Hi,

a friend of mine is building a project similar to mine BUT he still has his car with the OEM CR it is b16a2 also so it is 10.2:1 his intend is to build something like mine and as he dont speak english he asked me to ask you what is the relation between compression ratio and camshaft selection.

he want to get stage 2 cams without raising CR cause his car only has 70k miles o it and don´thave any oil consumption. will he get benefits by installing the cams or he definetly must raiso CR as I did and do a port an polish head work?


thanks all for you time and knowledge, we are newbies on this and every thing you say will be really usefull for us.

thanks!

 

he will see ALOT more benifets with a higher CR, those are some mildly agressive cams. If he wants too sick with his stock CR then he should look at some ITR/CTR cams.

 

Im in to know what is the exact relation between these 2 aspects

 

The bigger explosion the more power you will make, you can let in as much air as you want, but without compression the air is useless, that's why on the skunk2 site it suggests that you upgrade your pistons to higher compression to take full advantage of that cam profile.

 

I would read up on the term "compression" as I don't think you fully grasp it. When you increase the compression, the engine inhales more air than at a lower ratio. Compression is a way of "tricking" an engine into thinking it's bigger than it actually is by moving more air through it. Increasing compression will generally increase power made on any given set of cams but its main purpose is to help offset valve overlap/running larger primary lobes, like the Skunk pro series cams. Running cams like that bleeds compression and tends to make the car idle rough. Increasing displacement or compression ratio will help correct that and make more power (to a point).

 

Im running blox B's with a 1 ply headgasket on a b16a2 and it was a big difference over stock (butt dyno)

the reason you need more cr when going to bigger cams is that with more duration (valve stays open longer, as well as closes later) you loose some dynamic compression (dynamic compression states that your dynamic compression ratio doesn't start until the last intake valve is closed) as opposed to static compression ratio stating the piston travels up and down so far and the area left in the head when piston is tdc is the left over so how ever many x times you displace that much is your ratio.

 

Tell your friend to Google information on "dynamic compression ratio" and it will start to make more sense.

 

How exactly do you move more air by increasing compression?

I believe you pack the AFR better so the burn speed is faster, and stronger, thus, increasing power.

Now, I can't relate that to increasing the amount of air that comes in.


I'm not saying I don't believe you, actually I know it work's like this, but I can't figure out why.

 

as always thanks!!!! that was a very clear explanation! (but I don´t know the word grasp I will have to search the translation for grasp in the dictionary jajja)

 

valve stays open longer, as well as closes later) you loose some dynamic compression (dynamic compression states that your dynamic compression ratio doesn't start until the last intake valve is closed)

I knew that but I haven´t thought it, now it make me clearer some things!

 

so the bigger the cams the higher the CR then as a matter of saying you NEED more CR, supponging I want to intallar Pro 1s or 2s I would need 11.5:1 minimum. in my country the best fuel has 91 octanes, it is imposible to run that CR without retarding ignition to prevent detonation and at least my basic knowledge are leaner the meaner and it is all about ignition timing

 

I believe you guys have Ethanol there, don't you?

 

11.5:1 compression is actually considered to be an "ideal" ratio for the street that'll allow you to run full timing on typical supreme pump gas. I run that now on 91 octane without any issues.

 

more compression does not trick the engine into thinking it is moving more air, it makes the engine more efficient. by compressing your air/fuel mixture more you are putting more potential energy into a smaller area, when this energy is turned into kinetic energy(by igniting it) you are using more of that energy to push the piston down(as opposed to just making the engine hot), which thru the rod turns the crank and makes you go.

your cam determines how much air you can pack into the cylinder in the time the intake valve is open. and how much exhaust gases you can get out when the exhaust valve is open.

so what we are talking about here are 2 different things, compression ratio has a direct effect on thermodynamic efficiency and cams have a direct effect on volumetric efficiency. but BOTH are linked together. for example if you have a tiny cam but a high compression ratio then at after a certain rpm you will start LOSING both volumetric AND thermodynamic effiency because you arent filling the cylinders completely. now if you have a huge cam and low compression then you will run into the velocity vs volume roadblock.

so to sum it all up, you will make more power with an aggressive cam and stock compression, but you will most likely lose low end torque too. so my advice is to pick a more mild cam and stick with your current compression. you wont pick up as much HP, but you will keep or gain low end torque which in a street car is essential. id do like the advice of getting some ITR cams to put in it. upgrade springs and retainers at the same time. rev it to 8500 or so and i think you will be very happy.

