Using my secret brain, here's my two cents on static vs dynamic
compression ratios -
Both of the above measure the compression of an engine but use different
methods one ideal, one realistic.
Static Compression Ratio is a mathematical measurement where you measure
and compare the volume above the piston at TDC & BDC using a graduated
cylinder and a burrette.
Dynamic Compression Ratio also measures the compression ratio but under
"real" conditions. Dynamic takes into account may variables such as
camshaft lift & duration, type of induction... etc. Therefore, how
efficently the cylinders are being filled with AFM as the motor is running.
Basically Dynamic Compression Ratio is in fact the real compression
ratio - what is really happening compression-wise as you drive your Tr6
happily down the street!!
I hope my Can. .02 cents makes some sense!!
Aivars Berzins
Jim Swarthout wrote:
>The list is nearly complete
>
>If any of you on the current list below have participated just for the
>enjoyment of the Q&A session thats great! I have enjoyed the answers
>and learned some things too! If you really dont want to participate in
>the Roller Rocker installplease say somaybe Ill post a few new
>questions and open it up a little. I would never claim to know it
>allnobody does, but I can come up with some things to think about.
>
>Many people have asked.why do it?
>
>Well think about it!
>
>If I tell you that I have RRs for sale and they are the best thing
>since sliced breadwill you believe me of course not. It is already
>the general consensus that they dont really make a big difference, (all
>on their own), in performance.
>
>If a dozen unbiased TR-6/250 owners tell you they work great youll be
>more inclined to believe it.
>
>
>Current standings:
>
>Stephen Hanselman
>#1) There were 2 reasons why electronic ignition was developed? Both
>answers pertain to the consumer, or auto owner.
>
>In my mind electronic ignition was developed first to get a hotter
>spark. My dad put together one back in '67 or '68 for our Plymouth. He
>found one problem in the spark coil though, the fire was so "hot" it
>would arc over the top of the spark coil. Now days, driven by the left
>coast again, emission requirements are so tight that pulse to pulse
>timing is a major factor in controlling what comes out the tail pipe.
>This, of course, has no relation to any increase/decrease of power out
>of the engine.
>
>Alan Salvatore
>#2)A hydraulic cam is supposed to be quieter running and have less
>maintenance then solid lifters, (the manuel tappet adjustments required
>by stock tr6's)i have read that they are supposed to deliver power over
>a wider rev range and maintain idle quality. Too bad they don't make
>valve covers out of lexan. lol Al Salvatore 76tr6 daily
>
>
>Jamie Palmer
>#3) What does an air pump, (smog pump), do? Explain the entire
>cycle...peripheral equipment!
>
>Pumps in air into the exhaust stream right at the cylinder head,
>theoretically allowing unburned hydrocarbons to combust while in the
>exhaust stream.
>
>#4) Why will roller rockers of an equal ratio open the valves further?
>
>
>Vance Navarrette
>#5) What performance gains will you achieve by installing a long
>duration...(extended period of valve opening)..., high lift, camshaft
>in an otherwise stock engine?
>
>As they say, "It depends". All other things being equal, this is true.
>But if the cam is *TOO* big, you will end up with less power everywhere
>in the RPM range, because the rest of the motor cannot keep up with the
>big camshaft's demands at high RPM.
> Also, if the lobe centers are closer together, a longer duration
>cam might give more power *LOWER* in the RPM range than the stock cam.
>Lift and duration are not the only cam parameters that affect amount of
>power and it's location in the RPM range. Two other very important
>factors are lobe center spacing (in degrees) and valve acceleration (in
>inches/degree**2).
> The above is why it is simply impossible to compare cam timing
>and draw conclusions about which is "best" except in a very broad sense.
>The cam manufacturers do not give enough information about their
>profiles to make precise comparisons. You cannot even compare the rated
>durations because there is no standard method for measuring them. The
>only values you can compare are net lobe lift and duration at a known
>lobe lift (most often 0.050"), and many cam makers will not even give
>you that much information!
>
>
>
>Jeff Dewey/John
>
>Jeff Dewey
>#6) Why does an early Ferrari have a "true" 5-speed gearbox, but a 2003
>
>
>
>>Toyota does not?
>>
>>
>The early Ferrari 5-speed gearboxes had a 5th gear with a 1:1 final
>ratio.
>Late model production gearboxes could best be described as 4-speed +
>overdrive, ie, 4th gear is the 1:1 ratio and 5th is an o/d ratio.
>
>John
>I'd say the Toyota doesn't really have a 5-speed because the 5th ratio
>is an overdrive so it's a 4-speed with overdrive. I'm sure the ferarri
>5th gear is 1:1, and the lower ratios are nicely spaced for performance.
