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Discussion Starter #1 (Edited by Moderator)
Because there are a few very important misconceptions out there on how engine properties are determined I made the attached spreadsheets. One of the big ones being how CR affects cylinder pressure, which is not a direct multiplication of CR times the pressure at bottom dead center (BDC). It is an exponential relationship and there is a LOT of pressure in there, and heat. Additionally, the aftermarket has dumbed down the very important relationship of air and fuel flow to engine horsepower.

They use cubic feet per minute (CFM) of air and cubic centimeters (cc) of fuel, which are volume flow rates. Volume flow is an inaccurate way to make design choices, but are easier concepts to grasp, because its also how most of the equipment is rated. Most technical desicions should be made based in terms of mass flow, especially when using turbo compressor maps and determining the required boost to a fuel rate. Also, turbochargers do not put out what they are rated for unless the engine flow demands it, dependent on displacement and intake air temp.

So I combined 2 of my updated older calculators with a new Air Flow calculator in one Excel file that is attached.

Sheet 1: Affects of boost, compression ratio (CR), intake air temp (IAT) have on cylinder air pressure and temperature at top dead center (TDC). This can used to compare any engine.

Sheet 2: Air volume and mass flow, and air to fuel ratio (AFR). This can be used with any engine. Has elevation compensation, as well as fuel source (density) and temperature compensation based on user input. Remember the engine is a fixed air pump, boost increases the engine's mass flow, but only RPM and displacement can change an engine's air volume flow. This is why the terms are separated into pre compression (air filter and turbo flow) to post compression (engine and intercoolers).

Sheet 3: 6.5 CR calculator, using shaved or dished pistons or thicker gaskets, or even intake valve closure changes. This was specifically made with the 6.5 diesel engine in mind, but could work closely for other IDI diesels with similar flat top pistons with Ricardo swirl bowls.

These should help give an understanding on design choices when rebuilding an engine, why you would select a lower CR, selecting a turbo, intercooler or air filter, and determing injector pop pressures, boost limits, ect....

Sometimes, knowing these values, can give insight into the design choices that the manufacturer made.

Let me know if you find any issues with the calculations, or have any questions. And let me know if it helps anyone!
 

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Discussion Starter #2 (Edited by Moderator)
Already have an update

Updated version available in post #5 below.
 

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Discussion Starter #3
I have another update for the air flow calculator. Added more atmospheric pressure range, and more air temp density granularity above 200F. Also added a Volumetric Efficiency input, because right now the calculator assumes that if you have 10psi of boost that the cylinder will fill to 10psi before the intake valve closes. It might not be quite that perfect, especially when you have no boost, so the optional input is there.

I just need to figure out the best method for file updating in the posts. I might just have a moderator delete the previous attachments and put down a message to say go look in latest post.
 

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Question, does the cylinder pressure change over the RPM range? I mean with only so much time to get air in and out?

I dont like what I see for cylinder presures and recommended pop psi. With a better turbo +20psi isnt unheard of.

BTW, you have way too much time on your hands for coming up with this crap! lol
 

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Discussion Starter #5 (Edited)
Question, does the cylinder pressure change over the RPM range? I mean with only so much time to get air in and out?

I dont like what I see for cylinder presures and recommended pop psi. With a better turbo +20psi isnt unheard of.

BTW, you have way too much time on your hands for coming up with this crap! lol
Well luckily, this isnt all original thought, just application of physical sciences. It does take some time to take everything into account, use the right units, conversions, ect.... I had much worse in school, and this is a little more fun. I use it myself, and it saves me time when trying to research something, rather than constantly thinking of it in my head and a running all calculations on a regular calculator.

Yes, the pressure does change over time, it gets closer to the calculated value as RPMs increase. The less time air has to escape the more pressure will be retained. The value in the calculator is a peak pressure value if your rings are in good shape and spinning at about 3000rpm.

At cranking RPM, 60% of the pressure escapes to get a 400psi compression check. I've added some of this info into that calculator. I discuss it some in the CR calculator, trying to describe what the dynamic CR is, but its a moving target with respect to blowby.

Pushing 20psi of boost is part of the reason I recommend 2300 to 2500psi injector pop pressure, but also for better atomization in general.

Here I have attached a cleaned up version, with updates as mentioned in the previous post and cosmetic appearance of the AFR calculator.
 

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Nice work, I think I am on the right track with my build using 18:1 pistons and a Pro40 hybrid @ around 20psi boost and setting 311 tips at 2300psi ???????????
 

