Just noticed your post. I recalled that Roger Bywater posted something on this a LOOONG time ago. I dug it out of the archives as follows:
Blockquote
It may be completely normal.
It was known quite early on that there was a difference in the combustion qualities of the two cylinder banks because B bank consistently produced higher HC emissions than A bank.
It was thought to be due to the way piston rock across TDC created a squish condition from above the top ring that favoured the spark plug location on one bank more than the other. Of course the piston motion was related to crank rotation which was inwards on A bank and outwards on B bank.
…
It was concluded that there wasn’t much that could be done about the phenomenon but it might explain why slant six experimental prototypes were configured as for A bank and the split-twelve project used A bank as the continuous running bank.
I would not be surprised if there were other V engines that displayed a similar discrepancy.
I would have put it down to the different thermal characteristics of the two banks. But I will defer to Mr. Bywater. I know he is not able to take on as much work as he used to, I really hope he has an apprentice scheme in place.
Thanks, Mark! Yes, that is indeed very interesting. The theory quite suits my experiments, as even on the ECU which produces less CO on B bank, the values on B bank exceed those from A bank under some load (higher RPMs). The same applies to idling after a session using higher RPMs. It will be interesting to see if the exhaust temperatures fit into the picture.
It might be relevant that some work I did on a custom piston highlighted an issue with the large area that is shrouded down the side of the piston due to how deep down the piston barrel the first ring sits. Comparing this to a modern piston suggests more hydrocarbons would be trapped and left unburnt due to the shrouding that occurs. The numbers I had showed about 7.5% of the combustion volume was trapped down the side of the piston. On more modern designs (ie Chevy LS7) this was reduced to slightly less than 3%. It wasn’t something that was easy to change, at least on the flat head with valve pockets required in the top of the piston. Not sure if the HE suffered the same issue.
And no doubt this impacts combustion efficiency also. Which was sadly not great.
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
27
I seem to recall seeing pistons that were a slightly smaller diameter above the top ring. One would expect that to result in a clearance between piston and cylinder wall even with the piston loaded against one wall. That might enable flame propagation down that clearance, possibly resulting in a more complete burn in that area.
Yes. Generally they are slightly smaller dia above the top ring. This is presumably for thermal expansion in the hottest part. But also when the combustion event occurs, pressure builds down the side of the piston and along the top of the axial clearance of the top ring, before forcing the ring out from behind in the top ring groove. (Figure 8)
Gas Ports and Ring Flutter
Combustion pressure aids in sealing the ring to the cylinder wall. Cylinder pressure enters the ring groove from the crevice volume, traveling across the top of the ring and down behind it to force it out against the cylinder wall. The pressure above the ring seals it against the bottom of the groove.
Also from the same Wiseco blog a note on the contact reduction grooves seen in many modern pistons, which mentions piston rocking and detonation prevention …
Contact Reduction Grooves
Contact reduction grooves reduce friction by minimizing piston material in contact with the cylinder wall above the top ring. They also disrupt pressure spikes caused by detonation.
These grooves are machined into the top ring land above the top ring to minimize contact drag when the piston rocks over upon reversal. They add minimal volume to the crevice volume and they also help resist detonation by disrupting flame travel into the crevice volume where pressure spikes might unseat the ring.
