Piston Speed Question/Quiz

[grumpyvette]

lets make this much simpler

http://www.slowgt.com/Calc2.htm#PistSpeed

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http://www.slowgt.com/Calc2.htm#PistSpeed

http://www.axis-engineering.com/Geometry.htm

http://www.geocities.com/rustyatv/sampcalc.html

 

http://www.ucalgary.ca/~csimpson/Tech/Calculations.html

 

http://www.victorylibrary.com/mopar/rod-tech-c.htm

 

http://www.prestage.com/carmath/calc_pistonspeed.asp

 

http://www.rapidline.com/calc/engine/pcpiston.htm

 

http://www.stahlheaders.com/Lit_Rod%20Length.htm

[Guest Anonymous]

This thread is killing me! I started reading and now all I'm doing is searching high and low for my old SCC Mag. There was a great write up about piston speed in the 'Techno-Babble section', and I can't find it anywhere!

 

Untill I read the article, I always thought the piston reached its peak speed when there was a 90 degree angle from the center of the crank to the bottom center of the connecting rod to the top center of the rod...

 

I hope that makes sense...

 

I guess I never gave it much thought, or I would have used more math...

 

Anyways, if you guys can find that article, it gives you the equation for finding the peak speed of the piston as well as where in the stroke the peak speed occurs.

[Kevin Shasteen]

...I always thought the piston reached its peak speed when there was a 90 degree angle from the center of the crank to the bottom center of the connecting rod to the top center of the rod...

 

DD,

 

I believe you are confusing max piston speed for the "Stroke to Rod Ratio" at that 90* angle. The 90* angle relationship, between the crank main journal and the con.rod journal is where, the smarter than I, folks believe that max torque should take place. This is where the micro-annalysis of crank stroke to rod length angle ratio relative to cylinder displacement is so heavily researched.

 

So "Stroke to Rod Ratio" is what you should be diagnosing at that 90* angle and its benefits: providing your ratio is correct for the displacement of your engine's cylinder.

 

Kevin,

(Yea,Still an Inliner)

[Guest POKINATCHA]

Hey guys. I'm a total math junky and I love crunching the numbers on this kind of stuff. I have a question about the relationship between rod/stroke ratio and the static compression of an engine. I have seen writeups that indicate a longer rod will allow for a higher compression ratio in an engine without detonation. My question is, is this a function of the piston's speed in relation to the velocity of the flamefront as it moves across the combustion chamber?

[Guest Anonymous]

Yes POKINATCHA;

 

It appears you have sent all the boys into the woods to collect more sticks.....LOL

 

I too have read an article about the issue you mention, and wondered. The guy should have explained better. I think Grumpyvette has given a URL that covers this one but I do not remember for sure.

 

It seems to me that as it was explained, so it is as you surmise. I would suggest it is a matter of the rod/stroke idea used in combination with the crankshaft rotation in degrees and further coupled with the "Flame Front" as you say. It appears that the longer stroke results in a very slightly slower piston speed in a very narrow band around TDC and this gives the initial power burst of the flamefront a little more time to disperse itself more evenly over the top of the piston as it builds up it's thrusting power.

 

I can see where this would tend to offset some detonation, but how much?.....I beleive Grumpy mentioned that it would be so small in amount that it would really not be worth taking into account under such conditions when actually, correct tuning of the cam or ignition would do the same thing (cunsidering such a small amount).

 

That would bring us back to the original reason for the existence of the R/S ratio in the first place and cause that writers particular comment to have been a "side comment" and therefore in the small change department overall.

 

By the way; how would you explain a flame front appearing in a STATIC compression ratio calculation?

[Guest POKINATCHA]

Well, I actually misspoke when I mentioned the flamefront in conjunction with the static compression ratio. The exact article I was referring to is the same AFR article that I've been whoring around in the small block forum. In said article a small block with a r/s of approxiamately 1.9:1 ran a static compression ratio of 11:1 on 87 octane. This engine fascinates me to no end. I can't seem to wrap my brain around how this was accomplished, and its possible applications in my L-series buildup. As near as I can figure, it was, as has been stated by others, a slower piston speed around TDC that had the primary impact. Obviously the cam, heads, and any other number of factors had an influence, but I've never seen that combination of CR and crummy gas in any engine, so they're doing something right.

[Guest Anonymous]

Ha ha ha Nick!

 

But I absolve you my Son!.....LOL

 

PREAMBLE:

 

I always smell for a rat when I start getting confused and, I usually find one! When it comes to compression on the street, I go by one simple cardinal rule and that is: As detroit builds all their cars to run on USA pump gas and that best is 92-93 octane, I can build my engine for 8.5:1 dynamic compression (more or less 11:1 Static)and not one bit more without additives!

 

If you look in an old motor mmanual at engines built by detroit during the muscle car years, you will find some with (as advertized) 11:1 Static compression ratios (but they don't say Static). So what is this? 11.1 minus 2.37 = 8.73:1 Dynamic. It has to be because they run on pump gas! These detroit guys are also slinging the crap around and have been for many many years; Somtimes to suit themselves and lately to satisfy Congress and pressing smog problems.

