Ok, upon a preliminary inspection of the short block, this is what I noticed. I do not think my engine builder changed the secondary porting so this should be what was there before rebuild. I could be wrong but I thought he just touched up the primary ports. Anyway, I am having a problem with blowby pushing large amounts of oil out of my crankcase and into my catch can. It started after the rebuild. I did not have the problem before but also had a different turbo. I now have a gt40r. The blowby gets worse as boost goes up.

with the motor still together and the intake manifolds removed, I noticed the following. Looking in through the secondary intake port on the front rotor, I can see the outer oil control ring pass onto the inner edge of the port opening. It does not pass completely over to where it opens on the other side but about half way. I mean that the thickness of the oil ring is half exposed as it passes over the port. As you keep turning the motor by hand, it receeds back to where it disappears .

I know that I had a low compression reading on one of the front rotor faces after rebuild. This would indicate a side seal and would only be problematic under the combustion stroke. This is where the apex and side seals completely seal all 4 corners of the rotor face for the combustion process. If a side seal is weak or not up to par, then combustion would make it to the outer oil ring.

After the combustion and exhaust stroke, the rotor is coming up to the intake port. The side seals actually pass over the intake port as the rotor opens the port for the intake stroke. It is at this point that I can see the side of the rotor face and the outer half of the outer oil control ring. If this is too much encroachment, then Boost pressure in the intake manifold is pushing past the oil control ring and presurrising the crankcase. I would be getting fresh air in the crank case. This does seem to make sense as it does it more as boost gets higher.

I am unsure how much combustion pressures rise for each pound of boost, but I would imagine that it wouldn't be much different for 10 lbs vs 15 lbs. on the other hand 10 lbs vs 15 lbs directly on the oil ring might be the difference between blowby and no blowby.

Any input on port clearance would be helpful.

Mike

 

if you are getting blowby into your catchcan chances are the combustion pressure in your engine is leaking probably by the sideseal cornerseal gap and pressurizing your crankcase. generally a pressurized crankcase makes the oil not drain properly from the turbo centerhousing and you may get oil past the rear turbo bearing into your exhaust. less probable is the porting as your engine has a mild port job. do get a compression test and let us know what you find.

good luck

howard coleman

 

howard, we are going to inspect the side seals and corner seals. Compression on the front rotor was 75/75/65. This does indicate a side/corner seal issue. I do not want to rule out a bad port job. This is why I was curious about the interplay between the oil ring and port openning. I have done a little seaching and the diagrams I have found say that the port can be opened up inward to the point of the oil seal track. It does not say if you can lay onto the seal to a certain point.

The info on the corner seal end says that you need at least 50% of the corner seal supported but that you can port up to a point of half of the corner seal. Otherwise it will fall in. I can physically see the Oil ring through the port as the rotor comes around. I was curious as to wether it should be 100% behind the port (none of the ring exposed to boost pressure.)

The car seems worse at higher boost levels. How much is the combustion pressure increased by running higher boost? Shouldn't I see blowby wether I run 10 or 15 lbs. I would think it would leak under both cases as there is a lot of pressure at the point of ignition regardless of boost level.

Mike

 

Oh, THIS will do it.
You're not supposed to see any parts of the oil control ring.
It looks like your builder ported "toward the center", which is a big no-no in my book.
Porting towards the center intrudes into the oil control ring path and definitely increases blow-by.


-Ted

 

ah ****, I thought so. Well, this means new housings and more porting.

Hey Ted, any idea how much more pressure is created in the combustion chamber when you increase the boost? Does it rise significantly?

Mike

 

In my experiences, porting within 1/2 of the outter oil seal track has zero affect on blowby gas production or visible blue smoke. Several other engine builder's have expressed similar experiences. This was discussed a little while ago in no pistons.

 

Combustion chamber pressures are on order of about 1,000psi, so we're talking serious pressure here!


-Ted

 

I seen a 13B-REW ported 1mm max into the center.
Even with a 0.5mm buffer, it's still 0.5mm of intrusion into the oil control seal path.
This motor turned the engine oil black in 500 miles; a testimony of the serious amounts of blow-by by the port.


-Ted

 

Ted, what I am interested in knowing is how much more the pressure rises in the combustion chamber when the boost is turned up.

example. Boost = 10psi then combustion pressure = 1000 psi
Boost = 15 psi then combustion pressure = ?(1005 psi or exponential inc)

If it doesn't rise that much then it would seem that blowby due to corner seal/side seal gap shouldn't get worse with added boost. I mean, whats the difference between 1000 psi and 1050 psi. Both should leak.

