2.5L 6A12 - Easy to make??

spetz · Post by **spetz** » Fri Sep 29, 2006 1:55 am

So I have been thinking...
Talking to RPW I was told that to make the 2.5L MIVEC needed 3 engines, 6A12 MIVEC, 6A13 SOHC and 6A13 DOHC plus a host of custom parts.

What I am thinking...
Why not ust use the 6A12 block to begin with?
A brand new 6A13 crankshaft from Mitsubishi is $800
This means the engine has been stroke but bore is still the same. But the bore is over 78mm as is and the 6A13 only has less than 3mm bigger bore, so the stock 6A12 MIVEC pistons can be kept just to keep cost down. Anyway, if someone wants to go bigger/custom forged pistons to actually make this a 2.5-2.6L they can, just by boring the 6A12 which should be safe if the same block already comes with a larger bore. At the same time, maybe the 4G92 MIVEC pistons fit? Same bore. Only problem I see is the valve seats on the dome may cause issues. Honda B series pistons might fit too.

Now we are left pretty much with just the conrod length. I doubt the MIVEC ones can be used, even though I realise rods can be somewhat resized I do not think over 10mm to compensate for the stroke. So maybe custom rods might need to be made. I got quoted $1600 for custom steel rods with ARP bolts included

I bet everyone is thinking now, but the 2.5L block is physically taller than the 2? Fair enough, but if anyone has also seen a RB26 stroker kit to 3L they would see that the kit comes with a sort of plate that goes on between the block and head. A CNC machined piece basically made out of alloy and o-ringed/sleeved block so there are no gaps.
Hopefully the cam belt tensioner leaves enough gap to use the original timing belt

I don't really know whether this is a worthwhile effort for anyone to do, as they would have to account the cost of a rebuild etc, and I think realistically 10K for the RPW 2.5 is an extremely reasonably priced engine considering the bits it has, however what really triggered this thought was that I am having my 6A12 completely rebuilt. And also a friend works in metal place where he can CNC plates and so on.
So, the way I see it for me, if I had forged out $800 for a 6A13 crank, and $1600 for custom rods, I could have had a 2.4 MIVEC.

Basically, not considering the engine rebuild cost which obviously is getting done anyway, it would be $2400 more to up the capacity to 2.4(ish).
2K for custom forged pistons and this could be a 2.6 engine.

What do people think? This is just a discussion of interested and I am not doing it, but just thinking if what came to my head is a good idea or not.

Let the arguement begin, am I an idiot or not!

PS. I don't think this is a worthwhile exercise to do if you need to pay to strip the engine apart. And I think that if you want 2.5L the RPW engine can't be beat. In fact if people add up the cost of only the parts in that engine, 10 grand is damn cheap for it

Liquidity · Post by **Liquidity** » Fri Sep 29, 2006 4:16 am

honestly, i dont know much techy about engines. Well, the basics, but not how to apply specifics to various scenarios.

But logically, if you have a larger capacity, you can now suck in more air, have a bigger bang, and squish out more exhaust. Why would the standard 6a12m cams be okay for that?

Wouldnt you need custom cams to account for the increased flow etc?
What about strength. Would you be able to retain the sky high redline?

How about engine management. Would you have to go full standalone, do ya think? Beefier engine mounts? Or redesigning them to sit the engine lower? (Bonnet clearance).

I can see an advantage of this, not having to worry so much about oil galleries etc between different model engines.

lawrencetlc · Post by **lawrencetlc** » Fri Sep 29, 2006 12:07 pm

seems like it may work for me... but again, does a bigger capacity with the same engine shows more power?

because of the Mivec engine which means the valve actually changes accordingly at different RPM, i dont think it is just as simple as to increase the capacity of the engine that will increase the power or maybe torque. the timing of the valves to be opened and closed will be changed, the speed of the RPM will be changed, and the computer chip will just be confused about its own engine, wouldnt it??

anyway, i dont know much about engine, shaft, cam, valves and all.. but that's what i think might happen...

