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Author Topic: 2.7t content - what pistons/rods with 3.0 crank?  (Read 12102 times)
julex
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« on: May 21, 2011, 07:31:45 PM »

I am seeking concrete info on running 3.0 crank and shorter rods and what pistons can these drive.

The reason why is that I don't want to bore out the stock block for various reasons, and certainly not to 3.0l size.

Keeping stock bore size and using 3.0 crank yields cool 2.9l which I would be fine with considering what extra 100cc can cost when something goes wrong.

If somebody can confirm that a combo of 3.0l crank, vast custom (or whoever else's) shorter rods for 3.0l crank and stock pistons is possible, I would definitely give them an internet kiss.

Thanks.
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smeege
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« Reply #1 on: May 23, 2011, 09:37:36 PM »

Why are you concerned about keeping the stock bore diameter? I don't believe sealing will be much of an issue for the head gaskets, regardless of bore dia. I've had WAY too many heads off of Audi 30v motors, and I can say firsthand that Audi's head gaskets are extremely strong.

From a mechanic's point of view, I don't see why running a stock bore would be a problem, IF the cylinders all measure within spec. Since most of us are rebuilding a used block, I think it makes sense to bore the cylinders to give the pistons a nice, new home. I'm not certain on price difference, but once you're buying new pistons (right?), I don't recall a huge diff in price. The only thing to consider is piston CC for compression ratio, Usually the shorter rods bring the comp ratio back to reasonable levels.

I'm sure I'm stating the obvious here, sorry for lack of concrete 2.7 info. I already have a 3.0 crank waiting in the corner for me, so I'm very interested in how this all goes for you. If this is going in your Allroad, I definitely wouldn't skimp on engine internals, your engine's working a lot harder than it would in an S4.
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julex
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« Reply #2 on: June 09, 2011, 03:40:34 PM »

Update.

Obtained a VERY nice 3.0 crank already balanced and micro polished.
Looking at piston choices. It seems like AMD has a piston configuration for 92.8mm stroke that inorporates stock length rods and 81.5mm bore which would be perfect for my purposes. Only 0.5mm over stock bore, enough to basically refresh stock bore and get rid off any wear and tear surfaces.

I am not sure what's the compression on that is but I am assuming 8.5-8.8 as all stroker pistons to mitigate knocking at insane boost levels.

Bump in displacement should offset loss in compression for off the boost driving.

If I go with that, I will end up with 2.9L engine while retaining almost stock amount of block lining which in my mind is #1 priority.

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Matt Danger
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« Reply #3 on: October 02, 2011, 08:17:16 AM »

very cool, can't wait to see more of your progress. FWIW gearhead is using AMD's pistons with his 3.0L build.
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s-company
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« Reply #4 on: October 03, 2011, 02:10:47 AM »

its no problem.
shorter rods
3L crank
stock pistons and you are ready :-)

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Matt Danger
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« Reply #5 on: October 03, 2011, 07:45:00 AM »

its no problem.
shorter rods
3L crank
stock pistons and you are ready :-)



My understanding was that the short rod setups caused oil consumption or some other problems.
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Dvlax41
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« Reply #6 on: October 03, 2011, 03:23:35 PM »

Julex , u still have that old 2 liter ?
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s5fourdoor
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« Reply #7 on: October 03, 2011, 11:29:35 PM »

Wouldn't the shorter rods really mess up the rod-to-stroke ratio which is already being smacked up due to the increase in stroke size?  Seems like that wouldn't be optimal by itself, but I'd need to see the numbers to be sure.  I'm assuming that Jason@AMD had a reason not to design shorter rods, also some of which has to do with the fantastic availability of reasonably-priced good-quality stock length aftermarket rods.

Just my guess on things, but man I am excited to see this build progress further!
« Last Edit: October 03, 2011, 11:31:49 PM by nehalem » Logged
Snow Trooper
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« Reply #8 on: October 10, 2011, 12:13:43 PM »

Shorter rods are stupid on these engines.  If you look at the 3.0 setups and how deep they pull the piston already you will realize why all the guys who barely arent eating engines are on long rods.

end of story on the rod length issue, seriously.  The short rod thing was some idiotic attempt at being proprietary and cost cutting at the same time.  it hasnt worked out well for the most part on engines that are ran hard.

as for pistons, its been hit or miss with everything other than mahle it really seems.

whatever you do, i 100% agree that increasing bore as little as possible is a good idea if you want big boost.  if you want to rev to the moon on medium amounts of boost then stay stock stroke and bore if you have to, stay light in the piston and pin and rod.
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Snow Trooper
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« Reply #9 on: October 12, 2011, 01:57:26 PM »

btw, i sorta missed your main question at the end, but no you cannot run any variation of a 3.0 crank with a stock piston.
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