 

I had a shitty port job like that done when I had my FC , the guy cut the lower side of the port , toward the centre where the oil ring passed . I had the same experience , so much blow-by under boost that oil would sometimes end up on my front wind screen . On start- up there was also a lot of blue smoke , which would clear up when the it warmed up (and the oil pressure dropped) . The motor eventually died a natural death , locking up after some high revs !!!

 

I think the simplified equation is ratio of intake pressure = ratio of combustion pressure.
In actuality, it's more a function of airflow, cause we all know 15psi of boost on one turbo is not the same for all turbos.


-Ted

 

I concur with Scathcart. Part of the porting work I do all but usually required matching the nitride facing to the cast iron port "below". This almost certainly requires going into the outer oil control ring track. I've never experienced any problems with any of my customers' engines having excessive crankcase blow-by.

By the way, since when do you see 1000psi of pressure on the intake port side? It's a moot point. Going in even half-way on the outer oil control seal track won't have any effect at all.

B

 

The intake air/fuel charge that enters the engine is more or less the same as the boost pressure.

Once compressed by the rotor, the pressure would be pretty much (((Boost Pressure + Ambient Pressure) * Compression Ratio) - Ambient Pressure).

 

ok, and then it ignites and we see what amount of pressure from the "explosion" Obviously the power stroke is seeing much more psi than just the intake psi x the compression ratio. The question is, is this combustion psi (100psi much higher when boost is turned up only 5 psi.? Does 1000 psi go to 3000 psi or something like 1005 psi?

This would account for a huge difference in blow by for side seal issue.

Mike

 

I would look, then, for another problem realting to combustion blowby. I port farther than that into the oil seal track, and I have never experienced any oil darkening outside the normal amber colour on 5000 km oil change intervals. I am not describing a single engine, either: I have built multiple engines using this porting technique and none of them have experienced such levels of blowby.

 

also clearancing can have alot to do with getting more or less blowby.

 

after losing four motors in 04 due to sideseal issues (builder was dremel happy) i am well acquainted w blowby issues. yours sound a bit similar.

my motors always died when i was under boost. i am early stage tuning my twin garretts so boost was limited to approx 10 psi. as far as blowing out either poorly supported sideseals or poorly clearanced sideseals it appears that the effects of boost aren't linear.

if you have low compression on one rotor face it is probably a sideseal issue and not a poorly supported oil ring. there's lots of opinions on sideseal clearance but after 4 motors and listening to all i will be setting mine at no more than 1.5 thou. my last motor had between 4.5 and 8 thou sideseal to corner seal and had 65-75 compression.

howard coleman

 

Are we talking about strictly street / extended ports, or you guys mentioning bridges or more exotic ports?


-Ted

 

I went over to look at my motor and it hasn't been pulled apart yet. When I was looking in through the front secondary port, I noticed that there is a chunk of metal missing along the inside edge of the port. The side that lays onto the oil seal track. I do not remember this being there when we cleaned the housings for assembly. The ports were nice and smooth and had a very sharp edge (90deg right angle) on this side of the port.

I can not see how this could have happened after the engine was back together. It would have destroyed the motor and turbo. I have to see what it looks like when the housing is off. It looks like it was sheared from the existing metal. I will try and take some pictures. This is the same place where I can see the oil control ring pop into view.

This should have nothing to do with the low compression #'s on the one face. I still have to look at this as well.

I was reading on no pistons.com about port shaping. Judge ito was talking about how the closing side of the port should be radiused to help protect the incoming side seal. Evidently, the factory port does not cross the leading edge of the incoming side seal. However, when you increase the length and size of the closing edge with a large street port, it now lies in the pathe of the corner seal and front edge of the side seal. The spring under the side seal pushes it against the side housing and it can snag on the sharp (90 deg ) edge of the port. This is why he recomends radiusing the closing edge of the port to act as a ramp to flow the side seal back into the gap and not catch the leading edge. This is supposed to extend the life of the motor.

Mike

 

I am describing my largest street porting.

 

I think you would need a p/V Diagram to determine the peak combustion pressure, but the BMEP would give you an average pressure to work with.

I'm not a mechanic, so I wouldn't know about the side seal issue.

 

Intake port opening timing increased?
Approximately how much?
Or idle vacuum numbers?

The motor I was describing had a very mild street port done to it, putting down 16" to 18" of vacuum at idle.


-Ted

 

Yes... for street-ports this large, I don't worry too much about overlap, or the associated crappy idle. I leave 1mm off the scribed edge of the leading edge of the side seals... intake opening is ~20 degrees ATDC... stock is 32 degrees ATDC, IIRC.

Vacuum on these engines is usually 12-14 inches.

I wonder if the condition of the side seals or the irons would have been a more signficant cause of the blowby.

I run about a half thou clearance on the side seals; just enough to get them to fit move in and out smoothly, and I always replace the corner seals. They wear into each other very quickly, and I haven't had a sideseal stick on me yet.