Turbo Boost Pressure vs. Exhaust Back Pressure

[Otis2]

(Or as an alternate thread title: "One person's 15 psi is not the same as another person's 15 psi...")

I assume that when people talk about the amount of turbo boost pressure they are running, they are usually running a stock restrictive exhaust.  

Since an engine is more or less an air pump, if you attempt to ram more air into it (via turbo), part of the "p.s.i." that the turbo is pushing is just overcoming the exhaust restruction.  In other words, if you set up your turbo on stock exhaust to pump 15 psi into the engine as shown on your boost gauge, and then afterwards you change to a low restriction exhaust AND CHANGE NOTHING ELSE, your boost gauge will read much lower.  

My assumption here (please feel free to correct) is backed up by my own experience with my intercooled AAZ engine, with 2.5" mandrel bent exhaust.  I noticed in Smog's DIY, he says when you crank the lock nuts on the wastegate rod out to the extreme end, you should be pumping up about 15 psi, and sometimes his car sees 20.  Well, I have my lock nuts so far out on the threaded part of the rod that I'm worried they might rattle off with vibration, several threads further out than what Smog's photo shows, but still I can only get 10 psi of boost (from the stock tiny KKK turbo mounted on the AAZ).  I assume this is because with the low exhaust restriction, there simply is not enough exhaust backpressure to produce 15 psi of positive boost, even with the wastegate adjusted to maximum rod length.

So my question is, am I stressing the engine components as much at 10 psi with low back-pressure exhaust as someone else with stock exhaust and 15 psi?  That is, does 10 psi on my engine = 15 psi stress on a stock-exhausted engine?  

When Quick TD said elsewhere that he runs 15 psi but "insane" boost would be in the mid-20s, where does that leave someone using low-restriction exhaust... if my "10" is his "15", would my proposed "15" equate to his "insane range"?

One issue that strikes me is that I assume the turbo's turbine must be rotating at a much faster speed in my car to produce 15 psi of positive boost, than in a car with more restrictive exhaust.  I expect this would lead to increased shaft wear and possibly all manner of other evils, such as super-heating the intake air perhaps, forcing the turbo outside of its efficiency range?  If I'm wrong on this assumption, please enlighten me.

On gasoline-powered turbo cars, installing low-restriction exhaust can be a danger as well as an opportunity.  The engine will knock (detonate) more readily, and at lower boost pressure than before.  I'm a bit concerned the diesel is the same way, but I simply don't understand the comparative physics in each type of engine.

So, before I hook up a Grainger valve to "cheat" my wastegate into letting me get up to 15 psi (given that the rod is too short to get me there the easy way), I'd be curious to hear some ideas thrown around on this point.

In case it matters to the equation, my pump has been modified by Giles at Superior Fuel Injection, and the LDA is set up to produce sufficient fuel up to 15 psi positive boost.  The question is whether the other parts of the engine will stand up to that, since I suspect that "my 15 is not your 15".

[Patrick]

Believe it or not, you've got it backwards. Boost is on the intake side, and the only thing freeing up the exhaust side did for you is allow the exhaust side wheel spin better, hence better boost. How clean is your exhaust when you've got your maximum boost? The problem may be the wastegate/controls/settings(maybe the spring in the controller is weak/broken/different than his),or you might just need more fuel to make more heat!

[QuickTD]

might just need more fuel to make more heat!

This would be my guess. Either that or you have a very weak wastegate spring. I have no trouble at all making 15psi with a 2" exhaust, in fact I can have all 15lbs by about 1600rpm. You should be able to do better with a very low restriction exhaust. Boost always gets better with low restriction after the turbine, never the opposite.

[VWRacer]

It may well be that all you need is more fuel, but there is one other easy-to-fix possibility. Does your intake manifold have a pop-off valve? My engine was only making about 9-10 psi boost until I disabled this valve, then it would make about 12 psi.

Here's a photo of what I'm talking about.



And here is the fix...



If this isn't an issue, do yourself a favor and take a look at the compressor wheel on your turbo. This turned out to be the reason my engine will only make 12 psi boost. The wheel itself is knackered...  :cry:

[Otis2]

Thanks for the replies, guys, and for setting me straight.  I have to throw out several incorrect hypotheses for every correct one.

I understand what you're saying about inadequate fuel, but what stops me from jumping on that conclusion right away is that Giles set this pump up, and I have to assume he knew what he was doing for fueling settings.  That's what we pay him for, right?

I had a word with Giles today, who says that the original wastegate spring on the little KKK turbos mounted on AAZ engines was rated to only .75 bar.  So that may account for why I could only see 10.5 psi or so on the boost gauge (.75 bar = 10.8 psi).  He suggests going with a grainger valve first, before I monkey with his fuel settings.  So I'll see what the g-valve does first.  It will take me a week or two before I buy a g-valve and have time to fool with the boost again, but I'll report back here the result.

