There are many discussions about compression ratios, but what are the engine components which factor in? And which factor in the most? Heads? Cam? Timing?

And how much does compression matter, on a BBC?

I realize this might be a bit much to explain, here, so any links to videos or sites with this kind of info is appreciated.

 

You forgot the pistons! Not enough compression and you will be driving a fluffy soggy performing car, too much and you will be driving a grenade

 

Compression makes power! However, too much compression and you cant get good enough fuel to use it.
swept displacement to combustion chamber volume is compression ratio.

 

everything from how much the block was decked to how thick the head gasket is to how much meat was removed cleaning up the chambers. and this is after the orig chamber and dome size were set in the first place. now, what you need to know. 9;1 regular gas iron heads. 10;1 premium iron heads. add 1 point for aluminum heads. you lose 3% from 9 to 10. you lose 2% from 10 to 11. on a street driver you will never feel the diff.

 

Just to add to it. The shape of the valve heads and how deep they are in the seats, also how far the top compression ring is placed in the piston higher or lower down from the deck.

 

Just to add to it. The shape of the valve heads and how deep they are in the seats, also how far the top compression ring is placed in the piston higher or lower down from the deck.

 

What effects compression? Anything that changes that changes the amount of "squeeze" in the cylinder from BDC to TDC........but that is just part of it, they call this "measured" or "static" compression. Now to throw a crowbar in the turbine, there are many other factors that determine how much actual charge is in the cylinder before it gets squeezed.......and how much can get squeezed. It takes a lot of seeing it in your head working to describe everything and I know some that have been in the industry for 30 years that still do not have a good grasp on it........
In short, compression is efficiency.......the more charge you squeeze, the more potential energy you have after ignition. A higher compression engine will make more power everywhere on the curve than its twin with lower compression......everywhere, because it has to, it is releasing more energy. BUT, more compression means more heat, and heat can ignite fuel, this is how a diesel works.......so the fuel in a high compression engine must have better resistance to ignition, before it ignites. Octane is the threshold where a fuel will be on the tipping point of igniting.......lower octanes burn easier than high octanes. This is why running the lowest octane number you can get away with makes more power........if you run 110 octane fuel in a 10:1 engine because you think it runs better, you are an idiot.
NHRA Pro Stock runs about 17:1 now....and uses a 116 octane fuel to sustain it......my 10.2:1 LS powered Truck requires no more than 89 octane to sustain it.
Manufactures used high compression in the 50's and 60's to boost power output because 100 fuel was plentiful and easy to make, and because their knowledge of cylinder head chambers and ports was dismal.......nowadays, there are many other ways to make up the power loss from high compression......or make high compression viable on pump fuel.......the new Vette has an 11.5:1 compression ratio.....on pump fuel it makes 500 horsepower, because of a LOT of mechanical and electronic assistance......and the heads on this engine flow what a 90's NASCAR engine flow......

Ok.....now it is 2020, and the tech that applies to LS engines applies to GEN1 SBC and Mark IV engines as well.........building L-88 12.5:1 engines is a thing of the past, and completely unnecessary........I have 10.5:1 in my own 406 GEN1 SBC and it is fine.....a LOT of this is the chamber design in the Dart heads......they are so much better that it make peak power at 33 degrees lead rather then the normal 36 that is an engine builder staple.

Like I said.....a lot to cover, and I am getting off track.....so shutting the faucet off now.

Jebby

 

This is a HUGE subject. So big as jebby says there are guys with 30 years in the industry and still don’t have a solid grasp on it.
no way, none whatsoever that it could all be covered here.

get this book. I spent a year studying and reading just to rebuild my engine to insure getting the results I wanted. Not saying it should take that long, but a guys gotta work too, lol.
there is plenty more info out there but this book will get all the basics and plenty advanced stuff covered for you.

 

Are you asking for a specific reason - as in engine build decisions - or is this merely academic? If for a specific reason, a more direct set of questions may get you answers that are applicable to your needs.

 

I'll quote this, because it does cut to the nut of why I made this post. Jebby had some great info, and I thank you for that, too.

Mostly, this is academic. Ultimately, it might translate to specific approaches, but, for now, I'm mostly curious how cam and timing can affect it. And even if I need to worry about the compression numbers at this point. But, again - academic... except for how it might impact my understanding of adjusting timing.

The discussion got started from a chat I had with a friend of mine, when I was wondering how much a cam affected compression. He told me a cam does NOT affect compression, which seemed to fly in the face of what I intuitively understand about the subject.

The basic physics concept, without crunching numbers and getting into fluid dynamics, is easy to understand. The way gas is squeezed, how combustion relates to power, etc. BUT - it seems to me that many factors would come into play, as to just how much compression you would have; including the rate of combustion of the fuel, the volume to which it expands, and, it seems to me, the specific timing and duration of when valved are open and shut would have a direct impact, not only compression, but the amount of energy imparted to the piston (which wouldn't just be based on maximum compression, but the duration of this pressure throughout the stroke of the piston).

On this level, in my head, it's all theoretical, but the real-world mechanics of how this all occurs, and the mechanisms used to manage it, is what I'm interested in learning more about.

