Compression Ratio explained by Scott The Viking

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Bugfuel
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Post by Bugfuel » Thu Aug 02, 2007 5:53 pm

Piledriver wrote:Sorry, I thought the acronym would be clear..

DCR would be Dynamic Compression Ratio, and what I said was in context. as it includes valve timing.

This proves one of the points raised in the discussion before... people are not used to talking about Dynamic CR...

I totally missed it and thought you were talking about Static, like 99.9% of people, which is the de facto standard!!

LOL sorry, my bad.

Things would be easier and there'd be less chance for misunderstandings (like this), if people would stick to just static CR. For most, it is difficult to calculate or measure accurate Dynamic CR.

My recommendations above were for STATIC CR.

Jan

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Post by Piledriver » Fri Aug 03, 2007 1:38 am

My goal is to get them to STOP discussing static CR as it is an essentially meaningless number.

The motor is oblivious to it, so why should we care? :D
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Post by Bugfuel » Fri Aug 03, 2007 6:42 am

Piledriver wrote:My goal is to get them to STOP discussing static CR as it is an essentially meaningless number.

The motor is oblivious to it, so why should we care? :D

You will never get there, the static CR is such an established standard across all the car enthusiasts and engine builders that you'd only be hitting your head against a stone-age wall.... :)
(and creating confusion).

Save the DCR discussion to the secluded, private, invitation-only, smoky backrooms where only people who truly understand these issues hang out. Most US builders seem to be way off when CR and cam relations are concerned anyway.

And please do make it clear which one you mean by spelling it out: DYNAMIC CR.

It was way too easy for me to miss the DCR acronym..... since you practically never see it used.

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Post by scott the viking » Fri Aug 03, 2007 11:45 am

This really IS a great topic. I enjoy a good sensible discussion about topics such as this...It is (and will be) very informative when other need to know the facts of something.

Now...I have been holding off on turning this into a bunch of gearhead jargon that only a handful of us will understand....because that just confuses and discourages people. However, the topic is kind of on a runaway course toward the feeling that static compression means absolutely nothing and is just a term uninformed people use. I know that is a fun way to think of it...because most of us want to be the guy who is smarter and thinks "outside the box". Unfortunately, it's not an entirely true statement...So prepare yourself for some gearhead math...I will try and make this as readable as possible.

As mentioned in previous posts, the dynamic compression ratio changes with engine RPM and is VERY difficult to measure for your average guy. Yeah, you can get in the ballpark. Here's the thing...you CAN get the dynamic compression ratio in the same vicinity as the static compression ratio. It's really all about Volumetric efficiency. As most of us know, the timing on a camshaft is set up to work on sort of "supercharging" the air/fuel into the cylinder. It works on inertia. Now...Lets say your are able to cram your cylinder full to it's capacity with air, like 100% full, that would be what we call 100% volumetric efficiency. This and even higher than this, are common place in well built turbo charges engines. So when an engine reaches full Volumetric efficiency (can happen) then the static compression ratio absolutely has meaning. I'll tell you another spot where the static compression ratio has meaning, and that is the expansion ratio. The expansion ratio is what is taking place after the piston has reached the top and the lit air and fuel is expanding and pushing the piston down. The expansion rate and the static compression ratio are equal to one another. If your engine has a higher efficiency ratio, The job of the air fuel mix is done sooner, this means you can open the exhaust valve sooner. Opening the exhaust valve sooner will aid in reducing pumping losses. What does that mean? Well...since the expansion ratio is important and has meaning, then the static compression ratio has meaning just the same.

Now, about the dynamic compression ratio that many refer to just the same as others refer to static. What they are really referring to is something we call "effective compression ratio". This is the compression ratio BEFORE the inertia and supercharging effect of the cam kicks in at different RPMs.

When I read and hear talk of guys claiming to have (X) amount of a dynamic compression ratio on their engine, I feel that they are just as misinformed as those that claim static compression ratio numbers. The fact remains that the dynamic compression ratio of an engine operating at 2500 RPM is going to have a different dynamic compression ratio as the same engine operation at 6000 RPM. The engine WILL hit it's top Dynamic compression ratio somewhere in the RPM range and this is just due to it's mechanical design.

That being said...I will say this. It is absolutely a fact that you can run pump fuel on a high static compression ratio IF the dynamic compression ratio is kept at a decent level. BUT...when you have an engine that is capable of operation at 100% volumetric efficiency, all that goes right out the window. The formula given is VERY extreme egghead stuff and would really have no meaning to guys who aren't Steven Hawking, So that will be skipped.

