Boost/Compression Ratio Calculator

[tomtaylor]

Please tell me can you make up for the loss of efficiency with the lower CR by introducing propane injection? Thanks.

[DmaxMaverick]

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).

[tomtaylor]

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?

[tomtaylor]

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?

[DmaxMaverick]

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.

[More Power]

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...

[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?