What Is 'blow By'

[tridentt150v]

Did someone say fight..........[running fast but careful not to spill or drop stubby]!!!

I usually put my hand over the rocker cap area and measure how hard it trys to blow it off, but then I'm no garage or tuning expert.....thats how bad I think its choofing and the thread starter didn't ask for any specific car, just what is blowby? Just as long as we don't start on negative crankcase pressure hey.......

[FineLine]

*slowly walks out of thread backwards being sure not to take eyes of GT and ER*

Chickenshit. Stay and watch the manhug when they realise they are on the same page.

[Simon-S14]

excessive is filling a 2L catch can after a few laps of a track with oil.. simple as that haha.

i got a question tho.. does hitting the limiter have an affect on blow by at all?

[3LGODZILA]

guilt-toy post up ur opinion mate, would be good to hear some more of the experienced guys opinions

on a side note, is it really possible to get an idea of the condition of a engine by going off the oil filler cap? and are all engines the same/similar? (elite mentioned before nissans are sealed engines). or is it better to compression/leak down test. main reason i wanna know is when your going to look at a car to buy alot of the time its hard to a full on compression test over all the cylinders, testing via the filler cap would be a much much quicker way

[The Dan]

excessive is filling a 2L catch can after a few laps of a track with oil.. simple as that haha.

i got a question tho.. does hitting the limiter have an affect on blow by at all?

Well.....not really.

There is a difference between excessive blowby and cam cover filling. If you have a high volume oil pump, your engine can still be very healthy and fill a 2L catch can.

Fineline's R33 is a perfect example......cannot get one drop of oil to go into his primary can on the street or on the dyno but throw a high speed corner in and 2L in 3 laps of QR. That's purely because the head is hydraulic and we can't limit the oil to the top otherwise he will wear out his valvetrain components. It's also a street car so we need to consider this as well.

Excessive blowby will do things like push the dipstick out (but even that can be tricky as most of the dipsticks are old these days so they don't seal so well anyway). It can pump oil into catch cans but if it's not doing it in a straight line at high rpm then I suggest the problem may not be excessive blowby.

Yes detonation or missfiring or hitting the rev limiter will cause torsional ring twisting which does create excessive blowby but only for the duration of the missfire.

[Simon-S14]

leakdown test will tell you more then anything else will

[Simon-S14]

Well.....not really.

There is a difference between excessive blowby and cam cover filling. If you have a high volume oil pump, your engine can still be very healthy and fill a 2L catch can.

Fineline's R33 is a perfect example......cannot get one drop of oil to go into his primary can on the street or on the dyno but throw a high speed corner in and 2L in 3 laps of QR. That's purely because the head is hydraulic and we can't limit the oil to the top otherwise he will wear out his valvetrain components. It's also a street car so we need to consider this as well.

Excessive blowby will do things like push the dipstick out (but even that can be tricky as most of the dipsticks are old these days so they don't seal so well anyway). It can pump oil into catch cans but if it's not doing it in a straight line at high rpm then I suggest the problem may not be excessive blowby.

Yes detonation or missfiring or hitting the rev limiter will cause torsional ring twisting which does create excessive blowby but only for the duration of the missfire.

cool thanks for that.

last track day i got a tiny bit of oil in catch can but motor saw limiter a lot and was over full as well.. think they were factors.

in my CA18's case.. few laps it would empty the sump. oil everywhere, dipstick went flying and run like shit.... pretty sure that was excessive blowby

[The Dan]

+1 ^

leakdown test result of up to 20% is acceptable, anything more and you should go digging for issues

[The Dan]

cool thanks for that.

last track day i got a tiny bit of oil in catch can but motor saw limiter a lot and was over full as well.. think they were factors.

in my CA18's case.. few laps it would empty the sump. oil everywhere, dipstick went flying and run like shit.... pretty sure that was excessive blowby

Haha yeah I think that CA was trying to tell you something.

[femno]

Haha I can see alot of people reading this thread will be lifting oil caps and rushing over to their mechanics with possible engine failures!

[Granthem]

Many of you have absolutely no idea what 'excessive' is.

If you take the oil cap off any of the nissan turbo engines, the engine should drop in idle or nearly stall. They are a sealed engine. The cylinder head is SPLASH lubricated so it is meant to throw oil around there.

Fumes coming from the rocker cover breathers or out of the oil cap means nothing. The engine has to breathe. If it doesn't, bad shit happens.

The most annoying thing to hear is ppl going on about so called 'blowby' because when the car is on the dyno, fumes were coming out of their catch can breather or from their cover breather.

I know cars drop revs. But this thing was dying, no two ways about it.