RedlineGX · Post by **RedlineGX** » Fri Sep 29, 2006 4:16 pm

Sounds interesting but in reality by the time you're done you would near or even surpass the cost of turbocharging and not get the same power from it. But hey i wouldn't mind if someone else tried it

spetz · Post by **spetz** » Sat Sep 30, 2006 2:49 am

Obviously this is for people who are trying to avoid going forced induction

As far as power, computer, cams, redline go...
Power, kw might not increase but torque would most definately increase. Peak power would be at a lower rpm along with MIVEC switch over point and torque output
Redline, not taking cams into consideration, the crank and rods wouldn't have an issue, it is just the pistons that would be somewhat of a problem. But realistically pistons are always built stronger than they need to be, and it would still have a piston speed similar to the 4G92 MIVEC.
Computer, I am sure a simple piggyback would work fine
Oh, and you can get 6A12 1mm oversize piston from Mitsubishi at $75 each, this would already make the engine fairly close to being perfectly square, and with molybednum coating, and cams, I do not see why the factory 8000rpm redline couldn't be retained

droidy · Post by **droidy** » Sat Sep 30, 2006 3:26 am

Just use the factory rods from a 6a13 twin turbo. Right length and are all ready built fairly strong- way beefier than the stock 6a12 rods.

Liquidity · Post by **Liquidity** » Sat Sep 30, 2006 3:40 am

i beleive thats what the 6a13DOHC engine is, and the point is it may be cheaper to get them than to source a whole engine to rip the rods out of.

right?

spetz · Post by **spetz** » Sat Sep 30, 2006 5:08 pm

Sorry I forgot to mention can use stock 13 rods. That engine redlines at 7000rpm I think, so with the longer stroke I would assume 7000rpm would be fine on the pistons too, and with stock cams peak power should be somewhere there

And realistically, they would be cheap as from Mitsubishi (considering pistons are $75 each)
So then, if rebuilding a 6A12, for roughly $1500 more you can have 2.4 litres!

droidy · Post by **droidy** » Sat Sep 30, 2006 5:30 pm

hmmm interesting spetz

... I was planning to buy one of these 2.5 motors shortly- now I'm wondering if you're theory of just adding on an extra chunk to the top of the block of 12 motor would work- would it be cheaper though? I always thought that might be possible but wasn't sure.. why couldn't it be 2.5 instead of 2.4?

droidy · Post by **droidy** » Sat Sep 30, 2006 5:33 pm

btw- the 13 crank is definitely different to the 12 one is it? I didn't even realise that!

hmmm

Post by **Bennoz** » Sat Sep 30, 2006 8:21 pm

Theory is sound spetz, but I have a few buts...

If you increase the stroke, then you have a need to alter the ignition timing & valve opening points. Ie your cams would be way out. With the time taken for the piston to comlete the longer stroke, the standard profile of the 6A12 cams would have rotated, opened the intake valves & the spark fired prior to the piston hitting Top Dead Centre.

Which leads me to think, If you could mimic the exact internal dimensions of the 6A13 in terms of bore and stroke, you could then use the 6A13 cams..... but are the 6A13 cams any good? You could also use the 6A13 pistons & rods. Does the 6A12 block have enough meat in the bores to be bored out that much? Alternate to using a 6A13 crank, can the 6A12 crank be offset ground on the big end journals... effectively Â´strokingÂ´the existing crank to give the same stroke length as the 6A13?

Our only limiting factor in all of this is cams (once again!) If someone could grind Ã©m the way we wanted them, then IÂ´d be in the machine shop tearing reams of metal outta my block!

bigpitty1 · Post by **bigpitty1** » Sat Sep 30, 2006 11:27 pm

2.4L, I wonder if it could be done with ease like the Nissan RB20, I hear of people putting in a RB26 crank in to make a RB24 2.4L, I reacon there will be sites on making the RB24 as the nissan motors are very popular around the world, see what they are doing for some ideas on the doing the same conversion with the 6A motor, I reacon it can be done.

FTO338 · Post by **FTO338** » Sat Sep 30, 2006 11:50 pm

I've seen a RB20 with a 2.4L stroker kit, that thing rip big time. There is a RB24 engine from Nissan though, but i was told its a SOHC.

