Compression Ratio Explained - Roadkill Extra
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- Опубликовано: 27 окт 2017
- This installment of the Roadkill Extra engine-building series gets a bit theoretical: What is compression ratio, how does it affect engine performance, and how can it be altered? But it’s also very practical, as David Freiburger explains every detail you need to understand in order to use a compression-ratio calculator. Tune in and tune up!
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As you watch, cruise over to Facebook.com/RoadkillShow, follow us, and watch for future posts asking for your questions to be answered on the Q&A sessions that are posted every Friday on Roadkill Extra Powered by Dodge. 
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It is outstanding how today you can learn more effectively from 14 minute video on RUclips than from many classes on an 4 year engineer at University. Freiburger communication skills are flawless. If only half the professors had the natural talent he was gifted.
Motorola Phone you don't learn anything from this honestly static compression don't matter it's dynamic compression and volumetric efficiently
Robert Beene Absolutely. You are right. However I was referring more about ease of reach and accesibility and com skills. Matter of topic i think is relatively subjective tho.
That is true I mean at least he did get the basics
Yep but he was completely right on everything. Obviously smaller cc heads, dome pistons etc give you more compression. He did bring up camshafts influencing compression ratio's so he's aware of it i would say
Yes!
Ladies and gents, I give you David "One Take" Freiburger.
Damn impressive performance right there, sir.
Rip headphone users who turned up the volume in the beginning..
their editing really dropped in quality
Seems like they hired the person who used to upload top gear videos
watching the comment first is a good idea lol thanks!
Thanks for the heads up!
First World problems.
This guy teaches from the heart.very enriching lecture
He has a name
Learnt more about c/r in 13 minutes than i have in my whole life... keep up the how too's, love em 💪
In Kontrol how to's
I think.
In 13 minutes you've been able to explain this subject much much better than my professor did in 1,5 hours of lesson at the university (I'm studying mechanical engineering)! Totally in love with this kind of videos! \m/
One of the best explanations of static compression ratio I’ve ever heard.
im impressed on how good of a teacher he is very easy to follow
I love this dude. I wish my shop teacher was this good.
Freiburger just explained compression ratios far better than my engineering instructors... I'd be down to get some more educationally focused videos from him! He knows his stuff and, more importantly, knows how to explain it.
David is the best automotive teacher I’ve ever heard.
You are my hero! We love you here in the DRC! Thank you so much for the many years of knowledge you have given us!
please do more of these engine explain videos on youtube
harman klop the whole series is on mtod so it probably wont happen
Extremely thorough and accurate explanation! Thanks!
I've been a big fan of David Freiburger since his articles in Car Craft. I was subscribed from 1987 until 2009. It got to where they had done everything there was to be done and just rehashing the same old stuff.
Great explanation. I'm sending this to my grandson since he's a budding gearhead.
That was fantastic. Straight to the point, easy to understand. Cheers from Queensland Australia. 👌
Great stuff Dave! This is stuff I hear all of the time but I never fully understood how it all tied together
Anyone else here because you’re studying the army’s asvab auto & shop portion of the test? After I read a section, I go to RUclips to help me visualize what I just read. It works. ❤ first read and then watch a video to help out your thoughts into visual perspective. I’m 18
These just keep getting better and better
It’d be nice to have dynamic compression ratio explained and how cam shaft lift/duration/lobe separation angle can change the dynamic compression ratio
He spelled it "smallor" because mechanic
What a great explanation on compression ratio calculations. Kudos to the writer(s) for doing such a fantastic job in 13 minutes. And kudos to Freiburger for spitting it out so clearly and quickly. People don't realize how difficult it is to convey so much information is such a short time and be thorough and accurate. I knew how much info needed to be communicated and I was thinking, "There's no way he can do it justice in 13 minutes and change." I stand corrected.
wirenutt57 And there are no “writers.”
Hands down the clearest description!! You sir are what a GOOD teacher looks and sounds like. God bless you!!
I have never worked on an engine before but would like to someday and this was very helpful for someone who knows very little about engines. Thank you.
I know this is very late, but you can get a 70s and 80s v8 engine from a van of truck for stupid cheap. Either a u pull it or a used engine supply facility.
He's good eh!! Top tutorial👍👍💥
After all the year at school i did, Roadkill are the best professor i had yet :D
Hi From Australia, That's a beautiful video showing in straight terms the science. Great work.
Every time he said lower compression all I could think was boost
Yea never mind more we want less!
Strap that turbo on !!
