I know there has been some interest out there about the topic of boring and stroking a 6G75, and the fact there is a bored 4.2lt currently running, so below is some general info.


Before anyone asks me questions I can't answer, please note this isn't my engine and I haven't been in the car.

I've heard some early results of the 4.2lt so far, which at this stage is running a basic 95 tune (which isn't fully tailored to the engine yet).

In early stages a dyno run has been completed and there are obvious good improvements in torque, at least 10% more torque in mid range, also Maximum Kw is up around 6% at approx 5500rpm (a bit over 10Kw @ Wheels it seems anyway).


Fuel economy looks like it could be slightly better than 3.8 also.

However this is very early results and there is still potential for probably another 3-8Kw with a bit more specific tuning and as the engine settles in (not to mention if you look at a 98 octane tune to take advantage of the higher compression etc).


On the separate issue of stroking:
There is a possibility of a 98mm stroker crank fitting ok, which combined with the bored 4.2lt would give 4.6lt (but big money for getting cranks done, in the vicinity of $2500).

Wow, a 0.4l overbore on the 3.8. Im guessing that is the absolute limit? I read that the metal thickness between the cylinders and jackets is rather slim. I guess any form of turbo/supercharging is a no-no

A custom billet crank would be good, imagine 4.6l lol

Sounds like a lot of money and effort for pretty poor gains :/

As far as I know yes it's the limit you'd go. Reliability testing will show if it's too much i guess.

No boost is the only option I'm sure!


I don't know the full costs (but expect about $1800 for forged pistons, quality rings and the bigger bore gasket etc I guess, then add cost of boring etc).
Not something you would do as a first mod, but probably suit someone going down the rebuild / new pistons etc route anyway.
Also, once you do cams and inlet and exhaust etc you start to run out of N/A options.

Viability will depend on the final figures, but up to a possible 15-18Kw is a good gain, and you may also find a much better spread of torque and power.
If nothing else the extra torque down lower would help driveability once you start going for more aggressive cams. and the like


Remember too that people spend $700+ on a set of extractors to only gain 3Kw or so.

With these kinds of mods you would have to start looking at better inlet, perhaps triple or six throttle bodies with custom management. Cram as much air in as possible with some big injectors.

Shurly spending this sort of money on a 75, wouldn't it just be cheaper to customise a gearbox or something to suit the 3.5l v6 out of an aurion? If v6 power is the way you want to go that would be the way. I don't know how possible it is though.

Shurly spending this sort of money on a 75, wouldn't it just be cheaper to customise a gearbox or something to suit the 3.5l v6 out of an aurion? If v6 power is the way you want to go that would be the way. I don't know how possible it is though.

Why would you do that?

Why would you do that?

power.... with custom intake and exhaust they push more power then a cammed 380 motor.....

because for the amount of money that all of that work cost, you are getting sfa gain. 10 -20 kw at the wheels is very little for something that costs so much.

i know.... but this is the same lol

I spent $3000 and gained over 140HP ATW.. forgive me for not understanding the pursuit for such measley gains :S

I do however respect people trying new and wonderful mods so power to you!

Well why not?

Geez, don't shoot the guy down. If you're rebuilding and replacing the pistons etc then it is an option. It also opens up your mind to new ways to pull power out of the car. Yeah, it's not that big of a gain for a lot of cash and work, but I'm sure some people will find that info useful.

Its not his car, I'm not shooting him down :)

Its not his car, I'm not shooting him down :)

I know, I know.

Compared to some people I have seen trying to create power (installing an intercooler on an na car or drilling holes in their bonnet) this plan doesn't rank too bad. Its good for an na build and might give out some inspiration on how to push the '75 to its limits.

Oh for sure! I'd definitely raise an eyebrow if stroking starts getting played with.. however I imagine it wouldn't rev overly hard

...I'd definitely raise an eyebrow if stroking starts getting played with...

You would too, wouldnt you?

Interesting stuff..

BOoya had a run at an N/A 6G72 and had results that were barely worth the money spent if you're chasing power & driveability.

I vaguely remember Ben saying that it was a pig to idle and the power gains were not what you would call "Bang for Buck"

Nice to see the 6G75 getting major surgery..

Is it a workshop build or a garage build?

Hopefully if more information comes to light, you'd be happy to pass this on too.

SuFz :ninja:

Sounds good to me!.......... theres more to life than a wheezy Sprintex!

Sounds good to me!.......... theres more to life than a wheezy Sprintex!

Wheezy? lol ..... I think this + stroking PLUS the "wheezy" sprintex :P

Wheezy? lol ..... I think this + stroking PLUS the "wheezy" sprintex :P

That's exactly what I've been thinking since I first heard about this 'project' some time back!

I'd go the 4.2, would be a good "all rounder" as is, but would like it strong enough to handle the odd shot of nitrous. :happy:

That's exactly what I've been thinking since I first heard about this 'project' some time back!

I'll think about it when my 75 is in....

I'll think about it when my 75 is in....


So You're going a '75 eh Chow?? Have you got the motor already?? Keep us posted. Love to know how a '75 in an AWD goes.

So You're going a '75 eh Chow?? Have you got the motor already?? Keep us posted. Love to know how a '75 in an AWD goes.

Mal is doing it and has done Atleast another two into AWD's

So You're going a '75 eh Chow?? Have you got the motor already?? Keep us posted. Love to know how a '75 in an AWD goes.


They go brilliant :) The AWD should have had the '75 from factory. Well worth the $$, IMHO.

just doing some looking whilst procrastinating, i know that the 4.2 has been in progress and with the cams and all that, just wondering if anyone knows if there has been any progress on the 4.6L stroking at all?

Still some ideas being looked into apparently but the main problem is the expense.

Just isn't the demand to be made in bulk like there is with more tradional performance cars.

It is something that probably will get to the testing stage eventually, but I imagine no time real soon. Then it will just depend on actual costs versus results.

Hmm cause it seems very interesting...any very rough ideas on prices overall? And maybe any expected gains? or is it just a conplete stab in the dark at this stage?

With the gains listed, was the ecu tuned for the stroked out engine? I'm guessing the gains listed were on a standard tune, with a proper tune there'd probably be a lot better indication on the real gains.

With the gains listed, was the ecu tuned for the stroked out engine? I'm guessing the gains listed were on a standard tune, with a proper tune there'd probably be a lot better indication on the real gains.


Think it is all in the earlier post, but note the engine wasn't stroked, just bored.
Afaik it was a custom 95 tune but i think it was done before the engine was bored, so there could be some gains but not a real lot.

Gains and prices....again see earlier posts, as that is it at the moment.

He is looking at ideas that may be cheaper because he knows the cost versus gain equation is so important for everyone, himself included.
At the end of the day it is more a hobby/passion to him, and he doesn't run a workshop/business.

Yeah fair enough...be interested to see what he comes up with!