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ENGINE REBUILD. 6G75 DOHC MIVEC


 
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T04GTR
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Age: 43
Zodiac: Sagittarius
Joined: 29 Jun 2013
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Location: AUSTRALIA,sydney

PostPosted: Wed Jun 10, 2015 12:45    Post subject: ENGINE REBUILD. 6G75 DOHC MIVEC Reply with quote

hey all i am going to embark on a 3.8 bottom end upgrade using my evo heads.

after some sleepless weekends balls deep in the bottom end of my evo, (changed out bearings for "preventative" reasons) and the death rattle heard on cold start up)
i have decided to change out the 3.5 for a 3.8.

i picked up a "water damaged" pajero 6g75 non-mivec long motor from pick and payless for 500 bucks.
stripped it down and found it has done *beep* all kays.

after doing some online searching for pajero mivec cylinder head pictures/specs i came up abit short.

so over the queens bday long weekend i decided to pull a head off the check the cc of the combustion chambers.
welll... it appears they are quite small.
i measured them at ~46cc
had in just slapped the heads on the 75 bottom end i would have a comp ratio of approx 12.75:1, abit to high and avgas is abit pricey..

so a search for pistons began.
turbo vq35de 95.5mm pistons could work. but they would protrude from the block by .1mm leaving only 0.3mm to the head. depending on what brand comp ratios would vary between 10.5:1 to 12:1 but they are all quite bulky and deep. + the taller deck height by 0.5mm could be an issue.

i have settled on 95.5mm wossner yamaha XT600 atv pistons.(could of used stock 95mm xt600 but bores need a clean up, these are a nice flat top 28mm pin height, offset pin, light weight forging with good modern rings. 1,1.2 2.5mm, they will net me a nice bump in comp to 11.5:1 not to high for 98 ron fuel with there -0.5cc valve reliefs.

some measurements i have found/taken while the head was off and sump from earlier

same crank as regular 3.5 rwd. as are bearings. (one of my upper bigend shells were installed backwards!! and had flattened the locating tab)
6g74-mivec specs.
head gasket 1.1mm 94mm bore mls steel. (cleaned and re-used with vht copper gasket cement spray)(so i could put the thing back together and get to work)
head combustion chamber 46cc (syringe and cd case with grease seal measured with oil)
piston cc -11cc dish regular 6g74 pin height i think, 0 deck height. (calculated off advertised 10:1 comp and other data from head and gasket dimensions)
same valves and springs as 4g93 mivec/6g72 mivec
same "cam holder" as 6g72 mivec
same port shape/bolt spacing as regular dohc 3.5 bar a squared off injector relief
lower intake manifold to upper gasket same as 6g75 non mivec except the front m8 bolt hole is offset. all the rest are the same.
dual runner intake manifold is very close the 6g75 non mivec bar "high speed" larger runners are longer on evo manifold
uses same "yellow" injectors and rail as normal 3.5
plug lead boots are longer than normal engine.
cam seals same as sohc 3.5 (35mm)
mivec lash adjusters same as fto 6a12/4g93/6g72 mivec. not sure on rocker arms
cams are SMALLER lift/duration on high cam than 6g72 mivec. but same on low speed (timing is different as cam timing marks on drivers side bank are different) i will use these cams on this engine once i degree the stock mivec ones. i ran out of time to do that this time

6g75 non mivec.
pistons 29.1mm pin height (40.1mm from crown to bottom of installed 22mm pin)
0.4mm deck clearance
-2.5cc dish ish
rods are the worst things ive ever seen. even the big end caps fall off when the nuts are loosened. ie: not much of a positive dowel action from studs
crank has cross drilled mains but single drill for big ends.
same bearings as 3.5. not sure on thrust
valves are alot bigger than mivec 3.5 as are intake ports.
62cc combustion chamber
head gasket mls 0.4mm thick 96.25mm bore
fuel injectors are better denso 12 hole 60 um droplet same fuel rail bodies but full height injectors. not 3/4 height as 3.5
the sump is different as i will have to mod the 3.5 one to fit the 3.8. dipstick location is the reason i cant use it i think.

once i start the build il post pics etc,

fyi. i used the later 3.8 black injectors by spacing up the rail. and using the 3.8 reg.
air fuel ratios are far more consistent. i will report of fuel economy is better. exhaust note is deeper and less raspy for some reason to...???

so that was my long weekend lol. Very Happy
this calc has gotten a workout by me lately

ENGINE COMP RATIO CALC
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PostPosted: Wed Jun 10, 2015 12:45    Post subject: Google Ads keep the POCUK free to join!


