1.9 boost levels? help!new gauge, new car, almost 20 psi!!!! - IDI Engine - VWDiesel.net The IDI, TDI, and mTDI source.

• 1.9 boost levels? help!new gauge, new car, almost 20 psi!!!! by Dano on 03 Feb, 2006 05:24
• :shock: just put a boost gauge in my car that I just installed a 1.9l in, I have the max fuel screw turned out so the car idles around 950 with the idle screw all the way out, and I modified the spring under the cap on the top of the pump by clipping it shorter to soften it up.  Took the car for a drive and it shows around 18-19 psi at around 5000 rpm.  The car seems to fly along pretty well, but what I am wondering is that if this is normal as I havent even installed the boost bleed yet.  How much boost will these little motors handle?? I havent got my egt gauge yet, but the only driving it is seeing is short bursts up and down my road. (in the country)
• #1 by DubAllStar on 06 Feb, 2006 09:45
• i don't have a tach, but on my supposedly stock 1.9 TD, i'm pushing 17/18 PSI at about 170 km/h
• #2 by vwmike on 06 Feb, 2006 10:48
• Don't be too concerned about boost. The biggest thing I would worry about is the EGT's. If exhaust temperatures aren't too high then you should be safe. My car hits 27 psi daily and others run much more.
• #3 by DieselMonkey on 06 Feb, 2006 16:27
• Guys,
  
  I'm glad this topic has come up.
  
  I've been wondering this for a while now. How can we get away with 2 bar of boost on an IDI yet the guys over on TDiClub.com have to lower their compression to keep the peak cylinder pressure down ?
  
  Obviously ours has increased, but we have no choice but to keep it that way. I noticed a big difference in starting when warm when having a 3 hole gasket on my 1.9 GTD a year ago. To lower the compression any more would make its starting very hard.
  
  So, keeping the stock 22.5:1 compression ratio and up to 2 bar of boost, what gives ?
  
  Why dont the TDi guys keep there stock compression ratios also ? VW have made loads of changes over the years with the TDi compression ratios. The 1Z was 19.5:1, the PD 130 and 150 is down now to somthing like 18.0:1 , I think.
  
  When I started out I simply figured that you lowered the compression to stop the turbo from over speeding or to keep the head gasket in one peice, but that doesnt make any sense to me anymore. The wastegate (or VNT with ECU software) will keep the turbo speed set, and increasing boost will simply lower the exhaust temperature, reducing the chance of headgasket failure. Is peak cylinder pressure the important thing here ?
  
  I figured, if the motor has compression of around 400 - 450 PSi, another 28 - 29 psi wouldnt hurt would it ?
  
  Sorry to hijack the thread.
  
  Regards. Matt
• #4 by vwmike on 07 Feb, 2006 01:18
• Part of the issue on TDI's is that they use comparably small turbos. The tiny VNT turbos are excellent for driveability but aren't really intended to put out a ton of boost. I've seen spikes over 20 psi but it quickly comes back down. It is common to have to incorporate a bleed setup to reduce boost response in order to keep the turbo from shooting past it's surge limit and self destructing.
  
  IDI engines are much more prone to heat loss and as a result the high compression ratio is necessary for reliable starting. An ideal balance would be more like 18:1 as written in the development papers, but starting would be difficult. The TDI uses ~18:1 compression and start up is excellent.
  
  Increasing boost pressure does much more than you'd think on the surface, but...
  
  "I figured, if the motor has compression of around 400 - 450 PSi, another 28 - 29 psi wouldnt hurt would it ?"
  
  At atmospheric pressure which is 14.7 psi, or 1 bar, we add 2 bar of boost (29.4 psi) to get a manifold pressure of 3 bar, or 44.1 psi absolute (psia). So, we just tripled the amount of air that the engine will ingest. Of course there are mitigating factors such as a reduction in air density due to heat from compressing the air with both the turbocharger and in the combustion chamber, but you get the idea. Cylinder pressure will skyrocket as a result. Is this bad? Yes, it puts more stress on the engine. Is it so bad that you wouldn't do it? That's a decision that each of us needs to come to individually.
  
  Reducing compression has it's upsides and it's downsides. The primary purpose in a gas engine is to reduce engine knock. 3 psi of boost pressure roughly equates to 1 compression point. I run 16-18 psi in my gas Rabbit with 9:1 compression. Do the math on that one.
  
