Hi guys,
I have acquired a 1981 Mitsubishi Sigma Super Saloon and I'm currently building up the factory engine to try and squeeze as much as I possibly can out of it while keeping it NA.
It has got the mighty 4G63a 8 Valve SOHC engine in it.
Any advice or information on this engine will always be greatly appreciated, however here is my current plan:
Upon removing the engine from the car and timing it in on an engine stand I found the engine to be non-interference (The stock valve motion has 10.3mm of lift and 212 degrees duration @ 1mm" and my piston to valve at TDC is almost 11mm, I double checked this by rotating the cam with pistons and TDC and they did not touch). This gives me heaps of room for modifications both to valve motion and compression ratio.
The stock head runs a 43mm intake valve and 35mm exhaust valve and the stock ports flow 178 CFM and 122 CFM respectively.
I have calculated the stock compression ratio to be 8.99 : 1, which matches the factory number of 9 : 1 for these engines.
I am currently planing to run a custom camshaft, with 13.5mm valve lift and 256 degrees @ 1mm. This will require some machining of valve guides and retainers to be able to fit this much lift but shouldn't be too difficult.
I will be porting the head to maximise CFM on the intake side with the stock 43mm valve. I then plan to run Toyota 22R exhaust valves which are 37mm diameter, same length, same stem size followed by a port of the exhaust port.
The stock engine comes with an intake manifold that is just awful. When I pulled the head from my engine it was very plain to see that the inner two cylinders had been running rich while the outer cylinders had been much leaner.
For this reason I have purchased a set of Yamaha YZF R1 carburetors. (If anyone has installed these on this head before and found a good way to re-route the water cooling that used to go through the inlet manifold I am all ears).
Upon stripping the block I discovered a bit more wear than I had hoped for, Including a seized balance shaft which had spat out a bearing into the sump. I don't plant to run the balance shafts anyway but this does indicate that the engine has probably been run very low on oil at some point. The bores are also pretty scraped up and the crank journals will need polishing. I am going to get the block bored to run oversize 85.5mm pistons, not sure which pistons to get yet though.
After this I will time the engine back in on the new camshaft, check my new valve to piston numbers and see how much material I can safely take of the block and head to increase my compression ratio.
Finally I plan to make a a custom exhaust manifold. 4 into 1, 36mm primary into 2.5 inch, or there about.
Feel free to let me know any thoughts people have about this project.
Also, if anyone has one of these engines and would like me to check any other measurements or values, let me know and I will see what I can do.
