piston

[okieraptor]

I'm goin to turbo the 6.2. 6.5 crank and rods. With that said can I machine the pistons down far enough to end up with 19.25:1 without hurtin the stuctural integrity of them? If yes them how much would I have to take off? I would like to run 15lbs on it everyday allday reliably.

 

[fallout232]

A 6.5 crank and rods in a 6.2 will get you a 6.2. There is no difference between the stroke of the crank or length of the rods between either motor. I would also add that depending on the year of the 6.2 block a 6.5 crank with a 1 piece rear main seal will not fit in an early style 6.2 block.

The difference in displacement between the motors lies soley in the bore.

I would not "shave" a factory piston down as being aluminum they are already prone to cracking and having holes blown in them from excessive cylinder temps. Most new replacement pistons have teflon skirts and a hard anodized crown to help with heat rejection. Any sort of machining will remove the coating. The proper way is to lower the piston wristpin hole down a couple of thousandths of an inch to lower the compression or go with a custom casting. 15 Pounds is attainable in the 6.5 but your motor without major modifications is not going to last "All day" every day at that power level. Without an aftermarket turbo the factory setup (Turbine housing) becomes horribly inefficient at those boost levels.

Not trying to steer you away just giving you some info on these motors. I have been wrenching on them since 1997.

 

[okieraptor]

Ok ill move the wrist pins then. Will that weakin the skirts?

As far as the turbo ill either have a hy or hx35 with a dmax pedistal and custom headers.
I figured a 6.5 replacement crank and rods would be stronger due to the fact they are factory boosted crrect?

 

[okieraptor]

It will also get full arp treatment and girdle. Along with massive radiator and twin stats.
Also planing on an intercooler.

Will a stock 6.2 injection pump handle that or would I need to up grade it to a 6.5 pump?

 

[Wrench]

The pump you're looking for if you want to stay reasonable with cost is a DB2831-4911, the mechanical Stanadyne pump that was used on the first couple years of 6.5 turbo engines. If you want big horsepower, and you don't mind spending, get a marine DB2 from Peninsular or Kennedy.

It's my understanding that you can machine a bowl into the top of the piston to lower compression without reducing the integrity of the piston, it's been done many times before. I would however re-coat the tops of the pistons again before using them.

I have a Banks setup that i'm thinking of trying an HX35 or even an H1C on . . . full studs as well, but it's an AMG 6.5 engine . . . .

 

[Wrench]

Oh, and as long as you have head studs, the 15PSI limit that gets thrown around for these motors has everything to do with the stock turbo and nothing to do with the motor itself. The Banks manifold is nice because it sets the turbo forward and down, allowing a lot more room for a straight out downpipe, and you have to pick a turbo that fits your use. High HP/high RPM turbos need a bigger exhaust housing than the stock ones, no matter what engine you're running, 5.9 or 6.5.

 

[MaxPF]

A 6.5 crank and rods in a 6.2 will get you a 6.2. There is no difference between the stroke of the crank or length of the rods between either motor.

This is absolutely incorrect, as I found out when I did my build a while back. The stroke on a 6.2 crank is 3.80", while a 6.5 has a 3.82" stroke. Rods are identical. The added stroke doesn't affect displacement much, but the compression height is different between 6.2 and 6.5 pistons. The result is that 6.2 pistons will sit .010" higher out of the hole vs. a 6.5 piston. This ends up being a big deal. You can use the .010" thicker head gasket on an unmilled block to compensate, but you will still have stock 21.3:1 compression.

I would also add that depending on the year of the 6.2 block a 6.5 crank with a 1 piece rear main seal will not fit in an early style 6.2 block.

This is correct, but a 1 piece crank can be modified to fit in a 2 piece block.

The difference in displacement between the motors lies soley in the bore.

Nope, although the bore is responsible for most of the difference.


As for the OP's question about compression, as a rule every .010" you reduce in compression height will lower your compression approx. 1 point. Since 6.2 pistons were never hard anodized you can mill the crown down .020" to get roughly 19.3:1 compression. Another method is to call up Cometic and order a set of thicker head gaskets. You can also have either end of the rods offset bored to reduce the overall rod length .020". This will make an insignificant change to the rod-stroke geometry while giving you the increased compression height. Keep in mind that if you are using a 6.5 crank you will need to remove an additional .010" to compensate for the slightly increased stroke, so you would need a reduction in compression height of .030" total. You can either get the extra .010 at the same place you are getting your .020", or you can use the .010" thicker head gaskets (assuming you don't deck the block).

One other alternative is what I did. I unshrouded the area around the valves, and the added combustion chamber volume gave me 18:1 compression. You could have this done to a slightly lesser extent to get the 19.5:1 that you want.

Finally, fallout232 is correct that an otherwise stock engine with nothing other than lower compression isn't going to live long at a sustained boost level of 15psi. Intermittent, yes (assuming EGT's stay in check), but not continuous.

 

[okieraptor]

Thanks max I was hopin you would get in on this. When I said 15psi "all day everyday" that didn't mean it would always have 15psi on it. Just want it to live when pulling the trailer or gettin into and out of traffic. Pressure spikes might 18-19 but very short lived. Just want a stout simple motor I can tow with that will last. Sorry if I confussed e everyone.