V12 Swap Research

So, I have a twin turbo Dodge Stealth (re-badged Mitsubishi 3000GT) that blew an engine last year.
After tearing into it I discovered that the previous owner botched the transmission swap.
Though possible and fairly straightforward, fixing it properly would cost far more then the car is worth, and I’d be left with a drive line I’m afraid to abuse.

I had the idea to build a destroked turbo LS for it, using a Nissan FS6R31A (6 speed good to 500ft-lb).
That would get me a cheap drive line I wasn’t afraid to abuse. But the LS is kind of boring, and the whole point of this car is to have fun.

This might be a stupid idea, but after seeing one cheap on kijiji I’m considering a high RPM twin turbo v12. I’ve already cheeked, and the engine bay has room.

I’m aiming for 6-800hp and an 8000rpm redline. I’m unsure if I can get the valvetrain to like anything more then ~7k, but it’s what I’ll strive for.

For that goal I will need to lighten the reciprocating mass, fix valve float, increase head flow, and reinforce the main bearing caps.
Fuel and ignition will be handled by a Megasquirt, I’ve built a few and it’s a cheap/easy and effective solution.
For now I’m going to leave the valvetrain design until after I get an engine to play with.

I have the facilities to do most of the machining, and I like tinkering more then the driving.
So parts costs are my main concern, not labour.

As suggested elsewhere in this forum, LS rods and Subaru pistons could be used.
Bore and hone liners for Subaru ej20 pistons (92mm bore 32.6mm compression height)
Drill/ream pistons to accept 4th gen 4.8l LS rods (159.4mm long)
192mm deck height, stock is 191.9mm
Grind V12 crank to accept LS rods and with a slight offset to bring the pistons to the correct height.
The only thing I should need a shop to do is grind the crank.

I read somewhere that the stock stock main caps are week and tend to flex. The fix for this is of course some sort of girdle. I can’t find a good example but possibly one like this:

But with how deep the crank sits in the block on the V12 I would design it to be flush with the bottom of the block.

So, any thoughts on my crazy idea?


8000 rpm is easy, the stock V12 will do that. Anything past about 500 hp is much tougher because this head doesn’t breathe too well.

Funny enough it was you (in a vary old post) that gave me the idea of using Subaru pistons.

And ya, heads will need a fair bit or work, valve size seams fine.
Cams will be ground for ~0.45" lift.
Some work around the valves (I don’t mind lowering compression here).
As for porting as I understand it, intake side is fairly easy to port but there’s little room to modify the exhaust side.


Lifted from another forum, does anyone have a cross section of the HE heads?
It does look like there’s at-least some room to remove material from that roof.

You probably should talk to Norman Lutz. He’s on here somewhere. He likes to put Chevy 305 pistons in V12’s.

@Norman_LUTZ paging Norman Lutz.

Hmm, I hope tagging him sends an e-mail appears he hasn’t bean on in months.

Chevy 305 pistons would require new sleeves.

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How do you do that? I’ve seen it done before, wanna know how it’s done.

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Aaaaargh! Well, perhaps look up some of his old posts. He apparently has built a lot of V12’s for better performance.

If you know the user name, or just the first letter, just add the @ symbol before it (no space) and start typing it

Here is my screen, I am paging you @Kirbert


20 characters

Hi Nick, can you address all correspondence to etypefactory@ outlook.com


Norman Lutz

Yes, 305 Chev pistons require new liners. Stock liners can only be safe ly bored to 92mm.

Blake, we had a 48V engine in a boat that made 850HP @ 8500rpm with a stock bottom end,
Running an engine in a race boat is like having it on a Dyno, it’s full load nearly all the time.

So it sounds like a 92mm overbore would be alright on the stock sleeves.

Ya, the bottom end makes the LS look wimpy, and those manage 1200hp.
So I wasn’t too concerned.

I did want to run higher rpm then might be typical.
Partly because because my transmission is a bit on the weak side for this HP, but mostly I like the idea/sound of a high rpm v12 in this car.

Blake, high RPM will not hurt your trans. Trans are not HP rated, they are Torque rated.

I know that, you miss read what I wrote.
I want a higher rpm, because for the same hp that equals less torque.

Blake, I didn’t miss read, your quote was as follows.

"Partly because because my transmission is a bit on the weak side for this HP’.

HP is purely a calculated figure, =Torque X RPM X 5250, the only load on the transmission is Torque.
And if you accelerate at the maximum rate the vehicle can achieve you will load the transmission with maximum Torque.

The full quite is:
“I did want to run higher rpm then might be typical. Partly because because my transmission is a bit on the weak side for this HP.”

As you pointed out and I tried to imply an engine with a HP peak at a higher rpm will have a lower peak torque.

Take two engines
800HP = 525ft-lb at 8000rpm
800HP = 700ft-lb at 6000rpm
If you adjust final gear ratio to match the engines peak hp rpm the second transmission will have to endure 1.33x the torque of the first.

I’m aware that an engine’s peak torque will be at a different rpm then peak hp, but tuning for a higher RPM will lower torque. Besides that modern aftermarket ECUs (as I will be using) are capable of boost limiting based on gear and RPM. So the faster I can run this engine the more HP I can put through the transmission without exceeding it’s maximum torque rating.

Blake, the only effect that the final gear ratio has, is on the Torque applied to the rear axle, MAX Torque to the trans is applied by the Engine.

I recall Chad Bowles, back in the day, saying that the V12 crank was the hardest he had ever seen in terms of regrinding.