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rskram
Cochise

265 Posts

Posted - 19 Jun 2012 :  6:34:25 PM  Show Profile  Reply with Quote
I ran some numbers for my cam/compression on Wallace's calculators and came up with this:

Static compression ratio of 10.75:1.
Effective stroke is 3.55 inches.
Your dynamic compression ratio is 9.14:1 .
Your dynamic cranking pressure is 188.74 PSI.

Later I researched that it's recommended that DC should be between 7.5-8.5 for 91+ octane but idk. Now I'm second guessing this, does anyone know if my DC is too high?

These are my cam specs:
Lobe separation 110deg
Duration at .020 I 290, E 298
Gross valve lift I .600" E .620"
Valve lash hot .016" & .016"
degree intake lobe to 106

Timing at .050"
I---OP 24 CL 56
E---OP 68 CL 20
260 deg I
268 deg E
44 degrees overlap

forgot to add...solid flat tappet



Edited by - rskram on 21 Jun 2012 11:50:15 PM

screamingchief
Tribal Scout

USA
30 Posts

Posted - 20 Jun 2012 :  7:12:47 PM  Show Profile  Reply with Quote
You're making the same mistake tons & tons of guys make when computing and comparing DCR numbers.

Problem #1 is everyone has to be using the same valve event timing to accurately compute & compare DCR numbers.

That means you cant use the .050" valve event timing and accurately compare your DCR numbers to another fella that computed his DCR numbers using the advertised (seat) valve event timing,as that almost always will skew your DCR numbers significantly higher than his.

Also,understand that comparing DCR numbers between a hydraulic cam combo and a solid cam combo can also affect that dynamic,as they rate the advertised duration numbers at differing lifts (example: hyd @ .006" vs. solid @ .020").

Honestly,IMO using DCR calculation as a precise tool is virtually useless.

There are just too many variables it does'nt account for @ all.

So it's really just more of a "loose guide" than an absolute truth.

Anyhow,use the event timing for the advertised duration to calculate the IC event for the cam you have and your DCR numbers will look more realistic,and be slightly closer to what you mentioned as being "acceptable" for pump gas use.

290 degrees of duration @ .020" on a 106 ICL will have an IC event of 71 degrees ABDC.

Wallace racing says:

Static compression ratio of 10.75:1.
Effective stroke is 3.12 inches.
Your dynamic compression ratio is 8.16:1 .
Your dynamic cranking pressure is 163.11 PSI.

Does that look any better now?

So the moral of the story here is to never use the .050" valve event info to compute the DCR of a combo,always use the "seat" valve event timing instead.

HTH

Bret P.

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rskram
Cochise

265 Posts

Posted - 20 Jun 2012 :  9:22:13 PM  Show Profile  Reply with Quote
Cool, that's a big relief then! Thanks for explaining how that works Bret, I appreciate it.
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Blued and Painted
Chief PONTIAC

USA
2542 Posts

Posted - 21 Jun 2012 :  10:21:38 AM  Show Profile  Reply with Quote
I like this one http://www.kb-silvolite.com/calc.php?action=comp

Bull Nose Formula, 461, R44TS, DEX/MERC
69 GTO-400/670/373/4Spd
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screamingchief
Tribal Scout

USA
30 Posts

Posted - 21 Jun 2012 :  1:36:58 PM  Show Profile  Reply with Quote
quote:
Originally posted by Blued and Painted

I like this one http://www.kb-silvolite.com/calc.php?action=comp


That one can be quite inaccurate as well,as it "approximates" the seat (advertised) valve events by telling you to "add 15 degrees" to the .050" IC event number,but that wont always be accurate as many cams will have far more (or less) than 15 degrees difference between the .050" IC event and the seat/advertised IC event.

It's only pure luck that rskram's numbers would work OK with that KB DCR calculator.

But it's fairly common for a given cam to have more (or less) seat timing with the same basic .050" duration.

Just look at the difference between an 041/RAIV style hydraulic cam once and you'll see why the "add 15 degrees" thing is woefully inaccurate.

IC for that cam @ .050" is 47 degrees ABDC.
IC for that cam @ the seat is 86 degrees ABDC.

That's a 39 degree difference...

