effects of DCR on available SCR

It's late/early? and I've been thinking about this stuff for a long time, so hear me out. I've been playing with Pat Kelley's and RSR's (link (http://www.rbracing-rsr.com/comprAdvHD.htm) Dynamic Compression Ratio calculators ever since I first got the links to them on here.

Let's start out with a relatively basic combination. A stock aluminum head LT1 with the LT4 Hotcam. 10.5:1 compression 272/281 seat duration (somebody on here told me that...) 112LSA 109ICL IVC65 (Example A)

PK's DCR Calc gives a DCR of 8.365:1. RSR gives 8.37:1 so they both use the same calculations.

OK, here is what has me thinking...
Let's say we change the pistons to get the highest pump-gas friendly DCR of 9.0 that has also been stated many, many times on here. Static compression goes up to 11.32:1.

OK, let's say we think the engine is under cammed. So we advance the cam 3 degrees and install it straight up. 112LSA 112ICL 68IVC oops, DCR has dropped down to 8.78:1. So we can increase static compression up to 11.606:1 and be safe. (Ex. B)

Everybody in here will agree that, as long as you don't have any problems, higher compression makes more power, right? So, let's say you increase the compression ratio up to 12.0:1 but if you change the duration, the RPM range will move up too much. So, what do you do to keep it pump gas safe? Change the LSA and ICL, right? OK, do you make it tighter or wider?

Generally a tighter LSA will increase power, or at least that's what is commonly stated. And many people think that a tighter LSA will work better for a higher compression engine since there is more overlap.

So, let's say we move the lobes around on the cam to 106 LSA 106 ICL. IVC changes to 62 ABDC and the dynamic compression jumps UP to 9.29:1 :eek: Dang, here comes problems on pump gas! So we have to change the pistons to get a static compression ratio of 11.06:1. :irk: Hmm... (Ex. C)

OK, I've got an idea, what if we spread the lobes apart and set them at 115LSA 115 ICL which puts the IVC@71 ABDC. That allows us to run a static compression ratio of 11.92:1 which is much higher than the much tighter LSA/ICL allowed. (Ex. D)

So, I'm wondering which would make more torque through the range.
106LSA 106ICL and 11.06:1 or 115LSA 115ICL and 11.92:1

Now I open up DynoSim and make 2 identical engines with the hotcam and get a baseline of 380hp@5500 and 424tq@4000 which coincidently is Example A. Since we are all familiar with the Engine Masters Challenge, I'll go ahead and give what the score for each engine is. In this case the engine would score 689.5

Example B 405hp@5500 & 433tq@4000. EM score is 721.7

Example C 394hp@5500 & 422tq@4000. EM score is 706.8

Example D 404hp@5500 & 436tq@4000. EM score is 721.1

For what it's worth, Just changing the LSA/ICL and keeping them straight up and adjusting the SCR to keep DCR at 9.0 or a hair under the best combination that these programs is giving me 113/113 406 434 722.4


Now that I've gotten all that out, I'm a bit confused. I used to believe that a tighter LSA would bleed off cylinder pressure more and allow running higher compression. That seems to be the opposite of these findings.

Generally a tighter LSA will increase power, or at least that's what is commonly stated.

I'm at work, waiting on our Solar Winds scan to finish; so I might as well jump in the conversation...

I don't know how much of an impact this has on your thoughts, but I don't think it's actually correct to say that a smaller (lower number) Lobe Seperation Angle increases power.

Power might increase, but that's a secondary effect.

Any given cylinder, under optimum conditions; can only produce "X" amount of power. You can get more power out of the engine by increasing the RPMS, because naturally, you'll get more power out of 20 combustion events per minute than you will out of 10. Not per event mind you, but over a given period of time.

A tighter LSA will often have the effect of increasing power, but that's because it raises the RPM range at which the engine can make power, or make it's optimum power.

Of course, as you stated, there are a lot of variables.


The thing that confuses me is that people who know about these things tend to say that a DCR of about 9:1 is the limit for pump gas.

Well, that is presumably 9:1 at 100% volumetric efficiency.

Thing is...
Do these calculators take that into account? I dunno...

How would they anyway?

Now, if you have 9:1 DCR @ 100% VE and you run the engine at 90% VE, that's cool...

However, what about those times when your engine runs at say; 110% VE?

With a really well matched head/intake/cam system, that probably happens more often than we think.

There have been some great posts on this site regarding LSA and IVC.
I do have a favorite from over at LS1tech though.

http://www.ls1tech.com/forums/showthread.php?t=394947

Specifically post #61

I'll quote SStrokerAce from that thread.

"Actually LSA is a byproduct and shopping by LSA is a bad thing to do, now doing it by overlap that's a better idea." SStrokerAce 10-17-2005

Now that I've gotten all that out, I'm a bit confused. I used to believe that a tighter LSA would bleed off cylinder pressure more and allow running higher compression. That seems to be the opposite of these findings.

How does the tighter LSA bleed of cylinder pressure? Really think about that and you are going to be way ahead of the game here.

Or in other words what I should be saying is how does a tighter LSA (meaning more overlap) have to do anything with bleeding off cylinder pressure? The tighter LSA also makes the IVC earlier which raises the DCR.

Hint look at the valve events and what happens when you just change the LSA.

Lame,

Your getting on something with the higher VE, that should really be taken into account. 9:1 DCR might be way more than what the motor can handle on pump gas, so I really don't think we should make that the general rule. You CAN do it but not all the time, different gas and different DA's play into it.

GetaZforgetGT,

Thanks for quoting me! Cheston, you really have to look at the different Overlaps in your different examples. The higher SCR will need less overlap but you sticking too much to LSA and Duration and NOT vavlve events which is where you might really find something in your example.

Bret

9:1 DCR might be way more than what the motor can handle on pump gas, so I really don't think we should make that the general rule. You CAN do it but not all the time, different gas and different DA's play into it.

DA's ?

Density Altitude

There have been some great posts on this site regarding LSA and IVC.
I do have a favorite from over at LS1tech though.

http://www.ls1tech.com/forums/showthread.php?t=394947

Specifically post #61



FWIW, you can link to a specific post by clicking on the post number then copying that window:

http://www.ls1tech.com/forums/showpost.php?p=3676150&postcount=61

Of course that leaves out the rest of the thread so your link is probably better.

Density Altitude


Thank you.