Chamber mod intended to improve combustion
Posted: Fri Sep 13, 2019 10:57 am
I didn't/don't like some aspects of the chambers on my BBM Victors and carved on them a little bit.
- Exhaust transition into quench area rather abrupt
- The standard 15* valve angle is a plus, but the standard location of the valves vs the exhaust-side cylinder wall is too close
- The chamber's location with respect to the bore results in a pretty large quench area -- all on the cam-side of the chamber -- and at some point I know the quench ratio can be too big
- Most current 2-valve inline chambers I've looked at have a more gradual transition from the exhaust valve into the chamber, unless they're very small for high compression
- Stuff I've read regarding "reverse flow" into quench area indicates improving this characteristic helps overall combustion and can reduce amount of ignition lead required, too
So, I decided to lay back the transition area similar to many LS-type heads; here are the before & after shots. FWIW, I don't do much porting these days anyways, and never felt comfortable messing with the chambers. I had an old head to practice on before I touched the Victors to get a better feel for how to not botch it up too badly.
I wasn't worried about the loss in compression, since it was close to 11.5 before. A quick chamber volume check showed the mod added about 2 cc to the total, bringing the CR down to about 11.25. It's primarily a pump-gas engine anyway, so the less $$$ fuel I need to blend to keep things happy the better.
"Before" pics of chambers and piston tops after dyno time to show burn patterns:
. . .
"Before" pic of chamber scribed to lay back exhaust transition to quench pad:
. .
"After" pic of chamber with work completed:
. .
Two more "after" pics showing different chambers / view:
. . .
- Exhaust transition into quench area rather abrupt
- The standard 15* valve angle is a plus, but the standard location of the valves vs the exhaust-side cylinder wall is too close
- The chamber's location with respect to the bore results in a pretty large quench area -- all on the cam-side of the chamber -- and at some point I know the quench ratio can be too big
- Most current 2-valve inline chambers I've looked at have a more gradual transition from the exhaust valve into the chamber, unless they're very small for high compression
- Stuff I've read regarding "reverse flow" into quench area indicates improving this characteristic helps overall combustion and can reduce amount of ignition lead required, too
So, I decided to lay back the transition area similar to many LS-type heads; here are the before & after shots. FWIW, I don't do much porting these days anyways, and never felt comfortable messing with the chambers. I had an old head to practice on before I touched the Victors to get a better feel for how to not botch it up too badly.
I wasn't worried about the loss in compression, since it was close to 11.5 before. A quick chamber volume check showed the mod added about 2 cc to the total, bringing the CR down to about 11.25. It's primarily a pump-gas engine anyway, so the less $$$ fuel I need to blend to keep things happy the better.
"Before" pics of chambers and piston tops after dyno time to show burn patterns:
. . .
"Before" pic of chamber scribed to lay back exhaust transition to quench pad:
. .
"After" pic of chamber with work completed:
. .
Two more "after" pics showing different chambers / view:
. . .