What everyone needs to understand is that an engine does'nt really begin compress the cylinder very well till the intake valve is fully closed,and that's why using the seat IC event is the prefered method to compute the DCR.

Now that KB calculator can work fine using the seat IC event,you just gotta know what the seat/advertised IC event is for the given cam,and then you ignore the "add 15 degrees" non-sense and just put that known IC event number into that box and it'll spit out the same numbers as the Wallace DCR calculator does.

See,that's why I said DCR is mostly useless for a comparison tool,as each DCR calculator likes to compute the DCR in a slightly different manner,and that tends to confuse the users of those tools,when the truth is,if it's to be a truly accurate tool,they all need to kick out eactly the same numbers,otherwise one of those calculators is working with the GIGO principle.

FWIW

Bret P.
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Blued and Painted
Chief PONTIAC

USA
2542 Posts

Posted - 21 Jun 2012 :  2:49:02 PM  Show Profile  Reply with Quote
Boy you guy's from Texas are smart. Lot of good drag racing down there too. I think what we are trying to do is make sure our engine, or virtual engine is within the established norms to run pump gas. I'v seen calculators with alot more inputs but venders dont always make wrist pin placement and other info readaly avalable. I did not bookmark that one because i had to guess a to many values. So for the weak end warrior, not running a cam from the stone age, these calculators Can tell us if we have wandered out side the Box. IMO- B&P

Bull Nose Formula, 461, R44TS, DEX/MERC
69 GTO-400/670/373/4Spd
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screamingchief
Tribal Scout

USA
30 Posts

Posted - 21 Jun 2012 :  7:01:45 PM  Show Profile  Reply with Quote
Well,wrist pin placement would'nt have much of anything to do with calculating the DCR,except as how that might vaguely affect it via calculating the static compression ratio,strictly based on how that given spec relates to the engines deck height measurements or such.

As for cams from the stone age,that's just missing the point completely,sorry.

Inaccuracy is still inaccuracy no matter whether you care to acknowledge it or not.

And FYI: That + 15 degrees @ .050" deal would also be an issue with "modern" cams as well.

Take the Comp xe274 hydraulic cam,as it's a pretty popular "modern" cam,is it not?

IC for that cam @ .050" is 41 degrees ABDC.
IC for that cam @ the seat is 63 degrees ABDC.

That's still 22 degrees of difference.

Clearly the "+ 15 degrees" thing is inaccurate,be they "modern" cams or "stone age" cams.

I could even show you some cams (modern or not) that would be less than 15 degrees of difference between the .050" and seat.

All I'm doing is pointing out how inaccurate that KB calculator is at trying to approximate the actual IC event.

And I'm trying to explain how ya'll can use that KB calculator so that it will match the output data of most of the other DCR calculators out there,simply by ignoring the "+ 15 degrees" nonsense and just using the actual seat IC event for the cam.

It's exactly that sorta inaccuracy that gets guys into deep trouble when trying to use these sorta DCR numbers,especially when they are out there trying to compare them to another fella's DCR calculations.

As for vendors not supplying specific info,like the cam co.s not giving out their cams seat IC event,that's NBD really,things like that typically just involves doing some simple math.

To calculate the IC event (seat or .050"),just divide the given duration by 2,add to that the ICL,then subtract 180.

Example: Lets calculate rskram's solid cams seat IC event.
290 divided by two = 145
145 + 106 = 251
251 - 180 = 71

So the IC for that cam @ the seat is 71 degrees ABDC.

It's not all that hard to get the DCR numbers to jive calculator to calculator,but everyone has to be using the same basic formula and inputs to allow any comparisons to be even remotely accurate.

If all this seems too overly complicated,then I'm almost certain that trying to use DCR as an effective engine building tool will be a real bad idea for any given fella.

It's not really complicated @ all.

Me,I feel messing with this DCR stuff is mostly a waste of time,JMO.

But that's largely because I understand it pretty well too.

P.S. I'm not trying to be a pr!ck here,I'm just trying to give the best info so all can use this DCR stuff as well as is possible.

