Competition Compression Ratio Question

[KLockliear]

and most of the time my facts are right

 

[Hohn]

17. 3:1 for all the 3rd gens. 17. 3:1 for the HO 2nd gens, 16. 5:1 for the SO 2nd gens.

Ya know, for all the typing you spew out, at some point you should see about getting some facts. I'm not knocking any vendor out there, but YOU sure are. Get the facts before you post.

Th ironic thing here is that Cummins claims the HO was 17:1 even (2nd gen) and SO 2nd gen was 16. 3:1

http://dodgeram.org/tech/dsl/Facts/01ISBspecs.html

I believe the 17. 3:1 is correct for the HPCR trucks, but don't have a reference for it.


Was someone saying something about checking facts???

 

[Hohn]

ON compression ratio:


You need to recognize that on our engines, compression ratio and expansion ratio are the same. IOW, you have to look at the effects on ALL FOUR STROKES of the four-stroke cycle.

On the intake stroke, a higher compression ratio will enhance breathing for a given head/cam/valve combination. This is because the rapid expansion from TDC produces a faster and greater pressure drop, which initiates cylinder filling both earlier and with greater velocity.

On the exhaust stroke, you will see the same benefit in reverse. The rapidly decreasing volume near TDC will cause peak pressure to rise faster and farther. This means that exhaust leaves the chamber more efficiently and at higher velocity.

On the compression stroke, higher compression means that you will achieve a sufficient temperature for combustion earlier in terms of crankshaft degrees. This allows for more timing advance if you want it. Since the chamber is sealed on the compression stroke, higher compression means that you will have higher temp and pressure at TDC (ignoring fuel injection). It also means that for a given amount of injection timing, you will have higher peak combustion temps, more NOx, and a faster rate of burn (given enough O2).

ON the power stroke, a higher compression/expansion ratio means that the engine will extract more of the available power from the fuel. The greater expansion also means you will see lower EGT, ceteris parabus.



In general, a higher compression ratio is good. But not all by itself-- the engine has to be set up for it. And many times, higher compression brings costs that outweight benefits. For example, trying to keep the head on with higher compression AND advanced timing is quite a challenge. There's also the diminishing returns aspect mentioned by COMP461-- if you are throwing 80psi at the engine, higher compression isn't going to help you much, and may HURT because your compressing the charge to the moon when you don't need to. If you need 600psi peak compression for efficient combustion, then squeezing to 900psi isn't giving you much return on investment.

Also, if the engine has super restrictive breathing, then higher compression will make better use of the available breathing, but OTOH, the restrictive breathing really limits the benefits of higher compression. Higher compression REALLY shines in situations where you have big, high-flow heads that are free flowing.

Compression and timing are things that have to two-step together. Note how the factory CRs keep getting higher, even as injection timing gets more retarded. This is because of emissions management (have to keep peak temps low for NOx emissions), so timing is grossly retarded, and higher compression helps to offset that somewhat.


Compression and timing can only be optimized for ONE combination of RPM and BOOST pressure. Everything else is a compromise. Different boost pressures at the same RPM need different compression ratios and timing. And different RPM at the same boost pressure need different compression and timing.


Thus, there's no ideal compression ratio you can build in. Almost every engine will pass through its "ideal" for a split second from time to time.

It's just nothing but compromises of compromises.

 

[Mhannink]

ON compression ratio:





You need to recognize that on our engines, compression ratio and expansion ratio are the same. IOW, you have to look at the effects on ALL FOUR STROKES of the four-stroke cycle.



On the intake stroke, a higher compression ratio will enhance breathing for a given head/cam/valve combination. This is because the rapid expansion from TDC produces a faster and greater pressure drop, which initiates cylinder filling both earlier and with greater velocity.



On the exhaust stroke, you will see the same benefit in reverse. The rapidly decreasing volume near TDC will cause peak pressure to rise faster and farther. This means that exhaust leaves the chamber more efficiently and at higher velocity.