When you go to build an engine, you will have to make a decision on what you want your engine to do. How and where you want the torque, and what kind of torque curve and where you want the horsepower at. My point is, if you build an engine with a high static compression ratio and then attempt to cover up the high static ratio with a big cam (lower dynamic compression ratio) then I personally feel that you have just wasted your time. In example...if a guy was to build an engine with a 17 to 1 Static and then lowered that number with a cam...Sure...it CAN be done...but what if that camshaft does not run in the rpm range that he wanted it to...but he really did not have much choice in the matter DUE to the extremely high Static compression ratio.

You guys make a VERY good point about dynamic compression ratio and for most cases it's a good rule of thumb. The only reason I thought I should chime in about it is that I was concerned that someone new to the engine building scene would read it and build something that did not run at ALL the way he had originally anticipated because he did not understand the trade offs that go along with trying to hit the right Dynamic compression ratio by choosing a cam that is not fitting for him. In other words, (and something you guys already know) build it from the cam, not the pistons. This will work out the way you want it if you chose the right camshaft for you.

So, dynamic vs. static is really not as cut and dried as most think.

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Post by Piledriver » Fri Aug 03, 2007 1:58 pm

Everything is a compromise.
Always.

One reason one would want a really long/late intake event+high static CR and a short ex event would be fuel economy, in NA form, Miller Cycle is called Atkinson Cycle, and you'll find it in a Prius etc...

No bottom end, but reduced pumping losses...better efficiency.

The good "rules of thumb" and things like that calculator exist to hopefully prevent the (probably frequent) event that someone builds a motor with a big, bad cam and wonders why the motor runs like a slug, and is hard/impossible to tune.

There are quite a few "rules" out there, exhaust sizing, carb sizing, cam timing vs powerband, probably others.

Would be good to have them all in one place for new folks, and for the old folk to argue over ;-)
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Post by Bugfuel » Fri Aug 03, 2007 8:12 pm

I'll say this for the less experienced:

You don't start by deciding a CR. It means absolutely nothing and tells you nothing about the nature or power potential of the engine. It is just a useless number out of context. You may hear people boasting with 11:1 CR, and go all 'ooh, aah' about it... but it is entirely possible that for the engine in question, it is not nearly enough, and makes it run like crap.
(like it would in the 13:1 CR engines I've built).

You need more variables in the discussion.


Plan out the entire engine on paper first, and start with:

What do I want the engine to do?

What is the nature of the beast that you are looking to build, what is the intended purpose? Are you willing to sacrifice longevity and comfort on the altar of horsepower? Race or street? How narrow are you willing to let the safety margins go, without losing sleep? What if you go out on a road trip and are running almost empty, and for some reason the nearest gas stations don't sell the high octane fuel you optimized your weekend warrior for?


The cam has a central role in the whole thing, but it too has dependencies to other parts. You could start your plans by choosing the cam first. To me it determines the nature of the engine. Make your cam choice based on what you want out of the engine, and it will limit or dictate your choices of other parts and things like CR.

Sure, there's a huge difefrence between 1200cc and 2200cc, everything else being equal. But with a stock cam, it's still a mild mannered slug, compared to what it could be. In SOME cases though, that might be what you want. (Large displacement for increased low and mid range torque for a parts hauler & trailer tow vehicle)

A 'big' cam generally makes the engine lose some low end power, they pretty much all move the powerband higher up on the rpm scale. If you choose a huge cam for a small engine, you may be disappointed with the low end performance. The same cam in a 2000cc and bigger engines would work much better, since there is so much more 'natural' low end torque to sacrifice. So displacement is also linked to cam choice somewhat.


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Post by Piledriver » Fri Aug 03, 2007 11:41 pm

Very well said!

The other thing that gets people "in trouble" is big, bad heads/ports, and or NOT having big/bad heads to match the cam//powerband, and the biggest killer of all..

Undrivable powerband...

The biggest enemy of a first time engine builder is wanting that "killer" motor NOW and simply not grasping that an engine is a complex SYSTEM...

The heads/cam/CR/intake/carbs/exhaust ALL MUST PLAY WELL TOGETHER in the powerband you NEED, and realistically, that's going to be far lower/wider RPM band than you want for the "sacrifice to the altar of HP" unless the car is a 100% trailer queen.

(Note that the gearing/tire dia. plays a role here too)
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Post by jdavis » Sun Apr 06, 2008 10:08 am

High compression causes a change in flame speed. Increased flame speed increases combustion pressure. yields more power but also more heat. quenching remains the same so head runs hotter. There is no change of state (vapor to liquid) to absorb heat after combustion.

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Post by Piledriver » Sun Apr 06, 2008 1:29 pm

jdavis wrote:High compression causes a change in flame speed. Increased flame speed increases combustion pressure. yields more power but also more heat. quenching remains the same so head runs hotter. There is no change of state (vapor to liquid) to absorb heat after combustion.
But it's not a simple relationship.

