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Best way to lower compression ratio down to 18:1 on the 6.5 diesel

Dave, he is talking about the Ricardo swirl chambers on the tops of the pistons......the tops of the pistons can be shaved down between .020" and .040" (depending on original piston portrusion). This will get you in the neighborhood of 18:1 compression. This will lessen the depth of the swirl chambers, but not too drastically. Really needs to be calculated to get exact, but is not necessary. The idea is to lower the compression so asa to be able to add more boost with less stress on the motor.

Another method, is to shorten the connecting rods. This way the pistons do not need to be modified at all.

We have used mls gaskets (multi-layer steel), some like them some do not.

The fourth that can be used (as GM did) to lower some, not all, is pre-cup modifications.

In our present build now, we are using a combination of piston and pre-cup mods (looking for 17.5:1)
 
Best way or cheapest?

Best way; buy some new 18:1 pistons.

Pennisular is one source but there are others.

Cheapest way is probably diamond prechambers and thicker head gaskets.

Given that you already have indicated you intend to run up the boost, I wouldn't recommend that route....
 
I would prefer to shorten the conn rods, were it me; hard-anodizing or DIY coating are both a little bit iffy for my anxiety levels...

I'd rather keep the pistons intact and not worry about them. Not sayin' it won't work, but I wouldn't want to do it.
 
turbonator said:
dave, he is talking about the ricardo swirl chambers on the tops of the pistons......the tops of the pistons can be shaved down between .020" and .040" (depending on original piston portrusion). This will get you in the neighborhood of 18:1 compression. This will lessen the depth of the swirl chambers, but not too drastically. Really needs to be calculated to get exact, but is not necessary. The idea is to lower the compression so asa to be able to add more boost with less stress on the motor.

Another method, is to shorten the connecting rods. This way the pistons do not need to be modified at all.

We have used mls gaskets (multi-layer steel), some like them some do not.

The fourth that can be used (as gm did) to lower some, not all, is pre-cup modifications.

In our present build now, we are using a combination of piston and pre-cup mods (looking for 17.5:1)
Click to expand...


thanks turbonator u guys have helped me a lot already
 
I would un shroud the valves right up to the fire ring. Kills two birds. Lowers compression, improves head flow. A fly cutter for gassers could be adapted. Is the 6.2/6.5 valve stem the same as some common gasser? BB Chev?
 
L98TPI said:
I would un shroud the valves right up to the fire ring. Kills two birds. Lowers compression, improves head flow. A fly cutter for gassers could be adapted. Is the 6.2/6.5 valve stem the same as some common gasser? BB Chev?
Click to expand...

This would work and you'd be helping the airflow for sure.

Moving the pre-cup away from the Ricardo bowl will do the opposite of what an IDI diesel need for efficiency and power. The 'close collision' of the piston zooming towards the head and cup creates some serious turbulence.

A similar principle is used in some hot rod gasser engines. They call it a 'zero deck' build using flat top pistons, zero deck height and 'thin-ish' HG's.
 
I shaved my first set of pistions right off 0.70, my second set I had CNC work done to dish them out and maintain the firing ring around the edge and had them powder Coated
 
3500GMC said:
A similar principle is used in some hot rod gasser engines. They call it a 'zero deck' build using flat top pistons, zero deck height and 'thin-ish' HG's.
Click to expand...

Back in the day we went for 50 thou piston to head clearance on the Chrysler wedge engines. The "squish" generated worked great allowing us to run higher CR's on the crappy gas of the day. Humm, oh yea we still have that crappy gas.

Un shrouding the valves on the 6.2/6.5 has the additional advantage of you dont have to touch the pistons, even pull then out of the engine. IIRC someone here has already tried this on a 6.5, remember reading it don't remember who.
 
I've been staring at a set of standard 18:1 pistons and I bet john kennedy (kennedy diesel) could point you in the right direction. I highly doubt they completely stopped making them since Penninsular still has 18:1 marine engines going out the door.


I like this idea of Unshrouding the valves but have actually never heard this term before. Any "quick" description of what this entails? 6.x engines arent know for their high qualtity heads so I would like to know where the metal would be taken from.

a dished piston is the best option imo to preserve the piston hight at the edges
 
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