  The primary benefit is that the larger combustion chamber means more compressed air can fit into the combustion chamber and therefore more fuel can be injected. Thus, more power is produced. On a diesel the lower compression can mean hard starting. Lower compression can also reduce torque which would be detremental to driveability. That is usually something you want to retain as much of as you can.
• #5 by Dano on 07 Feb, 2006 07:05
• verrry interesting..  :idea: I know more fuel will give more boost, but the wastegate should dump it off between 8 to 10 psi or so, should it not?  The wastegate could possibly be stuck shut, I will have to verify.  It seems strange that it would make 17 psi without touching the wastegate and just adjusting the fuel some.   One thing I have learned is that the boost created in an engine depends on load and backpressure.  The exhaust is stock, and may be partial to blame as the boost can back up in the engine.  With a free-flowing system and a light car, the boost will read lower.  We supercharged a 1923 t bucket and the engine would only max about 10 psi boost in the car, but could show 15 psi loaded on the dyno.  The engine in a heavier car would cause a higher reading as the engine has to work to move the weight.  When it was in the t bucket, it would almost be like free revving the engine because it was so light.  (like 1500 lbs or so)  fun car though, with 450 hp.. like a shopping cart with a jet engine.  The egt gauge will tell...
• #6 by vwmike on 07 Feb, 2006 10:33
• Comparing boost characteristics of turbochargers and superchargers is like comparing apples and oranges. Low backpressure exhaust can cause boost to run away on some smaller turbos as the wastegate just can't flow enough to keep the turbine speed in check with the increased pressure differential across the wheel.
  
  The wastegate probably is bad. Can you reach back there and move the wastegate actuator arm (assuming your car has one like in smogs make your 1.9 faster thread). If it doesn't want to move then it might be stuck like you say. If it does then you can test the actuator with a bit of shop air. Put 10 psi into it and see if it moves then steadily increase the pressure to around 20 where the boost level is. It is also possible that the diaphragm in the wastegate actuator has broken so listen for leaks back there.
• #7 by Dano on 07 Feb, 2006 10:55
• HMMM..... More to ponder... how do you figure that comparing would be like apples and oranges?   They both produce boost.  Granted that one is crank driven and will only produce so many rpms depending on crank speed.  Do you not agree that boost will be higher depending on load imposed on the engine, regardless of type of forced induction?  Low backpressure is not the case causing the higher boost level, because the car has stock exhaust with converter and two mufflers.  Thanks for the advice on checking the wastegate.  The problem doesnt really bother me alot, (at least till I check the egt) but it did have me curious as to what would be causing it.  Likely is the wastegate. I will check it to see.
  The opposite could be true in that low backpressure will cause a smaller turbo not to flow enough cfm to fill the engine to the requested boost, causing lower boost psi, no?
• #8 by vwmike on 07 Feb, 2006 11:40
• I'm assuming that was a roots blower you guys used? That would be positive displacement. The only way I can see boost changing is through heat (density ratio) or a change in the volumetric efficiency of the supercharger or the engine. Smaller exhaust may reduce VE and cause more boost to build.
• #9 by Dano on 07 Feb, 2006 12:24
• Actually it was a weiand 144 procharger.  The engine would show 15 + psi on the dyno, but in the light car would only top out around 10 or so.  The problem is that the car would not load it enough to create the boost numbers.  The engine was very free flowing with a very efficent ve, causing the boost number to be low because the boost can't "back up" in the engine under load.  Put the same engine with the same charger in a 5000 lb impala, it will show the psi.  or... put a larger blower on it so there is an abundence of air and the boost will be higher for the same driven speed.  The maximum boost didnt change, unless the ambient temp. varied alot.  It's just a matter of proper sizing for application, same as a turbo.  Too small a turbo, lotsa boost at low rpm, chokes at a low rpm. Big turbo, not as much on the bottom, more lag, but more flow at high rpm.. Its all just a trade off. :wink:
• #10 by vwmike on 07 Feb, 2006 14:04
• I'm pretty sure the procharger is a centrifugal blower. It isn't positive displacement and like a turbo it actually has a compressor map. As for the specific reason behind the difference in boost pressure I'm still not sure.
• #11 by Dano on 08 Feb, 2006 05:05
• The weiand blower was a tiny roots style blower, they dont make it anymore, they were bought by holley, they make the same blower with holley on it. its only like 144 ci or so.