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C1 Practical Compression Ratio

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  • John T.
    NCRS Financial Officer
    • January 1, 1983
    • 290

    C1 Practical Compression Ratio

    I am getting ready to rebuild the engine in my 62 FI car. What is the practical maximum compression ratio to run without aviation gas or other chemical aids ? I am keeping the original camshaft and would like to come as close to the original feel and sound as I can. Anyone been there done that ?
    Thanks
    John
  • Ed Jennings

    #2
    Re: C1 Practical Compression Ratio

    John, my 62 FI car has domed pistons in it and the original heads. I don't know the exact CR, but it has to be in the 10.5-11 range. My engine has a Comp Cams H280, which is supposed to be a pretty agressive street cam, maybe roughly equivalent to the original in terms of overlap. I didn't build or spec the engine, so I'm not exactly sure of the exact specs. Anyway, the cars runs fine on pump premium. Timing is set to 36* total advance. I think you would run into more trouble with a 300 HP engine with 10:1 comperssion and a mild cam than the SHP engine with the more radical cam, despite the slightly higher compression.

    Comment

    • Ted S.
      Expired
      • January 1, 1998
      • 747

      #3
      Re: C1 Practical Compression Ratio

      John, it depends on the cam you plan on running. You can run 11:1 static compression ratio on pump gas if you have a cam with enough duration/appropriate timing events to bleed off some of the compression. Also you need to keep the quench to about .040. In order to run 11:1 compression on pump gas you'd need to have a cam that closes the intake around 74* - 75* after bottom dead center. This would put the dynamic compression ratio (DCR) at around 8:1 which is the max for pump gas with iron heads. You can bump the DCR to 8.5:1 with aluminum heads. If you'd like to discuss this more e-mail me since this is off the "restoration" path.

      Comment

      • Larry Parylla #41143

        #4

        Comment

        • Ed Jennings

          #5
          Re: C1 Practical Compression Ratio

          If you stick with the stock combination of parts or something very close to them, you will not likely encounter any difficulties. GM started adding a double head gasket to reduce compression slightly during this time frame, so the 11:1` compression was a little stout, even in days of yore. On your fuelie, stick with the stock cam or something very close to it. If you can't generate at least 12-13" of vacuum at idle, the FI won't operate properly. If you don't want to use the solid Duntov cam, a lot of folks have sucessfully used the 327/350 (151) hydraulic cam as a substitute.

          Comment

          • Ted S.
            Expired
            • January 1, 1998
            • 747

            #6
            Re: C1 Practical Compression Ratio

            Larry, This is what I emailed John. Somehow on my initial response I was thinking 283s so my calcs are slightly off. I guess I was thinking of my '58. Others more knowledgeable than myself please weigh in.

            John had stated that he would be using a stock "spec" cam with an intake closing event at 72* ABDC. Here was my response:

            "John, If my '62 motors manual is correct in showing the '62 fuelly ran 11.25 compression you'll have problems on pump gas with the spec cam with an intake closing event at 72*ABDC. That combination will put you up around 8.38 DCR. I had a 327 in a '60 running about that same DCR and it was taking 93.5 - 94 octane to run without pinging. To run 11.25 in a 327 you'd need a cam with an intake closing around 77* abdc. Some guys claim they have gotten slightly over the 8.0 but it requires polishing the chambers, smoothing the pistons, tight quench, etc to keep hot spots out of the combustion chamber. Dome pistons create more problems than flat tops. If you went to something like the TRW L2166 it would put your static compression ratio some where around 10.5 with your 62cc heads which would run with the stock spec cam. You may want to install the cam at 4* advanced which would help out on the lower end. Is the engine std bore, if not how much over? Has the block been decked? Maintaining the quench at .040 or slightly less is key as it helps the combustion speed to prevent detonation. If the block hasn't been decked this means running the steel shim gaskets. If I remember right Fel Pros are .022 thick. The normal composition head gaskets are too thick to maintain the quench if the block hasn't been decked."

            Hope this helps

            Comment

            • Duke W.
              Beyond Control Poster
              • January 1, 1993
              • 15610

              #7
              Re: C1 Practical Compression Ratio

              Before pulling the pistons/rods, carefully measure the deck to piston crown clearance. This dimension is critical to computing the actual CR.

