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Discussion Starter #1
I was trolling the internet looking for information on modifications to the B20 head (just because I am curious, not because I am considering it!). I found a few examples with photos where people had raised the roof of the exhaust port around the area where the valve guide enters the port. It seems that in all the examples that I found, the valve guide had been cut off flush with the new roof of the port.

It would seem that the ports modified in this manner would intuitively flow better (recognizing that fluid dynamics does not always yield to untutored intuition). From the photos, it was not clear what they had done to deal with that rather nasty right angle turn that occurs between the valve seat and the floor of the port. However, the merits of the port modification on flow is not my primary question. If anybody has done this, I am curious as to whether the significantly shorter valve guide results in accelerated wear of the guide due to the smaller bearing area for the side thrust imparted by the rocker motion? Finally, given the rather poor shape of the port, would leaving the guide untouched and extending into the port make a material difference to the potential flows through the port?
 

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This is a good question, I have been thinking about it quite a bit lately. Been spending some time looking at a spare head I have sitting in the basement. To give some perspective at what you are dealing with here is a cross section of the intake and exhaust ports. Not sure if this is actually a B20 head but it looks darn close.





Intake port looks decent, exhaust looks terrible. Of course 'looks' don't necessarily mean anything.

Air will want to flow to the outside of the turn, that bump in the exhaust not only restricts flow from a cross section perspective but forces the exhaust to make a couple of direction changes. The exhaust port is not centered around the the valve stem so it does get a smoother path on the side away from the intake. Opening that side up could increase flow with less extreme modifications.

I think the only way to handle the short side radius is to weld up the floor like Phil S does.

As far as valve/guide wear is concerned, the only major source of side loading on the valve stem is the rocker. Assuming a very stiff guide and semi flexible stem, a guide with material removed from the bottom side should not affect wear due to rocker forces.

Looking at the other end of the valve, there will be a longer section of the valve that is unsupported. This could lower the frequency of any lateral vibration and increase it's amplitude, which could cause some wear issues both at the guides and the seat. However, considering that the intake valve is larger, and has more mass dangling around at the end I doubt shortening the exhaust guide would make it perform any worse than the intake. Of course this is all arm-chair engineering.

One has to ask themselves why did Volvo make the heads like this in the first place. Perhaps they were concerned about heat in the exhaust valve. Having the guide fully encased by the head would pull more heat away, this obviously is not a concern on the intake side and the guide sticks out from the head casting. They may have intended to put that restriction there to provide an EGR effect without complex plumbing.
 

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Discussion Starter #3 (Edited)
That photo you showed is a section of a B20 head. The photo below is a section of the exhaust port on one of the B20 heads that was discussed on the Speedtalk forum.



The section through the port appears to be offset from the guide compared to the one you show; however, what is apparent is that the boss or protrusion in the casting around the valve guide looks like it has been completely eliminated with the objective of raising the roof of the port. Other photos of similarly modified ports show that the valve guide has been cut flush with the new roof of the port, significantly shortening the guide.

There is approximately 0.003" valve stem to valve guide clearance which will result in some side to side movement of the valve stem within the guide due to the side thrust applied by the rocker at the valve tip. A shorter valve stem definitely will result in greater side to side movement of the valve head and the side load applied at the end of the guide in the exhaust port. I have not bothered to plot the geometry of the motion of the rocker and the valve stem (too much like mechanical engineering - all that instant centers of motion stuff) and it may be that the side thrust while present, is immaterial. I was just curious as to whether ports modified in this manner experienced excessive guide wear (which if they did, I expect would also experience unevenly worn valve seats). It is also possible that if these port modifications end up in performance / racing engines, perhaps the heads come off frequently enough for other reasons that the guide wear does not become a limiting issue.

I agree that intuitively the exhaust port looks poorly designed and that the comments of Phil S. and others suggest that its flow characteristics are limiting. However, the purpose of EGR is to dilute the intake charge under certain operating conditions as a means of reducing NOX. NOX control definitely was not in the picture when the B18 and B20 were designed so I don't think that is the objective of the protrusion supporting the exhaust valve guide. As you note, the encased guide should enhance the heat transfer from the valve to the head and perhaps enhance the durability of the engine which sends me back to my original thoughts on the effect of the shorter guide on the durability of the guide!
 

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I like doing a little math. I didn't bother measuring anything just made a bunch of assumptions that would result in correct orders of magnitude. Angular deflection of a valve due to the 0.003" difference in guide and stem diameter would result in a linear displacement at the valve of about .000001". Shortening the guide length by 1/4" will increase that number by 50%. Throw in the fact that there will be a small fluid film of oil, the numbers are so small to not be a concern.

I would guess heat would be the biggest issue, but if you have a AFR meter and avoid super lean conditions you will probably be fine. The factory had to ship engines that would have to deal with some wild swings in AFR.

Even if the reduced area would increase wear by 10-20%, the life of a valve and guide would still be well into the hundreds of thousands of miles. I doubt most classic owners are going to need hundreds of thousands of miles of reliability, and if you are, you probably should not be seeking performance modifications.

This page has some nice shots of a heavily ported head with dyno results and port castings. This was from a swedish head porting contest

http://www.topplocksverkstan.se/flodestavling2010.html


http://forum.savarturbo.se/viewtopic.php?f=46&t=48429&start=210
 

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B20 exhaust modifications

I was trolling the internet looking for information on modifications to the B20 head (just because I am curious, not because I am considering it!). I found a few examples with photos where people had raised the roof of the exhaust port around the area where the valve guide enters the port. It seems that in all the examples that I found, the valve guide had been cut off flush with the new roof of the port.

It would seem that the ports modified in this manner would intuitively flow better (recognizing that fluid dynamics does not always yield to untutored intuition). From the photos, it was not clear what they had done to deal with that rather nasty right angle turn that occurs between the valve seat and the floor of the port. However, the merits of the port modification on flow is not my primary question. If anybody has done this, I am curious as to whether the significantly shorter valve guide results in accelerated wear of the guide due to the smaller bearing area for the side thrust imparted by the rocker motion? Finally, given the rather poor shape of the port, would leaving the guide untouched and extending into the port make a material difference to the potential flows through the port?
Hi 142 guy,
In the late 60's early 70's in Sydney Australia a number of dirt track sprint cars ran B20 engines, One of the front runners at the time had the roof of the exhaust port raised (similar to the pictures in the other thread) and metal added to the floor of the port. The side view of the exhaust ports showed two square holes. The added metal increased the radius of the inside exhaust port curve. The exhaust port hole size appeared to be slightly reduced but must have given increased air flow. Hope this helps
 
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