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Thread: 18:1 Compression Ratio

  1. #1
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    Arrow 18:1 Compression Ratio

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




    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 TDR
    So 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.
    Last edited by More Power; 11-16-2007 at 23:13. Reason: add to

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    Quote Originally Posted by More Power View Post
    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...
    '94 GMC 6.5TD K1500 4L80E 2-Door Yukon SLE 221K
    '93 Chevrolet 6.5TD K2500HD NV4500 Std. Cab Longbed 187K
    '85 Toyota 22R RN60 4x4 Std. Cab Shortbed 178K (Currently retired for rebuild)
    Diesel Page Member #2423

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    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.



    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
    Last edited by More Power; 04-08-2010 at 08:39. Reason: add stuff

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    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!
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    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.

    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
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    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
    1995 K1500 Tahoe 2 door, 6.5LTD, 4L80E, NP241, 3.42's, 285/75R16 BFG K02's; 1997 506 block; Kennedy OPS harness, gauges, Quick Heat plugs, and TD-Max chip; Dtech FSD on FSD Cooler; vacuum pump deleted, HX35 turbo, Turbo Master, 3.5" Kennedy exhaust, F code intake; dual t/stats, HO water pump, Champion radiator; Racor fuel filter

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    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.
    '94 Barth 28' Breakaway M/H ("StaRV II") diesel pusher: Spartan chassis, aluminum birdcage construction. Peninsular/AMG 6.5L TD (230HP), 18:1, Phazer, non-wastgated turbo, hi-pop injectors, 4L80E (Sun Coast TC & rebuild, M-H Pan), Dana 80 (M-H Cover), Fluidampr, EGT, trans temp, boost gage. Honda EV-4010 gaso genset, furnace, roof air, stove, microwave/convection, 2-dr. 3-way reefer. KVH R5SL Satellite. Cruises 2, sleeps 4, carries 6, and parties 8 (parties 12 - tested).

    Stand-ins are an '02 Cadillac Escalade AWD 6.0L and an '06 Toyota Sienna Limited.

  8. #8
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    Arrow Is 18:1 CR always the best choice?

    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

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    Quote Originally Posted by More Power View Post
    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
    Last edited by CareyWeber; 11-28-2007 at 10:59. Reason: added "know"
    97 GMC K3500 6.5TD 4.10 CC SRW +259K miles; Kennedy Fan Clutch w/ 20" steel fan; Greg's Oil Cooler Lines; Racor 445 30 micron w/ heater pre-lift pump; Dr Lee By-pass Oil Filter; High Idle Switch; Revo 265/75/R16 Load Range E tires

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    Quote Originally Posted by CareyWeber View Post
    Jim,

    Do you know where to get pistons anodized?

    Carey
    Shouldn't be too hard to find a source. Googling produced this list.

    Jim

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    Default 18:1 feature article question

    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.
    97 5spd K2500 Ext Cab short Bed ~160K miles.
    TM, 3" downpipe & 4" exhaust, remote FSD, remote oilfilter, Gauges: EGT, Boost, Fuel Pressure, B&W Gooseneck Turnover ball, Prodigy Brake Controller. Hi-Temp Hydraulic Oil Cooler Lines.

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    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
    (1) 1995 Suburban 2500 4x4
    (1) 1997 Astro
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    THIS IS BOW TIE COUNTRY

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    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

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    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

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    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
    (1) 1995 Suburban 2500 4x4
    (1) 1997 Astro
    (1) 2005 Suburban (Papa Smurf)
    THIS IS BOW TIE COUNTRY

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    Quote Originally Posted by More Power View Post

    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
    1995 K1500 Tahoe 2 door, 6.5LTD, 4L80E, NP241, 3.42's, 285/75R16 BFG K02's; 1997 506 block; Kennedy OPS harness, gauges, Quick Heat plugs, and TD-Max chip; Dtech FSD on FSD Cooler; vacuum pump deleted, HX35 turbo, Turbo Master, 3.5" Kennedy exhaust, F code intake; dual t/stats, HO water pump, Champion radiator; Racor fuel filter

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    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

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    Quote Originally Posted by arveetek View Post
    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

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    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

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

    Jim

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