18:1 Compression Ratio

[More Power]

This short tech piece first appeared in The Diesel Page in 1999.

http://www.thedieselpageforums.com/photopost/data/500/18to1pistons.jpg


The 18:1 Peninsular pistons were incorporated into our project 6.5TD engine build in late 1998. The advantages of a lower compression ratio (in the context of the project engine) is, a lower CR allows for more boost, and more boost allows for more fuel, which means the engine will make more power, while maintaining reasonable combustion temperatures & pressures. The folks at Peninsular Diesel told me they use a lower CR for the durability advantage.

In 1997, Kevin Emery (part owner of Peninsular Diesel at the time) told me that during engine dyno tests they found that a 6.5TD equipped with the factory 21.3:1 CR would suffer piston failure when run at rated power for about 3 hours. Installing 18:1 pistons would allow the 6.5 to run at rated power indefinitely.

I've always wondered why a lower CR improved durability. Now I know...... I read an analysis in the Turbo Diesel Register recently concerning compression ratios that finally put it all together and explained why a lower CR does what it does.

In issue twenty three (winter 1999) issue of the Turbo Diesel Register, columnist and automotive journalist Kevin Cameron discussed what effect compression ratios have on diesel engine design and efficiency. Cameron's column focused on the "law of diminishing return" and how that is applied to engine design in general, and to compression ratios in particular. Cameron explained that efficiency increases as compression ratios increase, to a point (of diminishing return), then other less desirable side effects become an issue. One paragraph I found particularly interesting is where he said;

"But there's another effect to consider here, which redoubles the law of diminishing returns. That is overall heat loss (amount of heat that needs to be dissipated) during combustion. As we raise the compression ratio, we also raise the peak flame temperature, and that pushes heat through the head and piston crowns faster. A couple paragraphs ago, we made a curve of theoretical energy recovery versus compression ratio - and how this heat loss effect causes that curve to flatten out even faster. The result is, that for diesel engines, that peak efficiency comes somewhere pretty close to 17 to one compression."
Kevin Cameron TDRSo now, it all comes together. A lower CR lowers the peak flame temperature, reducing the heat load on the pistons and cylinder heads. Voila, improved durability!
Cameron also explained that an indirect injected engine (like the 6.2/6.5), with its added surface area of the IDI pre-chamber, will dampen the heating of the compressed fuel/air mixture during the compression stroke, thus affecting cold starting with a theoretical most-efficient 17:1 CR. He went on to say that most IDI diesels use a 20:1 or higher CR to offset this cooling effect and allow reliable cold starts. Not surprisingly, beginning in the 1997 model year, the L65 6.5 turbo diesels were equipped with 20.2:1 pistons, and the newest 1999 model L65 pistons were scheduled to be reduced further to 19.5:1 (which didn't occur because the 6.5L diesel was dropped from the engine option list in the new GMT-800 new Silverado body style trucks). The first L65 6.5TD engines ran with 21.3:1 pistons. The use of lower CR pistons (20.2:1) is GM's attempt to strike a balance between cold starting ease, glow cycle times and engine durability - for the masses.

[trbankii]

In 1997, Kevin Emery (part owner of Peninsular Diesel at the time) told me that during engine dyno tests they found that a 6.5TD equipped with the factory 12.3:1 CR would suffer piston failure when run at rated power for about 3 hours.

I'm assuming this is a typo... :)

[More Power]

Yes, a typo. It should read 21.3:1 CR....

I should add that our 18:1 6.5 was first run during the winter of 1998/99, and that I scrambled to increase the first glow cycle duration in an effort to improve cold starts. With an unmodified glow controller, the first glow cycle at a cool 25 degrees F was approximately 8 seconds in duration. The 18:1 6.5 produced way too much white smoke with that combination of temperature and glow time. Increasing that first glow cycle time to 12 seconds produced typical 6.5 cold starts at that temperature. In later years, we increased glow time to about 15 seconds (w/AC-60G glow plugs). This is easy to do with the 1985-93 glow controller, and there are a couple of articles currently available in the subscriber areas that show how to increase glow cycle times for the 1994 or newer electronic trucks.