A tight deck (squish) allows a higher CR by providing better mixing/more turbulence. The increased turbulence and higher compression result in an efficient, faster, hotter burn, which MUST logically start later in the cycle (needs far less advance).

...All of which results in less negative work, more power, and frequently a COOLER running engine if the planets are all aligned.
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Post by Ford428CJ » Tue Oct 27, 2009 7:40 pm

jdavis wrote:High compression causes a change in flame speed. Increased flame speed increases combustion pressure. yields more power but also more heat. quenching remains the same so head runs hotter. There is no change of state (vapor to liquid) to absorb heat after combustion.

Not quit true. You would have to have a slower Flame travel on a higher C/R. The reason being, the higher octane fuel (because of the compression ratio)! It burns slower. Also you would have to advance your timing as well (starting the burn earlier). JMHO

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Re: Compression Ratio explained by Scott The Viking

Post by Sgt_Smoke » Tue Apr 27, 2010 4:55 pm

Hey guys, I am new to this board I have never rebuilt a engine before but have my stock 1600dp torn apart. The reason i can see that when you increase your c/r that the temps goes up is because you are "NOT" using the same input of fuel/air, my reasoning,
I build a fire with 10 logs i get a fire giving out so many btu i build a fire with 11 logs and i get a higher btu rating, the higher btu's contribute to the higher temps. 10/1 or 11/1 when i increase my cr in an engine the ability to push for air/fuel is higher adding more fuel to the combustion chamber meaning bigger boom. even thought it is more efficient it is still higher temps. WOW i dont think i can dumb it down any more.

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Re: Compression Ratio explained by Scott The Viking

Post by Piledriver » Tue Apr 27, 2010 6:32 pm

Higher CR==slower burn: Bloody Unlikely.
Higher CR==higher temps: Not necessarily.

A higher CR typically involves a proper squish area, in any case, you have the same air/fuel mix in a smaller area, usually with more turbulence.
This results in a faster burn, requiring LESS timing, sometimes much less.

CR vs head temps are also along the same line: How fast it burns (and how much power you get for your fuel vs. heat) and when it burns can have a fairly profound effect on CHT, esp on aircooled engines.

Jake Raby once said +/-2 degrees of timing off optimal can be worth 50F CHT, (higher OR lower) and I have found that to be about right.

Wanna see a hot pig? 6.5:1 semi hemi chambers (no squish) needs ~40+ degrees of timing at 3500.
Slow inefficient burn==hot heads and a slug on wheels off boost.
(on boost squish matters less, as you have enough turbulence)

Too much of any good thing can be bad though.
(Unless she's ~19)
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Re: Compression Ratio explained by Scott The Viking

Post by Mase_00_7 » Fri May 28, 2010 4:17 pm

VERY interesting thread.

I am currently building an 1800 T4 motor and am umm'ing and arr'ing over what sort of compression ratio I should go for.

I am using stock 32-34 PDSIT's, stock exhaust/heat exchangers but have gone for a Web Cam with *i think* 270 degrees duration and 0.423" of lift (234 degrees at 0.050").

The engine will be going in a 71 Westy that will be fully loaded for 6 months touring around Europe so I will need it to run cool and reliable.

Considering the 93mm bore, 66mm stroke and 0.063" of deck (7.8:1 compression) does this sound right for plenty of low down grunt and decent CHT's? Jets on the carb are 135 mains, 155 air correction and 50 pilots.

Cheers, Paul. :mrgreen:

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Re: Compression Ratio explained by Scott The Viking

Post by Piledriver » Fri May 28, 2010 4:48 pm

Mase_00_7 wrote:VERY interesting thread.

I am currently building an 1800 T4 motor and am umm'ing and arr'ing over what sort of compression ratio I should go for.

I am using stock 32-34 PDSIT's, stock exhaust/heat exchangers but have gone for a Web Cam with *i think* 270 degrees duration and 0.423" of lift (234 degrees at 0.050").

The engine will be going in a 71 Westy that will be fully loaded for 6 months touring around Europe so I will need it to run cool and reliable.

Considering the 93mm bore, 66mm stroke and 0.063" of deck (7.8:1 compression) does this sound right for plenty of low down grunt and decent CHT's? Jets on the carb are 135 mains, 155 air correction and 50 pilots.

Cheers, Paul. :mrgreen:
Pull any head gaskets or base shims and recheck your deck... .063" is (IMHO) about 50% too much.
(make sure you check fully torqued etc...Even if you can get it to ~.050 it's an improvement.)

~8-8.5:1 on a Web 86 is about right.
Note: A given cam seems "bigger" in a smaller motor.
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Re: Compression Ratio explained by Scott The Viking

Post by fusername » Fri May 28, 2010 10:24 pm

8-8.5 for a bus sounds a bit much to me, or am I missing something on this?
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