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  • #16
    On Wednesday October 31, 2001 7:04 AM, hyc GTS wrote:

    Assuming you have an answer to that question, then clearly the higher boost engine will produce more power at a fixed dynamic CR, because it ingests more air mass than the other.
    That's what I thought at first as well, but on reflection its not quite so clear. Two engines of identical displacement but different CR will produce different amounts of power from the same volume of air (and thus gas), so clearly the efficiency of the engine has changed. If this relationship is better than linear, then changing the compression ratio is better than changing boost because boost is less than a linear improvement (unless you have 100% efficient intercooling).

    Former PGT-turbo owner... now 2010 VW Golf TDI

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    • #17
      FYI, CR cam be changed as simply as increasing the hieght of the piston with no other changes. Displacement remains the same. Internal volume of combustion chamber is slightly less (ie. less air), but is compressed more for larger forces to "drive" pistons down.

      Here are some "rules of thumb" I have heard from the "detroit iron scene". Take these lightly but you can certainly see that gains from increasing static CR are not near the gains of increasing manifold pressure. Also note than increasing CR can be done in alot of different ways, and each offers different results. Some of the "numbers" I have heard range from 1-5% gain for each point CR increase (ie. increasing CR from 8:1 TO 10:1 would increase power 2-10%.)

      Enjoy,






      Bryan Pendleton | LeMons Racing Blog | BPi Flow Stacks | Facebook Anyone?

      Comment


      • #18
        On Wednesday October 31, 2001 10:57 AM, BryanPendleton wrote:

        Here are some "rules of thumb" I have heard from the "detroit iron scene". Take these lightly but you can certainly see that gains from increasing static CR are not near the gains of increasing manifold pressure. Also note than increasing CR can be done in alot of different ways, and each offers different results. Some of the "numbers" I have heard range from 1-5% gain for each point CR increase (ie. increasing CR from 8:1 TO 10:1 would increase power 2-10%.)

        Enjoy,





        Where are the rules? :grin:

        Comment


        • #19
          On Wednesday October 31, 2001 2:03 PM, BigProbe wrote:
          On Wednesday October 31, 2001 10:57 AM, BryanPendleton wrote:

          Here are some "rules of thumb" I have heard from the "detroit iron scene". Take these lightly but you can certainly see that gains from increasing static CR are not near the gains of increasing manifold pressure. Also note than increasing CR can be done in alot of different ways, and each offers different results. Some of the "numbers" I have heard range from 1-5% gain for each point CR increase (ie. increasing CR from 8:1 TO 10:1 would increase power 2-10%.)

          Enjoy,





          Where are the rules? :grin:

          Some of the "numbers" I have heard range from 1-5% gain for each point CR increase (ie. increasing CR from 8:1 TO 10:1 would increase power 2-10%.)

          Bryan

          Bryan Pendleton | LeMons Racing Blog | BPi Flow Stacks | Facebook Anyone?

          Comment


          • #20
            On Wednesday October 31, 2001 11:17 AM, BryanPendleton wrote:
            On Wednesday October 31, 2001 2:03 PM, BigProbe wrote:
            On Wednesday October 31, 2001 10:57 AM, BryanPendleton wrote:

            Here are some "rules of thumb" I have heard from the "detroit iron scene". Take these lightly but you can certainly see that gains from increasing static CR are not near the gains of increasing manifold pressure. Also note than increasing CR can be done in alot of different ways, and each offers different results. Some of the "numbers" I have heard range from 1-5% gain for each point CR increase (ie. increasing CR from 8:1 TO 10:1 would increase power 2-10%.)

            Enjoy,





            Where are the rules? :grin:

            Some of the "numbers" I have heard range from 1-5% gain for each point CR increase (ie. increasing CR from 8:1 TO 10:1 would increase power 2-10%.)

            Bryan

            <-- feeling jipped.

            I was looking for a canonical listing of CR truths :smile:

            Comment


            • #21
              OK, assuming the engines are otherwise identical, and CR is the only difference.
              (I.e., both are at the same boost level, or both have the same air/fuel mixture and mass.) In this case, the difference in power output between the two will be proportional to the thermal efficiency of the two. This efficiency is calculated as
              Code:
                       1
                1 - -------
                    CR^0.35
              For 8.0:1, efficiency is 51.7%
              For 9.2:1, efficiency is 54.0%
              For 10:1, efficiency is 55.3%

              So given equal mixtures, the 10:1 CR engine will output 55.3/54.0 = 1.024 times as much power (2.4% more power) as the 9.2:1 engine.

