63- 327/340 Engine Builder Questions

[Stuart F.]

Re: 63- 327/340 Engine Builder Questions

Michael;

Interesting post that sheds some light on the mystery of the dual head gaskets. I can't say for certain, as I mentioned, but I'm inclined to believe from my feeler gauge measurement that mine has just one gasket. It certainly performs with the best stock C-2 Small blocks thru 65, including fuelies. I always attributed it to the tall 3.36 final and the fact that so many "back in the day" ran the drag gears and thought they were invincible. They would fall for the 30 up every time. I, like yourself, have been around performance cars since I was a 16 year old kid, and I know the sound and feel of high compression - specially against your pant leg and in your chest.

Stu Fox

[Timothy B.]

Re: 63- 327/340 Engine Builder Questions

Joe,


I have never understood how valve timing @.050 can be used to establish static compression ratio. I understand it's used for comparasion of camshafts but the J604 SAE .006 tappet lift measurements seem to be better suited to establish the point when the valve is effectively closed. At .050 the cylinder is not building any pressure..

Looking at your other post the static compression on the 151 and the 097 cams are different IMO reflecting the difference in the J604 measurement which really reflects effective timing to establist a target 8-1 dynamic compression.

[Joe C.]

Re: 63- 327/340 Engine Builder Questions

Joe,


I have never understood how valve timing @.050 can be used to establish static compression ratio. I understand it's used for comparasion of camshafts but the J604 SAE .006 tappet lift measurements seem to be better suited to establish the point when the valve is effectively closed. At .050 the cylinder is not building any pressure..

Looking at your other post the static compression on the 151 and the 097 cams are different IMO reflecting the difference in the J604 measurement which really reflects effective timing to establist a target 8-1 dynamic compression.

That's right.

"J604D"/advertised duration/"seat-to-seat" duration all seem to be used interchangeably as the metric for "real" duration, which is, actually, when the valve is lifted high enough off of the seat to effectively bleed off cylinder pressure. There too, I believe that J604D is defined differently, depending on lifter type:
For a hydraulic, it's fairly easy to understand.............it is simply the duration between .006" valve lift, since hydraulics effectively have zero lash. For a solid, it gets more complicated, and lash take-up must be figured into the calcs. J604D duration for a solid tappet lobe equals the duration between .004" valve lift, which is actually true rocker arm ratio @ POML (which for a SBC is 1.44) X valve lash, + .004".

Cam manufacturers seem to have a labyrinthine methodology of defining cam lobes, which may or may not be intended to confuse the buying public. Various metrics for duration are: J604D (in its many representative labels), .050" lobe lift, .015" lobe lift. There are probably more.

Rate of cylinder pressure rise is not only a function of J604D, and 0.050" durations, but also what is happening between these two points, and up to POML. Ramp speed............the faster the ramp, the sooner the opening/closing event is completed, and the faster the cylinder can build pressure during the compression stroke, and relieve it during the exhaust stroke. More "area under the curve" as advertisements refer to it. Roller tappets have a huge advantage here, since there is no sliding friction to factor in, ramp rates can be much faster and stiffer springs are used to keep the tappet in contact with the lobe. As a matter of fact, some engine builders like to have a certain amount of "lofting" occur after POML, which keeps the valve wide open longer. Most race cams ("cheaters") have what is known as "POML duration", which defines how long the valve is held wide open. Seat bounce is never acceptable, so choice of valve springs, IMHO, should place greater emphasis on seat pressure than on pressure @ POML.

I like to use the difference, in degrees, between J604D and 0.050" durations as a rule of thumb for estimating ramp rates. Most of the original GM vintage cams have deltas here on the order of 60 degrees (like the 30-30 and the exhaust lobe of the LT1), and 42 degrees (like the 097). Naturally, the shorter this number, the faster the ramp. Some aftermarket flat tappet cams, like Comp's "High Energy" series, have deltas in the low to mid thirties; very fast ramps for a flat tappet cam. Some special race "cheater" cams have deltas in the high twenties, but, of course, these cams are not warranted, and are usually changed after every race.

Because of differences in ramp rates, and how it affects cylinder pressure, there are different durations used as the point where "closure" is considered to have taken place. Some programs use J604D and others use 0.050" plus 15 degrees, others use J604D plus 20 degrees, and there may be more. This is why I said that the "quick chart" in the other post should be used as a relative, not absolute measure. For that chart, I used J604D as the closure point for DCR.

One thing that you probably noticed from that "quick chart", is how little margin was left for detonation resistance in the design of the 327 with 097 cam. It's no wonder that there were detonation complaints from customers owning 1962-63 L76/L84 engines.