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6.2L/6.5L Diesel - Upgrades & Service Questions Answered Tech forum containing the best technical information about these engines. Please post in the related 6.2L & 6.5L forums. We'll transfer the best topic threads to this forum.

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  #1  
Old 07-20-2007, 05:13 PM
afgunn afgunn is offline
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Default Boost/Compression Ratio Calculator

http://rbracing-rsr.com/compression.htm

The link above is for a boost psi to compression ratio calculator. I ran a couple and was quite surprised at what I got!

1) A typical stock 6.5L
You are running 8 PSI of boost at an altitude of 1000 feet. Your motor's static compression is 21.2 :1. At this boost level and altitude your effective compression ratio is 32.54 :1, and without altitude correction your compression ratio would be 32.74 :1.

2) Reduced 18:1 CR and 15 PSI boost
You are running 15 PSI of boost at an altitude of 1000 feet. Your motor's static compression is 18 :1. At this boost level and altitude your effective compression ratio is 36.17 :1, and without altitude correction your compression ratio would be 36.37 :1.

Do these seem correct!? Can our 6.5s (or any diesel) handle this CR!? I was a bit shocked at the effective CR. I came upon this because I was wondering what is the effect of boost to CR. I knew it had to go up but, still I am very shocked.
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  #2  
Old 07-21-2007, 12:45 PM
afgunn afgunn is offline
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Default Update

Another way of looking at this by "first level approximation" is:
15psi = 2 atmospheres
8psi ~= 1.5 atmospheres

18 CR x 2 atmospheres = 36 CR
21.2 CR x 1.5 atmoshperes = 31.8 CR

So, I believe the calculator is accurate! I would never have thought that any engine would be able to handle a 32 much less a 36 CR. This is why it is necessary to reduce the typical 6.5L CR from 21.2 to ~18 to run 15psi of boost.
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  #3  
Old 07-21-2007, 01:29 PM
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Those who run their stock CR of 21.3:1 at 15 psi:

Quote:
You are running 15 PSI of boost at an altitude of 1000 feet. Your motor's static compression is 21.3 :1. At this boost level and altitude your effective compression ratio is 42.83 :1, and without altitude correction your compression ratio would be 43.03 :1.
That's why lowering the 6.5 CR to 18:1 improves durability..... Now, imagine what would happen if you raised CR above stock...

Jim
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Old 07-29-2007, 07:42 PM
Randy H Randy H is offline
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So why lower the static compression ratio and run more boost when the end result is the same?

Randy
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  #5  
Old 08-07-2007, 01:31 AM
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Thumbs up Same dynamic CR, Same A/F density, More A/F Volume

more boost = more combustion O2 avilable. Add a corresponding amount of additional fuel and you have a more powerful combustion event. Even though the dynamic compression ratio remains the same +/- , the lower height of the 18-1 piston crown creates a larger effective combustion chamber volume, which is now packed with the same density of air/fuel as the smaller area that was provided by the stock piston compression height.

Same DCR + Same A/F Density+ more A/F Volume = bigger bang per power stroke
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Old 08-09-2007, 03:48 PM
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Ennybody compare that to 3000psi combustion pressures?
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Old 08-11-2007, 01:45 PM
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No problem, as long as that pressure rise occurs at the right time!
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Old 08-13-2007, 10:21 PM
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Quote:
Originally Posted by robscarab View Post
more boost = more combustion O2 avilable. Add a corresponding amount of additional fuel and you have a more powerful combustion event. Even though the dynamic compression ratio remains the same +/- , the lower height of the 18-1 piston crown creates a larger effective combustion chamber volume, which is now packed with the same density of air/fuel as the smaller area that was provided by the stock piston compression height.

Same DCR + Same A/F Density+ more A/F Volume = bigger bang per power stroke
Nicely said! Although the fuel-air "density" isn't the same, but the fuel-air ratio is - unless as you mentioned, more fuel and more air is introduced. Without more fuel/air, with the larger volume, the density is decreased. The fuel-air mixture has, however, the same energy content, but it will be slightly less efficient than at the higher compression ratio. But this will decrease BMEP (Brake Maximum Effective Pressure) in the cylinders, which reduces piston head pressures and does, in fact contribute to longevity at the cost of a slight decrease in efficiency. With a bit more fuel/air, the power will be nearly the same, but still with less stress on piston crowns and skirts.