              As Ed said, beginning sometime in '62, Flint installed two steel shim gaskets to address customer detonation complaints, and this cut compression by about half a point, but increased the quench zone clearance. For a given CR having minimum quench distance is desireable, but it is tough to achieve if you are doing a restoration and don't want to deck the block.

              It's normal to use a composition head gasket for a rebuild (but this might be detectable in judging and cost a few points), as they are more forgiving of minor surface variations or flaws on and old engine. With freshly machined surfaces, shim gaskets seal fine, which is why Flint used them, and since it is desireable NOT to mill the heads and DEFINITELY NOT deck the block if the original pad stamp is intact, you will end up with a little less compression than the engine originally had with the two shim gaskets (about .015" compressed) versus the typical .038"-.040" for a composition gasket.

              The blueprint crankshaft centerline to deck dimension for the SB is 9.025", but in production they were probably closer to .020" high, so with one steel shim gasket you are at the minimum recommended quench clearance of .035". With a composition gasket the quench clearance will be about .060", but that's not the end of the world. The drop in compression more than offsets the increase in quench zone clearance and the engine will have less tendency to detonate.

              Once you know your deck clearance you can compute the CR for both the Speed Pro OE forged domed pistons (FM has the volume data) or look at flattop piston alternatives, and I recommend shooting for an ACTUAL computed CR of 10.5:1.

              I also recommend the LT-1 cam to replace the Duntov cam. It will yield better torque bandwidth, and the slightly later closing inlet valve will reduce dynamic compression a bit. The following are the computed dynamic compression ratios, assuming a 10.5:1 static ratio, computed by the Engine Analyser simulation program using the SAE J604d valve timing points (.006" valve lift).

              Duntov cam - 8.47:1
              LT-1 cam --- 7.99:1

              The LT-1 cam will provide very similar idle characteristics and vacuum (about 12" @ 900) as the Duntov cam because it has about the same effective overlap, and as long as you don't blab about it even the best PV judge would be very unlikely to detect that the engine doesn't have the OE Duntov cam.

              If I were to build the "ideal" SHP 327 from a pile of parts I would use flat forged pistons with just two valve notches, shave the block down to achieve -.005 deck clearance (pistons sticking out ,005") and use a compostion gasket. This would yield a quench clearance of about .035". I'd top it off with a nice set of pocket ported three angle valve job 461s and since I'm probably too cheap to spring for a '63-'65 FI system, I'd use a '63 SHP manifold with an AFB and 2.5" exhaust manifolds. Of course, it would have an LT-1 cam.

              Duke

              Comment

              • Duke W.
                Beyond Control Poster
                • January 1, 1993
                • 15610

                #8
                Correction (I think)

                I looked at the data and ran the numbers. The Federal Mogul (formerly TRW) L2166F-XX OE replacement forged pistons have a 1.675" compression height and the net dome volume is +5.3 cc, so if everything is nominal the piston crown to deck clearance should be .020" (9.025 - (1.625 + 5.7 + 1.675)).

                For a .030" overbore 327 with everything nominal and a .015" shim gasket the quench clearance is .040" and the CR with a 62 cc head is 11.58.

                Add a second .015" gasket (.055" quench) and its 11.03.

                Use a .040" (.065" quench) composition gasket: 10.74.

                Machine off the piston dome with the compostion gasket: 9.73.

                Consider these "ballpark" figures, and use as a guideline. You should always make exact measurements on your engine and run the numbers.

                Duke

                Comment

                • Duke W.
                  Beyond Control Poster
                  • January 1, 1993
                  • 15610

                  #9
                  Re: Correction (I think)

                  Nominal deck clearance would be .025", not .020". The calculations are correct (I think). Sorry, it's late and I'm getting tired.

                  I did the calcs with and engine parameter calculator called "Virtual Engine Calculator". I downloaded it from a link I got from Clem.

                  Duke

                  Comment

                  • Ted S.
                    Expired
                    • January 1, 1998
                    • 747

                    #10
                    Re: Correction (I think)

                    Duke, What are you using for the head gasket bore, piston to cylinder wall clearance, and the top ring land height? The first calcs for DCR, I used the trw "stated" compression ratios (i.e. 11.2 for 58CC and 10.35 for 64cc). I never have figured out what calcs they use to come up with their compression ratios. Using the 5.3cc for the dome I come in closer to the calcs you had. Also, what are you using for the intake closing on both cams?