              Now we take the case of unequal cylinder charge, but same dynamic CR. For the dynamic CR to be the same, we want to choose boost pressures such that
              • volume X * 9.2 = volume Y * 10.0
              • or volume X = volume Y * 1.087


              So to get the same dynamic CR, the 9.2:1 engine will be at a higher boost level, allowing it to ingest 8.7% more air than the 10.0:1 engine at a given dynamic CR. Note that the particular dynamic CR you choose is irrelevant, it all cancels out.

              The bottom line: the thermal efficiency shows how much energy we can harness from a given air/fuel charge. The dynamic CR determines how much air/fuel charge we have to work with. The product of these two is proportional to the total power output in this scenario.
              • 54.0% * volume X vs 55.3% * volume Y
              • with volume X = 1.087 volume Y
              • 54*1.087 = 58.7 which is greater than
                55.3

              As I said before, clearly the lower CR engine with higher boost will produce more power than the higher CR engine with less boost.

              -- Howard Chu Chief Architect, Symas Corp. Director, Highland Sun
              17x8" BBS RC/Leda 24-way adj. coilovers/Hye-Dra-Cyl Big Brakes: Wilwood 4-piston calipers+12.2x1.25" rotors/RRE FSTB/Top-End Racing FW/MazdaSpeed-Quaife LSD/65mm TB/Autophysics CAI/Groundz/Bi-Xenon HID headlamps/Euro Clear-corners+amber LEDs/Blazer amber fogs/JVC KDMX3000+CHX1200/Pioneer 6x9s/BCT7 Scanner/etc...

              Comment


              • #22
                On Thursday November 1, 2001 8:25 AM, hyc GTS wrote:

                As I said before, clearly the lower CR engine with higher boost will produce more power than the higher CR engine with less boost.
                Yeah, but you didn't back it up with math before. Assuming your equation for CR -> efficiency is correct (and no reason to believe it isn't -- where do you get these wonderful equations?) then its obvious that increasing boost is far more effective at increasing power. Indeed, browsing through my copy of Bosche's handbook seems to indicate that CR is increased primarily to improve fuel efficiency. This should drive the nail in the coffin of those who think boosting a ZE is a good idea... better to just use the manifold and port match it.

                Power comes from more air, and more air comes from displacement and/or boost.

                Thanks Howard.


                Former PGT-turbo owner... now 2010 VW Golf TDI

                Comment


                • #23
                  On Thursday November 1, 2001 3:57 PM, ASword wrote:
                  Power comes from more air, and more air comes from displacement and/or boost.
                  That sums this hole topic up the best. I love it. . . short and sweet.

                  Bryan
                  Bryan Pendleton | LeMons Racing Blog | BPi Flow Stacks | Facebook Anyone?

                  Comment


                  • #24
                    On Thursday November 1, 2001 11:19 AM, BryanPendleton wrote:
                    On Thursday November 1, 2001 3:57 PM, ASword wrote:
                    Power comes from more air, and more air comes from displacement and/or boost.
                    That sums this hole topic up the best. I love it. . . short and sweet.

                    Bryan
                    I entirely agree. More boost with lower compression will make more maximum power with all other variables the same. Irrefutable fact of math.

                    But where is that power?

                    ...area under the curve...
                    David Coleman #1891 - Fastest of any Gen @ MMIV:
                    1993 Mazda MX-3 GSR Special Edition - PINK POWA! - KLZE powered!

                    The Idiot Post Patrol :
                    Fighting ignorance one post at a time [this public service brought to you by Nikki and David]

                    Comment


                    • #25
                      (fyi, the formula for thermal efficiency comes from page 407 of my 4th Edition Bosch handbook.)
                      -- Howard Chu Chief Architect, Symas Corp. Director, Highland Sun
                      17x8" BBS RC/Leda 24-way adj. coilovers/Hye-Dra-Cyl Big Brakes: Wilwood 4-piston calipers+12.2x1.25" rotors/RRE FSTB/Top-End Racing FW/MazdaSpeed-Quaife LSD/65mm TB/Autophysics CAI/Groundz/Bi-Xenon HID headlamps/Euro Clear-corners+amber LEDs/Blazer amber fogs/JVC KDMX3000+CHX1200/Pioneer 6x9s/BCT7 Scanner/etc...

