Compression Ratio explained by Scott The Viking

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drmiller101
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Re: Compression Ratio explained by Scott The Viking

Post by drmiller101 » Sat May 29, 2010 9:38 pm

i've been reading about DCR on the bug forums, and it has been puzzling me. There sure seems like a LOT of really bright folks arguing about something which makes not much sense to me....

Compresssion causes more heat for 2 reasons. One was mentioned above - when you raise the compression ratio, you are effectively stuffing the same amount of swept volume into a smaller volume, and you get more heat and more pressure due to physics described by the Ideal Gas law. So, if you go from 8:1 up to 9:1, you will have 10 percent more heat and pressure from compressing the charge 10 percent more.

But wait. There's more.

When the sparky goes off, it will ignite the charge. Because there is more heat and pressure, the mixture WILL ignite faster. Another way to say this is the flame front will spread more quickly, and all the little fuel molecules will catch fire more quickly.

When the fuel ignites quicker, it causes even more pressure. When all done, the average temperature and pressure inside of the combustion chamber will be substantially higher. A higher temperature means more heat will get transferred to the head.

Further, because more of the fuel was ignited quicker, it is actually hotter inside the combustion chamber longer. After all, for a given RPM, the piston is moving at a fixed speed. If you light off a hotter fire and have it burn quicker, there is more time for heat to transfer to the head.

The good news is a higher, earlier pressure means more power.

So why not always run more compression? Eventually you run into detonation. Detonation is when the fuel air mixture ignites before the spark tells it to ignite.

Well, actually, this is not quite true. What is actually happening is there are many many types of carbon fuel molecules in your combustion chamber. Some of them light at low pressure/temperatures then others. Also, there are hotter spots in the combustion chamber.

If we let the fuel sit around the hot spots too long, eventually they get enough of the "lighter" carbon chains to ignite some heavier chains, and a chain reaction takes off, and the engine Knocks.

Modern engines count on a lot of turbulence to keep the mixture moving quickly around the hottest spots so the carbon chains don't start feeding off each other. If one molecule ignites, then gets blown into the next area, no big deal, as it gets "blown out" by getting cooled off in another area of the chamber. If you want to read more about my "molecules" read about free

Also, modern engines use tuned intakes and exhaust to manage the intake and exhaust flows. At some RPM the engine will be most efficient, and at this RPM your dynamic compression ratio will be the highest. The cam shaft, intake design, exhaust design, valve shrouding, and the RPM will all determine when that RPM happens.

I think the max DCR will happen at peak torque.
However, at peak torque, your RPM is high enough the light carbon molecules don't have much time to ignite, so peak torque probably is not when you have knock issues.

I can sit here and talk theory with the best of them, but when all done, the old farts who have been building race engines and street engines for many years KNOW what works, and what blew up.

I have nothing but respect for those who REALLY know what works and what doesn't. My post is nothing but theoretical guesses as to WHY some things work and some don't.

For more reading, read about free radicals detonation squish

http://www.motorcycle.com/how-to/wrench ... -3420.html

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Piledriver
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Re: Compression Ratio explained by Scott The Viking

Post by Piledriver » Sat May 29, 2010 10:42 pm

Excellent post..

What some of are trying to say (indirectly) that too LITTLE compression and/or a huge deck (no squish) can turn an engine into a hot runner that doesn't produce any power, anywhere, and sucks gas to boot. (not to mention hard>impossible to get a clean tune NA)

In other words... Higher CR=higher CHT is not a linear, or even necessarily true generalization.

A 103x74mm T4 motor I have been piecing together ended up having the dreaded "semi-hemi" cut chambers... But the custom JEs cone shaped domes match that perfectly for plenty of squish in a saving move.
A dish in the domes match the remaining "chamber" area... and I have even more squish area that I would have with flattops/flat chambers, and any squish is all "aimed" right at the center of mass of the now very compact combustion chamber, just off the plug tip.
(~1/2 of the effective chamber volume now resides in the piston)

Google for tapered squish...

A higher CR/higher squish engines NEEDS less advance---sometimes MUCH less... a result of this is that the chamber will see higher pressures and indeed, higher temps, but for a significantly shorter time, and properly during the expansion phase (not all in the chamber), rather than an extended period of the compression stroke, in what unintentionally works out to be a lame attempt to "fake" a higher CR with more advance.

Turbulence+more CR=Less advance needed>faster,later burn ...also gives far less time for free radical shenanigans.

On boost, you have plenty of turbulence, and the effective CR is much higher, so one must look for a happy medium...

Everything in moderation etc.

Note:
Oddly, some really big bore motors w/very high levels of squish appear to have a tendency to not respond "right" to tuning in another sense...
...as in almost utter failure to give a damn what you do--- they just run how they run, timing or jetting make little >no difference (within reason)

Modern production engines typically use a very tight deck, as much squish as practical, and as high a CR as possible for efficiency reasons as well as emissions reasons.
Modern STOCK HP/CI in ratios would have been considered bloody unlikely>impossible 30 years ago.