                    Using John's 77* ABDC and the 5.3 cc dome, .015 gasket with 4.1 bore, .025 deck, etc. I'm coming in at 11.17 static and 7.93 dynamic. In that situation you would want the cam straight up not advanced per my earlier post. With that it should run on fine on 91 - 92 octane as long as quench is maintained. I have an engine with a .041 quench running a 7.9 DCR on 91 octane without any problems.

                    The guys running "pump gas" racing engines say .040 or less quench is critical. A higher compression ratio with a .035 - .040 quench will have less tendency to detonate than a lower compression ratio with a greater quench.

                    Comment

                    • Duke W.
                      Beyond Control Poster
                      • January 1, 1993
                      • 15610

                      #11
                      Re: Correction (I think)

                      "The guys running "pump gas" racing engines say .040 or less quench is critical. A higher

                      compression ratio with a .035 - .040 quench will have less tendency to detonate than a

                      lower compression ratio with a greater quench."

                      You have to take this with a grain of salt. It's not quantitative. For example, by building the SHP engines with two shim gaskets (as Flint implemented during the '62 model year), quench increases from .040" to .055", but the CR dropped 0.5, and the detonation problem was addressed. The statement is correct in the sense that if you want to push the CR to the absolute limit, you have to be at minimum quench, but that's tough to do when restoring and engine unless you deck the block. I know my '63 L-76 was built with two gaskets, and I never had any detonation problems on '60s premiums, even after I updated the timing map to '64 SHP spec, which brought all the centrifugal in at 2350, and I never had to use the available "super premiums" like Chevron Custom Supreme.

                      The calculator I used leaves something to be desired. It does not have provision for deck clearance, so I just added deck clearance to gasket gasket thickness and input the gasket bore the same as cylinder bore. Thus, the calculated numbers are high because they don't allow any shape for the gasket bore increase over cylinders bore and piston crown to ring land. That's why I said they are ballpark figures, and each individual engine should be carefully measured and more accurate calculations conducted. The other inputs for the calculator are head cc and dome cc.

                      The stated CRs in the Speed Pro catalog are based on compressed gasket thicknesses of .038". There is no mention of deck height, so I ASSUME that they use nominal dimensions, which would yield a deck height of .025". With a 64 cc head and a .030" overbore their table says 10.35, which is fine.

                      As you probably know, and others need to realize is that a very small change in clearance volume has a big impact on CR. For example, if head cc is increased from 62 to 64 the drop in CR is about a quarter point.

                      The dynamic CRs that I extracted from Engine Analyser are based on the SAE J604d closing points that are 71.5 and 63.5 ABC for the LT-1 and Duntov cams respecitively.

                      Anecdotal evidence indicates that many are running original or rebuilt SHP engines with the original type pistons and no detonation with, perhaps, some slight timing map adjustments, so I feel comfortable running the OE type pistons with a compositon gasket. Boring 30 over raises the CR slightly, but reseating the valves and relieving the chambers where they overhang the bore (which I recommend) increases clearance space a handful of cc.

                      Dennis and I were discussing this last week. He likes to build about 9.5:1 compression, so he doesn't have to worry about octane availability and can drive his Corvettes anywhere with no hassle. My philosophy is to push the CR as high as I can (I don't like leaving any torque or power on the table), and make timing adjustments as necessary. Both philosophies are okay. It's just a matter of personal preference.

                      Duke

                      Comment

                      • Ted S.
                        Expired
                        • January 1, 1998
                        • 747

                        #12
                        Link for write up on this topic

                        For those interested in this topic, here's a pretty good write up on static vs dynamic, quench, etc.