Man what !!!? That’s all I kept thinking too
haha, me too..lol
Cool, explaines it all, keep up the good work 8-)
That was AMAZING! David did that all in one take...and almost all in one breath! Great information and knowledge!
Excellent. The internet needed this. Great contribution to human knowledge.
Very good and informative video!
what you left out that some might not be able to figure out is what the numbers mean in a 10:1, or 9:1 , etc. ratio. The volume of the cylinder at piston TDC is the "1". How many of those that will fit in the cylinder at piston BDC is the 1st number in the ratio. So the higher the ratio the more volume your are squeezing into that "1" in the ratio. More volume means more fuel and air means more energy and power.
james rudolph That's what I was waiting for too. Also, I thought he would give an example of why compression ratios were dropped in the larger engines in the 70's compared to the higher ratio of those same engines in the 60's. Was it to increase fuel economy or decrease emissions or both? And how would lowering compression ratio affect that? Well at least I have a better understanding of HOW compression ratios are determined. I just wish he also covered the WHY'S as well. I know I can pretty much figure it out, but still, if you're going to explain something, explain it all.
A lot of the switch to lower compression was that high octane gas with lead was being fazed out to lower emissions. Lowering compression is a good way to lower emissions because can go with lower emission gasoline. You could get good gas back in the day but it was full of unhealthy gasses. Another main reason was insurance rates got very high in the high HP cars due to so many people smearing themselves all over the road in their hemi cudas. Fuel economy was a small part of the change since all the domestics had a high mileage option or 2 for those worried about it.
james rudolph Awesome. Thanks for the info.
thank you!
Best explanation I've heard in a long while
Watched this video a few times now and still cannot believe you did this in one take 🤘
Jason has a whiteboard, David has cardboard.
Jack Duno. haha... doing it old skool
This is the basic stuff that you can figure out by yourself easily; the size of the combustion chamber affects the CR. Soooo, how about dynamic compression ratio, effective compression ratio, volumetric efficiency and how do different camshaft profiles and the engine RPM come into play? ;)
Audiojack YES......more on this ^!
Audiojack i just posted about the same thing vid is a very very basic model
Agreed! I'm planning a street motor and am wanting great torque, good horsepower, and at the same time decent fuel economy. I want to benefit from the greater dwell time with the longer 6" rod, and less valve over lap for fuel economy. The counter to that is big cam lift and likely Chevy aluminum fast burn heads or similar after market. I want to figure out how much the valve timing and piston dwell at tdc will effect detonation. How far can I push the compression given other extenuating factors.
Joe Robinson Just use a proven hci combo and move foward, what's the big deal?
Freiburger just makes it so simple. Spent an hr scouring googled pages with lengthy reads - this distilled all I needed to know in half the time lol
Finally got a chance to sit down and watch one of these. Good job guys. Very clearly explained.
Will try to watch the rest on MTOD.
"Up to the point of detonation..." And the diesel guys start laughing.
Whats diesel guys
Dr. Love HAHAHAHAHA
@@DrLoverLover me
than the methanol guys proceed to laugh at the diesel guys
Already know about all this just like these dudes videos
Dan Bertholf im sure you do muppet
Dan Bertholf says the skater boy grow up child
Most use episode of Roadkill extra ever.
I really have to say thank you to all of you guys for doing these videos. I know it is a fresh way of keeping up with tech and expanding on what things like magazines used to do , but Just something as simple as this video where a guy that I know and have watched for years explains something that I kind of knew but he does a great job of breaking it down and giving the facts. It is nice to just chill and eat some chow and watch and feel like my knowledge level of the hobby (life) that I love going up. Same for the Nitrous vid and the exhaust vid I just watched. There are things that I am semi knowledgeable about but everything that Roadkill does and Engine masters and extra etc etc really feels like I am there with friends having fun and learning and growing in my craft and hobby. I just hope all of you involved know how much these stupid RUclips videos really mean to all of us. Keeping up with the medium and keeping Hod Rod culture fresh and viable means a lot to all of us.
So if you want displacement and compression get a stroker kit.
Compression ratio is like a night out. 1 night 8 beer......meh. 12beer - 1 now your having fun. 18 beers equals detonation. How those beer effect your night depends on your body size, skinny guy is gonna have a wicked buzz, 300lbs guy is gonna want more beer
Great way to explain it
I'd take the piss, but this was a spot-on metaphor
You are a wise man.
Does that mean whisky is equivalent to nitrous?