Bret P.
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Blued and Painted
Chief PONTIAC

USA
2542 Posts

Posted - 21 Jun 2012 :  7:45:25 PM  Show Profile  Reply with Quote
I think rskram ran a calculator, saw somthing out of wack, and asked a question. Good Job. S chief, we apreciate your input on the in's and out's of DCR calculators. Do you think rskram's cam selection is right for 10.75 SCR

Bull Nose Formula, 461, R44TS, DEX/MERC
69 GTO-400/670/373/4Spd
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BearGFR
Bear

USA
569 Posts

Posted - 21 Jun 2012 :  9:55:04 PM  Show Profile  Visit BearGFR's Homepage  Reply with Quote
I can relate to the dilemma, rskram. When I started planning out my engine I was obsessing over compression, DCR, and everything related to it. The best thing I did was to license a copy of Performance Trends Engine Analyzer and start playing with it. It didn't take long to see that on "my" 461, even as much as a 5-points of compression (going from 9.5 up to 10.) was only worth a whopping 7 HP (on an engine that was already making 500 HP). From that revelation, it was easy to decide that trying to push the envelope on compression for me just wasn't worth the associated risk. Yeah, if everything was "perfect" I could get away with it - probably, but if everything turned out not to be "perfect" then I'd be looking at having to tear it down and address the problem, possibly having to buy different pistons to get more clearance volume, etc. On a max-effort race car where every ounce of power matters, you bet. But on a (mostly) street car? Like I said, for me the few HP that would have been gained just wasn't worth the associated risk.

"Your mileage may vary, void where prohibited by law, past performance is no guarantee of future return".... and all that.

Bear

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rskram
Cochise

265 Posts

Posted - 21 Jun 2012 :  11:44:48 PM  Show Profile  Reply with Quote
Blue I think according to his first post it looks okay?
Bear I hear ya, it looks like I may have it nailed down pretty close looking at all the info that Bret posted. Time to stop worrying so much, I do have 93 octane available so I will try that first.
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Cliff R
Cochise

USA
470 Posts

Posted - 21 Jun 2012 :  11:55:12 PM  Show Profile  Visit Cliff R's Homepage  Reply with Quote
The "rule of thumb" for adding compression is that you also add apprx 10 degrees more camshaft with each full point of compression ratio increase. This simply means that a cam that works good in a 9 to 1 CR 455 is too small for a 10 to 1 CR 455. To act the same, idle, off idle, etc, it needs to be about 10 degree larger if we increase the CR by apprx one point.

Folks continue to look at the power improvements as just a few HP. You have to get away from the computer for just a minute, and think about this topic in terms of what it takes to make power. Compression can be your friend, and running high compression simply means that you do things so that the engine never sees any more dynamic compression at low rpms than it would with less compression and a smaller camshaft.

Peak torque is a critical point in the engines rpm range when cylinder pressure is highest, engine load is highest, and octane requirements are greatest. Beyond peak torque, cylinder pressure falls off, as the events are happening quicker, so there is less time for cylinder filling.

We choose larger cams on wider LSA's for our higher compression ratio engines. They spread out power, lower peak torque numbers, and move peak torque higher in the rpm range. These things are all positives as far as octane requirements are concerned. They are also positives for vehicle performance. The broader the power curve, the more power we can apply to the pavement over a greater rpm range. Moving power up in the rpm range also helps with traction, and when we use good flowing heads, the engine enjoys strong upper mid-range and top end power. Smaller cams on tight LSA's pull power down, tighten up the power curve, and "spike" high dynamic compression at a lower rpm.

What is even more suprising is how strong an engine "feels" with a small cam on a tight LSA. These cams throw all their power at you right off idle, and fool the user into thinking vehicle performance is much better than what it really is. This is why any sort of "seat of the pants" evaluation is completely useless for predicting how well any particular part works vs another. This includes cams, intakes, carburetors, exhaust systems, and combination of all of the above. It takes a drag strip and good traction to really find out what's going on with these things....IMHO

Anyhow, bigger cams, wider LSA and higher compression produce a very strong/broad power curve. There is no real "rush" of power anyplace, the power is more "locomotive" like in contrast to the strong/quick "nitrous" type power produced by the smaller cams.

With my own car I still can't believe how quick it runs at the track. The power is very smooth, no real "rush" of power anyplace, just pulls hard all the way down the track, in any gear at any rpm.