On the compression stroke, higher compression means that you will achieve a sufficient temperature for combustion earlier in terms of crankshaft degrees. This allows for more timing advance if you want it. Since the chamber is sealed on the compression stroke, higher compression means that you will have higher temp and pressure at TDC (ignoring fuel injection). It also means that for a given amount of injection timing, you will have higher peak combustion temps, more NOx, and a faster rate of burn (given enough O2).



ON the power stroke, a higher compression/expansion ratio means that the engine will extract more of the available power from the fuel. The greater expansion also means you will see lower EGT, ceteris parabus.







In general, a higher compression ratio is good. But not all by itself-- the engine has to be set up for it. And many times, higher compression brings costs that outweight benefits. For example, trying to keep the head on with higher compression AND advanced timing is quite a challenge. There's also the diminishing returns aspect mentioned by COMP461-- if you are throwing 80psi at the engine, higher compression isn't going to help you much, and may HURT because your compressing the charge to the moon when you don't need to. If you need 600psi peak compression for efficient combustion, then squeezing to 900psi isn't giving you much return on investment.



Also, if the engine has super restrictive breathing, then higher compression will make better use of the available breathing, but OTOH, the restrictive breathing really limits the benefits of higher compression. Higher compression REALLY shines in situations where you have big, high-flow heads that are free flowing.



Compression and timing are things that have to two-step together. Note how the factory CRs keep getting higher, even as injection timing gets more retarded. This is because of emissions management (have to keep peak temps low for NOx emissions), so timing is grossly retarded, and higher compression helps to offset that somewhat.





Compression and timing can only be optimized for ONE combination of RPM and BOOST pressure. Everything else is a compromise. Different boost pressures at the same RPM need different compression ratios and timing. And different RPM at the same boost pressure need different compression and timing.





Thus, there's no ideal compression ratio you can build in. Almost every engine will pass through its "ideal" for a split second from time to time.



It's just nothing but compromises of compromises.

ok so lets say that peak is 600psi then if your running 100psi boost then you want to back your compression off so you can still run peak and if your turbos can spool quick then it will opereate at peak longer as long as you hold the boost you've desined it for right

 

[KLockliear]

Th ironic thing here is that Cummins claims the HO was 17:1 even (2nd gen) and SO 2nd gen was 16. 3:1



Was someone saying something about checking facts???

Without spending the time to find it all out, apparently there are sources that disagree on these numbers.



Case in point: https://www.turbodieselregister.com/forums/showthread.php?t=113176

 

[rivercat]

Cams, If you aren’t doing cams you way behind the power curve, the best cams out there now are for either John Russin at Buddha power, or Bullet Racing cams. Everything else is just these two groups reverse engineered stuff, and is out of date with the new stuff being worked on so when you want to buy one of these so called new cams that are the be all to end all cams , remember these are two to three year old designs . The newest stuff is light years better. But it is only a mater of time before some of the reverse engineering pros on here will con some one, LIKE ME in to being helpful and selling them a grind and then copying it.

You know who you are, and you know if funny the big be all to end all cam, you copied and now are marketing is one of John Russin 3 to 4 year old design cam I had laying around after progressing to the next 3 or 4 cams John has helped me with. The Cams John is now doing are dramatically better on spool up and power.

Ya know, for all the typing you spew out, at some point you should see about getting some facts. I'm not knocking any vendor out there, but YOU sure are. Get the facts before you post.

sounds like a guilty dog barking

 

[Hohn]

Without spending the time to find it all out, apparently there are sources that disagree on these numbers.

Case in point: https://www.turbodieselregister.com/forums/showthread.php?t=113176

Fair enough, but the 17:1 and 16. 3: is also consistent with my owner's manual.....

I can't vouch for the 3rd gen #, other than I *think* it's 17. 3:1 like you said.

 

[KLockliear]

sounds like a guilty dog barking

Yeah, that's it.