Fahad

Hi Everyone,

I have two question and can you answer and explain, How or What the point is?

Did the camshaft do more compression on the CR of the engine? for example, if we change the camshaft did we can see any difference in CR at the same engine?

Here's another question:

What's that mean >>> 114+ or 112+ LSA

When i see some cams i found their lobes with + mark!!!

Thanks in advance

elias_799

iTrader: (8)

lsa stands for lobe separation angle

i am pretty sure you can not increase static compression with a camshaft.

if you want to increase you static compression here are some common ways : mill the heads, change pistons, get heads with a smaller combustion chamber.

108dragon

iTrader: (5)

A lot of overlap in a cam bleeds off comp or boost at lower rpms and is sometimes employed for such effect. But I've never heard of any cam that will increase compression any more than what is already there.

TORCHD 02 TA

iTrader: (39)

Here is a newb question...

What is the stock CR?

Fahad

I'm sure same you but i discussed that with some friends and they're very sure that is any cam could make more compression on the engine because they already do a test by their selfs.

Your ways about increase more compression is all correct but how about my question???

Please ask and let us know what's the right answer.

LS1 engines 10.0:1 and LS6 engines 10.5:1

5.3LJimmy

iTrader: (10)

Different cams change the Dynamic compression ratio of the engine and will change cranking compression(as measured with a compression tester). Generally the larger the cam the less cranking compression and DCR the engine will have. The 112 and 114 you commonly see are the lobe separation angles and the +/- is usually used to show the intake centerline(or cam advance/retard).
Ex: 112+4 is a cam with a 112 lobe separation angle and 4 degrees of advance ground in and an intake centerline of 108
110-1 is a cam with a 110 lobe separation angle and 1 degree of retard ground in and an intake centerline of 111

elias_799

iTrader: (8)

just like i said before you can NOT increase STATICK compression with a camshaft.

ls1 engine 10.2:1 CR not 10.0:1

TORCHD 02 TA

iTrader: (39)

The excel spread sheet that someone made back in 2007 or so for VE. When i typed my cam values into the formula my SCR is 10.37 : 1 and my DCR is 8.28 : 1

108dragon

iTrader: (5)

+1 Nicely stated

108dragon

iTrader: (5)

I'm not sure what you are asking. 5.3LJimmy about pegged it. I'm pretty sure you can only decrease DCR with a cam, but not increase it. The only exception is if you actually STARTED OUT with the big cam and weren't maxing out your possible DCR to begin with. Then it would seem like the cam change to the smaller cam actually increased the DCR.
I asked a similar question about cam overlap a while back and never really got a solid answer.

5.3LJimmy

iTrader: (10)

I was assuming this to be the case. It sounds to me like they were swapping different aftermarket cams to maximize the DCR of an engine without changing static compression. It's very easy to pick up 20-40 PSI of cranking compression by moving the intake valve closing point.

Fahad

wow that is very nicely.. This's what i looking for...

So the cam do a compression dynamicly, right?

108dragon

iTrader: (5)

Again, the only way for a cam change (or physical install advance) to INCREASE the dynamic compression is if the cam in the engine now was REDUCING it. Advancing the cam a few degrees in this case should increase the dynamic compression and increase low end power, all things being equal. But there are other factors that play into this as well.

I remember reading several arcticles where people have used this to thier advantage in builds. But the following exerpt from a Kieth McCord arcticle came to mind first:

"Camshaft choice is probably the number one problem that people run into, and many times, the wound is self-inflicted. As we all know, a camshaft is more than just a Saturday night cruise sound maker; it can make or break an engine for performance, economy and drivability. Too many customers install the biggest camshaft possible without thinking about the engine as a whole.

For this design, Riskovsky chose an off-the-shelf LS7 stock camshaft from the Z06 Corvette. The specs of the camshaft are 211/230 @ .050 120 LSA, .558(1.7)/.588 (1.8). The camshaft was installed at 5 degrees retarded. By installing the camshaft retarded by five degrees, the dynamic compression was dropped from a risky 8.2:1 down to a boost-friendly 7.65:1. This also allowed the shape of the power curve to be shifted higher, which favors this setup due to the fact that it will make gobs of torque down low, but due to the smaller turbos, may run out of breath on the top end."