 

Cam timing does affect compression......it is called Dynamic Compression, and it is an extremely controversial subject with different engine builders including myself....not because I challenge the science of it to obtain a "dynamic" number.......but this number can change depending on how well the induction system works. It does not take into account VE (Volumetric Efficiency).....which in layman's terms is how fast the incoming charge is moving after the piston stops at BDC. It is a good reference number.......but nothing beats dyno proven combos.
I once built a 477 BBC (4.500 bore x 3.760 stroke) for a Nostalgia Exhibition 67' GTO that ran in Ontario......the customer wanted me to shorten the stroke so the tire would not hit so hard out of the hole......BUT....I put the Brodix Race Oval Port CNC head on it which is crazy high velocity....SO.....even with a belly button .748/.714 bracket shelf cam and 12:1 compression......this engine made 650ft/lbs. torque from 3500 to 7100 RPM.....and 835 horsepower. Pretty good eh? Well.....we did the math after the dyno pulls and this engine was making 108% VE.......which means 8% more fill was happening in the cylinder after BDC, it is inertia supercharging.....this did not change the dynamic compression, but it made more power than a 540 "cookie cutter" 13:1 deal because the heads were out of sight......more power than an engine with "more" cam and "more" head.........Ironically, the customer was kinda pissed LOL....but not really, he just pulled a bit of timing out of it. Cylinder fill is just as important as how much you squeeze it.......
So to answer your question......bigger cams bleed pressure, but not necessarily how much is "squeezed"........does that make sense? LOL!

Jebby

 

Your friend is right in that a cam does not have an effect on Compression Ratio of the engine. That is a fixed number once the engine is built. Dependent on things like stroke distance, bore size, head chamber volume, piston top volume, gasket thickness etc.

The point I think you are getting at is that the cam has an effect on ultimate cylinder pressures reached. YES, this is true. It also has an effect on when that pressure is reached, and at what RPM.
A short duration cam with wide lobe separation is going to have less overlap and achieve high cylinder pressures much sooner in the RPM range than the opposite, a cam with higher duration, more valve lift and a narrower lobe separation giving it lots of valve overlap.
Cam science is very dynamic, lots and lots of differing opinions on what cam is best suited to what situation, so much so that people make their living by designing and selling cams from minor differences in the design that can have a major effect on how the engine behaves.
For that you will need to study cams and their designs.
Again David Vizard has a book just on that, and the book I recommended covers cams designs and choices as well. He is not the final word on such things, but he has thousands of dyno runs under his belt and has been doing this his whole life, so he is an expert in the field. Many others are as well. But they have not written books about it and the exact science of their cams is not public info. For the obvious reason that they need to make a living and cannot afford to disclose all their knowledge.
If this is academic then reading and scouring the internet is the way to learn. 1 out every 100 may understand cams relatively well, most go with recommendations of people who do know.
I will never have a level of understanding that a professional does and do not desire to. But I do feel I have a pretty good grasp on cams, due largely to reading and much less so my experience thus far with them. There is no replacement for experience. Academics is a starting point, actually doing it hands on is real learning.

 

I completely agree with your take on octane but I've always been curious about something related to the discussion in this thread. Is compression ratio the definitive gauge for what octane level to use? For example, my new Smeding 383 came with a 9.8:1 compression ratio and made 423 HP on the engine dyno with an Edelbrock Pro Flo 4 XT EFI system. Smeding was sending this same engine out the door with a carb or throttle body EFI making 388 HP. Does the additional HP in my engine require any additional octane as compared to the other two engines I mentioned?

DC

 

Nope.....it is 100% overall pressure at TDC.
180-190 psi is the threshold of pump gas 93 octane with old style chambers and engine management. (Carb, mechanical distributor).
The actual horsepower threshold of pump gas is about 1600 horsepower.....on a V8....it simply requires too much pressure to make more than 200 horsepower per cylinder......
There are some that claim 2000 horses with turbos on pump gas.....but that is half bullshit as they inject the inlet tract with Alcohol.

In fact....on your EFI....you may be able to get away with 87 or 89 octane.....you would have to keep an eye on the timing software to see if it pulls any timing running that though......

Jebby

 

Thank you. Smeding was advertising this engine as running on 87 octane initially but have since taken that statement off their website. When I received my engine, they recommended I use 91 octane. It would be nice if I could at least use 89.

DC

 

Don't forget about swirl/turbulance and quench action. Or what is sometimes called mechanical detonation prevention.

A straight dish piston vs a D-dish piston, big difference in quench action and detonation prevention.

.040" squish distance vs .060" squish distance, big difference in detonation prevention and quench action also.

You can achieve much higher cylinder pressures and prevent detonation given good quench and visa versa you can have much less cylinder pressure and still run into detonation given poor quench action.

 

Agreed.....but for the sake of my explanation, I will keep the engine in question Apples vs. Apples....
That is why 180 to 200 is acceptable on pump gas for psi.....200 would work for the newer LS stuff.....
180 for Gen 1 SBC.....and evening 170-175 for early 350 horse 327’s and 350’s.....with cast iron heads and crummy chamber.

Jebby

 

primarily bore, stroke and combustion chamber volume (when at TDC); most of the rest can be modeled via CFD.
Computational Fluid Dynamics: free software

https://openfoam.org/

 

Agreed, stock my 77 detonated despite the very low compression (7.7:1) blowing about 150 ish psi with the crap pistons that provided no quench and cast iron heads.

Now I run 10.6:1 D-dish pistons .038 squish distance and a couple cylinders push 200 psi. AFR heads, No detonation no matter what I do. I've even run 87 octane still no detectable or evidence of detonation.

Not too many people know that lower octane produces more power as you pointed out.