jarkko
10-19-2015, 11:41 AM
Hi
Thought of asking about Yamaha squish bands because I have been looking at several 2.6 L 150...225 Yamaha heads today and many of them have different squish band diameters. Some 150 and 225 ones have around the same diameters but the total depth of the chambers is different. So the depth differentiates the volumes. One 150Hp was around 69 ml and one 225Hp was around 60 ml (top ones compared only). Maybe the same shape core has been used in both 150 and 225 casting processes. But... If I remember right the older 220 and 225 heads had smaller diameter cups in the chambers. Don't want to go and disassemble one to double check. And some 175 heads also have smaller diameter cups than the mentioned 225 ones.
There has been several messages here at S&F on what is the best head type for an old 2.6L Yamaha. Some say Excel heads, some Special heads. Now, if heads were to be machined down to the same volumes which one would then be better? I suppose there's many aspects to be considered like other modifications done to the engine. But if the final volumes were identical, would you go for the narrow and deep cup or the wide and shallow cup? I have been thinking about piston cooling, detonation, mixing of the air fuel mixture in the 2 different approaches - what has been there driving the changes over the years in the design and what would be the best core for machining some test heads, wide or narrow cup ones? I would not like to go through testing them all :). I'll let you all know if I do any testing.
Thanks, JJ
Thought of asking about Yamaha squish bands because I have been looking at several 2.6 L 150...225 Yamaha heads today and many of them have different squish band diameters. Some 150 and 225 ones have around the same diameters but the total depth of the chambers is different. So the depth differentiates the volumes. One 150Hp was around 69 ml and one 225Hp was around 60 ml (top ones compared only). Maybe the same shape core has been used in both 150 and 225 casting processes. But... If I remember right the older 220 and 225 heads had smaller diameter cups in the chambers. Don't want to go and disassemble one to double check. And some 175 heads also have smaller diameter cups than the mentioned 225 ones.
There has been several messages here at S&F on what is the best head type for an old 2.6L Yamaha. Some say Excel heads, some Special heads. Now, if heads were to be machined down to the same volumes which one would then be better? I suppose there's many aspects to be considered like other modifications done to the engine. But if the final volumes were identical, would you go for the narrow and deep cup or the wide and shallow cup? I have been thinking about piston cooling, detonation, mixing of the air fuel mixture in the 2 different approaches - what has been there driving the changes over the years in the design and what would be the best core for machining some test heads, wide or narrow cup ones? I would not like to go through testing them all :). I'll let you all know if I do any testing.
Thanks, JJ