Discussion
Hi.
I would like to fit bigger valves in my Austin metro's 12H engine, when I have tryed to buy the valves I have discovered that there are two diferent lengths.
Which is the better length for fit in my head (I have also a 1.5 roller rocker, but I don't know if it's important).
Thantks for all
I would like to fit bigger valves in my Austin metro's 12H engine, when I have tryed to buy the valves I have discovered that there are two diferent lengths.
Which is the better length for fit in my head (I have also a 1.5 roller rocker, but I don't know if it's important).
Thantks for all
I didn't know that the 3-groove valves came in different lengths. I do know that there can be a difference in length between the single-groove and the 3-groove valves, but the single groove ones also have different taper angles in the valve caps and can't be mixed-and-matched.
The best thing is to phone Rich at Mini Spares in Harrogate (that's 1071Guru on here) and ask him what he has in stock.
For a good road engine it's not worth going over 35.5mm on the inlets as there can be issues with the unleaded valve seats.
The best option is probably to use 35.5 mm inlets with a roller rocker shaft and, ideally, a cam to suit your desired use. For the road an MG Metro cam is super, or a Kent 266 or a Piper 270. A hotter cam than this will a lower diff ratio for road use (unless you like constantly slipping the clutch in traffic).
More important than valve diameter is the porting of the inlet throat, especially where it does its 90 degree turn into the valve. Unless that's well modified the mixture can't flow in efficiently whatever the valve size and if the throat is not properly modified all that happens is that the mixture slows down as it enters through the bigger valves and the swirl/mixture velocity is insufficient to fill the chamber properly.
In fact, yesterday I spent a lot of time porting and modifying the head of an MPi which will have 33.5 mm inlets with a 1.5:1 roller rocker. With the standard cam there really is no need for the 35.5 mm inlets if fitting the high-lift rockers.
I hope all this helps.
Peter
The best thing is to phone Rich at Mini Spares in Harrogate (that's 1071Guru on here) and ask him what he has in stock.
For a good road engine it's not worth going over 35.5mm on the inlets as there can be issues with the unleaded valve seats.
The best option is probably to use 35.5 mm inlets with a roller rocker shaft and, ideally, a cam to suit your desired use. For the road an MG Metro cam is super, or a Kent 266 or a Piper 270. A hotter cam than this will a lower diff ratio for road use (unless you like constantly slipping the clutch in traffic).
More important than valve diameter is the porting of the inlet throat, especially where it does its 90 degree turn into the valve. Unless that's well modified the mixture can't flow in efficiently whatever the valve size and if the throat is not properly modified all that happens is that the mixture slows down as it enters through the bigger valves and the swirl/mixture velocity is insufficient to fill the chamber properly.
In fact, yesterday I spent a lot of time porting and modifying the head of an MPi which will have 33.5 mm inlets with a 1.5:1 roller rocker. With the standard cam there really is no need for the 35.5 mm inlets if fitting the high-lift rockers.
I hope all this helps.
Peter
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