There are at least two ways to reduce compression. 1- Use pistons that were manufactured with relocated piston pin bores, which is what Peninsular does. 2- Machine the piston crown.

http://www.thedieselpageforums.com/photopost/data/500/schoolcraft3-33.jpg

The above Ron Schoolcraft image shows how KennedyDiesel.com does it. Ron sectioned a stock 6.5 piston (not shown) to show how thinning the crown doesn't appear to materially affect the strength or durability enough to matter. The factory 6.5 piston crowns are anodized, so machining the tops removes the factory coating. I'd recommend either ceramic coating or anodizing the tops to return the piston's heat tolerance, which I think KD does.

Which method of reducing CR is better? Good question. One could make a good argument for either method, as each has been proven during real-world durability tests.

I've heard mention of raising 6.5 compression ratio above stock. Other than being a really bad idea, I'd like to know how someone would raise CR? The head deck is already flat, and stock 6.5 pistons can already protrude a few thousandths above the block deck at TDC. This is why thicker head gaskets are available (are required) for those engines that have had their blocks decked during a rebuild.

Jim

[a5150nut]

Low compression and lots of boost is how the older Cat equipment motors were built. They also had a glow cycle. The in line 6 cylinders produced more usable torque than the newer V-8's. The 633 C model scrapers had an in-line 6. When they came out with the 633 D's the old 6's would out work them in the hills. You could pull them down in the RPM range and build power back up. The V-8's you had to run more like the old 2 stroke Detroit's. Keep them near max RPM or the bottom would fall out of the power range.

Then there was the duel overhead cam V-12's in the 660's and triple 6's!

[Robyn]

The later Cat truck engines (3406E model) have been a real treat.
For an inline 6 these little monsters will flat turn out some power.

I was yacking at a fellow from up Noth of the border that routinely pulls some very serious weigh on a Super "B" train and has had his 3406 spooled up to produce close to 1000 HP.
I have no way to verify this particular case but I know from chatting with the Cat boys that the outlaw shops can and will do it for a price.

The 3406E 600 HP engine can easily get there. It produces a whopping 2000 Ft lbs torque at 1200 RPM in stock trim so I can only imagine what it will do when the computer gets mauled by the outlaw programers. :eek:

My Western Star was purchased a little too early in production to be able to get the 600.
I even had to wait to get the 500, which is a real monster when it comes to hauling heavy loads.
^^
I can have it tweeked to 575 at the local Cat > . < Shop.
--
The 600 has a different bore and stroke I believe.

The 500 I have is rated at 1800 ftlbs torque at 1200RPM and its stock.

The boys over at the tranny companies were literally crappin in their shorts when the Cats, Cummins and Detroits showed up with all that power.
The only tranny I had as an option was the 18718B fuller 18 speed.
The clutches were another bone of contention too.
Full ceramic was the order of the day but now the new Kevlar is the hot ticket. I bought one 2 years ago and its is a super clutch. This thing does not eat the flywheel and the pressure plates and will hold the power well.

The boost on the kitty is right at 30 PSI.
The new low emission cats are running 50 PSI with twin turbos.

Back to the 6.5
I personally think that if you are going to build a custom 6.5 that the 18:1 pistons and some serious boost are the way to go.

Myself I am very happy with the stock compression along with the power chip and the turbo master.
The DaHooooley is very responsive and also very civilized. I personally have dusted off some late 90's 7.3 strokes and there was no doubt that the little 6.5 was in command of the situation.

The DS4 pump is the limiting factor IMHO and with the chip its giving the engine all that it can deliver. 91mm

If I were serious about a build I would think about a DB4 pump that can really stuff the fuel at the little diesel Rat.

