Compression Ratio Question...

[More Power]

I received the following email this morning about the 6.5 compression ratio. I thought that putting the message and my reply here might help answer any questions 6.5 owners might have about this subject and allow me to respond to some of the comments being circulated.

I have a '95 6.5TD in need of a rebuild. In looking for a long block, I came across a website, and was impressed with their product. In their FAQs, they specifically said 18:1 pistons are a bad choice in a vehicle. I emailed requesting more info., and this is what they sent me.

"Nate,
We agree that the loss of power and economy from lowering the compression cannot be made up with the factory turbo. 18:1 is strictly a Marine compression ratio for engines that will be required to run wide open for many hours on end. The marine applications use the lower compression ratio to keep from building up too much heat in extended duration full throttle situations. This will never be the case with a truck.

Over the years there has been many fixes, band-aids and patches for the old weak 6.5 blocks: main girdle kits, arp stud kits, sleeving, lowering the compression ratio, using the old 6.2 block and a myriad of other expensive and ineffective substitutes for fixing the real issue, the metallurgy of the block.
With the introduction of the improved block and heads from AM General, (and now available through aftermarket companies like us), there is no need to sacrifice power and economy for durability.
-- Jeff"

I respect your opinion. Please help me sort this out.
~Nate

Hi Nate,

Businesses push what they sell and usually put down what they don't. That’s just the nature of selling engines and engine components. We don't sell engines or engine components. Our goal is to help 6.5 owners achieve the best possible result by providing the best possible information – based on facts and real-world tests (that's what the Pull-Offs were all about). But, we do have advertisers who sell engines equipped with either compression ratio, so we don’t necessarily push one compression ratio over another. Both have their place.

I've seen several 6.5s (running performance programming) with damaged pistons due to excessive heat and I've heard of many-many more. Piston durability is the primary reason for using 18:1 pistons, and lower compression could have helped to prevent those failures. This is why we put together a performance-oriented 18:1 engine in 1999 - to learn more about lower CR. The truck this engine was installed in accumulated more than 250,000 miles before we sold the truck, so we know quite well how a lower CR engine performs.

Our 4.10 geared K2500 with an 18:1 6.5TD would routinely deliver 17+ mpg, with a best tank of 21-mpg. I've owned and/or driven enough 6.5s to know this is as good as or better fuel economy than most. Not till the more recent use of the Holset turbochargers on the 6.5 has the on-road performance of our original 18:1 engine been surpassed. We used a factory GM-8 turbo on our 18:1 engine, which was not the best choice for ultimate performance. I could go on with other examples of how an 18:1 engine performed better than the stock CR 6.5... we’ve written extensively about this subject.

In my opinion, cold start-ability is the only drawback to lower CR. The 18:1 engines require a longer initial glow cycle to start well in temperatures lower than about 30 degrees F. With a 15-17 second first glow, it'll start normally down to about 10 degrees. All 6.5 owners, owning either 18:1 or stock CR engines, should use the block heater at lower temps anyway.

I recommend the 18:1 CR if the truck will be used to tow heavy a significant portion of the time. If your use of the truck is mostly not towing, then a stock 20.2:1 CR might be a better choice because the glow system won't need to be modified and cold start-ability will be what 6.5 owners are used to.

You can read more about the 18:1 CR here:
http://www.thedieselpageforums.com/t...ad.php?t=29913

Jim

[Pelado]

Cristal clear for me, Thanks

[rustyk]

I replaced my stock ('92 version) 6.5L TD (21.1:1) with a Peninsular/AMG with 18:1 and a non-wastegated turbo. Fuel economy went from 9.2 to avg of 10.7. This is in a 16K lb. MH. The differenc in power is dramatic, and starting is slightly tougher when it's really cold (<25°F)

[TAG]

I installed my 18:1 penninsular back in 2006, No regrets here even in Michigan winters. I have started mine at -5f without plugging in, it isnt pretty, but it fires off quick. Just smokes & shakes for a bit. Fuel mileage has only improved with every mod i have done to the truck since 2000.

[jerry598]

Darn, wish I had done my rebuild with those lower compression pistons. If milage and durability are improved, I would opt for that over quick starts in cold weather. Next time for sure, as long as the Iranians don't block the gulf and cause a 3 fold price increase in diesel fuel.

[Robyn]

My rebuild last summer of my 6.5 ended up with about 19.5:1 compression and its great.

If I was going to tow real hard and long especially in hot weather I would have done 18:1.

