Engine Tuning: BDH Motor - Is it Possible?

Hello experts,

Here's the vehicle information:

Passat Variant 4 motion, 2004, 2.5L engine, bore 78.3mm, stroke 86.3mm, compression ratio 19:1, 132 kW, mileage 362,000 km, engine code BDH, transmission code GLX, no known major repairs needed.


Yesterday I went to the mechanic and had the compression checked because I suspect that some of the hydraulic lifters on the left cylinder bank might be faulty. It wouldn't be surprising at this mileage.


Target values for compression pressures:

New condition: 27 - 33 bar, wear limit: 24 bar.
My readings: cylinder 1, 20.8 bar; cylinder 2, 20.6 bar; cylinder 3, 20.6 bar.
4. 20.4 bar, 5. 18.5 bar, 6. 18.6 bar.
For reference, here are the consumption figures for this condition: city 5.8 to 7.0, highway 7.0 to 8.0, combined 7.1.

Since the values are now well below the wear limit, I'm considering a revision.
It occurred to me that, while I'm at it, I should also increase the engine displacement, if it's even possible with the electronic monitoring system and the VP44 injection pump.
The block is still a gray cast iron block, so it would be possible to drill it out if there's enough material remaining.

What do you think about it? Has anyone here ever tried this and had success, or tried it and had bad luck?
All experiences and suggestions, as well as risks.
are welcome.

Let's go, and thank you for reading.
Best regards,
Michi

Of course, it would technically be possible to do, but I'm not aware of any larger pistons or head gaskets available for a V6 engine.

The values are below the wear limit, but at least the differences are only a maximum of about 2 bar; 5 bar would be acceptable.

Economically, an overhaul at that age and mileage is probably not worth it, but ultimately, you have to decide if the car is worth it to you. However, it's important not to forget that at 362,000 km, it's not just the engine that is worn out.

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The values are indeed below the wear limit, but at least the differences are only a maximum of ~2 bar; 5 bar would be allowed

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...which might also suggest that it's an inaccurate measuring device. I would just let it run as is for now, as long as it's doing what it's supposed to.

Here are some component dimensions:

BDH Piston - Bolt diameter 26 mm, bolt length 58 mm, bore diameter 78.31 mm, compression height 47.83 mm, length 71.83 mm, groove depth 17.4 mm, valve pocket depth 1.38 mm, groove diameter 37 mm.

BSG (2.7 TDI) Pistons - with recess for oil injector, coated, valve bridge height 10 mm, bowl depth 13.6 mm, pin diameter 30 mm, bowl diameter 45 mm, compression height 46.45 mm, with cooling channel, with piston ring groove, pin length 68 mm, length 70.95 mm, cylinder bore 83.01 mm.


You see, there are definitely some differences, especially the bolt diameter is problematic, as the small connecting rod eyelet cannot be drilled out very much.
Besides that, BSG no longer has any bronze bushings for the piston pin guides.

Now, one could compare the connecting rods to see if any from BSG might be compatible, but that would be quite a complicated process.

The head gasket would be the least of the problems, or it would be the most likely solution.

@ VW mechanics
Do you happen to know if there's enough "material" available for drilling?
It's definitely an interesting topic, which is why I included it here.
For example, the VP44 pump has a maximum stroke of only 50mg per stroke. This might be too little for a modified engine.
And then there's the turbo, which you should also choose a larger size for, and you should adjust the exhaust pipe diameter accordingly, otherwise there's a risk of back pressure.
Not to be forgotten are the injectors, which I believe are 5 or 6-hole injectors in the BDH engine.
And the MSG (Message Service) certainly needs to be modernized or adapted as well.

I can, of course, just let it run like that until it stops working.

In another thread I read today that, according to some people, an engine with such poor compression values shouldn't be able to run or start.
I don't have that problem at all. It always starts up immediately.
It was also suggested that there might have been a measurement error (it was an engine with only 91,000 km), but I don't believe that's the case given my car's mileage of 362,000 km.
Let's see what other messages and suggestions are posted here. I'm very excited and curious to find out.