 

For example, to appease congress, Detroit at first lowered their advertized horsepower simply by using what they called the "SAE NET" figures as taken fron the rear of the transmission with all accessories on!.....LOL. So in 1972 when this all started, the advertized HP ratings dropped substantually, but (I'm guessing) half of it was in the swap from Gross to Net.

 

BODY:

 

I would suggest to you that when using the Static compression ratio formula, we can surely not exceed a resultant figure of more than about a 2.37 increase in numbers above the ruling 8.5:1 dynamic value as long as we are using only preimum pump gas! Let's see.....That would be about 10.87:1? (almost 11:1).

 

Now, detonation is not at all like pre-ignition.....No, detonation is very subtile. I would say that one could run a Static 12:1 ratio for some time before the dues would have to be paid. One sign of detonation would be unusual overheating. But if the cooling system is very good, this indication may not be present in the dashboard. Nevertheless the excessive heat is in there on top of the piston. Some guys think they have stumbled upon a secret way to extra power; But it is not a secret that too high a compression on pump gas puts the engine into hidden detonation and early engine failure. Eventually, these guys will smoke up the highway during a race and have to be towed back to the garage! But maby they know, and do it anyway to win races an make their buddies scratch their heads.

 

Now wrwp your mind around this Nick: Running 11:1 static on 87 octane is foolish because of hidden detonation and subsequent excessive chamber heat and subsequent short engine life. But look here! 11:1 static (obtained by calculation only) is only around (as previously stated) 8.73:1 Dynamic, and that is where it counts! And so they are alittle high there Dynamic compression-wise and using 87 octane (a little low there), and so they are in shall we say mild/medium detonation? Now what's the big deal about that? Since you know the real skinny now, do what you will.....LOL.

[Kevin Shasteen]

Well, I had a detailed reply but the board would not allow me to enter it; so I will make this quick-dont feel like restating what I attempted earlier.

 

SPIIRIT,

 

You can not obtain a legit DCR by simple subtraction of 2.37 from your SCR. Your ratio is a measured unit divided into another measured unit to obtain a mathematical relationship between the two measured units.

 

The purpose for utilizing the DCR and SCR as tools to building an engine is to allow the user to obtain somekind of an educated guess of cylinder pressures at cranking speeds. The DCR/SCR at cranking speeds tells the builder, once a moderate understanding is gained, what to expect at any other engine speed. Simple addition nor simple subtraction will gain you that understanding nor the correct ratio for that matter. The DCR is obtained, thru calculating the Compression Formula, based on your Cam's Duration...not from simply deducting a 2.37 from your SCR.

 

Dont try to rationalize the math: no short cuts as JUNK IN = JUNK OUT. Remember that the DCR differential between a street engine and a dedicate racer is only .5: Street Engine 8.5 while a dedicated racer is 9.0. If you are simply deducting 2.37 then you have already scewed your .5 On a Small Block this difference could be anything from 15-30cc's on the Total Combustion Chamber side of the Compression Formula; with even larger Total Combustion Chamber cc's difference on a Big Block engine.

 

As far as the 350 Chevy Should Have Built, they started w/a 400 .030 SBC, 327 SBC's stroke, 305 SBC's Cylinder Heads-by AFR which had 190cc Intake Ports, smaller 1.990 Intake Valves and a Dual Plane Intake Manifold along w/a mild 270 Roller Cam. I say mild Roller Cam because a Roller can use more Duration w/out the lope due to its "Roller" design-opens and closes the valve quicker. All this yielded a quicker Airflow Velocity at lower rpms...this equates to better cylinder filling which supplements the Cylinder Pressures. Add this to a piston that dwells at TDC longer due to a longer Rod-and your engine utilizes more of the Combution on the Combustion Stroke.

 

It is the same principle of adding an Intercooler on a Turbo engine. The intercooler makes the air molecule denser-thus more efficient. Same thing is happening on the test engine in the article "350 Engine Chevy Should Have Build"...just on a smaller scale. Better cylinder filling equates to a denser charge, then allow your piston to dwell longer at TDC and your combustion during the combustion cycle will be better utilized for your rpm range-based on the cam duration of your cam profile.

 

Kevin,

(Yea,Still an Inliner)

[Guest Anonymous]

DELETE

[Guest zrl]

 

r = stroke/2

L = rod length

 

O = theta = crank angle (0 at TDC)

w = omega = crank rotational speed (radians/sec)

 

yp (piston position) = r*cos(O) + L*cos[(r/L)*sin(O)]

 

(O) should be: theta = omega * time

 

 

I think this part of the formula is wrong for PP: L*cos[(r/L)*sin(O)]

it should be L*sin{arccos[(r/L)*sin(O)]}.

 

here is mine PP formula:

if we set the piston position at TDC as:

crank angle (o) = 0 (radian)' date='

piston position (z) = 0 (cm)

 

then, the piston position from TDC should be:

 

piston position (z) = [u'][® + (r/2)] -[/u] {[®^2 - (r/2)*sin(o)]^(1/2) + (r/2)*cos(o)}

 

[i use big 'R' for the rod length]

 

if you omit the underlined part, it would only tells you the distance of the piston from center of the crank shaft to the pivot point on the piston. BTW, all length should be measured from the center of the pivot points.

 

if you want piston speed, take the first derivative of the equation. and second derivative for acceleration. (you should know)

[Guest zrl]

I've corrected the formula. [sine & consine were interchanged]