I've read several threads on the topic but would like to maybe read or hear more on the topic. Regarding cams roughly in the area of the typical stage 2 cam, how should you go about picking the right compression? I obviously want to pick a nice medium number.

Cams in question are 278/284 (in/ex) duration, 12.4/12.0mm (in/ex) lift... 238/240@ 0.050"... My best judgement tells me somewhere around 12:1 is a good number, but I would like some experienced builders/tuners to tell me how much of a difference going a point over or under will make in the real world, (my cometic gives me 12.7:1, stock gives me 11.7:1

I have 94 or 93 available in the area...


Modified by teg92 at 10:20 AM 4/29/2004

 

You need to find out how much the cam has the intake valves open After Bottom Dead Center.

The idea is that you don't want to reduce dynamic compression/cranking pressure from stock, and increasing it up to 25% is going to be beneficial. (25% increase in cranking pressure is about the sweet spot.)

Here is a calculator that you can plug in static CR and find dynamic compression and cranking pressure.

http://www.rbracing-rsr.com/comprAdvHD.htm

Unfortunately I can't tell you how much a point or half a point will make, though I imagine it is very dependent on your whole engine package, and where the CR is. (ie 10:1 to 9.5:1 vs 12:1 to 11.5)

 

I'm not too clear here... What should I be aiming for in dynamic compression ratio and cranking pressure using that website? 25% higher than stock figures for the engine? So I would keep plugging in static CR's until I get the desired dynamic comp ratio and cranking pressure?

 

A 25% increase in cranking pressure from stock is generally the sweet spot in terms of performance. Any higher than that and you tend to blunt your high end power due.

Since you are also concerned about running pump gas, you might want to pay attention to what CR you are at per what %, and pick a reasonable pump gas CR.

As long as you are not LOOSING cranking pressure compaired to stock, you should be good. But going higher will result in more power.

So basicly plug in your stock numbers and find out what the DC is stock, and the cranking pressure stock, and start increasing compression and see where it puts you.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote &raquo;</TD></TR><TR><TD CLASS="quote">
II. Stock ABDC Spec (in crankshaft degrees)

LS (b18a) 31

LS (b18b) 23.5

GSR (b17a & b18c1) 40

ITR (b18c5) 45

JDM ITR (1b8c) 45
</TD></TR></TABLE>

 

@ 100 ft

Stock 11.1:1 JDM ITR gives 10:1 DYN w/212PSI

11.7:1 b20z gives me 10.84:1 DYN w/234PSI
12.1:1 b20z gives me 11.21:1 DYN w/245PSI
12.7:1 b20z gives me 11.77:1 DYN w/260PSI

11.7:1 static yields 10% increase in Cranking pressure and a 8% increase in DYN CR over stock.
12.1:1 static yields 16% increase in Cranking pressure and a 12% increase in DYN CR over stock.
12.7:1 static yields 23% increase in Cranking pressure and a 18% increase in DYN CR over stock.

So what does this all tell me in actual torque and power curves...

 

Well its the same idea as increasing compression in any situation. This way you just know if you are overcamming or undercamming your engine, and where you are in relation to the sweet spot.

When you increase compression you increase thermodynamic efficiency. So you will see gains all along the powerband.

 

Something for you guys to think about.

More compression makes the motor "breathe/flow" better.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Rocket &raquo;</TD></TR><TR><TD CLASS="quote">Something for you guys to think about.

More compression makes the motor "breathe/flow" better.</TD></TR></TABLE>

Could you explain further?

And what do you specifically mean by breathe/flow better (which is often referred to in terms of volumetric efficiency and specifically the head itself)

 

Think about what makes air flow from one end of a tube to another? Answer: Pressure differential.

You chemistry and engineerings major out there plot out the delta P (pressure) for a given crank rotation for a 9.0 CR vs. a 12 CR and you'll see what I mean.

Remember the relationship of the ideal gas law.

P*V = n*R*T

P = Pressure
V = Volume
n = number of atoms of gas
R = Boltzman Constant
T = Temp in Kelvins

If somebody here sees where I am going and can use this to explain what I mean by higher CR motors breath better, I'll be highly impressed.

Hint: delta V = V2 - V1 where V1 and V2 are the volume above the piston at subsequent crank rotations.


Modified by Rocket at 7:15 PM 4/30/2004

 

Gotta say I am lost on this one.

