DucatiDon

As most know I am in the early stages of a 383 build.
I have the block at the machine shop getting bored and clearanced.
I have manley forged crank, forged 6" rods and forged icon pistons ready to assemble once the block is ready.
I plan to use AFR 210cc heads.

My calcs put the Static Compression at 10.8

In selecting a cam, I am looking at Dynamic CR.

I have used a couple different calculators, one that uses the cam lobe separation angle, advertised duration, etc... while others only want to know the intake closing angle.

Which is correct? The first calculator is used http://projectpontiac.com/ppsite15/c...tio-calculator
shows that I need a smaller cam to get my DCR UP to 8.5

While the United Engine calulator https://www.uempistons.com/index.php...842e5f9b0a0ead shows I need a bigger cam to reduce the DCR from 9 ish.

Which is correct?

D

rklessdriver

Any of these calculators will get the job done but some are more accurate that others.

Pat Kelly's DCR calculator is about the best one I have used. It is a free download and easy to use althou you do have to switch between (3) different tabs.

http://members.uia.net/pkelley2/DynamicCR.html

It has alot of other useful functionality built into it

(1) The auto calcultion feature for camshaft opening and closing events
(2) The Static Compression ratio calculator

What these people are calling DCR is just an arbitrary measurement of the sweep volume availiable in the cyl when the intake valve reaches it's "closing pont" and compression of that volume (air/fuel mixture) begins.

To properly calculate this DCR you need as close to the actual Intake Valve Closing point as you can get.

Using the Advertised Duration (Seat Timing) and the "as installed" Intake Centerline is the best way IMO - Unless you get out a degree wheel and dial indicators.

I am not personally a big time DCR person. I don't really worry about it too much and I definatly don't build my engines around a specific DCR number. It's useful as a guideline.... It can be easily checked and compared to other similar engine builds.

In the end it's just a number and wether that number will "work" or not has a million other variables attached to it. Many of them more important than the number itself.

Before DCR was around, some people though cranking compression was a magic measurement and before that it was Static Compression ratio.....

Will

REELAV8R

I used this calculator when building my motor.

http://www.jeepstrokers.com/calculator/

I put in your numbers that I could find. Using the Straub cam suggested in your other thread, a .040 squish and 70* intake valve closing, all three calcs came up with 8.3 DCR.
I noticed that the uempistons wanted closing @50 + 15*, that must be for a lash closing so I used 70* and it agreed with the other two calcs.

DucatiDon

Hmm..I cant get that one to run on my Mac....

TheSkunkWorks

Kelley's fairly comprehensive calculator is virtually dead on to many decimals. That said...

While tDCR is certainly a value worth consideration (particularly for the lesser experienced), it isn't necessarily the holy grail for evaluating cam/CR combinations we might wish it to be.

Theoretical DCR (tDCR) as calculated per the intake valve closing event (IVC) is only an indication rather than a true "dynamic" measurement. Actual DCR in an operating engine is static CR x volumetric efficiency (VE%), the latter of which depends on given RPMs among numerous other variables.

Thus, it being possible to arrive at the same tDCR value with builds having radically different VE% profiles (resulting in significantly different peak cylinder pressures), it would be a mistake IMCO to conclude that any one particuler tDCR target range can be broadly applied to just any pump-gas build without leaving an increasingly wider margin the more potent is the engine in question. YMMV

REELAV8R

skunkworks makes some very good points.
Unfortunately for those of us who don't build 5 engines a year or have extensive experience on various combinations of heads, intakes, exhaust systems or a good way to test all the numerous variables involved in VE%, using a DCR calculator is a good way to compare various potential performance differences in cam designs. It's just one of many tools that can be used to estimate the outcome of the build.
So it is a good comparative tool for CR and cam events given all other variables are held relatively constant. Some cam dynamics may dictate differences in other parameters, but it can be kept in a certain "window" of operation. Street vs street/strip vs strip vs race vs radical race etc.

