VT Engine Break In

Gearbanger 101 said:

Actually, I've got to disagree with you with regards to that point. My Diamond pistons required much more clearance than the stock hypereutectic slugs did.....as do all forged pistons by comparison due to their increased expansion rate.

This is why forged engines often rattle during the first few seconds of cold start up. The nice thing about the Diamonds is that the wrist pin is offset, much like the factory piston to keep things quieter. But they do require more tolerance than the stock pistons just the same.

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With a 4032 piston, cold piston/cylinder wall tolerances are setup to ultimately be relatively tight once the motor is fully warmed up. Hot tolerances are the money tolerances. A 4032 piston and respective ring setup, should be setup to be relatively tight once fully warmed up. We like to use Diamond pistons as well and with the forged 4032's (most common street/astrip piston) we like to have a .0025"-.0030" clearance. The piston's diameter is measured at a point exactly .500" above the bottom point of the skirt.

Hyper's can be installed with tighter clearance from the factory but they are not. Especially when you factor in the stacked tolerances. This is why piston slap is one of the most common warranty complaints with GM and Ford vehicles. Hyper's only have a 15% more coeffcient of thermal expansion. So on that means on a motor using 4032's with a clearance of .0030", you would set up a hypereutectic piston with roughly .00255" of clearance. However, they are no where near that tight on our graphite skirt hyper's from the factory assembly line. On moly skirt hypers, the clearances can be slightly tighter, but not much.

Suffice it to say, once warmed up, a properly built performance forged bottom end will be tighter and show much less leakdown than a factory motor.

04YELLOWGT said:

My dyno is prolly 40-50 miles away from where Im having the block put in. So what should I do then just drive it there and then do it. Will that be ok to do it with 50 miles already on the engine?

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Having a competent tuner involved in the process of putting together a blown car is a critical factor that is often overlooked or not fully appreciated. You need to find a competent tuner to immediately apply a "base" tune once the blower goes on that at minimum includes your appropriate injector constants, spark/fuel tables, MAF curve (if using the stock MAF transfer function), and injector offsets if required. Then the key is just to keep it out of open loop where the 02 sensors are ignored. As long as you have your MAF and your O2 sensors working together in closed loop at part throttle, you will be fine to drive over to your dyno.

But like I said before, and I'll say it again, you want to apply varying light to moderate boost during the first 50 miles, after that it's too late.