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What Is 'blow By'

ryan101

By ryan101,
in Engines & Forced Induction

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The Dan Grand Master

The Dan

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Sorry, everyone talks about it I'm not sure wat it is?

Combustion gasses leaking past the piston rings. All combustion engines have blowby....it's 'excessive blowby' that means your engine MAY not be healthy. There are various other reasons for blowby including detonation (or hitting the limiter) and not really excessive blowby but gives the same result - incorrect plumbing of catch cans causing pressure build up in the crank case

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ryan101 Rising Star

ryan101

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Combustion gasses leaking past the piston rings. All combustion engines have blowby....it's 'excessive blowby' that means your engine MAY not be healthy. There are various other reasons for blowby including detonation (or hitting the limiter) and not really excessive blowby but gives the same result - incorrect plumbing of catch cans causing pressure build up in the crank case

What are signs of blow by then?

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hypergear Grand Master

hypergear

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What are signs of blow by then?

All engines has blow by. You means signs of excessive blow by or indication of engine failure?

If that is the case: Lots of oil throwing out of breathers, Poor engine compression, Engine noise, oil soaked spark plugs and etc.

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tridentt150v Grand Master

tridentt150v

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Heavy breathing......if you take your oil filler cap off and the motor is choofing hard [thee will be some blowby but you should be able to tell the diff]. If you dipstick keeps popping out, if you are using heaps of oil, if you are leaking oil out of the sump and/or rockers, if you have a burnt oil smell, if you have oil in your rocker tubing that drips out......there's probably a few I've missed.

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GTScotT Grand Master

GTScotT

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Heavy breathing......if you take your oil filler cap off and the motor is choofing hard

This can be misleading, its hard for a novice to know the difference so I would not advise this often.

Comp test and leak down are the most effective ways of testing for signs of wear that would result in excessive blow-by. Realistically if you are burning lots of oil, you can fit a catch can to try and rule out the turbo, then you can start thinking about blow-by.

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SiR_RB Grand Master

SiR_RB

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i remember seeing this r32 gtr begining of this year, at a first glance car looked neat, had it idling until it was warm and i took the oil filler cap off, it wasblowing white air/mist as high as the bonnet ! was making a "choofing" sound aswell...never seeing blow by that bad before..

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Granthem Mentor

Granthem

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i remember seeing this r32 gtr begining of this year, at a first glance car looked neat, had it idling until it was warm and i took the oil filler cap off, it wasblowing white air/mist as high as the bonnet ! was making a "choofing" sound aswell...never seeing blow by that bad before..

I checked out an S15, full service history for 120thou k. Everything looked great, took the oil cap off and dear son of rajab. The car couldn't idle, on the verge of stalling, and it was blowing oil drops out that high it covered the underside of the bonnet, and even made its way onto both front gaurds :down:

However, I've noticed many SR's have loads more blow by then any RB I've encountered.

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The Dan Grand Master

The Dan

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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.

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Guilt-Toy Grand Master

Guilt-Toy

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I hear you. So explain to us your opinion of "excessive" is ?

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.

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The Dan Grand Master

The Dan

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I hear you. So explain to us your opinion of "excessive" is ?

Impossible to explain it. It's something you know from seeing hundreds of cars over years of experience. Most ppl only see one or two and don't really have anything to compare it to. I'd be happy to take video of a 'normal' one so everyone can see what that looks like.

But, it's not like I can give some sort of measurement of gas flow or a quantity of fume lol.

You'd know from your experience with tuning anyway....why are you asking me?....or is it more for the readers?

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The Dan Grand Master

The Dan

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errr... Think its best i walk away from this one. Should have known better then to bother with someone with an attitude like that.

Wasn't having a go at you mate. Just saying there is no practical way to type in text how much is too much. I was also pointing out that I assumed you would know how much is too much as you have a lot of experience especially with cars on dynos.

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the_jack Enthusiast

the_jack

Posted
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my rb30e+t (high comp) is a massive fan of blow by lol. pops the dip stick out all the time, oil in the intake, fouls plugs, oil even seeps out of the sump and around the oil pump now aswell.

putting a new rb30et in this weekend and pulling the old block down to build a 25/30 if it is still any gd :P

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