates wide open, Car 2 would make more power therefore more volume of exhaust gas and therefore spin the turbos faster, which without wastegate control could get out of hand. I know you said 'no boost creep' but if one Car1 was holding one bar the same setup on Car2 wouldn't. Going back to your car, I'm not too sure why your boost will have dropped. Most boost controllers only take manifold pressure into account, irrespective of air density. Maybe you have reached the limit of something else: turbo/wategate/boost control design? Cheers, Nathan TDI PLC Link to comment Share on other sites More sharing options...
Ian C Posted June 5, 2003 Author Share Posted June 5, 2003 Originally posted by Nathan Yes. You would still have to have a wastegate operating to some degree though but I'd be nit-picking. Say Car 1 has a manifold pressure of 1 bar at a temperature of 100 deg. C. And say Car 2 has a manifold pressure of 1 bar at a temperature of 25 deg C. Car 2 has many molecules of oxygen available to burn than car 1, hence the possibility of more power, even though they are at the same pressure. I understand that but you are approaching it from a different angle, you have temperature and pressure as givens and are working out resultant air density. What I'm saying is: Car1's turbo is spinning at 100,000rpm and it's output is intercooled to 25degC Car2's turbo is spinning at 100,000rpm and it's output is intercooled to 60degC Do they both show the same pressure at the manifold? The nitpicking comes in because if you welded the wastegates wide open, Car 2 would make more power therefore more volume of exhaust gas and therefore spin the turbos faster, which without wastegate control could get out of hand. I know you said 'no boost creep' but if one Car1 was holding one bar the same setup on Car2 wouldn't. I was painting a theoreticaly picture to remove such real-world variances from the equation in order to try and understand what's going on. Maybe a better way of putting it would be - take one car, run it on the dyno at a constant load/boost pressure, and then spray a very cold airstream onto the intercooler - does boost pressure drop *at that moment* as the air going into the manifold is suddenly denser? Yes, OK, the system will probably react by winding the boost back up and adding more fuel so you end up with more power at the same boost, but at the moment of spraying the intercooler, what happens? Going back to your car, I'm not too sure why your boost will have dropped. Most boost controllers only take manifold pressure into account, irrespective of air density. Maybe you have reached the limit of something else: turbo/wategate/boost control design? You may be onto something - the boost controller I have has always been a bit of a bugbear for me, I thought you could dial in a boost pressure and it would try and hold it there in a sort of feedback loop. Instead, I have to give it a base duty cycle and a tweak to this for each gear. And whatever I set this duty cycle to dictates what boost I get so why the hell I also plug in a desired boost level is beyond me, seems pointless. It everso slightly adjusts up and down the rev range in 'self learning' mode, and even then varies about 0.05bar depending on what day of the week it is or something. Very disappointing bit of technology in my opinion. However, in this case it works to my advantage as I haven't changed the duty cycle and the dumb bloody thing isn't trying to automatically keep to 1.3bar by any stretch of the imagination. So, if I drop the intake pressure by better intercooling, it won't punt the boost pressure back up before I know what's going on. It just keeps the exhaust flowing as before, with the duty cycle I already had set up. Does that make any kind of sense?! Like I said earlier, I should do a back-to-back test and see if that gives a definitive result, as we could chunter on like this for ages All very interesting! -Ian Link to comment Share on other sites More sharing options...
Digsy Posted June 5, 2003 Share Posted June 5, 2003 Does a turbo work like a volume displacement device or is the boost pressure dependant upon the density of the air? If you replace the turbo with a supercharger with no internal compression (so it is just moving x cm3 of air from one place to another) then you have a fixed volume V of air going into the engine at a fixed pressure P, at a fixed temperature T. This gives you a mass or air M for use in the combution process. From the gas laws, PV = nRT, but nR sort or equates to mass, so you might say PV = MT (probably not strictly true but, hey). So if your intake volume is limited, V will be constant. We know you have lowered T through the imporved ducting. So to maintain the balance, either M needs to increase, or P needs to decrease. I'm not sure but I would hazard a guess that the volume of exhaust gas produced in both cases will be the same if the mass of the intake air remains the same in both cases, so if the turbo should be spinning at the same rate and M will be constant. Therefore P has to decrease, which is what you are seeing. Not sure why your boost controller isn't making up the difference though. MAYBE if you change the density of the air going through the turbo you get a different pressure out than the controller was seeing when it was set up? Link to comment Share on other sites More sharing options...