A few years ago we built a 455 with a 247/254/108 cam thinking the big cam on a tight LSA would be a good way to go. That engine to date was far and above the winner by any sort of "seat of the pants" evaluation. It took the big 455 and made it much better at what they are already good at, throwing ALL their power at you right off idle, and STRONG mid-range power. I thought for sure after driving the car and pulling thru the gears on the street, that it would run almost into the 10's at the track. That engine had the most "explosive" mid-range power curve I've ever seen in any engine, I really did think it was going to push the car into the 10's or close to it. Instead, it was a low to mid 12 second ride at 109-110mph.

We yanked the cam out, went to a smaller HR cam on a wider LSA. We also changed the heads which lowered the compression ratio slightly, but flowed better. The new set up was "boring", much smoother idle, no more big rush of power anyplace, just pulled hard till you felt like moving the shifter. The very first outing netted 11.70's at 118mph! BIG improvment in actual vehicle performance, car felt considerably SLOWER than it did with the tight LSA camshaft.....FWIW.....Cliff


If you can read this, thank a teacher. If you can read this in English, thank a Veteran.
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Steve C.
Crazy Horse

1171 Posts

Posted - 22 Jun 2012 :  01:49:37 AM  Show Profile  Visit Steve C.'s Homepage  Reply with Quote
Another valid opinion, this from Ken Keefer (Pontiac Dude) who for years has made a living building Pontiac combos and racing pontiac combos.....

"I have found in my experiences tied to track results that higher compression and then having to go to longer LSA to kill the compression/bleedoff has resulted in lower ET's at the track."

"Just from my experience. I would rather run less compression and the slightly tighter LSA and more cam profile, then up compression with a wider LSA. Dyno numbers will look better but track ET's won't show it. Been there and done that. And not knowing what is in the pump from Summer to winter. Better safe then sorry. Make up compression hp loss elsewhere. There is a point of too low an LSA for the bore/stroke and rod ratio deals too. And we aren't talking aftermarket super high flowing heads in this post. Just basic Pontiac iron head technology. Peak HP doesn't win races."
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Cliff R
Cochise

USA
470 Posts

Posted - 22 Jun 2012 :  07:24:55 AM  Show Profile  Visit Cliff R's Homepage  Reply with Quote
Steve, might want to put up some results that Bob Davis found going from a solid roller on 110LSA to a smaller HR on a 112 LSA?

We also had a Canadian customer swap out his "Old Faithful" HR camshaft for a larger solid roller spec'd out for his combo. He said it ran no quicker anyplace. About all he got for his efforts were a "rough" idle and about the same ET and MPH at the dragstrip.

These topic come up frequently, and there are always plenty of opinions on either side. The only thing I find interesting about it, is that even very experienced engine builders don't understand what's really going on here.

If you run lower compression then make it back up with a smaller cam on a tighter LSA, and end up making the same power in the lower rpm range as a larger cam on a wider LSA, then the octane requires are exactly the same. What happens in many cases, is that you can actually pull power down lower with a tight LSA cam, and make more peak torque with lower compression. More peak torque at lower rpm's INCREASE octane requirements.

I like to use examples, and here is a very good one. 455, 71 HO round port heads, 9.5 to 1 compression, Comp 224/230/110 HR camshaft. At a glance the cam seems well chosen. We were hired to help the owner tune this engine, as it was pinging no matter what he did. We started adding fuel and pulling out timing till the pinging stopped. Total timing had to brought down to 26 degrees using the highest octane pump gas available to get it to stop hammering at full throttle. Power production was pathetic at that point. We were able to add a few degrees timig by installing higher ratio rocker arms, picked up some power, but it was still unacceptable for the combination of parts.

The Comp HR cam was simply too good at what it was doing, which is cylinder filling. The intake lobe was too far advanced, intake closing point too early, and dynamic compression too high. I've had dozens of calls into the shop with very similiar experiences, where folks used what they thought was a "safe" compression ratio, installed the wrong cam, and experienced detonation issues.

