18:1 Compression Ratio

[jggiedeman]

Baqk to the 18:1 CR ratio. When I decided to lower my CR I bought some Zollner pistons off of E-Bay. Excellent pistons and they would have dropped my CR to around 20:1. Too bad the machine shop decided to ignore my written instructions and force me to move to a larger set of pistons. I went back to Peter Bierman's article of how he measured his final CR. The heads I bought from CCH had a larger precup area so I decided to CC the heads and Ricardo cup area. Wound up with 29 cc's for the head, 4 for the piston ricardo area and area from piston ring up, .010 gasket and piston depth(machined .020 off and ceramic coated) at -.018 gave me another 15 cc's. 820 + 48 / 48 gave me around 18.1:1. I also reduced my height of the pistons by relocating the pin bushings .010.

I'm glad that I had to do some measuring after the machine shop so efficiently hosed me over because my CR might have been much lower had I gone with true 18:1 pistons. Lots of ways to get down there and my method is far from the ideal setup but was born out of necessity.

[Robyn]

Sounds like you have it handled.
Should work sweet.

I have seen this trick done in several ways and as long as the results are the same it works.

???? did you ballance the rotating assembly after all the mixing matching and machining.

Having a good ballance on the engine is a must.

Be sure to ballance using the Damper, pulley and flex plate you will use.
Witness mark the pulley assembly to the ballancer in case its off a little.
Make good and sure that your ballancer rubber is in good shape as well as the dampened pulley assembly.

Good luck

Robyn

[6.5 Detroit Diesel]

I am looking at a few kits off E-bay right now. The one has an option to upgrade to Mahle 18:1 pistons for $300 more. How much boost can you run with the stock compression. If I could run between 12-15 psi with an intercooler I would be happy to stick with the stock pistons. Do you need to go with 18:1 to run that much boost?

[Kenneth]

With the stock CR you can run the boost up to 15 psi efficiently with a chip and intercooler. Over 15 psi and you need to lower the CR to 18:1 to handle the higher boost.

[Robyn]

Im running a performance chip in my DaHooooley and it has stock ratio.
Turbo master and 3-1/2 exhaust without CAT.
10-12 PSI max without charge cooler.
Truck runs great and plenty of power.

I personally would keep the boost with stock comp at no more than 12 PSI

With 18:1 and a charge cooler the addition of a better turbo can allow 15 plus.
The issue is the IP supplying the fuel. I DB2 pump can be turned up quite a bit.
The DS4 will only allow what the performance chip will give it.
I believe 91mm is the limit on these pumps.

I would recommend a max of 12 PSI even with a cooler and enjoy a long engine life.
Keep the EGT in line and do the cooling system mods as well and your little creature will buzzz along as happy as can be all day long.

Looking for the last ounce of horse pressure from a 6.5 can be like searching for Eldorado, "Not a fruitful endeavor"
The engine reliability can and will go south when you take it out on the edge of the envelope.

Even with reasonable mods and 12-14 PSI absolute max boost, you will need to drive this thing on the gauges. The truck will not be a get in and plant your boot on it and go thing. You will be fine cruising light but once the load goes on it you will have to pay attention to the gauges like a hawk.

There are many good people here myself included that will help anyway we can with advice.

My most important piece of advice in powering up a 6.5 is DONT GET GREEDY

The edge of the envelope can be flirted with but there is a bridge too far and its hard to tell exactly when your there until something melts.

Have fun and enjoy

Robyn

[6.5 Detroit Diesel]

Ya, it is the DB2 pump so I don't have a chip and can tune the pump myself. Or at least try to.... From what I have been reading unless big bucks are spent it is dangerous to cut corners and try to make power while keeping reliability. I want the truck to last and so I want to get the most power possible without driving down the reliability level. I figure while doing a rebuild on it I might as well go all the way and build it for what I want. I think an intercooler is a good idea especially if I am going to keep the stock CR.

[jggiedeman]

E-Bay Mahle 18:1 pistons. Seems I spoke with a few salesmen on these pistons. Currently aftermarket Mahle sells two versions of their 6.5 turbo pistons. Standard height and -.010 to accomodate machined deck. If you buy the reduced height pistons, add +.010 gaskets, larger precup haeds you might be able to get into the low 19:1 ratio. Before you buy those pistons for $300.00 get a part number and Google Mahle. Thier website has piston CR heights, etc. For all headaches invloved with machine shops, machining new pistons, I would just go with the JK pistons or Peninsular'sa 18:1.