Best regards,
Michi

I would like to perform a second compression measurement. Decisions like these should not be based on a single measurement.
Technically, everything is possible, but the financial and time investment is somewhat beyond the norm. Aside from the legal aspects...

As long as it runs and the performance and oil consumption are acceptable:
Don't look to see why!

...because he who measures, measures manure.

The values are consistent, and that's the most important thing.

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Do you happen to know if there would be enough "material" available for drilling?

Unfortunately, almost 5mm is quite a lot... It's much easier to go from a 1.9-liter four-cylinder engine (79.5mm bore, 1896cc) to a 2.0-liter engine (81mm bore, 1968cc).

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For example, the VP44 injector has a maximum stroke of only 50mg per injection.

That might be too little for a modified engine.

That's your least concern. With 50mg/H @ 4000 1/min, you're already achieving over 200 horsepower. The maximum amount is simply a limiting factor for power and torque. Even with a 2.7/3-liter engine, you could inject only 20mg/H, but in that case, the engine would only produce around 90 horsepower.
Mechanically/hydraulically, your VP44 can definitely handle significantly more .

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And let's not forget the nozzles; I believe the BDH engine has 5 or 6-hole nozzles.

You probably already have the six-hole nozzles. The next step would be to determine if the spray angle is compatible with the other mold shape (experimental setup).

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I can, of course, also let it continue running like this until it stops working.

Exactly – save your money and buy a 2.7 or 3.0 TDI if your 2.5 eventually stops working.

First of all, thank you to everyone involved for the information and tips.

I'm going to get a second measurement done first and see what the results say. I'll write it here afterwards.

On the other hand, I'm also aware that it no longer produces 180 horsepower at the wheels, nor 370 Nm of torque, but the fuel consumption is still surprisingly low. My Passat usually weighs around 2 tons, and yet it has such low fuel consumption.

Regarding the overhaul: A close friend of mine owns a machine shop that specializes in engine rebuilding, so it would cost me about 50% of the regular price. I would remove the engine myself and bring him the bare engine block, and we would work together to disassemble it, perform the overhaul, and reassemble it. I would also have the crankshaft and, if possible, the balance shaft precision balanced, as well as the vibration damper. If the engine has a dual-mass flywheel (ZMS), it should also be balanced, preferably along with the clutch.
I would replace the oil pump and its chain drive, as well as install a new clutch that can easily handle 500 Nm of torque. I would also have the injectors removed and checked by a Bosch service center. New stretch bolts for the cylinder heads are a must. I've already purchased new hydraulic lifters and roller rocker arms because there's a ticking noise on the left bank, but only at idle and under partial load.
180 original horsepower is actually enough. Maybe add a chip, and you'll get around 210 horsepower and 420 Nm of torque. That's quite enough, really.

If the second measurement yields the same result as the first, I don't think it's a good idea to essentially push the engine to its limit with each stroke, as this could potentially damage other components.

Or what?

Best regards,
Michi

First of all, thank you to everyone involved for the information and tips.

I'm going to get a second measurement done first and see what the results say. I'll write it here afterwards.

On the other hand, I'm also aware that it no longer produces 180 horsepower at the wheels, nor 370 Nm of torque, but the fuel consumption is still surprisingly low. My Passat usually weighs 2 tons, and yet it has such low fuel consumption.

