[Mk2] [Turbo] Crankcase pressure

[kev8611]

Hi,

I'm a little worried about the amount of crankcase pressure my engine seems to have.

It was rebuilt a few months ago with 86.25 cp's and was fine with the standard turbo etc but only after fitting a power fc, 800cc injectors and a GT3076r externally wastegated turbo with 3" downpipe into an apexi gt exhaust it seems to have increased. I have also noticed that the timing on the power fc is at 53deg. This surely isnt correct?

I have had it compression tested again at the weekend and all cylinders are high 160's and also the leak down test showed no signs of any concern.

I am just wondering if its normal for the crankcase pressure to increase when doing these mods to the car. I have not driven the car yet, it has just been ticking over. No signs of excessive blue smoke, I know it needs an oil change but surely this wouldnt be the cause unless the oil is extremely contaminated or welll broken down?

Thanks

[fingers99]

Generally you'd expect excess crankcase pressure to be a function of bore/ring wear. But you've ruled those out.

There are ways of measuring crankcase pressure, but I don't know of any baseline figures.

What makes you think yours is excessive?

[kev8611]

It seems to be more than what it was before lol

It only seems excessive on idle. It calms down when I rev the car.

[Marf]

How do you know you have excessive crank case pressure?

[kev8611]

I dont know thats why I am asking. When I take the oil filler cap off there seems to be alot of air rushing out compared to before. It goes away when revs increase a little.

Maybe its always been the case and im now paranoid but i cant remember it being like that

[Marf]

If the leakdown test is fine then don't worry IMO

[^Trickster^]

Your ring gaps are probably a little too big, I had this issue myself and I had excessive blowby, it never caused any problems tho, but it is annoying.

Who gapped your rings? and what gap did they make them?

Graeme

[kev8611]

If the leakdown test is fine then don't worry IMO

Yeah your probably right. It seems to run fine still and isnt popping the dipstick out.

Trickster. My local engine builder/motor engineer bored my block to the cp piston (the rings came with) spec sheet. He then checked the ring gaps etc for me and noted them down. I cant remember what they were but he had to file them a little. I then built the rest of engine myself.

Thanks

[^Trickster^]

Yeh well both my builds I had with cp pistons and done to their specs breathed a lot, but always ran fine, on high power engines you do run larger ring gaps, they maybe filed them on the larger side to be safe which is why the breathing is more noticeable on idle but when your giving it full power and there is a lot of heat the gaps close up. You do them larger on higher power builds so under hard load the ring ends don't touch under expansion.

Graeme

[kev8611]

Yeh well both my builds I had with cp pistons and done to their specs breathed a lot, but always ran fine, on high power engines you do run larger ring gaps, they maybe filed them on the larger side to be safe which is why the breathing is more noticeable on idle but when your giving it full power and there is a lot of heat the gaps close up. You do them larger on higher power builds so under hard load the ring ends don't touch under expansion.

Graeme

Thanks Graeme. Makes sense.

Also noticed that once the engine is up to temp it is not as severe, so maybe what you are saying is the case and as I say, it seems to run pretty well apart from needing a map obviously. I will have a look at the notes I got from the workshop and compare them to the spec sheet and cp's instruction on ring gaps etc.

Atleast now I can proceed with mapping

thanks again folks

[^Trickster^]

If you have rebuilt it and not mapped it you really need it on the dyno asap, itll bed the rings in properly, dont put loads of miles on it with casual driving, the cylinders need high pressure to bed the rings better.

Graeme

[kev8611]

It has already been ran in on the standard turbo. I rebuilt the engine earlier in the year.

Once it was run in the standard turbo didnt last long so I replaced it with the new one and done fuelling etc at same time. which is when i noticed something wasnt quite right (or so i thought)

its getting booked onto the dyno next week