I like to run a 150ml shot so that I can keep my beer ratio lower, it makes my tank last longer.
shedboyz21 and tequila is nitrous
This was an excellent episode with very straightforward directions/information delivery. I actually learned something from it, which is a challenge for me because of certain reasons. I would say that something like this is reason enough to pay the $5 a month (half of spotify which is only music) and contains stuff like this. Not only this, but the main episodes are very entertaining as well among with everything else in Motor Trend.
A lot of people may not even think about it but I just want to applaud this man. A perfect one take shot and never making a mistake or just saying... uhhh.. uhhh... every 10 seconds or so. If he taught an online class for engine building id be on that lol. You a legend Freiburger!
I think the newer Mazda engines (skyactive whatnot) have like a crazt high ratio of 14:1 or something like that
Yes but they have a delayed intake valve timing to reduce it artificially.
That's not crazy high.....
Its not really crazy even for a 4 cyl, and its not even crazy high for a race motor lol, in my dads daily driver he ran a 540 ford big block stroker with a 16.5:1 compression, this was 20 years ago and everytime the car started it shot 15 foot flames out of the 4 4" pipes out back, was canadas fastest street legal car
pseudo-Atkinson cycle, Toyota Hybrid cars use it as well.
For a stock regular commute car, it's kinda high
12,5:1 is the compressionratio of a Toyota 86 4cyl boxer. it needs premium gas, but i think having 100hp per liter of displacement is quite decent for a naturally aspirated machine.
My 12:1 Focus makes nearly 200 hp at the crankshaft.
Honda F20C, 11.5.1 and 123.5hp per liter
Yes it is but you’re not going to find many car people that care about how much power is at the crank so that subie boxer is only putting about 200 to the wheels (power to the wheels is the only thing that matters lol)
F20C makes horsepower by spinning to 9,000 rpm. That's not common.
plenty of engines with 160-180hp/L, NA. doesn't make you wrong, though.
edit: to the wheel.
I like the way you teach! Very easy to follow, my 6 yr watches with me, and he's already learned as much as me! Thank you!
Thank you for posting this. Looking into for the first time into doing a project motor for my Camaro, and all of these numbers make my head spin after reading and researching for a while. Going for big cam, 6-71, stroked and forged LS3 block. This expertly and in Lamens terms told me everything I needed to know about the plus or minus CC Rating on pistons. Thank you!
That is one bent connection rod.
Roadkill style.
Oh what a world we live in. This type of information used to be part of the mandatory sixth grade shop class back in the early seventies.
Just makes me wonder what was so important that this was removed from the basic curriculum.
Richard phillips ikr! They don't even teach girls how to cook and clean anymore!
GeebusCripes
Have heard about that too. Somehow back in '72 we were "progressive" enough that all sixth graders had to take half a year of basic shop (how all types of motors work, woodworking, and basic metal smithing) and then a half year of introductory home ec. It's when I learned how to make a French omelet, and the basics of using a sewing machine, along with using a needle and thread. Then from 7-9th we got to pick from either for one class per semester.
With all of this being deleted it's no wonder that we now have college kids so helpless that they can't change their own tires or oil, nor make a meal or do their laundry.
I just don't understand what it has been so important to teach our kids that basic life skills have been deleted.
If it has been social justice and political correctness, they are very poor substitutes and no wonder that the millennial generation has such a bad reputation.
Fortunately I have been able to retire in a rural area of the country and things aren't as crazy as what is seen from California or the other big urban areas. But we still have some kids that can't seem to walk around without a phone in front of their face but fortunately enough of the parents don't let theirs do it that it's still bearable to go out in public.
This type of content is worth signing up to mtod for. Before this I wasn’t interested in signing up but now I’m really considering it. Great work
I am so glad I found this. You really break it down into simple terms.
Kids today are soooo lucky.
Man O War jep
How, exactly.
wel if i whant to know somting i just can search for it
Thefreakyfreek what you didn't have libraries with books in them? Sure it took a little longer but same thing
Demonslayer20111 true and i did i had many info books and i was at the library onse a week as a 10 year old however i i repear electronics for a jop and having evry datasheet and conetor pinout in a second is faster than asking a reseller for a catalogue
i had many from vellman and conrad but it just cant compare
I'm a little sad you didn't talk about why you need to reduce compression ratio on older turbo charged engines.
Mikey R bro do you know why?
Its due to risk of detonation. Older turbo engines can not detect detonation and change timing and fueling to stop it. Less compression =less risk of detonation
Cheers Don Krawallo, so it's the ECU that allows a blown engine to run at higher compression ratios? Thanks for explaining!
And, as a follow up question, can an older engine be fitted with a supercharger or turbo, if you replace the distributor with an ECU?
