SCR DCR, CAM... ?'s

okay, i'mve trying to grasp everything, but atm i'm stuck on this...
my specs are
264/274 .450 110* 4* advanced

CI sheet, http://classicinlines.com/200Z.asp#D40
chamber =50 ,50
IVC = 60 , 65
SCR = 9.35 , 9.35
DCR = 7.74 , 7.47

but i'm a 62 IVC i'm probably ~ 7.63 DCR
If i'm not mistaken a stock 65 engine has DCR = ~7.14

Now here comes another question... i'm 4* advanced, does that add to the 62* or take away from the 62*?
my cam is ground stright up, so 4* makes it .050 so what does that affect? or does it not?

to increase DCR I can increase SCR but won't that hurt the engine? or to increase the DCR can't I advance the cam? or change the cam to a less duration...

i'm farting around Ideas before i do anything... already did a quick build but didn't pay attention to what i should've thought thru...

Hey MPG:
Using my figures
Bore: 3.71
Stroke: 3.126
Rod Length: 4.715
I came up with the fallowing:

MPGmustang
to increase DCR I can increase SCR but won't that hurt the engine?

Click to expand...

SCR of 9.0:1.
Effective stroke is 2.54 inches.
DCR is 7.30:1 .

If you raise SCR
SCR to 9.4:1.
Effective stroke is 2.54 inches.
DCR is 7.63:1 .

If you maintain both I don’t see the harm??

MPGmustang
or to increase the DCR can't I advance the cam? or change the cam to a less duration...

Click to expand...

I don’t know the answer to the above.
But
You may be able to find it HERE That’s where I got the answers I gave you above.. :mrgreen:

how does advancing the cam affect DCR?

I understand it lowers the opperating RPM spectrum, I just wonder if it changes the DCR...

I'd like to increase the DCR, without extra expenses.

Thanks in advance for any info

Richard

MPG as I understand cam timing, advancing the cam will have the effect of lowering the DCR..

Examples as I understand it:
If you had a cam with the IVC at 60*
Your dynamic compression ratio would be 7.63:1 .

If you advance that same cam 5* to IVC at 65*
Your dynamic compression ratio would be lowered to 7.36:1 .

If you retard that same cam to 55*
Your dynamic compression ratio would be higher at 7.89:1 .

So to increase your DCR as I understand it, you would need to advance the cam.
Also I have read if you advance any cam more than 6* you would need a different cam.

I would think you should be able to increase your DCR with out extra expense. If you do not exceed 6*?
Problem: I’m not sure my understanding is correct…
SO
Earth calling all cam experts..... now is the time for all cam experts to come to the aid...

hmm hoping to get more repiles

if your cam has the IVC @ 62* and you advance it 4*, does that mean it now closes @ 58*?

MPGmustang":229ghw95 said:

hmm hoping to get more repiles

if your cam has the IVC @ 62* and you advance it 4*, does that mean it now closes @ 58*?

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That is a good question.. I’m glad you put it that way because now I’m not sure.

If you move 62* to 66* you would have more volume in the chamber.??
If you move 62* to 58* you would have less volume.??

So which way represent advancing. ?
I will be needing that cleared up soon.

I have looked at my degree wheel so long I think the reflection has etched an image on my forehead..

Just thinking out loud here:
When you advance the ignition timing the timing mark move CCW.

Using that same concept. I put the DEGREE WHEEL with BDC at TDC.
Looking at where the IVC at 62º, and using the CCW theory for a 4º advance it would move CCW and become 66º.
Ant saying this is correct.. It’s just my simple reasoning..

According to those "in the know", the single most important detail in cam design is the Intake Valve Closing (IVC) event; keeping this in mind, advancing the cam will close the intake sooner, thereby enhancing lower rpm torque (relatively speaking) and retarding the cam closes the intake a bit later which means the air flow will keep ramming in just a bit longer at higher speeds.

This is all a very general statement, YMMV.
Joe

Thank you Lazy JW, that was very helpful to me.
I’m just looking for conformation here..
If the cam is grind for IVC at 62º and I waned to advance it 4º would I degree it in at 66º ??

I spoke with Mile a couple days ago.. He said if the cam has a 110º LC you would set it on Center Line at 106º for a 4º advance…

echo1955":2006r9zo said:

Thank you Lazy JW, that was very helpful to me.
I’m just looking for conformation here..
If the cam is grind for IVC at 62º and I waned to advance it 4º would I degree it in at 66º ??

Click to expand...

EDIT: I may have misunderstood your question, but I hope the info helps nonetheless. I believe the previous post more directly addresses your degreeing question and the intake centerline value...

Read LazyJW's post again. If you ADVANCE the cam, the intake valve closes SOONER. For example, your IVC is 62, you advance 4 degrees (sooner), your IVC is now 58 degrees.