Liquidity · Post by **Liquidity** » Sun Oct 01, 2006 12:07 am

also....wouldnt you be altering the angle between the crank and the rods, (...incident angle???) against the cylinder?

I've heard that can lead to fast wear and premature wear in engines that are "Aftermarket" stroked. Ie, more of the pistons movement is "against" the wall of the cylinder.....do people get what i mean?

spetz · Post by **spetz** » Sun Oct 01, 2006 6:21 am

Droidy, of course the crank is different, 12 has 69mm stroke and 13 has 80.8mm stroke

Bennoz, I don't see your theory being right with the cams. The cams will ALWAYS rotated at half the speed of the engine. With these MIVEC cams I am assuming that peak power/torque would be at roughly 20% lower rpm in each cam lobes

Liquidity, you are correct with rod angle and piston stroke etc, but no mater what engine you use, you get more power out of it, you lose longetivity

Now, droidy, you are always interesting to have these discussions with
Here goes:
Yes you could use the 13 block, but that is another things you will need to find. With the 12 block the beauty of it is:
1. You can skim the head to a larger extent so that a large portion of the block can be made into alloy
2. This is a cheap option. If you use the 13 block you will need to go 81mm pistons. Which means that neither the 12 original pistons can be used, and neither the 13 original pistons as they are low compression and and the valve clearences wont go good
3. There is a host of other things needed to make a 12/13 frankenstein which no one but RPW know for sure. Think of the money in research this would take

If what I am saying is all correct, and to my knowledge it seems to be correct, then this should be a very easy upgrade to someone who is doing an engine rebuild

The only downside I see here is it wont be that peaky hard revving monster anymore

Liquidity · Post by **Liquidity** » Sun Oct 01, 2006 3:20 pm

spetz wrote: Liquidity, you are correct with rod angle and piston stroke etc, but no mater what engine you use, you get more power out of it, you lose longetivity

2jz-gte

spetz · Post by **spetz** » Sun Oct 01, 2006 11:47 pm

You'll still lose longetivity...

But yes best damn 6 cylinder ever made! I agree!!

However, look up GTO VR4's in the states
Stock internals and people are doing 9's in them and revving them to 9000rpm. I think another great 6 cyl engine greatly overlooked by people

Mitsiman · Post by **Mitsiman** » Tue Oct 10, 2006 10:22 pm

Just gonna supply one bit of information which would explain why you need to use the three engine blocks

(A) THe longer stroke of the 6A13 crank, whilst it will fit into the 6A12 engine block barely, won't clear the bottem of the bores cause that is where the 6A13 engine block is taller so that the conrod throw section of the crank can clear the bottem of the engine block.

(B) Because of the height of the 6A12 Mivec heads, you need to use the 6A13 SOHC engine block components as the 6A13 DOHC components are too short in some areas especially on the V section of the engine block as the taller 6A12 Mivec heads have a much wide V angle, which is made worse with the taller engine block especially with the water heater rail unit that bolts onto the back of each of the cylinder heads and is non adjustable

(C) Some of the 6A13 SOHC stuff is also needed on the timing belt area to again accomodate the variation in engine block height and V Angles etc. Finding a timing belt for this engine took us a very very long time.

Final information - conrod and main bearings are different between the 6A12 and 6A13 engines, hence need to use all 6A13 internals, or custom ones as we did to make this hybrid work. You can use 6A12 pistons on the 6A13 rods cause of different piston pin diameters.

This is only touching the surface but as you can see, this is why this particular motor took us 3 years to develop and is a rare beast and one that we would only ever do with someone with a lot of money.

Post by **BorepYano** » Tue Oct 10, 2006 10:55 pm

@_@ so confused now

sorry for being ignorant (i know how to change oils and flush radiator fluids and that's about my mechenical skills

), so what's the final verdict on this?

Liquidity · Post by **Liquidity** » Wed Oct 11, 2006 12:33 am

in short, for the same price you could put a 4g63t with an lsd in...

this motor's only for those who SERIOUSLY dont want to go turbo