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I saw this mentioned in another thread, but I didn't want to hi-jack that one. I was looking for clarification on what bay-to-bay breathing is and why it's important. Can someone explain this please? Is this only a concern with longer stroke crankshafts? Is it just a high rpm issue?
I heard that the truck engines don't have bay-to-bay breathing, but the car engines do. So, does that mean that the LS3 and L92 are not identical blocks? I know trucks and cars commonly use different intakes and cams, but I always thought the engine blocks were the same. Does anyone have a photo that shows the differences?
The bay-to-bay helps with Hi RPM breathing so on a truck less of an issue. Also the bay-to-bay does weaken the block a little so that is why they keep them on the trucks. The ls2, l3, ls7 and the ls9 all have the bay-to-bay breathing. The issue with the LS block because of its Y shaped and shallow oil pan design and 6 bolt mains the individual bays don't breath as well as the old school small/big block chevy. Basically, the non bay-to-bay blocks compartmentalizes the cylinder banks too much. The new Dart LS block does not have bay-to-bay but has the old 4 bolt main cap design and a wider bottom skirt. I think GM did tests and it was worth like 10-15 hp on the 505 hp ls7 motor. The bay-to-bay helps equalize the cylinders as it relates to the crankcase stuff. On blower motor guys are more interested in strength the crankcase breathing stuff that is why they use the 5.3 blocks. So the holes you see above are filled in on the 5.3 and truck blocks.
Ok, thanks for the photos and explanation. I saw the "coves" cut out of the side of the block right where the cross bolts go into the main cap (pic below). I thought that was the breathing window. So, I need to look closer to the cylinder to tell if a block has a breathing window. Does anyone have a pic of an L92 block? They are more common than the LS3 or L99.
Ok, thanks for the photos and explanation. I saw the "coves" cut out of the side of the block right where the cross bolts go into the main cap (pic below). I thought that was the breathing window. So, I need to look closer to the cylinder to tell if a block has a breathing window. Does anyone have a pic of an L92 block? They are more common than the LS3 or L99.
You're correct, the "coves", or scallops, do aid in the bay to bay breathing. Earlier LS1 blocks didn't have the cast in windows in their blocks, but had a rather large hole machined thru the main caps. Some builders swear that the early blocks ('99-'00) are stronger due to not having the cast in windows. IIRC, all the Gen 3 blocks had the coves/scallops. If I'm wrong, I'll soon be corrected!!
There might be some truth to that as I've seen three LS7 blocks that were used in high boost applications where some of the crank saddles had been torn from the sides of the block.
There might be some truth to that as I've seen three LS7 blocks that were used in high boost applications where some of the crank saddles had been torn from the sides of the block.
I got my info from Steve@Race Engine Development. I value his experienced opinion.
The front three bulkheads are drilled through the front of the block. The block is then reversed to drill two matching holes from the back of the block. This takes roughly three hours of set up time and machining. Also a photo of a factory drilled LS1 block being machined for MID sleeves many years ago showing the same hole in the front of the block. You would need a huge machine to drill from the back avoiding the front bulkhead. Even GM didn't do it that way.
Which operational circumstances would most benefit from "bay windows"?
I can see converting Gen IV aluminum 5.3L blocks to LS6 specs via this operation and boring to 5.7L spec. Usable LS6 blocks are getting rare.
I was thinking about using a small angle grinder with a short drill bit, then a small burr. I'm not sure how much room is in there though. I don't have a big enough drill to go in from the ends.
It just looks like the holes in the end would allow oil to get into the front and back covers. I know those covers can leak, and I wouldn't want to encourage them more. I probably won't try this after all. It looks like something that is better left to the professionals.
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Bay-to-bay breathing question 