The slight reduction to 19.5 was just enough I think to help things out some.

The biggy is to have a turbo that can build some serious boost and couple that with a charge cooler.

With these mods the 18:1 really shines.

As mentioned for a stocker that is not going to see towing or hard use, the stock compression is fine.

A good freeflowing exhaust along with a reflash on the programing helps a bunch too.

The 6.5 is not all that forgiving when it comes to HEAT. You must keep the coolant temp under 220F and the EGT at a max of 1000F pre turbo.

Far too many 6.5 engines literally get Melted.

Running the EGT's upwards of 1000F for more than a few seconds can wreak havoc.

The 18:1 pistons will keep the peak burn temperature down a lot and this really helps.

My old engine would see EGT's around 950F Post turbo when working hard.

Now its hard to get it much over 700F on a hard pull.


Missy

[DaveBr]

Hey Robyn

I was just wondering why you say that max egt pre turbo should be 1,ooo*? From the articles and opinions I have read on this site the school of thought is 1250* max. That is a big difference as well as a big cutback in power. Again... just wondering why this is your max limit.

[Robyn]

IMHO
That last little bit of TEMP does not make all that much more usuable HP and makes all the difference in the world between melting stuff and not.

You can make gobs of power with the right programming in the ECM and the proper turbo and not even need to get up to those temps.

The 1250F MAX is just that, MAXXXXXXXXXXXXXXXXX
After that all bets are off and stuff starts going away.

Remember, not all cylinders are going to be running the same temp.
Have one or two in the bunch that are maybe running a tad warmer due to ??????????/ all of a sudden you have a failure.

One cylinder could actually be running closer to 1300-1400 or ?? .

The magic 1250F is indeed the upper limits.

1000F preturbo can be done all day long and not have issues.

If you need the last tid bit of power, go for it, but the closer you get to the breaking point, the easier it is to go past it for a tinny winny bit too long and PHHHHHHHHHHHHHT MELTVILLE

Rather than worry about being able to get the last bit of power out on the edge of the envelope, why not spend a few $$$$ and get more power with less heat.

A charge cooler, better turbo, big free flowing exhaust and other stuff that can make power without all that terrific heat.

There is simply no advantage to having the heat when you can do the same work without all that thermal stress on the engine.

Remember, the 6.5 will and does crack heads and other things. Heat in copious quantity is certainly a contributing factor.


Example

My old Cat 3406 A MODEL would run 1200F Post turbo on a hard pull.
My 1995 Cat 3406 E model runs more power and will never see the temps at the same spot in the pipe run over 800F

Getting the intake temps down and keeping the turbo driveside back pressure lower is the answer (charge cooler and a better turbo)


Just my two cents worth.


Missy

[DaveBr]

Thanks for your thoughts on the subject Robyn. The thing about max temps is there should always be a safety factor built into them. Any lifting device or tow strap has a max rating. This rating is far below the actual amount that can be lifted or pulled before damage occures. It would be my hope that max egt's would be rated the same way. How does one go about finding out if there is a buffer built into the max temps?

[More Power]

You can run 1250 (before turbo) for as long as it takes to pull the longest grade in North America, without hurting the engine or producing any cumulative damage. It's not a problem. A sustained 1500 degrees EGT and 250 degrees engine temp will burn it down in 30 seconds to 1 minute. I've run the 6.5 to 1400 for a few seconds, then backed out of the pedal till temps stabilized at 1250, and I've done this enough times to know nothing was damaged. The many Pull-Off events we held through the years provided lots of valuable information regarding the 6.5 and max EGT. Those who limit max EGT to 1000 are giving up performance for no good reason. Try it yourself the next time you tow to see what you're giving up.

Jim

[rogers]

Are max EGTs affected by dropping compression? In other words, can the hotter temps be run longer due to lower compression? I know that 1000* is 1000* but perhaps the space allotted to the burn affects heat migration into the heads and water jackets?

[DmaxMaverick]

Are max EGTs affected by dropping compression? In other words, can the hotter temps be run longer due to lower compression? I know that 1000* is 1000* but perhaps the space allotted to the burn affects heat migration into the heads and water jackets?