                        Static vs. Dynamic

                        Comment

                        • Clare Carpenter

                          #13
                          Re: Link for write up on this topic

                          If I understand the basics after reading thru the article one time, a 327 SHP with the .097, the 30-30, or the LT-1 cam should have no problem running 11:1 pistons if everything is attended to and set up properly. The relatively light weight of the early Corvettes, especially if geared low and with a manual transmission, helps to prevent detonation along with the overlap of the above mentioned cams. Add to that the right sized head gasket, proper quench, some polishing of the combustion chambers and piston tops, proper dial in of the cam and timing curve, good cooling and the right plugs and we should be good to go with pump premium. The article mentioned decking the block to .00, I think. Obviously not something we'd want to do with a vintage engine but also not necessary unless building for the optimum compression and performance of any given combo. Did I leave anything out? Duke, if you read this can you explain the decking process and how it relates to quench? I always assumed the block was decked just to square it up. Apparently there's alot more to it?

                          Comment

                          • Duke W.
                            Beyond Control Poster
                            • January 1, 1993
                            • 15610

                            #14
                            Re: Link for write up on this topic

                            Your summary is okay.

                            The "quench clearance", sometimes just called "quench", is the sum of the "deck clearance" - distance between the piston crown (top surface of the piston excluding dome or recesses, if any) and the block deck with the piston at TDC - and the compressed thickness of the head gasket. If the piston crowns are below the level of the block deck, clearance is positive; if above, clearance is negative, and the arithmetic sum is the quench clearance.

                            For for a given CR the one with the least quench will usually exhibit the greatest detonation resistance. For example it is better to run a flattop piston with minimum quench than a domed piston with greater quench. Assuming both yield the same static CR the flattop/minimum quench conifuration should be more resistant to detonation than the domed piston with greater quench.

                            Chevrolet specifies the minimum quench as .035" -.040" to allow some clearance for thermal expansion and rod stretch under load, but I think Clem has built drag racing engines with near zero quench.

                            If we add up the nominal dimensions of a SHP engine, the quench with a single steel shim gasket should be in the above specified range. When Chevrolet added the second gasket, they increased the quench by .015", but the nearly half-point loss of compression solved the detonation problem even though the quench was greater.

                            I believe NHRA allows "stock class" engines to be built to minimum quench, so builders can do this by some combination of gasket thickness and deck clearance. Since builders prefer composition gaskets to the OE steel shim gaskets, in order to achieve minimum quench and maximum CR with OE pistons, the block deck must be milled ("decked") to reduce deck clearance in order to achieve minimum quench with a compostion gasket. This legally achieves the maximum possible CR with the OE pistons.

                            For restoration, we don't want to machine the block deck in order to keep the original engine stampings intact, so it's not possible to achieve minimum quench with a composition gasket, but the small sacrifice in power is worth the benefit of keeping the engine "original" with intact block stampings, and the thick gasket reduces CR a bit from even the double steel shim gaskets.

                            The combination of a factory machined block with the OE domed pistons and a composition gasket will only yield about a 10.5 static CR, and with the late closing valves of the SHP cams (which results in a relatively low dynamic CR for a give static CR), a SB should run without signficant detonation on pump premium. If not, the timing map can be juggled by slowing the centrifugal curve, or reducing the initial timing and increasing the total centrifugal timing so that total WOT timing is near the ideal 36-38 degrees at high revs, or some combination of both strategies can be used

                            Detonation is more of a problem at low revs than high revs, so timing map changes can usually solve low rev detonation problems.

                            BBs, with their larger bores have more tendency to detonate, so owners may have to do more - such as reducing CR by using flat pistons, or blending some high octane avgas or racing gas with unleaded premium with the OE type domed pistons.

                            With any precise engine build I always STRONGLY recommend that the proper dimensions and volumes be measured to determine actual CR. This begins with measuring the deck clearance upon engine disassembly - before the pistons/rods are removed. If the deck heights are within a small range front to rear you can be fairly confident that the deck is parallel to the crankshaft. A slight out of parellel situation will show itself with decreasing or increasing deck clearance, front to rear.

                            Machine shops often often want to "take a small cut" on the decks to ensure they are parallel to the crankshaft centerline. IMO it would be rare to finds OE machined decks that are sufficiently out of parallel to cause problems. As an alternative the actual crankshaft centerline to deck distance should be measured at all four corners to check for parallelism, and this will require some special tools.