I used it in estimating the performance of my engine during the planning stages and am pleased with the outcome.
If anything I was a bit too conservative on the DCR @ 8.2 with aluminum heads.

scottyp99

I would like to chime in just to make sure I am understanding this properly. Are you saying that an engine with a SCR of 10:1 with a volumetric efficiency of 80% would have an actual DCR of 8:1? (I'm basing that on 10 x 80%)

BTW, the only thing I hate about Pat Kelly's calculator is that it doesn't remember the last sets of input you typed in. If you shut it down, and then later want to compare all the same variables, except using a smaller combustion chamber, you have to start from scratch all over again. Not that big of a deal, but it can be annoying. Overall, it's such a good enough program that I keep using it, though.

Scott

DucatiDon

I guess Id like to know what folks have experienced with DCR below 8, and high 8....issues? Or am i being over analytical?

The larger cam I had intended would reduce the DCR to mid 7s....

The Straub cam looks smaller than I had considered but probably the winner here...

D

REELAV8R

I don't think that is what he is saying Scott. DCR is a physical measurement based on how much of the swept volume is left after the intake valve closes vs the total swept volume which is CR.
I think all he' saying is that if 100 % VE is not achieved then your DCR will not actually be achieved, lowering cylinder pressure.
Flip side being if you achieve greater than 100% VE you will achieve something greater than calculated DCR and thereby greater cylinder pressures.
The variables involved in either are many and not able to be calculated with absolute certainty.

scottyp99

Well, in that case, here is what is confusing me. In his post, Skunkworks says :

"Actual DCR in an operating engine is static CR x volumetric efficiency (VE%),"

Now, skunk is a pretty sharp dude, and so are you, av8r, so I don't know what to think. Are you saying it's not an actual equation, just sort of a concept? I could wrap my head around that.

Scott

rklessdriver

DCR is an actual physical measurement but of only one point in time - of one point during the compression stroke.... it happends to be a pretty important point (ie when the intake valve closes) but still...

What about when the intake valve opend?

What about how long the intake runner is?

What about how dense the air (ie water grain, Density Altitude, ect) is?

DCR is just one point in time and it dosen't take into account everything an "actual DCR measurement" of a running engine would. You can see the concept of DCR is something much bigger....

When an engine is actually running you have to take into account RPM and VE. The amount of volume deposited in the cyl is affected by things that vary by RPM. Like "ram effect" in the intake port. This plays directly to the VE% of an engine.

So like I said, DCR is useful, it's just not something to hang your hat on.

When you look at building a pump gas engine you should be more focased on looking into things like the combustion chamber shape, plug location, plug heat range, cyl pressure, specific Gravity of the fuel used, temp of the chamber, spark advance... I can go on and on because even things in the intake tract affect this.....

These things control when the mixture starts to burn, how fast the flame front moves across the chamber, how completely it burns and when the burn is finished.

Will

REELAV8R

Agreed skunk is a sharp dude and likely knows more about this than I do. I'm not educated in this area nor am I very experienced, but have read a lot, built a few engines (only one Chevy non stock built V8) and can wrap my head around some concepts in this area by using what I've learned so far and some common sense.
I think he is comparing theoretical DCR to actual DCR once the engine is built.
We can only compute the theoretical DCR of an engine based on some general parameters like cam dynamics, rod length,bore, stroke etc.
However the realized DCR of a completed and running engine is going to be different, and VE% is part of what is going to affect that actual DCR.
VE% in itself is affected by numerous influences and will vary depending on what descisions are made to the items that influence VE%.
We may want a shorter duration cam for good street driving and nice bottom end torque, but that is going to reduce the VE% at 6000 RPM. It's a trade-off for what we want from the engine.
An inadequately sized intake for an engine will certainly starve the engine for air and negatively affect it's VE%.
Each build has to take into account what is the desired outcome and where do we want the engine to perform (RPM range).
Best VE% is typically achieved at the max torque number. Above that RPM it begins to fall off. The cylinders cannot fill to capacity due to the limited time available to fill the cylinder as RPM's increase.

Sounds like rklessdriver knows what he's talking about and I agree with what he is saying.
Density altitude is one item I see almost no one mention on this forum even though it is a significant factor in the design of the engine and it's operating environment.

REELAV8R

Just though it might be useful to provide a definition of VE%.

Since each cylinder has one intake stroke every two revolutions of the crankshaft, then the theoretical maximum volume of air it can ingest during each rotation of the crankshaft is equal to one-half its displacement. The actual amount of air the engine ingests compared to the theoretical maximum is called volumetric efficiency (VE). An engine operating at 100% VE is ingesting its total displacement every two crankshaft revolutions.