I am running 12 PSI max on DaHooooley and with this the EGT is about 800 (POST TURBO) on a hard pull with a 6K trailer behind.

Anything more than this needs a cooler on the inlet air to help out.

I am also a fan of the coated pistons that use a relocated wrist pin to do the honors of dropping the compression.

Good luck with any projects

Robyn

[arveetek]

I had the Diesel Depot make me some custom .040" over bored low-comp pistons. They machined the surface down and then ceramic-coated the tops. After decking the block and using standard 6.2L gaskets and topped pistons, I ended up with a CR of around 19.75:1.

I'm running a 6.5L pump, 6.5L injectors, DSG gear drive, Garrett turbo, 3" downpipe to 4" exhaust, and with these components I see a peak boost of 18 psi at 1050 degrees. I have very responsive boost as well. Just touch the throttle and she jumps to life. I can build around 12 psi when doing a powered launch.

The 19:1 pistons were cheaper than the 18:1 Peninsular pistons, and I believe is a good compromise for anyone building a 6.2L on a budget that will be seeing any kind of towing use. In reality, any CR drop is good!

Casey

[rustyk]

Back to the 6.5
I personally think that if you are going to build a custom 6.5 that the 18:1 pistons and some serious boost are the way to go.

I'd have to agree. The AMG/Peninsular Engines 6.5L TD I had installed was significantly better than the stock 6.5L TD (Early '92 manufacture). At first, I was a bit disappointed in the ability to use all the power in the replacement engine (230HP vs. 190HP stock), but the issue was a defective turbo, which failed eventually; the replacement has given me the engine I expected. With the original turbo, I would hit the EGT wall fairly frequently, even on moderate grades. With the new turbo, I've yet to be able to hit redline. I attribute the performance to the lower CR pistons.

The 18.1:1 pistons give up nothing in performance over the stock CR, and economy is up 15%. Now, I don't engage my 17K lb. motorhome in drag racing, but it'll suck the doors off of gaso-powered RVs in the same size range.

[More Power]

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

[CareyWeber]

The factory 6.5 piston crowns are anodized, so machining the tops removes the factory coating. I'd recommend either ceramic coating or anodizing the tops to return the piston's heat tolerance, which I think KD does.

Which method of reducing CR is better? Good question. One could make a good argument for either method, as each has been proven during real-world durability tests.

I've heard mention of raising 6.5 compression ratio above stock. Other than being a really bad idea, I'd like to know how someone would raise CR? The head deck is already flat, and stock 6.5 pistons can already protrude a few thousandths above the block deck at TDC. This is why thicker head gaskets are available (are required) for those engines that have had their blocks decked during a rebuild.

Jim

Jim,

Do you know where to get pistons anodized?

Carey

[More Power]

Jim,

Do you know where to get pistons anodized?

Carey

Shouldn't be too hard to find a source. Googling produced this list (http://www.google.com/search?hl=en&q=anodizing+aluminum). :)

Jim

[Hubert]

I believe lower compression is better when elevated fuel and boost is used and think I understand the 6.5 pretty good now just a little curious...

In the article about 18:1 compression. It says something about piston failure at stock compression ratio occurred within hours at rated power levels. But with 18:1 cr the engine could run almost indefinately at rated power.

Is that the 190 hp or 250 hp + rating? What were EGT's running when premature failure occurred?

Do marine units and or this test stand run an oil cooler? And if not do you think cooler oil along with the oil sprayers would that have extended the run time with stock CR? Not indefinate but how about significantly????

I am just thinking about stuff can 2 different setup engines with the same EGT reading have significantly different thermal load on the piston and/or combustion chamber with the same load and fuel rate? I think so depending mostly on compression ratio.

[Robyn]

If the 6.5 is run at the conservative power levels that GM sold them at they will be fine.

When the boost is elevated and the power level is jacked up the piston skirts can seaze due to the heat that is created by the extra boost.
The oil coolers are a necessary item on these engines but even with the sprayers will not take away enough of the heat to help.

