Bennoz wrote:I reckon it would be easy do-able, but I dont reckon you'd need anything fancy.
These are the main differences between the 6A13 & 6A12:
* The 6A13 bore & stroke is larger than 6A12. The 6A13 is an 81mm bore x 80.8mm stroke, while the 6A12 is an 78.4mm bore x 69.0mm stroke.
* The 6A13 has a much larger stroke crank which would go hand in hand with longer rods, as you can see by the big difference in stoke figures above, Plus all Mitsubishi factory turbo engines had the cranks nitrided (heat treated for strength.)
* The 6A13 had all the oil & water galleries in the block for the turbos tapped & drilled. These galleries come off the block & are present on the 6A12, but they are not drilled & tepped. They are just cast in.
* The 6A13 also has a larger capacity oil pump.
Now, the things we know will fit:
* 6A12 heads will bolt to the block - they have the same stud patterns
* The standard 6A12 Mivec timing assembly will fit on
if - you change over the crank timing belt pulley from the 6A12 to the 6A13 crank
and - you do not alter the deck heights of either the block or head (ie no major decking / machining of either.)
So, that gives us:
* 6A13 block (81mm bore)
* 6A13 crank and rods (80.8mm stroke)
* 6A12 heads, cams & timing assembly
What are we missing from that list? - Pistons.
The 6A13's lower compression ratio is governed by a combination of the 'dished style' top of the piston, the piston deck height (ie, the top surface of the piston relative in distance to the gudgeon or connecting pin to the rod) and head chamber design. These are the 2 things you'd need to look at carefully when bolting it all together. We'd need to source a 81mm diametre piston with the correct deck height, plus we'd need to match the combustion chamber size to the larger bore. Remebmer, the 6A12 heads are expecting to meet a 78mm piston, not an 81mm piston.
By piston deck height, this is what I mean:
The lower that distance is, the lower the compression will be (6A13's pistons are lower, hence the lower compression & why we can't use 6A13 pistons)
The higher the distance is, the higher the compression will be (As we require a compression ration of 10:1 or even higher for this application.)
Here are some piston top designs:
A 'dished piston' allows for a volume of air in the surface of the piston, as well as the combustion chamber volume. This larger total volume of the combined combustion area vs the stroke of the motor = lower compression. Example:
A 'Flat top' piston does not have the dished area & as such has no extra volume to add to the combustion volume. Therefore comrpession will be higher. Example:
Then there are other pistons such as 'hemi' pistons that actually go right up & almost mate with the combustion chamber face. These are obviously designed for motors that are much less of an interference fit. This example has machined sections out to allow for valve clearance:
The Mivec pistons are closer to being a flat piston, with valve clearances machined into them. Also keep in mind those valve clearances will also alter the compression ratio as they allow for more volume of air into the combustion area. Ideally, what we need for this application, is simply a copy of the 6A12 piston, just bigger - ie 81mm in diametre.
Now everyone goes on & on & about 'custom pistons' - which I think is bullshit. I've built dozen of hybrid monster engines over the years & not once have I needed custom made pistons. My mini was bored out to 74mm, so we found a Japanese 74mm Omega piston from an old Suziki that fit, I also build a 2 litre Datsun z24 motor which was bored out & we managed to fit Holden red motor pistons. If you think about it, we have been building internal combustion engines of the same design for the last 100 years. I
garauntee there will be an off the shelf piston out there that will fit.... and the only people that will be able to tell you what piston will fit, will be the crusty old guy with a beard thats been assembling motors for 40 years in the machine shop. Not the internet.
In order to check all these values out, the engine will need to be 'dry' assembled a number of times to determine the clearances. The other item to check during a dry assembly will be the valve clearances to the piston top. We may well have to scallop out valve area in the top of the piston as it will be an interference engine. Again, this is cheaked by a dry assembly (with cams in) & rotating the engine. They usually put a product called 'beraring blue' on the valve faces, which will leave a mark on the piston if they hit, telling us where to remove metal from the piston surface.
The last thing that will need to be checked is the combustion chamber size. Being a 6A12 head, the chamber diametre is 78.mm, yet the piston diametre will be 81mm, therefore the chamber size will need to be machined out to match. Thats no problem, however it will need to be done to all 6 chambers. Once thats done, all 6 chambers will need to be "cc'ed" - that means they turn the heads upside down & fill each chamber with fluid & measure the fluid volume. All chambers must match, so if one chamber contains less fluid than the others, then more metal will need to be removed for it to match. All 6 must match.
So there you go, that the machining & assembly part of the black. I'd say there'd be 2-3 grands worth of work there.
The rest of the Mivec ancillaries such as starer motors, alternator, AC compressors etc will all just bolt on. Even the standard exhaust will bolt on - you will however need to port it out slightly as the 6A13 exhaust ports are slightly larger. Personally I'd modify a set of 6A12 hurricane or RPW extractor to fit.
Lastly, this will need some sort of engine management to get the most out of it. Seeing as we're using Mivec, I'd keep the standard Mivec ECU, but just add a piggy back to get the fueling right. Haltech interceptor or similar. It should all wire up to an exisitng GPX / GPvR loom. At the end of the day, all labour, parts (including buying the 6A13 motor) gaskets, belts, fluids, clutch? tuning etc I reckon you'd be up for 8 grand. which I reckon is pretty good... Especially when rpw wanted over 10 grand just for the motor alone.
then sell the engine out of that.