Boost/Compression Ratio Calculator

[chevytuff]

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.

[BigFj40]

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?

[More Power]

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

[Robyn]

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

Missy

[BigFj40]

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

How did you increase your glow time? Did you buy an adjustable glow controller relay?

[spdgofast]

Ok since I have all of you guys on the same page (sort of) about CR does anyone have any suggestions on the best way to obtain a 19:1 CR on a new P400 6.5 engine? Is there only two ways to do this as of this date? Machine piston crowns or machine the block deck? If I intend on running an HX 35 @ 20 to 25 lbs of boost do I need to go to 18:1 CR? How much efficiency and or fuel mileage will I loss going with 18:1? I feel that with the envelope being pushed and the present day and future improvement of the DB2 and or the DB4 pumps that it will be much easier to increase boost pressures (than change CR) to match fuel input by tuning the turbo (such as changing turbine and compressor wheels & housings) and the HX 35 has many different choices to increase air charge much further than the 6.5 structural integrity can withstand, even the P400. Or even better if you could fit a 351VGT with a Fleece controller on one of these would be even better but I'm not sure that could be made to fit in a DD and retain the A/C and Heater assembly. I guess the platform and use has alot to do with the settup like Robscarab says, a half ton grocery getter could probably run 20lbs of boost with 20:1 CR in a P400 but my 3/4 ton 4X4 Suburban towing will probably need the 18:1 CR if I want it to live. I guess I am just looking for verification of what I am allready thinking by some of you others with a little more experience with these motors than I and or any info on any 19:1 CR pistons that I don't now about or should I really just go with the 18:1 s for longevity? I have a size 13 shoe and two 5.9 Cummins 12 valve trucks and one 5.9 common rail (all 3 turned up) so I'm sure when I step out of one of those into my Burb it's going to be real hard to have a lite foot. Oh and after seeing that dodge IC in one of these I'm thinking of replacing my spearco with the dodge mounted behind the grill but I'm not sure the gains will be worth the expense, we'll see when I get my grill off, but my Burb is big and black and needs all the A/C it can get so I'm not sure I want to cut down the condenser.
I appreciate any and all comments and opinions
Thanks, Dave

[Robyn]

This Fascination with high boost on the 6.5 is really an interesting subject.

The 6.5,even the P400 was never ment to run those high boost pressures.

The end result is going to be trouble that you are not going to like.

The P400 has a great bottom end now and the metalurgy in the block and heads has been improved too.

The issue is that there is still the same number of head bolts and these engines still use the same head gaskets that have been around for a few years now.

Running boost levels of 20 PSI is a one way trip to engine destruction, not if but when.

The Dmax, the Cummins and the Power Stroke have a little different layout in the heads and can handle more boost.

The fact that you have an IDI engine (precup) is one issue and the lack of more head bolts is the other.

The P400 was never meant to handle 20 ++PSI Boost, the P400 was designed to create a useable platform that could produce around 300-325 HP and 500 Ft lb torque at most and do it over the long haul.

I would leave the compression as it comes from AMG change the turbo out to reduce the backpressure on the drive side and then run about 14-15 PSI absolute max with no more than 1000F EGT showing at the Ports.

SO turn up the fuel or chip it depending on if your using a DB2 or a DS4 until the black smoke just starts to show at WOT on a pull.

As has been mentioned, BLACK smoke is $$$$$$$$$ going up the stack thats not making HP, Only heat.

Now, there are a couple ways to reduce compression ratio.\
Cut the tops of the pistons, or use a piston that has a wrist pin C/L thats slightly higher (lowers the piston in the hole)

You do not cut the block unless the block deck has been damaged or worn from time and use.\

There are .010" thicker gaskets but these are designed to be used when the deck has been cut, to restore the head to the standard location and also to keep the comp ration right.

A stock engine in the mid /late 90's vintage will have only about .045-.050 clearance between the piston and the head at TDC.

The late engines (AMG) are at 19:1 compression ration.

This is a great place to be unless you are planning on putting this thing on a 8% grade with the throttle wide open and keep it there for half a day, all while pulling 10,000 pounds.

My suggestions are not made because I dont like BOOST, I do like keeping an engine together far more than wrenching on them.

A well built 6.5 with things working right can do 300-320 HP and live.

This can be done handily with boost pressures no more than 15 PSI and EGT's kept in check.

Your new P400 has limitations. Live with them and you will be happy, step out on the edge of the envelope and you will find a place you dont want to be.

Its called MELTVILLE

Above 15 PSI the need for an aftercooler come into view really quick.

The 6.5 was just not designed to do more than what I have spoken of.

Your P400 is not going to make a D max nervous and only give a good running powerstroke of the 97 vintage a bit of a tussle.

Sorry about the bucket of cold water on the campfire, but I hate to see a fella plunk down the $$$$$$$$$$$ for a new P400 and then send it off to destruction.

At the power levels spoken of the P400 should be able to serve really well for 250K miles or more with some care.

The head gaskets are still the fuse, even on these engines.

Been with this engine family since 82 when they came out and owned a bunch of them. Broke some too.

Hope this helps.

Missy

[spdgofast]

Thanks for the reply Missy, as always I value your input and advice, can you tell me where you recieved the information that the P400 comes with 19:1 CR? The only reason I ask is that just two weeks ago I was told by Ron Taylor of W.W. Williams that the P400 came with a 20.2:1 CR and that a lower CR was not available and this information came directly from Brad Bee of AM General in an email dated 11/09/11. I would be tickled pink if they came with a 19:1 CR, I think I will have to try and call Brad Bee directly this week and verify this again. On the other hand, hasn't Ron Schoolcraft been successfull running 20 to 25 lbs of boost with his engine without failure and I believe he did quite well in one of the last pulloffs with his Suburban or am I mistaken, has it blown? I know you have more experience than I and are more up to date as to what others are doing with the 6.5 performance scene, so please correct me if I am wrong. All I do know is that 15 lbs of boost is not enough boost to clear up the black smoke with my exsisting Peninsular DB2 Marine pump putting out less than 90 Cubic millimeters of fuel per stroke and I am reading and getting told how many others are getting more than 100 cubic millimeters of fuel per stroke out of some DS4 and DB4 pumps. Therfore if it is true and you can actually get this kind of fuel out of these pumps then I want to be able to use that fuel and put it to power and not black smoke, and my present tune tells me that if I can't give it more boost then I can't use one of these pumps putting out that kind of fuel, but yet others are? So they say.
Like I said, I value your experience and advice and I will let you know if I find out anything different from AM GENERAL this week.
Thanks Again, Dave