OK I have deleted all that I posted. it is obvious that I know nothing about it and this way there is no chance of my causing anyone to be confused. Sorry to have cluttered up the thread.
I'm open to a ear full of knowledge about strip and Street engines anytime my friend. The way my old Hot Rod buddy (from Paw Paw Michigan) the late Big John Richardson use to say "there's those that do it right and win races and then there's those that just talk and like to be heard" You my friend have a wealth of information and knowledge from year's of building and playing around with the SBC on the strip and Street. Just write it down I will read it... "The young can be knowledgeable but not always wise... Wisdom comes with Age..." 1Nimrod
At the engine manufacturer I spent my career at, I don’t think I ever heard the term “dynamic compression ratio”. “Effective Compression Ratio” is the more technically correct term. Compression ratio, as described by others, is simply the geometric relationship between swept volume and clearance volume . Effective compression ratio takes valve motion into effect, considering only “trapped” cylinder volume and clearance volume in the calculation. What happens before valves seal as the crankshaft rotates is irrelevant for the effective compression ratio discussion. The valve timing of an aggressive camshaft reduces the “trapped” volume and thus the effective compression ratio. Aggressively crammed engines make up for this reduced effective compression ratio by decreasing the clearance volume (volume above the top ring at TDC), ie increasing the static compression ratio, with reduced chamber volumes, ( pop up pistons come to mind in a gas engine). Effective compression ratio is an important design parameter for things like cold starting on Diesel engines. The other important design parameter is peak firing pressure, which is rarely mentioned outside of engineering design discussions. Peak firing pressure (PFP) is, I suppose, a little like “dynamic compression ” , but it is measured in a running /firing engine and is the culmination of design /geometry and calibration. The engine structure, ie block strength, rod strength, piston strength, gasket integrity.. and so on are all designed with certain parameters in mind. Firing pressure is determined by static / effective compression ratio, combustion chamber design, timing, rate of injection, and boost pressure, among other things.
I'm just a wrench bender shop manual Tune Up kinda guy. Start talking camshafts stuff, forget it, I gotta go laydown and take a nap for a while. But I noticed something, a loose & worn timing chain with "late" valve timing will definitely affect cold starts in a gasoline engine too. Kinda "Lazy". It also retards engine manifold vacuum. Replacement timing set on the Y block, dialed it in where it's spoda be to be, NOW we're talkin'! cold starts were nice and crisp. Vroom!! Compression went from 135/140 to 150 psi.
Makes sense. Ignition timing will also be late unless compensated for beforehand. When I was varying valve timing in the staging lanes, the most hurried part was getting out the timing light and retiming afterward.
You can change the power characteristics of an engine by advancing or retarding the cam. Advance - increases cylinder filling and power at low speeds, retard - increases cylinder filling and power at high speeds. Stock car racers used to use this trick to get a useful power increase at the desired RPM range, "rocking" the torque curve about its midpoint. For better street performance with a hot cam you can advance and retard the cam with offset bushings or keys, test compression and go with the setting that gives the most compression. This will enhance idle and low RPM power.
Right, I "knew" that, but it took me a while to figure out what a vacuum gauge was telling me. Timing chain slop is part of it but so is overall factory tolerances and keyways. The stock camshaft was maybe 6 to 8 degrees retarded all told, something like that. I could get the manifold vacuum into the green by compensating for this late camshaft timing, by advancing the ignition timing, but it had to be way beyond factory spec. I wasn't paying attention to the book. Because that ain't how a Tune Up spec or test is derived. When the book sez '18" to 20" at Idle' what they mean is that this is the spec at say, 3° or 6° BTDC. Not 16° BTDC. See where I'm goin' with that? Like I said, I don't understand any of this shit but the engineers who designed it did, and if something ain't setup according to Hoyle it will not work as advertised.
It is definitely worth verifying "where" a camshaft is installed, even a stock engine with a stock camshaft. It can be way off from the factory or a bungled overhaul, and it sure makes a difference on how it runs and performs. And a lot of engines were purposefully crippled at some point on this matter, and replacement parts sometimes even have these defects ground in. So it pays to check. Shimming up valve springs to achieve even mild stock factory spring height & pressures also helps get a good sharp tune. A lot of stuff sometimes gets overlooked because it is considered to be "racing" tech or maybe too much work, when it is just part of a good basic setup.
You ha e a lot of great advice for old motors that have set for many years. Mine’s set for 30 years. It’s a 331 Hemi. I’ve tuned it up, rebuilt the carb and it seems to run pretty good but the motor seems to miss a bit. I think I’ve got a solid motor but can’t figure out the miss. And it squeals. If it’s idling it’s quiet but driving down the street is embarrassing. I’ve Used soap and belt dressing but nothing has worked so far. I’ve tightened the alternator belt too. Any ideas?
I added 64 cc heads to my 355 SBC with flattops... Compression came out at 9.7-1 My cam wanted 9.5 minimum. Do timing changes need to take place when increasing compression?
They might. Increasing compression can lead to detonation. If it was operating on the edge of detonation before the increase, it might be a problem now and require some changes to the total timing, and/or the advance curve.
The Jalopy Journal
