Hey everyone, hopefully I can get some draw through turbo gurus to give some advice here. I own a 1978 Datsun 280z with a draw through turbo setup, I have ran this draw through for well over a year without any issues until recently when I blew the rings on my last motor which I swapped out for my spare one. Ever since swapping the motors (exact same l28 block and head) I have had some serious oil smoking issues due to the turbo leaking oil into the intake through the compressor oil seal during high vacuum deceleration. Yes I do know about dynamic and carbon seals and that it is recommended to use a carbon sealed turbo for draw through turbo setups, but I had never had oil smoking issues with dynamic sealed turbos if I used a proper sized restrictor and oil return line. Until now of course..

As I mentioned I have smoking issues during deceleration/engine braking and only during that when it's in high vacuum, otherwise it drives great and boosts great with no smoke on acceleration. I've noticed that the smaller a restrictor I run the less it smokes, but I've gone all the way down to a .030" restrictor (for testing purposes) and it still puffs a bit of smoke on decel. When I swapped the engines I installed a new high pressure oil pump which I'm thinking might be my problem as that's the only thing different on the new engine. It's not the turbo itself I know this because I originally had a t04e on it and changed to a hx35 recently with no difference in the smoking. With the new oil pump I'm seeing over double the PSI my previous oil pump was operating at. Now after eliminating the turbo as a suspect for the smoking I'm looking at the oil pump and honestly considering swapping my old oil pump to my current engine. Not ideal but neither is smoking out the car behind me and having to fill up the oil every tank of gas.

Considering you cannot get a carbon seal for the hx35 are there any other things I can do to help it reduce the oil pulled into the intake? I've read about one guy with an air-cooled VW running a hx30 draw through setup and what he did was installed staggered gap oil rings, .060" restrictor, and increased the oil return line to 3/4". I've never rebuilt a turbo so I'm a little hesitant to mess with it like that.

 

What size drain do you have?

 

Ah knew I forgot something. The drain is a 5/8" ID, I know this seems a little small for a hx35 but I ran this size drain on my t04e as well which didn't smoke until the engine replacement. I might go with a 3/4" eventually but I'd really rather not pull the oil pan off again if I don't have to.

 

The carbon seals are required in draw through specifically to prevent the oil draw your fighting with now. In an application where the turbo is before the throttle body the seal never sees a vacuum, so the seals aren't designed to handle it. It could be your old engine was just so beat you weren't pulling a lot of vacuum.

If your drain routing is a nice straight vertical path, and your pan feed is up high in the pan you will have to figure out how to get carbon seals in the turbo. Allied metrics makes custom carbon seals, I don't know if they have a production minimum or not but it would be worth reaching out.

 

Better ring seal will pull more oil, if previous engine was tired, its something to think about. Restrictors in a journal bearing turbo are a bad thing, and asking for trouble. They make a pressure reducing valve that you could add in the feed which would allow lower pressure.

 

There is another option.
Modify the compressor seal to twin piston ring seals and run a line from atmosphere (in the air filter) to the space between the seals. The difficulty of this modification varies with different turbo makes and models. Most turbos do not require the welded insert shown in Fig. 4 below.
https://www.semanticscholar.org/pap...tson/6bc88bdc48b361a5a5db4e8eb0c88ae744773819 (See Figure 4)

 

Just a small update: swapped my old oil pump on and it dropped oil pressure to a reasonable number, smoking has got much better but still not 100% although it's a definite improvement. I think it's a combination of things here causing it, first higher oil pressure due to a turbo 280zx oil pump (removed now) and new rod bearings, also higher vacuum because the cylinder head is rebuilt. These two combined must have been the cause of the smoking because my last engine did NOT smoke or consume any oil even with a dynamic sealed turbo.

I like the idea of two piston ring seals, basically works like the staggered ring gap seals I've seen on eBay. How would one modify the turbine shaft for these? Also can I just mark the nut/compressor wheel/turbine shaft and reassemble it aligning the mark or would it have to be rebalanced?

 

If you are talking about piston ring seals with a stepped end, that is a totally different concept. No shaft mods needed. Did you look at fig 4 in the link?

 

I had to look at the figure 4 a few times to wrap my head around it, but I think it makes sense now. Wouldn't piston rings with a stepped gap achieve the same result as double rings though?

Also I'm wondering if my 5/8" ID return line is sufficient for this hx35 or should I increase it to 3/4"?

 

A stepped gap ring is interchangeable with a normal ring. Either will do the job in the twin-ring setup.

 

Well turns out my supposed 5/8" oil drain actually bottlenecks down to 14mm at the oil pan. I think this is why it doesn't smoke on first startup and for a few minutes of driving but after about 2 miles will start to puff a bit of oil smoke, I assume because the return line is getting backed up and the turbo is unable to drain properly. Working on installing a full 21mm ID drain using a 16an braided hose.

 

Most 10an drains are going to be less than that, there was a recent conversation about this, I think someone measured as small as ~.450. They usually work fine.

 

Well finished the 16an drain (one of the hardest things I have ever installed no joke) and switched the oil to 10w30 non synthetic and it's no longer smoking again!! HUGE difference in driving the exhaust smells better and it drives much better without the oil in the intake no surprise. Oil pressure is at a much more reasonable level now which I'm sure is helping too.