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Compression vs Boost FYI

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  • ZE, less boost, less duty cycle, smaller injectors, less lag = more torque early on, less room for tuning error

    DE, more boost, higher fuel requirements, more lag = less wheel spin early on, more room for tuning error
    1993 Mercury tRacer :Vitara & mFactory: In progress 2020
    1994 Mercury TrAcer Wagon : '02 SPI swap : sold
    2014 Factory Five 818s: sold
    2000 Lotus esprit v8TT : 1st v8 on MS3x, ls1 coils,h20 injection
    1997 Nissan 200sx 1.6L : 35-39mpg the daily

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    • Thanks, Mac. I wish I would've just made it to the "compression ratio" chapter in Maximum Boost first before I posted that. heh-heh. But very well said, sir. Thanks.
      1993 MX6 LS

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      • i am going to put a f2 block with a ported f2t head. probably add 70- 90 H.P.

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        • it's too bad i sold and parted out my probe shortly after i broke in the engine. well i did buy a ZE mx3 and i have a most of a mr.speed supercharger kit i think i'll have to revisit this topic before doing a rebuild
          1993 built ZE/DE n2o 2380lbs probe GT = parted out 2004
          1996 klze mx-3

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          • Originally posted by crazyprobe View Post
            it's too bad i sold and parted out my probe shortly after i broke in the engine. well i did buy a ZE mx3 and i have a most of a mr.speed supercharger kit i think i'll have to revisit this topic before doing a rebuild
            It's a trap!! lol
            1995 Probe GT. Colt 218* cams, Tein SS Coilovers with EDFC, K6F trailing arms, CZT traction bars, ZE Intake Manifold, Milly MAF/OBDII swap, GReddy SP1, Probe Addiction Strut braces, RX8 Seats, PRD STS, PRD door sills, ACT 6puck, MFactory LSD,Built trans, Euro tails and corners, Walbro 255, and more to come
            Stage 3 Audi S4 pushing about 425awhp

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            • I still have your engine if you want it. Not many more miles than when you sold it to me.

              Just couldn't stay away, welcome back to the madness.
              Last edited by Silver93GT; April 15, 2016, 10:39 AM.

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              • ^^^ wow look at all the manitobans !!!!
                1994 Project PGT

                M62 In Progress...

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                • hey you the guy on prairie talk that was thinking of selling is probe?
                  1993 built ZE/DE n2o 2380lbs probe GT = parted out 2004
                  1996 klze mx-3

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                  • Sure am. I think about selling that car every other week lol. It jus needs some minor body work and paint. But I get sick of storing it.

                    I have lost a lot of interest when MPI said I need an immobilizer to insure it. I don't really want some auto mechanic drop out hacking up my wiring harness.
                    Last edited by Brizzy; April 27, 2016, 09:21 AM.
                    1994 Project PGT

                    M62 In Progress...

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                    • Originally posted by BigProbe View Post
                      why does lower compression + high boost make more power? That's absurd. Go w/ the HIGHER Compression piston, if you aren't making a track-only car, which will never see RPMS below, say, 4K.
                      If you want high specific output, you want low CR + high boost.


                      There are a few things happening with each setup that explains this.

                      Higher dynamic compression (by whatever means) always means a more efficient combustion process. With that fact established, let's quickly look at what happens in each scenario.

                      With a higher mechanical compression ratio and a lower boost pressure, a higher cylinder pressure is achieved with less intake pressure at all times vs low CR engine (dynamic compression) ie. 14.7psia [sea level] + x psig from turbo/sc.

                      The benefit is obvious; more torque all around.
                      The cons are less obvious ie. most of the compression work that is done to achieve the same dynamic CR is done in-cylinder, at TDC. That's bad bc compression work occurring in-cylinder liberates heat in-cylinder, exactly where we DO NOT WANT it. Therefore knock limits are reduced for a given fuel's AKI rating.
                      And finally, the combustion chamber is indeed smaller, allowing a lower mass of cylinder charge at the fuel's knock limit.

                      With a lower mechanical compression ratio and a higher boost pressure
                      , off-boost torque output is reduced relative to the prior setup.
                      However, a higher boost pressure does more of the compression work in the compressor, and not in the combustion chamber. In this case, much more heat is shed is shed at the intercooler, therefore the charge is not only at a higher pressure at the intake valve, but it is also denser and cooler.
                      This alone staves off preignition/detonation and lowers the fuels instability threshold.
                      Then on top of that, the mechanical volume of the combustion chamber is also larger in volume, allowing more of that dense and cool charge to enter the combustion chamber that simply equals more chemical energy per combustion event.

                      So you choose which setup you want based on your expectations.

                      -If you want a nice efficient daily with a reasonably larger turbo then high CR and low pressure boost would do you well. The last thing you want with high CR is a small turbo that spools quickly.
                      WARNING:Low and mid-range RPMs are NOT where you want jam boost into a high CR engine, especially with a smaller turbo with a smaller compressor wheel putting out a higher temperature charge.

                      -If you want a safer setup and want higher specific output numbers ie. racing/dyno then you want low CR + high boost. Simple as that.
                      Plus you get to use pump fuel and your nice dyno numbers can also translate to the street/track rather than being limited to running pure C16 on the dyno for bragging numbers, and crippling power on the street running pump like would happen with a high CR engine.

                      95' 323C 'Neo' V6 BA23
                      stock KLZE/retuned VRIS0/deleted VRIS1/retuned VRIS2/MX3 rear suspension/0.680 5th gear/3deg retard cam timing/manual TBT

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