Hey guys,

thanks for the help on my post about compression ratios.. but I have another question that you can hopefully help me get a better understanding on..

I really want to fully build a B20 vtec.. but now I am trying to pick out a set of cams..

where the hel! do I start? I was planning with going either skunk2 or blox.. but there are so many options.. Tuner vs. Competition? and then interrpreting the lift/duration specs..

so again Im trying to figure out.. what is to stop people from throwing a stage 3 cam in their head? you're pbviously trying to make power.. so why would you go stage 2 or 1?

how high can you go to keep a reliable DD?

again, I am just trying to get a better understanding on my options..

of course valves/springs/retainers are going to be upgraded.. and I was able to rule out turbo series cams all on my own =b (all motor)

any help is appreciated!

-Chris

 

research and look at other similiar builds...its really your decision on what cams you want

 

There is much incorrect information on this subject floating around the web. Most ‘hobbyists’ just guess based on marketing. Designing a camshaft or choosing a camshaft from a catalog is easily the most complex part of building an engine. Camshaft companies seem to be reluctant to provide information on what factors are involved in selecting the right camshaft for your build – or they just don’t know. Many cam companies outsource the design of the cam to a specialist and then outsource the actual manufacturer either to a specialty grinder or to China.

However, armed with a little knowledge you can choose a cam that *should* perform well in your engine. Because this is such a complex subject I can only list the basics here.

Some terminology:

Valve lift: The maximum amount of lift the valve achieves from a given cam.

Cam lift: The difference between the base circle and the nose of the cam. When this dimension is multiplied by your rocker ratio, it equals the valve lift.

Seat-to-seat duration: The amount of time, in crank degrees, that the valve is open from the instant it is lifted off its seat to the instant it touches the seat again when it closes.

Duration @ .050”: The amount of time, in crank degrees, that the valve is open beginning when the valve has been opened .050” inches and ending when it is on it’s way closed and is .050” from closing. Some manufacturers specify this at the valve and some specify it at the cam, although typically it is at the valve. It was originally proposed by Harvey Crane of Crane Cams to be used with seat to seat duration. If two cams had the same seat to seat duration, say 280 degrees, but one cam had 240 degrees duration at .050” and the other cam had 250 degrees at .050”, the one with 250 degrees duration at .050” was said to have ‘more area under the curve’ because it was accelerated open faster. Unfortunately, manufacturers have taken a liking to just listing duration at .050” which is rather useless. Some manufacturers have said there is little airflow under .050” valve lift, which is complete non-sense especially on a 4-valve engine.

Port flow: The amount of air, in cubic feet per minute (CFM), that a port will pass at a given valve lift. This is very important when it comes to what your maximum valve lift should be because there is a point when opening the valve any further does not yield any significant gain in port flow so there is no reason to open the valve any more. In reality, the valve must be open further because camshafts are first of all design to satisfy a mechanical need and then designed to satisfy airflow demands. If you stopped lifting the valve the instant that port flow stopped increasing the rocker arm would be lofted off the nose of the cam and then coming slamming back down onto it due to spring pressure.

Overlap: The amount of time, in crank degrees, that both intake and exhaust valves are open at the same time.

Effective compression ratio: The compression ratio measured the instant the intake valves close.

The two most important factors when choosing a cam are peak port flow and effective compression.

Most people who build an engine never have the heads flow tested. Many people who are supposed to be experts in porting heads have never used a flow bench. I f you look at a graph from a head that has been flowed (and I will try to add a pic of what that looks like later) you will see that the is a point that lifting the valve further does not yield any significant increase in port flow. The valve needs to be lifted at least this high and usually higher. The valve also needs to be lifted this high by the time the piston reaches maximum velocity. This point is when the connecting rod is at a right angle to the crank throw. This is when the engine takes its biggest ‘gulp’ so it makes sense that having the valve open to the point of maximum port flow will yield the most air flow. Opening the valve further than this is only done so the valve can decelerate smoothly at so no allow it to bounce off the cam.

The effective compression ratio is important because it has a big effect on how thermally efficient the engine is. Again, the effective compression ratio is measured at the instant the intake valves close. It is important to get this ratio right because if it is too low, you are not taking maximum advantage of the fuel. If it is too high, the fuel could detonate which could destroy the engine. On a 4-valve aluminum head engine using pump gas this ratio is typically 8.5:1. Calculating the effective compression ratio involves some trigonometry so most people building engines are not up to calculating it. Most guys who call themselves professional engine builders have never calculated it.

There are many factors that determine what the seat to seat duration and lift for a specific build should be. So many factors, that I do not have time to list everything. Some factors are more important than others for specific builds. Some other things that come into play when choosing cams are rod/stroke ratio, percentage of intake flow vs. exhaust flow, static compression ratio, bore/stroke ratio, among other. I simply do not have time to list everything because this subject could easily fill several books but if you have other questions about cam theory please let me know.

 

Scott Tucker. Lots of great info! thank you for taking the time to type all that out.

I dont have time to read it right now as I am studying for finals.. But ill re-check tomorrw and make sense of it.

Thanks Again!!

 

Here is a picture of a portflow graph:



As you can see portflow levels off significantly at about .625" valve lift. Opening the valve further than this will not yield much more flow. However, the intake valve needs to be open this much by the time the piston reaches maximum velocity in order to maximize volumetric efficiency.

You'll also notice that this graph plots % of intake flow vs % of exhaust flow. This is important when considering how much duration the exhaust cam should have, how much overlap there should be, etc. I do not claim to be an absolute cam expert. That is why I consult with someone who is. However, I know what information is important to him and can provide him this information so together we can make the best decision.

 

Wow, that was great info. Thanks again!

I had a few quick questions.

why would it be lifted off the nose of the lobe? it seems like the pressure would be neutral no?


can you explain that in a little more detail?

awesome info! Thanks

 

No, the word was 'lofted' as in 'thrown off' due to the inertia of the valvetrain. What I meant was that in an ideal world the valves would open instantaneously to the point of maximum port flow and then when it was time to close it would close instantaneously also. Obviously, that's not possible because the valves would slam into the piston, among other reasons. The cam has to open the valves as smoothly as possibly and, more importantly, close them as smoothly as possible. It needs to do this with as little 'jerk' (change in acceleration) as possible. If the cam stopped pressing against the rocker as soon as the valve was at the point of maximum port flow it would continue opening due to inertia. Sorry if this isn't the best explanation, it's been a long day.


Yes, if you pretend your arm, from your shoulder to your elbow, is the crank throw and from your elbow to your hand is the connecting rod it would be when they are at right angles to each other. I'll try to post a diagram later. Or did you want a better explanation as to why this affects volumetric efficiency?

 

no that was alot better thanks. But getting back to the origional points you brought up, how much of that can you actually have a say in / adjust?

you mentioned measuring the port flow? does that give you an idea of how much valve lift you should have from a cam , and in turn an idea of which cams to buy?

again, great info!

much thanks!