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crankcase pressure


TimW

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bq3,

There is always blow-by of the piston rings, that cuases increased crankcase pressure.
If the piston rings are leaky, it will be worse.
You will notice anything from a breeze to a might wind blowing out of the open filler cap, and a compresison test will tell you, leaky rings.
OR
The normal amount of blow-by is not vented.   It should go either to the atmosphere in early cars or to the intake manifold, where the negative presuure will draw it in.   That's what Paudman was referring to, as opening the filler allows more air into the intake, changing the mixture and altering the idle speed.  Most GT6s have an "emission control valve" between rocker cover and manifold.   The pipe, or the valve can be blocked, raising crancase presure and preventing any chnage in speed when the filler is opened.

JOhn

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JohnD wrote:
bq3,

There is always blow-by of the piston rings, that cuases increased crankcase pressure.
If the piston rings are leaky, it will be worse.
You will notice anything from a breeze to a might wind blowing out of the open filler cap, and a compresison test will tell you, leaky rings.
OR
The normal amount of blow-by is not vented.   It should go either to the atmosphere in early cars or to the intake manifold, where the negative presuure will draw it in.   That's what Paudman was referring to, as opening the filler allows more air into the intake, changing the mixture and altering the idle speed.  Most GT6s have an "emission control valve" between rocker cover and manifold.   The pipe, or the valve can be blocked, raising crancase presure and preventing any chnage in speed when the filler is opened.

JOhn



John

The piston and rings are new (120 miles)

I do not have any sort of "emission control valve" just a pipe from the rocker cover to the inlet manifold balace pipe rarther than into the su carbs. could this be the cause?

I will check again tonight to see what happens to the engine revs when i remove the rocker cap.

Thanks for the help

Tim

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At just 120 miles some excess blowby could be expected as the rings are unlikely to have fully seated yet - reckon on 5 - 600 +.  Shouldn't be too much at idle though.  There are several breather possibilities and they are simply there to recycle the blowby fumes.  Are you sure it is positive pressure in the rocker box and not negative (from breather)

Nick

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Nick_Jones wrote:
At just 120 miles some excess blowby could be expected as the rings are unlikely to have fully seated yet - reckon on 5 - 600 +.  Shouldn't be too much at idle though.  There are several breather possibilities and they are simply there to recycle the blowby fumes.  Are you sure it is positive pressure in the rocker box and not negative (from breather)

Nick


Nick

Today i have bought a pipercross crankcase breather filter and fitted it to the rocker cover.  I have also blanked off the manifold take off.  I am now getting some smoke from the new rocker cover filter but do not get a release of pressure when removing cap.  

I think i am down on power because it seems to take an age to rev with a nasty flat spot at about 3000rpm.  The car is booked in with hispec motorsport next Wednesday for a session on their rolling road.  Maybe this will sort me out!!

Tim

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Tim I have seen a little bit of smoke when you take the top off a brand new Bentley and other petrol cars when they have been running for a while so I am not sure it's somthing to worry about

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I'd try and get a few more running-in miles on it before showing it the rolling road.  But then on the other hand if it's not going at all well a gentle run may get things in order and sort out any timing issues or lean running that might cause damage anyway.....    Just be sure the RR guy knows how little work it's done and treats it respectfully! Is it a TR5 cam you've got fitted?  What CR?

If you were getting positive pressure before and are not now with the atmospheric vent, it rather suggests that you breather arrangement didn't go anywhere!  (one way valve wrong way around?)

Nick

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Nick_Jones wrote:
I'd try and get a few more running-in miles on it before showing it the rolling road.  But then on the other hand if it's not going at all well a gentle run may get things in order and sort out any timing issues or lean running that might cause damage anyway.....    Just be sure the RR guy knows how little work it's done and treats it respectfully! Is it a TR5 cam you've got fitted?  What CR?

If you were getting positive pressure before and are not now with the atmospheric vent, it rather suggests that you breather arrangement didn't go anywhere!  (one way valve wrong way around?)

Nick



Nick

I will be using it to go to prescott on sunday so that will put a few more miles on it.  We didn't change any of the bearings in the engine at all when we did the rebuild and the engine builder said just take it easy on the rings and cylinders for a hundred miles or so

Yes it's a tr5 cam

The compression is 10:1

I didn't have any sort of valve fitted just a hose from rocker to balance pipe/servo take off on manifold

I know that the ignition curve will not be quite right as i am still using the delco dizzy from the 2.0l engine but i'm not sure if thats why it feels so flat.

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i had a filter on the rocker cover but soon took it off after the smell of fumes became annoying when in traffic,but the main problem was it filling with oil after a blast on the road and then the crankcase pressure eventually forced the oil through the filter and ran down over the manifold,very alarming when the smoke billows from under the bonnet.
have now got a decent sized oil seperator which is plumbed back into the atmospheric pressure side of the intake,use large bore tube and you won't have any pressure problems.

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