This isn't very technical so don't yell at me if my figures are a little off.

Lets say you have an ITR intake cam.

When the secondary kicks in, you have some nice power.

At 5,500 RPM you make good power. The intake valve is opening 45 times a second.

At 9,000 RPM its not making power anymore. The intake valve is opening 75 times in one second. The duration and lift felt good at x-over, but aren't feeling so hot now are they...

If you don't understand that, breathe through a straw. Alright isn't it? Now start doing jumping jacks. Gets harder to breathe though it right?


Now that we have that out of the way. Why aren't you making low end with big cams ("stage 3's" so we're not picking on any particular brand other than Skunk2 since thats how they label their cams ). 4,000 RPM, intake valve is opening 33 times a second. But you have 13mm of lift and 500 duration. Do you think the piston is traveling fast enough to make use of all that specage?

This can be taken a lot of ways, "overcamming/undercamming" etc. Just take it how it is. If the edumacated people want to get real technical with it, this forum could use some more of that.

 

perhaps this is also a good time to give a, forgive the expression, dumb'd-down, explaination of lift vs. duration ?

seems somewhat simple, but at the same time i could see people getting the terms mixed up


Duration - # of degrees the valve is off the seat.

Lift - the total height the valve reaches from its original location.


i dunno, i can't think of a nice simple simile like you, but you see where i'm going with this?

like the whole keeping it simple idea, btw.

 

 

If you try to keep this simple, people are only going to get a small picture of what you are trying to explain..

 

informative thread. A+

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by shad0wslay3r &raquo;</TD></TR><TR><TD CLASS="quote">If you try to keep this simple, people are only going to get a small picture of what you are trying to explain..</TD></TR></TABLE>

People had to learn their ABC's before they ever wrote essays right? I know I searched Howstuffworks.com 10 times a day whenever I was tying to understand a specific area within just one of many of the enormously elaborate discussions between some of the "heavy hitters" in the all motor forum.

 

aquafina

 

9 times out of 10 anytime you do something to an engine there is a compromise.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Aquafina &raquo;</TD></TR><TR><TD CLASS="quote">This isn't very technical so don't yell at me if my figures are a little off.

Lets say you have an ITR intake cam.

When the secondary kicks in, you have some nice power.

At 5,500 RPM you make good power. The intake valve is opening 45 times a second.

At 9,000 RPM its not making power anymore. The intake valve is opening 75 times in one second. The duration and lift felt good at x-over, but aren't feeling so hot now are they...

If you don't understand that, breathe through a straw. Alright isn't it? Now start doing jumping jacks. Gets harder to breathe though it right?


Now that we have that out of the way. Why aren't you making low end with big cams ("stage 3's" so we're not picking on any particular brand other than Skunk2 since thats how they label their cams ). 4,000 RPM, intake valve is opening 33 times a second. But you have 13mm of lift and 500 duration. Do you think the piston is traveling fast enough to make use of all that specage?

This can be taken a lot of ways, "overcamming/undercamming" etc. Just take it how it is. If the edumacated people want to get real technical with it, this forum could use some more of that.</TD></TR></TABLE>

good explaination!! Now explain what compression has to do with all of this...and how they work together, I need some schooling....

 

It all boils down to Volumetric efficiency, Thermodynamic efficiency, and Mechanical efficiency of any said setup.

There's also a very large debate on velocity of air vs. quantity of air. I'd imagine the best breathing engines utilize a great balance of both.

An engine is just a glorified air pump.

 

*ahem* increase cyl pressure, you increase power..

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by pomansouth01 &raquo;</TD></TR><TR><TD CLASS="quote">

good explaination!! Now explain what compression has to do with all of this...and how they work together, I need some schooling....</TD></TR></TABLE>

Google dynamic compression

 

The bigger the cam (laymans = valve opened more) the more compression that is bled off.

 

ok, I googled that and read the wiki on it....I understand what makes dynamic compression, I understand that the timing of the intake valve closing event has a big effect on the compression raito....I somehow cant find the link between an intake valve opening and closing 75 times in a second and the compression ratio being at a different level and making a difference. I understand that the higher the compression, the higher the amount of power that is available up to the limits of detonation and also higher compression is more "efficient" and creates more heat or "power"....but what does a valve opening a certain amount from the head and for a certain degree have to do so DIRECTLY with compression.... I know why certain cams are "too big" for motors but I dont know why compression can make that same cam more suitable for the same motor. I know it works this way because I see it all the time but I dont know why it works this way....the question is WHY???