Also, Stan, thanks for the ideas, but this is a known good turbo, the vanes are in great shape and there's no ugly play in the shaft.  Also, I don't think any of the AAZ engines had those pop-off valves on their intake manifolds.  At least mine didn't, and I don't think I saw one in Smog's photos of his AAZ engine, either.  May be black magic for the 1.6TD cars only.

[Otis2]

Patrick & QuickTD had it in one.  Not enough fuel.

First I tried installing a g-valve by itself, as Giles suggested, and effectively prevented any pressure signal from reaching the wastegate from the compressor housing.  I cranked that g-valve so tight I might as well have just pulled off the wastegate hose entirely.  But I still couldn't get much above 10 psi at full throttle & high rpms.  

I then broke the security collar, cranked up the "smoke screw", and turned down the idle speed screw accordingly.  I then took another test drive, after re-setting the g-valve to a more reasonable level.  Wow, what a difference.  

Looks like Giles left the main fueling screw alone when he rebuilt the pump.  I had to break off the original metal anti-tamper collar, and it was full of rust & still glued to the threads of the screw, even though the rest of the pump was spotless after the rebuild.  Probably hasn't been touched since the pump was first assembled.  This was a mail-order rebuild, so I didn't have the benefit of having Giles fine tune everything once it was reinstalled on the car.

Another symptom that I noticed before I played with the main fueling screw, but did not report earlier, is that boost was made very quickly from 0 to 5 psi, but then built much slower from 5 to 10 psi.  In my diesel ignorance, I thought that was normal.  Now that I have the main fuel screw turned up, the boost builds at the same (fast) rate from 0 to 14 psi, for as long as I hold the gear.  This slow building of steam from 5 to 10 psi might be a tell-tale sign for others who have this kind of problem, and who are not sure of the solution.

Thanks, guys.

[JimK]

(Or as an alternate thread title: "One person's 15 psi is not the same as another person's 15 psi...")

I assume that when people talk about the amount of turbo boost pressure they are running, they are usually running a stock restrictive exhaust.  

Since an engine is more or less an air pump, if you attempt to ram more air into it (via turbo), part of the "p.s.i." that the turbo is pushing is just overcoming the exhaust restruction.  In other words, if you set up your turbo on stock exhaust to pump 15 psi into the engine as shown on your boost gauge, and then afterwards you change to a low restriction exhaust AND CHANGE NOTHING ELSE, your boost gauge will read much lower.  

My assumption here (please feel free to correct) is backed up by my own experience with my intercooled AAZ engine, with 2.5" mandrel bent exhaust.  I noticed in Smog's DIY, he says when you crank the lock nuts on the wastegate rod out to the extreme end, you should be pumping up about 15 psi, and sometimes his car sees 20.  Well, I have my lock nuts so far out on the threaded part of the rod that I'm worried they might rattle off with vibration, several threads further out than what Smog's photo shows, but still I can only get 10 psi of boost (from the stock tiny KKK turbo mounted on the AAZ).  I assume this is because with the low exhaust restriction, there simply is not enough exhaust backpressure to produce 15 psi of positive boost, even with the wastegate adjusted to maximum rod length.

So my question is, am I stressing the engine components as much at 10 psi with low back-pressure exhaust as someone else with stock exhaust and 15 psi?  That is, does 10 psi on my engine = 15 psi stress on a stock-exhausted engine?  

When Quick TD said elsewhere that he runs 15 psi but "insane" boost would be in the mid-20s, where does that leave someone using low-restriction exhaust... if my "10" is his "15", would my proposed "15" equate to his "insane range"?

One issue that strikes me is that I assume the turbo's turbine must be rotating at a much faster speed in my car to produce 15 psi of positive boost, than in a car with more restrictive exhaust.  I expect this would lead to increased shaft wear and possibly all manner of other evils, such as super-heating the intake air perhaps, forcing the turbo outside of its efficiency range?  If I'm wrong on this assumption, please enlighten me.

On gasoline-powered turbo cars, installing low-restriction exhaust can be a danger as well as an opportunity.  The engine will knock (detonate) more readily, and at lower boost pressure than before.  I'm a bit concerned the diesel is the same way, but I simply don't understand the comparative physics in each type of engine.

So, before I hook up a Grainger valve to "cheat" my wastegate into letting me get up to 15 psi (given that the rod is too short to get me there the easy way), I'd be curious to hear some ideas thrown around on this point.

In case it matters to the equation, my pump has been modified by Giles at Superior Fuel Injection, and the LDA is set up to produce sufficient fuel up to 15 psi positive boost.  The question is whether the other parts of the engine will stand up to that, since I suspect that "my 15 is not your 15".