 

I am too tired at the moment to fully address what you said, but I want to clarify that I did NOT say raising the compression tricks the engine into thinking it's moving more air. I said it tricks the engine into thinking it's bigger than it really is. There's a difference.

When you increase an engines displacement, what happens? The increase in displacement allows more air and fuel to be burned which makes more power. When you raise compression it allows an engine to extract more mechanical energy from a given mass of air/fuel mixture due to its higher thermal efficiency. High ratios place the available oxygen and fuel molecules into a reduced space along with the adiabatic heat of compression, causing better mixing and evaporation of the fuel droplets. This allows increased power at the moment of ignition and the extraction of more useful work from that power by expanding the hot gas to a greater degree.

Extracting more power from the available air/fuel is more efficient but it can also be accomplished by increasing displacement which is not as efficient, but is just as effective. The larger the displacement an engine is, the "smaller" it makes a cam. So a pro 3 cam on a 1.6L B-series will require a lot more compression to support it than on a much larger 2+L B-series with lower compression. This is what I was getting at.

EDIT- I just re-read what I posted previously above and can see that what I meant and what I said, seem to be two completely different things. The post I just made here is what I was really trying to say. Maybe I was more tired when I wrote that than when I wrote this...lol

 

it is amazing what I can learn from you guys, a few months ago I almost couldnt change a spark plug, now I know things that I surprise myself and gave me courage to build my own car.

everything I learnt with this thread will help me and a lot of newbies that want to learn as me I am really happy to belong to a comunity of people that knows that much, I wish people in my country knew half of the things you know and share it as you do

 

Compression ratio and camshaft selection go hand in hand, although it is an extremely complex topic and one that can't be thoroughly explained in an internet forum.

The first thing to consider is that the static compression ratio is directly related to where in the rpm band peak torque will be made. The higher the compression ratio, the higher in the rpm peak torque will be made in a properly designed engine. If you have an engine with a relatively low redline, say 6000 rpm, a high static compression ratio will do you no good. You will make better power by keeping the static compression ratio low. Conversely, if you have a relatively high redline, say 9000, you need a correspondingly high static compression ratio. Determining how fast you can spin your engine is another topic entirely (search 'piston speed').

There are 3 compression ratios the engine sees:

The first is the 'static compression ratio'. The formula for this is:

cylinder displacement + combustion chamber volume / combustion chamber volume

This is the 'normal' compression ratio you will hear guys talk about (i.e. 9:1, 10:1, 13.5:1, etc.).

The second is the 'effective compression ratio'. The effective compression ratio is the compression ratio that is measured starting the moment the intake valves touch down on their seats. It has been incorrectly stated above that this is the 'dynamic compression ratio'. This is not true, I will get to that in a bit. Calculating the effective compression ratio involves some trigonometry so I will not go into the exact calculations here. Because the air/fuel mixture entering the cylinder has inertia, it wants to keep moving in the cylinder even though the piston has stopped once it reaches bottom dead center. This is a good thing because we can use this inertia to our advantage to cram as much air and fuel into the cylinder as possible (termed 'inertial supercharging') which creates more power. By keeping the valve open even though the piston is now moving up the cylinder we can let this inertia continue to fill the cylinder. Eventually, this inertia will be overcome and we need to close the intake valve to prevent the air fuel mixture from reverting back out the intake valves. The point in the rpm band where the mixture is just about to start coming back into the intake port but the intake valve closes will be peak torque. Once we have the mixture trapped and the compression stroke begins we need to compress it as much as possible before igniting it to keep thermal efficiency as high as possible. If we compress it too much, it will begin burning on it's own (generically termed 'detonation') due to the excessively high pressure and temperature of the mixture. If we don't compress it enough we are not extracting as much energy from the fuel as we could be and therefore we will not make as much power as we would like to. The ideal effective compression ratios on pump fuel (racing gas effective compression ratios will be higher, thus the increase in power when building an engine that will be using racing fuel) depends on a few factors. A small block Chevy for example will have an effective compression ratio of about 7:1. This is due to the fact that it uses an iron block and head which does not transfer heat as well as aluminum. An engine that uses an iron block but an aluminum head, like many Hondas, can use an effective compression ratio as high as 8:1. A well prepared aluminum block/aluminum head 4-valve engine may go as high as 8.5:1. Again, these numbers are for pump fuel of 91 - 93 octane (AKI).