Mikey R
Yes you can. Engine management harnesses (ECU, sensors, wiring harness) can be had for almost any kind of engine.
My head exploded learning this by books/reading(hobbyist) and you nail it in a 13 mins relaxed viewing. I finally fully get it.Awesome stuff
Blow a head gasket?
hahaha
Too much cranial compression. haha
This is one of the most helpful videos I've found for stuff like this
If you need a home for that project car when you're done, let me know!
And that's not all! There's also the effective compression ratio which is effected by valve timing
He mentioned that, but didn’t go into it for simplicity’s sake. Many people don’t understand the basics he presented.
I don't understand why your show or whatever you call it isn't in prime time tv times, it is more real than most others , and you do show good info in a way that all most anyone can understand and yes that is a great thing
Great presentation Friburger. Thanks 4 explaining.
My girlfriend increases my displacement and compression ratio
Still? After years witth mine, my displacement and compression ratio has decreased. I think i may need to find myse lf a new motor. Too much work to rebuild at this point.
So she's pegging you?
Mats Granqvist 😂😂😂
Yo David do a Chevrolet 4.8 vortec on engine masters
fuzzy wuzzy I was sayin content I wanna see, you're saying content you wanna see.
Chris, Chief Cope clever reply sir
fuzzy wuzzy lol thanks buddy took me a few mins to think of it :P
Same performance as the 5.3 just at 300rpm higher.
Clean Cut, Very Informative, Covers All Gas Engines!
Great Video! Thanks Fri Man!
What a great video. I’m learning about engines and this gave me enough info to be able to understand compression in an engine. I feel informed now! Thanks
For years I've watched Roadkill and Hot Rod Garage spend a fortune on cardboard and Sharpies. I am forced to pay alimony, child support, and pedigreed pet support times 7. Is it possible for me to build a quality car without cardboard or Sharpies, or am I doomed to an existence of drunken wishful thinking?
Doom.
I use stick in dirt method, works gudder enough fer me... geterdone
travel. elect a place to stay 'off-grid'. find a field of work/train to do something people are willing to pay for in cash (AC servicing; fix and sell used cars yourself; etc.). get a fake id (with legit tie-in--birth certificate, SS number if possible). kiss your past life good-bye. begin anew...sans child support, etc. (Or....wait till all those eons of child support obligation pass. ugh.)
Says "smaller" writes "smallor"
Call the police, grammar's been shot.
he just fucked up due to the thicker marker and holding the paper
his brain said "e"....but his hand drew like it was using a normal pen
you can see the start of the center bit of the e their.....but not much
and me being dyslexic.....if ya understand what he ment..... then the point of written language is fulfilled
its all good if you understand the idea
Lmao y'all beefin I'm just saying what he wrote I wasn't making fun of him
Wow. Amazingly explained. Great way to teach beginner gearheads. Cheers.
I couldn't ever ask for more free information. Thank yall so much
My Mazda 3 runs 13:1 on regular pump. Never understood it.
Mazda 3 is not turbo.
Ford Focus runs 12:1.
Mazda 3 runs 13:1.
Mazda CX-5 2.0L runs 14:1 with the long tube header system that doesn't fit under the Mazda 3.
Thats because mazda designed a special extractor for the exhaust
It's a header system that promotes efficient exhaust scavenging.
Lifted_Above i think they also use a different shaped pistion
Mazda uses 14:1. 2014 is when they swapped from 13:1 (pre kodo design)
Audio, guys. Fix it. This is video production 101 stuff.
Excellent video to explain one of the most key concepts of the automotive world.
Mechanical Engineer approved explanation! Well done sir!
Diesel and direct injection reach higher than 12:1. Diesel might go to 20:1
Georg Antonischki we are not talking about diesel here , are we ?
Robin Vanoppen, I prefer more information over less. if this overloads you or others I'm terribly sorry. Fortunately I didn't bring up charging :-)
i have a 3litre turbo diesel its 23:1
That would make sense since diesels use compression to ignite, as opposed to a spark plug
But the new Mazdas are lowering compression for Skyactiv D
You didn't cover a main aspect of compression ratio: the tendency to knock with higher CR. This is an important factor, especially with turbocharged cars. If you reduce the compression ratio on those, you can increase boost pressure. This is something to neglect when talking about naturally aspirated cars (you "kind of" mentioned it with the race fuel), but i feel like you should have mentioned it to a bigger extent.