 I am late posting on this subject however drive pressure is a big part of performance on a diesel engine. I dont know what the maps of the compressor are but at some given pressure the compressor will make a lot of heat. Example is a HX 35, it usable pressure is about 32 lb. It is better to have a drive pressure close to boost pressure, to make more boost pressure usually requires more heat= fuel   Or that is what I have been told   good luck JimK

[HarryMann]

Resurrecting an old thread  :wink:

I am late posting on this subject however drive pressure is a big part of performance on a diesel engine. I dont know what the maps of the compressor are but at some given pressure the compressor will make a lot of heat. Example is a HX 35, it usable pressure is about 32 lb. It is better to have a drive pressure close to boost pressure, to make more boost pressure usually requires more heat= fuel Or that is what I have been told good luck JimK

What do you mean by drive pressure?

Also, it looks from reading through the above, that it could definitely be a BOV valve limiting this if there's one fitted... I've blocked mine on an AAZ and am now getting 12.5 psi with no extra fuelling, and a simple wastegate bleed valve set open to spill until no further boost can be gained this way... I assume therefore this agrees with the 12 psi mentioned above, as typical without extra fuelling. It also goes up there quickly from 2500~3000 rpm - and no smoke, so you don't need the fuel to get above 10psi.
2 1/4" JX vanagon style exhaust (short) reducing t about 2 at the silencer.

[Otis2]

Harry, it is indeed an ancient thread.  I can't speak for JimK and "drive pressure", but I can tell you that I subsequently overcame the problems I mentioned above here by adjusting the mechanical linkage to the wastegate.  

My AAZ had a small K03 turbo, with a threaded arm connecting the wastegate to the diaphragm can (see Smog's "How to make your AAZ faster" thread in the FAQ for photos of this set-up).  I backed out the nuts to the extreme end of the threaded arm, effectively bottoming out the wastegate.  This turned out to be the solution.  After that, I could instantly get 15 to 17 lbs of boost by 2000 RPM.  The big camper van run far faster then, and no smoke in the exhaust.  

But it didn't last.  Within 1000 km of cranking down the wastegate, the turbo blew.  This was a surprise to me, since I kept seeing people here say the K03 was good for 15 psi (although at the end of its efficiency range by that point).  Maybe 15 psi with a functioning wastegate is easier on the turbo than 15 psi with a bottomed-out wastegate.

It may also have been be related to a problem with the oil return line (collapsed engine mount meant the drain flowed slightly upwards where it entered the block).  Or, it may have been just too much boost too often in the heavy van.  I noticed that it would never boost LESS than about 5 psi when accelerating, and held a steady 10 psi at 100 km/h cruise.  It seemed to act more like a supercharger than a turbocharger.

Dave Cross at Passenger Performance replaced the blown K03 with a Garrett T3 (very nice work by Dave).  Pros and cons to this mod.  The much larger turbo really lags in the van relative to the K03, and requires the injection pump fueling to be turned up smoky to get any kind of spooling response.  At 100 km/h cruise, this set-up is only boosting between 0 to 5 psi, but it runs significantly faster in the top end of the RPM range.  Third/fourth gear merge ramps into speeding highway traffic are far easier now... but then again, the injection pump fueling is cranked up much higher now, which kind of masks a low-boost problem.

Interestingly, the oil temps seem to be much lower with the T3 installed, but I have a lot more testing & tweaking to do before I decide how much I like it.  Everything is a compromise.  I'm toying with disabling the wastegate on the new T3 to see whether the lag is improved (as it did with the old K03).  But obviously I don't want a repeat blown turbo so soon after the last one.

[RabbitJockey]

also i'd like to point how you were saying that the engine must over come the back pressure of the turbo while getting boost, but the amount of back pressure  created is really nothing when you think about how the air going into the engine is around 100-200 degrees, while the air coming out of the engine can be in the thousands,  theres alot of expansion, so the turbo is getting pushed by more air (although it's really the same amount in away)  but it's being pushed by way more air than what the amount it has to suck in, not to mention that once ur turbo is already spooled there would be a bit of a push me pull me effect from the momentum of the turbine.

[QuickTD]

Dave Cross at Passenger Performance replaced the blown K03 with a Garrett T3 (very nice work by Dave).

Otis, you don't happen to know the turbine A/R and the compressor trim for the particular T3 you're using do you? Dave, if you're reading feel free to chime in as well.

I'm surprised to hear about the K03 letting go, mines been making 15psi for a few years now with a few shots past 30 while setting up a boost controller :shock: I like the quick spooling of the K03 but I've seriously outgrown in in my quest for power...

[hillfolk'r]

um,a couple of things,,that turbo in the pic of the banjos example looks like its been used for a wood chipper for some side money :shock: ,to me insane boost would be like over 40 onthe street,,compound setup oh yea and another thing,,theres somethin to reading the pressure in the exhaust manifold,versus "boost" theres somethin about it needing to be =      or the turbo is not efficient or somethin,,do some looking around about this,i dont know alot about that subject,,if its a good diagnostic tool for turbosizing,that would b great!!rabbittree has both psi guages in his dodge,and when he was on it they were about the same :?