The third is the 'dynamic compression ratio' which is the effective compression ratio multiplied by the volumetric efficiency. This is the actual 'running' compression ratio the engine sees while it's doing its thing, hence the term 'dynamic'.


So what ends up happening is, the higher your static compression ratio, the later you want those intake valves to close to keep the effective compression ratio around 8:1 (for an iron block/aluminum head). This means by definition your cam will have more duration. So the higher the static compression ratio, the more duration your cam will need. The lower your static compression ratio, the less duration it will need.

Of course there are a million other factors such as how early to open the intake valves before top dead center and how far after top dead center to close the exhaust valves (collectively called 'overlap') but that goes beyond what the OP was asking and could easily fill several books.

When you buy a cam for your build you need to talk to the cam manufacturer and ask them what is best for your engine. Picking it yourself based on how slick the marketing terminology is, is a recipe for disaster and unfortunately that is how most cams are chosen. Terms like 'tuner' and 'stage 1' or 'stage 2' are marketing terms only and give no indication whatsoever about how the cam will perform in your engine. It doesn't mean that cam companies that use terms like this don't make good cams it just mean be careful. Don't fall for the 'stage 2 will make me more power than stage 1 because it's one number higher' mentality. The cams that most of the major companies sell are a 'best guess' only. They are trying to market to a wide audience of engine builders who are using a wide variety of parts, compression ratios, etc. There is truly only 1 cam that is perfect for your application and that that cam would likely have to be custom ground. There is no one who is a hobbyist who is willing to pay for the R and D required to design that optimal cam. However, there are companies out there who will 'custom grind' a cam for you that will be damn close based off of the hundreds of cam masters they keep in stock for their cam grinding machines. Even the off the shelf cams will come close enough and will likely make you happy unless you are a dedicated professional racer.

 

not trying to jack ur thread but wat cams would u guys use on a ls vtec 13:1.0 comp

 

i completely agree

and dont worry about it, we all get a like that from time to time.

 

another option alot of people might overlook is calling up comp or crane cams and they will ask you all kinds of questions about what you have and what you want and then grind you custom cams for about the same price as off the shelf cams, just have to wait longer for them. both of those companies have been engineering casm for a LONG time

which is pretty much what you said in the last bit.....sorry, just now read that.

thanks for the info i didnt realize that there was a third compression figure to consider. im glad to see theres someone else that understands that a naturally aspirated engine can run at more than 100% VE. id also like to add that by your(and my new) definition of dynamic compression.........that figure changes as rpm changes because VE changes with rpm.............which is based in a large part on the camshaft(lift, duration, overlap, timing) but also by things like valve size and number as well as valve job(which btw is the most critical element of head flow because it is that last obstacle the air must get through to get in the cylinder, and the first thing the ehaust gases have to get through on its way out), combustion chamber size and shape(including quench areas), piston dome size and shape and distance from the cylinder walls to the valves, port size length and shape, IM size, length and shape, exhaust size length and shape........and a number of other variables that im forgetting. and on those where i mentioned shape, im referring mostly to the bends in the piece that make the air change direction........air gets pissed off and does crazy stuff when you change its direction more than about 4 degrees......but i do also mean the shape of the profile of that path, is it round, oval, square, does it change from square to oval to round, etc......all of these things affect how well a head will flow air at a given rpm. and any knowledgeable engine builder will tell you that MOST of your engines potential power is in the head.

and just to throw this out there, nitrous IS NOT naturally aspirated.

 

Agreed.

And you are correct, Nitrous is not naturally aspirated as you are artificially adding more oxygen and that is how it makes power.

I've learned what I know about camshafts from Dema Elgin of D. Elgin camshafts. He has 50 years of camshaft engineering experience and his cams have won literally thousands of races, including the Daytona 500. He doesn't advertise so not many people know his name especially in the Honda world.

 

yeah i went to nascar tech and i learned alot, unfortunately one of the most knowledgable ppl there that used to be an instructor is now an education manager.......he has 2 indy 500 wins as an engine builder and 2 daytona 500 wins as an engine builder so i wish i had the opportunity to pick his brain.

 

whats the main purpose of the engine? is it a hot DD? a street car that sees weekend strip use? autox? road course?

does idle/low end matter to you at all? do you want maximum top end power?

what redline do you want to run?

the reason im asking these questions is because while vtec is a pretty good way to get both low end and top end power, it isnt perfect and therefore compromises must be made.

 

Yeah, I've heard good things about that school. Dema grinds some camshafts for them.

 

Dave Hsu knows Dema and his work.