Competition Compression ratio - why lower it???

[GOT-Torque]

If you were building a full time pulling truck or dragracing truck, why would you lower the compression ratio of the engine?



Before you jump in and say...



to support lots of boost (100 psi +)

OR

to support high rpm (5000 rpm +)

OR

to support head gasket longevity

OR

so you have to use Ether to start it on a 80 degree day

OR

whatever else there is???



Try and add why exactly lower compression is needed to support your statement.

 

[GWoody]

I'm sure someone can explain it scientifically, but the way I understand it, when you lower the compression ratio it results in more open area within the cylinder bore so you can cram a larger volume of fuel and air in to which creates a bigger boom, hense more hp. If you could hold the head on it wouldn't be necessary to lower the compression ratio, but anytime you add more fuel and air the cylinder pressure increases.

 

[Cummins724]

You want to lengthen the stroke, but keep close to stock compression. More room for combustion & this also allows you to add enhancements like nos. The engine runs cooler with lower compression too. Back when my friend was building a 383 stroker engine a few guys told him that you will want to run stock compression if you want to run nos. More area=more fuel can be dumped in=more power.

 

[biggy238]

Laws of thermo dynamics. Elevated pressure=elevated temp given a certain volume.



You can apply that however you want but in the end, it means that to run more fuel without melting, you have to drop the pressure.

 

[GO 4LO]

With lower compression ratio, the pressure drop along the downward path of the piston is less. If boost level is not changed, then lower CR results in lower power. The key is that the lower CR allows you to run more boost for the same peak cylinder pressure, and the motor gains more power from the extra boost than it loses due to CR.



The power changes like this: (1-2. 5/(New CR))/(1-2. 5/(Stock CR)). If you lower the compression ratio, then what that means is that air is compressed less inside the cylinder, and expands less during expansion. That is, for the same peak cylinder pressure, the average downward force during the power stroke is greater for a low CR / high boost engine than a low boost / high CR engine.



Chris

 

[GOT-Torque]

If boost level is not changed, then lower CR results in lower power.

This reaffirms what I was thinking. If you change nothing else and just lower compression, you will lose hp.



However, like you stated,

The key is that the lower CR allows you to run more boost for the same peak cylinder pressure

If I use the equation stated above and compare a stock CR of 17. 3 to a new CR of 15 I get 0. 974. If I compare 17. 3 to 10 I get 0. 877. So does this mean the CR of 15 will produce 97. 4% of the power that 17. 3 did and that the CR of 10 will produce 87. 7% of the power that 17. 3 did? (if nothing else changes)

 

[Snoxracer]

Does it have anything to do with the timing too?

 

[GOT-Torque]

The simple answer is yes. That is actually what was driving me to ask the original question. The higher the cylinder pressure, the sooner it will reach the ignition point in the compression stroke, thus increasing the timing.



See my other thread for more info on timing.

 

[TCluff]

The whole purpose behind it is to maintain the same Boost level, the same cylinder pressure but have more volume! Even if you lower your compression and increase your power (add more air and fuel) your cylinder pressure is the same or similar, more air + more volume = same pressure! So why would you need to adjust your timing?

 

[biggy238]

Because A cylinder of a different volume will reach ignition pressure at a different point during the compression stroke.



This is why turbo-ed gas motors detonate. The fuel air charge ignites too soon, trying to force the piston back down when other cylinders on the crank are forcing it upward. The result is cylinder pressure (and thus heat) that the metal cannot support.







However, in a diesel engine, we have the luxury of controlling when the fuel enters the sequence.

 

[NGilardoni]

Pistons

Has anyone put low compres. heat treated pistons in a 24 val. ? I want to do that nexted.

 

[NoSeeUm]

The simple answer is yes. That is actually what was driving me to ask the original question. The higher the cylinder pressure, the sooner it will reach the ignition point in the compression stroke, thus increasing the timing.



See my other thread for more info on timing.

I believe your statement above might have been somewhat of a missing link for me in my struggle to understand advanced timing. My concept was that injecting fuel just before TDC, @TDC or PTDC was generally optimum.



But now... . :{ :{



As I know, the fuel will not ignite until it as 1) vaporized and 2) reached ignition temperature. By pre-loading the combustion chamber with fuel BTDC those two conditions have to wait to happen simultaneosly for ignition. As opposed to only having to wait on the fuel vaporization. The most critical condition would be timing the point of ignition temperature and I suppose not having too large of a percentage of the fuel vaporized (which I am guessing could be very bad). Too soon and the force on the piston tries to make it reverse direction BTDC.



You probably remember this thread , GOT-Torque. Fest3r added some very good imput there as well.



For my part I have decided there is simply too much that I do not understand, it won't fit in my cranial space anyways, and my "I believe button" is currently fully depressed.



Jim

 

[ThrottleJockey]

Has anyone put low compres. heat treated pistons in a 24 val. ? I want to do that nexted.

Got 'em in a daily driver, no less. Mine are forged and coated.

 

[GOT-Torque]

Got 'em in a daily driver, no less. Mine are forged and coated.

How low of compression are you running and did you notice any changes after the install? Any negative effects?

 

[ThrottleJockey]

How low of compression are you running and did you notice any changes after the install? Any negative effects?

AT startup, theres white smoke until the temps come up, or you start driving. Takes about a half mile for me. At idle, it's very quiet, but sounds great when the fuel is added. It actually starts when it's below zero, (but it aint easy!) Plugged in, it fires right up. I had to modify the grid heaters.



That's all the bad stuff. Scheid says theyr'e around 15 to 1. EGTs are unaturally low, even when towing. With a Smarty and a PDR cam, I outgrew a PDR HX-40. Running Jammer 2s. Close to being the perfect tow rig with the twins. I just can't figure out what to do to it next.



Oh. You'll lose some low end torque but it still snowplows great. Gotta love a Cummins.