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Dunno, 18:1 is usually for marine engines where you sustain high rpms for a long time.

The way a truck engine is used is completely different, and I'm not sure 18:1 are better than 20/22:1...

Just a thought...
 

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Well. I think a lower comp adds less stress for a heavy tow application motor maybe...

I took the last 3 lines of the chart and changed the comp to 18,20,21,--- made the boost 20psi for all 3, and IAT 200 for the 3, and compared them and the numbers for TDC PSI are way different, TDC air temp only changed 130* over the high range, the cylinder psi, ( IMO) adds a lot of stress to the motor everywhere, the lower psi I think will help a motor that is seeing very hard work 99% of the time.... just my thoughts, it sounds good to me thou....
 

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Discussion Starter #9 (Edited)
There is definately merit in dropping the CR on something that like a boat, may need to be hauling 3000rpm for 10min to 2 hours climbing a mountain. This is common in the Western US. But unlike a boat we dont have an unlimited amount of cool water, and perhaps a bit less aerodynamic and heavier. The temperature drop is also nice, you'll need every bit of EGT room as possible for some loads. Me personally, I would like to maintain at least 20:1 CR, but I dont tow anything real heavy or often.
 

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Discussion Starter #10
Recommended Maintenace and Modifications

I figured this would be a good place to also have my list of recommendations, most items are applicable to all 6.5 diesels.

It covers timing the IP to upgrading the turbo, and other routine stuff like the alternator and engine mounts.

Enjoy! :bounce:
 

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Very nice information, not shure how to use the TDC calc..

Børge
 

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Discussion Starter #12
The TDC calculator is to show you the air temp and pressure at TDC. When you input the engine parameters, it calculates those values from compression. It is informational, to understand the system that you could be modding with other chemical fuels such as propane or methanol, or understanding how boost affects blowing the head gaskets off. And how IATs affect cylinder temps.

You can modify any of the engine parameters on the left columns.
 

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The TDC calculator is to show you the air temp and pressure at TDC. When you input the engine parameters, it calculates those values from compression. It is informational, to understand the system that you could be modding with other chemical fuels such as propane or methanol, or understanding how boost affects blowing the head gaskets off. And how IATs affect cylinder temps.

You can modify any of the engine parameters on the left columns.
Thanks for clarifying, will try it...
 

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Very nice!
 

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Discussion Starter #15
New List of Recommendations

Attached a tweaked list of recommendations. Not a whole lot different, but refined.
 

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Nice writeup. :)

Børge
 

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:cool2:
 

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Good work Buddy, we saved it in the folder with your other calculators....LOL. How come no 6.2L's or any 6.2/6.5L's with over 50 psi boost ? Guess we will have to calculate that for ourselves....:)
 
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There was one, it bent the rods and crank every oil change. Timing is really important and the guy was playing w/ injection pumps at the same time and had trouble keeping them dialed in.
 

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Discussion Starter #20 (Edited)
There was one, it bent the rods and crank every oil change. Timing is really important and the guy was playing w/ injection pumps at the same time and had trouble keeping them dialed in.
And thus is part of the reason for providing a calculator like this, so people understand the high pressure values they are dealing with. Some guys may think you have 15psi atmospheric plus 15psi boost times 21 for the compression ratio and you get like 630psi in the cylinder, when its actually 2000psi, or for 50psi boost you are looking at 4500psi. And if the fuel ignites before TDC that pressure can double, because that was just air pressure before the boom. And what pressure is the injector nozzle popping in comparison, or the IP have to push get the fuel out and is it coming out in nice spray like when GM had it popping higher than the cylinder air pressure or pushing out a thick stream.

The AFR calculator has come in handy for me. If you assume an 80% volumetric efficiency, then I have noticed good power and clean exhaust at 18:1 AFR, like other modern diesels. If you assume a high VE, because it is turbo charged after all, then the 6.5 seems to like around 22-24:1 AFR for clean power. This calculator also demonstrates the reason for measuring air in mass flow, and how IAT and CFM combine to get the whole AFR picture and how each component needs to rated wrt flow. This calculator can tell you how much boost you actually need if you know the fuel rates and IATs. Together, the calculators point out that it may be wiser to intercool after you hit 20-25psi of boost, because more boost is more cylinder pressure, but cooler air is more air mass with less cylinder pressure. With the intercooler there is less engine stress from less boost, with the same or more power when youre getting into fuel rates that high. Although I think most 6.5s with normal fueling setups can get by just fine with 20psi of boost and no intercooler.
 
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