Help me out with some math here something’s not adding up. Someone was going to use (.010) thicker head gaskets and valve modification to get his motor down to about 19:1. Additionally it has been said that (.050) is removed from factory pistons to get the compression down to 18:1. Siv-o-lite has pistons for our engine that have a reduced compression height by (.010).



I must be missing something it seems like changing the compression height by (.010) head gaskets or siv-o-lites would only have 1/5 the effect on lowering the compression ratio as shaving (.050) off the factory pistons.



21.6:1 CR stock

Shave (.050) = 18:1

Now applying this to the (.010) change offered by Thick head gaskets or Siv-o-lite Pistons

1/5*3.6 = .72

Compression should only drop by about .72 points for every (.010) decrease in compression height.

So thick head gaskets or Siv-o-lite pistons would only get you down to about 20.9:1 or 20.2:1 if combined.



The reason I'm asking all this is that I wanted to go slightly lower than 18:1 possibly 17.75:1 or 17.8:1 so I can really hit it with a lot of boost (possibly a BD-twin setup for the Cummins).



I just want to make sure I'm doing my math right for when I build my engine. I've rebuilt motors before, but not modified internal components outside of factory specs.



I'm sure someone out there won't think this is a good idea, but I don't want them to fill up this thread with a bunch of "It can't or shouldn't be done" I really don't want to hear about whether or not this is a good idea or be told that the engine can't handle it.
Except from someone who has personally done it to his own motor and it failed.

question . . . would dropping CR too far make starting an IDI more difficult?

I think yes. I refigured the .010" gasket and valve pocket releif to ~20.2:1 combined.

That's one of the rumors I've heard but I'm prepared to extend glow times and/or install a start-up intake air heater like the first gen Cummins.

careful with that thicker head gasket and a lot of boost. THe thicker the head gasket, the more likely it is to blow out in the weak spots which is about everywhere in my opinion. I am getting ready to build a low compression 18:1 motor and am considering o-ringing the heads so i can really put the hurt on the engine. I am also making an custom girdle for the main caps to hold things together a little better. That is if I find a good block one of these days:eek:

Good point with the thicker head gasket I'll have to do some research on any failure analysis, but I have heard only good things about the fel-pro gaskets both stock and thick.

If .050 yields 18:1 with stock thickness head gaskets then some possiblities for getting the lower compression I'm looking for could include something like

.053-.055 removed, but this might make the crown too thin for reliability.

Maybe .050 removed with QM's idea of removing material from the valves

Or Siv-O-Lite claims that even though their pistions are .010 lower compression height they maintain factory crown thickness. So by my quick math removing .043-.045 would yeild a total change in compression height of .053-.055 compared to stock while at the same time leaving .005-.008 more crown thickness than a stocker shaved by .050 and factory thickness head gasket can still be utilized.

Better yet S-O-L shaved by .040 and QM's modified valves would yield the thickest crown with lower than 18:1 CR while still using the factory thickness head gasket.This option seems like it could be the best of both worlds.

I don't remeber who, but someone on this site had a bad experience with Siv-O-lite piston comming apart and toasting their engine, so I do have some reservations about the S-O-L pistons.

I personally would not remove material from a Siv-O-Lite piston. They are an inexpensive piston made to be better than oem pistons. For what you plan to do, I would check for a better piston than the Siv-O-Lite. If you do shave (flycut) the pistons, check into getting a protective coating on the combustion side (dome) applied. Just my 2 cents.

That's two that seem to be against S-O-L. No matter what pistons I use they will be professionally ceramic coated, on the top and possibly teflon coated on the skirts as well.

I could still go with .050 shaved Mahle's stock thickness head gaskets and some valve work.

Aren't the stock pistons coated as well guys? :confused:

They appear to be anodized on top and the top ring area also.

To the best of my knowledge all pistons available for the 6.5, both factory and aftermarket have ceramic coating on top. I wouldn't dare run a turbocharged engine of any kind without coating at least the crowns.

I haven't found any pistons that get down the compression to the 18:1 area without shaving, so re coating is a must.

I also don't understand why it is impossible to find forged pistions, this seems like a logical fix for cracked pistons, which I have seen at least a half dozen times on this site.

Oh well, I will do the best I can with the parts available.

It would cost too much time and money to try and re-invent the wheel, I am just trying to have some fun here with my 6.5.

Has anyone looked into custom pistons? A friend had some done for his turbo stealth when he shattered the stock ones. I believe the company was Ross? The things looked like jewels when he got them and i think they were about $900 iirc. Something to think about. He beat the crap out of the thing afterward and never had a meltdown.