No. The temperature ceiling doesn't change. Lowering the compression ratio increases the combustion chamber volume, allowing more air into it at the same boost level. The increased volume reduces the resultant EGT, all else being equal. However, the combustion temperature will have the same effect on the combustion components, regardless of how it is achieved. With a larger combustion chamber, the heat volume (BTU) can be greater, although the measured temperature (EGT) remains constant. This translates to more heat energy to drive the turbine at the same measured temperature. So, at the same temp, a lower compression ratio allows for a more efficient use of the heat energy, within the efficiency envelope of the turbine design. Too much in either direction is not a good thing. This is also a major contributing factor in keeping the EPA happy, and why we have all this smog stuff on later models. The emissions vs. performance goal is to operate at the optimal (typically lower than the norm) EGT while maintaining the heat energy available to drive the turbine at maximum efficiency. There's a lot more to it, but this is the basic concept.

More Power is correct. 1250° is a safe EGT limit, measured at the exhaust port (manifold). This value becomes a bit fuzzy if measured at other locations. The downpipe is a popular location, but not as accurate as the manifold. Measuring post-turbo will be even less precise, as there are many factors that can effect the EGT on the way there. Simply, further from the combustion event is less accurate.

[JohnC]

Measuring post-turbo will be even less precise, as there are many factors that can effect the EGT on the way there.

IMHO, measuring post turbo is very difficult to interpret. There is a temperature drop across the turbo and the amount of drop increases the harder you work the turbo, so, the harder you work it the greater the error, and the error is not on the side of safety!

[DaveBr]

Thanks everyone for clarification. I have always used 1250 pre turbo as my limit however I must confess there has been a few times I have pulled on steep grades and forgot to check egt's and have gone over the max. I finally blew a piston on a hiway run with a load but it was flat all the way with egt's averaging 600. Pulling engine this week so I should be able to do a visual for heat related damage. Will keep you posted.

[rustyk]

Peninsular Diesel, which built my engine, recommends 850°F max continuous, 1050° max intermittent. At 12 psig boost, the EGT doesn't hit more than 800°. AT 17 psig (Floored), the EGT barely hits 1075°, well under the rule-of-thumb 1250° ceiling.

[Robyn]

The resulting "Damage" from excess heat, IMHO is an accumulative thing and such items as pistons and cylinder heads reach a point from which there is no return.

In other words, one "Burned" piston is a good idicator that all 8 have likely reached the end of their useful lifespan.

I have a set of old pistons here in the shop that came from a 6.5 that had seen far more abuse than the law allows. These pistons when looked over carefully show a lot of very small heat checks and such in the anodized surface coating.

To reuse any of these would be a poor choice.
Over time the structural integrity of the piston goes away too (like bending the paper clip)

These are a cast piston and not forged. Although good pistons they do have a limit of service.

The cylinder heads also can be right on the verge of leaving the scene and really not show much.

Common indicators of trouble in the 6.5 heads are cracks between the valves (very common)
The issues really compound when you find cracks in the "Firedeck" of the head.
Any cracks across the face of head is a death sentence.

The small cracks between valves can be repaired with a bronze sleave inserted into the water passage.

With as much good aftermarket parts available now, I would not even consider trying to reuse any old 6.5 heads, unless they are in pristine condition.

While your checking over your engine, be very careful to evaluate the mainline.

Many center mainweb cracks can go almost unoticed.

The best way to find them is to clean the webs real good (Block bare) with a very good solvent (BrakeKleen) and then warm the area around the outer bolt holes (center 3 mains)
Heat this area with a propane torch until hot to the touch. Once hot, let stand for a few minutes, any cracks will oooooze oil and show a fine little black line.

Heat/overheat in a 6.5, either coolant temp or EGT will wreak havoc on things.

These engines just will not tollerate overtemp conditions.

A very short duration (very short) overheat of the coolant on a hard pull is generally not going to kill the little beast, but regular running above 220F is tough on them.

These engines are in fact "delicate" and need to be treated with a little more respect than what one might do with heavy duty diesels.

I say this after having sorted through the carnage of many 6.5 corpses.

Back in the day when Banks came out with their turbo kits for the 6.2 the attitude was OHHHHHHH BOYYYYYYYYYYYY all sorts of power.

Many treated these engines as the "Little engine that could"
I have seen these things pushed and pushed and pushed some more until they literally started melting things.

Bottom line in my book is that 1000F is a good stopping place for the EGT even pre turbo.

Beyond this point the point of NO RETURN can be reached in seconds and from there its game over.

As Maverick mentioned, the lower compression does many things, the greatest advantage is the lower "PEAK BURN TEMPERATURE"

One of the other grand things to do on these engines is to change the turbo and the exhaust outlet to one that allows the exhaust gasses to flow more freely with less "Heat retention"

The GMx series turbos have some serious flow probalems once you get into some serious boost.