Compression ratio is a compromise - higher CR results in higher theoretical efficiency, but at the cost of longevity - and sometimes it's too high, such that decreasing it raises efficiency. Which is why I elected to get my new engine with 18:1 pistons. And fuel economy is up 15% over the original GM 6.5L TD.
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Old 08-13-2007, 10:50 PM
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For an IDI (InDirect Injected) diesel to produce clean cold starts with short glow cycle times, and to produce fewer cold start emissions, the CR has to be somewhere at/above 20:1. The precups/prechambers cool the charge on a cold engine. A DI (Direct Injected) diesel (like the current Duramax, Cummins, PSD) starts well cold at a lower CR. So, a higher CR was an IDI cold start compromise.

I did a short tech piece on the Isuzu C240 I-4 2.4L diesel recently. It too is an IDI engine that has 20:1 CR. Its 3.0L DI siblings are, on the other hand, just over 18:1 CR.

Diesel farm tractors with DI diesels (of those I've seen the data for) run at anywhere between 15 and 18:1 CR. The Duramax/PSD/Cummins ran at 17.5-18:1 (the LMM Duramax is now at 16.8:1). The marine 5.9L Cummins runs with 15:1 CR. And so on...

Tractor, industrial and marine diesels are usually run at high load for extended periods, and tend to have a lower CR for improved durability. Certainly, tractors, OTR diesels, marine diesels and so on are also designed with efficiency in mind. If a higher CR was a better compromise (for efficiency), they'd all have a higher CR. They don't because of increasing durability issues with increasing CR.

Jim

PS - Here's a question for the thinkers out there.... What effect does CR have on crankshaft harmonics?

Last edited by More Power; 07-05-2011 at 12:23 AM. Reason: Add question...
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  #10  
Old 08-14-2007, 01:58 PM
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The reason I selected the 18-1 pistons is so I could safely run higher boost while towing. I have increased the amount of fuel injected per power stroke by "turning up" my DB2 and installing marine injectors. The only time I will benefit from these mods is when I am accellerating a load or pulling it up a grade at cruise. I chose the comprimise of less efficient low boost performance and harder starting. If I was not using this rig exclusively for towing than I would have stayed with the stock compression ratio. If I had a half ton used for running around town empty,I personally would not go with lower compression. I live in Canada where we have periods of very cold temps and starting a 6.5 at -35C can be a challange at the best of times!
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  #11  
Old 05-20-2011, 08:52 AM
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Default Dynamic static CR

Please tell me can you make up for the loss of efficiency with the lower CR by introducing propane injection? Thanks.
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Old 05-20-2011, 12:11 PM
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Please tell me can you make up for the loss of efficiency with the lower CR by introducing propane injection? Thanks.
No. Propane is just another fuel. Propane injection only adds to volume of fuel(s) already in the combustion. Adding more/other fuels until the combustion process uses all the available O2 available can be advantageous from an economic standpoint (if the alternative fuel is significantly less expensive than the primary fuel), but offers little more than that. Also, consider that almost all fuels (capable of being used in an internal combustion engine) other than #2 Diesel have a significantly lower BTU potential (translates to less "efficient"). Mixing alternative fuels into any engine may have an economic advantage, but you'll have to consider all the costs involved. Most often, the ROI (Return On Investment) is far beyond what you will actually ever achieve.