I have not seen an RB with such an issue yet. I can assure you none of my Rb25s (or mates RB20s) have come anywhere near dying when removing the cap, nor is there any real "choofing", and as for oil, one or two splatters on the cam cover only.

The S15 was in a different league, and yes, thats a different league to the other S15's I checked. A friends S15 checked the very next day seemed to have a nice choof, but would idle, and not splatter oil everywhere. The S15 I checked was litterally blowing oil out, not splattering it.

IMO thats excessive. Also noticable on shut off, the engine ran on, which I saw as a sign of low compression in the cylinders.

Everyone has different ideas on blow by and what is 'excessive', but I'm happy I walked away from that one! I won't be touching one again in that state either

[The Dan]

I know cars drop revs. But this thing was dying, no two ways about it.

I have not seen an RB with such an issue yet. I can assure you none of my Rb25s (or mates RB20s) have come anywhere near dying when removing the cap, nor is there any real "choofing", and as for oil, one or two splatters on the cam cover only.

The S15 was in a different league, and yes, thats a different league to the other S15's I checked. A friends S15 checked the very next day seemed to have a nice choof, but would idle, and not splatter oil everywhere. The S15 I checked was litterally blowing oil out, not splattering it.

IMO thats excessive. Also noticable on shut off, the engine ran on, which I saw as a sign of low compression in the cylinders.

Everyone has different ideas on blow by and what is 'excessive', but I'm happy I walked away from that one! I won't be touching one again in that state either

Yes certainly if there is a noticeable difference between two cars the same, you can use that to gauge what is excessive or not. Only using your post as an example because the revs will drop and oil will come out if you take the cap off. Some ppl will read about the revs dropping and soon everyone will be stressing about buggered engines lol.

Engine dying is not a good sign, unless the idle was a little too low already. But yeah, referring to your better judgement is always best. My main point in all of this is basically that not everyone should stress out about oil in a catch can or fumes coming from breathers.

Hopefully the ppl who read from top to bottom of this thread will be able to use the information to be able to gauge it for themselves when they inspect a car. But when in doubt, get a leakdown test done.....they don't lie when done properly

[GTRPowa]

Keep in mind too that small amounts of fuel is mixed with that air being "blown by". This dilutes your oil eventually turning it into a fuel-oil mix hardly offering protection.

Keep an eye on your oil's quality if driving with "excessive blowby"

[gotRICE?]

The revs dropping and/or stalling is due to the PCV and exh cam cover -> turbo inlet breather correct? And the fact they are drawing in a small amount of un-metered air?

Assuming that the car still has the factory AFM setup that is.

[The Dan]

The revs dropping and/or stalling is due to the PCV and exh cam cover -> turbo inlet breather correct? And the fact they are drawing in a small amount of un-metered air?

Assuming that the car still has the factory AFM setup that is.

Yes that is correct.

However, it's not like introducing a direct leak to an intercooler pipe, it's more of an in-direct addition of unmetered air through the ventilation system into the intake

[The Bogan]

The more boost I run the more hydrocarbon liquid stuff ends up in the can, 11 psi I get nothing but at 15 psi after a track day I get 100-200 mils, no oil though, Im running R compound tyres but I don't think my balls and skills are up to pushing to many G,s.

Think I need to go faster through corners but someone keeps staying on the brake for to long.....................

[Mandingo]

The more boost I run the more hydrocarbon liquid stuff ends up in the can, 11 psi I get nothing but at 15 psi after a track day I get 100-200 mils, no oil though, Im running R compound tyres but I don't think my balls and skills are up to pushing to many G,s.

Think I need to go faster through corners but someone keeps staying on the brake for to long.....................

Easier said ten done, but commitent is the key! go into a corner half arsed and its all over from the start. Commitment to the terrain seperates the man from the sheep!

[The Bogan]

[meet07]

What about white smoke out of the tail pipe. I just noticed some more than normal blowby after installing this turbo. Im thinking its blowby from the piston rings b/c at idle im seeing smoke from the cacth can and the oil dipstick when removed. It could be the turbo but doubt it. The dipstick hasnt popped out after hard acceleration yet..

[GTRPowa]

What about white smoke out of the tail pipe. I just noticed some more than normal blowby after installing this turbo. Im thinking its blowby from the piston rings b/c at idle im seeing smoke from the cacth can and the oil dipstick when removed. It could be the turbo but doubt it. The dipstick hasnt popped out after hard acceleration yet..

You'll see some smoke in any and all cases, just differing amounts.

A compression test + leakdown test will answer your question properly.

Ask any decent mechanic who is wearing his mechanics uniform in the mechanics workshop.

Not the internet mechanics with propeller hats. Try and imagine every post ITT coming from the same person. lol..

(I am an internet mechanic with a propeller hat as well so you know.)