Hopefully, this example sheds some light on the subject from an application standpoint.

hammertime

iTrader: (1)

The name "static compression" implies that it is fixed by the mechanical components in the engine. At Bottom Dead Center you have a cylinder with volume A, plus a combustion volume B. As the crank turns, the sum of those two volumes is compressed to occupy only the volume B.

A = Cylinder 3.9 diameter and 3.62 length
A = [(3.9/2)^2] * 3.1415927 * 3.62 = 43.24ci

B = 78cc combustion volume = 4.76ci

(A+B)/B = (43.24+4.76)/4.76 = 48/4.76 = 10.08:1 compression ratio

Unless these fixed dimensions change (bore, stroke, combustion volume) you cannot change the "static compression ratio".

I know what you are thinking, where did 78cc's come from, LS1 heads are 67cc's? Note that the combustion volume is actually the sum of several volumes, including the chamber on the head, the volume of the head gasket when compressed, any dish or dome on the piston, and any positive or negative deck clearance (depth the piston is down the hole at TDC, or above the deck at TDC).

Dynamic compression is the same formula as static, applied to the volume of the cylinder when the intake valve closes. Since it is impractical for us to close the valve at BDC, dynamic compression will always be less than the static compression ratio. Lots of complex math can allow you to calculate the exact point at which the valve closes for a given camshaft and intake centerline angle. You can advance or retard a cam to change the dynamic compression. For many years, major manufacturers would always advance their grinds to bring the dynamic compression up a point or two.

If you have two different cams, you'll need to calculate the intake valve closing point to compare dynamic compression.

220/220 .551/.551 112+0 [(220/2)+112]-180 = 42* ABDC
224/224 .563/.563 114+0 [(224/2)+114]-180 = 46* ABDC
230/236 .592/.602 112+0 [(230/2)+112]-180 = 47* ABDC
242/248 .608/.612 110+0 [(242/2)+110]-180 = 51* ABDC

As the valve events change, you can see that larger lobes tend to push the IVC farther and farther ABDC. Tightening the LSA becomes necessary to maintain dynamic compression.

All the above examples are at +0 advance, so the ICL is the same as the LSA. Adding advance decreases the intake centerline. If you have different cams, with the same lobes, but different LSA, the dynamic compression will depend on the intake centerline angle.

224/224 .563/.563 114+4
224/224 .563/.563 112+2
224/224 .563/.563 110+0

All use the same 110 degree ICL, and therefore all have the same dynamic compression. Remember, you cannot change the LSA without regrinding the cam; you can only change the ICL by advancing or retarding the camshaft.

Some will also tell you that dynamic compression is changed when boost is added to the equation. There are calculators online that show this as well.

01ssreda4

iTrader: (96)

^^^nicely put....

108dragon

iTrader: (5)

Yeah.. to an automotive engineering major. I'm gathering that the OP's native language isn't English... ergo my method of explanation. But that post is definately worth a cut n paste to the engineering files. lol

hammertime

iTrader: (1)

Your post came as I was trying to get mine together. You and 5.3LJimmy both gave great answers for the OP.

And yes, I'm betting some of the terminology in my post may be lost on someone who hasn't read english text on automotive performance for 20 years. I attribute the bulk of my knowledge to this forum - just have to use a good BS meter at times.

108dragon

iTrader: (5)

Actually there are guys like you here on LS1tech that provide information specific to our cars that I CAN'T find anywhere else...which is why I cut n pasted your post to my "engineering files". Truthfully, I've even learned from the guys I've gone rounds with on these threads. Minus the trolls you run into every now and again (which are on all forums), LS1tech has consistantly drawn the sharpest and most experienced people for the rest of us to gain knowledge from.

01ssreda4

iTrader: (96)

the way i see it some answers just arent so simple like the OP wants. not everything is cut and dry. And sometimes when you ask complicated questions you get complicated answers

108dragon

iTrader: (5)

This is also true... but more often the case is people making things harder than they have to be. It has been said that if it cannot be explained in mathematical terms it is not science, but opinion. I would prefer to substitute the words "an art" for the word "opinion"... Often times "art" is easier to grasp than "science". lol