Why do you want to advance it 4 degrees? Just curious?

59F100":1shluc0u said:

Why do you want to advance it 4 degrees? Just curious?

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Howdy 95F100,
I'm really asking to see what my true DCR (Dynamic Compresion Ratio) is. @ 58* I have a higher DCR than if it wasn't advanced. I was really looking at what it could do for me if I advanced it 2* more. Rule of Thumb, more DCR = more Torque and the need of higher octaine but most importantly a lower RPM range of operation. I don't mind using the 91 at the pumps, it's the race octaine I am trying to advoid and there for seeing how far I could Advance the cam to keep lowing the RPM range, Granted it's not smart to advance it 8* so I only wanted to know the 6* DCR, currently it wouldn't be enough for me, I would have to raise SCR to increase DCR and still advance the cam to 6* or increase SCR for the DCR and add a 1.65 roller rockers as they effectivly do the same effect as advancing the cam but with the benefit you keep your higher RPM range.

I hope I made sense, and for anyone that "understand" what i'm trying to accomplish does this sound correct? or am i just rolling on my own understanding wrong.

Okay I see what you are trying to do. I would be careful to just advance your cam however without carefully determining the effect it will have (which you are trying to do).

Raising your static compression ratio (i.e. shaving heads, using domed pistons,...) WILL raise your dynamic compression ratio without making changes to the camshaft or valve train.

Raising your dynamic compression ratio (i.e. advancing your cam...making the intake valve close sooner) WILL NOT have any effect on your static compression ratio.

What you are dealing with is three values.

V1 = Volume of one cylinder at TDC (top dead center)
V2 = Volume of one cylinder at BDC (bottom dead center)
V3 = Volume of one cylinder at IVC (intake valve closing)

SCR = V2/V1
DCR = V3/V1

SCR is a fixed value that never changes.
DCR is a theoretical value that "can" change with engine rpm. The compression value or PSI (air pressure) in that cylinder will vary due to the "ram effect" (air rushing into the cylinder). The calculated DCR based on the IVC will be slightly lower than the DCR with engine running. Thus the "dynamic" CR designator.
A roller cam with roller rockers will not change your calulated DCR value which is based on the IVC point. But it will have the effect of raising DCR during engine operation because they hold the valve closer to full open for a longer period of time (they dont have to start closing the valve as soon as a flat tappet cam would) because of their steeper ramps. This is dependent on the cam lobe only, not the rockers.

With all that said, there is a "rule of thumb" DCR value that you should shoot for based on the gas you plan to use, engine design, intended use, etc...For a typical street engine and regular unleaded I THINK that value is somewhere aroung 7.5-7.8 DCR...dont quote me on that.

59F100":1bvtrm6p said:

echo1955":1bvtrm6p said:

Thank you Lazy JW, that was very helpful to me.
I’m just looking for conformation here..
If the cam is grind for IVC at 62º and I waned to advance it 4º would I degree it in at 66º ??

Click to expand...

EDIT: I may have misunderstood your question, but I hope the info helps nonetheless. I believe the previous post more directly addresses your degreeing question and the intake centerline value...

Read LazyJW's post again. If you ADVANCE the cam, the intake valve closes SOONER. For example, your IVC is 62, you advance 4 degrees (sooner),
your IVC is now 58 degrees.
Why do you want to advance it 4 degrees? Just curious?

Click to expand...

Thank you 59F100. You have answered my question clearly.
It’s not that I want to. It’s because I have been advised to advance the cam 4º from more than several sources. I am told this will lower the power band and give better performance at lower rpm.. while sacrificing very little on wot.

Other than that I have no rhyme or reason. Just trying to fallow those with the experience for maximum performance….

Thanks again you were a big help…


ps: The simple to experience is complex to me…. Thus the reason I ask simple questions..more than once

59F100":1kzvw9xe said:

A roller cam with roller rockers will not change your calulated DCR value which is based on the IVC point. But it will have the effect of raising DCR during engine operation because they hold the valve closer to full open for a longer period of time (they dont have to start closing the valve as soon as a flat tappet cam would) because of their steeper ramps. This is dependent on the cam lobe only, not the rockers.

Click to expand...

This sounds more correct that what I have been thnking it does, it keeps it all the same but because it CAN hold it fully open SOONER & LONGER & still open/close at the same times it would increase the air volume due to it's ram air effect

I think when I pull the head again for the 2bbl conversion i'll ask to shave some off, this will increase SCR and DCR making more HP and letting me use 91 (currently 89 summer 87 winter). I'll also eventually invest, although not quickly, in the roller rockers for the engine. I got some other projects that I need doing first

http://www.wighat.com/fcr3/index.html

A lot to read through, but very well worth the effort.