To answer an earlier question on balancing my rotating assy. When the chop and shop machine shop was finished hosing me over I went to another more reptuable shop. They listened to my concerns, smiled and said they would take care of me. It took about a week and a half over Thanksgiving weekend to balance my rotating assy. #1 machine shop balanced twice. 2nd time was to correct for new psitons since they overbored my block. I was pretty skeptical with the results since absolutely no maching marks were found on the pistons. Found this hard to believe that Mahle would precision manufacture 8 pistons. Seems #2 machine figured MS #1 was lying. The results that came back was that the rods were off by 4 grams, the pistons(after moving pins around to get even weight distribution) were off 6 grams, and the crank was off by 50 grams. #2 figured they probably didn't mount the ring gear, harmonic dampner, and serp belt isolater to achieve this level of incompetence. This motor would have shaken apart before I would be able to drive it.

Parts are still rolling in and the date keeps moving back on it's christening. Money, or the lack there of, plays a big part also. I haven't taken any pictures but I do plan on writing a short story about the experiences I have had to date. I will say that the explosion of e-commerce has lowered my overall costs some but beware of trying to send something back (broken upon arrival) or to get missing parts from a multi shipment. Better have the patience of an oyster.

Jerry

[arveetek]

Jerry,

Sounds like you're having the same problems I did about 3 years ago. Machine shop bored my block to book measurements instead of piston measurements. So when I got my custom ordered, .030" over lower comp pistons, they fit too loose in the bores!! Aaarrrgghh!!

The shop bored the block again, but this time I made them wait until the .040" pistons came in and they bored each hole to fit each piston to my specs.

It sure is hard to find local machine shops that know what they're doing. I don't know how they stay in business.

Casey

[Chevrolet4x4s]

How much needs to be machined off of the pistons to get an 18:1 ratio.Has anyone ever tried 17 or 17.5:1? Also just a hairbrained idea but could one double headgasket a 6.5 or would that be beging to blow a headgasket.
Shane

[Ratau]

Did anyone tried to make head gaskets out of annealed pure copper plate?

[78Chev]

No...

If you do a lot of heavy towing, or are building a more performance oriented 6.5 and plan to run up to 20 psi boost pressure, then yes 18:1 CR would be a good choice. The 18:1 pistons are no more expensive than factory CR pistons (Peninsular type pistons).

If you use your 6.5 as a people mover, commuter car, soccer mom rig, or grocery getter, then you could be better served with the factory 20.2:1 CR.

Jim

I understand from this discussion what the benefits of 18.1 are but what are the advantages of staying stock 20.2 as you suggest for people moving, commuting, etc.? Is the only downside of 18.1 the potentially harder cold starts? I will be in a 6.5 rebuild project soon. I plan to tow a horse trailer a few times a year over the mountains to Montana so I am still trying to make up my mind. Thanks.
Randy

[More Power]

I understand from this discussion what the benefits of 18.1 are but what are the advantages of staying stock 20.2 as you suggest for people moving, commuting, etc.? Is the only downside of 18.1 the potentially harder cold starts? I will be in a 6.5 rebuild project soon. I plan to tow a horse trailer a few times a year over the mountains to Montana so I am still trying to make up my mind. Thanks.
Randy

People need to modify their glow system to include longer glow cycles when running 18:1s. Plus, there could be an advantage (possibly a small fuel economy benefit) when sticking with 20.2:1 for light-duty use. This is purely theoretical - based on thermodynamics. Our 18:1 6.5 would deliver fuel economy as good as or better than an identically equipped stock 6.5 powered truck. If your existing engine runs well, I'd leave it alone, except for maybe some external bolt-ons if you choose to.

Jim

[78Chev]

Jim - I am doing a rebuild anyway on this 6.5 so I have to buy new pistons. So the question is, since I'm doing the rebuild does it make sense to go with the 18:1 anyway given that the costs are the same? Thanks again.
Randy

[93GMCSierra]

I think I would, because getting a longer glow cycle is not a problem, and running higher boost will pack that combustion chamber so the compression is there and you get cleaner burn and lower egt with more air.
And of course the option for more power if and when it will be needed.

[More Power]

The gray areas of vehicle use make it hard to know what to do. One gray area would be a 6.5TD family rig that tows only occasionally, but is used as a people mover most of the time. I'd probably lean more toward a fuel economy build for that vehicle - small port precups & factory CR. Plus I'd add a few other items that improve efficiency - like gear drive timing set, intake/exhaust mods, etc.

On the other hand, if the vehicle will be used to tow heavy more than occasionally, I'd build in some durability and power - like 18:1s and hi-cap cooling, along with turbo & fueling mods.

A little trivia.... The Duramax began with 18:1 CR in 2001, and now is at 16.8:1 CR.

Jim