 

Read my above reply.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by IntegraType-R &raquo;</TD></TR><TR><TD CLASS="quote">*ahem* increase cyl pressure, you increase power..</TD></TR></TABLE>

Not always true. There is a point where amount of compression is not as import as the manor in which you're achieving that compression. Where that point is.....is difficult to say. It really depends on alot of factors including piston design and combustion chamber design.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Aquafina &raquo;</TD></TR><TR><TD CLASS="quote">The bigger the cam (laymans = valve opened more) the more compression that is bled off.</TD></TR></TABLE>

so is there basically a certain dynamic compression that is most efficient on certain types of gas?? is there a certain formula that says that you should shoot for a certain cylinder pressure at combustion??

if so, then should you pic your cam before you pick your compression??

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by bambam &raquo;</TD></TR><TR><TD CLASS="quote">Not always true. There is a point where amount of compression is not as import as the manor in which you're achieving that compression. Where that point is.....is difficult to say. It really depends on alot of factors including piston design and combustion chamber design.</TD></TR></TABLE>

I understand what you are aiming at or atleast I think I do....the flam front needs to be able to travel over the piston easily and when you have a peek and deep valve pockets to make clearance, you kinda screw that up right??

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by pomansouth01 &raquo;</TD></TR><TR><TD CLASS="quote">

if so, then should you pic your cam before you pick your compression??</TD></TR></TABLE>

Exactly....now you're starting to understand what most people on this site simply don't get. They go for a big compression ratio and then throw cams at their engine and expect to make power....it doesn't work like that...it's the other way around. Match the compression to the cam requirements, the better the job you do here, the more power you can make.

 

The general concensus for Honda cars on 91-94 octane is 11-11.5:1.

Some cams might like more compression than that, but then you get into pulling timing.

IMO, you don't pick one component, then another. The engine needs to work as a whole. You know the whole teamwork, you fall we all fall speech coaches give?

As far as flame travel/compression (again, this is MO), good flow doesn't stop once it enters the cylinder. You have a good VJ and chamber, but what good does that do if it runs into a hillside? You can get flow from valve pockets, troph relief.

 

If you want to really understand how engines work then I suggest you begin to read the following:

Internal Combustion Engine Fundamentals by John Heywood.

It is one the best books available on the subject.

And if you what to understand the induction and exduction process, I suggest

Design and Simulation of Four Stroke Engines by Gordon Blair
Computional Gasdynamics by Culbert Laney

After reading and understanding those you should be able to write you own engine sim.

 

Rocket once told me something but I forgot how he worded it.

Reading is good for knowledge.

Doing is good for know-how.

I think I just butchered the immortal hell out of that.

 

good thread!

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Aquafina &raquo;</TD></TR><TR><TD CLASS="quote">Rocket once told me something but I forgot how he worded it.

Reading is good for knowledge.

Doing is good for know-how.
</TD></TR></TABLE>

I don't think you understand.

Those books will provide you with a funadamental basis for understanding engine design and performance.


They give you the basis to design from scratch or evaluate other designs. Further, they can help you to understand the underpinnings of engine simulation.

When you look at the major motor sports teams all over the planet, jtcc, btcc, etcc, nascar, indy, F1, etc they all use some design program along with a testing program. This applies to every major manufacture also.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Aquafina &raquo;</TD></TR><TR><TD CLASS="quote">The general concensus for Honda cars on 91-94 octane is 11-11.5:1.

Some cams might like more compression than that, but then you get into pulling timing.

IMO, you don't pick one component, then another. The engine needs to work as a whole. You know the whole teamwork, you fall we all fall speech coaches give?

As far as flame travel/compression (again, this is MO), good flow doesn't stop once it enters the cylinder. You have a good VJ and chamber, but what good does that do if it runs into a hillside? You can get flow from valve pockets, troph relief.</TD></TR></TABLE>

ok, lets take my motor for instance that I am building....I am using pro 2 camshafts with a portflow gsr head, ls long rods in a 84mm benson block. Skunk2 reccomends 12:1 or higher for their pro2 cams....most people say that 11.5 to one in a honda motor is right about the limit before timing has to start being pulled. Is this 11.5:1 dynamic or static?? How do I figure out what the dynamic compression is with a setup at a certain degree on the intake cam?? If I know what my dynamic compression is, can I from that figure out my cylinder pressure at combustion under high load and lower rpm?? What is the highest pressure a pump gas motor can get to before detonation takes place?? I like to read about this stuff but I also like to be able to turn to someone as I am reading and say...what does XXX have to do with YYY and explain the correlation between the two...