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OFF RD
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Joined: 11 Jul 2014
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Location: Auckland NZ

PostPosted: Thu Jun 11, 2015 8:46    Post subject: Reply with quote

Oh my goodness  Shocked only the keen aye...

You have the bug big time.  I have been wondering about injectors.

What cc rating are the 3.8 ones?  Thanks for sharing the knowledge.  I will have to get on the hunt, I have done the air box aswell and wouldn't mind a bit more gas.  Clutch never lasted long after air box mods lol.
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T04GTR
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Age: 43
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PostPosted: Thu Jun 11, 2015 9:38    Post subject: Reply with quote

i think they are the same, but at a higher pressure.
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T04GTR
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Age: 43
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Location: AUSTRALIA,sydney

PostPosted: Thu Jun 11, 2015 9:39    Post subject: Reply with quote

well im comitted now, just orderd the pistons and spool h beam rods. (only off the shelf 6g74/75 rods i could find)
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NJV6
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PostPosted: Thu Jun 11, 2015 11:13    Post subject: Reply with quote

Cool project, explain the death rattle you talk of with the 6G74, mine has a rattle when cold which is not lifters and it goes away after about 3-5 minutes, took the flaps out of the inlet manifold as they were rattling away when warm, not sure that it performs quite as well now and road speed, 2500-3000 rpm.
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OFF RD
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Joined: 11 Jul 2014
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Location: Auckland NZ

PostPosted: Fri Jun 12, 2015 2:42    Post subject: Reply with quote

I have an ungodly sound if my truck hasn't been started for couple days.  It only happens on cold start up and only when the engine fires.  It's just a couple solid knocks that made me *beep* my pants the first time I heard it, so if it is what T04GTR is talking about then I am happy it's not just mine so I will be watching this thread closely.

I do not have any other rattles or knocks and I feel the noise is coming from the bottom end.  It's not every time either, I now change my oil more regularly I.e. 5k kms..
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T04GTR
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Age: 43
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Joined: 29 Jun 2013
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Location: AUSTRALIA,sydney

PostPosted: Fri Jun 12, 2015 10:01    Post subject: Reply with quote

yes, i believe all the evo motor is set up with some larger oil clearance. and when the car is left for a few hours the oil drains back out of the main oil galleries, and it takes abit for the oil pump to suck it up on start up.
it is big-end bearing noise. but i checked mine and they were ok.

evan after putting brand new mains and bigends in, it still rattled on cold start. oil viscosity made no difference. and going by the oil pressure. it taps out at 6 bar ish hot by 3000rpm,, but idles at 2 bar. with a 10w40 oil. so one might say a 5-30 is more applicable to this engine. but i wouldn't do it.

i installed an external electric oil pump via a -10 fitting welded to the steel sump plate, feeding into a remote oil filter i fitted. with a hobs switch that primes the engine before each start. taking power from the starter solenoid then switching power thru to the starter once 20 odd psi is reached via a solid state relay. works a treat, with no rattle.
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T04GTR
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Age: 43
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Joined: 29 Jun 2013
Posts: 70
Location: AUSTRALIA,sydney

PostPosted: Wed Jul 15, 2015 9:33    Post subject: Reply with quote

engine build is on hold for a while.
i have all the bits ready to go.

i hack sawed my left index finger extensor tendons all the way threw on the top of my hand. will be 10 weeks till i can use it properly again.. 6 weeks till the splint comes off. Smile thats if i dont pull the stitches holding my tendons together.
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Rob546454
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PostPosted: Wed Jul 15, 2015 16:10    Post subject: Reply with quote

Ouch, good luck with the build sound pretty epic.
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aLARGEfarva
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PostPosted: Sun Aug 30, 2015 4:17    Post subject: sick! Reply with quote

Sorry to hear about your finger bud! Looking forward to seeing your project finished! How exciting Very Happy
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dervmonster
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PostPosted: Sun Jan 31, 2016 23:05    Post subject: Reply with quote

Just reading this, funny thing, I had No3 upper shell in wrong two, coincidence?  
Has got a little hot, snatched the pair and spun them about 15-20 degree or something which in turn had blocked the journal. still set of shells £80 at the time, now I have a minute small end knock on a cold start up.  still I use a straight 15w/60 or even a 70 if I can get it, these heads rely on good oil pressure for the vvt.
lurk
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JOHNVOR
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PostPosted: Tue Oct 03, 2017 10:18    Post subject: Reply with quote

Any updates of this project? How it went?
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