Bottom line here, you can get into trouble with these things, even if you lower the compression to normally accepted "safe" limits. The camshaft is equally as important as the compression ratio....FWIW......Cliff

If you can read this, thank a teacher. If you can read this in English, thank a Veteran.

Edited by - Cliff R on 22 Jun 2012 07:25:34 AM
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cortcomp
Coyote

USA
4369 Posts

Posted - 22 Jun 2012 :  09:37:03 AM  Show Profile  Visit cortcomp's Homepage  Reply with Quote
" even if you lower the compression to normally accepted "safe" limits. The camshaft is equally as important as the compression ratio"


I think that the two need to be considered as a matched pair. Someone pointed out that people were reading cam cards and putting 670s on 400s and 428s and then putting XE grinds in. I did exactly that, thought that it would be better than the summit generigrind in there.

Cranking pressure was like 215! It didn't spark knock, but it had a host of other issues (timing set was off, valves were kissing pistons, gasket leak at one head) so i never got to find out WHY it didn't knock like crazy before i tore it down.

The cam companies and magazines don't tell the whole story when swapping cams.
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Steve C.
Crazy Horse

1171 Posts

Posted - 22 Jun 2012 :  10:04:50 AM  Show Profile  Visit Steve C.'s Homepage  Reply with Quote
"Steve, might want to put up some results that Bob Davis found going from a solid roller on 110LSA to a smaller HR on a 112 LSA?"

For the most part he was pleased because of the slightly improved idle quality (and according to him less exhaust fumes in the garage).... all attributed to the wider LSA. Never apples-to-apples testing at the track because of changes to the car itself with the cam change, ran 'about' the same, no significant improvement. He changed from a XE 242/248 at .050" solid roller to the Old Faithfull hyd roller cam, I'll bet if your were to look at the actual duration 'at the valve' under running conditions the two cams would very be similar.

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Cliff R
Cochise

USA
470 Posts

Posted - 22 Jun 2012 :  10:27:09 AM  Show Profile  Visit Cliff R's Homepage  Reply with Quote
Agreed, actual valve movements and airflow potential are most likely nearly identical. You might also have added that we worked with him long enough that he got his Q-jet running the same ET and MPH as his custom Holley (Bear Claw?), while delivering improved throttle response and off idle smoothness/efficiency along the way. We talk frequently, and he couldn't be happier with the end result, much like I am with my own car......Cliff

http://www.youtube.com/watch?v=6zVdoLR-VzM&feature=plcp

If you can read this, thank a teacher. If you can read this in English, thank a Veteran.
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Steve C.
Crazy Horse

1171 Posts

Posted - 22 Jun 2012 :  11:44:18 AM  Show Profile  Visit Steve C.'s Homepage  Reply with Quote
Cliff your right. Bob is tickled pink with all the changes to his car, he only intends to race it twice a year at our local BOP event and wanted to make it more street friendly. He feels the Q-jet has better throttle response or improved off idle performance... or something to that effect. As far as having about the same performance, again I'm not 100 percent sure if he performed any back-to-back carb testing on the same day. I'll suggest it was like the time when you sent me one of your trick 800 cfm Q-jets to test on my car compared to my Holley HP950, my actual back-to-back track testing did indeed indicate it ran ABOUT the same. No arguement from me about the Q-jet in that realm. That said, I personaly did not experance any vast off idle improvement with my limited street driving using the Q-jet. But my 'worked' HP950 is fantastic in regards to throttle response. I removed the Q-jet because it is so ugly :)

Bob has also changed from the 'chrome colored' fiberglass bumpers to his origional factory chrome bumpers. He changed from his TH400 to a TH200-4R overdrive transmission. Changed back to power steering. Changed from his 3.89 gears in the Ford 9-inch to 3.50's. Changed to Tri-Y type headers for improved ground clerance along with complete change in the exhaust system from a 3.5" to a smaller 3" diameter tubing. And drum roll..... he installed aftermarket air conditioning. Last BOP outing he ran 11.70's, but that doesn't mean much since weather anf track conditions always vary.

Yes, he is a happy camper !
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Blued and Painted
Chief PONTIAC

USA
2542 Posts

Posted - 22 Jun 2012 :  11:45:30 AM  Show Profile  Reply with Quote
If I may jump into this love fest, Is rskram big flat tapet with quick ramps and relativley short overlap sutable for his compression level? Are there other flat tappet grinds he may want to look at? I ask because i have a hi compression build on the bench for later this year.