                      Comment


                      • #26
                        to FE3T - of course, there's no reason to go overboard in lowering CR just to raise boost. As many other posts have already stated, you want a reasonably high CR for off-boost driveability...
                        -- Howard Chu Chief Architect, Symas Corp. Director, Highland Sun
                        17x8" BBS RC/Leda 24-way adj. coilovers/Hye-Dra-Cyl Big Brakes: Wilwood 4-piston calipers+12.2x1.25" rotors/RRE FSTB/Top-End Racing FW/MazdaSpeed-Quaife LSD/65mm TB/Autophysics CAI/Groundz/Bi-Xenon HID headlamps/Euro Clear-corners+amber LEDs/Blazer amber fogs/JVC KDMX3000+CHX1200/Pioneer 6x9s/BCT7 Scanner/etc...

                        Comment


                        • #27
                          On Thursday November 1, 2001 6:39 PM, hyc GTS wrote:
                          (fyi, the formula for thermal efficiency comes from page 407 of my 4th Edition Bosch handbook.)
                          Harumph... I can't find it in my 5th edition.


                          On Thursday November 1, 2001 4:19 PM, BryanPendleton wrote:
                          On Thursday November 1, 2001 3:57 PM, ASword wrote:
                          Power comes from more air, and more air comes from displacement and/or boost.
                          That sums this hole topic up the best. I love it. . . short and sweet.
                          We aim to please. Verbal efficiency. :smile:

                          Former PGT-turbo owner... now 2010 VW Golf TDI

                          Comment


                          • #28
                            Great String, thanks for the info/equations. I knew anecdotally that the lower CR/higher boost was correct, but haven't seen the equations before.

                            Anyone know how lower CR will affect emissions. (I'll be running 8.5:1; finally received my new Eagle rods and Ross pistons on Wed.)
                            '94 Rio Red GT, 2.65l, 8.8:1 CR, Eaton M90, Quaife limited slip, TEC GT, 370 cc's (Still have the short block, all else gone)
                            '98 Pearl White 3000GT VR4 (with a few mods) SOLD
                            '93 Bone Stock MX-6 Sold (in '05) sadly to the crusher in 2010
                            '92 Midnight Blue Porsche 968

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                            • #29
                              On Friday November 2, 2001 12:11 PM, Joe Kamman wrote:
                              Great String, thanks for the info/equations. I knew anecdotally that the lower CR/higher boost was correct, but haven't seen the equations before.

                              Anyone know how lower CR will affect emissions. (I'll be running 8.5:1; finally received my new Eagle rods and Ross pistons on Wed.)
                              Lower compression will help your NOx emissions. Since you're going to a stand alone [or however the TEC II is classed] I don't think you'll have any problems passing emissions. Tuning! :smile:


                              David Coleman #1891 - Fastest of any Gen @ MMIV:
                              1993 Mazda MX-3 GSR Special Edition - PINK POWA! - KLZE powered!

                              The Idiot Post Patrol :
                              Fighting ignorance one post at a time [this public service brought to you by Nikki and David]

                              Comment


                              • #30
                                That's good news. I recently realized when I made the decision to lower the CR a little, I hadn't thought about passing the local testing (my post above is wrong, I'll actually be at 8.8:1).

                                Since the shop that is doing the rebuilding didn't want to mess with the TEC II, I've had to set up the engine to run off the factory computer. I had set up everything to run in parallel, so it was quite simple. About all I've had to do is bolt back on the OEM injectors/rails, and put the factory airbox, and VAF back on. So, my backup plan for passing emissions, hopefully it won't be an issue with the TEC II properly tuned, would be to set it back up to run on the factory computer to pass the test.
                                '94 Rio Red GT, 2.65l, 8.8:1 CR, Eaton M90, Quaife limited slip, TEC GT, 370 cc's (Still have the short block, all else gone)
                                '98 Pearl White 3000GT VR4 (with a few mods) SOLD
                                '93 Bone Stock MX-6 Sold (in '05) sadly to the crusher in 2010
                                '92 Midnight Blue Porsche 968

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