                            The parallelism issue can be important if the shop uses boring equipment that indexes off the deck. Better shops have equipment that indexes off the crankshaft centerline, so a slight out of parallel condition will not adversly affect the finished bore angle, which should be dead on perpendicular to the crankshaft axis.

                            Grinding valve seats and relieving the combustion chamber on the heads will increase chamber volume a cc or two, which will reduce CR. When building for max CR the head ccs are usually equalized to a uniform larger than OE minimum spec, then the head is milled to bring them down to minimum spec. For a restoration rebuild, I would equalize all at the largest measured value with a little grinding along the chamber edges. This will ensure that all cylinders are are at a uniformly "low" CR. If the heads measure flat with a machinist's bar and .0015" feeler gage, they will seal with a composition gasket, so only mill the heads if absolutely necessary to to ensure the head sealing surface is flat.

                            With your final dimensions for deck height on the assembled short block, compressed gasket thickness, and head chamber cc, you can compute the final static CR, with a target of about 10.5:1 for SHP engines and no more than 10:1 for medium performance engines.

                            A "restoration rebuild" is different from a typical "commercial rebuild". For the former, only the minimum machining should be done, and this minimum level is determined by taking the measurements discussed above. Ideally, no machining of deck and head sealing surfaces will be needed.

                            On a typical commercial rebuild the block deck and head are machined to ensure parallelism and flatness without taking any initial measurements. This typically is the quickest and cheapest way to do the job, and is the typical procedure used by commercial machine shops. For a restoration rebuild, you must work with the shop and ensure that they understand your objective and do it your way.

                            If you get the "we do them all this way" ( the commercial way) response, keep shopping until you find a machine shop that understands restoration rebuilding.

                            Obviously, a "restoration rebuild" is a labor intensive endeavor, and will be very expensive if professionally done, however, an enthusiast with the motivation to do his disassembly and assembly can do most of the hand work. Once the engine is disassembled (and you have the deck heights of all eight cylinders) you can disassemble the heads and reinstall them on the block with a few bolts. Then from the underside you can mark the chambers for any necessary relieving. This would be any head material that ovehangs the bore, especially on the exhaust side.

                            With the head on the bench grind away the overhang taking into account the little extra you will need for the overbore. 461 heads originally equipped with 202 valves were relieved with a cutter on the exhaust side in production, but they should be checked, and the 1.94" valve versions will need some work. All this can be done in conjunction with pocket porting and port matching to the manifolds, and while you are spending hours in your garage doing this, you can send the out block and bottom end components for magnaflux inspection, dimension checking, boring, and whatever other rework is required.

                            Crankshafts should only be ground if ABSOLUTELY necessary. This is especially true for SHP cranks as grinding them will remove the Tufftride surface hardening. Journals can all be measured with a 2-3" micrometer, and your machine shop should check the crankshaft for straightness.

                            Early small bearing rods should be used as paperweights. Brand new late design small bearing rods should be purchased, or, for a bulletproof bottom end, a set of Crower Sportsman rods. The entire rotating/reciprocating assemble should then be balanced along with the damper, flywheel and clutch.

                            Once you've finished you head work you can send them out for seat and valve guide work, but performing a final chamber relief check with the bored block is a good idea to make sure your relief job is okay. While the heads are out the the short block can be assembled and measure final deck clearance. Once the heads are back from the machine shop, assemble them and measure the chamber ccs, then perform additional grinding as necessary to equalize all chamber cc at the highest value of the eight.

                            You should end up with a very nice running and reliable engine that will probably end up outliving many of us, and you'll have the satisfaction of knowing that you properly planned and managed the project and did a lot of the work with your own two hands.

                            Duke

                            Comment

                            • Clare Carpenter

                              #15
                              Duke's write-up is worth printing as a reference.

                              Thanks Duke. This is an article worth printing and keeping for future reference. Whether someone wants to do some work themselves or farm it all out, Duke's explanation will help anyone in choosing a machine shop and communicating with their builder, as well as better understanding the processes and what should or shouldn't be done to end up with the best overall "blueprinted" job.

                              If you elect to use your original rods, make sure to have them magnafluxed, resized and shotpeened for extra strength. I'd do the same with a set of new later design Chevy rods before installing. Crower Sportsmans already have this work done, I believe.

                              Comment

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