None of this will change the "efficiency" of a charge-air system. The principals, function and science are very simple. X amount of exhaust gas energy will produce X amount of charge-air energy. On the charge-air side, you will hit a wall on compressor efficiency, at which point, it will fall off exponentially (the value of the fuel input compared to the output power depart very quickly). None of this takes into consideration the mechanical limitations of the platform involved (the point an efficiency increase start breaking things).
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  #13  
Old 05-22-2011, 01:29 PM
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Default Dynamic CR & Static CR

Thanks for the feedback on fuel types

In reading here about Dynamic vs Static compression ratio the conversation sounds more like cylinder pressure than compression ratios. Isn't Dynamic CR only affected by cam profile?

if that is true then DCR is cool to understand but wouldn't really matter since we can't change it unless we are out grinding custom cams as we don't have a plethora cam options like a SBC 350?
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Old 07-04-2011, 06:26 PM
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Default Ehtanol injection systems

Was just talking to a guy that says ethanol inj will have a dramatic impact on fuel economy. He claims it burns the fuel more completely producing more power. Anyone trying this, any truth to it?
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Old 07-04-2011, 06:47 PM
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Originally Posted by tomtaylor View Post
Was just talking to a guy that says ethanol inj will have a dramatic impact on fuel economy. He claims it burns the fuel more completely producing more power. Anyone trying this, any truth to it?
If you aren't making smoke, you are burning all the fuel. Adding more fuel (such as ethanol) will only increase power up to the point it uses all the remaining O2. Adding #2 up to that point will yield more power. Once it begins making smoke, you have crossed that line, and are pushing incompletely burned fuel out the pipe.

The only way to burn "more completely" is to improve atomization and distribution within the combustion chamber. Adjusting (optimizing) the combustion event timing can help, to some degree. Late model Diesels (Duramax, Cummins, PS) achieve this by increasing the fuel injection pressure (10X), and optimize the fuel duration (also with multiple injection events) and timing dynamically controlled by a computer.

Once again, adding ethanol is adding fuel. Adding fuel will take advantage of available O2. Nothing more. Using a more costly, less efficient (lower BTU) fuel to use up extra air is a cost, in the end. If your goal is economy, this is a net loss, no matter how long your test period is.
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Old 07-05-2011, 12:12 AM
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Quote:
Originally Posted by tomtaylor View Post
Was just talking to a guy that says ethanol inj will have a dramatic impact on fuel economy. He claims it burns the fuel more completely producing more power. Anyone trying this, any truth to it?
Like DmaxMaverick said, if alcohol was cheaper than diesel (by cost per BTU, not volume), there might be an economic advantage to its use.

Water/alcohol injection can be advantageous for other reasons (i.e. to cool the intake air charge). This can be helpful to reduce detonation in a gas engine, or it can be used to reduce EGT in a diesel. For either engine, its not so much about economy, but more about power.

That said, if someone was selling water/alcohol injection systems, and the diesel fuel costs at the pump were high, they might try to make the case about fuel economy to make the sale. Ask them to provide verifiable data... I'm just saying...
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Old 07-11-2011, 06:06 PM
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Quote:
Originally Posted by tomtaylor View Post
Thanks for the feedback on fuel types

In reading here about Dynamic vs Static compression ratio the conversation sounds more like cylinder pressure than compression ratios. Isn't Dynamic CR only affected by cam profile?

if that is true then DCR is cool to understand but wouldn't really matter since we can't change it unless we are out grinding custom cams as we don't have a plethora cam options like a SBC 350?
Compression Ratios, static and dynamic directly correlate to cylinder pressure ratios. Which when combined with Combusting fuel correlate to cylinder pressure. In a naturally aspirated induction system, your DCR will always be less than your SCR.

With Forced induction you can increase the DCR over the SCR with the added benefit of having more Oxygen available for the fuel to use to combust.

DCR can be controlled as the calculator More power posted by increasing the level of boost. or mechanically altering your SCR Via increasing the combustion chamber volume size which is commonly done by shaving pistons down, using thicker gaskets, or using pistons which have the wrist pin placement raised on the skirt thereby shortening the stroke of the piston.

Custom cams and grinds for diesel motors are not very prevalent because almost always they will move the power band further up into the rpm range given the same timing correlation of the valve events to a stock motor. Retarding or advancing the cam to bring it back into a useable RPM range is often not practicle With the relatively small piston to valve clearance in a diesel motor. Due to the sheer mass of the components and already high cylinder pressures within a diesel motor RPM's are typically 3/4 to half of what a gasoline motor operates off of so any work to adjust the level down to an area where it is most useable becomes fruitless as parts will start contacting.