Regarding the overhaul: I should mention that a good friend of mine owns a machine shop that specializes in engine rebuilding. Therefore, it would cost me about 50% of the regular price. I would remove the engine myself and bring him the bare engine, and we would work together to disassemble it, have it overhauled, and then reassemble it. I would also have the crankshaft and, if possible, the balancer shaft precision balanced. The vibration damper should also be balanced. If the engine has a dual-mass flywheel (ZMS), it should also be balanced, preferably along with the clutch.
I would replace the oil pump and its chain drive, as well as install a new clutch that can easily handle 500 Nm of torque. I would also have the injectors removed and checked by a Bosch service center. New stretch bolts for the cylinder heads are a must. I've already purchased new hydraulic lifters and roller rocker arms because there's a ticking noise on the left bank, but only at idle and under partial load.
180 original horsepower is actually enough. Maybe add a chip, and it will have about 210 horsepower and 420 Nm of torque. That's actually quite enough.

If the second measurement yields the same result as the first, I don't think it's a good idea to essentially push the engine to its limit, which could potentially damage other components.

Or what?

Best regards,
Michi

amishorty wrote:

On the other hand, I also notice that it no longer has 180 horsepower at the wheels, nor 370 Nm of torque.

Perhaps it would be a good idea to check the mass airflow sensor (MAF), especially since, by its design, it's practically a wear item, particularly in the V6 TDI engines.

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Regarding the revision, one more thing. A good friend of mine owns a machine shop, so it would cost me about 50% of the regular price.

Have you already calculated the ratio of the cost of the planned overhaul to the vehicle's value?

Do you think that the alleged compression loss, which currently seems questionable, is the reason for the performance loss? I think it's more likely that this (as mentioned before) is related to the engine itself.

With supposedly low compression values, it would start cold and be difficult to get going, and it would smoke a lot after starting . I think the measuring device is probably showing a lower value than it should!
It's more likely that the VP 44 and the camshaft will wear out (take both valve covers off) than the engine block.
Save yourself the trouble, keep going! Lack of performance: Am I also going to rely on an LLM like everyone else here?

Based on my experience, the 2.5 TDI V6 is unfortunately the most disastrous TDI.
That's why I prefer to drive a 4-cylinder engine.

I had a second measurement taken today. The first one was done in Switzerland, and the second one was done in Germany.
Here's the result:



4-cylinder: 31.6
1-cylinder: 20.5
5th Cylinder: 22.5 ------------ 2nd Cylinder: 29.3
6-cylinder: 31.1
3-cylinder: 33.0


"Well, this looks completely different now. Now, the +/- 5 bar pressure variation is really significant. Based on this measurement, my suspicion is that the intake camshafts for cylinders 1 and 5 are worn. It's also possible that the hydraulic lifters for those cylinders are defective."
And frankly, I can't quite understand why it's specifically cylinders 1 and 5 that are so worn.
Is there a known weakness related to the cylinder arrangement that might cause premature wear in BDH engines?

What about the firing order? With the sequence 1-5-3-6-2-4, the first two cylinders would fire first. However, if the sequence is 1-6-2-5-3-4, the firing is more evenly distributed.
What do you think? Should I remove the valve springs, take out the camshafts, and measure the cam profiles on the prism? I would need to know the lift and the cam height in the original condition.

It's certain that the camshafts, lifters (roller type), and hydraulic lifters are the original ones, because my repair history doesn't list any replacements for them. Of course, it's possible that those parts were replaced somewhere without a receipt, which I can't rule out.

Please provide your assessment of the situation. If you need any further information, please feel free to ask.
Thank you for reading.

Best regards,
Michi

The engine is likely running very unevenly due to the values. The LLRR should also be at its maximum on both cylinders. It's worth checking; I suspect there's a measurement error.

@ Michael
Thanks for your assessment. I'll check the LLRR today and post the results here.
The testing device from the friendly mechanic was only recalibrated a week ago. If the initial measurement was correct, the engine shouldn't be running at all. However, it is working as it should, starting up every time, even when cold. It doesn't run unevenly, just with slight, noticeable irregularities. During a cold start, it idles at approximately 1000 RPM, and after 2 minutes, it drops to 900 RPM. When warm, it idles at 720 RPM.
I don't believe there were only two measurement errors, because the power is there, just not the full amount.

Michi.