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Old 27-07-2007, 01:41 AM   #1
fastleno
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Default AU I6 bottom end 7500rpm and 10:1

will a balanced standard AU I6 bottom end put up with 7500rpm and 10:1 compression ratio ?, cheers.

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Old 27-07-2007, 07:47 AM   #2
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Yes. But not for long
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Old 27-07-2007, 08:11 AM   #3
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10:1 is nothing, thats only a tick over factory anyhow. As for 7500rpm, it had want to be a lot more than just balanced, the rods simply will not sustain that sort of abuse for any length of time before letting go (and probably giving the block a few fancy speed holes). Not to mention the head wont deal with those sorts of RPM anyhow without some work done to it, at least the springs and probably lifters. Oh, and it had want to be a major wild cam to even bother going to those RPMs as most will drop off quickly after about 6000-6500.

Can the 4.0 rev to 7500rpm? Yep, but it had better be a rebuild bottom end and a rebuild head.
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Old 27-07-2007, 08:21 AM   #4
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Quote:
Originally Posted by Casper
10:1 is nothing, thats only a tick over factory anyhow. As for 7500rpm, it had want to be a lot more than just balanced, the rods simply will not sustain that sort of abuse for any length of time before letting go (and probably giving the block a few fancy speed holes). Not to mention the head wont deal with those sorts of RPM anyhow without some work done to it, at least the springs and probably lifters. Oh, and it had want to be a major wild cam to even bother going to those RPMs as most will drop off quickly after about 6000-6500.

Can the 4.0 rev to 7500rpm? Yep, but it had better be a rebuild bottom end and a rebuild head.
I second this, and would also add that balance is critical on any 7000plus RPM I6 motor - torsional vibration has a habit of shaking bolts loose - ask anyone that has raced an XU1 or simillar on a circuit track - flywheel, balancer bolts can have problems.

You need to look at dual crank keyways, and at least 2 dowels on the flywheel/flexplate.

Also, there is a secret to the balance.

All this does cost more money than the obligatory prepped or aftermarket rods and pistons and sump, etc.
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Old 27-07-2007, 09:51 AM   #5
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yes i pretty much agree with above but here is my rundown you need better rods forged pistons a steel crank (not iron like the factory one) major head work a fairly wild cam which wouldn't be real streetable, injectors and reg once you screw this together the sort of power you're likely to be puting out will kill your tranny so you'll need to rebuild it with go fast bits I belive casper is the one with the knowledge on this. then we come to the tailshaft which will need to be a custom job to handle the speed and diff which will need toughening up so time to see the bank manager for that 2nd mortgage
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Old 27-07-2007, 11:17 AM   #6
fastleno
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no worries thanks fella's, its a speedway engine so the driveability in the low rev range dont bother me also it will be running a toploader and the head isnt a issue i have that under control, the cam's power range is 4000-8000rpm i want to make themost of it, i am also running custom adjustable rockers, i was thinking about skimping on the bottom end and hopeing to get away with a balance since the motor will only see 7500 for a few seconds at the end of the straights. oh and also its getting a 390 4bbl holley and EDIS and megajolt.
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Old 27-07-2007, 01:05 PM   #7
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I think you'd get away with balancing, and a stud girdle to be honest, because its a race motor, wont be driven or thrashed every day, I think it would last.
The stock bottom ends in these things are strong, very strong. I wouldnt bother with forgies or anything like that, rods MAYBE.. you can get these motors for 300 dollars from a wrecker, I wouldnt worry about pouring big $$$ into it.
Plus ive seen an i6 free rev and accelerate to a little over 8000rpm, stock motor with a tune.. and its been like that for a while now.
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