That is also why turbocharged cars come with lower compression ratio than NA. Also, the compression ratio isn't the only factor which will determine how a turbocharged engine feels, in my belief the lower CR but therefore more possible boost pressure can change the engine characteristics dramatically. In my case, i already have my second engine waiting to get into the car. From 5-cylinder 230hp/320nm 2521ccm 9:0 CR / ~0.85bar boost i will go to ~440hp/6xx nm 2521ccm 8.5:1 / ~1.6bar boost. Without the reduced CR, it would be more complicated to write a software for this boost pressure without knocking, also the new camshafts have more lift and require the additional space the other pistons offer.
CentaX cause is all about v8s, hp, tq, and displacement bruh. Go talk about turbo ricer engines on your honda forums. This is big boy talk.
Throw a turbo on a V8, face the same aspects
The higher the boost, the hotter the intake temps, the more prone to detonation. Also less timing due to higher temps meaning less power per psi of boost utilized. This is why 11.5:1 compression ratio stock Coyote mustangs , make 600 horsepower with minimal boost. For every one full point of compression raised on a N/A engine is also roughly 4% increase in engine power. Therefore, lowering an engines CR to allow higher boost, means your having to also crank up boost to account for the loss in CR statically. Plus more Compression without boost means more torque sooner, expelling spend gases faster , spooling the turbo sooner.
This was an incredibly informative bit. Great job in this one.
Thank you so much for breaking this down for me.
I had to do all my math by hand when figuring this out. We didn't have computers or apps to do it for us! Kids today have it so easy, maybe that's why they feel so entitled.
Robert Ries hahahahaha, spot on
Piston is not at BDC/TDC. Rod is not straight in either drawing.
Ryan Miller it was a conceptual drawing
I'll hazard a guess that he knows it. :D
It's conceptually wrong, ha ha!
Damn literalists.
It isn't necessarily wrong. In some engines the rod is not parallel to the bore centerline at bottom or top dead center. How is this possible? Some engines such as the Flathead V8 have cylinder centerlines that do not intersect the crank centerline. Also many engines have a wrist pin that's offset from the center of the piston and the bore centerline. This makes the rod have a slight angle at bottom and top dead center.
thank you, very simply and clearly explained
You're good at explaining this stuff, Freiburger.
And what most people don't know is most of what he's talking about totally doesn't matter, the static compression only vaguely resembles the running compression. It's the running compression that will determine the power it puts out as well as the grade of gas the engine needs, basically the more efficiently the engine works the better power it has. I had a toyota 4age engine i built that on paper had 10.5-1 comp but the static comp was 215 psi per cyl, with that compression i should have had to run it on at least premium if not 102 octane, but it ran perfectly on the crappy 87 octane we get in cali. Never pinged or knocked and had better power then my buddies 4agze which was stock but supercharged
dposer10 I call BS ! Don't believe it ! FFS! BMP!
Toyota!? Get the heck out of here commie!
Compression ratio and a compression test isn't the same
dposer10 You sure?🤔
Valve timing, people. Static compression means fuck all. It's *effective* compression that matters, and if part of your charge gets pushed out you will have less effective compression (and possibly higher velocity which affects breathing and so on).
“Smallor” Is that how Americans spell it?
KRD_015 no, it's smaller. Are you asking that due to his pronunciation or the comment above?
Just being a smartass lol
Justin M He writes it down at 5:03 .
KRD_015 And he fucks up cylinder as well. I thought he was an editor in chief/ journalist masquerading as a car guy but now I just accept it as Murrica just fuck it up as we go along. You guys can't even deal with the metric system correctly. It's LITRES and METRES people.
But it could be smallor.
Very well explained by Dave, Thanks Dave.
DF just covered a semester of shop class in 13:30. The guy's a G!!!!
The angle of the rods in the drawing triggers me.
BDC and TDC and the rod is on an angle.. should be straight, unwatchable.
Correct. However, due to math and geometry, the vertical piston movement near TDC is very minuscule for large rod swing angles.
Friburger, thanks man. This is something I could never comprehend
Absolutely outstanding!!! Very well explained in simplistic terms.
Nothing I didn't know but so well explained. Thank you!
Freiburger we appreciate your knowledge!!
outstanding way of breaking it down!
Best Roadkill extra yet!
Great information! Thanks for the video. Love the series!
Really good explanation, thanks so much.
Great video! Very impressed that you aren't sweating like a mad man in a 90 degree shop with no fans running!
This is wonderful information and literally I don’t think you could have explained it more clearly tyyyy
I love this show !! You are the best sir I'm green with envy !!! Watch you every time I can !!! Thanks for the no B's info