Bull Nose Formula, 461, R44TS, DEX/MERC
69 GTO-400/670/373/4Spd
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bdrac
Tribal Scout

1 Posts

Posted - 22 Jun 2012 :  11:51:44 AM  Show Profile  Reply with Quote
before i added a significant amount of weight with ac ,power brakes ,extra insulation my car ran the same track numbers with the comp solid roller vs old faithful cam. the old faithful cam has a better idle and smoother too with better vacuum.
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Steve C.
Crazy Horse

1171 Posts

Posted - 22 Jun 2012 :  12:20:51 PM  Show Profile  Visit Steve C.'s Homepage  Reply with Quote
"before i added a significant amount of weight with ac ,power brakes ,extra insulation my car ran the same track numbers with the comp solid roller vs old faithful cam. the old faithful cam has a better idle and smoother too with better vacuum."

There you go, straight from the horses mouth :) But important to note the small XE 242/248 solid roller with a 110 lobe separation did not have a poor idle quality. It was a pussy cat, and drove very nice ! Bob has also previously used XE solid rollers with 248 and 254 degrees intake duration.

And a link to a previous magazine article on Bob's car:
http://www.popularhotrodding.com/features/0804phr_bob_davis_64_pontiac_tempest_custom/index.html

Edited by - Steve C. on 22 Jun 2012 12:23:28 PM
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Blued and Painted
Chief PONTIAC

USA
2542 Posts

Posted - 22 Jun 2012 :  3:12:14 PM  Show Profile  Reply with Quote
Well there you go. If you dont like the cam, throw it out the back door and let it rust to dust.

Bull Nose Formula, 461, R44TS, DEX/MERC
69 GTO-400/670/373/4Spd
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screamingchief
Tribal Scout

USA
30 Posts

Posted - 23 Jun 2012 :  02:11:46 AM  Show Profile  Reply with Quote
I really dont know how this DCR thread got turned into a cam debate?

I say that as DCR theory as was being discussed earlier only takes a few given aspects of a given engine into consideration,and little else,the only aspect of the cam that DCR computations even care about is the intake closing event,and that given valve event can vary widely between cams,be they wide or narrow LSA cams,especially seeing that seat durations can vary widely as well.

Anyhow...

All DCR theory cares about is bore,stroke,rod length,static compression ratio,and the intake valve closing event timing,and any given atmospherics like boost or altitude.

DCR theory does'nt care what LSA the cam has,all it cares about is when the cam closes the intake valve.

Duration,LSA,and the installed centerlines all factor into determining when that happens.

Which is precisely why I said earlier that there is a whole lotta things that DCR theory wont account for @ all.

quote:
Blued and Painted:
Do you think rskram's cam selection is right for 10.75 SCR

I guess it's an decent enough cam for the most part,as far as how that cam relates to the rest of his combo.

As far as this DCR theory is concerned,it's certainly no worse a choice than the SD performance old faithful hydraulic roller would be,as an example for comparisons sake you can see that SD/OF/HR cam closes the intake @ 72 degrees ABDC when installed on a 108 degree intake centerline,now if rskram installed his solid cam @ 108 ICL like the SD cam uses,his would close the intake @ 73 degrees ABDC.

But that just shows how DCR theory kinda has tunnel vision...

The thing is,without any doubt,each of those given cams are going to behave quite differently than the other one will,and that is going to affect any combo to some degree for sure,and all this DCR theory stuff will never be able to tell you how either of those cams would affect that deal.

Some people seem to be confusing a running engine's cylinder pressure traits with all this DCR theory stuff,but they're really not the same things @ all.

Dynamic Compression Ratio is a kind of a poor name for what those formulas/calculators are all about,as it really is'nt all that dynamic at all.

DCR theory only concerns itself with a few very basic parameters,and that's in a very limited range of an engines operation,which is not necessarily even in a running engine,it really only tells you what's happening at lower (cranking) speeds when the engine is'nt even running,and other factors are'nt really in play.

