I am thinking about what to do next with my M20.
My current engine has 172000 miles. Use no oil and is sound with no running issues whatso ever. It is a UK spec 525e bottom end, that is 81mm crank, 130 mm con rods and 10.2:1 CR eta pistons. A 885 cylinder head is fitted along with the matching inlet manifold. A big bore TB is alos fitted (it sounds a lot better) and an 082 AFM with motronic 1.3. The problem is troque above 5500 rpm drops of quite rapidly. I would like to improve the high rpm performance to justify the fitting of a 3.91 final drive to replace the 3.46:1 which is fitted at the moment and suit the current torque curve well. The compression ratio of the engine is calculated to be 9.35:1. The only thing is I do want to sacrife too much low torque but losing a little will not be too much of problem witht the shorter diff.
The three options I am concidering, one of which will happen are;
1) fit a longer duration cam such as a cat cams 285/285 profile with 11.55mm of lift, a 325i exhaust manifold and down pipes (to replace the 525e exhaust manifold and pipes) and a new exhaust to match the larger down pipes.
2) As above but I would like to get the inlet manifold and inlet ports port matched as well. Also machine work to improve the air flow around the valves. I know the benefits of this are going to come at a price but I am also hoping it will allow me to get the best out the cam and exhaust.
3) Raise the CR back to 10.2:1 by using a 731 head with above cam profile and 42mm/36mm valves inserted to replace the smaller ones in this cylinder head. The inlet ports will need to be matched to the 325i inlet manifold. Mate this to the 325i exhaust manifold and down pipes job hopeful done.
Option three will be the most expensive, but will raising the CR give me any noticlable benefit over sticking with the 9.35:1 CR provided by the 885 head. It's not much of jump so this is why I am asking.
My current engine has 172000 miles. Use no oil and is sound with no running issues whatso ever. It is a UK spec 525e bottom end, that is 81mm crank, 130 mm con rods and 10.2:1 CR eta pistons. A 885 cylinder head is fitted along with the matching inlet manifold. A big bore TB is alos fitted (it sounds a lot better) and an 082 AFM with motronic 1.3. The problem is troque above 5500 rpm drops of quite rapidly. I would like to improve the high rpm performance to justify the fitting of a 3.91 final drive to replace the 3.46:1 which is fitted at the moment and suit the current torque curve well. The compression ratio of the engine is calculated to be 9.35:1. The only thing is I do want to sacrife too much low torque but losing a little will not be too much of problem witht the shorter diff.
The three options I am concidering, one of which will happen are;
1) fit a longer duration cam such as a cat cams 285/285 profile with 11.55mm of lift, a 325i exhaust manifold and down pipes (to replace the 525e exhaust manifold and pipes) and a new exhaust to match the larger down pipes.
2) As above but I would like to get the inlet manifold and inlet ports port matched as well. Also machine work to improve the air flow around the valves. I know the benefits of this are going to come at a price but I am also hoping it will allow me to get the best out the cam and exhaust.
3) Raise the CR back to 10.2:1 by using a 731 head with above cam profile and 42mm/36mm valves inserted to replace the smaller ones in this cylinder head. The inlet ports will need to be matched to the 325i inlet manifold. Mate this to the 325i exhaust manifold and down pipes job hopeful done.
Option three will be the most expensive, but will raising the CR give me any noticlable benefit over sticking with the 9.35:1 CR provided by the 885 head. It's not much of jump so this is why I am asking.
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