>
>#7) Suppose you have two 152ci TR engines side-by-side; same, make,
>model. One engine has a compression ratio of 12:1 with a static
>compression of 90. The other engine has a compression ratio of 7.5:1
>with a static compression of 200. Which engine would you want in your
>car? Why?
>
>#8) Is compression ratio related to static compression? Yes or No, and
>why?
>
>Shane Ingate
>#9) You're speedometer shows 60MPH. You're Tachometer shows 3000 RPM's.
>You're in 4th gear at 1:1 with a final drive ratio of 4:10. What is
>the outside diameter of your tires?
>
>27.56"? (I can only remember PI to 3 significant figures). If that9s
>the case, this is pretty close to a 215/70-15 tire.
>
>Erik Quackenbush
>#10) Casting is the art of pouring molten metal (hot!) into a mold to
>form a shape. Forging is the art of banging on a solid (usually hot)
>piece of metal to bend it into shape. Machining is the art of using a
>cutting tool (usually with a lathe or milling machine) to trim a piece
>of metal to shape. In general, forged parts are stronger than cast parts
>but they cost more to make. Forging is a good way to make connecting
>rods. Parts with internal passages are usually. Casting is a good way to
>make cylinder heads. Parts that are JUST machined (carved from a billet
>of solid metal) are the most expensive to make but not the strongest.
>Machining is a good way to make ONE of something. In real life most cast
>and forged parts have surfaces that that must be machined to make them
>useful (bores, decks, journals, etc.)
>
>
>Terry Geiger
>#11) How are engine firing orders determined by the engineers? Is it
>rhyme or reason?
>
>Firing order is set up to that the engine is "load balanced" on the
>crankshaft
>so that vibration and stress on the cranshaft is minimized.
>
>Easiest to understand example is a 4 cylinder engine:
>
>Cylinders 1 and 4 are 180 degrees from cylinder 2 and 3. When 1 and 4
>are at
>TDC, 2 and 3 are at the bottom of their stroke. For this arrangement to
>work
>the firing order would be 1-3-4-2 which would alternate the power
>strokes to
>where if 1 is in a power stroke 3 is in a combustion stroke, if 2 is in
>a power
>stroke 4 is in combustion and so on so that anytime 1 or 4 is in a power
>stroke
>then an opposing cylinder (cylinder 2 or 3) is simultaneously in a
>combustion
>stroke.
>
>
>Hugh Fader, Mark Hooper
>#12) If the piston to be fired is approaching compression...(15 degrees
>BTDC)... and you have the timing set to fire at 10 degrees Before
>TDC...why doesn't the piston travel back down...moving in the direction
>from which it just came...?
>
>First of all, the piston is not approaching compression. Compression is
>nearly completed at this point. The simple answer is that the force on
>the top of the piston is less than the force on the bottom.
>
>Forces on the bottom of the piston are: crankcase pressure acting on the
>piston bottom, inertia from the spinning crank transmitted through the
>connecting rod, and in a multi-cylinder, forces from other pistons on
>their expansion strokes transmitted through the crank and rod. Oh, I
>almost forgot the inertia of the piston itself.
>
>Forces on the top of the piston are: pressure due to the compressed gas
>acting on the piston top and pressure due to combustion.
>
>Now, in an optimally timed combustion event (MBT), peak cylinder
>pressure occurs about 12-14 degrees after top dead center. The
>combustion pressure has yet to even get started developing at ignition.
>So the force due to combustion cannot overcome the other forces at this
>point.
>
>Hugh Fader's answer while correct is limited in that he omits to mention
>that although ignition of the gases is almost instantaneous at the spark
>point, the flame front still has to propogate thoughout the compressed
>gases and then raise their temperature to the point where they press
>against the piston (and cylinder walls/head of course) with the force
>required to drive the overall engine. It is this propogation that
>actually chews up the time and requires the preset of the ignition. Once
>uniformly ignited, a correctly designed engine can have a much smoother
>action by having the gases continue to burn and heat well after TDC and
>with correct engine sizing, maintain anconstant cylinder pressure even
>though the piston movement is changing the cylinder volume in a
>non-linear fashion due to the eccentric crank movement.
>
>
>Thanks to those of you who have participated!
>
>If I have left anybody out...please let me know.
>
>I realize that many of you answered all of the questions...I simply
>chose one to attach to your name.
>
>3 more to go...
>
>4,7,8 remain...they have been inserted above...for those who do not have
>the original post.
>
>Don't copy/paste I'll know.
>
>Jim
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