There would be a ton of other factors that would need to be added to the DCR equations in order for it to be able to tell us what's going on in an actual running engine,things like VE would for sure come into play,as would head flow and exhaust scavenging and such,really too many things would need to be taken into account to figure that all out.

quote:
Blued and Painted:
Is rskram big flat tapet with quick ramps and relativley short overlap sutable for his compression level?

First thing first,that solid cam of his is'nt short on overlap @ all.

It has 74 degrees of total overlap,that's plenty of overlap.

That 44 degrees of overlap mentioned in his post is what his cam's overlap is @ .050",not what it has @ the seat.

And FYI that's ALOT of overlap @ .050".

For comparison,that SD/OF/HR cam has 73 degrees of total overlap,yet it has only 16 degrees of overlap @ .050".

That is bound to have some affect on all this,would it not?

But as I said earlier,DCR theory will never be able to tell us what affect that might be.

Simply put,this DCR theory stuff just does'nt account for everything that it needs to account for to be an accurate tool,so that's why I said that I feel that playing with these equations & DCR calculators are largely a waste of time,because they are'nt going to tell anyone what will work and what wont work,typically only hard earned experience will tell a person that sorta thing.

If anyone here is he!! bent on using this DCR theory stuff to choose how to build their engines,by all means they can be my guest,I just hope they wont be too surprised when the actual end results are'nt necessarily what those DCR calculators suggested they should be.

FWIW

Bret P.
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Cliff R
Cochise

USA
470 Posts

Posted - 23 Jun 2012 :  06:46:52 AM  Show Profile  Visit Cliff R's Homepage  Reply with Quote
"I say that as DCR theory as was being discussed earlier only takes a few given aspects of a given engine into consideration,and little else,the only aspect of the cam that DCR computations even care about is the intake closing event,and that given valve event can vary widely between cams,be they wide or narrow LSA cams,especially seeing that seat durations can vary widely as well."


True, but it's NOT that cut and dried. LSA, overlap, the area under the curve or "window" of opportunity to move air, flow capabilities of the heads, restrictions in the intake/exhaust systems, scavenging, and about a half dozen other things effect cylinder filling capabilities. True the intake closing point is a key player here, but even more important is how well the cylinder is filled when the valve actually closes, and everything above the piston gets compressed.

"Simply put,this DCR theory stuff just does'nt account for everything that it needs to account for to be an accurate tool,so that's why I said that I feel that playing with these equations & DCR calculators are largely a waste of time,because they are'nt going to tell anyone what will work and what wont work,typically only hard earned experience will tell a person that sorta thing."

+1

The biggest problem with this topic, is that folks continue to look at these things in "static", not "dynamic" terms. You have to consider the amount of cylces per second the valves are making as rpm's increase. The time required to move air in and out of the engine for each cycle is diminished as rpms increase. In the N/A engine, low rpms require mostly on the downward movement of the piston for cylinder filling. They get more help at higher rpm's from the cam design, meanng duration, overlap, and actual opening and closing events.......Cliff


If you can read this, thank a teacher. If you can read this in English, thank a Veteran.

Edited by - Cliff R on 23 Jun 2012 06:47:39 AM
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rskram
Cochise

265 Posts

Posted - 23 Jun 2012 :  09:34:31 AM  Show Profile  Reply with Quote
So it sounds like I made the correct cam choice for what I want out of my combo......I want a rowdy, 10 second street car. I've studied a lot of ""complete combo recipes" and I'm pretty confident that this one will give me what I want. Was gonna go solid roller but with budget constraints I decided not to. Really no need anyway, yes its overall a little better but the higher price tag doesn't justify it for me. Im pretty sure that I can be happy with the power produced with this combo and don't need to go with a roller to be happy. I know that most people like to go that route nowadays and thats cool, to each his own. But after a lot of thought and "lack of funds" this works for me. I saved between 1000-1500 bux because I'm too anal and would have had to have $800+ crower lifters, $300+ springs and such. I will make more power than a 242@.050 XE solid roller setup and probably have close to the same "idol with attitude" but spend a lot less money....that works for me. I know.....the XE cams don't require exotic valve train parts, but I would have put them in it anyway! Thank-you to the knowledgeable people for their input on this thread so far.