With forced induction this becomes even less important as you are increasing the engines ability to intake air which is where most of your power over stock grinds comes from with custom camshafts in Gas motors.

All of that being said If you were able clearance the pistons around the valves and change the overlap and allow the exhaust valve to hang open a bit longer it seems that some more power could be made. But would it increase your DCR and therefore your Pressure ratio to a point where things start breaking?

A simple way to visualise it assuming that a compression ratio is Adiabatic (Thermal dynamics phrase which doesnt take into effect heat introduced outside of the heat of compression as that math is overly complicated for this example). Your pressure ratio with a Compression ratio of 10:1 would be 25.12:1 CR of 15:1 would be 44.31:1 CR of 20:1 would be 66.29:1 CR of 25:1 would be 90.61:1 and a CR of 35:1 would be 145.11:1

Actual cylinder pressure when taking into account the combustion is hard to figure accurately without measuring it as so many different factors effect it. If you could figure the PSI of a metered amount of diesel combusting you can than get a general ideal of the cylinder pressure and its relatationship to compression above by using a simple formula.
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Old 09-28-2011, 03:44 PM
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So how hard is it to start when cold with 18 to 1 pistons? Are we talking 20 seconds of cranking? What glow plugs/relay parts are best to start a 18 to 1 motor? Could bad injection pump timing affect its ability to start easily (DB-2)?

Peculiar issue.... I have a '93 that was running to much boost. I believe I burned up a couple pistons. A compression test showed low numbers~150psi. The truck started great with the low compression test numbers and still ran with less power. Finally I swapped in a 18 to 1 long block, and now it cranks for 20 or 30 seconds before wanting to fire. All the components were swapped directly off the old motor onto the new motor, pump, hi pop injectors, glow stuff etc....

Anyone have any insight on why its so hard to start? Maybe I got some glow wiring screwed up?
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Old 09-29-2011, 10:36 AM
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Originally Posted by BigFj40 View Post
So how hard is it to start when cold with 18 to 1 pistons? Are we talking 20 seconds of cranking? What glow plugs/relay parts are best to start a 18 to 1 motor? Could bad injection pump timing affect its ability to start easily (DB-2)?

Peculiar issue.... I have a '93 that was running to much boost. I believe I burned up a couple pistons. A compression test showed low numbers~150psi. The truck started great with the low compression test numbers and still ran with less power. Finally I swapped in a 18 to 1 long block, and now it cranks for 20 or 30 seconds before wanting to fire. All the components were swapped directly off the old motor onto the new motor, pump, hi pop injectors, glow stuff etc....

Anyone have any insight on why its so hard to start? Maybe I got some glow wiring screwed up?
Where a 21:1 6.2/6.5 will start normally down to +20 degrees F with 8 seconds of glow, an 18:1 needs 15 or a few more seconds of glow to start more or less normally. Your engine block heater is your friend. With an hour or two of block heat, the 18:1 will start normally with 10-12 seconds of glow time down to about as low a temp as required (lower 48).

Jim
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Old 10-01-2011, 08:07 AM
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A lot of the engines ability to start is atributable to the condition of the engines mechanical condition, Glow plugs, injectors and the IP.

A couple years ago we had some NASTY winter weather with temps down in the low single digits (+5) Cold for us anyway.

Our 94 Burb 6.5 that was left out in the cold with no block heat would fire right off with one standard glow cycle and then twist its tail.

This was a standard compression 6.5 with the engine having been rebuilt a year prior.

The glows were the stock ones that came in the 94 trucks.

A friend has an 18:1 enigne and uses 60G plugs. His truck starts a tad balky even in 35F weather (fresh engne)

More glow time on some 60G plugs and the issue is not a big one.

Get those glow plugs really warm.

The condition of the Squirts is very important to good clean, easy starts.

Squirts that are low on POP pressure and that dont spray a nice fine mist of fuel will result in a very hard starting engine.

Squirts with POPs set at 2000 or so and a lovely fine cone of mist when they let fly will make things work really well.

Our 94 Burb sounded like a very UNHAPPY powerstroke when started in those low temps.

Have fun

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