Jump to content

ToMitz' 1981 Mitsubishi Sigma build ideas.


ToMitz

Recommended Posts

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.

 

   

 

 

  • Like 2
Link to comment
Share on other sites

Im not 100% sure where all of the parts will be coming from. However I am currently lucky enough to be one of the Cam design engineers for Kelford Cams, so I will be able to do alot of the head / valve-train work myself with help from some friends here at kelfords and others in the performance industry. I am currently working on sourcing as many other parts as I can through work (Pistons, rods, gaskets etc).  

Link to comment
Share on other sites

  • 2 weeks later...

no idea on the engine stuff but i saw a gold one of these/similar shape with gold meshies and lows, near whenuapai on the way to work today and it looked like perfect 80's tits.

 

i strained my neck as it went past the other way

 

please carry on

Link to comment
Share on other sites

  • 2 weeks later...

Decided to weld up the combustion chambers, with help from the guys at Royce Clive engineering. Should make it a bit easier to get this bad boy up to 12:1 compression or maybe more. Will machine the welds back ASAP and CC the head to find out what gains have been made.

  • Like 1
Link to comment
Share on other sites

Keen to see this take shape! Badass. I have no idea what I'm talking about, but 256 duration seems quite mild? Especially with the amount of compression you're going to run (also badass) if you're wanting to run it on pump gas.

 

Full stock suspension and wheels and high output motor? Sounds good.

Link to comment
Share on other sites

Keen to see this take shape! Badass. I have no idea what I'm talking about, but 256 duration seems quite mild? Especially with the amount of compression you're going to run (also badass) if you're wanting to run it on pump gas.

 

Full stock suspension and wheels and high output motor? Sounds good.

The 256 degrees was at 1mm of valve lift, the total seat to seat duration would have been about 305 degrees. I have actually decided to de-tune the cams a bit more now, sow they will now be 252 @ 1mm or 292 degrees advertised and 13.5mm lift on the intake. And 245 @ 1mm or 285 degrees advertised and 12.5mm lift on the exhaust. Designed to give me my desired intake opening point as well as 9:1 dynamic compression with 108 degrees seperation. 

It will start life on stock suspension and wheels. Will probably clean up and widen the stock rims after I get it running then look at suspension, brakes, diff etc.

Link to comment
Share on other sites

those recesses near the spark plugs are where the Jet valves would be if it were a MCA-Jet head.

Thanks for that information, I have been trying to find out what they were about. Glad I got them filled in now, this engine has no time for emissions control, even if it wasn't being used. 

Link to comment
Share on other sites

  • 1 month later...

I'm currently looking at options for a motor that's going to have a bit of power for weekend driving and occasionally daily it. Are these motors worth rebuilding or should I put in my cordia turbo motor? Looks like it's coming together pretty well dude nice car

Link to comment
Share on other sites

Hello Old school. Is there anyone out there who has any spare rocker arms for the 4G63a Sohc 8 valve engine? My build is currently on hold as one of the rocker arms is dead beyond repair and I am struggling to find a spare anywhere?

Any help will be much appreciated. 

Link to comment
Share on other sites

I'm currently looking at options for a motor that's going to have a bit of power for weekend driving and occasionally daily it. Are these motors worth rebuilding or should I put in my cordia turbo motor? Looks like it's coming together pretty well dude nice car

As of yet I am still unsure of the gains that will be achieved. Hopefully will have the engine done early next year and then will let you know my results so we can gauge whether or not it was worth it. 

Link to comment
Share on other sites

Hello Old school. Is there anyone out there who has any spare rocker arms for the 4G63a Sohc 8 valve engine? My build is currently on hold as one of the rocker arms is dead beyond repair and I am struggling to find a spare anywhere?

Any help will be much appreciated. 

Should be able to find some from any SOHC 4G62 and 4G63 at pick a part. Although I think the MPI engines used hydraulic lifters in the arms. 

 

PS where are you getting the 'a' from in 4G63a? Mitsubishi historically do not have designations for turbo, DOHC, EFI, valve number etc after the engine code. It's either G63B for the early naming system and somewhere around 1985 they switched to 4G63. There is no 4G63B or 4G63a for that matter..

Link to comment
Share on other sites

Should be able to find some from any SOHC 4G62 and 4G63 at pick a part. Although I think the MPI engines used hydraulic lifters in the arms. 

 

PS where are you getting the 'a' from in 4G63a? Mitsubishi historically do not have designations for turbo, DOHC, EFI, valve number etc after the engine code. It's either G63B for the early naming system and somewhere around 1985 they switched to 4G63. There is no 4G63B or 4G63a for that matter..

My bad, I don't even remember where i picked that up from but if its wrong its wrong.   G63B is what i'm meaning then. I will have a dig around pick a part then, hopefully there is something useful. 

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...