The drop in the comp ratio to 18:1 drastically lowers the flame temperture during operation and allows the boost and fuel values to be bumped up a bunch.

For most applications the stock ratio is fine. Now when you get hungry and are not content with 10-12 PSI max and start wanting to run 15-20 PSI and have plans for this little creature to really rotate the earth then the 18:1 and an intercooler is a must have.

EGT above 1000f is not good.
All this heat has to go somewhere and the block, heads, valves and pistons take a horrible beating and soon something will fail.

Bt reducing the comp ratio the power levels can be bumped up into the 300hp range in a truck. The issue is still heat but its whats in the radiator now thats the worry.
In marine applications there is not issue as the pond is full of cool water and the engine temp can be easily controlled as can the inlet air temps using a water cooled charge cooler.

Heat is the big enemy here no matter how you cut it.

18:1 is a nice way to go if you are looking to make big power and then use it to reef on something real heavy.

Hope this helps

Robyn

[john8662]

I know that the stock 6.5TD that is rated at 190HP is likely rated at WOT. I also know that if you take a stock 6.5TD, put it in 3rd gear (or 1:1, 4th for a stick) and put your foot in it and keep it there and run it up to near the governor for any real distance of time, the engine will come apart. The EGT's on a stock engine aren't very well controlled, so the temperature rises at that high compression ratio easily. This leads to the aluminum pistons expanding and consuming the clearance allowed and scoring the bore. Eventually the piston will expand to the point where it won't move in the bore, something usually gives out, usually a connecting rod.

J

[More Power]

The statement about "running at rated power" came to me by one of the owners of Peninsular Diesel in 1997. By "rated power" I assumed that to be 190 FW. They developed the 18:1 pistons for the 6.5 at a time when the engines ran a DB2 (mechanical fuel injection) - likely an on-road factory calibrated DB2-4911. Last year, while visiting Peninsular in Michigan, they demonstrated dyno'ing an NA 6.5 for us on their in-house engine dyno. It ran with a large fan-cooled engine oil cooler and a huge fan-cooled radiator (looked like a big-rig radiator). It's amazing how much heat these engines generate when loaded hard.

I don't remember EGT's coming up in that conversation back in 1997, when discussing piston durability, but like John said, it was likely whatever was developed at WOT (or whatever), to get rated power.


Do you think cooler oil along with the oil sprayers would that have extended the run time with stock CR? Not indefinate but how about significantly????

I am just thinking about stuff can 2 different setup engines with the same EGT reading have significantly different thermal load on the piston and/or combustion chamber with the same load and fuel rate? I think so depending mostly on compression ratio.

Piston oil spray can help improve piston durability, that's why it's used in most diesel engines. Will that reduce or eliminate the need for lower compression? I don't have a firm answer, except that I know lowering compression ratio improves piston durability. Combustion temperatures are lower with a lower CR. I've seen the results of overheated 6.5 pistons... It's not a pretty sight...

EGT.... I heard from a non-turbo 6.2L owner a few years ago who ran his truck at 1000-1100 degrees for several hours. He was in a hurry, towing a trailer, and ran it to the ECT limit all during that time. He reported it still ran fine afterward. I'd have been sweating.... ;)

Jim

[Robyn]

Let me add this tid bit.

This is pretty much gospel as far as I am concerned.
"IF" you are going to runa 6.5 TD with some major power improvements you need a good set of gauges on the beast.

WATER TEMP, EGT, BOOST

you have got to drive these little monsters on the gauges and there is really no exception especially when your asking them to work hard.

The Coolant temp, EGT can all get hinky depending on the outside conditions (TEMP, Altitude and the load)

This is not a plant your boot and forget it engine, you have to be on top of things.