Edited by - rskram on 23 Jun 2012 09:36:53 AM
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Cliff R
Cochise

USA
470 Posts

Posted - 23 Jun 2012 :  10:57:25 AM  Show Profile  Visit Cliff R's Homepage  Reply with Quote
Unless I missed it someplace, no mention was made of the rocker arm ratio being used?

Always modify the heads for high ratio rockers, whether or not you start out using them or not.

Going from 1.5' to 1.6's, or even on to 1.65's are like a small cam change, and will lower DCR a bit, all else being equal.

We have actually gained ground when tuning a few engines that pinged pretty hard on pump gas with 1.5 rockers by installing higher ratio rockers.

Using high ratio rockers on a flat cam, also make it act more like a roller cam with similiar specifications. Think of it like this. A roller cam doesn't have the limitations between the lobes and lifters like flat cams do. They can get the valve up to full lift quicker, hold it there longer, and drop it faster, moving more air with the same seat to seat timing as a similiar flat cam. Top your flat cam with high ratio rockers, and you make everything the lobes are doing now better, similiar to what roller cams do right to start with....Cliff

If you can read this, thank a teacher. If you can read this in English, thank a Veteran.
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rskram
Cochise

265 Posts

Posted - 23 Jun 2012 :  11:31:18 AM  Show Profile  Reply with Quote
I have some 1.6's that I planned on using so that might workout good. I have plenty of time to sort this out because its not gonna run til next spring.....unfortunately.
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phil400
Cochise

Canada
301 Posts

Posted - 23 Jun 2012 :  1:28:22 PM  Show Profile  Reply with Quote
This has been one the best topics ever,no bickering,just great knowledge being passed along

78T/A 4-SPEED
"sometimes I do what I want to do,rest of the time I do what I have to"
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Blued and Painted
Chief PONTIAC

USA
2542 Posts

Posted - 23 Jun 2012 :  4:10:23 PM  Show Profile  Reply with Quote
Cliff, I understand that if you make a recommendation on an informal sight like PSP, it puts your name on a project you know nothing about. Thanks for the Info.

Bull Nose Formula, 461, R44TS, DEX/MERC
69 GTO-400/670/373/4Spd
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Steve C.
Crazy Horse

1171 Posts

Posted - 23 Jun 2012 :  4:54:36 PM  Show Profile  Visit Steve C.'s Homepage  Reply with Quote
Interesting tid bit for intertainment.
The potential effects of a running engine on theoretical input into a DCR calculator. Using the following spec...

4.180 bore / 4.210 stroke / 6.625 rod
9.5 static compression ratio
90 percent VE%
Sea level / no boost

Single pattern hydraulic roller cam using Comp XE lobe 3315 (rated 230 at .050") Catalog rated duration 282 degrees

Calculator input specs:
282/282
110 lobe separation
106 ICL (4 adv)
1.5 rocker ratio on intake & exhaust
Intake close 67 degrees ABDC

Using the Wallace Racing calculators:

DCR 7.5 / 146.16 psi
Effective compression ratio 6.75

Now let's start the engine...

Based on Spin-Tron testing, under actual running conditions measured 'at the valve' that specific lobe actually has 277 degrees of duration. The Spin-Tron machine indicats 5 degrees less duration than the theoretical number. The reason, a hydraulic roller lifter under actual running conditions acts like a shock absorber, the lifter "gives" just a little as you load it under running conditions. This 'give' reduces the duration slightly.

Now use this 277 running degrees for input into the calculators...

Calculator input specs:
277/277
110 LS / 106 ICL
1.5 rocker ratio on intake & exhaust
Intake close 64.5 ABDC

DCR 7.64 / 149.73 psi
Effective compression ratio 6.88

( Note: A similar situation happens on a solid cam when you factor in valve lash. It effects the running duration at the valve )











Edited by - Steve C. on 23 Jun 2012 4:56:24 PM
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Total Jackass
Tribal Scout

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Posted - 14 Jan 2014 :  04:17:44 AM  Show Profile  Reply with Quote
Q. What did the pencil sharpener say to the pencil?
http://www.amreplicawatches.com/iwc-c-99.html

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