Just out of curiosity I have doing some thinking about raising my compression some more, and have a few thoughts. It is to my understanding that the when raising compression to hit what is called the sweet spot, you want to raise your cranking pressure by 20-25%. However, what does this mean to hit the sweet spot? If I am to go over this percentage will I harm my production of power or is this just an area where I will generally make good power gains without putting tons of stress on the engine? Or is this just a myth and should I not worry about this and increase my compresion more?

Stock, my crank pressure was 189 psi. To achieve the so-called sweet spot theory I would need a combination to raise my cranking pressure to 236 psi. Right now my cranking pressure is 219.49 psi and I can easily achieve 236 psi by simply milling the head .020" with my combination. However if it is safe to raise my compresion even further, I am considering CTR Pistons in my engine which will give me a cranking pressure of 261.1 psi, and a dynamic compression of 11.81:1.

Someone share their thoughts and comments with me please.

 

Interesting topic, I'd like to learn more also. So bump for more info.

 

bump!

 

^Bump to the top

 

Kb pistons have done some tech papers on compression/pressure.I've had positive results using this info in building engines. http://kb-silvolite.com/articl...id=36
Glenn

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by NJIN BUILDR &raquo;</TD></TR><TR><TD CLASS="quote">Kb pistons have done some tech papers on compression/pressure.I've had positive results using this info in building engines. http://kb-silvolite.com/articl...id=36
Glenn</TD></TR></TABLE>

Good info, but I already knew the difference between static and dynamic compression! Thats why I am asking about the 25% rule or if I should through it out of the window? Is anything over this 25% going to cause detonation?

Also my dynamic compression 10.3:1 right now(static 11:1) and to raise it to the 25% over stock, I can accomplish by milling the head .020" which will give me a dynamic compression of 10.8:1 and 236 psi cranking pressure.


Here is a Dynamic Compression Calculator:
http://www.rbracing-rsr.com/comprAdvHD.htm

 

Are you telling me that out of all the supposed engine builders here, no one knows anything about dynamic compression!

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by hybrid_vtec &raquo;</TD></TR><TR><TD CLASS="quote">Are you telling me that out of all the supposed engine builders here, no one knows anything about dynamic compression!</TD></TR></TABLE>

 

i woudlnt use ctr pistons, but if i wanted to get the compression from a piston i would use a nice forged unit with a better design.


in your case milling the head will work perfectly, i have heared this 20-25% before, but i dont see why more compression would hurt in any case if the proper fuel and tuning were used.

 

The cranking cylinder pressure number is what it is.I don't use that as a guideline for dynamic cylinder pressures.The intake closing numbers are more of a clue as to what the real number is.The ability to measure dynamic cylinder pressure of the actual engine would be a wonderful educational tool. http://www.tfxengine.com/index.html
Glenn

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by NJIN BUILDR &raquo;</TD></TR><TR><TD CLASS="quote">The cranking cylinder pressure number is what it is.I don't use that as a guideline for dynamic cylinder pressures.The intake closing numbers are more of a clue as to what the real number is.The ability to measure dynamic cylinder pressure of the actual engine would be a wonderful educational tool. http://www.tfxengine.com/index.html
Glenn</TD></TR></TABLE>

Cranking pressure is just as important as dynamic compression. I posted the site to the calculator I use to calculate my Dynamic Compression. It uses measurements to calculate Dynamic Compression: Bore, Stroke, Rod Length, Static compression, Degree Intake valves close at, and Altitude. These measurements must be inputed into the calculator in inches. If you need the measurements in inches I have them for all B-series and D-series(H-series too I think).

 

bump for some advice plz.

Refer to the question at the top, Thanx!

 

Now lock in your VTEC lobes and check the dynamic comp. You are checking with the primary lobes that have less overlap. Things to think about.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by DonF &raquo;</TD></TR><TR><TD CLASS="quote">Now lock in your VTEC lobes and check the dynamic comp. You are checking with the primary lobes that have less overlap. Things to think about.</TD></TR></TABLE>

I dont have Vtec in my hatch, its a B18b w/ 404 and P30 Pistons.

Im emailing you, I've got a question for you.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by hybrid_vtec &raquo;</TD></TR><TR><TD CLASS="quote">Just out of curiosity I have doing some thinking about raising my compression some more, and have a few thoughts. It is to my understanding that the when raising compression to hit what is called the sweet spot, you want to raise your cranking pressure by 20-25%. However, what does this mean to hit the sweet spot? If I am to go over this percentage will I harm my production of power or is this just an area where I will generally make good power gains without putting tons of stress on the engine? Or is this just a myth and should I not worry about this and increase my compresion more?

Stock, my crank pressure was 189 psi. To achieve the so-called sweet spot theory I would need a combination to raise my cranking pressure to 236 psi. Right now my cranking pressure is 219.49 psi and I can easily achieve 236 psi by simply milling the head .020" with my combination. However if it is safe to raise my compresion even further, I am considering CTR Pistons in my engine which will give me a cranking pressure of 261.1 psi, and a dynamic compression of 11.81:1.

Someone share their thoughts and comments with me please.</TD></TR></TABLE>

 

the sweet spot is maximum compression without detonation.


the graph is for a pontiac v8 but is a good example to show that there is no power loss from going too high, unless you start detonating. so dont be like omfg those percentages are wrong

 

Thanx! Im just trying to get some feedback b/c their are many that claim what I stated in my question.

 

[QUOTE=hybrid_vtec]

I dont have Vtec in my hatch, its a B18b

is this a daily drver ?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by wesisrael &raquo;</TD></TR><TR><TD CLASS="quote">
is this a daily drver ? </TD></TR></TABLE>

No, b/c the 6 puck disk clutch makes it a bitch to drive. And the cams barely idle.

but its still a fun car to drive!

 

I bet that they would idle just fine if you got AEM EMS/Chrome/Überdata/hondata/Neptune instead of that SAFC. If you ever go that direction try advancing the daylights out of the ignition at idle. I am running 30º advance but it tapers off real quick once you step on the pedal...

Pirate

 

I am playing around with Uberdata now but looking for a cheap laptop to use to datalog

 

This is a great topic, what kinda of power should a ls/vtec with CTR pistons and cams make to the wheels, because me and my co-worker built this motor for his DC2 and it just doesnt seem to make the power it should... and it makes great PSI on a compression test

 

Static compression, cam choice, valve lash, cam timing, will all decide your dynamic compression. From those numbers you decide total or optimal timing, according to the octane you are running. There are no simple answers. Run a looser valve lash and you will increase dynamic compression, but maybe kill the top end, just one of your choices. Lots of stuff to play with.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by DonF &raquo;</TD></TR><TR><TD CLASS="quote">Static compression, cam choice, valve lash, cam timing, will all decide your dynamic compression. From those numbers you decide total or optimal timing, according to the octane you are running. There are no simple answers. Run a looser valve lash and you will increase dynamic compression, but maybe kill the top end, just one of your choices. Lots of stuff to play with. </TD></TR></TABLE>

Thats the fun part!