Oil from blow-by

[Jebu]

Hi,

I'm experiencing some blow-by mostly during trackdays, esp high revving.

It normally does not happen during normal drives.

Does blowby automatically mean that something is wrong with my piston rings/or seals? Or is it normal on an RB25DET?

Could it be due to my BOV? It seems to be aggravated since I've installed the BOV.

I'm using Turbosmart.

When it first happened it kept on fouling my AFM, but I've since installed an oil catch tank.

Just wondering whether I should start saving for an engine rebuild!

[HoeTrain]

I think, and I’m open to correction, that you will experience a small amounts of blow by when high revs are applied.

The oil catch tank will only catch the oil the is mixed with air under the rocker covers which can accumulate as a result of blow-by from the engine and basically separates the air from the oil and either expels the air into the atmosphere or re-circulates it back into the intake excluding the oil. The oil air separator (catch tank) is a good little gizmo as oil in the oxygen that your engine breaths decreases the explosion in the cylinder, coats your intercooler internally with a film of oil reducing it’s efficacy, and can also clog up things like bleeder valves.

I guess it is dependant on how much blow-by you are getting. I'm not an expert, but someone who knows more might be able to help further.

[GTRgeoff]

It has been explained to me recently that the 25t has extra oil supply to the head compared to the 26dett. This causes oil surge and can result in some blowby oil if there is crankcase pressure due to blowby. No biggie if the oil quantity is small, but if is like 600 ml in 30 min of track time (from experience) it is time for rebuild.

The oil supply can be modified as I have to mine to the same as 26dett and so far no probs.

[Minorci]

I remember reading somewhere TOMEI have some oil restrictors or something like that (forgive my ignorance) but essentially they reduce the amount of oil travelling up to the head. Which can reduce blow by gas. But in my instance, it some dead rings...although i'll be taking this oppertunity to put some forgies in now.

[GTRgeoff]

The oil flow to block is the feed between 2-3 to the head on the inlet side. the very front one on a 25det is the cam change and the rear (3-4) should be left as is but I am not at all sure on the rb20.

[Jebu]

Hi guys,

I think currently oil quantity is small...

Haven't been to the track yet since its currently closed for the F1 race...

Will see how it goes in April.

But from the last track day, the quantity is definitely not as much as 600ml.. Hope I'll be ok though!

I'll start worrying when I see more blow by on normal hard driving...

Anyway, I was wondering (if I really had to), I was just to do a top overhaul would it be sufficient?

I don't think I want to change to forged pistons or anything like that.. stuff like that are quite expensive up here...

[Steg33]

Jebu, the best thing you can do to get it checked out is to get a compression test or a leak down test done on each cylinder to tell you how much blowby is occuring. If you're getting much white smoke out the back, it's a bad thing.

Anyway, I was wondering (if I really had to), I was just to do a top overhaul would it be sufficient?

I don't think I want to change to forged pistons or anything like that.. stuff like that are quite expensive up here...

If in the end you're getting a lot of blowby then it's your bottom end that needs doing and overhauling the top will not do a thing for you.

Do a search and some reading up if you want more info.

Get the checks done and then go from there.

[Jebu]

The oil flow to block is the feed between 2-3 to the head on the inlet side. the very front one on a 25det is the cam change and the rear (3-4) should be left as is but I am not at all sure on the rb20.

hi skylinegeoff,

I don't really understand what does this do, I've tried to search a bit about it but can't really understand how this is done.

I understand (correct me if I'm wrong) its purpose is to prevent too much oil to the head.. is this applicable for RB25det only?

how is it done for the RB25?

any pic?

I've got to go and explain it to my mechanic...

anyone care to explain a little bit more? sorry I'm a little bit lost...

thanks!

[Sydneykid]

Hi Jebu, blow by is compression leaking out of the combustion chamber. It either gets past the rings into the sump, through the head gasket or past the valves and seals. As engine wear they get more blowby, this is normal. If you run your engine at high rpm for long periods and you don't do the oil flow mods (as below) it will fill the cam covers up with oil. This happens all the time at track days. The compression blowby then agitates the oil and pushes it into the catch can.

Let me start off by saying that I am not a fan of cam cover baffles, whatever brand. They are a band aid, that simply covers up the real problem.

The first real problem is, at high rpm, too much oil is getting to the head and too much is staying there. You need to limit how much goes up and increase the flow rate of what goes down. Make sure you have the standard Nissan oil flow restrictor in place between the head and the block, some people remove it. On RB26s' one of the two holes is blocked off, this is a good mod on RB20/25's. It can only be done when you have the head off. We also increase the size of the oil return galleys to the sump, you can this with the head on. We run an external pipe from the rear of the head to the sump as well, you can only do this with the head off.

The most common cause of blowby is compression getting past the rings, real problem #2. When we see too much blowby we always do a leak down test on the engine, both wet and dry. This tells us if there is a problem and where it is. If your workshop does not know how to do a leak down test, then go to another workshop.

Hope that helps

[Jebu]

thanks for the explanation sydneykid.

I think the situation is much clearer now.

Just that my mechanic doesn't speak english very well so its hard to explain it in the local language if I don't really know what to do hehe

[ADM]

Guys,

I’m looking at using the Tomei oil restrictor "lug" for the RB26DETT, which restricts the oil feed diameter to the head down to 1mm. I’m concerned that it isn’t large enough to allow enough oil to the head during daily running conditions. Does anyone have an opinion on this or has anyone had experience with this mod.

ADM