We have all read the various articles on big bore heads on small bore motors and most get away with the water jacket misalignment and apparently recessing the exhaust valve a few thou to use a mid range cam doesn`t affect the flow any mentionable amount ?
Two perfectly clean surfaces and a quality gasket should hold back 10 psi of water, no ?
Then there is the debate over the loss of torque/ drive-ability at lower revs due to the inlet tracts being too large but considering a 950cc Fiesta has 33.20 mm inlets and 29.1 mm exhaust vales which is as near as damn it to a unleaded 1275 head and puts out 45 bhp in a nice manner begs the question why ?
It is said you have to keep the cam lift low if not pocketing the block which in turn defeats the object of increasing power but a lame cam never stopped an 850 from revving ?
Keeping the inlet manifold bore small as well as the carb 1 1/2" or Hif 38 should help the gas speed at lower revs ?
I can only ask questions as i`m not technical enough for the science part of it and look back on my own experience when I fitted a 1275 head on a 998 probably about 24 years ago on a budget weekend hooligan fun car ( £70 all in ). Standard HS4 with a drilled airbox, a kent cam of some sort and a Cooper three into one exhaust manifold with a Cherry bomb.
It went like stink and was great fun and never failed in the 12 months of abuse I gave it despite the lack of any care and attention to working clearances, just pure luck based on the fact it didn`t squash my Blu tack or Plasticine flat when I turned the motor over.
The fact is you can have all the best parts in the world but if they don`t compliment each other and are not set up right it won`t deliver the goods hence the need for a rolling road session. There are a lot of modified 1275`s out there that can`t keep up with a standard one.
I got lucky I guess in keeping the standard inlet manifold and i picked the right needle and the dizzy curve was OK and maybe I didn`t notice any loss in low down torque due to the 3.44 dif ratio keeping the revs up anyway. The speedo needle would willingly pass the ignition light and 70 mph in third was the normal gear change point but it behaved like a standard car and pulled up the hills in forth gear without complaint at 40 mph.
12g940 heads are cheap and if you get a factory unleaded one all the better as I personally wouldn`t risk hardened seats in a 12g295
Edited by absx2, 13 November 2019 - 09:19 PM.