Ian C Posted June 5, 2003 Author Share Posted June 5, 2003 Originally posted by Darren Blake Does a turbo work like a volume displacement device or is the boost pressure dependant upon the density of the air? If you replace the turbo with a supercharger with no internal compression (so it is just moving x cm3 of air from one place to another) then you have a fixed volume V of air going into the engine at a fixed pressure P, at a fixed temperature T. This gives you a mass or air M for use in the combution process. From the gas laws, PV = nRT, but nR sort or equates to mass, so you might say PV = MT (probably not strictly true but, hey). So if your intake volume is limited, V will be constant. We know you have lowered T through the imporved ducting. So to maintain the balance, either M needs to increase, or P needs to decrease. I'm not sure but I would hazard a guess that the volume of exhaust gas produced in both cases will be the same if the mass of the intake air remains the same in both cases, so if the turbo should be spinning at the same rate and M will be constant. Therefore P has to decrease, which is what you are seeing. Not sure why your boost controller isn't making up the difference though. MAYBE if you change the density of the air going through the turbo you get a different pressure out than the controller was seeing when it was set up? I read this three times and it actually made sense once I crowbarred the formulae into my head instead of skipping over them hoping that they weren't relevant (come on, own up, you all did it). So theoretically it's possible that my theory of, "all other things remaining constant, cooling the intake charge lowers the boost pressure", is correct? Blimey. However, I don't trust my own theory so I'm going to test it as I mentioned and I'll let you know what the results are! As for my boost controller, like I said, if I don't feed it the right duty cycle for a given level of boost it won't attain it. With whatever is going on now I need a higher duty cycle to reach the levels of boost I had previously. It can't figure this out itself, it seems. Apexi AVCr, by the way, if anyone knows differently let me know, but I've read the isntructions a few times and played with it enough to be pretty confident it's not the best/most intelligent bit of kit -Ian Link to comment Share on other sites More sharing options...
Nathan Posted June 5, 2003 Share Posted June 5, 2003 Maybe a better way of putting it would be - take one car, run it on the dyno at a constant load/boost pressure, and then spray a very cold airstream onto the intercooler - does boost pressure drop *at that moment* as the air going into the manifold is suddenly denser? Right, the following could be complete pants, but without going back to the deepest darkest depths of my memory I think it works like this: Your above example is good. There will be a pressure drop. Although for arguments sake the car will be making the same power because theres the same amount of air molecules, just at a lower pressure. Now, as most turbos use some form of pressure-controlled wastegate, turbine rpm will increase to 'make up' for the pressure that has been lost due to cooling. This isn't actually as bad as it sounds as most of us aren't using our turbos at max rpm (unless your Mk4 is making 1.4 bar on the stock IC..). And it is this which increases power for the same given boost level. Ta'daa! Thats why I said that your car doesn't make sense. The boost controller should see the pressure drop and adjust the wastgate accordingly, thus giving you the 'free' increased power which is the reason why we all fit uprated intercoolers. Now, continually increasing turbine rpm isn't ideal. It's better than using more and more boost to make hot air though, as worn turbos are preferable to worn turbos and detted engines. So now we start talking bigger turbos. We reduce the turbine speed AND use a decent intercooler to give us the best of both worlds, and you are now likely to be at the stage where you are actually making more power at less boost. You are simply shifting so much cold air. This is why a big twin can make 550bhp at 1.4 bar whilst a stock-turboed car will make less than that at 1.5 bar. Your boost controller sounds like pants mate. Sorry. Unless it's an air density-detecting one which would be pretty flash...you haven't been knocking something up in your lab have you....;-) Like I said I if anyone knows better please stand up. Cheers! Nathan TDI PLC Link to comment Share on other sites More sharing options...
Nathan Posted June 5, 2003 Share Posted June 5, 2003 Originally posted by Nathan you haven't been knocking something up in your lab have you....;-) Bugger..you're Ian not Adam. Sorry. Why do I always get you two mixed up? Similar humour/writing I feel. Cheers, Nathan TDI PLC Link to comment Share on other sites More sharing options...
Ian C Posted June 5, 2003 Author Share Posted June 5, 2003 I'd like to point out that Adam copies my style, not vica versa I'm with you on this, I was trying to suss out how hybrids do what they do a couple of years ago and eventually figured out the they shift the air without heating it up as much, therefore more power at lower boost. I'm totally comfortable with that. My boost controller is and Apexi AVCr and I swear it doesn't care what I tell it to boost to, it will just dumbly use the duty cycle if give it and that's that. If you know what I'm doing wrong, let me know -Ian Link to comment Share on other sites More sharing options...
Geezabloke Posted June 5, 2003 Share Posted June 5, 2003 Sorry guys for jumping in. I dont pretend to understand most of whats above but... Am i right in assuming that if i fit a FMIC i will get the same performance, at a lower boost level, therefore putting the engine under less stress, as more usable oxygen is being made available with less pressure due to the cooler particals being easier to burn? I would presumably then have more performance if i upped the boost thereafter? Phew got it all out............. And if so which is the best "without breaking the bank" FMIC to go for? Link to comment Share on other sites More sharing options...
Digsy Posted June 5, 2003 Share Posted June 5, 2003 For a given level of performance out the engine will be under the same mechanical stress no matter how you go about it. What a cooler intake charge does is increase the mass flowing into the engine so you can run lower boost and reduce the risk of detonation but still get the same power. Or you can run more spark advance get more power with the same risk, or you could up the boost and the fuel and get more power that way. A colder intake charge is better any way you look at it. Link to comment Share on other sites More sharing options...
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