GM planned the setup (Engine package) to produce around 190 HP and to use a max boost level of about 7-8 PSI.

We come along and add 4 inch exhaust, power chips and other goodies and then take the little engine that could deep into the land of 12-15 PSI boost.

At this level (not high by most standards) is still beyond what the design was meant to handle.

The drive side restrictions on the GMX turbo is reached quickly and along with this, the IAT (intake air temps) start heading off into the ozones and this just adds to the heat problem.

With the 18:1 pistons you do not solve the issues with the GMx turbo but you do get rid of that very high burn temp.

The addition of a better turbo with the ability to flow more air out as well as in allows the lower compression to do its job.

The technical numbers on all this stuff are best left for the folks that are into messing around with calculus and other such number crunching regimens.

Bottom line, keep the inlet air as cool as you can (the closer to ambient the better) and reduce drive side back pressure as much as possible.

With the proper turbo, 18:1 pistons and a charge cooler the 6.5 can handle 15 PSI boost levels.

The upper thermal limits still apply but when its all clicking, you should not be seeing those numbers even when making some very good power.

As was pointed out, the lower compression will allow you to burn more fuel with more boost and do so at a lower peak temp, thus keeping the temps out of the melt zone

High temperature does not mean more useable power, actually the reverse is true.

Now if you are running an electronic IP the limit of available power will be reached fairly quick as the DS4 has a finite limit.

Around 275-300 HP is gonna be about it.

A DB2 (4911 or one of the marine pumps) can get some serious fuel into the engine and 400HP is within reach.
Now the issue is keeping the coolant temps in check. This is near impossible in a truck due to the radiator size constraints.



Power chips like those offered by various folks (Kennedy,Heath) juggle injection timing curves around quite a bit and in so doing make a huge difference in the Heat picture.

Now Bill Heath is not a fan of lower comression and will tell you so, Point blank.

This all said, some of his theories are quite good when taken in the context of which they were given.

For severe service under max boost conditions, the 18:1 program is a good one.

I believe my engine (new pistons) were rated to yield 19:1, which for what I want from the thing is perfect.


Have fun with the project and keep us posted on what you find when you get into it.

Jim's "Power Project" rig was a fine example of what a 6.5 can do.

Myself I would not look to go beyond 300HP with a 6.5 as the life expectancy will fall off very quickly (except in marine applications)

Robyn

[ronniejoe]

A properly built 6.5 can handle quite a bit more than 15 psi. With the right internal components, the right turbocharger, an intercooler and the right fuel programming, one can make more power than most ever dreamed possible out of a 6.5 with zero heat problems. It doesn't come cheaply nor easily, but it is worth while in the end. Compared to the cost of a brand new truck, however, it is rather economical.

Here are a couple of videos (clip 1, clip 2, the quality isn't very good, but you get the point) that show my boost and egt gages along with the instrument panel on a trip last month towing my travel trailer.

Level cruise (the first clip) shows steady boost pressures of 22-23 psi running 72 mph and about 750 F pre-turbine egt.

Rolling into a small grade (the second clip) shows no loss of speed, boost climbs to 25 psi and egt maxes at a comfortable 900 F pre-turbine.

I have accumulated ~90,000 miles on this set-up working my truck like this nearly every day over the last four years.

Don't try this with your average 6.5...

[KrisL]

I'm still curious as to what the specs on your turbo are Ronniejoe? I don't recall seeing them posted anywhere on here.

[ronniejoe]

The specs are here.

[Robyn]

Ron
What you have achieved with your 6.5 is nothing short of amazing.

Time and research always pays off in the long run.

Really seems strange why a company as large as GM did not get far closer to these potentials with the 6.5.

The factory offering was rather pitiful by comparison for sure.

The 6.5 in my DaHooooley truck is no where near the moose that you have built up in your Burb, but does have many improvements over the stock configuration.

I have always placed the 15# limit as a good point of reference for those trying to get by with a stock turbo.

Most folks that inquire about more power are not willing to spend the $$$ required to get to the end results that you have achieved .

The additions of larger free flowing exhaust and a good chip in the ECM and some common sense generally is about the extent of what many are willing to pay for.

If you ever get out to the west coast, I would love to see your Burb in action.

With the new turbo setup, what sort of rear wheel HP are you seeing??

Robyn