There is much to be learned from modern engines, some of it even applies well to our old ACVWs.
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fasty67
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Re: Compression Ratio explained by Scott The Viking

Post by fasty67 » Sat Dec 10, 2011 1:55 am

COMPRESSION RATIO CALCULATOR
Static and Dynamic Compression Ratio
(Considers Cam Timing and Rod Ratio)


ENTER YOUR DATA CALCULATED DATA
Cylinder Head Volume (cc) Cylinder Head Vol (cubic in.) 3.050 I entered 50cc
Piston Head Volume (cc) Piston Head Vol (cubic in.) 0.000 I assume this is for domed or dished pistons
Gasket Thickness (in.) Swept Volume (cubic in.) 29.177 I put in 0. wouldn't change with a variety of #'s tried
Gasket Bore (in.) T.D.C. Volume (cubic in.) 3.555
Cylinder Bore Diameter (in.) Gasket Volume (cubic in.) 0.000
Deck Clearance (in.)Note: Neg. nubmer above deck, Pos. number below deck Deck Volume (cubic in.) 0.505
Stroke (in.) STATIC COMPRESSION RATIO 9.207
OPTIONAL DATA
Rod Length (in.) Adjusted Stroke (in.) 1.897
Intake Closing Point (degrees)ABDC @ 0.050 lift plus 15 degrees DYNAMIC EFFECTIVE COMPRESSION RATIO 6.988

This is a copy/paste from the link before to the calculator http://www.kb-silvolite.com/calc.php?action=comp
It looks funny here but the calculator is sweet!

I just threw a few estimated #'s, by forum research, that seemed to be average
Anybody still looking at this thread? It does have a ton of good arguments
Who knew to add in rod ratio? I'm not sure if that argument got brought up or settled here.
If it ain't broke, don't fix it! If it is, upgrade it!

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Piledriver
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Re: Compression Ratio explained by Scott The Viking

Post by Piledriver » Sat Dec 10, 2011 2:42 am

fusername wrote:8-8.5 for a bus sounds a bit much to me, or am I missing something on this?
8.5:1 static...
With a Web73, it would probably be iffy in a Bus w/o a really tight deck, but that tight deck would probably be required to make that static CR, and is likely to to mitigate the heat issue if the builder/tuner finds the right timing for the motor he has instead of using the numbers in the book for a stock engine he no longer has.

With a Web86, it's about right, or at least a good starting place..
...
With an 86b@112, it would barely be enough compression to run. :lol:
Who knew to add in rod ratio? I'm not sure if that argument got brought up or settled here.
It was "settled" decades ago, rod ratio has well known effects.

A link to the KB Silvolite calculator is on page 3 of this thread.
There's some good reading on the KB website.
I, for one, regularly embrace our new robot overlords, as I am the guy fixing the robots...

builtbychevy
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Re: Compression Ratio explained by Scott The Viking

Post by builtbychevy » Wed Mar 27, 2013 8:54 pm

A nitrous engine we can achieve that 3000 psi either with a relatively small amount of nitrous and a lot of compression or a lot of nitrous and a lot less compression. Let us assume we can go either way without any combustion difficulties. If this is the case then there is a clear cut argument for going with a low CR and a lot of nitrous. And it goes like this: The higher the expansion ratio (the downward stroke equivalent to the compression ratio) the faster the cylinder pressure decays. To make the point I will take two extreme compression ratios (and thereby expansion ratio's) – these are 15/1 and 2/1. If the 15/1 cylinder has 3000 psi at TDC (or there abouts) that 3000 psi has decayed to about 300 psi by the time the piston has traveled half way down the bore. So there is only 300 psi times the piston area pushing the crank around. The same scenario for the 2/1 cylinder is that from the starting point of 3000 psi it has only decayed to 1350 psi by the time the piston is half way down the bore. This means there is a lot more force there to turn the crank and the average torque throughout the expansion stroke is about 400% more than with the 15/1 cylinder.
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builtbychevy
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Re: Compression Ratio explained by Scott The Viking

Post by builtbychevy » Wed Mar 27, 2013 9:06 pm

Because our fuels are junk. They are low quality and have tons of extra stuff that we don't want because they are soo hard to burn and ignite. This is also why its best to use the highest quality fuel filter. Contaminations cause hot spots, cold spots, and interrupt the fuel burn pattern. A two valve engine you want the burn to swirl not tumble. That is why a nitro & methanol fuel mix are soo effective and allow us to use higher Cr ratios is because they burn faster, cleaner, and I suppose you may say cooler even though you do have to use it in a 2:1 volume compared to gasoline. But vapor before ignition is extremely important.

Of course a fuel filter or high quality components don't matter on a cheapo budget build that wasn't designed to live over 40,000 miles any ways...
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Re: Compression Ratio explained by Scott The Viking

Post by Steve Arndt » Thu Mar 28, 2013 10:16 am

Great, the Chevy guy from samba is here... :p

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RHough
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Re: Compression Ratio explained by Scott The Viking

Post by RHough » Wed May 15, 2013 6:42 pm

Available for Android:

Compression Calculator

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