Just my opinion

Robyn

[arveetek]

EGT.... I heard from a non-turbo 6.2L owner a few years ago who ran his truck at 1000-1100 degrees for several hours. He was in a hurry, towing a trailer, and ran it to the ECT limit all during that time. He reported it still ran fine afterward. I'd have been sweating.... ;)

Jim

Jim,

When my '81 was naturally aspirated w/stock CR, I saw EGT's up near 1300 and spikes up to 1400. Towing my 5th wheel (like I did to Dayton in '01), I regularly saw 1100 to 1200 for hours on end. It never seemed to bother the engine.

Installing the turbo and 4" exhaust with no other mods reduced the EGT's to a max. of 1050 at 14 psi.

When I rebuilt the 6.2L a few years ago, there was really very little cylinder scoring, just some slight piston damage. It was running just fine at the time. I only pulled it to replace lifters and a few other things, and then those "might as wells" started kicking in...

I don't think high EGT's are nearly as critical on a n/a engine as they are on a turbo'ed.

Casey

[john8662]

The dangerious aspect of what Jim described above is that two conditions were at their maximum. The EGT's were at the engines designed red line and the ECT (Engine Coolant Temperature) was at it's near maximum. Those two conditions can get things too toasty.

Usually when under high EGT, the ECT will quickly follow, which seems to be a good way of judging the working ability of the 6.2 without a pyrometer installed.

I don't know how regularly the 6.2 will see high EGT's in stock and properly calibrated form (designed fuel rate, not turned up).

But, as guessed, n/a high EGT's are less detremental to the engine than Turbo high EGT's, for many reasons.

N/A engine has less backpressure (lack of turbo-backpressure generator), lets the engine more easily dissipate the EGT heat, rather than heat soaking the engine.

A Turbo engine, with it's added plumbing and added backpressure, less cylinder scavenging will retain more of the engines heat, by design. This can be a disadvantage when EGTs are at the limit.

Relating this to the compression ratio. A turbo is a device that supplies more oxygen to the engine, but because it's pressure also acts as a method to give the engine a varying compression ratio. With the turbo 6.5 and the non-turbo 6.2 having nearly the same compression ratio, the turbo 6.5 engine under full-boost will be running at a higher compression ratio. More compression, more heat due to compressing the air.

I don't think I'd want a n/a 18:1 6.5 tho!

J

[More Power]

Jim,

When my '81 was naturally aspirated w/stock CR, I saw EGT's up near 1300 and spikes up to 1400. Towing my 5th wheel (like I did to Dayton in '01), I regularly saw 1100 to 1200 for hours on end. It never seemed to bother the engine.

Installing the turbo and 4" exhaust with no other mods reduced the EGT's to a max. of 1050 at 14 psi.

When I rebuilt the 6.2L a few years ago, there was really very little cylinder scoring, just some slight piston damage. It was running just fine at the time. I only pulled it to replace lifters and a few other things, and then those "might as wells" started kicking in...

I don't think high EGT's are nearly as critical on a n/a engine as they are on a turbo'ed.

Casey

Casey,

Your experience does illustrate how a turbocharger reduces EGT under similar load conditions. EGT is a reflection of combustion temperatures (a ratio of), where combustion temps could be 3000+ at max load, but it cools to 1100-1200 when measured in the exhaust manifold (due to expansion and absorption). This also dispels the notion that the NA precups will suffer excessive damage when turbocharged. The EGT limit remains the same for either a turbo or NA 6.2/6.5 engine.

Back in the MFI days, diesel engine manfacturers rated their engines at a specific EGT (usually 1100 degrees F) and sometimes also exhaust opacity. This isn't true so much anymore with electronic fuel injection systems.

Jim

[Chevrolet4x4s]

Has anyone ever tried the piston heat barrier coating(cant think of the name of it) and would this increase the safe egt capacity?
Shane

[More Power]

Yes, quite a few 6.5 pistons have received the ceramic crown treatment. :)

Jim

[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 :eek:

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.... :eek: 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