The only thing I can think of is the greater pressure from high compression (by increasing n and T) would create a stronger vacuum when the exhaust valves open. During overlap this could aid in scavenging?

 

Just assume for simplicity sake the n*R*T remains constant.

The trick is V2-V1 is different for lower CR vs. Higher CR.

 

Eh, I give up. lol.

Everything I try and graph doesn't lead me to any logical conclusions. The only thing I have been able to find on the subject else-were is someone saying that more compression reduces mixture volume.

 

I'm sorry I left out it's the percentage change represented as (V2-V1)/V1

Just do this. Calculate (V2-V1)/V1 for Low Comp and High Comp.

Low Comp Scenario:

V1 = volume above the piston at TDC = 41.6
V2 = volume above the piston a certain point after TDC say = 30 + 41.6

so (V2-V1)/V1 = (30 + 41.6 - 41.6)/41.6 = 72%

Meaning the volume above TDC increased 72%.


Hi Comp Scenario:

V1 = volume above the piston at TDC = 32
V2 = volume above the piston a certain point after TDC say = 30 + 32
so (V2-V1)/V1 = (30 + 32 - 32)/32 = 94%

Meaning the volume above TDC increased 94%.

I have to correct myself again, since n = atoms would be less at TDC in the high compression scenario v. the low compression scenario (smaller volume, same pressure, same Temp implies less atoms).

See the difference in %vol change. Given the ideal gas law the pressure would drop in direct proportion and thus cause flow. I mentioned graphing it since the further away you are from TDC the less dramatic the difference.

This is why high CR motors can tolerate bigger cams, because the motor will start pulling air in harder and earlier in the intake and conversely push harder and later on the exhaust stroke. This allows you to run bigger cams with more overlap (more lift near TDC).

I tip my hat to you for being the only one giving it a try


Modified by Rocket at 6:01 PM 5/1/2004

 

Definitely see what you are saying now.

I have only had time to play around with a few graphs using the equation you provided, but its very interesting to see the differences.

Thanks much for the information!

 

Vizard does a nice job explaining it from another angle for the domestic boys, and anyone here who may still be confused:

http://www.popularhotrodding.c...eeze/

I'm sure Rocket knows as much and more given his interests, profession, and colleagues, but you can't beat a nicely written article complete with pictures when it comes time to offer a little free advice.

 

Yeah, to get a really good idea how the % volume is changing would require doing some crazy calculus and and finding the derivative the change in volume above the piston with respect to crank angle. The derivative is hard to formulate but can be approximated using numerical analysis.

 

not really that crazy calculus, because we are already dealing with a cylinder... pi*r^2 *dH
(no triple integrals needed)
the derivative however...

 

Rocket, I think I understand what you're saying, I've only had high school chem when I was a sophomore 2 years ago, but in lamens terms you're saying that higher compression increases vacuum into the cylinder and more pressure pushing gasses out?

 

and to think i fell asleep in math class

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by jiahanhao &raquo;</TD></TR><TR><TD CLASS="quote">not really that crazy calculus, because we are already dealing with a cylinder... pi*r^2 *dH
(no triple integrals needed)
the derivative however...</TD></TR></TABLE>

Well the formula for piston position with respect to crank angle is a bit tough to differentiate but I have a formula for piston position. Numerically just calculate the rate of change of the piston and one can plot this out.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by itsallmotor &raquo;</TD></TR><TR><TD CLASS="quote">Rocket, I think I understand what you're saying, I've only had high school chem when I was a sophomore 2 years ago, but in lamens terms you're saying that higher compression increases vacuum into the cylinder and more pressure pushing gasses out?</TD></TR></TABLE>

Yep yep. On high compression motors the volume goes from being very small to large quicker. And guess what wants to fill the void.

Another term that is used is called "signal". Higher CR motors create a stronger sigal.

 

and StyleTEG, you're saying that increasing cranking pressure over 25% will hurt top end power, because I always thought the higher the compression the better(obviously barring the consequences of the fuel you use), I guess thats only true if you have the cams to match it

Edit: I guess what I'm saying is, I was under the assumption that letting your compression exceed your cams was better then your cams exceeding compression

 

Yeah, in general past 25% you end up "undercaming your engine"

As you increase the cylinder pressure, you are also effecting the momentum the piston has as it is squeezing the mixture.

 

Great info Rocket, StyleTEG