Even-ing up the mixture and other thoughts
Posted: Wed Feb 26, 2020 11:17 amSo I'm currently grinding in a new exhaust valve on No 4. Compression went to pretty much zero - just like that.
With the head off it was pretty apparent the edge of the valve had started to erode. Fortunately the valve seat was OK. Further digging - in the (EMPI) head - there was a significant casting "ridge" in the exhaust port (not present in the No 3 end). And the port itself was narrower than the gasket by a fair amount. Not at the No 3 end. So I'm thinking restricted exhaust flow got the valve hotter than optimal. EGT's were not suggesting anything however.
So I've ported and polished the No 4 end of the casting to get rid of the ridge and to "gasket match" the gasket and exhaust pipe. Some matching polishing in the other end to try and keep things even. No real metal removal required at that end.
Just waiting for the mail man with the new exhaust valves.
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So....... Let's go street car racing a moment. Look harder at the inlet side of things. You know - no street car mechanic would allow this mismatch between the red manifold (rectangular holes) and the aluminum (round) ports.
Street wisdom would have it that the mixture being delivered though the rectangular inlet - hitting the flat edges of the round holes in the ports would tumble and be restricted and inlet flow would be compromised. On the street - you would remove aluminum from the round holes till they were a rectangular match for the inlet manifold. Remove restriction, improve flow, more power. Usually this simple mod, with matching polishing of the inlet casting, would get 10% more power.
So let's take this a little further - could I try differential porting - making one hole match and not the other - to improve the differential mixture the engine suffers from and so even up the CHT and EGT's? Power improvement would be less of a factor (though there would be some). Getting all four cylinders to burn equally and so smooth the engine out would be a better longer term aim.
--
Two thoughts against:
- Over porting can actually decrease the inlet mixture speed.
- Perhaps the mismatch of the rectangle to round holes is a deliberate "design" way to increase mixture "tumbling" to get a more even mixture?
--
Anyone?
With the head off it was pretty apparent the edge of the valve had started to erode. Fortunately the valve seat was OK. Further digging - in the (EMPI) head - there was a significant casting "ridge" in the exhaust port (not present in the No 3 end). And the port itself was narrower than the gasket by a fair amount. Not at the No 3 end. So I'm thinking restricted exhaust flow got the valve hotter than optimal. EGT's were not suggesting anything however.
So I've ported and polished the No 4 end of the casting to get rid of the ridge and to "gasket match" the gasket and exhaust pipe. Some matching polishing in the other end to try and keep things even. No real metal removal required at that end.
Just waiting for the mail man with the new exhaust valves.
--
So....... Let's go street car racing a moment. Look harder at the inlet side of things. You know - no street car mechanic would allow this mismatch between the red manifold (rectangular holes) and the aluminum (round) ports.
Street wisdom would have it that the mixture being delivered though the rectangular inlet - hitting the flat edges of the round holes in the ports would tumble and be restricted and inlet flow would be compromised. On the street - you would remove aluminum from the round holes till they were a rectangular match for the inlet manifold. Remove restriction, improve flow, more power. Usually this simple mod, with matching polishing of the inlet casting, would get 10% more power.
So let's take this a little further - could I try differential porting - making one hole match and not the other - to improve the differential mixture the engine suffers from and so even up the CHT and EGT's? Power improvement would be less of a factor (though there would be some). Getting all four cylinders to burn equally and so smooth the engine out would be a better longer term aim.
--
Two thoughts against:
- Over porting can actually decrease the inlet mixture speed.
- Perhaps the mismatch of the rectangle to round holes is a deliberate "design" way to increase mixture "tumbling" to get a more even mixture?
--
Anyone?