$900 for six pistons probably means $1200 for eight. I'd love to do it if I could afford it.

http://www.kennedydiesel.com/images/lowcomppist_sm.jpg

JK, are these cast or forged??

That is what I want to know. Is there a forged set of pistons out there in order to sustain the large power numbers that us 6.5 freeks want? That makes me want to know how the 6.6 DMAX holds out to large power numbers without breaking all the time. Are those pitons forged or somthing?

I NEED ANSWERS:help2:

I plan on building my 18:1 motor soon with the Zollner .020 pistons that I purchased on ebay. I am just looking for the block to put them in. They have a moved pin location and a ceramicoated top and the sides have a teflon/graphite coating for high lubricity.

Couldn't help myself!

:funnypost I wonder how many will get it!

Guess a pacifier aint gonna stop that wailing, this time.......quick - somebody change that kid's diaper!

I remember D.Camilleri say in the past that he had some s-o-l pistons ruin a 599 block of his.

JE will make anything you want for $108 per piston. Just send them whatever ring stack you want and they can handle it, as far as a steel insert for the top ring......doubt they've ever had anyone ask. And .010" at bore diameter is around 2 cc's, so when your total combustion chamber (gasket, inverted dome, and cylinder head) = 39.5 cc's stock, it changes the compression in a hurry.

Top and top ring area is anodized in the stock piston. No steel or ceramics in piston.

BS, DP. Top of stock piston is anodized, but top ring land it a steel insert...

DP, I thought that Dcamelerri (spelling?) cut one open and it had a steel ring insert in it? Are you guys sure it is anodizing and not some type of coating? :confused:

the stock piston i cut in half had a steel land for the top ring.

Pistons come in various 'blends' of anodizing - high quality Diesel pistons are hard-anodized.

NASA specs similar anodize on aluminum surfaces for the spacecraft, for protection during re-entry.

Hard-anodize is non heat conductive thru the coating, but is conductive across the exposed surface, preventing temperature-rise in hot-spots.

Bohn and Zoller oem 6.5 turbo pistons are eutectic castings, with steel ribs across the wrist-pin areas for strength, with steel top-ring lands, and are hard-anodized.

Pistons with re-located wrist-pins retain that hard-anodize surface.

Any fly-cutting or CNC milling destroys the hard-anodized factory coating, and must be re-coated for survival in turbo application.

Some, maybe even most, use ceramic coating - some do not bother.
If you get a piston with a shiny aluminum top-surface, it is not protective-coated.

Sorry - I know you said no facts, just tell you what you want to hear, but that's the published specs....................

The stock piston I have on my desk has no steel in it.

my new 18:1 low cmpression pistons from Zollner have a ceramic coating on the top, a steel insert on the top ring and a silicon coating on the sides where there is side wall contact.

The stock piston I recently cut apart has no steel in it anywhere. All aluminum with no steel in top ring land or under the wrist pin. The top is hard anodized as are all factory 6.5 pistons.

The silvolite pistons that failed on me had no coating on the top of the pistons at all. At the time of failure, one piston cracked, resulting in compression loss into the crankcase. I tried to nurse the poor beast home(200 miles, stuborn driver) and the cracked piston disintegrated into small pieces of aluminum gravel. I heard that the military had similiar circumstances with silvolite and sued them.

My 18:1 pistons that I purchased from Penninsular, were suposedly stock piston blanks with the wrist pin off set to lower compression. This left the hard anodizing in place and didn't sacrafice any crown thickness. Those pistons were pricey, 1245.00 US for the set, but I never had any problems whatsoever with that engine, other than trying to keep it cool in the summer while towing. But that problem hasn't changed with my cummins either, although the 22 inch flexalite class 5 truck fan has helped a bunch, at the expense of a few ponies.

Anybody section a factory 6.5T piston vertically thru the wrist-pin bosses?

BTW - the top ring steel land is an upgrade piston - anybody verify for the '99-up engines, for the proposed 215hp upgrade?

I cannot see paying 65 bucks @ for several pistons to do some destructive R&D..................

The piston I cracked was a replacement but for all I know came from GM. I'd like to cut it in half, as it cracked clean through but didn't disentegrate, but I only drove it 1/2 mile when it went. I try cutting it in half and posting it later.

The one I cut in half is cut vertically through the wrist pin bores. I will try taking some pictures and posting it here. On stock pistons that I have cracked, the cracks extended all the way to the top of the wrist pin bore, so that the piston was basically cracking in half.

it is hard to see on a used piston unless it's bead blasted. The piston i cut in half was from a 2929 block, so that dates it 94-95 and it deffinately had a steel upper ring land.