shaving 6.2 pistons

[diaric]

any thoughts on how much to shave off my pistons to get to the 19:1 range, or how to calculate it?
engine has been bored 20 over.
heads have been resurfaced
adding ceramic coating to the tops would be about 1-2 thou thick

thanks

[More Power]

Welcome to the board! Tell us a little about your rig, your engine and what goals you have for it.

Have you read about measuring combustion chamber volume for either a gas or diesel engine? The methods are the same, whether gas or diesel. Once you know the volume, it's fairly easy to compute a new CR. All this has been discussed here before. A search would likely turn up some clues. If your search doesn't turn up the right thread, let us know.

Piston protrusion can vary somewhat engine to engine, depending on whether the block decks had been resurfaced. This affects the total CR to some degree. I know that for the 18:1 6.5 I built, that the piston crowns were about 1/16" below the block deck at TDC. Stock CR puts the crowns at flush or slight above the block deck at TDC.

Jim

[Robyn]

The only real way to have numbers that mean anything is to "cc" the thing

This entails finding the position of the piston at TDC and then calculating the total cylinder CC swept volume and then comparing this number to the volume left with the piston at TDC

This is with Gasket in place (crushed).
The head and precup chamber will need to be "cc'd" too.

Fairly simple although time consuming to get the CC numbers using a suitable oil.

Most Hot rod how to books that talk of figuring compression ratio will give a good tutorial on this with pictures and such.

In a nutshell. with the piston at BDC and the block assembly set so the deck is level fill the cylinder with light oil (ATF) and keep close account of exactly ho much it takes.
Write this number down. Now add the head and chamber numbers and add this to the total.

Now with the piston at TDC and the head bolted on with a crushed gasket (snug is fine and no need to place all the bolts in the thing)
Fill though the injector hole (glow plug in place) to a point that is where the injector stops.
You may need to burpit to make sure there are no air pockets (roll the block back and forth on the stand)

Now take the total volume of the cylinder and the chamber and divide it by the chamber only volume and you will have a number that represents the comp ratio.

Dont be supprised if you get some wierd number as the calcs are done originally from the blue prints and specs. Chambers (precups) can and do vary.

Replacement piston can and do vary some too.

Probably to get where you want will place the piston somewhere between .030" and .060" below the deck depending on if the deck has been cut.

To get very accurate numbers requires patience and care to measure the oil fill very precisely.

Just a few drops of oil can make a difference.

Doing this exercise was a requirement back in my college auto tech days.
Its messy to say the least.

***HINT***
Use a rubber Oring for the top ring to keep the oil from leaking past the piston and skewing the results :D

Have fun with it and let us know how it goes.

Best

Robyn

[diaric]

Thanks for the welcome. I'm doing a restoration on my fully loaded 87 jimmy. In the process of redoing the bodywork as well. Have a turbo setup off a 6.5 to install.
The block has not been decked. I have done searches and not come up with the answer, that is why I asked. I see in a couple of instances, in your tech section, people took .030 off 6.5 pistons to get into this ball park.
I have no way of measuring oil accuratly enough, so its not going to happen. I know it can be done with a special foam too, but I'm not going to waste a set of head gaskets at the price there at these days.
was hoping someone would have an idea of approximatly how much would be fairly close to this result. I could figure it out over a long period of time, but i don't know what the head chamber and precup volumes are.
It would be cheaper and easier to get 18:1 pistons, but I don't want to go this low as starting in the Canadian winter would become an issue. If I leave it all as it is, the CR is probably going to be over the stock 21:1, due to the overbore and head resurface, which is high to begin with.

[Robyn]

If you take off about .040" you will be close enough to make things happy.
The one issue that comes to my mind is the ballance on the rotating assembly after you remove the metal.

The engine will be down to do this so a ballance job should be part of the work to really make things proper.

A fluid damper would also be a nice touch too.

Good luck

Robyn

[diaric]

thanks. I'm not worried about acheiving an exact number of CR. just getting down to a better number for running better and longer with the turbo.
the new fluid damper is here in the box.

i was thinking about getting it balanced, probably a better job than me trying to weigh everything and do myself

[Robyn]

Sounds like a plan.

Good luck:D

[COMP]

any updates ???

[diaric]

not really. got 10 thou thicker head gaskets. was waiting for days off before i made a trip to the big city for a machine shop that can do it. have to decide how much to take off i guess. I will probably drop them off on monday