DCR like previously said is the swept volume after the intake valve closes, well really after .006" or .004"of lift or less in most cases since that is the advertised duration number.

scottyp99

I was just asking if that was literally the equation to figure out actual DCR. I think you guys have "over-answered" my question.

Still, I do feel a bit smarter now, so thanks!

Scott

REELAV8R

sorry bout that. Bad habit.

gkull

Before all the 10.8 how did you get this? flat tops and how many cc are these 210 heads?

To me DCR is kind of a close guess lots of variables. I used all the calcs and went wrong on my first 383 even with thermal coated pistons with 64 cc and a solid roller cam of 232/240 with Crane cams valve event numbers.

I was running a big 830 cfm carb single plane port matched that added the ram effect so my VME was really high with mid .600 lift valves. The stupid DCR clacs don't have allowances for better breathing like your 210 heads or mine.

my original 383 was a pinging POS the cure was a custom lobes of 236/242 with again mid 600 lift 112 and valve event number and lobes i chose out of the comp cams lobe charts

rklessdriver

Going back over this I just saw these questions.

All I can tell you going back to what I said in my very first response.

There is way more to these engines than just a number.... If pick a camshaft based off what the DCR will be you are in for a world of disapointment.

I can give you numerous examples of engines I have built (hell there are build threads - dyno threads and first track outing threads on this board for at least 2 of them) and yes the SCR/DCR numbers for them are listed.... won't do you a bit of good one way of the other unless you built a carbon copy of one of those engines.

Are you being over analytical?

I feel it's unfair to expect a regular guy that is only going to build a few good engines for himself over his lifetime to understand some of these concepts and understand their position in how important or unimportant they are.

When you are in that position the BEST thing you can do it find a smart guy and follow his direction. Try to lean what you can from him but don't dwell on what you don't understand. You can get tied up on something so insignifigant in the big picture that you miss an oportunity to lean something really important.

IMO DCR is such a thing. You will need some other background info/knowlege and to understand those other things before the DCR theroy will come into even a fuzzy focas... once it is you see that DCR is not important enough to base the descision of ANY other engine part around during an engine build.

If you have been working with Chris Straub on a cam - My advice would be to keep doing so. He will steer you in the right direction. We buy parts from him and have done so for a pretty long time. He's worked at a number of racing engine parts suppliers over the yrs before opening his own place.... He has never done us wrong.

Chris is a sharp cat and very knowlegable. He does this stuff for a living..... look at the people that he has helped with engine builds... here, z28, Yellow Bullet, Chevells.... there is plenty info on the internet about his work and I have yet to see something of his that didn't meet expectations. Disreguard his cam recomendation at your own peril....
Will

DucatiDon

gKull....I have AFR 65cc heads, flat top pistons with 6.4cc reliefs.

REELAV8R

If you are pushing the limits on DCR then it will become more critical to pay attention to things like, squish, intake temps, mixture, plug heat range, engine cooling etc to prevent detonation.
Squish distance should be at .040" for best results without risk of hitting the head. Mine is at .038" but in a 350 with 5.7" steel rods. I've read of guys going down to .030". I'm not that brave.
I also put a splash shield on the underside of the intake to keep hot oil on it to a minimum, and blocked the crossover heat.
Added additional cold air intake. Plugs in proper heat range, mixture just this side of rich at WOT, and timing 34*. Installed electric fans and extended the air dam for more air through the radiator to keep it at or near the 180* thermostat temp. All in an effort to stave off detonation while producing the most power I can get.
I'm not getting anywhere near 100% VE I suspect so that makes it easier to avoid detonation as well.
So far so good at 8.2 DCR, aluminum heads and 87 octane fuel equivalent.

hugie82

Don, you also need to factor how far down TDC is from the deck, head gasket thickness, cam duration and the other measurements you mentioned.
Also, if you're running aluminum head, most engine builders will say you can go that extra point of compression without detonation. Compared to iron heads.
For a street engine, I always lean towards the smaller cam. There's always tricks you can do to keep knock away (timing, cooler stat, 1:6 rockers, ect....)but go too big and you'll kill the fun factor with no low end torque
Summit has a pretty good calculator for compression